859 research outputs found
Macroscopic quantum tunneling in globally coupled series arrays of Josephson junctions
We present a quantitative analysis of an escape rate for switching from the
superconducting state to a resistive one in series arrays of globally coupled
Josephson junctions. A global coupling is provided by an external shunting
impedance. Such an impedance can strongly suppress both the crossover
temperature from the thermal fluctuation to quantum regimes, and the
macroscopic quantum tunneling (MQT) in short Josephson junction series arrays.
However, in large series arrays we obtain an enhancement of the crossover
temperature, and a giant increase of the MQT escape rate. The effect is
explained by excitation of a {\it spatial-temporal charge instanton}
distributed over a whole structure. The model gives a possible explanation of
recently published experimental results on an enhancement of the MQT in single
crystals of high- superconductors.Comment: 4 pages, 3 figure
Continuous measurements of two qubits
We develop a theory of coherent quantum oscillations in two, in general
interacting, qubits measured continuously by a mesoscopic detector with
arbitrary non-linearity and discuss an example of SQUID magnetometer that can
operate as such a detector. Calculated spectra of the detector output show that
the detector non-linearity should lead to mixing of the oscillations of the two
qubits. For non-interacting qubits oscillating with frequencies and
, the mixing manifests itself as spectral peaks at the combination
frequencies . Additional nonlinearity introduced by the
qubit-qubit interaction shifts all the frequencies. In particular, for
identical qubits, the interaction splits coherent superposition of the
single-qubit peaks at . Quantum mechanics of the measurement
imposes limitations on the height of the spectral peaks.Comment: 14 pages, 6 figure
Collective transport in the insulating state of Josephson junction arrays
We investigate collective Cooper-pair transport of one- and two-dimensional
Josephson junction arrays in the insulating state. We derive an analytical
expression for the current-voltage characteristic revealing thermally activated
conductivity at small voltages and threshold voltage depinning. The activation
energy and the related depinning voltage represent a dynamic Coulomb barrier
for collective charge transfer over the whole system and scale with the system
size. We show that both quantities are non-monotonic functions of magnetic
field. We propose that formation of the dynamic Coulomb barrier as well as the
size scaling of the activation energy and the depinning threshold voltage, are
consequences of the mutual phase synchronization. We apply the results for
interpretation of experimental data in disordered films near the
superconductor-insulator transition.Comment: 4 pages, 2 figures; typos corrected, new figures, an improved fit to
experimental dat
Inverse proximity effect in superconductors near ferromagnetic material
We study the electronic density of states in a mesoscopic superconductor near
a transparent interface with a ferromagnetic metal. In our tunnel spectroscopy
experiment, a substantial density of states is observed at sub-gap energies
close to a ferromagnet. We compare our data with detailed calculations based on
the Usadel equation, where the effect of the ferromagnet is treated as an
effective boundary condition. We achieve an excellent agreement with theory
when non-ideal quality of the interface is taken into account.Comment: revised, 7 pages, 3 figure
Weber blockade theory of magnetoresistance oscillations in superconducting strips
Recent experiments on the conductance of thin, narrow superconducting strips
have found periodic fluctuations, as a function of the perpendicular magnetic
field, with a period corresponding to approximately two flux quanta per strip
area [A. Johansson et al., Phys. Rev. Lett. {\bf 95}, 116805 (2005)]. We argue
that the low-energy degrees of freedom responsible for dissipation correspond
to vortex motion. Using vortex/charge duality, we show that the superconducting
strip behaves as the dual of a quantum dot, with the vortices, magnetic field,
and bias current respectively playing the roles of the electrons, gate voltage
and source-drain voltage. In the bias-current vs. magnetic-field plane, the
strip conductance displays what we term `Weber blockade' diamonds, with vortex
conductance maxima (i.e., electrical resistance maxima) that, at small
bias-currents, correspond to the fields at which strip states of and
vortices have equal energy.Comment: 4+a bit pages, 3 figures, 1 tabl
Thermal fluctuation field for current-induced domain wall motion
Current-induced domain wall motion in magnetic nanowires is affected by
thermal fluctuation. In order to account for this effect, the
Landau-Lifshitz-Gilbert equation includes a thermal fluctuation field and
literature often utilizes the fluctuation-dissipation theorem to characterize
statistical properties of the thermal fluctuation field. However, the theorem
is not applicable to the system under finite current since it is not in
equilibrium. To examine the effect of finite current on the thermal
fluctuation, we adopt the influence functional formalism developed by Feynman
and Vernon, which is known to be a useful tool to analyze effects of
dissipation and thermal fluctuation. For this purpose, we construct a quantum
mechanical effective Hamiltonian describing current-induced domain wall motion
by generalizing the Caldeira-Leggett description of quantum dissipation. We
find that even for the current-induced domain wall motion, the statistical
properties of the thermal noise is still described by the
fluctuation-dissipation theorem if the current density is sufficiently lower
than the intrinsic critical current density and thus the domain wall tilting
angle is sufficiently lower than pi/4. The relation between our result and a
recent result, which also addresses the thermal fluctuation, is discussed. We
also find interesting physical meanings of the Gilbert damping alpha and the
nonadiabaticy parameter beta; while alpha characterizes the coupling strength
between the magnetization dynamics (the domain wall motion in this paper) and
the thermal reservoir (or environment), beta characterizes the coupling
strength between the spin current and the thermal reservoir.Comment: 16 page, no figur
Coherent oscillations in a Cooper-pair box
This paper is devoted to an analysis of the experiment by Nakamura {\it et
al.} (Nature {\bf 398}, 786 (1999)) on the quantum state control in Josephson
junctions devices. By considering the relevant processes involved in the
detection of the charge state of the box and a realistic description of the
gate pulse we are able to analyze some aspects of the experiment (like the
amplitude of the measurement current) in a quantitative way
Single-electron transistor effect in a two-terminal structure
A peculiarity of the single-electron transistor effect makes it possible to
observe this effect even in structures lacking a gate electrode altogether. The
proposed method can be useful for experimental study of charging effects in
structures with an extremely small central island confined between tunnel
barriers like a nanometer-sized quantum dot or a macromolecule probed with a
tunneling microscope), where it is impossible to provide a gate electrode for
control of the tunnel current.Comment: 5 pages, 2 figure
On the exactness of the Semi-Classical Approximation for Non-Relativistic One Dimensional Propagators
For one dimensional non-relativistic quantum mechanical problems, we
investigate the conditions for all the position dependence of the propagator to
be in its phase, that is, the semi-classical approximation to be exact. For
velocity independent potentials we find that:
(i) the potential must be quadratic in space, but can have arbitrary time
dependence.
(ii) the phase may be made proportional to the classical action, and the
magnitude (``fluctuation factor'') can also be found from the classical
solution.
(iii) for the driven harmonic oscillator the fluctuation factor is
independent of the driving term.Comment: 7 pages, latex, no figures, published in journal of physics
Establishing Social Work Practices in England: The Early Evidence
Social Work Practices (SWPs) were established in England in 2009 to deliver social work services to looked after children and care leavers. The introduction of independent social work-led organisations generated controversy focused on issues such as the privatisation of children's services and social workers' conditions of employment. This paper reports early findings from the evaluation of four of these pilots, drawing on interviews with children and young people, staff, and local authority and national stakeholders. The SWPs assumed a variety of organisational forms. The procurement process was demanding, with protracted negotiations over matters such as budgetary control and providing a round-the-clock service. Start-up was facilitated by an established relationship between the SWP provider and the local authority. Once operational, SWPs continued to rely on local authorities for various functions; in most cases, local authorities retained control of placement budgets. Levels of consultation and choice offered to children and young people regarding the move to an SWP varied considerably. Children's understanding about SWPs was generally low except in the pilot where most children retained their original social worker. These early findings show some dilution of the original SWP model, while the pilots' diversity allows the benefits of particular models to emerge
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