3,069 research outputs found
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
Criticality and quenched disorder: rare regions vs. Harris criterion
We employ scaling arguments and optimal fluctuation theory to establish a
general relation between quantum Griffiths singularities and the Harris
criterion for quantum phase transitions in disordered systems. If a clean
critical point violates the Harris criterion, it is destabilized by weak
disorder. At the same time, the Griffiths dynamical exponent diverges upon
approaching the transition, suggesting unconventional critical behavior. In
contrast, if the Harris criterion is fulfilled, power-law Griffiths
singularities can coexist with clean critical behavior but saturates at a
finite value. We present applications of our theory to a variety of systems
including quantum spin chains, classical reaction-diffusion systems and
metallic magnets; and we discuss modifications for transitions above the upper
critical dimension. Based on these results we propose a unified classification
of phase transitions in disordered systems.Comment: 4.5 pages, 1 eps figure, final version as publishe
Infinite-noise criticality: Nonequilibrium phase transitions in fluctuating environments
We study the effects of time-varying environmental noise on nonequilibrium
phase transitions in spreading and growth processes. Using the examples of the
logistic evolution equation as well as the contact process, we show that such
temporal disorder gives rise to a distinct type of critical points at which the
effective noise amplitude diverges on long time scales. This leads to enormous
density fluctuations characterized by an infinitely broad probability
distribution at criticality. We develop a real-time renormalization-group
theory that provides a general framework for the effects of temporal disorder
on nonequilibrium processes. We also discuss how general this exotic critical
behavior is, we illustrate the results by computer simulations, and we touch
upon experimental applications of our theory.Comment: 6 pages (including 3 eps figures). Final version as publishe
Phase diagrams and universality classes of random antiferromagnetic spin ladders
The random antiferromagnetic two-leg and zigzag spin-1/2 ladders are
investigated using the real space renormalization group scheme and their
complete phase diagrams are determined. We demonstrate that the first system
belongs to the same universality class of the dimerized random spin-1/2 chain.
The zigzag ladder, on the other hand, is in a random singlet phase at weak
frustration and disorder. Otherwise, we give additional evidence that it
belongs to the universality class of the random antiferromagnetic and
ferromagnetic quantum spin chains, although the universal fixed point found in
the latter system is never realized. We find, however, a new universal fixed
point at intermediate disorder.Comment: 10 pages, 10 figure
Dissipation effects in percolating quantum Ising magnets
We study the effects of dissipation on a randomly dilute transverse-field
Ising magnet at and close to the percolation threshold. For weak transverse
fields, a novel percolation quantum phase transition separates a
super-paramagnetic cluster phase from an inhomogeneously ordered ferromagnetic
phase. The properties of this transition are dominated by large frozen and
slowly fluctuating percolation clusters. Implementing numerically a
strong-disorder real space renormalization group technique, we compute the
low-energy density of states which is found to be in good agreement with the
analytical prediction.Comment: 2 pages, 1 eps figure, final version as publishe
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
Rounding of a first-order quantum phase transition to a strong-coupling critical point
We investigate the effects of quenched disorder on first-order quantum phase
transitions on the example of the -color quantum Ashkin-Teller model. By
means of a strong-disorder renormalization group, we demonstrate that quenched
disorder rounds the first-order quantum phase transition to a continuous one
for both weak and strong coupling between the colors. In the strong coupling
case, we find a distinct type of infinite-randomness critical point
characterized by additional internal degrees of freedom. We investigate its
critical properties in detail, and we discuss broader implications for the fate
of first-order quantum phase transitions in disordered systems.Comment: 5 pages, 4 figure
Rare regions and Griffiths singularities at a clean critical point: The five-dimensional disordered contact process
We investigate the nonequilibrium phase transition of the disordered contact
process in five space dimensions by means of optimal fluctuation theory and
Monte Carlo simulations. We find that the critical behavior is of mean-field
type, i.e., identical to that of the clean five-dimensional contact process. It
is accompanied by off-critical power-law Griffiths singularities whose
dynamical exponent saturates at a finite value as the transition is
approached. These findings resolve the apparent contradiction between the
Harris criterion which implies that weak disorder is renormalization-group
irrelevant and the rare-region classification which predicts unconventional
behavior. We confirm and illustrate our theory by large-scale Monte-Carlo
simulations of systems with up to sites. We also relate our results to a
recently established general relation between the Harris criterion and
Griffiths singularities [Phys. Rev. Lett. {\bf 112}, 075702 (2014)], and we
discuss implications for other phase transitions.Comment: 10 pages, 5 eps figures included, applies the optimal fluctuation
theory of arXiv:1309.0753 to the contact proces
A cluster-based mean-field and perturbative description of strongly correlated fermion systems. Application to the 1D and 2D Hubbard model
We introduce a mean-field and perturbative approach, based on clusters, to
describe the ground state of fermionic strongly-correlated systems. In cluster
mean-field, the ground state wavefunction is written as a simple tensor product
over optimized cluster states. The optimization of the single-particle basis
where the cluster mean-field is expressed is crucial in order to obtain
high-quality results. The mean-field nature of the ansatz allows us to
formulate a perturbative approach to account for inter-cluster correlations;
other traditional many-body strategies can be easily devised in terms of the
cluster states. We present benchmark calculations on the half-filled 1D and
(square) 2D Hubbard model, as well as the lightly-doped regime in 2D, using
cluster mean-field and second-order perturbation theory. Our results indicate
that, with sufficiently large clusters or to second-order in perturbation
theory, a cluster-based approach can provide an accurate description of the
Hubbard model in the considered regimes. Several avenues to improve upon the
results presented in this work are discussed.Comment: 22 pages, 21 figure
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