230 research outputs found
Spontaneous magnetization and Hall effect in superconductors with broken time-reversal symmetry
Broken time reversal symmetry (BTRS) in d wave superconductors is studied and
is shown to yield current carrying surface states. The corresponding
spontaneous magnetization is temperature independent near the critical
temperature Tc for weak BTRS, in accord with recent data. For strong BTRS and
thin films we expect a temperature dependent spontaneous magnetization with a
paramagnetic anomaly near Tc. The Hall conductance is found to vanish at zero
wavevector q and finite frequency w, however at finite q,w it has an unusual
structure.Comment: 7 pages, 1 eps figure, Europhysics Letters (in press
Control of spin injection by direct current in lateral spin valves
The spin injection and accumulation in metallic lateral spin valves with
transparent interfaces is studied using d.c. injection current. Unlike
a.c.-based techniques, this allows investigating the effects of the direction
and magnitude of the injected current. We find that the spin accumulation is
reversed by changing the direction of the injected current, whereas its
magnitude does not change. The injection mechanism for both current directions
is thus perfectly symmetric, leading to the same spin injection efficiency for
both spin types. This result is accounted for by a spin-dependent diffusion
model. Joule heating increases considerably the local temperature in the spin
valves when high current densities are injected (80--105 K for
1--2A cm), strongly affecting the spin accumulation.Comment: 6 pages, 5 figure
Planar Superconductor-Normal-Superconductor Josephson Junctions in MgB2
Since the discovery of superconductivity in MgB2 considerable progress has
been made in determining the physical properties of the material, which are
promising for bulk conductors. Tunneling studies show that the material is
reasonably isotropic and has a well-developed s-wave energy gap (∆),
implying that electronic devices based on MgB2 could operate close to 30K.
Although a number of groups have reported the formation of thin films by
post-reaction of precursors, heterostructure growth is likely to require
considerable technological development, making single-layer device structures
of most immediate interest. MgB2 is unlike the cuprate superconductors in that
grain boundaries do not form good Josephson junctions, and although a SQUID
based on MgB2 nanobridges has been fabricated, the nanobridges themselves do
not show junction-like properties. Here we report the successful creation of
planar MgB2 junctions by localised ion damage in thin films. The critical
current (IC) of these devices is strongly modulated by applied microwave
radiation and magnetic field. The product of the critical current and normal
state resistance (ICRN) is remarkably high, implying a potential for very high
frequency applications.Comment: 7 pages including 4 figure
Correlation of tunneling spectra with surface nano-morphology and doping in thin YBa2Cu3O7-delta films
Tunneling spectra measured on thin epitaxial YBa2Cu3O7-delta films are found
to exhibit strong spatial variations, showing U and V-shaped gaps as well as
zero bias conductance peaks typical of a d-wave superconductor. A full
correspondence is found between the tunneling spectra and the surface
morphology down to a level of a unit-cell step. Splitting of the zero bias
conductance peak is seen in optimally-doped and overdoped films, but not in the
underdoped ones, suggesting that there is no transition to a state of broken
time reversal symmetry in the underdoped regimeComment: accepted to ep
Temperature and junction-type dependency of Andreev reflection in MgB2
We studied the voltage and temperature dependency of the dynamic conductance
of normal metal-MgB2 junctions obtained either with the point-contact technique
(with Au and Pt tips) or by making Ag-paint spots on the surface of
high-quality single-crystal-like MgB2 samples. The fit of the conductance
curves with the generalized BTK model gives evidence of pure s-wave gap
symmetry. The temperature dependency of the gap, measured in Ag-paint junctions
(dirty limit), follows the standard BCS curve with 2Delta/kTc = 3.3. In
out-of-plane, high-pressure point contacts we obtained almost ideal Andreev
reflection characteristics showing a single small s-wave gap Delta = 2.6 +/-
0.2 (clean limit). These results support the two-gap model of
superconductivity, the presence of a modified layer at the surface of the
crystals and an important and non-conventional role of the impurities in MgB2.Comment: 5 pages, 4 eps figures, SNS 2001 conferenc
Two-gap superconductivity in MgB: clean or dirty?
A large number of experimental facts and theoretical arguments favor a
two-gap model for superconductivity in MgB. However, this model predicts
strong suppression of the critical temperature by interband impurity scattering
and, presumably, a strong correlation between the critical temperature and the
residual resistivity. No such correlation has been observed. We argue that this
fact can be understood if the band disparity of the electronic structure is
taken into account, not only in the superconducting state, but also in normal
transport
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