2,739 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
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
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
Computational analysis of the behavior of atmospheric pollution due to demographic, structural factors, vehicular flow and commerce activities
According to the latest assessments made by the world health organization (WHO2016), the atmospheric pollution (air), has become one of the main causes of morbidity and mortality in the world, with a steep growth of respiratory diseases, increase in lung cancer, ocular complications, and dermis diseases [1,2,3]. Currently, there are governments which still underestimate investments in environmental care, turning their countries into only consumers and predators of the ecosystem [1,2,3]. Worldwide, several cities have been implementing different regional strategies to decrease environmental pollution, however, these actions have not been effective enough and significant indices of contamination and emergency declarations persist [1,2,3]. MedellÃn is one of the cities most affected by polluting gases in Latin America due to the high growth of construction sector, high vehicular flow, increase in commerce, besides a little assertive planting trees system, among other reasons [1,2,3]. With the purpose of providing new researching elements which benefit the improvement of air quality in the cities of the world, it is pretended to mathematically model and computationally implement the behavior of the flow of air, e.g., in zones in the city of MedellÃn to determine the extent of pollution by tightness, impact of current architectural designs, vehicular transport, high commerce flow, and confinement in the public transport system. The simulations allowed to identify spotlights of particulate tightness caused by architectural designs of the city which do not benefit air flow. Also, recirculating gases were observed in different zones of the city. This research can offer greater knowledge around the incidence of pollution generated by structures and architecture. Likewise, these studies can contribute to a better urban, structural and ecological reordering in cities, the implementation of an assertive arborization system, and the possibility to orientate effective strategies over cleaning (purification) and contaminant extracting systems
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
A Holographic Fractional Topological Insulator
We give a holographic realization of the recently proposed low energy
effective action describing a fractional topological insulator. In particular
we verify that the surface of this hypothetical material supports a fractional
quantum Hall current corresponding to half that of a Laughlin state.Comment: 4 pages, 2 figure
Strong-disorder renormalization-group study of the one-dimensional tight-binding model
We formulate a strong-disorder renormalization-group (SDRG) approach to study
the beta function of the tight-binding model in one dimension with both
diagonal and off-diagonal disorder for states at the band center. We show that
the SDRG method, when used to compute transport properties, yields exact
results since it is identical to the transfer matrix method. The beta function
is shown to be universal when only off-diagonal disorder is present even though
single-parameter scaling is known to be violated. A different single-parameter
scaling theory is formulated for this particular (particle-hole symmetric)
case. Upon breaking particle-hole symmetry (by adding diagonal disorder), the
beta function is shown to crossover from the universal behavior of the
particle-hole symmetric case to the conventional non-universal one in agreement
with the two-parameter scaling theory. We finally draw an analogy with the
random transverse-field Ising chain in the paramagnetic phase. The
particle-hole symmetric case corresponds to the critical point of the quantum
Ising model while the generic case corresponds to the Griffiths paramagnetic
phase.Comment: includes 12 pages, 4 figure
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