1,383 research outputs found
Critical currents in weakly textured MgB2: Nonlinear transport in anisotropic heterogeneous media
A model for highly non-linear transport in heterogeneous media consisting of
anisotropic particles with a preferred orientation is proposed and applied to
the current transport in weakly textured magnesium diboride, MgB2. It
essentially explains why, unlike in conventional superconductors, a significant
macroscopic anisotropy of the critical currents can be induced by the
preparation of MgB2 tapes. The field and angular dependence of the critical
current is calculated for various degrees of texture and compared to
experimental data
Nonzero temperature effects on antibunched photons emitted by a quantum point contact out of equilibrium
Electrical current fluctuations in a single-channel quantum point contact can
produce photons (at frequency omega close to the applied voltage V x e/hbar)
which inherit the sub-Poissonian statistics of the electrons. We extend the
existing zero-temperature theory of the photostatistics to nonzero temperature
T. The Fano factor F (the ratio of the variance and the average photocount) is
1 for T>T_c (bunched photons). The
crossover temperature T_c ~ Deltaomega x hbar/k_B is set by the band width
Deltaomega of the detector, even if hbar x Deltaomega << eV. This implies that
narrow-band detection of photon antibunching is hindered by thermal
fluctuations even in the low-temperature regime where thermal electron noise is
negligible relative to shot noise.Comment: 4 pages, 2 pages appendix, 3 figure
Flat-lens focusing of electrons on the surface of a topological insulator
We propose the implementation of an electronic Veselago lens on the
conducting surface of a three-dimensional topological insulator (such as
Bi2Te3). The negative refraction needed for such a flat lens results from the
sign change of the curvature of the Fermi surface, changing from a circular to
a snowflake-like shape across a sufficiently large electrostatic potential
step. No interband transition (as in graphene) is needed. For this reason, and
because the topological insulator provides protection against backscattering,
the potential step is able to focus a broad range of incident angles. We
calculate the quantum interference pattern produced by a point source,
generalizing the analogous optical calculation to include the effect of a
noncircular Fermi surface (having a nonzero conic constant).Comment: 6 pages, 6 figure
Who Do We Think We Are? A Qualitative Exploration of Social Work Identity
Since the inception of the profession, social workers have struggled to identify a common understanding of social work, its fundamental activity, and the role of the worker. Utilizing a phenomenological approach, this qualitative study explored the following questions: 1) What is the participant’s lived experience of social work identity? and 2) How is this understanding of identity similar to or different from that of other helping professionals who perform similar activities? Semi-structured interviews, conducted with 22 social workers in the field, diverse in education, practice area, and experience, highlighted parallels among the various experiences, and advanced the exploration of the essence of social work identity. The interviews investigated the presence of role ambiguity and/or role conflict in the experience of each social work participant, as well as their response orientation to conflict (i.e., expedient, moral, and moral-expedient) and the possible effects of ambiguity (i.e., super and sub identities) on their understanding of their own social work identity. While saturation was not reached, the analysis indicated that the participant’s job description, workplace expectations, and their inability to adequately distinguish their role and activity from that of other helping professional preforming the same task or occupying the same role affected the understanding many had of their own social work identity. Themes included super and subordinate orientations, role or activity dominance, identity modifiers, and conflict resolution orientations. These findings may be used to inform future research of social work identity as well as the unifying element(s) of the profession
Coherent pumping of a Mott insulator: Fermi golden rule versus Rabi oscillations
Cold atoms provide a unique arena to study many-body systems far from
equilibrium. Furthermore, novel phases in cold atom systems are conveniently
investigated by dynamical probes pushing the system out of equilibrium. Here,
we discuss the pumping of doubly-occupied sites in a fermionic Mott insulator
by a periodic modulation of the hopping amplitude. We show that deep in the
insulating phase the many-body system can be mapped onto an effective two-level
system which performs coherent Rabi oscillations due to the driving. Coupling
the two-level system to the remaining degrees of freedom renders the Rabi
oscillations damped. We compare this scheme to an alternative description where
the particles are incoherently pumped into a broad continuum.Comment: 4 pages, 3 figure
Anyonic interferometry without anyons: How a flux qubit can read out a topological qubit
Proposals to measure non-Abelian anyons in a superconductor by quantum
interference of vortices suffer from the predominantly classical dynamics of
the normal core of an Abrikosov vortex. We show how to avoid this obstruction
using coreless Josephson vortices, for which the quantum dynamics has been
demonstrated experimentally. The interferometer is a flux qubit in a Josephson
junction circuit, which can nondestructively read out a topological qubit
stored in a pair of anyons --- even though the Josephson vortices themselves
are not anyons. The flux qubit does not couple to intra-vortex excitations,
thereby removing the dominant restriction on the operating temperature of
anyonic interferometry in superconductors.Comment: 7 pages, 3 figures; Added an Appendix on parity-protected
single-qubit rotations; problem with Figure 3 correcte
Scattering formula for the topological quantum number of a disordered multi-mode wire
The topological quantum number Q of a superconducting or chiral insulating
wire counts the number of stable bound states at the end points. We determine Q
from the matrix r of reflection amplitudes from one of the ends, generalizing
the known result in the absence of time-reversal and chiral symmetry to all
five topologically nontrivial symmetry classes. The formula takes the form of
the determinant, Pfaffian, or matrix signature of r, depending on whether r is
a real matrix, a real antisymmetric matrix, or a Hermitian matrix. We apply
this formula to calculate the topological quantum number of N coupled dimerized
polymer chains, including the effects of disorder in the hopping constants. The
scattering theory relates a topological phase transition to a conductance peak,
of quantized height and with a universal (symmetry class independent) line
shape. Two peaks which merge are annihilated in the superconducting symmetry
classes, while they reinforce each other in the chiral symmetry classes.Comment: 8 pages, 3 figures, this is the final, published versio
Quantized conductance at the Majorana phase transition in a disordered superconducting wire
Superconducting wires without time-reversal and spin-rotation symmetries can
be driven into a topological phase that supports Majorana bound states. Direct
detection of these zero-energy states is complicated by the proliferation of
low-lying excitations in a disordered multi-mode wire. We show that the phase
transition itself is signaled by a quantized thermal conductance and electrical
shot noise power, irrespective of the degree of disorder. In a ring geometry,
the phase transition is signaled by a period doubling of the magnetoconductance
oscillations. These signatures directly follow from the identification of the
sign of the determinant of the reflection matrix as a topological quantum
number.Comment: 7 pages, 4 figures; v3: added appendix with numerics for long-range
disorde
Coulomb-assisted braiding of Majorana fermions in a Josephson junction array
We show how to exchange (braid) Majorana fermions in a network of
superconducting nanowires by control over Coulomb interactions rather than
tunneling. Even though Majorana fermions are charge-neutral quasiparticles
(equal to their own antiparticle), they have an effective long-range
interaction through the even-odd electron number dependence of the
superconducting ground state. The flux through a split Josephson junction
controls this interaction via the ratio of Josephson and charging energies,
with exponential sensitivity. By switching the interaction on and off in
neighboring segments of a Josephson junction array, the non-Abelian braiding
statistics can be realized without the need to control tunnel couplings by gate
electrodes. This is a solution to the problem how to operate on topological
qubits when gate voltages are screened by the superconductor
Observations Supporting the Role of Magnetoconvection in Energy Supply to the Quiescent Solar Atmosphere
Identifying the two physical mechanisms behind the production and sustenance
of the quiescent solar corona and solar wind poses two of the outstanding
problems in solar physics today. We present analysis of spectroscopic
observations from the Solar and Heliospheric Observatory that are consistent
with a single physical mechanism being responsible for a significant portion of
the heat supplied to the lower solar corona and the initial acceleration of the
solar wind; the ubiquitous action of magnetoconvection-driven reprocessing and
exchange reconnection of the Sun's magnetic field on the supergranular scale.
We deduce that while the net magnetic flux on the scale of a supergranule
controls the injection rate of mass and energy into the transition region
plasma it is the global magnetic topology of the plasma that dictates whether
the released ejecta provides thermal input to the quiet solar corona or becomes
a tributary that feeds the solar wind.Comment: 34 pages, 13 figures - In press Astrophysical Journal (Jan 1 2007
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