55,679 research outputs found
Domain - wall - induced magnetoresistance in pseudo spin-valve/superconductor hybrid structures
We have studied the interaction between magnetism and superconductivity in a
pseudo-spin-valve structure consisting of a Co/Cu/Py/Nb layer sequence. We are
able to control the magnetization reversal process and monitor it by means of
the giant magnetoresistance effect during transport measurements. By placing
the superconducting Nb-film on the top of the permalloy (Py) electrode instead
of putting it in between the two ferromagnets, we minimize the influence of
spin scattering or spin accumulation onto the transport properties of Nb.
Magnetotransport data reveal clear evidence that the stray fields of domain
walls (DWs) in the pseudo-spin-valve influence the emerging superconductivity
close to the transition temperature by the occurrence of peak-like features in
the magneto-resistance characteristic. Direct comparison with magnetometry data
shows that the resistance peaks occur exactly at the magnetization reversal
fields of the Co and Py layers, where DWs are generated. For temperatures near
the superconducting transition the amplitude of the DW-induced
magnetoresistance increases with decreasing temperature, reaching values far
beyond the size of the giant magnetoresistive response of our structure in the
normal state.Comment: 20 pages, 4 figure
Lasercooled RaF as a promising candidate to measure molecular parity violation
The parameter , which characterizes nuclear spin-dependent
parity violation effects within the effective molecular spin-rotational
Hamiltonian, was computed for the electronic ground state of radium fluoride
(RaF) and found to be one of the largest absolute values predicted so far.
These calculations were performed with the complex generalised Hartree-Fock
method within a two-component (quasi-relativistic) zeroth-order regular
approximation framework. Peculiarities of the molecular electronic structure of
RaF lead to highly diagonal Franck-Condon matrices between vibrational states
of the electronic ground and first excited states, which renders the molecule
in principle suitable for direct laser cooling. As a trapped gas of cold
molecules offers a superior coherence time, RaF can be considered a promising
candidate for high-precision spectroscopic experiments aimed at the search of
molecular parity-violation effects.Comment: 4.5 pages, 1 figure, 2 tables. Supplementary material can be
requested from the authors. Minor changes to version
A Radio Flare from GRB 020405: Evidence for a Uniform Medium Around a Massive Stellar Progenitor
We present radio observations of GRB 020405 starting 1.2 days after the
burst, which reveal a rapidly-fading ``radio flare''. Based on its temporal and
spectral properties, we interpret the radio flare as emission from the reverse
shock. This scenario rules out a circumburst medium with a radial density
profile \rho ~ r^{-2} expected around a mass-losing massive star, since in that
case the reverse shock emission decays on the timescale of the burst duration
t~100 s. Using published optical and X-ray data, along with the radio data
presented here, we further show that a self-consistent model requires
collimated ejecta with an opening angle of 6 degrees (t_j~0.95 days). As a
consequence of the early jet break, the late-time (t>10 days) emission measured
with the Hubble Space Telescope significantly deviates from an extrapolation of
the early, ground-based data. This, along with an unusually red spectrum, F_\nu
\~ \nu^{-3.9}, strengthens the case for a supernova that exploded at about the
same time as GRB 020405, thus pointing to a massive stellar progenitor for this
burst. This is the first clear association of a massive progenitor with a
uniform medium, indicating that a \rho ~ r^{-2} profile is not a required
signature, and in fact may not be present on the lengthscales probed by the
afterglow in the majority of bursts.Comment: Submitted to ApJ; 14 pages, 2 tables, 3 figure
Geometric phases in superconducting qubits beyond the two-level-approximation
Geometric phases, which accompany the evolution of a quantum system and
depend only on its trajectory in state space, are commonly studied in two-level
systems. Here, however, we study the adiabatic geometric phase in a weakly
anharmonic and strongly driven multi-level system, realised as a
superconducting transmon-type circuit. We measure the contribution of the
second excited state to the two-level geometric phase and find good agreement
with theory treating higher energy levels perturbatively. By changing the
evolution time, we confirm the independence of the geometric phase of time and
explore the validity of the adiabatic approximation at the transition to the
non-adiabatic regime.Comment: 5 pages, 3 figure
Cities in fiction: Perambulations with John Berger
This paper explores selected novels by John Berger in which cities play a central role. These cities are places, partially real and partially imagined, where memory, hope, and despair intersect. My reading of the novels enables me to trace important themes in recent discourses on the nature of contemporary capitalism, including notions of resistance and universality. I also show how Berger?s work points to a writing that can break free from the curious capacity of capitalism to absorb and feed of its critique
Electron beam induced radio emission from ultracool dwarfs
We present the numerical simulations for an electron-beam-driven and
loss-cone-driven electron-cyclotron maser (ECM) with different plasma
parameters and different magnetic field strengths for a relatively small region
and short time-scale in an attempt to interpret the recent discovered intense
radio emission from ultracool dwarfs. We find that a large amount of
electromagnetic field energy can be effectively released from the beam-driven
ECM, which rapidly heats the surrounding plasma. A rapidly developed
high-energy tail of electrons in velocity space (resulting from the heating
process of the ECM) may produce the radio continuum depending on the initial
strength of the external magnetic field and the electron beam current. Both
significant linear polarization and circular polarization of electromagnetic
waves can be obtained from the simulations. The spectral energy distributions
of the simulated radio waves show that harmonics may appear from 10 to
70 ( is the electron plasma frequency) in the
non-relativistic case and from 10 to 600 in the relativistic
case, which makes it difficult to find the fundamental cyclotron frequency in
the observed radio frequencies. A wide frequency band should therefore be
covered by future radio observations.Comment: 10 pages, 19 figures, accepted for publication in the Astrophysical
Journa
Asymptotics for the Wiener sausage among Poissonian obstacles
We consider the Wiener sausage among Poissonian obstacles. The obstacle is
called hard if Brownian motion entering the obstacle is immediately killed, and
is called soft if it is killed at certain rate. It is known that Brownian
motion conditioned to survive among obstacles is confined in a ball near its
starting point. We show the weak law of large numbers, large deviation
principle in special cases and the moment asymptotics for the volume of the
corresponding Wiener sausage. One of the consequence of our results is that the
trajectory of Brownian motion almost fills the confinement ball.Comment: 19 pages, Major revision made for publication in J. Stat. Phy
Microwave-controlled generation of shaped single photons in circuit quantum electrodynamics
Large-scale quantum information processors or quantum communication networks
will require reliable exchange of information between spatially separated
nodes. The links connecting these nodes can be established using traveling
photons that need to be absorbed at the receiving node with high efficiency.
This is achievable by shaping the temporal profile of the photons and absorbing
them at the receiver by time reversing the emission process. Here, we
demonstrate a scheme for creating shaped microwave photons using a
superconducting transmon-type three-level system coupled to a transmission line
resonator. In a second-order process induced by a modulated microwave drive, we
controllably transfer a single excitation from the third level of the transmon
to the resonator and shape the emitted photon. We reconstruct the density
matrices of the created single-photon states and show that the photons are
antibunched. We also create multipeaked photons with a controlled amplitude and
phase. In contrast to similar existing schemes, the one we present here is
based solely on microwave drives, enabling operation with fixed frequency
transmons
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