Non-linear quantum critical transport and the Schwinger Mechanism

Scaling arguments imply that quantum critical points exhibit universal non-linear responses to external probes. We investigate the origins of such non-linearities in transport, which is especially problematic since the system is necessarily driven far from equilibrium. We argue that for a wide class of systems the new ingredient that enters is the Schwinger mechanism--the production of carriers from the vacuum by the applied field-- which is then balanced against a scattering rate which is itself set by the field. We show by explicit computation how this works for the case of the symmetric superfluid-Mott insulator transition of bosons

Coulomb and Hard Core Skyrmion Tails

Quantum Hall skyrmions are quantized solitons of a ferromagnetic O(3) sigma-model. The reference, classical, solutions depend upon the interaction between the electrons and exhibit completely different asymptotic profiles for the physical Coulomb interaction than for the model hard core interaction frequently used to generate variational wavefunctions. In this note we show, by means of numerical calculations on (large) finite size systems at nu=1, that this physically important difference, crucial for a sharp definition of their statistics, persists for the quantized skyrmions at n=1.Comment: Revtex 9 pages, figs.ps files at ftp://landau.calstatela.edu/pub/tailfig

Health needs and co-morbidity among detainees in contact with healthcare professionals within police custody across the London Metropolitan Police Service area

Aims Detainees requiring access to healthcare services in police custody have been shown to suffer from poor physical and mental health, often exacerbated by substance misuse. This study examines the extent and nature of health needs in police custody across the Metropolitan Police Service (MPS), London. Methods A survey (n = 1657) was administered by Healthcare Professionals (HCP) for one month in 2015 across all MPS custody suites representing a 73% response rate. A logistic regression model was created using four binary outcomes (whether a detainee was a drug user, had mental health issues including self-harm and had an alcohol use disorder) with ten prognostics to test for co-morbid associations. A multiple imputation method using chained equations was used to manage missing cases. Findings High rates of physical health conditions, drug use, problematic alcohol use were noted but are within the upper range of existing studies. Mental health, self-harm and overall substance misuse levels (illicit drug user and a current drinker) were shown to be higher than other published studies. The logistic regression model found statistically significant associations between drug use, alcohol consumption and mental health including self-harm. Age was also found to be a key confounding factor. Physical health was broadly negatively associated with the four main outcomes. Discussion Levels of need for health interventions among the detainee population in London are broadly consistent with other European centres. There is a need for police custody staff to consider detainees' dual diagnosis needs. The development of integrated interventions alongside the enhanced clinical management of alcohol, drug use and mental health was considered

Correlation function diagnostics for type-I fracton phases

Fracton phases are recent entrants to the roster of topological phases in three dimensions. They are characterized by subextensively divergent topological degeneracy and excitations that are constrained to move along lower dimensional subspaces, including the eponymous fractons that are immobile in isolation. We develop correlation function diagnostics to characterize Type I fracton phases which build on their exhibiting {\it partial deconfinement}. These are inspired by similar diagnostics from standard gauge theories and utilize a generalized gauging procedure that links fracton phases to classical Ising models with subsystem symmetries. En route, we explicitly construct the spacetime partition function for the plaquette Ising model which, under such gauging, maps into the X-cube fracton topological phase. We numerically verify our results for this model via Monte Carlo calculations

Rare region effects dominate weakly disordered 3D Dirac points

We study three-dimensional Dirac fermions with weak finite-range scalar potential disorder. In the clean system, the density of states vanishes quadratically at the Dirac point. Disorder is known to be perturbatively irrelevant, and previous theoretical work has assumed that the Dirac semimetal phase, characterized by a vanishing density of states, survives at weak disorder, with a finite disorder phase transition to a diffusive metal with a non-vanishing density of states. In this paper we show that nonperturbative effects from rare regions, which are missed by conventional disorder-averaged calculations, instead give rise to a nonzero density of states for any nonzero disorder. Thus, there is no Dirac semimetal phase at non-zero disorder. The results are established both by a heuristic scaling argument and via a systematic saddle point analysis. We also discuss transport near the Dirac point. At the Dirac point, we argue that transport is diffusive, and proceeds via hopping between rare resonances. As one moves in chemical potential away from the Dirac point, there are interesting intermediate-energy regimes where the rare regions produce scattering resonances that determine the DC conductivity. We derive a scaling theory of transport near disordered 3D Dirac points. We also discuss the interplay of disorder with attractive interactions at the Dirac point, and the resulting granular superconducting and Bose glass phases. Our results are relevant for all 3D systems with Dirac points, including Weyl semimetals.Comment: Expanded version of 1307.3252 with many new results, including a new section showing how the results may be derived using a systematic saddle point calculatio

Quantum Hall Skyrmions with Higher Topological Charge

We have investigated quantum Hall skyrmions at filling factor \nu=1 carrying more than one unit of topological, and hence electric, charge. Using a combination of analytic and numerical methods we find the counterintuitive result that when the Zeeman energy is tuned to values much smaller than the interaction energy (g \mu_B B/(e^2/\epsilon\ell) < 9*10^{-5}),the creation energy of a charge two skyrmion becomes less than twice the creation energy of a charge one skyrmion, i.e. skyrmions bind in pairs. The doubly charged skyrmions are stable to further accretion of charge and exhibit a 10% larger spin per unit charge than charge one skyrmions which would, in principle, signal this pairing.Comment: 4 pages, 3 figures. Submitted to Phys. Rev. B, Rapid Communication

Nature of the spin liquid state of the Hubbard model on honeycomb lattice

Recent numerical work (Nature 464, 847 (2010)) indicates the existence of a spin liquid phase (SL) that intervenes between the antiferromagnetic and semimetallic phases of the half filled Hubbard model on a honeycomb lattice. To better understand the nature of this exotic phase, we study the quantum $J_1-J_2$ spin model on the honeycomb lattice, which provides an effective description of the Mott insulating region of the Hubbard model. Employing the variational Monte Carlo approach, we analyze the phase diagram of the model, finding a phase transition between antiferromagnet and an unusual $Z_2$ SL state at $J_2/J_1\approx 0.08$, which we identify as the SL phase of the Hubbard model. At higher $J_2/J_1\gtrsim 0.3$ we find a transition to a dimerized state with spontaneously broken rotational symmetry.Comment: 5 pages, 6 figure