CONDENSED MATTER SECTION SEMINAR on Disorder and strong electron correlations

[CM ICTP - Trieste]

CONDENSED MATTER SECTION

	
SEMINAR on

Disorder and strong electron correlations




Thursday, 24 January   -    11:00 a.m.


Seminar Room,  Main Bldg.- first floor




N. PROKOF'EV
( UMASS, USA )


"Diagrammatic Monte Carlo for resonant fermions "

Abstract

I will discuss two different Diagrammatic Monte Carlo schemes for 
solving the problem of the BCS-BEC crossover in a Fermi gas with 
resonant attractive interactions (the so-called zero-range universal 
limit). One is based on summation of relevant Feynman diagrams for the 
partition function of up to several hundred fermions.  We determine the 
normal-superfluid transition temperature in the BCS-BEC crossover 
region and  unambiguously confirm that the maximum of Tc is on the Bose 
–side and  at unitarity T_c/E_F=0.152(7) (in disagreement with other 
groups!) . The other scheme is based on direct summation of Feynman 
diagram for the proper self-energy. This approach is used to solve the 
problem of a single spin-down fermion resonantly interacting with the 
Fermi gas of spin-up particles. Though the original series based on 
bare propagators are sign-alternating and often divergent one can still 
determine the answer behind them by using two strategies (separately or 
together): (i) using proper series re-summation techniques, and (ii) 
introducing renormalized propagators which are defined in terms of the 
simulated proper self-energy, i.e. making the entire scheme 
self-consistent. Our solution is important for understanding the phase 
diagram and properties of the BCS-BEC crossover in the strongly 
imbalanced regime. On the technical side, we develop a generic 
sign-problem tolerant method for exact numerical solution of 
polaron-type models, and, possibly, of the interacting many-body 
Hamiltonians.