Thursday, 4 June 2009 - 15.00
SISSA Lecture Room A

ARRIGO CALZOLARI
( CNR-INFM S3 - Modena )

"AB INITIO STUDY OF HYBRID INTERFACES FOR PHOTOVOLTAIC APPLICATIONS"


Metal-oxide semiconductors have been profitably investigated for applications in catalytic systems and optoelectronic devices. In particular, it has been largely demonstrated that the metal-oxide nanoparticles may be sensitized to harvest visible radiation in working solar cells, upon molecular dye functionalization. However, at present the principles regulating interactions between the oxide layer and the rest of the photovoltaic cell are poorly understood. Indeed, upon molecular sensitization, the semiconductor surface interacts with molecules in a wide range of possible scenarios, driving the formation of hybrid interfaces with specific spatial arrangement and novel electronic properties, and affecting both light harvesting and the generation of the photovoltaic current.
In order to unravel these problems from a fundamental point of view, we provide a microscopic description of oxide-based interfaces, by means of first principles calculations, based on static and time-dependent Density Functional Theory.
We focus on non-polar ZnO(10-10) surface in connection either with natural organic dyes (anthocyanins) and with water solution. We first characterize the optoelectronic properties of the dye both in the gas phase in liquid water. Then, we consider the effects of water adsorption on the ZnO surface, and the role of surface defects. Finally, the adsorption of the molecular chromophore on the ZnO surface is presented.

 Wednesday, 1 April 2009 - 16.00
SISSA Lecture Room A

Alessandro Silva
( ICTP, Trieste )

"The fermi edge singularity, work statistics, and quantum quenches"


A quantum quench is a rapid change in time of a control parameter in the hamiltonian describing a quantum, correlated many body system. The physics of this paradigmatic nonequilibrium process, which has been realized recently in the context of cold atoms, is of particular interest in quantum critical systems. In this talk I will discuss a fundamental way to characterize these processes using basic thermodynamic variables and looking at their statistics. I will do so by first elucidating the connection between a classic problem in condensed matter physics, the Fermi edge singularity, the statistics of the work done in a quantum quench, and quantum Jarzynski equalities. Using this connection, I will then characterize the statistics of the work done on a quantum Ising chain by quenching locally or globally the transverse field, showing that for local quenches starting at criticality, the probability distribution of the work displays an interesting edge singularity.
I will finally show that similar singularities emerge in other quantum critical systems, such as the Dicke model..

 Tuesday, 15 July 2008 - 15.00
SISSA Lecture Room D

Prof. Mike Gillan
( Department of Physics and Astronomy, University College London )

"First-principles statistical mechanics of thermal desorption"


Temperature programmed thermal desorption (TPD) is one of the most widely used experimental methods for determining adsorption energies of molecules on surfaces. To interpret such experiments, there is a strong need for techniques for calculating thermal desorption rates from first principles. I will present some of the methods and results that we have we have obtained in the past two years at UCL.

Tuesday, 1 July 2008 - 16.00
SISSA Lecture Room A

GIANCARLO RUOCCO
( Dipartimento di Fisica and SOFT-INFM-CNR -Universita' di Roma "La Sapienza", Rome, Italy )

"THE PRESENT UNDERSTANDING OF THE VIBRATIONAL DYNAMICS IN GLASSES"


After almost ten years of application of the Inelastic X-ray Scattering (IXS) technique, which complements the well established Inelastic Neutron Scattering (INS) one, the main characteristics of the high frequency (THz) collective modes in topologically disordered systems have been identified. Despite the contributions of this new experimental tool, less is know about the collective excitations of glasses in a momentum-energy region that lie between the IXS (Q>1 nm-1) and the traditional Brillouin Light Scattering (Q<0.04 nm-1) domains. Very recently, the first steps were taken to fill this gap using the synchrotron based UV Brillouin spectroscopy.
In this talk I will review what is the present understanding on the high frequency dynamics of glasses, as obtained by INS and IXS experiments. Moreover I will briefly present new measurements of the sound attenuation in vitreous silica, performed by UV Brillouin spectroscopy, for Q values up to 0.12 nm-1. These data, partially bridging the gap between the existing BLS and IXS data, allows us to depict a coherent description of the sound attenuation phenomenon in v-SiO2, and to reveal the existence of very long range stress correlations in this material.

 Monday, 30 June 2008 - 16.00
SISSA Lecture Room A

LAZARO CALDERIN
( Dept. of Theoretical, Atomic and Optical Physics, Univ. of Valladolid, Spain )

"ON THE OVERALL SCALING OF THE KOHN-SHAM POTENTIAL AND THE VIRIAL AS A DENSITY FUNCTIONAL"


Consequences of uniform coordinate scaling of the independent particle kinetic energy functional of density functional theory, Ts[n], are discussed. A scaling relation for the functional derivative of Ts[n] is obtained which leads to a scaling relation for the Kohn-Sham potential, while the scaling of the Kohn-Sham equations also provides information on the scaling of the eigenvalues. It is also proved that the scaling relation for the functional derivative of Ts[n] can be obtained by direct functional differentiation of the scaling expression for Ts[n]. Numerical evidence of such a scaling are discussed. An expression is deduced for the virial in terms of the ground state density and the corresponding Kohn-Sham potential. It is shown that the virial is the kinetic energy functional for a 2-electron systems while for N-electron systems the external potential as a functional of the density preclude us from having an explicit form for the non-interacting kinetic energy functional. An approximated kinetic energy functional for N-electron systems, based on the virial, is presented and some numerical results discussed

 Monday, 30 July 2007 - 16.00
SISSA Lecture Room A

KEIVAN ESFARJANI
( Department of Physics, Sharif University of Technology and UC Santa Cruz Santa Cruz, CA 95064 USA)

"NEGATIVE DIFFERENTIAL RESISTANCE IN NANOCONTACTS AND ATOMIC CHAINS"


NDR has been observed in several devices, in particular resonant double barrier structures. It occurs when with increasing bias the resonant state moves in the gap of one of the contacts. In this talk, we present another mechanism, which occurs in nanocontacts or atomic chains connecting two contacts. It has to do with the localization of states at the two contacts and a reduction in the local density of states (LDOS) of the right/left leads. We express the conditions necessary for the observation of NDR, and as an illustration, present our findings for atomic chains bridging two capped carbon nanotubes or graphene ribbons.

 Thuesday, 24 July 2007 - 10.00
SISSA Lecture Room A

MATTEO FARNESI CAMELLONE
( EMPA, Materials Science and Technology, Electronic/Metrology Laboratory, CH-8600 Duebendorf, Switzerland)

"POINT DEFECTS IN CRYSTALLINE AND AMORPHOUS SILICA"


The essential limitations on gate insulator are related to the exponentially increasing gate current as the thickness of the oxide is reduced, and the effect of this current on both the functionality and reliability of devices and circuits. Quantum mechanical (ab initio) calculations models can be used to simulate the formation of  atomic defects of a realistic material structure. Density functional theory and Hartree Fock theory have been used to model and study the nature of point defects in crystalline and amorphous silica. The amorphous silica models have been generated using a combined classical and ab inito molecular dynamics simulation scheme. The nature of oxygen vacancies, the role of  hydrogen species and self trapped holes in SiO2 have been investigated.

Monday, 2 July 2007 - 16.00
SISSA Lecture Room A

EDUARDO LAMAS
( Texas A&M University, College Station, TX )

"COMPUTATIONAL STUDIES OF THE CATHODE REGION IN PROTON EXCHANGE FUEL CELLS"

Thursday, 28 June 2007 - 16.00
Aula A1 (3rd floor)
Dipartimento Scienze Chimiche
Università di Trieste, Edificio C11, Via Giorgeri 1

Prof. FRANCESCO GIANTURCO
( Università di Roma "La Sapienza" )

"Modelling radiation damage in biosystems mediated by slow electrons"

Wednesday, 13 June 2007 - 14.30
SISSA Lecture Room A

KATALIN GAÁL-NAGY
( Dipartimento di Fisica and ETSF, Università degli Studi di Milano )

"AB-INITIO STUDY OF THE BETA-TIN -> IMMA -> SH PHASE TRANSITIONS IN BULK SILICON"


We present an ab-initio study of the lattice statics and dynamics of Si near the high-pressure Β-tin→Imma→ sh phase transitions.
In a first step, temperature effects are neglected and the phases are investigated with respect to their stability. Herewith, the order of the phase transitions is determined and the results are compared with experimental data.
In a second step, the temperature dependence of the transition pressure is investigated within the quasiharmonic approximation. This yields a detailed study of the phonon frequencies of the structures where especially the phonon frequencies at the point are inspected in detail. We found a soft-mode behavior for the phase transition sh→Imma which can be used for the determination of the transition pressure. A comparison of our data with the experimental ones shows that the phase transitions Β- tin→Imma→sh have been already indicated in the Raman spectra even though not realized because of marginal changes.
The convergence of the phonon spectra is very important for the determination of the temperature dependence of the transition pressure and a new method for accelerating the convergence has been developed for that purpose. Furthermore, we have found a Kohn anomaly at the bct structure, however, beyond the stability range of the Β-tin phase.
This work has been performed using the plane-wave pseudopotential approach to the density-functional theory and the density-functional perturbation theory within the local density approximation as implemented in the Quantum-Espresso package.

Thursday, 7 June 2007 - 15.00
SISSA Lecture Room A

KEVIN LEHMANN
( University of Virginia )

"SOLVATION OF IMPURITIES IN LIQUID HE DROPLETS:
A MIXED THEORETICAL/EXPERIMENTAL PERSPECTIVE"

Friday, 11 May 2007 - 10:00
Aula E

E. VITALE
( IBM Zurich )

"Metal-Carbon Nanotube Contacts: the Link between Structure and Electrical Behavior from First-Principles Calculations"


Field-Effect Transistors based on Carbon Nanotubes are promising candidates for post-CMOS microelectronics. It has by now been established that transport in these devices is dominated by the Schottky barriers existing at the metal-channel interfaces. We used Car-Parrinello molecular dynamics calculations to investigate the actual structure of the metal-CNT interface for two representative metals, Al and Pd. Our calculated Schottky-Barrier Heights (SBH) show a good agreement with experiment. Moreover, we find a correlation between the SBH and the nature of the chemical bonding at the interface.

Monday, 18 September 2006, 10.00 SISSA Room A

GIANLUCA RASTELLI
(LEPES-CNRS Grenoble)

"WIGNER CRYSTALLIZATION IN LOW DIMENSIONAL AND POLARIZABLE SYSTEMS"


Among several possible microscopic scenarios for the metal-insulator transition, Wigner crystallization involves uniquely the long-range Coulomb repulsion betweenelectrons at low density. A considerable amount  of recent experiments have been accumulated in low dimensional (or anisotropic) compounds, where the charge ordering phenomena observed at low temperatures are either interpreted as some form of Wigner crystallization or ascribed to the presence of long range Coulomb interactions. Motivated by these observations, a model that describes Wigner crystallization in an anisotropic environment will be presented in the first part of the seminar. I will show that restricting the electron motion into layers or chains, while keeping the full three-dimensional nature of the Coulomb interactions, can result in a sizeable stabilization of the crystallized phase as compared to the usual  three or two-dimensional case. A sequence of structural transitions  of the electron crystal is also predicted  upon varying the density.In the second part, I will discuss the effect of electron-phonon interaction on Wigner crystallization. In real solids the phonon dynamics can be coupled to the electrons and in principle it cannot be neglected. When electron-phonon interaction is strong, polarons are formed leading to qualitative modification of the phase diagram.

Wednesday, 27 September 2006, 11.00 SISSA Room A

MASSIMO BONINSEGNI
(Department of Physics, University of Alberta)

"PHASES OF HARD CORE BOSONS IN OPTICAL LATTICES"


Impressive scientific and technological advances in trapping cold atoms in optical lattices (OL) render it now feasible to create in the laboratory remarkably close experimental realizations of model many-body systems long thought of as of mostly academic interest. The possibility of "fine-tuning" the interactions, as well as of carrying out a direct experimental verification of quantitative predictions, has prompted renewed effort in the theoretical study of "exotic" phase of matters that one may observe in OLs. In this talk, we shall discuss two examples of many-body systems  that display intriguing phase diagrams, both involving hard core bosons on two-dimensional lattices; one is a single-component system on a triangular lattice, the other an isotopic mixture on the square lattice. Results of recent studies based on numerical (Quantum Monte Carlo) simulations will be presented.

Monday, 3 July 2006, 10.00 Lecture Hall A
      ICTP Adriatico Guest House (lower level 1)

RICHARD M. MARTIN
(Dept. of Physics, University of Illinois
LSI Lab, Ecole Polytechnique)

"INSULATORS, METALS, AND FRACTIONALIZED STATES - THE MOTT TRANSITION AND THE LUTTINGER THEOREM"


Recent work has led to a new approach for understanding different states of condensed matter: insulators, metals, and states with fractional quantum numbers. The fundamental distinction for metals vs. insulators proposed by Kohn in 1964 is the sensitivity to boundary conditions.  This has been extended in two ways: A theory of the insulating state [1] that  provides a quantitative measure applying to all systems independent of interactions. Recent work [2] that generalizes the ideas  to the topological behavior of the wavefunction as a function of  boundary conditi This approach reveals the relation of fundamental concepts including the Aharonov-Bohm effect, Luttinger theorem, fractional quantum Hall effect, and other fractionalized states. This talk will give an overview of the ideas and will focus upon the nature of the insulating state, the Luttinger theorem on the volume enclose by the Fermi surface,  and new proposals for the nature of a Mott insulator.


1. I.Souza, T. J. Wilkens, and R. M. Martin, "Polarization and localization in insulators: generating function approach", PRB 62, 1666 2000).
2. See, for example, Oshikawa, "Insulator, Conductor, and Commensurability: A Topological Approach"  PRL 90, 236401 (2003).
3. A. Seidel, et al. "Incompressible Quantum Liquids and  New Conservation Laws", PRL 95, 266405 (2006).

Thursday, 29 June 2006, 11.00 SISSA Room B

MARCELLO CIVELLI
(ILL, Grenoble, France)

"INVESTIGATION OF THE MOTTNESS IN THE TWO DIMENSIONAL HUBBARD MODEL USING CELLULAR DYNAMICAL MEAN FIELD THEORY"


I present a study of the 2-D Hubbard model (relevant for H-Tc superconducting materials) close to the density-driven Mott metal-insulator transition, using a Cluster-extension of Dynamical Mean Field Theory (the CDMFT). In recent years, Dynamical Mean Field Theory (DMFT) has proved a successful tool to access the physics of strongly correlated electron systems. DMFT is a self-consistent mean-field approach which fully describes the quantum-dynamics on a single site, neglecting by construction spatial correlations. We show however that this is a too crude approximation in describing systems where spatial correlations are fundamental, like for example cuprate superconductor materials. Contrari to DMFT, CDMFT is able to take into consideration the short ranged physics by replacing a single site by a cluster or neighboring sites. We apply CDMFT to the 2-D Hubbard model, studying its anomalous normal-state properties and the anomalous superconductive state deriving from them.

Wednesday, March 29, 2006, 12:00 SISSA Room A

Fabio Pietrucci
(Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca)

"Ab-initio study of oxygen vacancies in yttria-stabilized zirconia"


Zirconia (ZrO2) stabilized in the cubic fluorite phase by the addition of yttria (Y2O3) is a fast oxygen conductor widely used in solid oxide fuel cells and oxygen sensors. Although the number of carriers (oxygen vacancies) increases with yttria content, the conductivity reaches a maximum for a dopant concentration of 9 mol%, decreasing beyond this value. This complex behaviour is believed to stem from interactions among defects, including the formation of a complex of two vacancies aligned along á111ñ directions. However, a compelling experimental evidence of this defect, which has also been tentatively associated to prominent signals in EPR and optical spectra of reduced samples, is still lacking. In fact, other extrinsic defects (such as Ti) have also been claimed responsible for the spectroscopic signals. In this work we have studied the electronic properties of paramagnetic di-vacancy complexes by computing the EPR g-tensor from density functional perturbation theory [1]. The paramagnetic defects are modeled in periodic supercells of zirconia with 10 or 14 mol% yttria containing up to 93 atoms. The calculated EPR g-tensors of both a Zr3+ or a Ti3+ ion at the center of a di-vacancy aligned along the á111ñ directions are compatible with the experimental EPR signal. However, comparison of experimental optical absorption spectra with calculated optical excitations at the B3LYP level allows us to decide in favor of the Ti3+ di-vacancy complex [2]. Preliminary results will be also presented on the diffusion processes of oxygen vacancies. The results show that the activation barrier for the diffusion of a single vacancy does not depend only on the vacancy-yttrium interaction as customarily assumed, but also on the position of the other vacancies, which might also promote concerted multiple jumps.



1. C. J. Pickard and F. Mauri, Phys. Rev. Lett. 88, 86403 (2002).
2. F. Pietrucci, M. Bernasconi, C. Di Valentin, F. Mauri, and C. J. Pickard, Phys. Rev. B, in press.

Friday, 20 January 2006 - 16:00
SISSA Room A

MARIALORE SULPIZI
( Cambridge University Centre for Computational Chemistry Department of Chemistry, University of Cambridge, UK )

"AB INITIO MOLECULAR DYNAMICS SIMULATIONS OF REDOX REACTIONS: THE MARCUS PERSPECTIVE"


Electron transfer reactions are among the most fundamental chemical processes and are crucial in a number of important biological processes, such as photosynthesis and cell respiration. Nature and chemists alike have been tuning electron donor (D) and acceptor (A) sites or their environments to obtain ET rates and relative D-A stabilities optimized for a particular task. Here we employ an atomistic theoretical approach based on molecular mechanics and ab initio molecular mechanics to quantitative predicts the effects of such modifications on parameters that control the ET within the Marcus theory perspective. In particular examples of ruthenium-amine complexes and quinones in different solvents will be discussed. Our work confirms recent experiments and helps to rationalize the differences in redox potentials and reorganization energies due to chemical substitutions and changes in the environments.

Thursday, 22 December 2005 - 11:00
SISSA room A

Fabrizio Anfuso
( Dept. of Physics, Chalmers University of Technology, Goteborg )

"IMPURITIES IN LOW-DIMENSIONAL ANTIFERROMAGNETS"


We present Monte Carlo results for the local magnetic response around vacancies in the 2D Heisenberg model and we outline their relevance to NMR and susceptibility experiment (Cond-Mat/0511001). We also discuss an effective potential between static vacancies and the relation of this quantity to the the local VB order of quantum AF(Cond-mat/0512001).

Monday, 26 October 2005 - 11:00
SISSA room A

Cyrus Umrigar
( Cornell University )

"OPTIMIZATION OF MANY-BODY WAVE FUNCTIONS AND TIME-EVOLUTION OPERATORS WITH AND WITHOUT THE FIXED-NODE BOUNDARY CONDITIONS"


I will (perhaps foolishly) try to discuss two disjoint topics in quantum Monte Carlo. First, I will discuss some recent progress on the optimization of wave functions and point out that different issues arise in optimizing the Jastrow and the determinantal parameters. In the case of the Jastrow parameters, it is possible to gain a few orders of magnitude in efficiency by adding terms to the straightforward expression for the Hessian of the energy that are zero when the integrals are performed exactly, but that cancel much of the statistical fluctuations for a finite Monte Carlo sample. I will also show the example of a very simple molecule for which we have learned by optimizing a multideterminantal wave function that



1. the fixed-node error of a single-determinant wave function is shockingly large and
2. that it is very important to reoptimize the coefficients multiplying the determinants that one gets from a MCSCF calculation.

In the second part of the talk I will use the toy example of a simple harmonic oscillator in 1-dimension to demonstrate


1. the differences in the EXACT propagator with fixed-node boundary conditions and the EXACT propagator without fixed-node boundary conditions
2. a better approximation to the fixed-node propagator than the one usually employed
3. a "cross-node" propagator that is a good approximation to the propagator without boundary conditions
4. present some speculative ideas on combining this cross-node propagator with Sandro Sorella's stochastic reconfiguration idea to improve upon Ceperley's release-node algorithm.

Acknowledgements: much of the first part of this talk was done in collaboration with Claudia Filippi and has been recently published with her and are closely related to work done independently by Sandro Sorella. The ideas for the 2nd part of the talk were developed in the 1990's with the help of Andreas Savin, but have not yet been published.

Monday, 29 August 2005 - 16:00
SISSA room A

Giuseppe Carbone
( Dipartimento di Ingegneria Meccanica, Politecnico di Bari, Bari, Italy )

"ADHESION AND FRICTION OF RUBBER"


The mechanical response of a compliant solid is of fundamental importance in a large number of industrial and hi-tech applications. Some examples are the structural adhesives used in the aerospace and automotive industries, the adhesive interlayer of laminated glasses, the thin films used as protective coatings in orthopedic implants, or a new class of adhesives which replicate the attachment systems of some biological species, e.g. the Gekko Gecko. In all such applications, the rheological properties of the layer, its thickness, the interfacial energy and the roughness of the substrate play a major role in controlling the "adhesive strength" of the joint. Adhesion is also fundamental in sliding friction on flat and rough surfaces and significantly affects the handling and breaking performances, e.g., of F1-tires. The interplay between adhesion and roughness is fundamental in all such applications. In fact, surface roughness can remove the adherence force almost completely: When an elastic body is brought into contact with a rigid rough substrate, a certain amount of elastic energy is stored at the interface, this elastic energy can be used to break the adhesive bonds, so that even a relative small surface roughness can completely remove the adhesion. In this talk it is shown how adhesion may be related to friction of rubber. We show that the sliding process may be considered as resulting from a closing crack at the front edge, and a opening crack at the trailing edge of the contact regions, and that, because of adhesion hysteresis, the amount of energy required to open the crack largely overcome the amount of energy recovered during crack closing, thus producing a net friction force. We also show that adhesion largely affect the area of real contact on rough surfaces even when the adherence force is completely removed, and explain why in many biological systems, showing a high adhesive ability, as for example the gecko foot pad, a very thin leaf-like plate (spatula) is found at the end of each thin fiber: The plate can easily bend to follow the long-wave surface roughness profile and therefore enhance adhesion.

Thursday, 26 May 2005 - 16:00
SISSA room B

Claudio Fernan
( Molecular Biology Department Max Planck Institute for Biophysical Chemistry )

"New insights into the Metallobiology of Parkinson\u2019s disease: Structural and biological implications of Cu(II) binding to a-Synuclein"


The objective of our studies is to establish the role of metal ions in synucleinopathies at the molecular resolution currently available for other amyloidoses. In this work, the interaction of Cu(II) with a-Synuclein (AS) was studied under physiologically relevant conditions using a battery of low and high-resolution spectroscopic techniques (CD, EPR and NMR) and chemical modification.1 Cu(II) levels in the micromolar range were effective in accelerating AS aggregation. The interactions of Cu(II) and AS were characterized at single-residue resolution by NMR. We showed for the first time that Cu(II) binds to a long-range preformed interface located in the N-terminal domain of AS (KD=100nM). A second binding motif is located at the C-terminus, were Cu(II) binds with lower affinity (KD=50mM). Cu(II) as an structural probe indicated that AS adopts in solution an ensemble of conformations characterized by a well-defined long-range topology in the N-terminal domain and isolation of the NAC region from the surrounding environment. Cu(II) might act as a conformational switch, releasing long-range interactions that are critical to stabilize a soluble native auto-inhibited conformation of AS.2,3 The new insights into the structural basis of copper interaction with AS support a tighter link with other amyloid-related disorders such as Alzheimer\u2019s disease and prion disease, indicating that perturbations in copper metabolism may constitute a more widespread element in neurodegenerative disorders than recognized previously.

1. Rasia et al, , Proc. Natl. Acad. Sci. U.S.A. (2005) 102, 2490.
2. Fernandez et al, EMBO J. (2004) 23, 2039.
3. Bertoncini et al, Proc. Natl. Acad. Sci. U.S.A. (2005) 102, 1430.

Tuesday, 17 May 2005 - 15:00 SISSA Room A

RAFFAELE RESTA
( INFM-DEMOCRITOS & Dipartimento di Fisica Teorica, Univ. Trieste )

"ORBITAL MAGNETIZATION AS A BULK PROPERTY OF EXTENDED SYSTEMS"


(Work performed in collaboration with D. Ceresoli, T. Thonhauser, and D. Vanderbilt) The magnetic dipole moment of any finite sample is well defined, while it becomes ill defined in the thermodynamic limit, due to the unboundedness of the position operator. Effects due to surface currents and to bulk magnetization are not easily disentangled. The corresponding electrical problem, where surface charges and bulk polarization appear as entangled, has been solved in the 1990s by the modern theory of polarization, based on a Berry phase. We proceed here on a similar path, and we derive a bulk expression for the orbital magnetization of a non-interacting (band) insulator with broken time-reversal symmetry.

Our theory is limited to the case of vanishing (or commensurate) magnetic field and zero Chern number. We start expressing magnetization in terms of localized orbitals: the dissipationless current circulating in the sample is decomposed in two contributions: the first arises from the internal circulation of bulk-like Wannier functions in the interior of the sample, while the second arises from net currents carried by the localized orbitals at the boundary of the sample. While the fist term has an exact electrical analogue, the second one is qualitatively new. We show that when both contributions are included, the result can be transformed into the Brillouin-zone integral of an elegant k-space expression, somehow reminiscent of the Berry connection and curvature appearing in electrical polarization and Hall conductivity. Our result is consistent with a recent semiclassical argument by Niu and coworkers (cond-mat/0502340).

Numerical implementations on a 2d tight-binding model are used to illustrate how circulating currents flow through the sample and to demonstrate the correctness of the derived formula.

Interestingly, our final expression remains well-defined even for metals, but it is not yet clear whether it is correct in that case. It is also unclear whether it can be extended to insulators with nonzero Chern numbers.

Tuesday, 22 March 2005 11:00 SISSA Room A

GAROLD MURDACHAEW
(Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA)

"Accurate ab initio intermolecular potentials with a focus on flexible water"


Understanding the weak van der Waals forces which act between molecules is essential for predicting the spectroscopy of clusters and the properties of molecular condensed phases. In the first half of the talk, I will briefly review the ab initio theory of intermolecular forces, in particular the symmetry-adapted perturbation theory (SAPT), followed by a discussion of intermolecular potentials and the approximations made in developing such potentials (e.g., level of theory, finite basis sets, and assumption of monomer rigidity). The second half of the talk will be devoted to describing the development of an accurate flexible potential for water explicitly dependent on the monomer geometries. Why does no such purely ab initio potential yet exist for this important and ubiquitous substance? The answer is dimensionality: developing a flexible water potential requires the sampling of a 12-dimensional configuration space, while for rigid water that space is only 6-dimensional. Since the accurate calculation of interaction energy at each configuration (grid point)is expensive, only recently have computational resources become available to allow us to generate the large number of points needed for an accurate 12-dimensional fit. The procedure used in selecting monomer and dimer configurations will be described. The final fit was performed to 240,000 grid points as well as to ab initio flexible multipole moments and van der Waals induction and dispersion coefficients. The fit will be analyzed and its preliminary predictions will be discussed.

Tuesday, 22 February 2005, 14.00 SISSA Room A

Luciano COLOMBO
(INFM-DEMOCRITOS, INFM-SLACS, and Dipartimento di Fisica Università di Cagliari)

"The atomic scale origin of crack resistance in brittle fracture"


We investigate the physical meaning of the intrinsic crack resistance in the Griffith theory of brittle fracture by means of atomic-scale simulations. By taking cubic SiC as a typical brittle material, we show that the widely accepted identification of intrinsic crack resistance with the free surface energy underestimates the energy release rate. Strain dependence of the Young's modulus and surface energy, as well as allowance for lattice trapping, improve the estimate of the crack resistance. In the smallest scale limit, crack resistance can be fitted by an empirical elasto-plastic model.



1. A. Mattoni, L. Colombo, and F. Cleri, Phys. Rev. B70, 094108 (2004)
2. A. Mattoni, L. Colombo, and F. Cleri, submitted for publication (Jan. 2005)

Thursday, 10 February 2005, 16.30 SISSA Room D

Anna Maria Capelli
(GlaxoSmithKline S.p.A., (Verona))

"Computational Chemistry"


Computational aspects of rational Drug Design in GSK

Tuesday, 1 February 2005, 16.00 SISSA Room B

Rodolfo Del Sole
(INFM and Dipartimento di Fisica, Università di Roma "Tor Vergata")

"Ab-initio theory of excited states: electronic and optical properties of surfaces"


The microscopic study of complex systems has nowadays reached a high level of accuracy that allows for a deep understanding of the electronic excitations. Ab-initio descriptions of experiments such as direct or inverse photoemission, optical absorption, electron energy loss, have become possible thanks to the huge progress in theory and the increased computational power. Charged excitations, as well as neutral excitations, can now be studied within the Many-Body Perturbation Theory based on the Green's function formalism. Optical spectra can be calculated with inclusion of the electron hole interaction by solving the Bethe Salpeter equation (BSE) within the framework of Green's function theory. Moreover, optical spectra can be nowadays also well described within the Time Dependent Density Functional Theory (TDDFT).

In this seminar we will present examples of calculations of surface optical properties according to different approximations. In particular, we will show that the BSE and TDDFT approaches yield the same optical spectrum for Si(111)2x1.

Tuesday, 7 December 2004, 11.00 Seminar Room ICTP M. B. (first floor)

C. Masciovecchio
(Elettra Synchrotron Light Laboratory, Trieste)

"Collective dynamics in disordered systems studied by inelastic UV scattering"


The recent construction of an Inelastic UltraViolet Scattering (IUVS) beamline at the ELETTRA Synchrotron Light Laboratory opens new possibilities for studying the density fluctuation spectrum, S(Q,E), of disordered systems in the mesoscopic momentum (Q) and energy (E) transfer region not accessible by other spectroscopic techniques. We will present first IUVS results obtained on two prototype samples such as liquid water and fused silica [1,2]. In water we were able to measure the temperature dependence of the structural relaxation time showing that the divergence in the transport properties does not need an underlying critical behavior but can be explained in the framework of the Mode Coupling Theory [3]. In vitreous silica IUVS experiments gave clear experimental evidence of the presence of a step in the sound attenuation coefficient at wavelengths between 6 and 60 nm thus locating a characteristic length where sound waves interfere with glass inhomogenities.



1. C. Masciovecchio, S.C. Santucci, A. Gessini, S. Di Fonzo, G. Ruocco, and F. Sette,  Physical Review Letters 92, 255507, (2004)
2. C. Masciovecchio, A. Gessini, S. Di Fonzo, L. Comez, S.C. Santucci and D. Fioretto, Physical Review Letters 92, 247401 (2004).
3. F. Sciortino, P. Gallo, P. Tartaglia, S. H. Chen, Physical Review E 54, 6331, (1996).  F.W. Starr, M.-C. Bellissent- Funel, H. E. Stanley, Physical Review Letters 82, 3629, (1999). F.W. Starr, F. Sciortino, H. E. Stanley, Physical Review E 60, 6757, (1999).

Monday, 29 November 2004, Università di Trieste, Aula Informatica del Dipartimento di Scienze Chimiche, Edificio C11, via Giorgieri 1

TRYGVE HELGAKER
(Department of Chemistry, University of Oslo)

"THE APPLICATION OF DENSITY-FUNCTIONAL RESPONSE THEORY TOLARGE MOLECULAR SYSTEMS"


With recent advances in computational techniques, it has become possibile to apply response theory to the calculation of properties of molecular systems larger than that of previous systems. In the present talk, some of these advances are reviewed, with emphasis on properites such asexcitation energies, polarizabilities, and indirect nuclear spin--spincoupling constants calculated using density-functional theory.

Thursday, 14 October 2004, 16.30 SISSA Room B

D. Beveridge
(Wesleyan University)

"Induced Fit and Entropy of Structural Adaptation in the Formation of Protein DNA Complexes"

Tuesday, 14 September 2004, 17.30 SISSA Room A

Marialore Sulpizi
(Lab. of Computational Chemistry and Biochemistry - EPFL Lausanne)

"Electron Transfer Description via charge restrain QM/MM dynamics"


We introduce a new quantum mechanics/ molecular mechanics based method to drive electron transfer reactions. Our approach uses the dynamically restrained electrostatic potential derived charges of the quantum atoms[1] as a reaction coordinate and allows an estimation of the free energy barrier of the electron transfer process. Moreover, it provides an accurate description of the electronic structure changes and of the nuclear reorganization associated with the reaction. We use the method to describe the electron-transfer induced dissociation of the m-chloro-cyano-benzene radical anion in aqueous solution. The reaction is triggered by solvent reorganization by a change in the coordination water shell around the cyano nitrogen atom. At the onset of the reaction, charge-spin segregation is observed. The negative charge is transferred to the leaving Cl, while the spin density localizes on the non-saturated carbon atom of the benzene ring. The calculated free energy barrier of dissociation is in good quantitative agreement with the experimental data. This type of study can be extended to the ET description in biological systems, and can find practical applications in cases in which a charge transfer is connected to a complex geometric reaction coordinate and therefore traditional thermodynamic integration is difficult to apply.

Tuesday, 14 September 2004, 10.00 SISSA Room A

Stefano Piana
(Nanochemistry Research Institute, Curtin University of Technology)

"Molecular mechanism of caspase activity regulation"


Cysteine-dependent aspartic proteases (caspases) are a family of enzymes, which play a crucial role in programmed cell suicide [1]. This is a key  cellular process for multicellular organisms. Indeed irregular apoptosis is  associated with diseases such as cancer or Alzheimer. Caspases are dimeric proteins that accumulate in all eukaryotic cells in the form of low-activity  proenzyme precursors. Proteolytic cleavage of specific sites triggers conformational changes that lead to full activation, and thus to the initiation of the apoptotic cascade. We have used a number of different  computational techniques, from QM/MM to classical MD to structure-based thermodynamic analysis methods, in order to  investigate the molecular mechanisms that regulate caspase and procaspase  activity. These studies allowed the characterisation of the chemical steps of the enzymatic mechanism[2] and of the structural rearrangements on the pathway  leading to a procaspase activation[3]. They also enabled the identification of  the most important residues for folding and dimerization in all the caspases  and procaspases[4]. Finally, the folding pathways of different classes of  procaspases were also studied [5]. A coherent picture emerges indicating that  the activation of procaspases is triggered by the formation of a small number  of interactions at the dimer interface that stabilize the active site loops in  the active conformation.

1. Earnshaw, W. C., Martins, L. M., and Kaufmann, S. H. Annu.Rev Biochem, 1999, 68, 383-424.
2. M.Sulpizi, A. Laio, J. VandeVondele, A. Cattaneo, U. Rothlisberger, P.Carloni. Proteins, 2003, 52, 212-224.
3. Piana, S. and Rothlisberger, U. Proteins, 2004, In Press
4. Piana, S., Sulpizi, M., and Rothlisberger, U. Biochemistry, 2003, 42, 8720-8728.
5. Piana, S., Taylor, Z., and Rothlisberger, U. Manuscript, Submitted.

Wednesday, 25 August 2004, 11.00 SISSA Room A

Renata M. M. Wentzcovitch
(Dept. of Chemical Engineering and Materials Science, Minnesota Supercomputer Institute for Digital Technology and Advanced Computations University of Minnesota, Minneapolis)

"Phase transition in MgSiO3 perovskite in the earth's lower mantle"


A new polymorph of MgSiO3 with the CaIrO3 structure and more stable than the Pbnm-perovskite phase has been identified by first-principles computations. High-temperature calculations within the quasiharmonic approximation indicate that this phase transition occurs at pressure-temperature ( P-T) conditions expected in the vicinity of the D" discontinuity. The D" region, i.e., the bottom 250 km of the mantle, is know for its abnormal properties attributed to the formation of a thermochemical boundary layer existent between two dissimilar regions, core and mantle, that don't mix today. Therefore, this phase transition provides a new paradigm for interpreting abnormal seismic properties of D". We have followed up with calculations of its thermoelastic properties and attempt to address some of these abnormal properties in light of this new information.

Monday, 30 August 2004, 11.00 SISSA Room A

Abdallah Qteish
(Department of Physics, Yarmouk University (Irbid- Jordan))

"Exact-exchange Kohn-Sham density functional theory calculations of the electronic Structure semiconductors"


The exact-exchange (EXX) Kohn-Sham density functional theory approach will be described and used to calculate the electronic structures of the zincblende phase of AlN, GaN and InN. In these calculations the Ga 3d and In 4d electrons are treated for the first time, within EXX, as valence states. For comparison with previous calculations they are also treated and as part of the frozen core. Our EXX results for AlN and GaN (obtained with the semicore Ga 3d electrons included as core states) are found to be in excellent agreement with the similarly calculated values and the available experimental data. When the semicore d electrons are treated as valence states, the EXX bandgap of ZB-GaN is found to be in excellent agreement with the GW results, and the results of the two methods are significantly smaller than experiment. For InN, where the application of the GW approach is problematic, due the negative LDA bandgap, the EXX approach allows a fully consistent treatment. Contrary to the common believe, the removal of the self-interaction, provided by the EXX approach, does not account for the large discrepancies between the LDA (or GGA) results for the position of the semicore d bands and experiment. The prospects of overcoming the difficulties encountered by relaxing the semicore d electrons by performing GW calculations based on EXX results will be discussed.

Friday, 20 August 2004, 11.00 SISSA Room A

Raghani Pushpa
(Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur, Bangalore, INDIA)

"Structural and Vibrational Consequences of Reduced Coordination"

Tuesday, 27 July 2004, 13.00 SISSA Room A

Enzo ALESSIO
(University of Trieste, Department of Chemistry)

"Anticancer Ruthenium Complexes"


In recent years several ruthenium compounds were found to have very promising anticancer activity, frequently different from that of Pt drugs, in animal tumor modelsand in preclinical trials. Two ruthenium compounds developed in Europe have nowbeen introduced into clinical trials: the first was NAMI-A, [imH][trans-RuCl4(dmso-S)(im)] (im = imidazole, Figure), a non-cytotoxic Ru(III)-dmso compound developed inTrieste, that proved to have excellent activity against disseminated solid tumors (metastases). NAMI-A has accomplished phase I clinical trials and will hopefully enter phase II soon. The second compound that reached phase I trials at the end of 2003 is [indH][trans-RuCl4(ind)2] (ind = indazole). This complex, developed by  Bernhard Keppler and termed KP1019, proved to be active against platinum-resistant colorectal tumors.The lecture will be focused on NAMI-A along the following topics: chemical behavior of NAMI-A in physiological solution and in the presence of  biological reducing agents; anticancer activity; mechanistic hypotheses; future  developments.For recent review on NAMI-A see refs 1 and 2.

1. E. Alessio, G. Mestroni, A. Bergamo, G. Sava in "Metal Ions and Their Complexes in Medication and  in Cancer Diagnosis and Therapy", Vol. 42 of Met. Ions Biol. Syst., A. Sigel and H.Sigel, eds., M. Dekker: New York, 2004, pp. 323-351.
2. E. Alessio, G. Mestroni, A. Bergamo, G. Sava, Curr. Topics Med. Chem., in press.

Monday, 26 July 2004, 11.00 SISSA Room A

Pavel HOBZA
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Prague

"True stabilization energies for H-bonded and stacked structures of DNA base pairs and aminoacids pairs: complete basis set calculations at the MP2 and CCSD(T) levels"


Stabilization energies of H-bonded and stacked structures of DNA base pairs and aminoacids pairs were determined in the gas-phase optimized as well as crystal structures. Stabilization energies were determined as the sum of the complete basis set  limit of MP2 stabilization energies and CCSD(T) correction term. Final stabilization energies are very substantional, much larger than published previuously what is especially true for stacked structures. On the basis of compariosn with experimental data it is conlcluded that the present values represent the lower boundary of the true stabilization energies. The surprising stabilization of stacked structures can significantly change the present view on the origin of stabilization of DNA and proteins.

Friday, 9 July 2004, 11.00 SISSA Room A

Carlos G. LEVI
(Materials Dept., University of California, Santa Barbara)

"FUNDAMENTAL ISSUES IN THERMAL BARRIER SYSTEMS FOR ADVANCED GAS TURBINES"


Thermal barrier coatings (TBCs) are applied to actively cooled gas turbine components to reduce the operating temperature of the structural superalloy, thus increasing its life, and/or to enable substantial increments in the gas temperature and thus the efficiency of the engine without compromising the durability of the component.  The coating system is multi-layer and multifunctional, comprising a refractory oxide "top coat" (10-250 µm) which provides thermal insulation, a thin (~5 µm) "thermally grown (aluminum) oxide" (TGO) interlayer that imparts oxidation resistance, and an underlying metallic bond coat (50-100 µm) whichs upplies Al to sustain the formation of the TGO.  The established systems are typically ZrO2+7±1%YO1.5 (7YSZ) on single-phase (Ni,Pt)Al or two-phase (Ni,Co)CrAlY bond coats. TBCs are arguably the single most important materials issue in enabling new advances in gas turbine technology, comparable in impact to the combined developments in superalloy design and cooling engineering over the past 30 years.  However, current systems are not exploited up to their maximum potential because of concerns about their reliability.  It is also evident that the demands of new engine designs will exceed their performance capabilities.  A host of alternate materials has emerged, primarily aimed at reducing the thermal conductivity and/or improving the morphological stability of the top coat.  In general, these materials fall into two groups, one based on co-doping of YSZ with one or more rare earth oxides, and the other on replacing YSZ with a rare earth zirconate, which may also be co-doped.  Implementation of these materials, however, is hindered by a paucity of fundamental understanding of several issues.  For example, there is inadequate understanding of the role of chemical composition on thermal transport through the top coat, on the evolution and/or stability of the microstructure vis-à-vis phase constitution and morphology, as well as on the mechanical properties of the oxide and related failure mechanisms.  The TGO, which is the major source of strain incompatibility within the system, often develops undulations of increasing amplitude with thermal cycling which result from a combination of in-plane growth stresses, cyclic plasticity and phase transformations in the underlying bond coat.  These undulations are a source of interfacial separations that eventually lead to spallation of the insulating layer.  Regarding the bond coat, the major issues result from interdiffusion with the substrate and the need for adequate diffusion barriers.  Interdiffusion results not only in changes in the phase constitution and mechanical properties of both bond coat and substrate, but also on the migration of deleterious impurities, e.g. S, to the metal/oxide interface with attendant loss of adherence. This presentation will discuss the general aspects of the problem and the issues needing further understanding, especially from the modeling arena, drawing on current research activities at UCSB and partner institutions. (Research sponsored by the Office of Naval Research and an international collaborative research program sponsored by the National Science Foundation and the European Commission.)

Tuesday, 6 July 2004, 11.00 SISSA Room A

Rainer D. BECK
(Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland)

"Quantum State Resolved Studies of Gas/Surface Reaction Dynamics"


Laser light is a powerful tool for detailed experimental studies of chemical reactions at the gas/surface interface. It can be used to prepare molecules incident on a solid surface in specific quantum states for state resolved reactivity measurements (1). We have used this technique to measure state resolved reaction cross sections for the dissociative chemisorption of methane (CH4 results provide a comparison of the efficacy of energizing different degrees of freedom of the incident molecule for crossing the reaction barrier (2). Different cross sections for nearly isoenergetic vibrational states clearly demonstrates the presence of mode specificity in this gas - surface reaction. For CD2H2, we observed up to a factor of 5 greater reaction probability with two quanta of excitation in one C-H bond versus one quanta in each of two C-H bonds (3) . These results clearly exclude the possibility of statistical models correctly describing the mechanism of this process and emphasize the importance of full-dimensional calculations of the reaction dynamics.  

1. M.P. Schmid, P. Maroni, R.D. Beck and T.R. Rizzo, Rev. Sci. Inst., 74, 4110 (2003)
2. M.P. Schmid, P. Maroni, R.D. Beck and T.R. Rizzo, J. Chem. Phys. 117, 8603 (2002)
3. R.D. Beck, P. Maroni, D. C. Papageorgopoulos, T.T. Dang, M.P. Schmid, T.R. Rizzo, Science, 302, 98 (2003)

Friday, 28 May 2004, 14.00 SISSA Room A

Leonardo SPANU
(Dipartimento di Fisica "A. Volta", Università di Pavia - INFM Pavia)

"Spin dynamics and spin-phonon effects in antiferromagnetic molecular rings"


Spin dynamics and spin-phonon effects in antiferromagnetic molecular rings NMR has proved to be a powerful tool to investigate both static and dynamic properties of magnetic systems. In particular in recent years NMR has been employed to study local spin dynamics in molecular magnets and nowadays a considerable body of experimental results are still unexplained. Actually the nuclear spin relaxation process in a finite system, with discrete energy levels, is a very general problem and it requires coupling the spin system to external degrees of freedom. For this purpose we have investigated both spin correlation functions and spin-phonon effects in antiferromagnetic (AF) molecular rings, which are a subclass of molecular magnets. Two parts of the work will be presented:

• AF ring coupled to adiabatic phonons. Lattice stability is investigated for different values of the spin and numbers of magnetic sites. The magneto-elastic transition is shown to be heavily affected by the spin value, displaying a qualitative difference in the nature of the instability for the spin one-half.
• Spin dynamics at finite temperature and coupling to external bath. We present results for the spin dynamics at finite temperature, showing in particular how the low-frequency dynamical correlation functions are influenced by the spin-phonon coupling.

Friday, 28 May 2004, 9.00 SISSA Room A

Mauro ROVERE
(Dipartimento di Fisica, Università Roma Tre - INFM Roma Tre and Democritos)

"Effects of confinement on static and dynamical properties of water"


Molecular dynamics results on water confined in a silica pore are presented.  Static and dynamical properties are studied upon supercooling and  by varying the hydration level.  Strong layering effects due to the hydrophilic character of the substrate are found. At high hydration levels a double dynamical regime is observed which corresponds to the presence of two subsets of water molecules with different static and dynamical properties.  In the low hydration regime, where most of the molecules reside close to the substrate, the molecules show an  anomalous diffusion with a sublinear behaviour at long time. This behaviour is strictly connected to the long time decay of the residence time distribution analogously to water at contact with proteins.

Tuesday, 18 May 2004, 11.00 SISSA Room D

Iain McLay
(GlaxoSmithKline Medicines Research Centre, Computational and Structural Sciences, Stevenage - Hertfordshire, UK)

"Understanding the Glucocorticoid Receptor"

Friday, 7 May 2004, 11.00 SISSA Room A

Matteo COCOCCIONI
(Department of Materials Science and Engineering, Massachusetts Institute of Technology)

"Electronic-enthalpy functional for first-principles simulations of finite systems under pressure(*)"


An electronic-enthalpy functional is introduced to allow for the ab-initio simulation of finite systems under pressure. Electrons are evolved according to a new operator H+PV_q describing the "quantum" enthalpy of the finite system, where H is the Hamiltonian of the system, P the external pressure and V_q the quantum volume occupied by the electronic cloud. The external pressure field acts directly on the electronic structure of the system to account directly for electrostatic and Pauli-repulsion interactions through which, in real compression processes, the external load is transferred to the particle. The Hellman-Feynman theorem directly applies, and the ions follow exactly an isoenthalpic dynamics. No external pressurizing medium is required resulting in low computational costs and straightforward implementations in any electronic structure code. Thin coatings of environmental ligands or ions can also be easily introduced if chemically relevant. We applied this method to the study of group-IV nanoparticles under shock waves, highlighting the significant differences in the plastic and elastic response of the diamond-cage under load.

(*) Work done in collaboration with Nicola Marzari and Gerbrand Ceder (DMSE - MIT) and Francesco Mauri (LCPM-Paris VI).

Wednesday, 5 May 2004, 11.00 SISSA Room E

Klaus KERN
(Max-Planck-Institut fr Festkörperforschung, Stuttgart)

"SUPRAMOLECULAR NANOARCHITECTURES AT SURFACES"


A key issue in nanotechnology is the development of conceptually simple construction techniques for the mass fabrication of identical nanoscale structures. Conventional top down fabrication techniques are both energy intensive and wasteful, because many production steps involve depositing unstructured layers and then patterning them by removing most of the deposited films. Furthermore, increasingly expensive fabrication facilities are required as the feature size decreases. The natural alternative to the top down construction is the bottom up approach, in which nanoscale structures are built from their atomic and molecular constituents by self-assembly. This approach relies on the exploitation of noncovalent directional interactions and is one of the key building principles of all living organisms.
Recent experimental advances have provided the unique ability to investigate and manipulate individual nanostructures on the atomic scale in a wide variety of environments and under extreme conditions. In this talk I will demonstrate the power of variable-temperature scanning tunneling microscopy to probe the dynamics of atomic/molecular motion and assembly. Scanned probes not only allow us to image molecules adsorbed at surfaces with unprecedented resolution but also to follow supramolecular self-assembly and chemical complex formation in real space and time. As examples we will discuss the building of supramolecular nanostructures at surfaces by hydrogen and coordination bonding. The steering of the self-assembly of the molecular components allows for the fabrication of distinct architectures. In particular the precise positioning of molecules and metal centres on a surface opens up new possibilities for the bottom-up fabrication of low-dimensional functional materials.

Tuesday, 4 May 2004, 11.00 SISSA Room A

Hande USTUNEL
(Laboratory of Atomic and Solid State Physics, Cornell University)

"AB INITIO PREDICTIONS OF NANOSCALE RESONATORS"


In recent years, considerable experimental effort has gone into the fabrication and characterization of resonators in the nanometer range. Theoretical methods have proven to be invaluable for understanding the results of these experiments. In this talk, I will present two examples of how we use various theoretical methods, including ab initio, to study different aspects of oscillations and mechanical loss in nanostructures.
One common defect in silicon devices is the divacancy. Although the single vacancy in silicon is mobile at fabrication temperatures, the divacancy is stable, and as a result is one of the primary causes of mechanical loss. I will present ab initio calculations of the elastic response of this defect and show how these results can be used to predict the mechanical loss caused by this defect in a silicon resonator.
Recent experiments have demonstrated oscillations of a suspended carbon nanotube under a gate voltage. We have conducted simulations of such a system in which we treat the nanotube as an elastic medium with parameters extracted from ab initio calculations. I will show results from these simulations, and show how they can be used to characterize experimental devices for which several parameters, such as length and diameter of the nanotube, are not known experimentally.

Wednesday, 21 April 2004, 11.00 SISSA Room A

Noboru Fukushima
(Technical University Braunschweig)

"Thermodynamic properties of quantum Heisenberg models by a series expansion"


Using a high-temperature series expansion, thermodynamic properties are analyzed for two different systems. (i) Mixed-spin chain systems with alternating magnetic moments: We focus on the difference between ferromagnetic (FM) and antiferromagnetic (AFM) exchange. For the specific heat, we find a multi-peak structure in the FM case which is strongly enhanced as compared to that known for the AFM case. Furthermore, we find indications of additional low energy scales in the FM case which are not observed for AFM exchange. (ii) SU(n) Heisenberg models represented by exchange operators: In three dimensions, we obtain the series coefficients as polynomials of n, and generalize n to a continuous variable. We analyze singularities of a correlation function, and report n-dependence of the transition temperature.

Tuesday, 30 March 2004, 16.30 SISSA Room A

Giorgia Brancolini
(Università di Bologna)

"Stability and spectroscopical activities of lithium-doped carbon nanostructures: a computational study"


The capacity of lithium ion batteries depends strongly on the lithium storage capacity of the carbon anode. In carbonaceous materials containing substantial hydrogen (soft carbon), the amount of lithium that can be stored is larger compared with pure graphite, and lithium appears to reversibly bind near hydrogen atoms. Owing to their graphene-like structure combined with a substantial hydrogen content, polycyclic aromatic hydrocarbons (PAHs) are suitable models for soft carbon. Thus, in this work we present a computational study on the stability and spectroscopical activities of a large PAH: hexa-peri-hexabenzocoronene (HBC), C42H18 doped with alkali metals. LinPAH complexes are investigated quantum chemically using the hybrid B3LYP functional, a level of theory that has been shown to provide reliable results. We explored a large number of lithium-interaction sites both above or above and below the HBC molecule and obtained equilibrium structures, binding energies and spectroscopical properties. It is shown that lithium interaction does not simply causes charge transfer to the PAH but also covalent bonding with the periphery of the hydrocarbon. Vibrational spectra of the lithium complexes are also computed and compared with those of undoped HBC and it is shown that remarkable changes in selected frequency regions can be attributed to the presence of lithium atoms. UPS spectra corresponding to di_erent degrees of lithium doping are also simulated and compared with available experimental data.

Friday, 27 February 2004, 14.00 SISSA Room A

Sandro MASSIDDA
(Dip.to di Fisica, Università di Cagliari)

"Application of density functional theory to the superconducting state"

Thursday, 26 February 2004, 16.30 SISSA Room A

Julien PILME
(Univ. de Paris 6)

"An approach of the chemical bond by the quantum topological analysis. Some applications to the first row transition metal compounds"


In point of view of the quantum mechanics the molecular system is a collection of interacting particles (electron and nuclei) and his behavior is ruled by the Schrdinger's equation. The chemical bond isn't an observable and the bonding properties are generally described by the traditionally orbitals model. However, the pragmatic conception of the chemist is linked to the Lewis picture where the molecule is constituted by several atoms which are connected by the chemical bonds. In addition, the bonds are classified (covalent, ionic..) following the nature of the atoms. In order to make a bridge between the two approaches of the chemical bonding, we can propose an alternative with the topological analysis of an local electronic localization function which is calculated from a wave function. Recently Richard Bader [1] (Atoms in molecules) has proposed to use the electronic density ((r) because it is an observable and it has a real physical significant. However, the extrema values of ((r) are essentially localized on atoms and its topological analysis cannot distinguish lone pairs and the bonding pairs. Another localization function has been proposed by Becke and Edgecombe (ELF). [2] The topological analysis [3] of the ELF function allows to share the molecular space which is consistent with the traditional conceptions of Lewis and VSEPR [4] models. I will propose some recent applications of the topological analyses (AIM and ELF) on a few transition metal systems because the electronic structures of this kind of compounds are generally not yet known. In this context, I will show most results on the binary carbonyl complexes M(CO) (M=Sc,..,Cu) [5]. Another example of ELF topological analysis will be presented with the system Cu2O22+ which is involved in the life mechanisms.



1. R.F. W. Bader. Atoms in molecules: A quantum theory. Oxford Univ. Press, 1990
2. A. D. Becke and K. E. Edgecombe, J. Chem. Phys. 92:5397, 1990
3. B. Silvi and A. Savin, Nature, 371:683, 1994
4. R. J. Gillespie and P.L.A Popelier, Chemical bonding and Molecular Geometry From Lewis to Electron Densities, Oxford University Press, 2001
5. J . Pilme, E. A. Alikhani and B. Silvi, J. Phys. Chem. A, 107: 4506, 2003

Thursday, 12 February 2004, 16.00 SISSA Room A

Mauro Stener
(Dipartimento di Scienze Chimiche - Università di Trieste)

"Time dependent Density Functional Theory of core electron excitations"


The Time Dependent Density Functional Theory (TDDFT) has proven as a common and powerful method to study electronic excited states. The basic equations of the methods will be reviewed, with particular emphasis to their numerical implementations in quantum chemistry computer codes, for efficient molecular calculations in basis set formulation. An extension of the method, suggested to study excitations of core electrons, will be described, together with its implementation in the ADF program. This extension includes the most important physical effects of TDDFT which are mandatory to describe excitations of 2p electrons of metal atoms, keeping the numerical effort within reasonable limits. This method is very promising to study large systems like, for example, molecules of biological interest, in particular regions of enzymes containing transition metal atoms. Preliminary results on a series of molecular models of the sulfite-oxidase enzyme obtained with this method will be presented and compared with recent experimental data.

Tuesday, 20 January 2004, 17.30 SISSA Room D

Michael L. KLEIN
(University of Pennsylvania)

"Computer Simulation of Self-assembling Systems: From Synthetic Di-block Copolymers to Natural Lipids."


The talk will review recent progress in using coarse grain interaction models to follow the self assembly in bulk solution of amphiphilic di-block copolymers into a variety of structures, including micelles, worms, and vesicles. The effect of polymer molecular weight on the latter structure will be discussed with emphasis on the properties of the bilayer (membrane thickness, etc.). These results will be contrasted with the behavior of naturally occurring phospholipids membranes, the self-assembly of which can also be demonstrated with coarse grain models. The effect of a surface on di-block copolymer self-assembly will also be discussed, with particular emphasis on the influence of the surface on the surfactant nanoscale supramolecular organization.

Friday, 12 December 2003, 11.00 SISSA Room A

Jordi Mur
(Università di Barcellona e Università di Pisa)

"Two-dimensional boson-fermion mixture"

Thursday, 27 November 2003, 16.00 SISSA Room A

Rafael Guardiola
(Universidad de Valencia)

"Droplets made of He-3 and He-4 atoms"


The talk presents a complete analysis of systems made of a varying number of He-3 and He-4 atoms, with the objective of determining a stability chart of such mixtures. The computations are carried out by means of the Diffusion Monte Carlo algorithm, using the Aziz potential. The most remakable outcome is the prediction of instability islands, which appear when the number of bosons is small. A comparison with experimental measurements (preliminary) is also done

Thursday, 6 November 2003, 11.00 SISSA Room A

Giovanni Vignale
(University of Missouri at Columbia)

"Time-dependent spin density functional theory"


Time-dependent density functional generally suffers from an ultra-nonlocality problem, namely the local density approximation or the exchange-correlation potential always fails, no matter how slowly varying is the density. I show that this problem is particularly severe in time-dependent spin density functional theory, but can be cured by switching to a formulation in which the spin currents are the basic variables. I discuss the physical significance of the new terms that appear in the spin-current dependent functional, and review the progress that has been made in providing explicit expressions for the latter.

Friday, 17 October 2003, 12.00 SISSA Room A

Anna Tramontano
(University of Rome La Sapienza)

"An evolutionary approach to the solution of the protein folding problem"

Monday, 22 September 2003, 15.30 SISSA Room C

Jan Kaspar
(Università di Trieste)

"CeO2-based nanostructured materials: applications, open issues and perspectives"


CeO2 based materials are extensively employed in many catalytic processes and particularly in the automotive pollution control. Their role in the current TWCs will be described. It will be shown that by designing the material at an intimate - nanoscale - level some chemical and physical properties can be properly tuned.

Thursday, 18 September 2003, 16.00 SISSA Room A

Andrea Callegari
(Physics Department of the Université de Lausanne)

"Nanomatter probes for probing nanomatter: Photochemically grown silver nanoparticles with wavelength-controlled size and shape"


Matter organized at the nanometer scale has very interesting properties that are not found either in bulk matter or in its elemental components, atoms and molecules. Since these peculiar properties arise from confinement effect and interplay between surface and bulk effects, both size and shape have a profound influence on the physical and chemical properties of nanoparticles (NPs). Linear and nonlinear optical properties, electronic and magnetic properties, chemical reactivity and catalytic activity, all show dramatic changes with these two parameters. Correspondingly, an increasingly large interest has been growing about controlling NPs size and shape by the preparation method. Here we report a photochemical method that transforms a colloidal solution of spherical silver nanoparticles into a solution of larger nanoparticles with different shapes. The photo-induced morphological changes are highly wavelength-specific, resulting in particles with size and shape that can be controlled by choosing the wavelength(s) of the light used to drive the photochemical growth. This method demonstrates that light can be used as a convenient control parameter in the chemical synthesis of NPs.

Friday, 8 August 2003, 14.00 SISSA Room A

Sumio Ishihara
(Department of Physics, Tohoku University)

"Electron Correlation and Electron-Lattice Interaction in the High Tc Superconductivity"


It has been a long standing question whether a correlated electronic model plane can explain the essential experiments of superconductivity in cuprates. Although such a model has been successful in explaining many physical properties, the phonons and lattice effects are clearly present in these materials. Recently, a kink structure is observed in the angular resolved photoemission spectra (ARPES) around 50-80meV which is interpreted to originate from the coupling of electrons with a half-breathing mode of the oxygen motion in conjunction with the data of the neutron experiments.
We have investigated role of electron-phonon interaction in the strongly correlated cuprates [1].
We have found that, by considering the off-diagonal electron-phonon coupling, which modulates the transfer of the Zhang-Rice singlet, the oxygen displacement contributes strongest to the d-wave paring in the region along the (00)-(q_x,0) and (0,q_y), where the strong coupling to the electrons is observed experimentally. Being based on the slave-boson picture applied to the t-J model and the Eliashberg theory, role of electron-phonon interaction in ARPES, tunneling spectra and optical conductivity is investigated. The vertex correction is essential to describe the electron-phonon interaction in the strongly correlated cuprates.

Reference
[1] Z.-X. Shen, A. Lanzara, S. Ishihara, N. Nagaosa, Phil. Mag. B 82 1349 (2002).

Tuesday, 29 July 2003, 11.00 ICTP Seminar Room, 1st floor

V. I. Anisimov
(Institute for Metal Physics, Yekaterinburg, Russia)

"Dynamical Mean-Field Theory for realistic systems"


LDA+DMFT is a novel computational technique for ab initio investigations of real materials with strongly correlated electrons, such as transition metals and their oxides. It combines the strength of conventional band structure theory in the local density approximation (LDA) with a modern many-body approach, the dynamical mean-field theory (DMFT). In the last few years LDA+DMFT has proved to be a powerful tool for the realistic modelling of strongly correlated electronic systems.LDA+DMFT method was successfully used for description of the Mott-Hubbard metal-insulator transition in the transition metal oxide V2O3 and the alpha-gamma transition in the 4f-electron system Ce.

Tuesday, 15 July 2003, 16.00 SISSA Room C

Akihisa KOGA
(Osaka University, Japan)

"Quantum phase transitions in the orthogonal-dimer spin system"


Frustrated quantum spin systems have attracted current interest. A typical example is the orthogonal-dimer compound SrCu₂(BO₃)₂, where novel magnetic properties were observed such as magnetization plateaus, excited states without dispersion. More recently, a new orthogonal-dimer compound Nd₂BaZnO₅ was synthesized, where the local moment J=9/2 shows an antiferromagnetic order below T_N=2.4K. In this talk, we discuss how a higher spin generalization (S>1/2), together with the competing exchange couplings, affects the ground state properties of the orthogonal-dimer spin systems.

Wednesday, 9 July 2003 - 16.00 ICTP SEMINAR ROOM - MAIN BUILDING (first floor)

R. Car
(Princeton University)

"Electron transport with dissipation in molecular wires"


It is usually assumed that electron transport in molecular wires is ballistic and that scattering with phonons is only important to thermalize the electrons in the substrate far from the molecular contact. Here I discuss an alternative formulation in which both tunneling and dissipation are explicitly taken int account. In this approach electrons are accelerated by an external electric field, but coupling to a heat bath prevents the electrons from moving far out of equilibrium. Many-electron dynamics is treated within time-dependent current-density functional theory, while dissipative effects are treated phenomenologically by a master equation. The approach is well suited to numerical simulations and allows us to study steady state and dynamical phenomena. In particular, in the case of a simple double-barrier resonant tunneling structure, I discuss how dissipative effects and the approximations made in the master equation affect the calculated electric current.

Tuesday, 8 July 2003, 16.00 SISSA Room A

Saverio Moroni
(Istituto Nazionale per la Fisica della Materia c/o Università degli Studi di Roma "La Sapienza")

"Solvent structure and impurity dynamics in doped Helium clusters"


Recent developments in Helium nanodroplet isolation spectroscopy witness increasing interest in studying the properties of the cluster as a function of the number of solvent atoms. Microwave and/or infrared spectral lines, assigned to doped clusters with given number of Helium atoms in the range from unity to about ten, offer insight into the structure of the first solvation shell and show evidence for the onset of Helium superfluidity.
Quantum monte Carlo simulations, based on accurate interparticle potentials, are used to enrich the picture inferred from experiments. We present a study of OCS@He_N, as well as preliminary results on CO@He_N. For the former we relate the rotational spectrum of the solvated molecule with the structural properties of the He atoms, and both with the onset of superfluidity.In the case of CO@He_N, the purely rotational spectrum --not available from measurements-- is discussed in terms of the cluster symmetry along the molecular axis.

Thursday, 3 July 2003, 16.00 SISSA Room A

Gian Paolo Brivio
(Istituto Nazionale per la Fisica della Materia and Dipartimento di Scienza dei Materiali, Universita` di Milano-Bicocca)

"Spectroscopic properties at a semi-infinite metal surface within the DFT framework"


Spectroscopic properties at a metal surface, either free or covered with adsorbates, are investigated by solving the Kohn-Sham equation of DFT. We use a formalism, based on the Green function embedding approach, which is able to take into account a semi-infinite solid and hence to describe the continuous spectrum of the system [1]. We show results of CCV Auger spectra of adatoms on simple metals by using different levels of approximation both for the substrate and the Auger rates [2]. We find that the Auger spectra are affected by the site of the adsorbed impurity, which may be a subsurface or a chemisorption one, and mainly depend on the local electronic properties of the interacting system. De-excitation spectra of a metastable He atom on alkali-metals of low work function on which the CVV Auger process is the dominant one, are also analysed and compared with available experimental data [3]. Finally, starting from the DFT Green function, we present a many-body GW approach to deal with semi-infinite jellium and demonstrate the presence of a surface resonance peaked at the position of the first image state [4].

References:


1. H. Ishida, Phys. Rev. B 63, 165409 (2001).
   
2. N. Bonini, M.I. Trioni and G.P. Brivio, Phys. Rev. B 64, 035424(1-7) (2001).
   
3. N. Bonini, G.P. Brivio and M.I. Trioni, Phys. Rev. B, in press (2003).
   
4. G. Fratesi, G.P. Brivio and L.G. Molinari, cond-mat/0305344 (2003), G. Fratesi, G.P. Brivio, P. Rinke and R.W. Godby, Phys. Rev. B submitted (2003), cond-mat/0305678
   

Wednesday, 2 July 2003, 16.00 AM SISSA Room A

Mickael Profeta
(Laboratoire de Mineralogie-Cristallographie de Paris, Universitè Pierre et Marie Curie)

"Calculation of ¹⁷O NMR parameters from first principles. Applications to silicates, crystalline polymorphs and glass"


¹⁷O NMR spectroscopy is a promising tool for the study of the microscopic structure of oxides. ¹⁷O is a quadrupolar nucleus and thus both chemical shiftand electric field gradient tensors can be exploited.

Until recently, the conventional approach to the theoretical interpretation of solid state NMR spectra was to approximate the infinite solid by a cluster, but the comparaison with experiment is poor.

Here we will present GIPAW method which is able to simulate periodic solids without the cluster approximation and perform first-principle calculation with pseudopotentials. We apply this method to SiO₂ crystalline polymorphs and perform the assignment of the spectra of the ferrierite zeolite. We also apply the method to an aluminosilicate glass with calcium cation, and show the impact of the level of the 3d orbitals of calcium on the chemical shift of ¹⁷O.

Thursday, 19 June 2003, 11.00 AM SISSA Room A

Elvio Carlino
(Center for Electron Microscopy - Laboratorio Nazionale TASC-INFM, Area Science Park, Trieste, Italy)

"Transmission Electron Microscopy: how to study the solid state at the highest spatial resolution"


Current efforts to develop nanostructured materials and devices are stimulating the implementation of new experimental probes of the structure and chemical composition of solids on the atomic scale. Over the years, transmission electron microscopy (TEM) has widely demonstrated its potential to study material structures with the highest spatial resolution available. More recently, the combination of scanning tunneling microscopy (STM) and high-angle annular dark-field imaging (HAADF) in scanning transmission electron microscopy has shown promise as an analytical tool that can provide /chemical/ information with comparable atomic resolution. The results obtained in the last year at the Center for Electron Microscopy will describe how different experimental conditions during a TEM experiment has to be used to achieve the relevant physical properties of a specimen.

last modified: 14.01.2010