80 research outputs found
Majorana spin liquids and projective realization of SU(2) spin symmetry
We revisit the fermionic parton approach to S = 1/2 quantum spin liquids with
SU(2) spin rotation symmetry, and the associated projective symmetry group
(PSG) classification. We point out that the existing PSG classification is
incomplete; upon completing it, we find spin liquid states with S=1 and S=0
Majorana fermion excitations coupled to a deconfined Z2 gauge field. The
crucial observation leading us to this result is that, like space group and
time reversal symmetries, spin rotations can act projectively on the fermionic
partons; that is, a spin rotation may be realized by simultaneous SU(2) spin
and gauge rotations. We show that there are only two realizations of spin
rotations acting on fermionic partons: the familiar naive realization where
spin rotation is not accompanied by any gauge transformation, and a single type
of projective realization. We discuss the PSG classification for states with
projective spin rotations. To illustrate these results, we show that there are
four such PSGs on the two-dimensional square lattice. We study the properties
of the corresponding states, finding that one -- with gapless Fermi points --
is a stable phase beyond mean-field theory. In this phase, depending on
parameters, a small Zeeman magnetic field can open a partial gap for the
Majorana fermion excitations. Moreover, there are nearby gapped phases
supporting Z2 vortex excitations obeying non-Abelian statistics. We conclude
with a discussion of various open issues, including the challenging question of
where such S=1 Majorana spin liquids may occur in models and in real systems.Comment: 19 pages, 8 figures. Typos corrected, references adde
Topological Entanglement Entropy of Fracton Stabilizer Codes
Entanglement entropy provides a powerful characterization of two-dimensional
gapped topological phases of quantum matter, intimately tied to their
description by topological quantum field theories (TQFTs). Fracton topological
orders are three-dimensional gapped topologically ordered states of matter, but
the existence of a TQFT description for these phases remains an open question.
We show that three-dimensional fracton phases are nevertheless characterized,
at least partially, by universal structure in the entanglement entropy of their
ground state wave functions. We explicitly compute the entanglement entropy for
two archetypal fracton models --- the `X-cube model' and `Haah's code' --- and
demonstrate the existence of a topological contribution that scales linearly in
subsystem size. We show via Schrieffer-Wolff transformations that the
topological entanglement of fracton models is robust against arbitrary local
perturbations of the Hamiltonian. Finally, we argue that these results may be
extended to characterize localization-protected fracton topological order in
excited states of disordered fracton models.Comment: published versio
Odd Fracton Theories, Proximate Orders, and Parton Constructions
The Lieb-Schultz-Mattis (LSM) theorem implies that gapped phases of matter
must satisfy non-trivial conditions on their low-energy properties when a
combination of lattice translation and symmetry are imposed. We describe
a framework to characterize the action of symmetry on fractons and other
sub-dimensional fractional excitations, and use this together with the LSM
theorem to establish that X-cube fracton order can occur only at integer or
half-odd-integer filling. Using explicit parton constructions, we demonstrate
that "odd" versions of X-cube fracton order can occur in systems at
half-odd-integer filling, generalizing the notion of odd gauge theory to
the fracton setting. At half-odd-integer filling, exiting the X-cube phase by
condensing fractional quasiparticles leads to symmetry-breaking, thereby
allowing us to identify a class of conventional ordered phases proximate to
phases with fracton order. We leverage a dual description of one of these
ordered phases to show that its topological defects naturally have restricted
mobility. Condensing pairs of these defects then leads to a fracton phase,
whose excitations inherit these mobility restrictions
Entanglement Entropy of 3-d Conformal Gauge Theories with Many Flavors
Three-dimensional conformal field theories (CFTs) of deconfined gauge fields
coupled to gapless flavors of fermionic and bosonic matter describe quantum
critical points of condensed matter systems in two spatial dimensions. An
important characteristic of these CFTs is the finite part of the entanglement
entropy across a circle. The negative of this quantity is equal to the finite
part of the free energy of the Euclidean CFT on the three-sphere, and it has
been proposed to satisfy the so called F-theorem, which states that it
decreases under RG flow and is stationary at RG fixed points. We calculate the
three-sphere free energy of non-supersymmetric gauge theory with a large number
N_F of bosonic and/or fermionic flavors to the first subleading order in 1/N_F.
We also calculate the exact free energies of the analogous chiral and
non-chiral {\cal N} = 2 supersymmetric theories using localization, and find
agreement with the 1/N_F expansion. We analyze some RG flows of supersymmetric
theories, providing further evidence for the F-theorem.Comment: 31 pages, 2 figures; v2 refs added, minor change
An SU(N) Mott insulator of an atomic Fermi gas realized by large-spin Pomeranchuk cooling
The Hubbard model, containing only the minimum ingredients of nearest
neighbor hopping and on-site interaction for correlated electrons, has
succeeded in accounting for diverse phenomena observed in solid-state
materials. One of the interesting extensions is to enlarge its spin symmetry to
SU(N>2), which is closely related to systems with orbital degeneracy. Here we
report a successful formation of the SU(6) symmetric Mott insulator state with
an atomic Fermi gas of ytterbium (173Yb) in a three-dimensional optical
lattice. Besides the suppression of compressibility and the existence of charge
excitation gap which characterize a Mott insulating phase, we reveal an
important difference between the cases of SU(6) and SU(2) in the achievable
temperature as the consequence of different entropy carried by an isolated
spin. This is analogous to Pomeranchuk cooling in solid 3He and will be helpful
for investigating exotic quantum phases of SU(N) Hubbard system at extremely
low temperatures.Comment: 20 pages, 6 figures, to appear in Nature Physic
Three-body interactions with cold polar molecules
We show that polar molecules driven by microwave fields give naturally rise
to strong three-body interactions, while the two-particle interaction can be
independently controlled and even switched off. The derivation of these
effective interaction potentials is based on a microscopic understanding of the
underlying molecular physics, and follows from a well controlled and systematic
expansion into many-body interaction terms. For molecules trapped in an optical
lattice, we show that these interaction potentials give rise to Hubbard models
with strong nearest-neighbor two-body and three-body interaction. As an
illustration, we study the one-dimensional Bose-Hubbard model with dominant
three-body interaction and derive its phase diagram.Comment: 8 pages, 4 figure
The Spin Liquid State of the Tb2Ti2O7 Pyrochlore Antiferromagnet: A Puzzling State of Affairs
The pyrochlore antiferromagnet Tb2Ti2O7 has proven to be an enigma to
experimentalists and theorists working on frustrated magnetic systems. The
experimentally determined energy level structure suggests a local Ising
antiferromagnet at low temperatures, T < 10 K. An appropriate model then
predicts a long-range ordered Q = 0 state below approximately 2 K. However,
muon spin resonance experiments reveal a paramagnetic structure down to tens of
milli-Kelvin. The importance of fluctuations out of the ground state effective
Ising doublet has been recently understood, for the measured paramagnetic
correlations can not be described without including the higher crystal field
states. However, these fluctuations treated within the random phase
approximation (RPA) fail to account for the lack of ordering in this system
below 2 K. In this work, we briefly review the experimental evidence for the
collective paramagnetic state of Tb2Ti2O7. The basic theoretical picture for
this system is discussed, where results from classical spin models are used to
motivate the investigation of quantum effects to lowest order via the RPA.
Avenues for future experimental and theoretical work on Tb2Ti2O7 are presented.Comment: Latex2e,6 pages, IOP format, introduction shortened and other minor
corrections, replaced with published version in the Proceedings of the Highly
Frustrated Magnetism 2003 Conference, Grenobl
Reversing non-local transport through a superconductor by electromagnetic excitations
Superconductors connected to normal metallic electrodes at the nanoscale
provide a potential source of non-locally entangled electron pairs. Such states
would arise from Cooper pairs splitting into two electrons with opposite spins
tunnelling into different leads. In an actual system the detection of these
processes is hindered by the elastic transmission of individual electrons
between the leads, yielding an opposite contribution to the non-local
conductance. Here we show that electromagnetic excitations on the
superconductor can play an important role in altering the balance between these
two processes, leading to a dominance of one upon the other depending on the
spatial symmetry of these excitations. These findings allow to understand some
intriguing recent experimental results and open the possibility to control
non-local transport through a superconductor by an appropriate design of the
experimental geometry.Comment: 6 pages, 3 figure
Gauge fields for ultracold atoms in optical superlattices
We present a scheme that produces a strong U(1)-like gauge field on cold
atoms confined in a two-dimensional square optical lattice. Our proposal relies
on two essential features, a long-lived metastable excited state that exists
for alkaline-earth or Ytterbium atoms, and an optical superlattice. As in the
proposal by Jaksch and Zoller [New Journal of Physics 5, 56 (2003)],
laser-assisted tunneling between adjacent sites creates an effective magnetic
field. In the tight-binding approximation, the atomic motion is described by
the Harper Hamiltonian, with a flux across each lattice plaquette that can
realistically take any value between 0 and . We show how to take advantage
of the superlattice to ensure that each plaquette acquires the same phase, thus
simulating a uniform magnetic field. We discuss the observable consequences of
the artificial gauge field on non-interacting bosonic and fermionic gases. We
also outline how the scheme can be generalized to non-Abelian gauge fields
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