27,901 research outputs found
A consistent interpretation of the Karmen anomaly
The Karmen anomaly can be interpreted as being due to a heavy neutrino of
mass around 137 MeV produced in decays. This interpretation is
consistent with the present limits on the couplings of such an object.Comment: 11pages, 4figures, Te
Possible acceleration of charged particles through the reconnection of magnetic field lines in interplanetary space
Prominent intensity spikes in the flux of protons and alphas with less than 0.5 MeV per charge were observed in the region several hours behind an interplanetary shock front. The small spatial scale of these events and the high anisotropy of the particle flux suggest local acceleration. The spectra of the particles, which are cut off at equal energy per charge, suggest acceleration through an electric field. The possibility is examined that these events have their origin in active magnetic neutral sheets in the shocked solar wind
High performance NbN nanowire superconducting single photon detectors fabricated on MgO substrates
We demonstrate high-performance nanowire superconducting single photon
detectors (SSPDs) on ultrathin NbN films grown at a temperature compatible with
monolithic integration. NbN films ranging from 150nm to 3nm in thickness were
deposited by dc magnetron sputtering on MgO substrates at 400C. The
superconducting properties of NbN films were optimized studying the effects of
deposition parameters on film properties. SSPDs were fabricated on high quality
NbN films of different thickness (7 to 3nm) deposited under optimal conditions.
Electrical and optical characterizations were performed on the SSPDs. The
highest QE value measured at 4.2K is 20% at 1300nm
Scale invariant correlations and the distribution of prime numbers
Negative correlations in the distribution of prime numbers are found to
display a scale invariance. This occurs in conjunction with a nonstationary
behavior. We compare the prime number series to a type of fractional Brownian
motion which incorporates both the scale invariance and the nonstationary
behavior. Interesting discrepancies remain. The scale invariance also appears
to imply the Riemann hypothesis and we study the use of the former as a test of
the latter.Comment: 13 pages, 8 figures, version to appear in J. Phys.
Mechanism for bipolar resistive switching in transition metal oxides
We introduce a model that accounts for the bipolar resistive switching
phenomenom observed in transition metal oxides. It qualitatively describes the
electric field-enhanced migration of oxygen vacancies at the nano-scale. The
numerical study of the model predicts that strong electric fields develop in
the highly resistive dielectric-electrode interfaces, leading to a spatially
inhomogeneous oxygen vacancies distribution and a concomitant resistive
switching effect. The theoretical results qualitatively reproduce non-trivial
resistance hysteresis experiments that we also report, providing key validation
to our model.Comment: Accepted for publication in Physical Review B, 6 twocolumn pages, 5
figure
Self-consistent microscopic calculations for non-local transport through nanoscale superconductors
We implement self-consistent microscopic calculations in order to describe
out-of-equilibrium non-local transport in normal metal-superconductor-normal
metal hybrid structures in the presence of a magnetic field and for arbitrary
interface transparencies. A four terminal setup simulating usual experimental
situations is described by means of a tight-binding model. We present results
for the self-consistent order parameter and current profiles within the sample.
These profiles illustrate a crossover from a quasi-equilibrium to a strong
non-equilibrium situation when increasing the interface transparencies and the
applied voltages. We analyze in detail the behavior of the non-local
conductance in these two different regimes. While in quasi-equilibrium
conditions this can be expressed as the difference between elastic cotunneling
and crossed Andreev transmission coefficients, in a general situation
additional contributions due to the voltage dependence of the self-consistent
order parameter have to be taken into account. The present results provide a
first step towards a self-consistent theory of non-local transport including
non-equilibrium effects and describe qualitatively a recent experiment [Phys.
Rev. Lett. 97, 237003 (2006)].Comment: 12 pages, 14 figures, 2 figures correcte
Low temperature irreversibility induced by thermal cycles on two prototypical phase separated manganites
We have studied the effect of irreversibility induced by repeated thermal
cycles on the electric transport and magnetization of polycrystalline samples
of La0.5Ca0.5MnO3 and La0.325Pr0.3Ca0.375MnO3. An increase of the resistivity
and a decrease of the magnetization at different temperature ranges after
cycling is obtained in the temperature range between 300 K and 30 K. Both
compounds are known to exhibit intrinsic submicrometric coexistence of phases
and undergo a sequence of phase transitions related to structural changes.
Changes induced by thermal cycling can be partially inhibited by applying
magnetic field and hydrostatic pressure.
Our results suggest that the growth and coexistence of phases with different
structures gives rise to microstructural tracks and strain accommodation,
producing the observed irreversibility. Irrespective of the actual ground state
of each compound, the effect of thermal cycling is towards an increase of the
amount of the insulating phase in both compounds.Comment: to appear in Journal of Alloys and Compounds (2003
Hysteresis Switching Loops in Ag-manganite memristive interfaces
Multilevel resistance states in silver-manganite interfaces are studied both
experimentally and through a realistic model that includes as a main ingredient
the oxygen vacancies diffusion under applied electric fields. The switching
threshold and amplitude studied through Hysteresis Switching Loops are found to
depend critically on the initial state. The associated vacancy profiles further
unveil the prominent role of the effective electric field acting at the
interfaces. While experimental results validate main assumptions of the model,
the simulations allow to disentangle the microscopic mechanisms behind the
resistive switching in metal-transition metal oxide interfaces.Comment: 14 pages, 3 figures, to be published in Jour. of Appl. Phy
Analysis of the feasibility of an experiment to measure carbon monoxide in the atmosphere
The feasibility of measuring atmospheric carbon monoxide from a remote platform using the correlation interferometry technique was considered. It has been determined that CO data can be obtained with an accuracy of 10 percent using this technique on the first overtone band of CO at 2.3 mu. That band has been found to be much more suitable than the stronger fundamental band at 4.6 mu. Calculations for both wavelengths are presented which illustrate the effects of atmospheric temperature profiles, inversion layers, ground temperature and emissivity, CO profile, reflectivity, and atmospheric pressure. The applicable radiative transfer theory on which these calculations are based is described together with the principles of the technique
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