671 research outputs found
Superconductivity and non-metallicity induced by doping the topological insulators Bi2Se3 and Bi2Te3
We show that by Ca-doping the Bi2Se3 topological insulator, the Fermi level
can be fine tuned to fall inside the band gap and therefore suppress the bulk
conductivity. Non-metallic Bi2Se3 crystals are obtained. On the other hand, the
Bi2Se3 topological insulator can also be induced to become a bulk
superconductor, with Tc ~ 3.8 K, by copper intercalation in the van der Waals
gaps between the Bi2Se3 layers. Likewise, an as-grown crystal of metallic
Bi2Te3 can be turned into a non-metallic crystal by slight variation of the Te
content. The Bi2Te3 topological insulator shows small amounts of
superconductivity with Tc ~ 5.5 K when reacted with Pd to form materials of the
type PdxBi2Te3
Critical points in edge tunneling between generic FQH states
A general description of weak and strong tunneling fixed points is developed
in the chiral-Luttinger-liquid model of quantum Hall edge states. Tunneling
fixed points are a subset of `termination' fixed points, which describe
boundary conditions on a multicomponent edge. The requirement of unitary time
evolution at the boundary gives a nontrivial consistency condition for possible
low-energy boundary conditions. The effect of interactions and random hopping
on fixed points is studied through a perturbative RG approach which generalizes
the Giamarchi-Schulz RG for disordered Luttinger liquids to broken left-right
symmetry and multiple modes. The allowed termination points of a multicomponent
edge are classified by a B-matrix with rational matrix elements. We apply our
approach to a number of examples, such as tunneling between a quantum Hall edge
and a superconductor and tunneling between two quantum Hall edges in the
presence of interactions. Interactions are shown to induce a continuous
renormalization of effective tunneling charge for the integrable case of
tunneling between two Laughlin states. The correlation functions of
electronlike operators across a junction are found from the B matrix using a
simple image-charge description, along with the induced lattice of boundary
operators. Many of the results obtained are also relevant to ordinary Luttinger
liquids.Comment: 23 pages, 6 figures. Xiao-Gang Wen: http://dao.mit.edu/~we
The UK risk assessment scheme for all non-native species
1. A pest risk assessment scheme, adapted from the EPPO (European and Mediterranean Plant Protection Organisation) scheme, was developed to assess the risks posed to UK species, habitats and ecosystems by non-native taxa.
2. The scheme provides a structured framework for evaluating the potential for non-native organisms, whether intentional or unintentional introductions, to enter, establish, spread and cause significant impacts in all or part of the UK. Specialist modules permit the relative importance of entry pathways, the vulnerability of receptors and the consequences of policies to be assessed and appropriate risk management options to be selected. Spreadsheets for summarising the level of risk and uncertainty, invasive attributes and economic impact were created. In addition, new methods for quantifying economic impact and summarising risk and uncertainty were explored.
3. Although designed for the UK, the scheme can readily be applied elsewhere
Assessment of recycled glass and expanded clay in a dual media configuration for drinking water treatment
S-matrices for Perturbed N=2 Superconformal Field Theory from quantum groups
S-matrices for integrable perturbations of superconformal field
theories are studied. The models we consider correspond to perturbations of the
coset theory . The perturbed models are closely
related to -affine Toda theories with a background charge tuned to .
Using the quantum group restriction of the affine Toda theories we derive the
S-matrix.Comment: 29 pages 2 figure
Exciting, Cooling And Vortex Trapping In A Bose-Condensed Gas
A straight forward numerical technique, based on the Gross-Pitaevskii
equation, is used to generate a self-consistent description of
thermally-excited states of a dilute boson gas. The process of evaporative
cooling is then modelled by following the time evolution of the system using
the same equation. It is shown that the subsequent rethermalisation of the
thermally-excited state produces a cooler coherent condensate. Other results
presented show that trapping vortex states with the ground state may be
possible in a two-dimensional experimental environment.Comment: 9 pages, 7 figures. It's worth the wait! To be published in Physical
Review A, 1st February 199
Lattice Analogues of Superconformal Models via Quantum Group Truncation
We obtain lattice models whose continuum limits correspond to
superconformal coset models. This is done by taking the well known vertex model
whose continuum limit is the conformal field theory, and
twisting the transfer matrix and modifying the quantum group truncation. We
find that the natural order parameters of the new models are precisely the
chiral primary fields. The integrable perturbations of the conformal field
theory limit also have natural counterparts in the lattice formulation, and
these can be incorporated into an affine quantum group structure. The
topological, twisted superconformal models also have lattice analogues,
and these emerge as an intermediate part of our analysis.Comment: 25 pages and 2 figure
The pressure of hot QCD up to g^6 ln(1/g)
The free energy density, or pressure, of QCD has at high temperatures an
expansion in the coupling constant g, known so far up to order g^5. We compute
here the last contribution which can be determined perturbatively, g^6 ln(1/g),
by summing together results for the 4-loop vacuum energy densities of two
different three-dimensional effective field theories. We also demonstrate that
the inclusion of the new perturbative g^6 ln(1/g) terms, once they are summed
together with the so far unknown perturbative and non-perturbative g^6 terms,
could potentially extend the applicability of the coupling constant series down
to surprisingly low temperatures.Comment: 18 pages. Small clarifications added. To appear in Phys.Rev.
Atom lasers: production, properties and prospects for precision inertial measurement
We review experimental progress on atom lasers out-coupled from Bose-Einstein
condensates, and consider the properties of such beams in the context of
precision inertial sensing. The atom laser is the matter-wave analog of the
optical laser. Both devices rely on Bose-enhanced scattering to produce a
macroscopically populated trapped mode that is output-coupled to produce an
intense beam. In both cases, the beams often display highly desirable
properties such as low divergence, high spectral flux and a simple spatial mode
that make them useful in practical applications, as well as the potential to
perform measurements at or below the quantum projection noise limit. Both
devices display similar second-order correlations that differ from thermal
sources. Because of these properties, atom lasers are a promising source for
application to precision inertial measurements.Comment: This is a review paper. It contains 40 pages, including references
and figure
Fluctuations, dissipation and the dynamical Casimir effect
Vacuum fluctuations provide a fundamental source of dissipation for systems
coupled to quantum fields by radiation pressure. In the dynamical Casimir
effect, accelerating neutral bodies in free space give rise to the emission of
real photons while experiencing a damping force which plays the role of a
radiation reaction force. Analog models where non-stationary conditions for the
electromagnetic field simulate the presence of moving plates are currently
under experimental investigation. A dissipative force might also appear in the
case of uniform relative motion between two bodies, thus leading to a new kind
of friction mechanism without mechanical contact. In this paper, we review
recent advances on the dynamical Casimir and non-contact friction effects,
highlighting their common physical origin.Comment: 39 pages, 4 figures. Review paper to appear in Lecture Notes in
Physics, Volume on Casimir Physics, edited by Diego Dalvit, Peter Milonni,
David Roberts, and Felipe da Rosa. Minor changes, a reference adde
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