5,692 research outputs found
RVB gauge theory and the Topological degeneracy in the Honeycomb Kitaev model
We relate the Z gauge theory formalism of the Kitaev model to the SU(2)
gauge theory of the resonating valence bond (RVB) physics. Further, we
reformulate a known Jordan-Wigner transformation of Kitaev model on a torus in
a general way that shows that it can be thought of as a Z gauge fixing
procedure. The conserved quantities simplify in terms of the gauge invariant
Jordan-Wigner fermions, enabling us to construct exact eigen states and
calculate physical quantities. We calculate the fermionic spectrum for flux
free sector for different gauge field configurations and show that the ground
state is four-fold degenerate on a torus in thermodynamic limit. Further on a
torus we construct four mutually anti-commuting operators which enable us to
prove that all eigenstates of this model are four fold degenerate in
thermodynamic limit.Comment: 12 pages, 3 figures. Added affiliation and a new section,
'Acknowledgements'.Typos correcte
Majorana Spin Liquids on a two-leg ladder
We realize a gapless Majorana Orbital Liquid (MOL) using orbital degrees of
freedom and also an SU(2)-invariant Majorana Spin Liquid (MSL) using both spin
and orbital degrees of freedom in Kitaev-type models on a 2-leg ladder. The
models are exactly solvable by Kitaev's parton approach, and we obtain
long-wavelength descriptions for both Majorana liquids. The MOL has one gapless
mode and power law correlations in energy at incommensuare wavevectors, while
the SU(2) MSL has three gapless modes and power law correlations in spin,
spin-nematic, and local energy observables. We study the stability of such
states to perturbations away from the exactly solvable points. We find that
both MOL and MSL can be stable against allowed short-range parton interactions.
We also argue that both states persist upon allowing gauge field
fluctuations, in that the number of gapless modes is retained, although with an
expanded set of contributions to observables compared to the free parton mean
field.Comment: 15 pages, 6 figures. Revised versio
Spin Bose-Metal phase in a spin-1/2 model with ring exchange on a two-leg triangular strip
Recent experiments on triangular lattice organic Mott insulators have found
evidence for a 2D spin liquid in proximity to the metal-insulator transition. A
Gutzwiller wavefunction study of the triangular lattice Heisenberg model with
appropriate four-spin ring exchanges has found that the projected spinon Fermi
sea state has a low variational energy. This wavefunction, together with a
slave particle gauge theory, suggests that such spin liquid possesses spin
correlations that are singular along surfaces in momentum space ("Bose
surfaces"). Signatures of this state, which we refer to as a "Spin Bose-Metal"
(SBM), are expected to be manifest in quasi-1D ladder systems: The discrete
transverse momenta cut through the 2D Bose surface leading to a distinct
pattern of 1D gapless modes. Here we search for a quasi-1D descendant of the
triangular lattice SBM state by exploring the Heisenberg plus ring model on a
two-leg strip (zigzag chain). Using DMRG, variational wavefunctions, and a
Bosonization analysis, we map out the full phase diagram. Without ring exchange
the model is equivalent to the J_1 - J_2 Heisenberg chain, and we find the
expected Bethe-chain and dimerized phases. Remarkably, moderate ring exchange
reveals a new gapless phase over a large swath of the phase diagram. Spin and
dimer correlations possess particular singular wavevectors and allow us to
identify this phase as the hoped for quasi-1D descendant SBM state. We derive a
low energy theory and find three gapless modes and one Luttinger parameter
controlling all power laws. Potential instabilities out of the zigzag SBM give
rise to other interesting phases such as a period-3 VBS or a period-4 Chirality
order, which we discover in the DMRG; we also find an interesting SBM state
with partial ferromagnetism.Comment: 30 pages, 23 figure
Superconductivity in zigzag CuO chains
Superconductivity has recently been discovered in
PrBaCuO with a maximum of about 15K.
Since the CuO planes in this material are believed to be insulating, it has
been proposed that the superconductivity occurs in the double (or zigzag) CuO
chain layer. On phenomenological grounds, we propose a theoretical
interpretation of the experimental results in terms of a new phase for the
zigzag chain, labelled by CS. This phase has a gap for some of the
relative spin and charge modes but no total spin gap, and can have a divergent
superconducting susceptibility for repulsive interactions. A microscopic model
for the zigzag CuO chain is proposed, and on the basis of density matrix
renormalization group (DMRG) and bosonization studies of this model, we adduce
evidence that supports our proposal.Comment: 10 pages, 5 figures; Journal-ref. adde
A Quantum Theory of Cold Bosonic Atoms in Optical Lattices
Ultracold atoms in optical lattices undergo a quantum phase transition from a
superfluid to a Mott insulator as the lattice potential depth is increased. We
describe an approximate theory of interacting bosons in optical lattices which
provides a qualitative description of both superfluid and insulator states. The
theory is based on a change of variables in which the boson coherent state
amplitude is replaced by an effective potential which promotes phase coherence
between different number states on each lattice site. It is illustrated here by
applying it to uniform and fully frustrated lattice cases, but is simple enough
that it can easily be applied to spatially inhomogeneous lattice systems
The reconfigurable Josephson circulator/directional amplifier
Circulators and directional amplifiers are crucial non-reciprocal signal
routing and processing components involved in microwave readout chains for a
variety of applications. They are particularly important in the field of
superconducting quantum information, where the devices also need to have
minimal photon losses to preserve the quantum coherence of signals.
Conventional commercial implementations of each device suffer from losses and
are built from very different physical principles, which has led to separate
strategies for the construction of their quantum-limited versions. However, as
recently proposed theoretically, by establishing simultaneous pairwise
conversion and/or gain processes between three modes of a Josephson-junction
based superconducting microwave circuit, it is possible to endow the circuit
with the functions of either a phase-preserving directional amplifier or a
circulator. Here, we experimentally demonstrate these two modes of operation of
the same circuit. Furthermore, in the directional amplifier mode, we show that
the noise performance is comparable to standard non-directional superconducting
amplifiers, while in the circulator mode, we show that the sense of circulation
is fully reversible. Our device is far simpler in both modes of operation than
previous proposals and implementations, requiring only three microwave pumps.
It offers the advantage of flexibility, as it can dynamically switch between
modes of operation as its pump conditions are changed. Moreover, by
demonstrating that a single three-wave process yields non-reciprocal devices
with reconfigurable functions, our work breaks the ground for the development
of future, more-complex directional circuits, and has excellent prospects for
on-chip integration
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