stability of time-reversal symmetry breaking spin liquid ...eszirmai/spin_liquids_part1.pdf ·...

Stability of time-reversal symmetry breaking spinliquid states in high-spin fermionic systems

Edina Szirmai

International School and Workshop on Anyon Physics of Ultracold Atomic GasesKaiserslautern, 12-15. 12. 2014.

Quantum simulations with ultracold atoms

Quantum simulation offundamental models (properties,phenomena).

Novel behavior, completely newphases are expected due to thehigh spin.

High-Tc superconductors

Quantum information

Simulation of gauge theories




A possible explanation for themechanism of high-Tc

superconductivity and their strangebehavior in the non-superconductingphase based on the strong magneticfluctuation in dopped Mott insulators.These fluctuation can be treated withinthe spin liquid concept.


Quantum information





In topological phases of spinliquids the quasiparticles havefractional statistics. They arenonlocal and resist well againstlocal perturbations. Promising qbitcandidates.


Quantum information





Low energy excitations above spinliquids can be described byeffective gauge theories. Aim: tostudy various gauge theories withultracold atoms.


Quantum information



Atoms loaded into an optical lattice

Periodic potential: standing wave laser light.

Interaction between the neutral atoms:

van der Waals interaction

in case of alkaline-earth atoms:spin independent s-wavecollisions

Easy to control the model parameters:

Interaction strength: Feshbach resonance

Localization: laser intensity

Lattice geometry

Outline

Part IFundamentals of high spin systemsSpin wave descriptionValence bond picture

Part IICompeting spin liquid states ofspin-3/2 fermions in a square latticeCompeting spin liquid states ofspin-5/2 fermions in a honeycomb lattice

Properties of chiral spin liquid stateStability of the spin liquid states beyondthe mean-field approximationFinite temperature behaviorExperimentally measurable quantities

In collaboration with:

M. Lewenstein

P. Sinkovicz

G. Szirmai

A. Zamora

PRA 88(R) 043619 (2013)

PRA 84 011611 (2011)

EPL 93, 66005 (2011)

Fundamental properties of high-spin systems


The Hubbard Hamiltonian with n.n hopping and on-site interaction:

H =−t ∑<i,j>

c†i,αcj,α +∑

iUα,β

γ,δ c†i,αc†

i,β ci,δ ci,γ .

S = |F1−F2|, . . . |F1 + F2|Sz =−S, . . .SF : individual atoms, S: total spin of 2 scattering atoms

4 components: ± 32 , and ± 1

2

F1 = F2 = 32

S = 0: Sz = 0,S = 1: Sz =−1,0,1

S = 2: Sz =−2, . . . ,2S = 3: Sz =−3, . . . ,3



H =−t ∑<i,j>

c†i,αcj,α +∑

iUα,β

γ,δ c†i,αc†

i,β ci,δ ci,γ .


2 components: ↑, and ↓

F1 = F2 = 12 ,

S = 0: Sz = 0,S = 1: Sz =−1,0,1



2

F1 = F2 = 32

S = 0: Sz = 0,S = 1: Sz =−1,0,1

S = 2: Sz =−2, . . . ,2S = 3: Sz =−3, . . . ,3



H =−t ∑<i,j>

c†i,αcj,α +∑

iUα,β

γ,δ c†i,αc†

i,β ci,δ ci,γ .



2

F1 = F2 = 32

S = 0: Sz = 0,S = 1: Sz =−1,0,1

S = 2: Sz =−2, . . . ,2S = 3: Sz =−3, . . . ,3


The Hamiltonian:

H = Hkin + Hint

where Hint contains manytypes of scatteringprocesses.

on-site interaction=⇒

Pauli’s principle

The only nonzero terms arecompletly antisymmetric forthe exchange of the spin ofthe two scattering particles.


The Hamiltonian:

H = Hkin + Hint



Pauli’s principle


spin-1/2 fermions

Only singlet scatterings areallowed:

Stot = 0 =⇒

H =−t ∑〈i,j〉(c†i,σ cj,σ +H.c.)+∑i U0P

(i)0 , and

P(i)0 = c†

i,↑c†i,↓ci,↓ci,↑ = ni↑ni↓ projects to the singlet

subspace Stot = 0.


The Hamiltonian:

H = Hkin + Hint



Pauli’s principle


spin-3/2 fermions

Classification of the allowedscattering processes:

Stot = 0

Stot = 2=⇒

H =−t ∑〈i,j〉(c†i,σ cj,σ +H.c.)+∑i

[U0P

(i)0 +U2P

(i)2

],

and P(i)S = c†

i,σ1c†

i,σ2ci,σ3 ci,σ4 P̂S , where P̂S projects to

the subspace Stot = S.T. L. Ho PRL (1998)

T. Ohmi and K. Machida JPSJ (1998)


The Hamiltonian:

H = Hkin + Hint



Pauli’s principle


spin-5/2 fermions

Classification of the allowedscattering processes:

Stot = 0

Stot = 2

Stot = 4

=⇒

H =−t ∑〈i,j〉(c†i,σ cj,σ +H.c.)

+∑i

[U0P

(i)0 +U2P

(i)2 +U4P

(i)4

],

and P(i)S = c†

i,σ1c†

i,σ2ci,σ3 ci,σ4 P̂S , where P̂S projects to

the subspace Stot = S.T. L. Ho PRL (1998)

T. Ohmi and K. Machida JPSJ (1998)

Effective spin models in the strong coupling limit


H =−t ∑<i,j>

c†i,αcj,α + U ∑

ic†

i,αc†i,β ci,αci,β .

spin independent interaction→ SU(N) symmetry

Strongly repulsive limit: U/t → ∞

repulsive interaction, f = 1/(2F +1) filling (# of particles =# of sites)

Perturbation theory up to leading orderwith respect to t .

t preserves S and Sz

nearest-neighbor hopping


Effective Hamiltonian (spin-F fermions in the U/t → ∞ limit)

H = J ∑〈i,j〉

c†i,αc†

j,β cj,αci,β

nearest-neighbor interaction

the same spin dependencethat has the original model

no particle transport

Mott state

Without long range spin order/preserved spin rotational invariance:spin liquid state


(ni = c†i,σ ci,σ , and Si = c†

i,σ Fσ ,σ ′ci,σ ′ )

Spin exchange appears explicitly in the Hamiltonian:

F = 12 : AFM Heisenberg model

Heff = J ∑〈i,j〉(SiSj − 1

4 ninj)

F = 32 :

HF=3/2eff = ∑〈i,j〉

[a0 ninj + a1SiSj + a2(SiSj)

2 + a3(SiSj)3]

F = 52 :

HF=5/2eff = ∑〈i,j〉

[a0 ni nj +a1Si Sj +a2(Si Sj)

2 +a3(Si Sj)3 +a4(Si Sj)

4 +a5(Si Sj)5]

Antiferromagnetic Heisenberg model: Heff = J ∑〈i,j〉(SiSj − 1

4 ninj)

Let us consider a two-site problem: Hi,j = JSiSj

Site-by-site picture:|Sz

i = F ,Szj =−F

⟩: Ei,j =−JF 2.

BUT this is not an eigenstate of Hi,j = JSzi Sz

j +J2 (S

+i S−j +S−i S+

j ).Let Sz fluctuate to gain energy

⇒ theory of AFM spin waves

Fluctuation above the classical Néel state:

Sublattice magnetization:〈Sz

A〉=−〈SzB〉= F −∆F .

Description: magnon picture.

Large ∆F → "quantum melting"→

Spin liquid

Antiferromagnetic Heisenberg model: Heff = J ∑〈i,j〉(SiSj − 1

4 ninj)

Basic concepts of AFM SWT and magnon picture:

Description of the spin wave excitations→ boson operators.

Holstein-Primakoff transformation (and similar for sublattice B):

S+A,j =

(2F −a†

j aj

)1/2aj S−A,j = a†

j

(2F −a†

j aj

)1/2

SzA,j = 2−a†

j aj

Interacting boson system→ (SiSj)p multiboson int.

Linear spin wave theory: no magnon-magnon interaction.

The ground state energy/bond (LSWT): Ei,j =−JF(F + ζ ), with0 < ζ < 1 and geometry-dependent.

If ∆F large from the LSWT:Beyond LSWT.

Change concept.


Even in case of the simplest F = 12 case there exist lattices that

have ground state without breaking of spin rotational invariance(no Néel order) in the thermodynamic limit.Various disordered spin sates occur in e.g.

finite systems,frustrated systems,systems described by Hamiltonian with higher power of SiSj .

To describe these disordered states a powerful possibility:

valence bond picture.

Valence bond pictureF = 1/2 AFM Heisenberg model

Fazekas, Electron Correlation and Magnetism (1999)

Affleck, Kennedy, Lieb, Tasaki, Valence Bond Ground States in Isotropic Quantum Antiferromagnets (1988)

Valence bond picture

Back to the two-site problem: Hi,j = JSiSj

Site-by-site picture:|Sz

i = F ,Szj =−F

⟩: Ei,j =−JF 2.

BUT this is not an eigenstate of Hi,j = JSzi Sz

j +J2 (S

+i S−j +S−i S+

j ).Let Sz fluctuate to gain energy

⇒ theory of AFM spin waves

Bond-by-bond picture:Hi,j =−JF(F +1)+ J

2 (Si +Sj)2 ⇒ |Si + Sj |= 0 (singlet)

Ei,j =−JF(F + 1)

⇒ theory of valence bonds


Let us consider now larger lattice: H = ∑〈i,j〉Hi,j = J ∑〈i,j〉SiSj

[ij] = 1√2[α(i)β (j)−β (i)α(j)] = ji singlet bond

Usually longer bonds appear.

VB state: |VB⟩

= ∏pairs|[i, j]

⟩RVB state: |RVB

⟩= ∑

conf∏pairs

A (|i− j|)|[i, j]⟩

Approximation: nearest-neighbor RVB

P. W. Anderson (1973)




Singlet state on a lattice: letting all spins participate in pair bonds.

The singlet basis for L = 4:

[12][34] =

1 2

4 3

and [23][41] =

1 2

4 3


VB state: |VB⟩

= ∏pairs|[i, j]

⟩RVB state: |RVB

⟩= ∑

conf∏pairs

A (|i− j|)|[i, j]⟩






Singlet state on a lattice: letting all spins participate in pair bonds.

The singlet basis for L = 4:

[12][34] =

1 2

4 3

and [23][41] =

1 2

4 3

One spin – one bond.

Only non-crossing bonds:

1 2

4 3

−=

1 2

4 3

1 2

4 3


VB state: |VB⟩

= ∏pairs|[i, j]

⟩RVB state: |RVB

⟩= ∑

conf∏pairs

A (|i− j|)|[i, j]⟩






F = 12

SiSj =−12P

(i,j)0 + 1

4 , and P(i,j)0 projects to the singlet subspace

P(2,3)0

=

1 2 1 2

4 3 4 3

or P(4,6)0

1 2

4 3

6

5

=

1 2

4 3

6

5

Resonate between different configurations of the valence (singlet) bonds.

L. Hulthén (1938)


VB state: |VB⟩

= ∏pairs|[i, j]

⟩RVB state: |RVB

⟩= ∑

conf∏pairs

A (|i− j|)|[i, j]⟩







VB state: |VB⟩

= ∏pairs|[i, j]

⟩RVB state: |RVB

⟩= ∑

conf∏pairs

A (|i− j|)|[i, j]⟩




1) Simplest RVB system:

L = 4, isotropic HM, F = 1/2

Ψ0 = 1√3

([12][34] + [23][41]) = 1√3

1 2

4 3

1 2

4 3

+

E0 =−2J.⟨Ψ0|Sz

j |Ψ0⟩

= 0, for ∀j .〈Φ0|Sz

1Sz2 |Ψ0〉=−1/6, and 〈Φ0|Sz

1Sz3 |Ψ0〉= 1/12

⇒ No magnetic order but there is an AF correlation.


2) One-dimensional J1-J2 isotropic AFM HM, F = 1/2

H(J1,J2) = J1 ∑j

SiSj+1 + J2 ∑j

SiSj+2

Majumdar-Ghosh model: J1 = 2J2 ≡ J.



H(J1,J2) = J1 ∑j

SiSj+1 + J2 ∑j

SiSj+2


H(J,J/2) =J4 ∑

j(Sj−1 + Sj + Sj+1)2 + const.

The total spin of a "trion of site j" is either Strion = 1/2 or 3/2

E = J4 LStrion (Strion + 1) + const. ⇒ S(j)

trion(GS) = 1/2.



H(J1,J2) = J1 ∑j

SiSj+1 + J2 ∑j

SiSj+2


H(J,J/2) =J4 ∑

j(Sj−1 + Sj + Sj+1)2 + const.

The total spin of a "trion of site j" is either Strion = 1/2 or 3/2

E = J4 LStrion (Strion + 1) + const. ⇒ S(j)

trion(GS) = 1/2.

Construct such a state.



H(J1,J2) = J1 ∑j

SiSj+1 + J2 ∑j

SiSj+2


S(j)trion(GS) = 1/2

3 4 5 6 7 81 2

2-fold degeneracy.

No magnetic order.

(Discrete) translational inv. is broken!

spin liquid↔

valence bond solid


3 4 5 6 7 81 2

Dimer order parameter: Dj = A〈Sj−1Sj −SjSj+1〉 (with A =−4/3J).

Why there is no resonance?

Cancellations only in the MG case (J2/J1 = 1/2).

Dimer ground state above J2/J1 ≈ 0.24.Inhomogeneous: Dj 6= 0.Only short range AF correlations.

RVB ground state below J2/J1 ≈ 0.24.Homogeneous: Dj = 0.⟨Sz

j Szl

⟩decay algebraically→ quasi-long range AF correlations.


3 4 5 6 7 81 2

Dimer order parameter: Dj = A〈Sj−1Sj −SjSj+1〉 (with A =−4/3J).

Why there is no resonance?

Cancellations only in the MG case (J2/J1 = 1/2).Dimer ground state above J2/J1 ≈ 0.24.

Inhomogeneous: Dj 6= 0.Only short range AF correlations.

RVB ground state below J2/J1 ≈ 0.24.Homogeneous: Dj = 0.⟨Sz

j Szl

⟩decay algebraically→ quasi-long range AF correlations.

stability of time-reversal symmetry breaking spin liquid ...eszirmai/spin_liquids_part1.pdf ·...

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