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SUMMARY:Diagrammatic Monte Carlo for resonant fermions. - Prof. Nikolay Pr
 okof'ev\, University of Massachusetts at Amherst
DTSTART:20080508T131500Z
DTEND:20080508T141500Z
UID:TALK10976@talks.cam.ac.uk
CONTACT:Dr Jonathan Keeling
DESCRIPTION:I will discuss two different Diagrammatic Monte Carlo schemes 
 for solving the\nproblem of the BCS-BEC crossover in a Fermi gas with reso
 nant attractive\ninteractions (the so-called zero-range universal limit). 
 One is based on\nsummation of relevant Feynman diagrams for the partition 
 function of up to\nseveral hundred fermions.  We determine the normal-supe
 rfluid transition\ntemperature in the BCS-BEC crossover region\, unambiguo
 usly confirm that the\nmaximum of T~c~ is on the Bose side\, and  at unita
 rity T~c~/E~F~=0.152(7). The other\nscheme is based on direct summation of
  Feynman diagram for the proper\nself-energy. This approach is used to sol
 ve the problem of a single spin-down\nfermion resonantly interacting with 
 the Fermi gas of spin-up particles. Though\nthe original series based on b
 are propagators are sign-alternating and often\ndivergent one can still de
 termine the answer behind them by using two strategies\n(separately or tog
 ether): (i) using proper series re-summation techniques\, and\n(ii) introd
 ucing renormalized propagators which are defined in terms of the\nsimulate
 d proper self-energy\, i.e. making the entire scheme self-consistent. Our\
 nsolution is important for understanding the phase diagram and properties 
 of the\nBCS-BEC crossover in the strongly imbalanced regime. On the techni
 cal side\, we\ndevelop a generic sign-problem tolerant method for exact nu
 merical solution of\nthe interacting many-body Hamiltonians.\n
LOCATION:TCM Seminar Room\, Cavendish Laboratory
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