1,342 research outputs found
Minimalist design of a robust real-time quantum random number generator
We present a simple and robust construction of a real-time quantum random
number generator (QRNG). Our minimalist approach ensures stable operation of
the device as well as its simple and straightforward hardware implementation as
a stand-alone module. As a source of randomness the device uses measurements of
time intervals between clicks of a single-photon detector. The obtained raw
sequence is then filtered and processed by a deterministic randomness
extractor, which is realized as a look-up table. This enables high speed
on-the-fly processing without the need of extensive computations. The overall
performance of the device is around 1 random bit per detector click, resulting
in 1.2 Mbit/s generation rate in our implementation
Spectroscopy of phonons and spin torques in magnetic point contacts
Phonon spectroscopy is used to investigate the mechanism of current-induced
spin torques in nonmagnetic/ferromagnetic (N/F) point contacts. Magnetization
excitations observed in the magneto-conductance of the point contacts are
pronounced for diffusive and thermal contacts, where the electrons experience
significant scattering in the contact region. We find no magnetic excitations
in highly ballistic contacts. Our results show that impurity scattering at the
N/F interface is the origin of the new single-interface spin torque effect.Comment: 4 pages, 5 figs., accepted for publication in PR
Advances in point-contact spectroscopy: two-band superconductor MgB2 (A review)
Analysis of the point-contact spectroscopy (PCS) data on the new dramatic
high-T superconductor MgB reveals quite different behavior of two
disconnected and electronic bands, deriving from their
anisotropy, different dimensionality, and electron-phonon interaction. PCS
allows direct registration of both the superconducting gaps and
electron-phonon-interaction spectral function of the two-dimensional
and three-dimensional band, establishing correlation between the gap
value and intensity of the high-T driving force -- the boron
vibration mode. PCS data on some nonsuperconducting transition-metal diborides
are surveyed for comparison.Comment: 17 pages, 30 figs., will be published in Low Temp. Phys. V.30 (2004)
N
Longitudinal magnetic excitation in KCuCl3 studied by Raman scattering under hydrostatic pressures
We measure Raman scattering in an interacting spin-dimer system KCuCl3 under
hydrostatic pressures up to 5 GPa mediated by He gas. In the pressure-induced
quantum phase, we observe a one-magnon Raman peak, which originates from the
longitudinal magnetic excitationand is observable through the second-order
exchange interaction Raman process. We report the pressure dependence of the
frequency, halfwidth and Raman intensity of this mode.Comment: 4 pages, 3 figures, inpress in JPCS as a proceeding of LT2
On the Selfconsistent Theory of Josephson Effect in Ballistic Superconducting Microconstrictions
The microscopic theory of current carrying states in the ballistic
superconducting microchannel is presented. The effects of the contact length L
on the Josephson current are investigated. For the temperatures T close to the
critical temperature T_c the problem is treated selfconsistently, with taking
into account the distribution of the order parameter inside the
contact. The closed integral equation for in strongly inhomogeneous
microcontact geometry ( is the coherence length at
T=0) replaces the differential Ginzburg-Landau equation. The critical current
is expressed in terms of solution of this integral equation. The
limiting cases of and are considered. With
increasing length L the critical current decreases, although the ballistic
Sharvin resistance of the contact remains the same as at L=0. For ultra short
channels with ( is
the Debye frequency) the corrections to the value of critical current I_c(L=0)
are sensitive to the strong coupling effects.Comment: 15 pages LaTex, 3 jpg figure
Static Solitons of the Sine-Gordon Equation and Equilibrium Vortex Structure in Josephson Junctions
The problem of vortex structure in a single Josephson junction in an external
magnetic field, in the absence of transport currents, is reconsidered from a
new mathematical point of view. In particular, we derive a complete set of
exact analytical solutions representing all the stationary points (minima and
saddle-points) of the relevant Gibbs free-energy functional. The type of these
solutions is determined by explicit evaluation of the second variation of the
Gibbs free-energy functional. The stable (physical) solutions minimizing the
Gibbs free-energy functional form an infinite set and are labelled by a
topological number Nv=0,1,2,... Mathematically, they can be interpreted as
nontrivial ''vacuum'' (Nv=0) and static topological solitons (Nv=1,2,...) of
the sine-Gordon equation for the phase difference in a finite spatial interval:
solutions of this kind were not considered in previous literature. Physically,
they represent the Meissner state (Nv=0) and Josephson vortices (Nv=1,2,...).
Major properties of the new physical solutions are thoroughly discussed. An
exact, closed-form analytical expression for the Gibbs free energy is derived
and analyzed numerically. Unstable (saddle-point) solutions are also classified
and discussed.Comment: 17 pages, 4 Postscript figure
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