766 research outputs found
Anomalous elasticity in a disordered layered XY model
We investigate the effects of layered quenched disorder on the behavior of
planar magnets, superfluids, and superconductors by performing large-scale
Monte-Carlo simulations of a three-dimensional randomly layered XY model. Our
data provide numerical evidence for the recently predicted anomalously elastic
(sliding) intermediate phase between the conventional high-temperature and
low-temperature phases. In this intermediate phase, the spin-wave stiffness
perpendicular to the layers vanishes in the thermodynamic limit while the
stiffness parallel to the layers as well as the spontaneous magnetization are
nonzero. In addition, the susceptibility displays unconventional finite-size
scaling properties. We compare our Monte-Carlo results with the theoretical
predictions, and we discuss possible experiments in ultracold atomic gases,
layered superconductors and in nanostructures.Comment: 6 pages, 4 eps figures included, proceedings of FQMT11, final version
as publishe
Smeared quantum phase transition in the dissipative random quantum Ising model
We investigate the quantum phase transition in the random transverse-field
Ising model under the influence of Ohmic dissipation. To this end, we
numerically implement a strong-disorder renormalization-group scheme. We find
that Ohmic dissipation destroys the quantum critical point and the associated
quantum Griffiths phase by smearing. Our results quantitatively confirm a
recent theory [Phys. Rev. Lett. {\bf 100}, 240601 (2008)] of smeared quantum
phase transitions.Comment: 7 pages, 10 eps figures embedded, final version as publishe
Transport properties in antiferromagnetic quantum Griffiths phases
We study the electrical resistivity in the quantum Griffiths phase associated
with the antiferromagnetic quantum phase transition in a metal. The resistivity
is calculated by means of the semi-classical Boltzmann equation. We show that
the scattering of electrons by locally ordered rare regions leads to a singular
temperature dependence. The rare-region contribution to the resistivity varies
as with temperature where the is the usual Griffiths
exponent which takes the value zero at the critical point and increases with
distance from criticality. We find similar singular contributions to other
transport properties such as thermal resistivity, thermopower and the Peltier
coefficient. We also compare our results with existing experimental data and
suggest new experiments.Comment: 4 pages, 1 figur
Local defect in a magnet with long-range interactions
We investigate a single defect coupling to the square of the order parameter
in a nearly critical magnet with long-range spatial interactions of the form
, focusing on magnetic droplets nucleated at the defect while
the bulk system is in the paramagnetic phase. Because of the long-range
interaction, the droplet develops a power-law tail which is energetically
unfavorable. However, as long as , the tail contribution to the
droplet free energy is subleading in the limit of large droplets; and the free
energy becomes identical to the case of short-range interactions. We also study
the droplet quantum dynamics with and without dissipation; and we discuss the
consequences of our results for defects in itinerant quantum ferromagnets.Comment: 8 pages, 5 eps figures, final version, as publishe
Infinite-randomness quantum critical points induced by dissipation
We develop a strong-disorder renormalization group to study quantum phase
transitions with continuous O symmetry order parameters under the
influence of both quenched disorder and dissipation. For Ohmic dissipation, as
realized in Hertz' theory of the itinerant antiferromagnetic transition or in
the superconductor-metal transition in nanowires, we find the transition to be
governed by an exotic infinite-randomness fixed point in the same universality
class as the (dissipationless) random transverse-field Ising model. We
determine the critical behavior and calculate key observables at the transition
and in the associated quantum Griffiths phase. We also briefly discuss the
cases of superohmic and subohmic dissipations.Comment: 11 pages, 3eps figures embedded, final version as publishe
Quantum phase transition between one-channel and two-channel Kondo polarons
For a mobile spin-1/2 impurity, coupled antiferromagnetically to a
one-dimensional gas of fermions, perturbative ideas have been used to argue in
favor of two-channel Kondo behavior of the impurity spin. Here we combine
general considerations and extensive numerical simulations to show that the
problem displays a novel quantum phase transition between two-channel and
one-channel Kondo screening upon increasing the Kondo coupling. We construct a
ground-state phase diagram and discuss the various non-trivial crossovers as
well as possible experimental realizations.Comment: 5+4 pages, 5+3 fig
The quantum phase transition of itinerant helimagnets
We investigate the quantum phase transition of itinerant electrons from a
paramagnet to a state which displays long-period helical structures due to a
Dzyaloshinskii instability of the ferromagnetic state. In particular, we study
how the self-generated effective long-range interaction recently identified in
itinerant quantum ferromagnets is cut-off by the helical ordering. We find that
for a sufficiently strong Dzyaloshinskii instability the helimagnetic quantum
phase transition is of second order with mean-field exponents. In contrast, for
a weak Dzyaloshinskii instability the transition is analogous to that in
itinerant quantum ferromagnets, i.e. it is of first order, as has been observed
in MnSi.Comment: 5 pages RevTe
Generalized contact process with two symmetric absorbing states in two dimensions
We explore the two-dimensional generalized contact process with two absorbing
states by means of large-scale Monte-Carlo simulations. In part of the phase
diagram, an infinitesimal creation rate of active sites between inactive
domains is sufficient to take the system from the inactive phase to the active
phase. The system therefore displays two different nonequilibrium phase
transitions. The critical behavior of the generic transition is compatible with
the generalized voter (GV) universality class, implying that the
symmetry-breaking and absorbing transitions coincide. In contrast, the
transition at zero domain-boundary activation rate is not critical.Comment: 7 pages, 7 eps figures included, final version as publishe
Dynamical conductivity at the dirty superconductor-metal quantum phase transition
We study the transport properties of ultrathin disordered nanowires in the
neighborhood of the superconductor-metal quantum phase transition. To this end
we combine numerical calculations with analytical strong-disorder
renormalization group results. The quantum critical conductivity at zero
temperature diverges logarithmically as a function of frequency. In the
metallic phase, it obeys activated scaling associated with an
infinite-randomness quantum critical point. We extend the scaling theory to
higher dimensions and discuss implications for experiments.Comment: 4 pages, 2 figures; (v2) minor typos corrected, published versio
Protecting clean critical points by local disorder correlations
We show that a broad class of quantum critical points can be stable against
locally correlated disorder even if they are unstable against uncorrelated
disorder. Although this result seemingly contradicts the Harris criterion, it
follows naturally from the absence of a random-mass term in the associated
order-parameter field theory. We illustrate the general concept with explicit
calculations for quantum spin-chain models. Instead of the infinite-randomness
physics induced by uncorrelated disorder, we find that weak locally correlated
disorder is irrelevant. For larger disorder, we find a line of critical points
with unusual properties such as an increase of the entanglement entropy with
the disorder strength. We also propose experimental realizations in the context
of quantum magnetism and cold-atom physics.Comment: 5 pages, 3 figures; published versio
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