210 research outputs found
Specific heat of single crystal MgB_2: a two-band superconductor with two different anisotropies
Heat-capacity measurements of a 39 microgramm MgB_2 single crystal in fields
up to 14 T and below 3 K allow the determination of the low-temperature linear
term of the specific heat, its field dependence and its anisotropy. Our results
are compatible with two-band superconductivity, the band carrying the small gap
being isotropic, that carrying the large gap having an anisotropy of ~ 5. Three
different upper critical fields are thus needed to describe the superconducting
state of MgB2.Comment: 4 pages, 4 figures - V2: Bibliography updated and some typo
corrected. One reference added - V3: version accepted for publication in PRL,
changes made in the tex
Finger patterns produced by thermomagnetic instability in superconductors
A linear analysis of thermal diffusion and Maxwell equations is applied to
study the thermomagnetic instability in a type-II superconducting slab. It is
shown that the instability can lead to formation of spatially nonuniform
distributions of magnetic field and temperature. The distributions acquire a
finger structure with fingers perpendicular to the screening current direction.
We derive the criterion for the instability, and estimate its build-up time and
characteristic finger width. The fingering instability emerges when the
background electric field is larger than a threshold field, , and the
applied magnetic field exceeds a value . Numerical
simulations support the analytical results, and allow to follow the development
of the fingering instability beyond the linear regime. The fingering
instability may be responsible for the nucleation of dendritic flux patterns
observed in superconducting films using magneto-optical imaging.Comment: 8 pages, 6 figures, accepted to Phys. Rev. B; (new version: minor
changes
Superconducting anisotropy and evidence for intrinsic pinning in single crystalline MgB
We examine the superconducting anisotropy
of a metallic high- superconductor MgB by measuring the magnetic
torque of a single crystal. The anisotropy does not depend
sensitively on the applied magnetic field at 10 K. We obtain the anisotropy
parameter . The torque curve shows the sharp
hysteresis peak when the field is applied parallel to the boron layers. This
comes from the intrinsic pinning and is experimental evidence for the
occurrence of superconductivity in the boron layers.Comment: REVTeX 4, To be published in Physical Review
Evidence for two-dimensional nucleation of superconductivity in MgB
According to the crystal structure of MgB and band structure
calculations quasi-two-dimensional (2D) boron planes are responsible for the
superconductivity. We report on critical fields and resistance measurements of
30 nm thick MgB films grown on MgO single crystalline substrate. A linear
temperature dependence of the parallel and perpendicular upper critical fields
indicate a 3D-like penetration of magnetic field into the sample. Resistivity
measurements, in contrast, yield a temperature dependence of fluctuation
conductivity above T which agrees with the Aslamazov-Larkin theory of
fluctuations in 2D superconductors. We consider this finding as an experimental
evidence of two-dimensional nucleation of superconductivity in MgB.Comment: 5 RevTex pages, 3 PostScript Figures ZIPed in archive Sidoren.zip.
Submitted to EuroPhys. Lett. December 3, 200
Thin film growth by pulsed laser deposition and properties of 122-type iron-based superconductor AE(Fe1--xCox)2As2 (AE = alkaline earth)
This paper reports comprehensive results on thin-film growth of 122-type
iron-pnictide superconductors, AE(Fe1-xCox)2As2 (AE = Ca, Sr, and Ba,
AEFe2As2:Co) by a pulsed laser deposition method using a neodymium-doped
yttrium aluminum garnet laser as an excitation source. The most critical
parameter to produce the SrFe2As2:Co and BaFe2As2:Co phases is the substrate
temperature (Ts). It is difficult to produce highly-pure CaFe2As2:Co phase thin
film at any Ts. For BaFe2As2:Co epitaxial films, controlling Ts at 800-850
{\deg}C and growth rate to 2.8-3.3 {\AA}/s produced high-quality films with
good crystallinity, flat surfaces, and high critical current densities > 1
MA/cm2, which were obtained for film thicknesses from 100 to 500 nm. The doping
concentration x was optimized for Ba(Fe1-xCox)2As2 epitaxial films, leading to
the highest critical temperature of 25.5 K in the epitaxial films with the
nominal x = 0.075.Comment: will be published in the special issue of Superconductor Science and
Technology, `Iron12
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
Electron-phonon interaction in transition metal diborides TB_2 (T=Zr, Nb, Ta) studied by point-contact spectroscopy
The electron-phonon interaction (EPI) in transition metal diborides TB_2
(T=Zr, Nb, Ta) is investigated by point-contact (PC) spectroscopy. The PC EPI
functions were recovered and the EPI parameters lambda<0.1 were estimated for
all three compounds. Common and distinctive features between the EPI functions
for those diborides are discussed also in connection with the superconductivity
in MgB_2.Comment: V2: minor changes, Ref.[21] added, publ. in PR
Effects of Al doping on the structural and electronic properties of Mg(1-x)Al(x)B2
We have studied the structural and electronic properties of Mg(1-x)Al(x)B2
within the Virtual Crystal Approximation (VCA) by means of first-principles
total-energy calculations. Results for the lattice parameters, the electronic
band structure, and the Fermi surface as a function of Al doping for 0<x<0.6
are presented. The ab initio VCA calculations are in excellent agreement with
the experimentally observed change in the lattice parameters of Al doped MgB2.
The calculations show that the Fermi surface associated with holes a the boron
planes collapses gradually with aluminum doping and vanishes for x=0.56. In
addition, an abrupt topological change in the sigma-band Fermi surface was
found for x=0.3. The calculated hole density correlates closely with existing
experimental data for Tc(x), indicating that the observed loss of
superconductivity in Mg(1-x)Al(x)B2 is a result of hole bands filling.Comment: 4 pages (revtex) and 4 figures (postscript
Assembling the puzzle of superconducting elements: A Review
Superconductivity in the simple elements is of both technological relevance
and fundamental scientific interest in the investigation of superconductivity
phenomena. Recent advances in the instrumentation of physics under pressure
have enabled the observation of superconductivity in many elements not
previously known to superconduct, and at steadily increasing temperatures. This
article offers a review of the state of the art in the superconductivity of
elements, highlighting underlying correlations and general trends.Comment: Review, 10 pages, 11 figures, 97 references; to appear in Superc.
Sci. Techno
Carbon-substitution dependent multiple superconducting gap of MgB: a "sub-meV" resolution photoemission study
``Sub-meV'' resolution photoemission spectroscopy was used to study
carbon-substitution dependence on the multiple superconducting gap of Mg(BC. Two features corresponding to and gaps are
clearly observed in the raw spectra up to carbon concentration x = 7.5 {%}. The
observed x dependence of the two gaps shows a qualitatively different behavior:
a marked change of the gap proportional to the variation and
a negligible one of the gap. This as well as the temperature dependence
can be explained with the two-band mean-field theory. Implications from the
present study are discussed.Comment: 4 figures, PDF onl
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