Nikolay Prokofiev, Umass, Amherst work done in collaboration with PITP, Dec. 4, 2009 DIAGRAMMATIC MONTE CARLO FOR CORRELATED FERMIONS oris Svistunov UMass Kris van Houcke UMass Univ. Gent Evgeny Kozik ETH Lode Pollet Harvard Emanuel Gull Columbia Matthias Troyer ETH Felix Werner UMass
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Nikolay Prokofiev, Umass, Amherst work done in collaboration with PITP, Dec. 4, 2009 DIAGRAMMATIC MONTE CARLO FOR CORRELATED FERMIONS Boris Svistunov UMass.
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Calculate irreducible diagrams for , , … to get , , …. from Dyson equations
+ + + ...0 ( , )G p
1 2( , )p
G U
+ Dyson Equation:( , )G p
Make the entire scheme self-consistent, i.e. all internal lines in , , … are “bold” = skeleton graphs
U +U
or(0)
+ U (0)G
+ + T
G(0) (0)GG G G
Every analytic solution or insight into the problem can be “built in”
-Series expansion in U is often divergent, or, even worse, asymptotic. Does it makes sense to have more terms calculated?
Yes! (i) Unbiased resummation techniques (ii) there are interesting cases with convergent series (Hubbard model at finite-T, resonant fermions)
- It is an unsolved problem whether skeleton diagrams form asymptotic or convergent series Good news: BCS theory is non-analytic at U 0 , and yet this is accounted for within the lowest-order diagrams!
Polaron problem: environmentparticl couplinge HHH H
quasiparticle
*( 0), , ( , ), ...E p m G p t
E.g. Electrons in semiconducting crystals (electron-phonon polarons)
e e
( )( 1/ 2)
( )
. .
q qq
q
p p
pqq
p
p q p
H p a
V
a
a a
p
h
b
c
b
b
electron
phonons
el.-ph. interaction
Fermi-polaron problem:
Fermi s
2
ea ( )' ( ') ' 2
H V rH nr r rp
md
0rr
( )V r
( )r
1/3 ~ /Fn k
0
~ 1
0F S
F
k
k r
a
Universal physics( independent)( )V r
Examples:
Electron-phonon polarons (e.g. Frohlich model)= particle in the bosonic environment.
Too “simple”, no sign problem, 210N
Fermi –polarons (polarized resonant Fermi gas = particle in the fermionic environment.
G
G
Sign problem! max 11N
self-consistent and G
self-consistent onlyG
=( )Fk a
0.615Confirmed
by ENS
“Exact” solution:
Polaron
Molecule
sure, press Enter
Updates:
/ 4U t / 1.5 0.6t n
/ 0.025 /100FT t E
2D Fermi-Hubbard model in the Fermi-liquid regime
Bare series convergence:yes, after order 4
Fermi –liquid regime was reached
2'
'
2
2 2
( ) (0) ( )6
( ) (0)6
F FF
F
TE T E E
Tn T n
/ 4U t / 3.1 1.2t n
/ 0.4 /10FT t E
2D Fermi-Hubbard model in the Fermi-liquid regime
Comparing DiagMC with cluster DMFT(DCA implementation)
!
/ 4U t / 3.1 1.2t n
/ 0.4 /10FT t E
2D Fermi-Hubbard model in the Fermi-liquid regime
Momentum dependence of self-energy
0( , ) x yT p p p along
/ 4U t ( ) / 1.5 1.35nU t n
/ 0.1 / 50FT t E
3D Fermi-Hubbard model in the Fermi-liquid regime
DiagMC vs high-T expansion in t/T(up to 10-th order)
Unbiased high-T expansion in t/Tfails at T/t>1 before the FL regime sets in
28
10
10
8
( )Fk a / 0.3
/ 0.152(7)F
C F
T E
T E
3D Resonant Fermi gas at unitarity :
Bridging the gap between different limits
S. Nascimb`ene, N. Navon, K. J. Jiang, F. Chevy, and C. Salomon
DiagMC
ENS data
/Te
0/P P
Seatlle’s Det. MC
700
600
500
400
300
200
100
1000900800700600500
2.5
2.0
1.5
1.0
0.5
0.0
Opt
ical
Den
sity
300250200150100500
Pixel [1.2 m]
DiagMC fit
Andre Schirotzek, Ariel Sommer, Mark Ku, and Martin Zwierlein
60
50
40
30
20
10
0
F0
3210-1-2-3
Universal function F0
Single shot data
3 Tn
Andre Schirotzek, Ariel Sommer, Mark Ku, and Martin Zwierlein
Conclusions/perspectives
• Bold-line Diagrammatic series can be efficiently simulated.