298 research outputs found
Nanoscale spin-polarization in dilute magnetic semiconductor (In,Mn)Sb
Results of point contact Andreev reflection (PCAR) experiments on (In,Mn)Sb
are presented and analyzed in terms of current models of charge conversion at a
superconductor-ferromagnet interface. We investigate the influence of surface
transparency, and study the crossover from ballistic to diffusive transport
regime as contact size is varied. Application of a Nb tip to a (In,Mn)Sb sample
with Curie temperature Tc of 5.4 K allowed the determination of
spin-polarization when the ferromagnetic phase transition temperature is
crossed. We find a striking difference between the temperature dependence of
the local spin polarization and of the macroscopic magnetization, and
demonstrate that nanoscale clusters with magnetization close to the saturated
value are present even well above the magnetic phase transition temperature.Comment: 4 page
Heat Resistance of Human Pathogens in Sous-Vide Products Studied in Model Nutrition Media
Sous-vide (French for ʽunder vacuum’) is a professional cooking method, by which, under oxygen-free conditions and precise temperature control, not only cooking but preservation is achieved. During the process the food matrix is vacuum-packed and undergoes a mild heat treatment, thus achieving an enhanced nutrition value and a better organoleptic character. Due to the mild heat treatment (55 to 90 °C), the high water activity, and the slight acidity of raw materials, the microbial quality assurance is a great challenge even for professionals. The heat treatment does not assure the inactivation of pathogen spores. In our experiments we used Clostridium perfringens representing the spore-forming pathogens, and Salmonella Enteritidis as a the food-borne infection bacterium. Effects of various temperatures were measured in normal and sous-vide type vacuum packaging. Higher thermal death rate in vacuum packaging was demonstrated for Salmonella Enteritidis and Clostridium perfringens
Electron Conditioning of Technical Aluminium Surfaces: Effect on the Secondary Electron Yield
The effect of electron conditioning on commercially aluminium alloys 1100 and
6063 were investigated. Contrary to the assumption that electron conditioning,
if performed long enough, can reduce and stabilize the SEY to low values (, value of many pure elements), the SEY of aluminium did not go lower than
1.8. In fact, it reincreases with continued electron exposure dose.Comment: 36 pages, 25 figures, submitted to JVST
Microwave Surface Impedance of YBCO:123 crystals: Experiment and comparison to a d-wave model
We present measurements of the microwave surface resistance Rs and the
penetration depth lambda of YBCO:123 crystals. At low T obeys lambda(T) a
polynomial behavior, while Rs displays a characteristic non-monotonic
T-dependence.
A detailed comparison of the experimental data is made to a model of d-wave
superconductivity which includes both elastic and inelastic scattering. While
the model reproduces the general features of the experimental data, three
aspects of the parameters needed are worth noting. The elastic scattering rate
required to fit the data is much smaller than measured from the normal state,
the scattering phase shifts have to be close to pi/2 and a strong coupling
value of the gap parameter 2\Delta(0)/kTc = 6 is needed. On the experimental
side the uncertainties regarding the material parameters lambda(0) and
Rs,res(0) further complicate a quantitative comparison.
For one sample does Rs,res(0) agree with the intrinsic value which results
from the d-wave model.Comment: uuencoded tar.Z, 11 pages with 5 figures, used style files: elsart
and graphicx, PS-file available at http://sagar.cas.neu.edu/preprints.htm
Transport properties and point contact spectra of Ni_xNb_{1-x} metallic glasses
Bulk resistivity and point contact spectra of Ni_xNb_{1-x} metallic glasses
have been investigated as functions of temperature (0.3-300K) and magnetic
field (0-12T). Metallic glasses in this family undergo a superconducting phase
transition determined by the Nb concentration. When superconductivity was
suppressed by a strong magnetic field, both the bulk sample R(T) and the point
contact differential resistance curves of Ni_xNb_{1-x} showed logarithmic
behavior at low energies, which is explained by a strong electron - "two level
system" coupling. We studied the temperature, magnetic field and contact
resistance dependence of Ni_{44}Nb_{56} point-contact spectra in the
superconducting state and found telegraph-like fluctuations superimposed on
superconducting characteristics. These R(V) characteristics are extremely
sensitive detectors for slow relaxing "two level system" motion.Comment: 4 pages, 5 figure
Infrared Conductivity in Layered -wave Superconductors
We calculate the infrared conductivity of a stack of coupled, two-dimensional
superconducting planes within the Fermi liquid theory of superconductivity. We
include the effects of random scattering processes and show that the presence
of even a small concentration of resonant impurities, in a -wave
superconductor, has an important effect on both the in-plane and -axis
transport properties, which could serve as signatures for -wave pairing.Comment: 18 pages in a RevTex (3.0) file plus 5 postscript figures
(uuencoded). Replaced with minor changes as it will appear in the Physical
Review B {\bf 52} issue 1 Oct. 199
Identification of the bulk pairing symmetry in high-temperature superconductors: Evidence for an extended s-wave with eight line nodes
we identify the intrinsic bulk pairing symmetry for both electron and
hole-doped cuprates from the existing bulk- and nearly bulk-sensitive
experimental results such as magnetic penetration depth, Raman scattering,
single-particle tunneling, Andreev reflection, nonlinear Meissner effect,
neutron scattering, thermal conductivity, specific heat, and angle-resolved
photoemission spectroscopy. These experiments consistently show that the
dominant bulk pairing symmetry in hole-doped cuprates is of extended s-wave
with eight line nodes, and of anisotropic s-wave in electron-doped cuprates.
The proposed pairing symmetries do not contradict some surface- and
phase-sensitive experiments which show a predominant d-wave pairing symmetry at
the degraded surfaces. We also quantitatively explain the phase-sensitive
experiments along the c-axis for both Bi_{2}Sr_{2}CaCu_{2}O_{8+y} and
YBa_{2}Cu_{3}O_{7-y}.Comment: 11 pages, 9 figure
Evidence for coexistence of the superconducting gap and the pseudo - gap in Bi-2212 from intrinsic tunneling spectroscopy
We present intrinsic tunneling spectroscopy measurements on small
BiSrCaCuO mesas. The tunnel conductance curves show both
sharp peaks at the superconducting gap voltage and broad humps representing the
-axis pseudo-gap. The superconducting gap vanishes at , while the
pseudo-gap exists both above and below . Our observation implies that the
superconducting and pseudo-gaps represent different coexisting phenomena.Comment: 5 pages, 4 figure
Connective neck evolution and conductance steps in hot point contacts
Dynamic evolution of the connective neck in Al and Pb mechanically
controllable break junctions was studied during continuous approach of
electrodes at bias voltages V_b up to a few hundred mV. A high level of power
dissipation (10^-4 - 10^-3 W) and high current density (j > 10^10 A/cm^2) in
the constriction lead to overheating of the contact area, electromigration and
current-enhanced diffusion of atoms out of the "hot spot". At a low electrode
approach rate (10 - 50 pm/s) the transverse dimension of the neck and the
conductance of the junction depend on V_b and remain nearly constant over the
approach distance of 10 - 30 nm. For V_b > 300 mV the connective neck consists
of a few atoms only and the quantum nature of conductance manifests itself in
abrupt steps and reversible jumps between two or more levels. These features
are related to an ever changing number of individual conductance channels due
to the continuous rearrangement in atomic configuration of the neck, the
recurring motion of atoms between metastable states, the formation and breaking
of isolated one-atom contacts and the switching between energetically
preferable neck geometries.Comment: 21 pages 10 figure
First experimental evidence of one-dimensional plasma modes in superconducting thin wires
We have studied niobium superconducting thin wires deposited onto a
SrTiO substrate. By measuring the reflection coefficient of the wires,
resonances are observed in the superconducting state in the 130 MHz to 4 GHz
range. They are interpreted as standing wave resonances of one-dimensional
plasma modes propagating along the superconducting wire. The experimental
dispersion law, versus , presents a linear dependence over the
entire wave vector range. The modes are softened as the temperature increases
close the superconducting transition temperature. Very good agreement are
observed between our data and the dispersion relation predicted by Kulik and
Mooij and Sch\"on.Comment: Submitted to Physical review Letter
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