705 research outputs found
Superconductor-to-Spin-Density-Wave Transition in Quasi-One-Dimensional Metals with Ising Anisotropy
We study a mechanism for superconductivity in quasi-one-dimensional materials
with Ising anisotropy. In an isolated chain Ising anisotropy opens a spin gap;
if inter-chain coupling is sufficiently weak, single particle hopping is
suppressed and the physics of coupled chains is controlled by a competition
between pair hopping and exchange interaction. Spin density wave and triplet
superconductivity phases are found separated by a first order phase transition.
For particular parameter values a second order transition described by SO(4)
symmetry is found.Comment: 18 pages, 1 figur
Issues and Observations on Applications of the Constrained-Path Monte Carlo Method to Many-Fermion Systems
We report several important observations that underscore the distinctions
between the constrained-path Monte Carlo method and the continuum and lattice
versions of the fixed-node method. The main distinctions stem from the
differences in the state space in which the random walk occurs and in the
manner in which the random walkers are constrained. One consequence is that in
the constrained-path method the so-called mixed estimator for the energy is not
an upper bound to the exact energy, as previously claimed. Several ways of
producing an energy upper bound are given, and relevant methodological aspects
are illustrated with simple examples.Comment: 28 pages, REVTEX, 5 ps figure
The BCS Functional for General Pair Interactions
The Bardeen-Cooper-Schrieffer (BCS) functional has recently received renewed
attention as a description of fermionic gases interacting with local pairwise
interactions. We present here a rigorous analysis of the BCS functional for
general pair interaction potentials. For both zero and positive temperature, we
show that the existence of a non-trivial solution of the nonlinear BCS gap
equation is equivalent to the existence of a negative eigenvalue of a certain
linear operator. From this we conclude the existence of a critical temperature
below which the BCS pairing wave function does not vanish identically. For
attractive potentials, we prove that the critical temperature is non-zero and
exponentially small in the strength of the potential.Comment: Revised Version. To appear in Commun. Math. Phys
Roper Resonance and S_{11}(1535) from Lattice QCD
Using the constrained curve fitting method and overlap fermions with the
lowest pion mass at , we observe that the masses of the first
positive and negative parity excited states of the nucleon tend to cross over
as the quark masses are taken to the chiral limit. Both results at the physical
pion mass agree with the experimental values of the Roper resonance
() and (). This is seen for the first
time in a lattice QCD calculation. These results are obtained on a quenched
Iwasaki lattice with . We also extract the
ghost states (a quenched artifact) which are shown to decouple from
the nucleon interpolation field above . From the
quark mass dependence of these states in the chiral region, we conclude that
spontaneously broken chiral symmetry dictates the dynamics of light quarks in
the nucleon.Comment: 10 pages, 5 figures, revised version to appear in PL
An Improved Upper Bound for the Ground State Energy of Fermion Lattice Models
We present an improved upper bound for the ground state energy of lattice
fermion models with sign problem. The bound can be computed by numerical
simulation of a recently proposed family of deformed Hamiltonians with no sign
problem. For one dimensional models, we expect the bound to be particularly
effective and practical extrapolation procedures are discussed. In particular,
in a model of spinless interacting fermions and in the Hubbard model at various
filling and Coulomb repulsion we show how such techniques can estimate ground
state energies and correlation function with great accuracy.Comment: 5 pages, 5 figures; to appear in Physical Review
Theory of parity violation in compound nuclear states; one particle aspects
In this work we formulate the reaction theory of parity violation in compound
nuclear states using Feshbach's projection operator formalism. We derive in
this framework a complete set of terms that contribute to the longitudinal
asymmetry measured in experiments with polarized epithermal neutrons. We also
discuss the parity violating spreading width resulting from this formalism. We
then use the above formalism to derive expressions which hold in the case when
the doorway state approximation is introduced. In applying the theory we limit
ourselves in this work to the case when the parity violating potential and the
strong interaction are one-body. In this approximation, using as the doorway
the giant spin-dipole resonance and employing well known optical potentials and
a time-reversal even, parity odd one-body interaction we calculate or estimate
the terms we derived. In our calculations we explicitly orthogonalize the
continuum and bound wave functions. We find the effects of orthogonalization to
be very important. Our conclusion is that the present one-body theory cannot
explain the average longitudinal asymmetry found in the recent polarized
neutron experiments. We also confirm the discrepancy, first pointed out by
Auerbach and Bowman, that emerges, between the calculated average asymmetry and
the parity violating spreading width, when distant doorways are used in the
theory.Comment: 37 pages, REVTEX, 5 figures not included (Postscript, available from
the authors
Gutenberg Richter and Characteristic Earthquake Behavior in Simple Mean-Field Models of Heterogeneous Faults
The statistics of earthquakes in a heterogeneous fault zone is studied
analytically and numerically in the mean field version of a model for a
segmented fault system in a three-dimensional elastic solid. The studies focus
on the interplay between the roles of disorder, dynamical effects, and driving
mechanisms. A two-parameter phase diagram is found, spanned by the amplitude of
dynamical weakening (or ``overshoot'') effects (epsilon) and the normal
distance (L) of the driving forces from the fault. In general, small epsilon
and small L are found to produce Gutenberg-Richter type power law statistics
with an exponential cutoff, while large epsilon and large L lead to a
distribution of small events combined with characteristic system-size events.
In a certain parameter regime the behavior is bistable, with transitions back
and forth from one phase to the other on time scales determined by the fault
size and other model parameters. The implications for realistic earthquake
statistics are discussed.Comment: 21 pages, RevTex, 6 figures (ps, eps
Orbital currents and charge density waves in a generalized Hubbard ladder
We study a generalized Hubbard model on the two-leg ladder at zero
temperature, focusing on a parameter region with staggered flux (SF)/d-density
wave (DDW) order. To guide our numerical calculations, we first investigate the
location of a SF/DDW phase in the phase diagram of the half-filled weakly
interacting ladder using a perturbative renormalization group (RG) and
bosonization approach. For hole doping delta away from half-filling,
finite-size density-matrix renormalization-group (DMRG) calculations are used
to study ladders with up to 200 rungs for intermediate-strength interactions.
In the doped SF/DDW phase, the staggered rung current and the rung electron
density both show periodic spatial oscillations, with characteristic
wavelengths 2/delta and 1/delta, respectively, corresponding to ordering
wavevectors 2k_F and 4k_F for the currents and densities, where 2k_F =
pi(1-delta). The density minima are located at the anti-phase domain walls of
the staggered current. For sufficiently large dopings, SF/DDW order is
suppressed. The rung density modulation also exists in neighboring phases where
currents decay exponentially. We show that most of the DMRG results can be
qualitatively understood from weak-coupling RG/bosonization arguments. However,
while these arguments seem to suggest a crossover from non-decaying
correlations to power-law decay at a length scale of order 1/delta, the DMRG
results are consistent with a true long-range order scenario for the currents
and densities.Comment: 24 pages, 17 figures. Follow-up to cond-mat/0209444. (v2) Some
revisions in text, improved presentation. Minor changes in title, abstract
and reference
Competing Orders in Coupled Luttinger Liquids
We consider the problem of two coupled Luttinger liquids both at half filling
and at low doping levels, to investigate the problem of competing orders in
quasi-one-dimensional strongly correlated systems. We use bosonization and
renormalization group equations to investigate the phase diagrams, to determine
the allowed phases and to establish approximate boundaries among them. Because
of the chiral translation and reflection symmetry in the charge mode away from
half filling, orders of charge density wave (CDW) and spin-Peierls (SP)
diagonal current (DC) and -density wave (DDW) form two doublets and thus can
be at most quasi-long range ordered. At half-filling, umklapp terms break this
symmetry down to a discrete group and thus Ising-type ordered phases appear as
a result of spontaneous breaking of the residual symmetries. Quantum disordered
Haldane phases are also found, with finite amplitudes of pairing orders and
triplet counterparts of CDW, SP, DC and DDW. Relations with recent numerical
results and implications to similar problems in two dimensions are discussed.Comment: 16 pages, 5 figures, 4 tables. Revised manuscript; a misprint in Eq.
B3 has been corrected. The paper is already in print in PR
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