22 research outputs found
Anisotropies of the lower and upper critical fields in MgB single crystals
The temperature dependence of the London penetration depth () and
coherence length () has been deduced from Hall probe magnetization
measurements in high quality MgB single crystals in the two main
crystallographic directions. We show that, in contrast to conventional
superconductors, MgB is characterized by two different anisotropy
parameters ( and ) which strongly differ at low temperature and merge at .
These results are in very good agreement with recent calculations in weakly
coupled two bands suprerconductors (Phys. Rev. B, 66, 020509(R) (2002).Comment: 4 pages, 4 figure
Unconventional superconductivity in YNi2B2C
We use the semi-classical (Doppler shift) approximation to calculate magnetic
field angle-dependent density of states and thermal conductivity kappa_zz for a
superconductor with a quasi-two-dimensional Fermi surface and line nodes along
k_x=0 and k_y=0. The results are shown to be in good quantitative agreement
with experimental results obtained for YNi2B2C.Comment: 5 pages, 3 figure
Hc2 of anisotropy two-band superconductors by Ginzburg-Landau approach
The purpose of this research is to study the upper critical field (Hc2) of
two-band superconductors by two-band Ginzburg - Landau approach. The analytical
formula of Hc2 included anisotropy of order parameter and anisotropy of
effective-mass are found . The parameters of the upper critical field in
ab-plane and c-axis can be found by fitting to the experimental data . Finally,
we can find the ratio of upper critical field that temperature dependent in the
range of experimental result .Comment: 10 pages. accepted by Physica
Ginzburg-Landau theory of vortices in a multi-gap superconductor
The Ginzburg-Landau functional for a two-gap superconductor is derived within
the weak-coupling BCS model. The two-gap Ginzburg-Landau theory is, then,
applied to investigate various magnetic properties of MgB2 including an upturn
temperature dependence of the transverse upper critical field and a core
structure of an isolated vortex. Orientation of vortex lattice relative to
crystallographic axes is studied for magnetic fields parallel to the c-axis. A
peculiar 30-degree rotation of the vortex lattice with increasing strength of
an applied field observed by neutron scattering is attributed to the multi-gap
nature of superconductivity in MgB2.Comment: 11 page
B NMR study of pure and lightly carbon doped MgB superconductors
We report a B NMR line shape and spin-lattice relaxation rate
() study of pure and lightly carbon doped MgBC for
, 0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe.
We show that while pure MgB exhibits the magnetic field distribution from
superposition of the normal and the Abrikosov state, slight replacement of
boron with carbon unveils the magnetic field distribution of the pure Abrikosov
state. This indicates a considerable increase of with carbon doping
with respect to pure MgB. The spin-lattice relaxation rate
demonstrates clearly the presence of a coherence peak right below in pure
MgB, followed by a typical BCS decrease on cooling. However, at
temperatures lower than K strong deviation from the BCS behavior is
observed, probably from residual contribution of the vortex dynamics. In the
carbon doped systems both the coherence peak and the BCS temperature dependence
of weaken, an effect attributed to the gradual shrinking of the
hole cylinders of the Fermi surface with electron doping.Comment: 8 pages, 6 figures, submitted to Phys. Rev.
MgB2 single crystals: high pressure growth and anisotropic properties
Single crystals of MgB2 with a size up to 1.5x0.9x0.2 mm3 have been grown
with a high pressure cubic anvil technique. The crystal growth process is very
peculiar and involves an intermediate nitride, namely MgNB9. Single crystals of
BN and MgB2 grow simultaneously by a peritectic decomposition of MgNB9.
Magnetic measurements in fields of 1-5 Oe show sharp transitions to the
superconducting state at 37-38.6 K with width of ~0.5 K. The high quality of
the crystals allowed the accurate determination of magnetic, transport and
optical properties as well as scanning tunnelling spectroscopy (STS) and
decoration studies. Investigations of crystals with torque magnetometry show
that Hc2//c is very low (24 kOe at 15 K), while Hc2//ab increases up to 140 kOe
at 15 K. The upper critical field anisotropy gamma = Hc2//ab/ Hc2//c was found
to be temperature dependent (decreasing from 6 at 15 K to 2.8 at 35 K). The
effective anisotropy gamma_eff, as calculated from reversible torque data near
Tc, is field dependent (increasing roughly linearly from 2 in zero field to 3.7
in 10 kOe). The temperature and field dependence of the anisotropy can be
related to the double gap structure of MgB2 with a large two-dimensional gap
and small three-dimensional gap, the latter being rapidly suppressed in a
magnetic field. Torque magnetometry investigations show a pronounced peak
effect, indicating an order-disorder transition of vortex matter. Decoration
experiments and STS visualise a hexagonal vortex lattice. STS spectra evidence
two gaps (3 meV/6 meV) with direction dependent weight. Magneto-optic
investigations with H//c show a clear signature of the smaller of the two gaps,
disappearing in fields higher than Hc2//c.Comment: 17 pages pdf only, 15 figures integrated (higher resolution
photographs available on request); submitted to Supercond. Sci. Technol.
(Proceedings of Boromag conference
The Upper Critical Field Hc2 in Advanced Superconductors with Anisotropic Energy Spectrum
A brief review of works on the microscopic theory of determining the upper
critical field in two-band isotropic and anisotropic superconductors is given.
The research is based on a set of the Ginzburg-Landau equations for the order
parameters in a magnetic field that are studied in terms of the classical
approach to a superconducting system in a magnetic field. Two inequivalent
energy bands with different topology of Fermi surface cavities overlapping on
the Fermi surface are discussed. The cases of the direction of the external
magnetic field H\to// the (ab) plane and H\to // the crystallographic c axis
are studied. The equations for determining Hc2(ab) and Hc2(c) for a pure
superconductor and a superconductor doped with electrons and holes are derived.
The analytical solutions to these equations in the vicinity of the
superconducting transition temperature (Tc - T<<Tc) and in the vicinity of zero
(T<<Tc) are found. The temperature and impurity dependences of the upper
critical fields Hc2(ab) and Hc2(c), as well as the anisotropy coefficient
\gammaH, are studied. The resulting theory is applied to determine the
dependences of the above magnetic characteristics of intermetallic compound
MgB2. The theory agrees qualitatively with experimental data.Comment: 24 pages, 6 fi
Nonlocal effects in angular dependence of in-plane magnetization of tetragonal superconductors
The reversible magnetization M of Lu(Ni1-xCox)(2)B2C with x = 0 and 0.06 is measured in a broad temperature domain as a function of field orientation theta in the basal crystal plane. The data are interpreted within London theory extended for nonlocality of the current-field relation in superconductors. The dependence M (theta) diminishes on warming, vanishes at T* T*. The low-T sign is opposite to what is expected from the known angular dependence of the upper critical field and therefore cannot be attributed to the latter. Upon doping with Co, the effect disappears with decreasing mean-free path in agreement with theory.This article is published as Kogan, V. G., S. L. Bud’ko, P. C. Canfield, and P. Miranović. "Nonlocal effects in angular dependence of in-plane magnetization of tetragonal superconductors." Physical Review B 60, no. 18 (1999): R12577.
DOI: 10.1103/PhysRevB.60.R12577.
Copyright 1999 American Physical Society.
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