17,257 research outputs found

### Schwinger Boson Mean Field Theories of Spin Liquid States on Honeycomb Lattice: Projective Symmetry Group Analysis and Critical Field Theory

Motivated by the recent numerical evidence[1] of a short-range resonating
valence bond state in the honeycomb lattice Hubbard model, we consider
Schwinger boson mean field theories of possible spin liquid states on honeycomb
lattice. From general stability considerations the possible spin liquids will
have gapped spinons coupled to Z$_2$ gauge field. We apply the projective
symmetry group(PSG) method to classify possible Z$_2$ spin liquid states within
this formalism on honeycomb lattice. It is found that there are only two
relevant Z$_2$ states, differed by the value of gauge flux, zero or $\pi$, in
the elementary hexagon. The zero-flux state is a promising candidate for the
observed spin liquid and continuous phase transition into commensurate N\'eel
order. We also derive the critical field theory for this transition, which is
the well-studied O(4) invariant theory[2-4], and has an irrelevant coupling
between Higgs and boson fields with cubic power of spatial derivatives as
required by lattice symmetry. This is in sharp contrast to the conventional
theory[5], where such transition generically leads to non-colinear
incommensurate magnetic order. In this scenario the Z$_2$ spin liquid could be
close to a tricritical point. Soft boson modes will exist at seven different
wave vectors. This will show up as low frequency dynamical spin susceptibility
peaks not only at the $\Gamma$ point (the N\'eel order wave vector) but also at
Brillouin zone edge center $M$ points and twelve other points. Some simple
properties of the $\pi$-flux state are studies as well. Symmetry allowed
further neighbor mean field ansatz are derived in Appendix which can be used in
future theoretical works along this direction.Comment: mistakes in Eq.(13) and Eq.(A17) corrected on top of published
version, 14 pages, 6 figure

### Realization of the Exactly Solvable Kitaev Honeycomb Lattice Model in a Spin Rotation Invariant System

The exactly solvable Kitaev honeycomb lattice model is realized as the low
energy effect Hamiltonian of a spin-1/2 model with spin rotation and
time-reversal symmetry. The mapping to low energy effective Hamiltonian is
exact, without truncation errors in traditional perturbation series expansions.
This model consists of a honeycomb lattice of clusters of four spin-1/2
moments, and contains short-range interactions up to six-spin(or eight-spin)
terms. The spin in the Kitaev model is represented not as these spin-1/2
moments, but as pseudo-spin of the two-dimensional spin singlet sector of the
four antiferromagnetically coupled spin-1/2 moments within each cluster. Spin
correlations in the Kitaev model are mapped to dimer correlations or
spin-chirality correlations in this model. This exact construction is quite
general and can be used to make other interesting spin-1/2 models from spin
rotation invariant Hamiltonians. We discuss two possible routes to generate the
high order spin interactions from more natural couplings, which involves
perturbative expansions thus breaks the exact mapping, although in a controlled
manner.Comment: 11 pages, 3 figure, 1 table, RevTex4, rewritten for clarity, error
corrected, references added

### Twisted Hubbard Model for Sr2IrO4: Magnetism and Possible High Temperature Superconductivity

Sr2IrO4 has been suggested as a Mott insulator from a single J_eff=1/2 band,
similar to the cuprates. However this picture is complicated by the measured
large magnetic anisotropy and ferromagnetism. Based on a careful mapping to the
J_eff=1/2 (pseudospin-1/2) space, we propose that the low energy electronic
structure of Sr2IrO4 can indeed be described by a SU(2) invariant
pseudospin-1/2 Hubbard model very similar to that of the cuprates, but with a
"twisted" coupling to external magnetic field (a g-tensor with a staggered
antisymmetric component). This perspective naturally explains the magnetic
properties of Sr2IrO4. We also derive several simple facts based on this
mapping and the known results about the Hubbard model and the cuprates, which
may be tested in future experiments on Sr2IrO4. In particular we propose that
(electron-)doping Sr2IrO4 can potentially realize high-temperature
superconductivity.Comment: 5 pages, 1 figure, RevTex4, updated reference

### Spin phonon induced colinear order and magnetization plateaus in triangular and kagome antiferromagnets. Applications to CuFeO_2

Coupling between spin and lattice degrees of freedom are important in
geometrically frustrated magnets where they can lead to degeneracy lifting and
novel orders. We show that moderate spin-lattice couplings in triangular and
Kagome antiferromagnets can induce complex colinear magnetic orders. When
classical Heisenberg spins on the triangular lattice are coupled to Einstein
phonons, a rich variety of phases emerge, including the experimentally observed
four sublattice state and the five sublattice 1/5th plateau state seen in the
magneto-electric material CuFeO$_2$. In addition we predict magnetization
plateaus at 1/3, 3/7, 1/2, 3/5 and 5/7 at these couplings. Strong spin-lattice
couplings induce a striped colinear state, seen in $\alpha$-NaFeO$_2$ and
MnBr$_2$. On the Kagome lattice, moderate spin-lattice couplings induce
colinear order, but an extensive degeneracy remains.Comment: 5 pages, 4 figure

### Spin Liquid States on the Triangular and Kagome Lattices: A Projective Symmetry Group Analysis of Schwinger Boson States

A symmetry based analysis (Projective Symmetry Group) is used to study spin
liquid phases on the triangular and Kagom\'e lattices in the Schwinger boson
framework. A maximum of eight distinct $Z_2$ spin liquid states are found for
each lattice, which preserve all symmetries. Out of these only a few have
nonvanishing nearest neighbor amplitudes which are studied in greater detail.
On the triangular lattice, only two such states are present - the first
(zero-flux state) is the well known state introduced by Sachdev, which on
condensation of spinons leads to the 120 degree ordered state. The other
solution which we call the $\pi$-flux state has not previously been discussed.
Spinon condensation leads to an ordering wavevector at the Brillouin zone edge
centers, in contrast to the 120 degree state. While the zero-flux state is more
stable with just nearest-neighbor exchange, we find that the introduction of
either next-neighbor antiferromagnetic exchange or four spin ring-exchange (of
the sign obtained from a Hubbard model) tends to favor the $\pi$-flux state. On
the Kagom\'e lattice four solutions are obtained - two have been previously
discussed by Sachdev, which on spinon condensation give rise to the $q=0$ and
$\sqrt{3}\times\sqrt{3}$ spin ordered states. In addition we find two new
states with significantly larger values of the quantum parameter at which
magnetic ordering occurs. For one of them this even exceeds unity,
$\kappa_c\approx 2.0$ in a nearest neighbor model, indicating that if
stabilized, could remain spin disordered for physical values of the spin. This
state is also stabilized by ring exchange interactions with signs as derived
from the Hubbard model.Comment: revised, 21 pages, 19 figures, RevTex 4, corrected references, added
4 references, accepted by Phys.Rev.

### Schwinger boson spin liquid states on square lattice

We study possible spin liquids on square lattice that respect all lattice
symmetries and time-reversal symmetry within the framework of Schwinger boson
(mean-field) theory. Such spin liquids have spin gap and emergent Z_2 gauge
field excitations. We classify them by the projective symmetry group method,
and find six spin liquid states that are potentially relevant to the J_1-J_2
Heisenberg model. The properties of these states are studied under mean-field
approximation. Interestingly we find a spin liquid state that can go through
continuous phase transitions to either the N\'eel magnetic order or magnetic
orders of the wavevector at Brillouin zone edge center. We also discuss the
connection between our results and the Abrikosov fermion spin liquids.Comment: 27 pages, 14 figures, mistakes in Section III corrected, references
updated on top of the published versio

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