2,987 research outputs found
Numerical evidence for the spin-Peierls state in the frustrated quantum antiferromagnet
We study the spin- Heisenberg antiferromagnet with an
antiferromagnetic (third nearest neighbor) interaction on a square
lattice. We numerically diagonalize this ``-'' model on clusters up
to 32-sites and search for novel ground state properties as the frustration
parameter changes. For ``larger'' we find enhancement of
incommensurate spin order, in agreement with spin-wave, large- expansions,
and other predictions. But for intermediate , the low lying excitation
energy spectrum suggests that this incommensurate order is short-range. In the
same region, the first excited state has the symmetries of the columnar dimer
(spin-Peierls) state. The columnar dimer order parameter suggests the presence
of long-range columnar dimer order. Hence, this spin-Peierls state is the best
candidate for the ground state of the - model in an intermediate
region.Comment: RevTeX file with five postscript figures uuencode
Fractionalized Fermi liquids
In spatial dimensions d >= 2, Kondo lattice models of conduction and local
moment electrons can exhibit a fractionalized, non-magnetic state (FL*) with a
Fermi surface of sharp electron-like quasiparticles, enclosing a volume
quantized by (\rho_a-1)(mod 2), with \rho_a the mean number of all electrons
per unit cell of the ground state. Such states have fractionalized excitations
linked to the deconfined phase of a gauge theory. Confinement leads to a
conventional Fermi liquid state, with a Fermi volume quantized by \rho_a (mod
2), and an intermediate superconducting state for the Z_2 gauge case. The FL*
state permits a second order metamagnetic transition in an applied magnetic
field.Comment: 4 pages, 1 figure; (v2) changed title and terminology, but content
largely unchanged; (v3) updated version to appear in PR
Turning a First Order Quantum Phase Transition Continuous by Fluctuations: General Flow Equations and Application to d-Wave Pomeranchuk Instability
We derive renormalization group equations which allow us to treat order
parameter fluctuations near quantum phase transitions in cases where an
expansion in powers of the order parameter is not possible. As a prototypical
application, we analyze the nematic transition driven by a d-wave Pomeranchuk
instability in a two-dimensional electron system. We find that order parameter
fluctuations suppress the first order character of the nematic transition
obtained at low temperatures in mean-field theory, so that a continuous
transition leading to quantum criticality can emerge
Ultracold atoms in one-dimensional optical lattices approaching the Tonks-Girardeau regime
Recent experiments on ultracold atomic alkali gases in a one-dimensional
optical lattice have demonstrated the transition from a gas of soft-core bosons
to a Tonks-Girardeau gas in the hard-core limit, where one-dimensional bosons
behave like fermions in many respects. We have studied the underlying many-body
physics through numerical simulations which accommodate both the soft-core and
hard-core limits in one single framework. We find that the Tonks-Girardeau gas
is reached only at the strongest optical lattice potentials. Results for
slightly higher densities, where the gas develops a Mott-like phase already at
weaker optical lattice potentials, show that these Mott-like short range
correlations do not enhance the convergence to the hard-core limit.Comment: 4 pages, 3 figures, replaced with published versio
Superconducting d-wave stripes in cuprates: Valence bond order coexisting with nodal quasiparticles
We point out that unidirectional bond-centered charge-density-wave states in
cuprates involve electronic order in both s- and d-wave channels, with
non-local Coulomb repulsion suppressing the s-wave component. The resulting
bond-charge-density wave, coexisting with superconductivity, is compatible with
recent photoemission and tunneling data and as well as neutron-scattering
measurements, once long-range order is destroyed by slow fluctuations or glassy
disorder. In particular, the real-space structure of d-wave stripes is
consistent with the scanning-tunneling-microscopy measurements on both
underdoped Bi2Sr2CaCu2O8+x and Ca2-xNaxCuO2Cl2 of Kohsaka et al. [Science 315,
1380 (2007), arXiv:cond-mat/0703309].Comment: 5 pages, 3 figs, (v2) final version to be published in PR
Infinite disorder scaling of random quantum magnets in three and higher dimensions
Using a very efficient numerical algorithm of the strong disorder
renormalization group method we have extended the investigations about the
critical behavior of the random transverse-field Ising model in three and four
dimensions, as well as for Erd\H os-R\'enyi random graphs, which represent
infinite dimensional lattices. In all studied cases an infinite disorder
quantum critical point is identified, which ensures that the applied method is
asymptotically correct and the calculated critical exponents tend to the exact
values for large scales. We have found that the critical exponents are
independent of the form of (ferromagnetic) disorder and they vary smoothly with
the dimensionality.Comment: 6 pages, 5 figure
Entanglement and particle correlations of Fermi gases in harmonic traps
We investigate quantum correlations in the ground state of noninteracting
Fermi gases of N particles trapped by an external space-dependent harmonic
potential, in any dimension. For this purpose, we compute one-particle
correlations, particle fluctuations and bipartite entanglement entropies of
extended space regions, and study their large-N scaling behaviors. The
half-space von Neumann entanglement entropy is computed for any dimension,
obtaining S_HS = c_l N^(d-1)/d ln N, analogously to homogenous systems, with
c_l=1/6, 1/(6\sqrt{2}), 1/(6\sqrt{6}) in one, two and three dimensions
respectively. We show that the asymptotic large-N relation S_A\approx \pi^2
V_A/3, between the von Neumann entanglement entropy S_A and particle variance
V_A of an extended space region A, holds for any subsystem A and in any
dimension, analogously to homogeneous noninteracting Fermi gases.Comment: 15 pages, 22 fig
Geometric phases and quantum phase transitions
Quantum phase transition is one of the main interests in the field of
condensed matter physics, while geometric phase is a fundamental concept and
has attracted considerable interest in the field of quantum mechanics. However,
no relevant relation was recognized before recent work. In this paper, we
present a review of the connection recently established between these two
interesting fields: investigations in the geometric phase of the many-body
systems have revealed so-called "criticality of geometric phase", in which
geometric phase associated with the many-body ground state exhibits
universality, or scaling behavior in the vicinity of the critical point. In
addition, we address the recent advances on the connection of some other
geometric quantities and quantum phase transitions. The closed relation
recently recognized between quantum phase transitions and some of geometric
quantities may open attractive avenues and fruitful dialog between different
scientific communities.Comment: Invited review article for IJMPB; material covered till June 2007; 10
page
Columnar Fluctuations as a Source of Non-Fermi-Liquid Behavior in Weak Metallic Magnets
It is shown that columnar fluctuations, in conjunction with weak quenched
disorder, lead to a T^{3/2} temperature dependence of the electrical
resistivity. This is proposed as an explanation of the observed
non-Fermi-liquid behavior in the helimagnet MnSi, with one possible realization
of the columnar fluctuations provided by skyrmion lines that have independently
been proposed to be present in this material.Comment: 4pp, 4 figure
Dynamical phases and intermittency of the dissipative quantum Ising model
We employ the concept of a dynamical, activity order parameter to study the
Ising model in a transverse magnetic field coupled to a Markovian bath. For a
certain range of values of the spin-spin coupling, magnetic field and
dissipation rate, we identify a first order dynamical phase transition between
active and inactive {\em dynamical phases}. We demonstrate that dynamical
phase-coexistence becomes manifest in an intermittent behavior of the bath
quanta emission. Moreover, we establish the connection between the dynamical
order parameter that quantifies the activity, and the longitudinal
magnetization that serves as static order parameter. The system we consider can
be implemented in current experiments with Rydberg atoms and trapped ions
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