29,267 research outputs found
Review of the environmental and organisational implications of cloud computing: final report.
Cloud computing – where elastic computing resources are delivered over the Internet by external service providers – is generating significant interest within HE and FE. In the cloud computing business model, organisations or individuals contract with a cloud computing service provider on a pay-per-use basis to access data centres, application software or web services from any location. This provides an elasticity of provision which the customer can scale up or down to meet demand. This form of utility computing potentially opens up a new paradigm in the provision of IT to support administrative and educational functions within HE and FE. Further, the economies of scale and increasingly energy efficient data centre technologies which underpin cloud services means that cloud solutions may also have a positive impact on carbon footprints. In response to the growing interest in cloud computing within UK HE and FE, JISC commissioned the University of Strathclyde to undertake a Review of the Environmental and Organisational Implications of Cloud Computing in Higher and Further Education [19]
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
Quantum vortex dynamics in two-dimensional neutral superfluids
We derive an effective action for the vortex position degree-of-freedom in a
superfluid by integrating out condensate phase and density fluctuation
environmental modes. When the quantum dynamics of environmental fluctuations is
neglected, we confirm the occurrence of the vortex Magnus force and obtain an
expression for the vortex mass. We find that this adiabatic approximation is
valid only when the superfluid droplet radius , or the typical distance
between vortices, is very much larger than the coherence length . We go
beyond the adiabatic approximation numerically, accounting for the quantum
dynamics of environmental modes and capturing their dissipative coupling to
condensate dynamics. For the case of an optical-lattice superfluid we
demonstrate that vortex motion damping can be adjusted by tuning the ratio
between the tunneling energy and the on-site interaction energy . We
comment on the possibility of realizing vortex Landau level physics.Comment: 14 pages, 10 figures, accepted by PRA with corrected references and
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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
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
Nernst and Seebeck effect in a graphene nanoribbon
The thermoelectric power, including the Nernst and Seebeck effects, in
graphene nanoribbon is studied. By using the non-equilibrium Green function
combining with the tight-binding Hamiltonian, the Nernst and Seebeck
coefficients are obtained. Due to the electron-hole symmetry, the Nernst
coefficient is an even function of the Fermi energy while the Seebeck
coefficient is an odd function regardless of the magnetic field. In the
presence of a strong magnetic field, the Nernst and Seebeck coefficients are
almost independent of the chirality and width of the nanoribbon, and they show
peaks when the Fermi energy crosses the Landau levels. The height of -th
(excluding ) peak is for the Nernst effect and is
for the Seebeck effect. For the zeroth peak, it is abnormal with height
for the Nernst effect and the peak disappears for the Seebeck effect.
When the magnetic field is turned off, however, the Nernst effect is absent and
only Seebeck effect exists. In this case, the Seebeck coefficient strongly
depends on the chirality of the nanoribbon. The peaks are equidistant for the
nanoribbons with zigzag edge but are irregularly distributed for the armchair
edge. In particular, for the insulating armchair ribbon, the Seebeck
coefficient can be very large near the Dirac point. When the magnetic field
varies from zero to large values, the differences among the Seebeck
coefficients for different chiral ribbons gradually vanish and the nonzero
value of Nernst coefficient appears first near the Dirac point then gradually
extents to the whole energy region.Comment: 8 pages, 7 figure
Current-induced torques due to compensated antiferromagnets
We analyse the influence of current induced torques on the magnetization
configuration of a ferromagnet in a circuit containing a compensated
antiferromagnet. We argue that these torques are generically non-zero and
support this conclusion with a microscopic NEGF calculation for a circuit
containing antiferromagnetic NiMn and ferromagnetic Co layers. Because of
symmetry dictated differences in the form of the current-induced torque, the
phase diagram which expresses the dependence of ferromagnet configuration on
current and external magnetic field differs qualitatively from its
ferromagnet-only counterpart.Comment: 4 pages, 5 figure
Chern number spins of Mn acceptor magnets in GaAs
We determine the effective total spin of local moments formed from
acceptor states bound to Mn ions in GaAs by evaluating their magnetic Chern
numbers. We find that when individual Mn atoms are close to the sample surface,
the total spin changes from to , due to quenching of the
acceptor orbital moment. For Mn pairs in bulk, the total depends on the
pair orientation in the GaAs lattice and on the separation between the Mn
atoms. We point out that Berry curvature variation as a function of local
moment orientation can profoundly influence the quantum spin dynamics of these
magnetic entities.Comment: 4 pages, 3 figure
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