1,084 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
Direct visualization of iron sheath shielding effect in MgB_2 superconducting wires
Local magneto-optical imaging and global magnetization measurement techniques
were used in order to visualize shielding effects in the superconducting core
of MgB_2 wires sheathed by ferromagnetic iron (Fe). The magnetic shielding can
provide a Meissner-like state in the superconducting core in applied magnetic
fields up to ~1T. The maximum shielding fields are shown to correlate with the
saturation fields of magnetization in Fe-sheaths. The shielding has been found
to facilitate the appearance of an overcritical state, which is capable of
achieving a critical current density (J_c) in the core which is larger than J_c
in the same wire without the sheath by a factor of ~2. Other effects caused by
the magnetic interaction between the sheath and the superconducting core are
discussed.Comment: 4 pages, 3 figure
Substitution induced pinning in MgB_2 superconductor doped with SiC nano-particles
By doping MgB_2 superconductor with SiC nano-particles, we have successfully
introduced pinning sites directly into the crystal lattice of MgB_2 grains
(intra-grain pinning). It became possible due to the combination of
counter-balanced Si and C co-substitution for B, leading to a large number of
intra-granular dislocations and the dispersed nano-size impurities induced by
the substitution. The magnetic field dependence of the critical current density
was significantly improved in a wide temperature range, whereas the transition
temperature in the sample MgB_2(SiC)_x having x = 0.34, the highest doping
level prepared, dropped only by 2.6 K.Comment: 4 pages, 6 figure
Symmetry and Temperature dependence of the Order parameter in MgB2 from point contact measurements
We have performed differential conductance versus voltage measurements of
Au/MgB2 point contacts. We find that the dominant component in the conductance
is due to Andreev reflection. The results are fitted to the theoretical model
of BTK for an s-wave symmetry from which we extract the value of the order
parameter (Delta) and its temperature dependence. From our results we also
obtain a lower experimental bound on the Fermi velocity in MgB2.Comment: 7 pages (Including figure captions) and 4 figure
Soft x-ray spectroscopy measurements of the p-like density of states of B in MgB2 and evidence for surface boron oxides on exposed surfaces
Soft X-ray absorption and fluorescence measurements are reported for the
K-edge of B in MgB2. The measurements confirm a high density of B
pxy(sigma)-states at the Fermi edge and extending to approximately 0.9 eV above
the edge. A strong resonance is observed in elastic scattering through a
core-exciton derived from out-of-plane pz(pi*)-states. Another strong
resonance, observed in both elastic and inelastic spectra, is identified as a
product of surface boron oxides.Comment: 7 pages total, 4 figures, submitted to Phys. Rev. Let
High-Resolution Photoemission Study of MgB2
We have performed high-resolution photoemission spectroscopy on MgB2 and
observed opening of a superconducting gap with a narrow coherent peak. We found
that the superconducting gap is s-like with the gap value of 4.5 meV at 15 K.
The temperature dependence (15 - 40 K) of gap value follows well the BCS form,
suggesting that 2Delta/kBTc at T=0 is about 3. No pseudogap behavior is
observed in the normal state. The present results strongly suggest that MgB2 is
categorized into a phonon-mediated BCS superconductor in the weak-coupling
regime.Comment: 3 pages, 3 figures, accepted in Physical Review Letter
Superconducting thin films of MgB2 on (001)-Si by pulsed laser deposition
Superconducting thin films have been prepared on Si-substrates, using pulsed
laser deposition from a target composed of a mixture of Mg and MgB2 powders.
The films were deposited at room temperature and post-annealed at 600 degrees
C. The zero resistance transition temperatures were 12 K, with an onset
transition temperature of 27 K. Special care has been taken to avoid oxidation
of Mg in the laser plasma and deposited film, by optimizing the background
pressure of Ar gas in the deposition chamber. For this the optical emission in
the visible range from the plasma has been used as indicator. Preventing Mg
from oxidation was found to be essential to obtain superconducting films
Casting Control of Floating-films into Ribbon-shape Structure by modified Dynamic FTM
We have developed a new method to obtain Ribbon-shaped floating films via dynamic casting of floating-film and transfer method (dynamic-FTM). Dynamic-FTM is a unique method to prepare oriented thin-film of conjugated polymers (CPs) which is quick and easy. This method has several advantages as compared to the other conventional casting procedure to prepare oriented CP films. In the conventional dynamic FTM appearance of large scale circular orientation poses difficulty not only for practical applications but also hinders the detailed analysis of the orientation mechanism. In this present work, pros and cons of this newly proposed ribbon-shaped floating-film have been discussed in detail from those of the conventional floating-film prepared by dynamic-FTM.12th International Conference on Nanomolecular Electronics (ICNME-2016), December 14-16, 2016, Kobe International Conference Center, Kobe, Japa
Influence of Rb, Cs and Ba on Superconductivity of Magnesium Diboride
Magnesium diboride has been thermally treated in the presence of Rb, Cs, and
Ba. Magnetic susceptibility shows onsets of superconductivity in the resulting
samples at 52K (Rb), 58K (Cs) and 45K (Ba). Room-temperature 11B NMR indicates
to cubic symmetry of the electric field gradient at boron site for the samples
reacted with Rb and Cs, in contrast to the axial symmetry in the initial MgB2
and in the sample treated with Ba.Comment: 3 pages (twocolumn), 2 figure
Superconducting and Normal State Properties of Neutron Irradiated MgB2
We have performed a systematic study of the evolution of the superconducting
and normal state properties of neutron irradiated MgB wire segments as a
function of fluence and post exposure annealing temperature and time. All
fluences used suppressed the transition temperature, Tc, below 5 K and expanded
the unit cell. For each annealing temperature Tc recovers with annealing time
and the upper critical field, Hc2(T=0), approximately scales with Tc. By
judicious choice of fluence, annealing temperature and time, the Tc of damaged
MgB2 can be tuned to virtually any value between 5 and 39 K. For higher
annealing temperatures and longer annealing times the recovery of Tc tends to
coincide with a decrease in the normal state resistivity and a systematic
recovery of the lattice parameters.Comment: Updated version, to appear in Phys. Rev.
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