Cardona second talk; 26/1 Salasnich

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

JOINT ICTP/SISSA CONDENSED MATTER SEMINAR





Seminar Room - ICTP Leonardo Building  (first floor)



Wednesday, 20 January -     4:00 p.m.





Manuel CARDONA  ( Max-Planck Institute for Solid State Research,  
Stuttgart )



"Effects of isotopic mass and temperature on the optical properties of  
semiconductors"



Abstract



The state of the art calculations of the dielectric function of  
semiconductors will be reviewed and compared with extant experimental  
data. Most of the comparisons available in the literature suffer from  
the fact that the calculations are performed with the atoms at fixed  
lattice positions, whereas the experimental data are often taken at  
room temperature. Even when the latter have been obtained at low  
temperatures, close to 0K, they are affected by the zero-point  
vibrations, a quantum mechanical effect which can be varied by  
changing the isotopic masses of the constituent atoms. In recent years  
samples with different isotopic compositions have become available.  
Measurements on such samples make possible to determine the  
renormalization of the dielectric function and other physical  
properties effected by the lattice vibrations (zero point and  
temperature effects). This renormalization is particularly large in  
crystals with light atoms (diamond, ZnO, GaN), a fact that signals a  
rather strong electron phonon interaction. This strong interaction is  
responsible for the superconductivity recently observed in boron-doped  
diamond (also Si and SiC).

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JOINT ICTP/SISSA STATISTICAL PHYSICS SEMINAR



Tuesday, 26 January   -   11:00 hrs.





Lecture Room D - SISSA Main Building





Luca SALASNICH    (Università di Padova)



"The amazing unitary Fermi gas"



Abstract



We discuss the amazing properties of the unitary Fermi gas, which is  
the dilute and ultracold gas of fermions with infinite scattering  
length. Then, we illustrate the zero-temperature density functional of  
the unitary Fermi gas we have recently introduced on the basis of  
Monte Carlo calculations. This functional includes a gradient term  
which is essential to describe accurately the surface effects of the  
system, in particular with a small number of atoms. We analyze in  
detail the role of this gradient term for the determination of density  
profiles and collective modes. Finally, we investigate the low- 
temperature thermodynamics of the unitary Fermi gas by using the zero- 
temperature spectra of both bosonic collective modes and fermonic  
single-particle excitations. We determine the superfluid fraction, the  
critical temperature, the first sound and the second sound, comparing  
our analytical results with other theoretical predictions and  
experimental data.