35,660 research outputs found
Magnetic Oscillations of a Fractional Hall Dot
We show that a quantum dot in the fractional Hall regime exhibits mesoscopic
magnetic oscillations with a period which is a multiple of the period for free
electrons. Our calculations are performed for parabolic quantum dots with
hard-core electron-electron interactions and are exact in the strong field
limit for smaller than the fractional Hall gap. Explicit expressions
are given for the temperature dependence of the amplitude of the oscillations.Comment: 11 pages, IUCM-004, plain te
Pauli-Limited Superconductivity in Small Grains
We report on an exploration of the mean-field phase diagram for Pauli-limited
superconductivity in small metallic grains. Emphasis is placed on the crossover
from the ultra-small grain limit where superconductivity disappears to the bulk
thin-film limit as the single-particle level spacing in the grain decreases. We
find that the maximum Zeeman coupling strength compatible with
superconductivity increases with decreasing grain size, in spite of a
monotonically decreasing condensation energy per unit volume.Comment: 4 pages of text, 6 figure
The Social Wasps (Hymenoptera: Vespidae) of Indiana
An updated taxonomic treatment of the social wasps (Hymenoptera: Vespidae) of Indiana is made. Illustrated identification keys are provided for species of Polistes, Vespa, Vespula, and Dolichovespula. New distributional records and biological notes are provided for each species
Magnetization orientation dependence of the quasiparticle spectrum and hysteresis in ferromagnetic metal nanoparticles
We use a microscopic Slater-Koster tight-binding model with short-range
exchange and atomic spin-orbit interactions that realistically captures generic
features of ferromagnetic metal nanoparticles to address the mesoscopic physics
of magnetocrystalline anisotropy and hysteresis in nanoparticle quasiparticle
excitation spectra. Our analysis is based on qualitative arguments supported by
self-consistent Hartree-Fock calculations for nanoparticles containing up to
260 atoms. Calculations of the total energy as a function of magnetization
direction demonstrate that the magnetic anisotropy per atom fluctuates by
several percents when the number of electrons in the particle changes by one,
even for the largest particles we consider. Contributions of individual
orbitals to the magnetic anisotropy are characterized by a broad distribution
with a mean more than two orders of magnitude smaller than its variance and
with no detectable correlations between anisotropy contribution and
quasiparticle energy. We find that the discrete quasiparticle excitation
spectrum of a nanoparticle displays a complex non-monotonic dependence on an
external magnetic field, with abrupt jumps when the magnetization direction is
reversed by the field, explaining recent spectroscopic studies of magnetic
nanoparticles. Our results suggests the existence of a broad cross-over from a
weak spin-orbit coupling to a strong spin-orbit coupling regime, occurring over
the range from approximately 200- to 1000-atom nanoparticles.Comment: 39 pages, 18 figures, to be published in Physical Review
Thin films of a three-dimensional topological insulator in a strong magnetic field: a microscopic study
The response of thin films of BiSe to a strong perpendicular magnetic
field is investigated by performing magnetic bandstructure calculations for a
realistic multi-band tight-binding model. Several crucial features of Landau
quantization in a realistic three-dimensional topological insulator are
revealed. The Landau level is absent in ultra-thin films, in agreement
with experiment. In films with a crossover thickness of five quintuple layers,
there is a signature of the level, whose overall trend as a function of
magnetic field matches the established low-energy effective-model result.
Importantly, we find a field-dependent splitting and a strong spin-polarization
of the level which can be measured experimentally at reasonable field
strengths. Our calculations show mixing between the surface and bulk Landau
levels which causes the character of levels to evolve with magnetic field.Comment: 5 pages, 4 figure
Asymptotically exact trial wave functions for yrast states of rotating Bose gases
We revisit the composite fermion (CF) construction of the lowest angular
momentum yrast states of rotating Bose gases with weak short range interaction.
For angular momenta at and below the single vortex, , the overlaps
between these trial wave functions and the corresponding exact solutions {\it
increase} with increasing system size and appear to approach unity in the
thermodynamic limit. In the special case , this remarkable behaviour was
previously observed numerically. Here we present methods to address this point
analytically, and find strongly suggestive evidence in favour of similar
behaviour for all . While not constituting a fully conclusive proof
of the converging overlaps, our results do demonstrate a striking similarity
between the analytic structure of the exact ground state wave functions at , and that of their CF counterparts. Results are given for two different
projection methods commonly used in the CF approach
Elementary Excitations of Ferromagnetic Metal Nanoparticles
We present a theory of the elementary spin excitations in transition metal
ferromagnet nanoparticles which achieves a unified and consistent quantum
description of both collective and quasiparticle physics. The theory starts by
recognizing the essential role played by spin-orbit interactions in determining
the energies of ferromagnetic resonances in the collective excitation spectrum
and the strength of their coupling to low-energy particle-hole excitations. We
argue that a crossover between Landau-damped ferromagnetic resonance and
pure-state collective magnetic excitations occurs as the number of atoms in
typical transition metal ferromagnet nanoparticles drops below approximately
, approximately where the single-particle level spacing, ,
becomes larger than, , where is the
ferromagnetic resonance frequency and is the Gilbert damping
parameter. We illustrate our ideas by studying the properties of semi-realistic
model Hamiltonians, which we solve numerically for nanoparticles containing
several hundred atoms. For small nanoparticles, we find one isolated
ferromagnetic resonance collective mode below the lowest particle-hole
excitation energy, at meV. The spectral weight of
this pure excitation nearly exhausts the transverse dynamical susceptibility
spectral weight. As approaches , the
ferromagnetic collective excitation is more likely to couple strongly with
discrete particle-hole excitations. In this regime the distinction between the
two types of excitations blurs. We discuss the significance of this picture for
the interpretation of recent single-electron tunneling experiments.Comment: 19 pages, 13 figure
Edge State Tunneling in a Split Hall Bar Model
In this paper we introduce and study the correlation functions of a chiral
one-dimensional electron model intended to qualitatively represent narrow Hall
bars separated into left and right sections by a penetrable barrier. The model
has two parameters representing respectively interactions between top and
bottom edges of the Hall bar and interactions between the edges on opposite
sides of the barrier. We show that the scaling dimensions of tunneling
processes depend on the relative strengths of the interactions, with repulsive
interactions across the Hall bar tending to make breaks in the barrier
irrelevant. The model can be solved analytically and is characterized by a
difference between the dynamics of even and odd Fourier components. We address
its experimental relevance by comparing its predictions with those of a more
geometrically realistic model that must be solved numerically.Comment: 13 pages, including 4 figures,final version as publishe
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