CONDENSED MATTER AND STATISTICAL PHYSICS SECTION announces two seminar tomorrow

[Cond.Matt. & Stat.Mech.Section]

SEMINAR on   Disorder and strong electron correlations



Thursday, 28 February -  11:30 a.m.



Luigi Stasi Seminar Room,  Leonardo Building - first floor

 
Elisa FRATINI     (ICTP)



"Resonant Bose-Fermi mixtures:  A T-matrix and Quantum Monte Carlo study"



Abstract


We develop the theoretical study of a novel physical system, in the context of ultracold gases.  We investigate the physical behavior of a resonant Bose-Fermi mixture, namely, an ultracold gas made of both bosons and fermions, with a strong attractive interaction between these two components.

We study homogeneous density and mass imbalanced mixtures from weak- to strong- coupling limit, comparing the results obtained with two different theoretical approaches, a many-body diagrammatic approach (the T- matrix approximation) and Quantum Monte Carlo method.

By using many-body diagrammatic methods we first obtain the finite-temperature phase diagram and the thermodynamic properties of the system.  We observe the presence of a quantum phase transition from the condensed (superfluid) to the normal (molecular) phase.  Developing the zero-temperature limit of the same Green’s function formalism we study the effect of density and mass imbalances for the Bose-Fermi mixture.  By using the corresponding retarded propagators we calculate the spectral weight functions and the dispersions of bosons and fermions.

We apply for the first time the Quantum Monte Carlo method with Fixed-Node approximation to investigate resonant Bose-Fermi mixture, from weak to strong boson-fermion attraction.  Two different nodal surfaces are used as trial wave functions in the two regimes.  We obtain the equation of state of a density imbalanced mixture and we observe the presence of the quantum phase transition through the crossing of the energies, calculated with their respective trial wave functions.  A phase diagram in the coupling and boson-fermion concentration variables is derived and the occurrence of phase separation is discussed.  We compare Quantum Monte Carlo results to T-matrix calculations, finding an interesting agreement between the results.

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JOINT ICTP/SISSA CONDENSED MATTER SEMINAR


 

Thursday, 28 February    -    3:30 p.m.

 

SISSA, Santorio Building,   Room 131 (1st floor)

 

Riccardo FERRANDO    ( Università degli Studi di Genova )



"Structures of metallic nanoparticles and nanoalloys"

 

Abstract


The structures of metallic nanoparticles are searched for by a computational methodology combining global optimization searches within an atomistic model and density-functional local relaxation. Different systems are considered, from single-metal nanoparticles adsorbed on magnesium oxide, to free and supported nanoalloys.

Metallic nanoparticles adsorbed on MgO are of great interest for applications in catalysis. Here we show that a variety of different morphologies can be obtained depending on the magnitude of the lattice mismatch and on the interaction strength with the substrate. We consider Au, Ag and Ni clusters on MgO(100) and compare our results with the experimental findings.

Nanoalloys with core-shell arrangement are of special interest in applications, such as in optics, catalysis, magnetism and biomedicine. Despite this wide interest, the physical factors stabilizing the structures of these nanoparticles are still unclear to a great extent, especially for what concerns the relationship between geometric structure and chemical ordering.

Here global-optimization searches are performed in order to determine the most stable chemical ordering patterns corresponding to the most important geometric structures, for a series of weakly miscible systems, including AgCu, AgNi, AgCo andC AuCo. Our calculations show that

a) the overall geometric structure of the nanoalloy and the shape and placement of its inner core are strictly correlated; b) centered cores can be obtained in icosahedral nanoparticles but not in crystalline or decahedral ones, in which asymmetric quasi-Janus morphologies form;

c) in icosahedral nanoparticles, when the core exceeds a critical size, a new type of morphological instability develops, making the core asymmetric and extending it towards the nanoparticle; d) multi-center patterns can be obtained in large polyicosahedral nanoalloys.

Analogies and differences between the instability of the core in icosahedral nanoalloys and the Stranski-Krastanov instability occurring in thin-film growth are discussed. All these issues are crucial for designing strategies to achieve effective coatings of the cores.