11,968 research outputs found
Metal-Insulator-Transition in a Weakly interacting Disordered Electron System
The interplay of interactions and disorder is studied using the
Anderson-Hubbard model within the typical medium dynamical cluster
approximation. Treating the interacting, non-local cluster self-energy
() up to second order in the
perturbation expansion of interactions, , with a systematic incorporation
of non-local spatial correlations and diagonal disorder, we explore the initial
effects of electron interactions () in three dimensions. We find that the
critical disorder strength (), required to localize all states,
increases with increasing ; implying that the metallic phase is stabilized
by interactions. Using our results, we predict a soft pseudogap at the
intermediate close to and demonstrate that the mobility edge
() is preserved as long as the chemical potential, , is
at or beyond the mobility edge energy.Comment: 10 Pages, 8 Figures with Supplementary materials include
Dual Fermion Dynamical Cluster Approach for Strongly Correlated Systems
We have designed a new multi-scale approach for Strongly Correlated Systems
by combining the Dynamical Cluster Approximation (DCA) and the recently
introduced dual-fermion formalism. This approach employs an exact mapping from
a real lattice to a DCA cluster of linear size Lc embedded in a dual fermion
lattice. Short-length-scale physics is addressed by the DCA cluster
calculation, while longer-length-scale physics is addressed diagrammatically
using dual fermions. The bare and dressed dual Fermionic Green functions scale
as O(1/Lc) so perturbation theory on the dual lattice converges very quickly.
E.g., the dual Fermion self-energy calculated with simple second order
perturbation theory is of order O(1/Lc^3), with third order and three body
corrections down by an additional factor of O(1/Lc^2)
Role of the van Hove Singularity in the Quantum Criticality of the Hubbard Model
A quantum critical point (QCP), separating the non-Fermi liquid region from
the Fermi liquid, exists in the phase diagram of the 2D Hubbard model
[Vidhyadhiraja et. al, Phys. Rev. Lett. 102, 206407 (2009)]. Due to the
vanishing of the critical temperature associated with a phase separation
transition, the QCP is characterized by a vanishing quasiparticle weight. Near
the QCP, the pairing is enhanced since the real part of the bare d-wave p-p
susceptibility exhibits algebraic divergence with decreasing temperature,
replacing the logarithmic divergence found in a Fermi liquid [Yang et. al,
Phys. Rev. Lett. 106, 047004 (2011)]. In this paper we explore the
single-particle and transport properties near the QCP. We focus mainly on a van
Hove singularity (vHS) coming from the relatively flat dispersion that crosses
the Fermi level near the quantum critical filling. The flat part of the
dispersion orthogonal to the antinodal direction remains pinned near the Fermi
level for a range of doping that increases when we include a negative
next-near-neighbor hopping t' in the model. For comparison, we calculate the
bare d-wave pairing susceptibility for non-interacting models with the usual
two-dimensional tight binding dispersion and a hypothetical quartic dispersion.
We find that neither model yields a vHS that completely describes the critical
algebraic behavior of the bare d-wave pairing susceptibility. The resistivity,
thermal conductivity, thermopower, and the Wiedemann-Franz Law are examined in
the Fermi liquid, marginal Fermi liquid, and pseudo-gap doping regions. A
negative next-near-neighbor hopping t' increases the doping region with
marginal Fermi liquid character. Both T and negative t' are relevant variables
for the QCP, and both the transport and the motion of the vHS with filling
suggest that they are qualitatively similar in their effect.Comment: 15 pages, 17 figure
Enhancing Transport Efficiency by Hybrid Routing Strategy
Traffic is essential for many dynamic processes on real networks, such as
internet and urban traffic systems. The transport efficiency of the traffic
system can be improved by taking full advantage of the resources in the system.
In this paper, we propose a dual-strategy routing model for network traffic
system, to realize the plenary utility of the whole network. The packets are
delivered according to different "efficient routing strategies" [Yan, et al,
Phys. Rev. E 73, 046108 (2006)]. We introduce the accumulate rate of packets,
{\eta} to measure the performance of traffic system in the congested phase, and
propose the so-called equivalent generation rate of packet to analyze the
jamming processes. From analytical and numerical results, we find that, for
suitable selection of strategies, the dual- strategy system performs better
than the single-strategy system in a broad region of strategy mixing ratio. The
analytical solution to the jamming processes is verified by estimating the
number of jammed nodes, which coincides well with the result from simulation.Comment: 6 pages, 3 figure
Characterisation of the Glass Fraction of a Selection of European Coal Fly Ashes
Fly ash largely consists of the inorganic content of coal that remains after combustion. The crystalline phases present in fly ash may form upon cooling of a molten alumino-silicate glass. This view is supported by the spherical shape of many fly ash particles, inferring that they have gone through a viscous fluid state. The amorphous content in fly ash is believed to dominate reactivity behaviour, under both alkaline and acid conditions, because glasses have a higher potential energy than the equivalent crystal structure and the variation of bond angles and distances in a glass makes the bond breakage easier. It is the degradation behaviour under alkaline conditions, and the subsequent release of silica from the glass phase, that is important in the use of fly ash for conversion to zeolites and for pozzolanic applications in cement. This research comprehensively studies the composition, quantity and stability of the glass phase in a series of nine fly ashes sourced from Spanish and Italian power plants. The quantitative elemental composition of the glass phase in each fly ash was determined. Samples of the ashes then underwent a series of tests to determine the internal structure of the ash particles. Heat treatment of most of the ashes results in mullite crystallising from the glass phase; this is the crystalline phase that is predicated to form by both the relevant phase diagrams and also by NMR spectroscopy. In the ashes, mullite is present as a spherical shell, tracing the outline of the particle but in some specific cases the mullite skeleton is made up of coarse crystals reach also the internal parts of the particles. The morphology and density of the mullite crystals in these shells varies greatly. This work has supported the view that some crystalline phases present in fly ashes, such as mullite, form upon cooling of the amorphous glass melt as opposed to direct conversion from existing mineral phases in the coal during the combustion process. © 2004 Society of Chemical Industry
Evolution of the superconductivity dome in the two-dimensional Hubbard model
In a recent publication, we identified a line of Lifshitz transition points separating the Fermi liquid and pseudogap regions in the hole-doped two-dimensional Hubbard model. Here, we extend the study to further determine the superconducting transition temperature in the phase diagram. By means of large-scale dynamical cluster quantum Monte Carlo simulations, we are able to identify the evolution of the d-wave superconducting dome in the hole-dope side of the phase diagram, with next-nearest-neighbor hopping (t′), chemical potential, and temperature as control parameters. To obtain the superconducting transition temperature Tc, we employ two-particle measurements of the pairing susceptibilities. As t′ goes from positive to negative values, we find the d-wave projected irreducible pairing vertex function is enhanced, and the curvature of its doping dependence changes from convex to concave, which fixes the position of the maximum superconducting temperature at the same filling (n≈0.85) and constraints the dome from precisely following the Lifshitz line. We furthermore decompose the irreducible vertex function into fully irreducible, charge and spin components via the parquet equations, and consistently find that the spin component dominates the pairing vertex function in the doping range where the dome is located. Our investigations deepen the understanding of the phase diagram of the two-dimensional Hubbard model and, more importantly, pose new questions to the field. For example, we found as t′ goes from positive to negative values, the curvature of the pairing strength as a function of doping changes from convex to concave, and the nature of the dominant fluctuations changes from charge degree of freedom to spin degree of freedom. The study of these issues will lead to further understanding of the phase diagram of the two-dimensional Hubbard model and also the physics of the hole-doped cuprate high-temperature superconductors. © 2013 American Physical Society
Skyrmions and edge spin excitations in quantum Hall droplets
We present an analysis of spin-textures in Quantum Hall droplets, for filling
factors . Analytical wavefunctions with well defined quantum
numbers are given for the low-lying states of the system which result to be
either bulk skyrmions or edge spin excitations. We compute dispersion relations
and study how skyrmions become ground states of the Quantum Hall droplet at
. A Hartree-Fock approximation is recovered and discussed for
those spin textures.Comment: RevTeX, four postscript figures appende
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