1,164 research outputs found
Specific heat of the S=1/2 Heisenberg model on the kagome lattice: high-temperature series expansion analysis
We compute specific heat of the antiferromagnetic spin-1/2 Heisenberg model
on the kagome lattice. We use a recently introduced technique to analyze
high-temperature series expansion based on the knowledge of high-temperature
series expansions, the total entropy of the system and the low-temperature
expected behavior of the specific heat as well as the ground-state energy. In
the case of kagome-lattice antiferromagnet, this method predicts a
low-temperature peak at T/J<0.1.Comment: 6 pages, 5 color figures (.eps), Revtex 4. Change in version 3: Fig.
5 has been corrected (it now shows data for 3 different ground-state
energies). The text is unchanged. v4: corrected an error in the temperature
scale of Fig. 5. (text unchanged
Dirty quantum Hall ferromagnets and quantum Hall spin glasses
We study quantum Hall ferromagnets in the presence of a random electrostatic
impurity potential, within the framework of a classical non-linear sigma model.
We discuss the behaviour of the system using a heuristic picture for the
competition between exchange and screening, and test our conclusions with
extensive numerical simulations. We obtain a phase diagram for the system as a
function of disorder strength and deviation of the average Landau level filling
factor from unity. Screening of an impurity potential requires distortions of
the spin configuration. In the absence of Zeeman coupling there is a
disorder-driven, zero-temperature phase transition from a ferromagnet at weak
disorder and small deviation from integer filling to a spin glass at stronger
disorder or large charge deviation. We characterise the spin glass phase in
terms of its magnetic and charge response, as well as its ac conductivity.Comment: 12 pages, 6 figures, REVTEX
Mott, Floquet, and the response of periodically driven Anderson insulators
We consider periodically driven Anderson insulators. The short time behavior
for weak, monochromatic, uniform electric fields is given by linear response
theory and was famously derived by Mott. We go beyond this to consider both
long times---which is the physics of Floquet late time states---and strong
electric fields. This results in a `phase diagram' in the frequency-field
strength plane, in which we identify four distinct regimes. These are: a linear
response regime dominated by pre-existing Mott resonances, which exists
provided Floquet saturation is not reached within a period; a non-linear
perturbative regime, which exhibits multiphoton-absorption in response to the
field; a near-adiabatic regime, which exhibits a primarily reactive response
spread over the entire sample and is insensitive to pre-existing resonances;
and finally an enhanced dissipative regime.Comment: 15 pages, 9 figure
Electron Interactions and Transport Between Coupled Quantum Hall Edges
We examine the effects of electron-electron interactions on transport between
edge states in a multilayer integer quantum Hall system. The edge states of
such a system, coupled by interlayer tunneling, form a two-dimensional, chiral
metal at the sample surface. We calculate the temperature-dependent
conductivity and the amplitude of conductance fluctuations in this chiral
metal, treating Coulomb interactions and disorder exactly in the weak-tunneling
limit. We find that the conductivity increases with increasing temperature, as
observed in recent experiments, and we show that the correlation length
characterising conductance fluctuations varies inversely with temperature.Comment: 4 pages, 2 figures, typos corrected, Ref. 17 added, minor changes
made for publicatio
Local pairing of Feynman histories in many-body Floquet models
We study many-body quantum dynamics using Floquet quantum circuits in one
space dimension as simple examples of systems with local interactions that
support ergodic phases. Physical properties can be expressed in terms of
multiple sums over Feynman histories, which for these models are paths or
many-body orbits in Fock space. A natural simplification of such sums is the
diagonal approximation, where the only terms that are retained are ones in
which each path is paired with a partner that carries the complex conjugate
weight. We identify the regime in which the diagonal approximation holds, and
the nature of the leading corrections to it. We focus on the behaviour of the
spectral form factor (SFF) and of matrix elements of local operators, averaged
over an ensemble of random circuits, making comparisons with the predictions of
random matrix theory (RMT) and the eigenstate thermalisation hypothesis (ETH).
We show that properties are dominated at long times by contributions to orbit
sums in which each orbit is paired locally with a conjugate, as in the diagonal
approximation, but that in large systems these contributions consist of many
spatial domains, with distinct local pairings in neighbouring domains. The
existence of these domains is reflected in deviations of the SFF from RMT
predictions, and of matrix element correlations from ETH predictions;
deviations of both kinds diverge with system size. We demonstrate that our
physical picture of orbit-pairing domains has a precise correspondence in the
spectral properties of a transfer matrix that acts in the space direction to
generate the ensemble-averaged SFF. In addition, we find that domains of a
second type control non-Gaussian fluctuations of the SFF. These domains are
separated by walls which are related to the entanglement membrane, known to
characterise the scrambling of quantum information.Comment: 22+7 page
Many-body delocalisation as symmetry breaking
We present a framework in which the transition between a many-body localised
(MBL) phase and an ergodic one is symmetry breaking. We consider random Floquet
spin chains, expressing their averaged spectral form factor (SFF) as a function
of time in terms of a transfer matrix that acts in the space direction. The SFF
is determined by the leading eigenvalues of this transfer matrix. In the MBL
phase the leading eigenvalue is unique, as in a symmetry-unbroken phase, while
in the ergodic phase and at late times the leading eigenvalues are
asymptotically degenerate, as in a system with degenerate symmetry-breaking
phases. We identify the broken symmetry of the transfer matrix, introduce a
local order parameter for the transition, and show that the associated
correlation functions are long-ranged only in the ergodic phase.Comment: 5+5 page
A hidden Goldstone mechanism in the Kagom\'e lattice antiferromagnet
In this paper, we study the phases of the Heisenberg model on the \kagome
lattice with antiferromagnetic nearest neighbour coupling and
ferromagnetic next neighbour coupling . Analysing the long wavelength, low
energy effective action that describes this model, we arrive at the phase
diagram as a function of . The interesting part of
this phase diagram is that for small , which includes , there is
a phase with no long range spin order and with gapless and spin zero low lying
excitations. We discuss our results in the context of earlier, numerical and
experimental work.Comment: 21 pages, latex file with 5 figure
Evaluation of matrix-assisted laser desorption ionisation time-of-flight mass spectrometry (MALDI-TOF MS) for the Identification of Group B Streptococcus.
Objective
Group B Streptococcus (GBS) is a leading cause of neonatal meningitis and sepsis worldwide. Intrapartum antibiotics given to women carrying GBS are an effective means of reducing disease in the first week of life. Rapid and reliable tests are needed to accurately identify GBS from these women for timely intrapartum antibiotic administration to prevent neonatal disease. Many laboratories now use matrix-assisted laser desorption ionisation time-of-flight mass spectrometry (MALDI-TOF MS) by direct plating or cell lysis for the identification of GBS isolates. The cell lysis step increases time to results for clinical samples and is more complex to perform. Therefore, we seek to evaluate the sensitivity and specificity of the quicker and more rapid direct plating method in identifying GBS.
Results
We directly compared swab isolates analysed by both direct plating and cell lysis method and demonstrated that direct plating has a sensitivity and specificity of 0.97 and 1, respectively, compared to an additional cell lysis step. We demonstrated that MALDI-TOF MS can be successfully used for batch processing by the direct plating method which saves time. These results are reassuring for laboratories worldwide who seek to identify GBS from swabs samples as quickly as possible
A Farewell to Liouvillians
We examine the Liouvillian approach to the quantum Hall plateau transition,
as introduced recently by Sinova, Meden, and Girvin [Phys. Rev. B {\bf 62},
2008 (2000)] and developed by Moore, Sinova and Zee [Phys. Rev. Lett. {\bf 87},
046801 (2001)]. We show that, despite appearances to the contrary, the
Liouvillian approach is not specific to the quantum mechanics of particles
moving in a single Landau level: we formulate it for a general disordered
single-particle Hamiltonian. We next examine the relationship between
Liouvillian perturbation theory and conventional calculations of
disorder-averaged products of Green functions and show that each term in
Liouvillian perturbation theory corresponds to a specific contribution to the
two-particle Green function. As a consequence, any Liouvillian approximation
scheme may be re-expressed in the language of Green functions. We illustrate
these ideas by applying Liouvillian methods, including their extension to Liouvillian flavors, to random matrix ensembles, using numerical
calculations for small integer and an analytic analysis for large .
We find that behavior at is different in qualitative ways from that
at . In particular, the limit expressed using Green
functions generates a pathological approximation, in which two-particle
correlation functions fail to factorize correctly at large separations of their
energy, and exhibit spurious singularities inside the band of random matrix
energy levels. We also consider the large treatment of the quantum Hall
plateau transition, showing that the same undesirable features are present
there, too
Anderson localisation in tight-binding models with flat bands
We consider the effect of weak disorder on eigenstates in a special class of
tight-binding models. Models in this class have short-range hopping on periodic
lattices; their defining feature is that the clean systems have some energy
bands that are dispersionless throughout the Brillouin zone. We show that
states derived from these flat bands are generically critical in the presence
of weak disorder, being neither Anderson localised nor spatially extended.
Further, we establish a mapping between this localisation problem and the one
of resonances in random impedance networks, which previous work has suggested
are also critical. Our conclusions are illustrated using numerical results for
a two-dimensional lattice, known as the square lattice with crossings or the
planar pyrochlore lattice.Comment: 5 pages, 3 figures, as published (this version includes minor
corrections
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