3,003 research outputs found
Absence of conventional quantum phase transitions in itinerant systems with disorder
Effects of disorder are examined in itinerant systems close to quantum
critical points. We argue that spin fluctuations associated with the long-range
part of the RKKY interactions generically induce non-Ohmic dissipation due to
rare disorder configurations. This dissipative mechanism is found to
destabilize quantum Griffiths phase behavior in itinerant systems with
arbitrary symmetry of the order parameter, leading to the formation of a
"cluster glass" phase preceding uniform ordering.Comment: 4+epsilon pages, 1 figure. Phys. Rev. Lett., in press (2005
Quantum critical dynamics of the two-dimensional Bose gas
The dilute, two-dimensional Bose gas exhibits a novel regime of relaxational
dynamics in the regime k_B T > |\mu| where T is the absolute temperature and
\mu is the chemical potential. This may also be interpreted as the quantum
criticality of the zero density quantum critical point at \mu=0. We present a
theory for this dynamics, to leading order in 1/\ln (\Lambda/ (k_B T)), where
\Lambda is a high energy cutoff. Although pairwise interactions between the
bosons are weak at low energy scales, the collective dynamics are strongly
coupled even when \ln (\Lambda/T) is large. We argue that the strong-coupling
effects can be isolated in an effective classical model, which is then solved
numerically. Applications to experiments on the gap-closing transition of spin
gap antiferromagnets in an applied field are presented.Comment: 9 pages, 10 figure
Spin dynamics across the superfluid-insulator transition of spinful bosons
Bosons with non-zero spin exhibit a rich variety of superfluid and insulating
phases. Most phases support coherent spin oscillations, which have been the
focus of numerous recent experiments. These spin oscillations are Rabi
oscillations between discrete levels deep in the insulator, while deep in the
superfluid they can be oscillations in the orientation of a spinful condensate.
We describe the evolution of spin oscillations across the superfluid-insulator
quantum phase transition. For transitions with an order parameter carrying
spin, the damping of such oscillations is determined by the scaling dimension
of the composite spin operator. For transitions with a spinless order parameter
and gapped spin excitations, we demonstrate that the damping is determined by
an associated quantum impurity problem of a localized spin excitation
interacting with the bulk critical modes. We present a renormalization group
analysis of the quantum impurity problem, and discuss the relationship of our
results to experiments on ultracold atoms in optical lattices.Comment: 43 pages (single-column format), 8 figures; v2: corrected discussion
of fixed points in Section V
Current driven quantum criticality in itinerant electron ferromagnets
We determine the effect of an in-plane current flow on the critical
properties of a 2d itinerant electron system near a ferromagnetic-paramagnetic
quantum critical point. We study a model in which a nonequilibrium steady state
is established as a result of exchange of particles and energy with an
underlying substrate. the current gives rise not only to an effective
temperature equal to the voltage drop over a distance of order the mean free
path, but also to symmetry breaking terms of the form in the effective action. The effect of the symmetry breaking on
the fluctuational and critical properties is found to be small although (in
agreement with previous results) if rotational degrees of freedom are
important, the current can make the classically ordered state dynamically
unstable.Comment: 4 pages, published versio
Quantum Phase Transitions and Matrix Product States in Spin Ladders
We investigate quantum phase transitions in ladders of spin 1/2 particles by
engineering suitable matrix product states for these ladders. We take into
account both discrete and continuous symmetries and provide general classes of
such models. We also study the behavior of entanglement of different
neighboring sites near the transition point and show that quantum phase
transitions in these systems are accompanied by divergences in derivatives of
entanglement.Comment: 20 pages, 6 figures, essential changes (i.e derivation of the
Hamiltonian), Revte
Thermal melting of density waves on the square lattice
We present the theory of the effect of thermal fluctuations on commensurate
"p x p" density wave ordering on the square lattice (p >= 3, integer). For the
case in which this order is lost by a second order transition, we argue that
the adjacent state is generically an incommensurate striped state, with
commensurate p-periodic long range order along one direction, and
incommensurate quasi-long-range order along the orthogonal direction. We also
present the routes by which the fully disordered high temperature state can be
reached. For p=4, and at special commensurate densities, the "4 x 4"
commensurate state can melt directly into the disordered state via a self-dual
critical point with non-universal exponents.Comment: 12 pages, 5 figure
Critical Exponents in a Quantum Phase Transition of an Anisotropic 2D Antiferromagnet
I use the two-step density-matrix renormalization group method to extract the
critical exponents and in the transition from a N\'eel
phase to a magnetically disordered phase with a spin gap. I find
that the exponent computed from the magnetic side of the transition is
consistent with that of the classical Heisenberg model, but not the exponent
computed from the disordered side. I also show the contrast between
integer and half-integer spin cases.Comment: 4 pages, 2 figure
Boson Core Compressibility
Strongly interacting atoms trapped in optical lattices can be used to explore
phase diagrams of Hubbard models. Spatial inhomogeneity due to trapping
typically obscures distinguishing observables. We propose that measures using
boson double occupancy avoid trapping effects to reveal key correlation
functions. We define a boson core compressibility and core superfluid stiffness
in terms of double occupancy. We use quantum Monte Carlo on the Bose-Hubbard
model to empirically show that these quantities intrinsically eliminate edge
effects to reveal correlations near the trap center. The boson core
compressibility offers a generally applicable tool that can be used to
experimentally map out phase transitions between compressible and
incompressible states.Comment: 11 pages, 11 figure
Nonlinear conductance of long quantum wires at a conductance plateau transition: Where does the voltage drop?
We calculate the linear and nonlinear conductance of spinless fermions in
clean, long quantum wires where short-ranged interactions lead locally to
equilibration. Close to the quantum phase transition where the conductance
jumps from zero to one conductance quantum, the conductance obtains an
universal form governed by the ratios of temperature, bias voltage and gate
voltage. Asymptotic analytic results are compared to solutions of a Boltzmann
equation which includes the effects of three-particle scattering. Surprisingly,
we find that for long wires the voltage predominantly drops close to one end of
the quantum wire due to a thermoelectric effect.Comment: 4+ pages, 3 figures plus supplementary material (2 pages, 1 figure);
minor changes, references correcte
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