66 research outputs found
Universal scaling of the Hall resistivity in MgB2 superconductors
The mixed-state Hall resistivity and the longitudinal resistivity in
superconducting MgB2 thin films have been investigated as a function of the
magnetic field over a wide range of current densities from 100 to 10000 A/cm^2.
We observe a universal Hall scaling behavior with a constant exponent of 2.0,
which is independent of the magnetic field, the temperature, and the current
density. This result can be interpreted well within the context of recent
theories.Comment: 4 page
Temperature- and magnetic-field-dependent resistivity of MgB2 sintered at high temperature and high pressure condition
We report the temperature- and magnetic-field-dependent resistivity of MgB2
sintered at high temperature and high pressure condition. The superconducting
transition width for the resistivity measurement was about 0.4 K, and the
low-field magnetization showed a sharp superconducting transition with a
transition width of about 1 K. The resistivity in the normal state roughly
followed T^2 behavior with smaller residual resistivity ratio (RRR) of 3 over
broad temperature region above 100 K rather than reported T^3 behavior with
larger RRR value of ~ 20 in the samples made at lower pressures. Also, the
resistivity did not change appreciably with the applied magnetic field, which
was different from previous report. These differences were discussed with the
microscopic and structural change due to the high-pressure sintering.Comment: 2 pages, 3 figures. Accepted by Physica
Temperature-scaling behavior of the Hall conductivity for Hg-based superconducting thin films
The Hall conductivities of HgBa_{2}CaCu_{2}O_{6+\delta}and
HgBa_{2}Ca_{2}Cu_{3}O_{8+\delta} thin films are investigated for a magnetic
field parallel to the c axis. The mixed-state Hall conductivity for these
compounds is well described by \sigma_{xy}=C_{1}/H+C_{2}+C_{3}H. The prefactor
C_1 shows a temperature dependence of the form C_1 = A(1-t)^n near T_c, where
t=T/T_c is the reduced temperature. Contrary to the previous results, C_2 also
follows a temperature-scaling behavior similar to that of the coefficient C_1.
The observed value of n = 1.8 - 2.3 is comparable to the previously observed
values for YBa_{2}Cu_{3}O_{7-\delta} and La_{2-x}Sr_{x}CuO_{4}.Comment: 5 pages, 4 eps figure
Critical flux pinning and enhanced upper-critical-field in magnesium diboride films
We have conducted pulsed transport measurements on c-axis oriented magnesium
diboride films over the entire relevant ranges of magnetic field 0 \alt H \alt
H_{c2} (where \hcu is the upper critical field) and current density 0 \alt j
\alt j_{d} (where is the depairing current density). The intrinsic
disorder of the films combined with the large coherence length and
three-dimensionality, compared to cuprate superconductors, results in a
six-fold enhancement of and raises the depinning current density
to within an order of magnitude of . The current-voltage
response is highly non-linear at all fields, resulting from a combination of
depinning and pair-breaking, and has no trace of an Ohmic free-flux-flow
regime.
Keywords: pair, breaking, depairing, superconductor, superconductivity, flux,
fluxon, vortex, mgb
In-plane Hall effect in c-axis-oriented MgB2 thin films
We have measured the longitudinal resistivity and the Hall resistivity in the
ab-plane of highly c-axis-oriented MgB2 thin films. In the normal state, the
Hall coefficient (R_H) behaves as R_H ~ T with increasing temperature (T) up to
130 K and then deviates from that linear T-dependence at higher temperatures.
The T^2 dependence of the cotangent of the Hall angle is only observed above
130 K. The mixed-state Hall effect reveals no sign anomaly over a wide range of
current densities from 10^2 to 10^4 A/cm^2 and for magnetic fields up to 5 T.Comment: 5 pages including 5 figure
Far-infrared transmission studies of c-axis oriented superconducting MgB2 thin film
We reported far-infrared transmission measurements on a c-axis oriented
superconducting MgB thin film in the frequency range of 30 250
cm. We found that these measurements were sensitive to values of
scattering rate and superconducting gap . By fitting the
experimental transmission spectra at 40 K and below, we obtained
(700 1000) cm and 42 cm. These two
quantities suggested that MgB belong to the dirty limit.Comment: submitted at May
Thermoelectric power of MgBBe
We investigated thermoelectric power of MgBBe (,
0.2, 0.3, 0.4, and 0.6). decreases systematically with , suggesting
that the hole density increases. Our band calculation shows that the increase
occurs in the -band. With the hole-doping, decreases.
Implication of this phenomenon is discussed within the BCS framework. While the
Mott formula explains only the linear part of at low temperature,
incorporation of electron-phonon interaction enables us to explain over
wide temperature range including the anomalous behavior at high temperature.Comment: 4 pages, 4 figure
Suppression of Superconducting Critical Current Density by Small Flux Jumps in Thin Films
By doing magnetization measurements during magnetic field sweeps on thin
films of the new superconductor , it is found that in a low temperature
and low field region small flux jumps are taking place. This effect strongly
suppresses the central magnetization peak leading to reduced nominal
superconducting critical current density at low temperatures. A borderline for
this effect to occur is determined on the field-temperature (H-T) phase
diagram. It is suggested that the small size of the flux jumps in films is due
to the higher density of small defects and the relatively easy thermal
diffusion in thin films in comparison with bulk samples.Comment: 7 figures Phys. Rev. B accepted scheduled issue: 01 Feb 200
Coherence lengths and anisotropy in MgB2 superconductor
Field and temperature microwave measurements have been carried out on MgB2
thin film grown on Al2O3 substrate. The analysis reveals the mean field
coherence length xi_{MF} in the mixed state and a temperature independent
anisotropy ratio gamma_{MF} = xi_{MF}^{ab} / xi_{MF}^c approximately 2. At the
superconducting transition, the scaling of the fluctuation conductivity yields
the Ginzburg-Landau coherence length with a different anisotropy ratio
gamma_{GL} = 2.8, also temperature independent.Comment: submitted to PR
A first-principles study of MgB2 (0001) surfaces
We report self-consistent {\it ab initio} calculations of structural and
electronic properties for the B- and Mg-terminated MgB (0001) surfaces.
We employ ultra-soft pseudopotentials and plane wave basis sets within the
generalized gradient approximation. The surface relaxations are found to be
small for both B- and Mg-terminated surfaces. For the B-terminated surface,
both B and surface bands appear, while only one B
surface band exists near the Fermi level for the Mg-terminated surface. The
superconductivity of the MgB surfaces is discussed. The work function is
predicted to be 5.95 and 4.25 eV for the B- and Mg-terminated surfaces
respectively. The simulated scanning tunneling microscopy images of the
surfaces are not sensitive to the sign and value of the bias voltages, but
depend strongly on the tip-sample distance. An image reversal is predicted for
the Mg-terminated surface.Comment: 3 pages, 4 figures, Revte
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