1,574 research outputs found
Algebraic vortex liquid theory of a quantum antiferromagnet on the kagome lattice
There is growing evidence from both experiment and numerical studies that low
half-odd integer quantum spins on a kagome lattice with predominant
antiferromagnetic near neighbor interactions do not order magnetically or break
lattice symmetries even at temperatures much lower than the exchange
interaction strength. Moreover, there appear to be a plethora of low energy
excitations, predominantly singlets but also spin carrying, which suggest that
the putative underlying quantum spin liquid is a gapless ``critical spin
liquid'' rather than a gapped spin liquid with topological order. Here, we
develop an effective field theory approach for the spin-1/2 Heisenberg model
with easy-plane anisotropy on the kagome lattice. By employing a vortex duality
transformation, followed by a fermionization and flux-smearing, we obtain
access to a gapless yet stable critical spin liquid phase, which is described
by (2+1)-dimensional quantum electrodynamics (QED) with an emergent
flavor symmetry. The specific heat, thermal conductivity, and
dynamical structure factor are extracted from the effective field theory, and
contrasted with other theoretical approaches to the kagome antiferromagnet.Comment: 14 pages, 8 figure
Interlayer coherent composite Fermi liquid phase in quantum Hall bilayers
Composite fermions have played a seminal role in understanding the quantum
Hall effect, particularly the formation of a compressible `composite Fermi
liquid' (CFL) at filling factor nu = 1/2. Here we suggest that in multi-layer
systems interlayer Coulomb repulsion can similarly generate `metallic' behavior
of composite fermions between layers, even if the electrons remain insulating.
Specifically, we propose that a quantum Hall bilayer with nu = 1/2 per layer at
intermediate layer separation may host such an interlayer coherent CFL, driven
by exciton condensation of composite fermions. This phase has a number of
remarkable properties: the presence of `bonding' and `antibonding' composite
Fermi seas, compressible behavior with respect to symmetric currents, and
fractional quantum Hall behavior in the counterflow channel. Quantum
oscillations associated with the Fermi seas give rise to a new series of
incompressible states at fillings nu = p/[2(p \pm 1)] per layer (p an integer),
which is a bilayer analogue of the Jain sequence.Comment: 4 pages, 3 figure
Gapless layered three-dimensional fractional quantum Hall states
Using the parton construction, we build a three-dimensional (3D) multilayer
fractional quantum Hall state with average filling \nu = 1/3 per layer that is
qualitatively distinct from a stacking of weakly coupled Laughlin states. The
state supports gapped charge e/3 fermionic quasiparticles that can propagate
both within and between the layers, in contrast to the quasiparticles in a
multilayer Laughlin state which are confined within each layer. Moreover, the
state has gapless neutral collective modes, a manifestation of an emergent
"photon", which is minimally coupled to the fermionic quasiparticles. The
surface sheath of the multilayer state resembles a chiral analog of the
Halperin-Lee-Read state, which is protected against gap forming instabilities
by the topological character of the bulk 3D phase. We propose that this state
might be present in multilayer systems in the "intermediate tunneling regime",
where the interlayer tunneling strength is on the same order as the Coulomb
energy scale. We also find that the parton construction leads to a candidate
state for a bilayer \nu = 1/3 system in the intermediate tunneling regime. The
candidate state is distinct from both a bilayer of \nu=1/3 Laughlin states and
the single layer \nu = 2/3 state, but is nonetheless a fully gapped fractional
quantum Hall state with charge e/3 anyonic quasiparticles.Comment: 11 pages, 1 figur
The Roton Fermi Liquid
We introduce and analyze a novel metallic phase of two-dimensional (2d)
electrons, the Roton Fermi Liquid (RFL), which, in contrast to the Landau Fermi
liquid, supports both gapless fermionic and bosonic quasiparticle excitations.
The RFL is accessed using a re-formulation of 2d electrons consisting of
fermionic quasiparticles and vortices interacting with a mutual
long-ranged statistical interaction. In the presence of a strong
vortex-antivortex (i.e. roton) hopping term, the RFL phase emerges as an exotic
yet eminently tractable new quantum ground state. The RFL phase exhibits a
``Bose surface'' of gapless roton excitations describing transverse current
fluctuations, has off-diagonal quasi-long-ranged order (ODQLRO) at zero
temperature (T=0), but is not superconducting, having zero superfluid density
and no Meissner effect. The electrical resistance {\it vanishes} as
with a power of temperature (and frequency), (with ), independent of the impurity concentration. The RFL phase also has a full
Fermi surface of quasiparticle excitations just as in a Landau Fermi liquid.
Electrons can, however, scatter anomalously from rotonic "current
fluctuations'' and "superconducting fluctuations'', leading to "hot" and "cold"
spots. Fermionic quasiparticles dominate the Hall electrical transport. We also
discuss instabilities of the RFL to a conventional Fermi liquid and a
superconductor. Precisely {\it at} the instability into the Fermi liquid state,
the exponent , so that . Upon entering the
superconducting state the anomalous quasiparticle scattering is strongly
suppressed. We discuss how the RFL phenomenology might apply to the cuprates.Comment: 43 page
Conductance fluctuations at the integer quantum Hall plateau transition
We study numerically conductance fluctuations near the integer quantum Hall
effect plateau transition. The system is presumed to be in a mesoscopic regime,
with phase coherence length comparable to the system size. We focus on a
two-terminal conductance G for square samples, considering both periodic and
open boundary conditions transverse to the current. At the plateau transition,
G is broadly distributed, with a distribution function close to uniform on the
interval between zero and one in units of e^2/h. Our results are consistent
with a recent experiment by Cobden and Kogan on a mesoscopic quantum Hall
effect sample.Comment: minor changes, 5 pages LaTex, 7 postscript figures included using
epsf; to be published Phys. Rev. B 55 (1997
Universal point contact resistance between thin-film superconductors
A system comprising two superconducting thin films connected by a point
contact is considered. The contact resistance is calculated as a function of
temperature and film geometry, and is found to vanish rapidly with temperature,
according to a universal, nearly activated form, becoming strictly zero only at
zero temperature. At the lowest temperatures, the activation barrier is set
primarily by the superfluid stiffness in the films, and displays only a weak
(i.e., logarithmic) temperature dependence. The Josephson effect is thus
destroyed, albeit only weakly, as a consequence of the power-law-correlated
superconducting fluctuations present in the films below the
Berezinskii-Kosterlitz-Thouless transition temperature. The behavior of the
resistance is discussed, both in various limiting regimes and as it crosses
over between these regimes. Details are presented of a minimal model of the
films and the contact, and of the calculation of the resistance. A formulation
in terms of quantum phase-slip events is employed, which is natural and
effective in the limit of a good contact. However, it is also shown to be
effective even when the contact is poor and is, indeed, indispensable, as the
system always behaves as if it were in the good-contact limit at low enough
temperature. A simple mechanical analogy is introduced to provide some
heuristic understanding of the nearly-activated temperature dependence of the
resistance. Prospects for experimental tests of the predicted behavior are
discussed, and numerical estimates relevant to anticipated experimental
settings are provided.Comment: 29 pages (single column format), 7 figure
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