120 research outputs found
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
Chiral Surface States in the Bulk Quantum Hall Effect
In layered samples which exhibit a bulk quantum Hall effect (QHE), a
two-dimensional (2d) surface ``sheath" of gapless excitations is expected.
These excitations comprise a novel 2d chiral quantum liquid which should
dominate the low temperature transport along the field (z-axis). For the
integer QHE, we show that localization effects are completely absent in the
``sheath", giving a metallic z-axis conductivity. For fractional filling , the ``sheath" is a 2d non-Fermi liquid, with incoherent z-axis transport
and . Experimental implications for the Bechgaard salts
are discussed.Comment: 4 pages, RevTeX 3.0, with two encapsulated postscript figures, which
can be automatically included in-text if desired. The complete postscript
file is available on the WWW at http://www.itp.ucsb.edu/~balents/sheath.p
Dual Order Parameter for the Nodal Liquid
The guiding conception of vortex-condensation-driven Mott insulating behavior
is central to the theory of the nodal liquid. We amplify our earlier
description of this idea and show how vortex condensation in 2D electronic
systems is a natural extension of 1D Mott insulating and 2D bosonic Mott
insulating behavior. For vortices in an underlying superconducting pair field,
there is an important distinction between the condensation of flux hc/2e and
flux hc/e vortices. The former case leads to spin-charge confinement,
exemplified by the band insulator and the charge-density-wave. In the latter
case, spin and charge are liberated leading directly to a 2D Mott insulator
exhibiting *spin-charge separation*. Possible upshots include not only the
nodal liquid, but also a novel undoped antiferromagnetic insulating phase with
gapped excitations exhibiting spin-charge separation.Comment: 16 pages, 2 figure
Quasiparticles in the 111 state and its compressible ancestors
We investigate the relationship of the spontaneously inter-layer coherent
``111''state of quantum Hall bilayers at total filling factor \nu=1 to
``mutual'' composite fermions, in which vortices in one layer are bound to
electrons in the other. Pairing of the mutual composite fermions leads to the
low-energy properties of the 111 state, as we explicitly demonstrate using
field-theoretic techniques. Interpreting this relationship as a mechanism for
inter-layer coherence leads naturally to two candidate states with
non-quantized Hall conductance: the mutual composite Fermi liquid, and an
inter-layer coherent charge e Wigner crystal. The experimental behavior of the
interlayer tunneling conductance and resistivity tensors are discussed for
these states.Comment: 4 Pages, RevTe
Creep via dynamical functional renormalization group
We study a D-dimensional interface driven in a disordered medium. We derive
finite temperature and velocity functional renormalization group (FRG)
equations, valid in a 4-D expansion. These equations allow in principle for a
complete study of the the velocity versus applied force characteristics. We
focus here on the creep regime at finite temperature and small velocity. We
show how our FRG approach gives the form of the v-f characteristics in this
regime, and in particular the creep exponent, obtained previously only through
phenomenological scaling arguments.Comment: 4 pages, 3 figures, RevTe
Nonequilibrium Steady States of Driven Periodic Media
We study a periodic medium driven over a random or periodic substrate. Our
work is based on nonequilibrium continuum hydrodynamic equations of motion,
which we derive microscopically. We argue that in the random case instabilities
will always destroy the LRO of the lattice. In most, if not all, cases, the
stable driven ordered state is a transverse smectic, with ordering wavevector
perpendicular to the velocity. It consists of a periodic array of flowing
liquid channels, with transverse displacements and density (``permeation
mode'') as hydrodynamic variables. We present dynamic functional
renormalization group calculations in two and three dimensions for an
approximate model of the smectic. The finite temperature behavior is much less
glassy than in equilibrium, owing to a disorder-driven effective ``heating''
(allowed by the absence of the fluctuation-dissipation theorem). This, in
conjunction with the permeation mode, leads to a fundamentally analytic
transverse response for . Our results are compared to recent experiments
and other theoretical work.Comment: 39 PRB pages, RevTex and 9 postscript figures, uses multicol.st
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
Fractionalization in an Easy-axis Kagome Antiferromagnet
We study an antiferromagnetic spin-1/2 model with up to third
nearest-neighbor couplings on the Kagome lattice in the easy-axis limit, and
show that its low-energy dynamics are governed by a four site XY ring exchange
Hamiltonian. Simple ``vortex pairing'' arguments suggest that the model
sustains a novel fractionalized phase, which we confirm by exactly solving a
modification of the Hamiltonian including a further four-site interaction. In
this limit, the system is a featureless ``spin liquid'', with gaps to all
excitations, in particular: deconfined S^z=1/2 bosonic ``spinons'' and Ising
vortices or ``visons''. We use an Ising duality transformation to express vison
correlators as non-local strings in terms of the spin operators, and calculate
the string correlators using the ground state wavefunction of the modified
Hamiltonian. Remarkably, this wavefunction is exactly given by a kind of
Gutzwiller projection of an XY ferromagnet. Finally, we show that the
deconfined spin liquid state persists over a finite range as the additional
four-spin interaction is reduced, and study the effect of this reduction on the
dynamics of spinons and visons.Comment: best in color but readable in B+
Vortex Glass Phase and Universal Susceptibility Variations in Planar Array of Flux Lines
Some of the properties of the low temperature vortex-glass phase of
randomly-pinned flux lines in 1+1 dimensions are studied. The flux arrays are
found to be sensitive to small changes in external parameters such as the
magnetic field or temperature. These effects are captured by the variations in
the magnetic response and noise, which have universal statistics and should
provide an unambiguous signature of the glass phase.Comment: 11 pages and no figures; revtex 3.
Andreev current in finite sized carbon nanotubes
We investigate the effect of interactions on Andreev current at a
normal-superconducting junction when the normal phase is a Luttinger liquid
with repulsive interactions. In particular, we study the system of a finite
sized carbon nanotube placed between one metallic and one superconducting lead.
We show that interactions and finite size effects give rise to significant
deviations from the standard picture of Andreev current at a normal-
superconductor junction in the nearly perfect Andreev limit.Comment: 4 pages, 2 figure
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