110 research outputs found
Superconducting gap parameters of MgB2 obtained on MgB2/Ag and MgB2/In junctions
MgB2 superconducting wires with critical temperature Tc approaching 40 K were
used for preparation of MgB2/Ag and MgB2/In junctions. The differential
conductance vs. voltage characteristics of N-S junctions exhibit clear
contribution of Andreev reflection. Using modified BTK theory for s-wave
superconductors two order parameters 4 meV and 2.6 meV have been determined
from temperature dependencies. Surprisingly, larger order parameter vanishes at
lower temperature ~20 K than smaller one with Tc 38 K. Both the magnitudes of
the order parameters and their critical temperatures are in good agreement with
theoretical calculations of electron-phonon coupling in MgB carried out by
Liu et al.Comment: revised manuscrip
Studies of resistance switching effects in metal/YBa2Cu3O7-x interface junctions
Current-voltage characteristics of planar junctions formed by an epitaxial
c-axis oriented YBa2Cu3O7-x thin film micro-bridge and Ag counter-electrode
were measured in the temperature range from 4.2 K to 300 K. A hysteretic
behavior related to switching of the junction resistance from a high-resistive
to a low-resistive state and vice-versa was observed and analyzed in terms of
the maximal current bias and temperature dependence. The same effects were
observed on a sub-micrometer scale YBa2Cu3O7-x thin film - PtIr point contact
junctions using Scanning Tunneling Microscope. These phenomena are discussed
within a diffusion model, describing an oxygen vacancy drift in YBa2Cu3O7-x
films in the nano-scale vicinity of the junction interface under applied
electrical fields.Comment: To be published in Applied Surface Science
MgB2 radio-frequency superconducting quantum interference device prepared by atomic force microscope lithography
A new method of preparation of radio-frequency superconducting quantum
interference devices on MgB2 thin films is presented. The variable-thickness
bridge was prepared by a combination of optical lithography and of the
scratching by an atomic force microscope. The critical current of the
nanobridge was 0.35 uA at 4.2 K. Non-contact measurements of the current-phase
characteristics and of the critical current vs. temperature have been
investigated on our structures.Comment: RevTeX4. Accepted in Appl. Phys. Let
Superconducting properties of MgB2 thin films prepared on flexible plastic substrates
Superconducting MgB2 thin films were prepared on 50-micrometer-thick,
flexible polyamide Kapton-E foils by vacuum co-deposition of Mg and B
precursors with nominal thickness of about 100 nm and a special ex-situ rapid
annealing process in an Ar or vacuum atmosphere. In the optimal annealing
process, the Mg-B films were heated to approximately 600 C, but at the same
time, the backside of the structures was attached to a water-cooled radiator to
avoid overheating of the plastic substrate. The resulting MgB2 films were
amorphous with the onset of the superconducting transition at T_(c,on) about 33
K and the transition width of approximately 3 K. The critical current density
was > 7x10^5 A/cm^2 at 4.2 K, and its temperature dependence indicated a
granular film composition with a network of intergranular weak links. The films
could be deposited on large-area foils (up to 400 cm^2) and, after processing,
cut into any shapes (e.g., stripes) with scissors or bent multiple times,
without any observed degradation of their superconducting properties.Comment: 3 figure
Time-resolved dynamics of the superconducting two-gap state in MgB2 thin films
Femtosecond pump-probe studies show that carrier dynamics in MgB2 films is
governed by the sub-ps electron-phonon (e-ph) relaxation present at all
temperatures, the few-ps e-ph process well pronounced below 70 K, and the
sub-ns superconducting relaxation below Tc. The amplitude of the
superconducting component versus temperature follows the superposition of the
isotropic dirty gap and the 3-dimensional gap dependences, closing at two
different Tc values. The time constant of the few-ps relaxation exhibits a
double-divergence at temperatures corresponding to the Tc's of the two gaps.Comment: Accepted for publication in Phys.Rev.Let
Low-temperature performance of semiconducting asymmetric nanochannel diodes
We present our studies on fabrication and electrical and optical characterization of
semiconducting asymmetric nanochannel diodes (ANCDs), focusing mainly on the temperature dependence of their current–voltage (I–V) characteristics in the range from room temperature to
77 K. These measurements enable us to elucidate the electron transport mechanism in a nanochannel. Our test devices were fabricated in a GaAs/AlGaAs heterostructure with a twodimensional
electron gas layer and were patterned using electron-beam lithography. The 250-nmwide, 70-nm-deep trenches that define the nanochannel were ion-beam etched using the photoresist as a mask, so the resulting nanostructure consisted of approximately ten ANCDs
connected in parallel with 2-μm-long, 230-nm-wide nanochannels. The ANCD I–V curves
collected in the dark exhibited nonlinear, diode-type behavior at all tested temperatures. Their
forward-biased regions were fitted to the classical diode equation with a thermionic barrier, with the ideality factor n and the saturation current as fitting parameters. We have obtained very good
fits, but with n as large as ~50, suggesting that there must be a substantial voltage drop likely at
the contact pads. The thermionic energy barrier was determined to be 56 meV at high temperatures. We have also observed that under optical illumination our ANCDs at low temperatures exhibited, at low illumination powers, a very strong photoresponse enhancement that exceeded that at room temperature. At 78 K, the responsivity was of the order of 104 A/W at the nW-level light excitation
Phonons in MgB2 by Polarized Raman Scattering on Single Crystals
The paper presents detailed Raman scattering study of the unusually broad E2g
phonon mode in MgB2 crystal. For the first time, it is shown by the polarized
Raman scattering on few-micron-size crystallites with natural faces that the
observed broad Raman feature really does obey the selection rules of an E2g
mode. Raman spectra on high quality polycrystalline superconducting MgB2 wires
reveal a very symmetric E2g phonon line near 615 1/cm with the room temperature
linewidth of 260 1/cm only. Additional scattering of different polarization
dependence, observed in certain crystallites is interpreted as weighted phonon
density of states induced by lattice imperfections.Comment: 4 pages + 7 figure
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