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ArnaudKoetsier
Floris van Liere
Henk Stoof
The imbalanced antiferromagnet in an optical lattice
2
Introduction
• Fermions in an optical lattice• Described by the Hubbard model• Realised experimentally [Esslinger ’05]• Fermionic Mott insulator recently seen [Esslinger ’08, Bloch ’08]• There is currently a race to create the Néel state
• Imbalanced Fermi gases• Experimentally realised [Ketterle ’06, Hulet ’06]• High relevance to other areas of physics (particle physics, neutron stars, etc.)
• Imbalanced Fermi gases in an optical lattice ??
3
Fermi-Hubbard Model
Sums depend on:Filling NDimensionality (d=3)
On-site interaction: U Tunneling: t
Consider nearest-neighbor tunneling only.
The positive-U (repulsive) Fermi-Hubbard Model, relevant to High-Tc SC
H = −tPσ
Phjj0i
c†j,σcj0,σ + UPjc†j,↑c
†j,↓cj,↓cj,↑
4
Quantum Phases of the Fermi-Hubbard ModelFi
lling
Frac
tion
0
0.5
1
Mott Insulator (need large U)
Band Insulator
Conductor
Conductor
Conductor
• Positive U (repulsive on-site interaction):
• Negative U: Pairing occurs — BEC/BCS superfluid at all fillings.
5
• At half filling, when and we are deep in the Mott phase.Hopping is energetically supressedModel simplifies: only spin degrees of freedom remain (no transport)
• Integrate out the hopping fluctuations, then the Hubbard model reduces to the Heisenberg model:
U À t
Mott insulator: Heisenberg Model (no imbalance yet)
kBT ¿ U
J =4t2
U
Szi =1
2
³c†i,↑ci,↑ − c†i,↓ci,↓
´S+i =c
†i,↑ci,↓
S−i =c†i,↓ci,↑
H =J
2
Xhjki
Sj · Sk
Spin ½ operators: S = 12σ
Superexchange constant (virtual hops):
6
Néel State (no imbalance yet)
• The Néel state is the antiferromagnetic ground state for
• Néel order parameter measures amount of “anti-alignment”:
• Below some critical temperature Tc, we enter the Néel state and becomes non-zero.
0 Tc0
0.5
T
⟨n⟩
0 ≤ h|n|i ≤ 0.5
h|n|i
h|n|i
nj = (−1)jhSji
J > 0
7
• Until now, • Now take — spin population imbalance. • This gives rise to an overal magnetization
N↑ 6= N↓
• Add a constraint to the Heisenberg model that enforces
Heisenberg Model with imbalance
Effective magnetic field (Lagrange multiplier):
H =J
2
Xhjki
Sj · Sk −Xi
B · (Si −m)
m = (0, 0,mz)
mz = SN↑ −N↓N↑ +N↓
(fermions: )S = 12
N↑ = N↓ = N/2
hSi =m
B
8
• ground state is antiferromagnetic (Néel state)Two sublattices:
• Linearize Hamiltonian:
• Magnetization:
• Néel order parameter:
• Obtain the on-site free energy subject to the constraint (eliminates )
J > 0⇒
Mean field analysis
A, B
m =hSAi+ hSBi
2
n =hSAi− hSBi
2
SA(B)i = hSA(B)i i+ δS
A(B)i
f(n,m;B)
∇Bf = 0 B
B
A B
A
A
B
AB
B
B
A
A
9
Phase Diagram in three dimensions
0 Tc0
0.5
T
⟨n⟩
Add imbalance0.0
0.2
0.4mz0 0.3 0.6
0.91.2kBT�J
0.0
0.2
0.4n
mz
k B T
/J
0 0.1 0.2 0.3 0.4 0.50
0.5
1
1.5
m
nIsing:
m
n
Canted:
n 6= 0
n = 0
10
Spin waves (magnons)
• Spin dynamics can be found from:
• Imbalance splits the degeneracy:
−π2
π
2kd
h̄ω/Jz
00
0.1
0.2
0.3
0.4
0.5No imbalance: Doubly degenerate antiferromagnetic dispersion
Antiferromagneticmagnons: ω ∝ |k|
ω ∝ k2Ferromagneticmagnons:
dS
dt=i
~[H,S]
Gap:(Larmorprecessionof n)
11
Long-wavelength dynamics: NLσM
• Dynamics are summarised a non-linear sigma model with an action
• The equilibrium value of is found from the Landau free energy:
• NLσM admits spin waves but also topologically stable excitations in the local staggered magnetisation .
S[n(x, t)] =
Zdt
Zdx
dD
½1
4Jzn2
µ~∂n(x, t)
∂t− 2Jzm× n(x, t)
¶2− Jd
2
2[∇n(x, t)]2
¾
F [n(x),m] =
Zdx
dD
½Jd2
2[∇n(x)]2 + f [n(x),m]
¾
• lattice spacing:• number of nearest neighbours:• local staggered magnetization:
d = λ/2z = 2Dn(x, t)
n(x, t)
n(x, t)
12
• The topological excitaitons are vortices; Néel vector has an out-of-easy-plane component in the core
• In two dimensions, these are merons:• Spin texture of a meron:
• Ansatz:
• Merons characterised by:Pontryagin index ±½VorticityCore size λ
Topological excitations
n =
⎛⎝ pn2 − [nz(r)]2 cosφ
nvpn2 − [nz(r)]2 sinφ
nz(r)
⎞⎠nv = 1
nv = −1nv = ±1
nz(r) =n
[(r/λ)2 + 1]2
13
Meron size
• Core size λ of meron found by plugging the spin texture into and minimizing (below Tc):
• The energy of a single meron diverges logarithmically with the system area A at low temperature as
merons must be created in pairs.
0 0.1 0.20.3
0.4mz0.3
0.60.9
1.21.5
kBT�J0
2
4
6Λ
d
F [n(x),m]
Jn2π
2ln
A
πλ2
mz
k B T
/J
0 0.1 0.2 0.3 0.4 0.50
0.5
1
1.5
Meronspresent
Meron core size
14
Meron pairs
Low temperatures:A pair of merons with opposite vorticity, has a finite energy since the deformation of the spin texture cancels at infinity:
Higher temperatures:Entropy contributions overcome the divergent energy of a single meronThe system can lower its free energy through the proliferation of single merons
15
Kosterlitz-Thouless transition
• The unbinding of meron pairs in 2D signals a KT transition. Thisdrives down Tc compared with MFT:
• New Tc obtained by analogy to an anisotropic O(3) model (Monte Carlo results: [Klomfass et al, Nucl. Phys. B360, 264 (1991)] )
mzk B
T/J
0 0.05 0.10
0.02
0.04
0.06
n 6= 0
mz
k B T
/J
0 0.2 0.40
0.2
0.4
0.6
0.8
1
KT transition
MFT in 2D
16
Experimental feasibility
• Experimental realisation:Imbalance: drive spin transitions with RF fieldNéel state in optical lattice: adiabatic cooling [AK et al. PRA77, 023623 (2008)]
• Observation of Néel stateCorrelations in atom shot noiseBragg reflection (also probes spin waves)
• Observation of KT transitionInterference experiment [Hadzibabic et al. Nature 441, 1118 (2006)]In situ imaging [Gericke et al. Nat. Phys. 4, 949 (2008); Würtz et al.arXiv:0903.4837]
17
Conclusion
• Tc calculated for entering an antiferromagnetically ordered state in mean field theory
• Topological excitations give rise to a KT transition in 2D which significantly lowers Tc compared to MFT.
• The imbalanced antiferrromagnet is a rich systemferro- and antiferromagnetic propertiescontains topological excitationsmodels quantum magnetism, bilayers, etc.merons possess an internal Ising degree of freedom associated toPontryagin index — possible application to topological quantum computation
• Future work:include fluctuations beyond MFT for better accuracy in three dimensionsinvestigate topological excitations in 3D (vortex rings)incorporate equilibrium in the NLσMgradient of n gives rise to a magnetization
18
• On-site free energy:
where• Constraint equation:
• Critical temperature:
• Effective magnetic field below the critical temperature:
Results
f(n,m;B) =Jz
2(n2 −m2) +m ·B
− 12kBT ln
∙4 cosh
µ |BA|2kBT
¶cosh
µ |BB |2kBT
¶¸BA (B) = B− Jzm± Jzn
B = 2Jzm
m =1
4
∙BA|BA|
tanh
µ |BA|2kBT
¶+BB|BB |
tanh
µ |BB |2kBT
¶¸
Tc =Jzmz
2kBarctanh(2mz)
19
Anisotropic O(3) model
• Dimensionless free energy of the anisotropic O(3) model [Klomfass et al, Nucl. Phys. B360, 264 (1991)] :
• KT transition:
• Numerical fit:
βf3 = −β3Xhi,ji
Si · Sj + γ3Xi
(Szi )2
γ3(β3) =β3
β3 − 1.06exp[−5.6(β3 − 1.085)].
1.0 1.2 1.4 1.6 1.8 2.00.0
0.2
0.4
0.6
0.8
1.0
b3
g 3êH1+g
3L
20
Analogy with the anisotropic O(3) model
• Landau free energy:
0.0 0.1 0.2 0.3 0.4 0.50.0
0.5
1.0
1.5
2.0
2.5
3.0
m
gHm,bLê
J
βF =− βJ
2
XhI,ji
ni · nj + βXi
f(m,ni,β)
'− βJn2
2
XhI,ji
Si · Sj + βn2γ(m,β)Xi
(Szi )2
β3 =Jβn2
2
γ3 =2β3Jγ(m,β)
Mapping of our model toAnisotropic O(3) model:
Numerical fit parameter
0.02
1/Jβ =
0.2
0.4
0.60.8
Tc