15 research outputs found
Surface Enhancement of Superconductivity in Tin
The possibility of surface enhancement of superconductivity is examined
experimentally. It is shown that single crystal tin samples with cold-worked
surfaces represent a superconductor with a surface-enhanced order parameter (or
negative surface extrapolation length b), whose magnitude can be controlled.Comment: 8 pages, 4 figure
Enhancement of the superconducting transition temperature in Nb/Permalloy bilayers by controlling the domain state of the ferromagnet
In (S/F) hybrids the suppression of superconductivity by the exchange field
h_ex of the ferromagnet can be partially lifted when different directions of
h_ex are sampled simultaneously by the Cooper pair. In F/S/F trilayer
geometries where the magnetization directions of the two F-layers can be
controlled separately, this leads to the so-called spin switch. Here we show
that domain walls in a single F-layer yield a similar effect. We study the
transport properties of Ni_0.8Fe_0.2/Nb bilayers structured in strips of
different sizes. For large samples a clear enhancement of superconductivity
takes place in the resistive transition, in the very narrow field range (order
of 0.5 mT) where the magnetization of the Py layer switches and many domains
are present. This effect is absent in microstructured samples. Comparison of
domain wall width \delta_w to the temperature dependent superconductor
coherence length \xi_S(T) shows that \delta_w ~ \xi_S(T), which means that the
Cooper pairs sample a large range of different magnetization directions.Comment: 4 pages, 5 figure
Penetration of the magnetic field into the twinning plane in the type I and II superconductors
It is demonstrated that in the type I and II superconductors with
low-transparent twinning planes (TP) the penetration of external parallel
magnetic field into the region of the twinning plane can be energetically
favorable. In the type I superconductors the twinning planes become similar to
Josephson junctions and the magnetic field penetrates into the center of the TP
in the form of soft Josephson-like vortices. This leads to increase in the
critical magnetic field values. The corresponding phase diagram in the
parameter plane "temperature - magnetic field" essentially differs from the one
obtained without taking the finite value of the magnetic field near the TP into
account. Comparison between obtained phase diagrams and experimental data for
different type I superconductors can allow to estimate the value of the TP
transparency, which is the only fitting parameter in our theory.Comment: 5 pages, 2 figure
Magnetic field induced polarization effects in intrinsically granular superconductors
Based on the previously suggested model of nanoscale dislocations induced
Josephson junctions and their arrays, we study the magnetic field induced
electric polarization effects in intrinsically granular superconductors. In
addition to a new phenomenon of chemomagnetoelectricity, the model predicts
also a few other interesting effects, including charge analogues of Meissner
paramagnetism (at low fields) and "fishtail" anomaly (at high fields). The
conditions under which these effects can be experimentally measured in
non-stoichiometric high-T_c superconductors are discussed.Comment: 10 pages (REVTEX), 5 EPS figures; revised version accepted for
publication in JET
Superconducting properties of [BaCuO_x]_2/[CaCuO_2]_n artificial structures with ultrathick CaCuO_2 blocks
The electrical transport properties of [BaCuO_x]_2/[CaCuO_2]_n
(CBCCO-2xn)underdoped high temperature superconducting superlattices grown by
Pulsed Laser Deposition have been investigated. Starting from the optimally
doped CBCCO-2x2 superlattice, having three CuO_2 planes and T_c around 80 K, we
have systematically increased the number n up to 15 moving toward the
underdoped region and hence decreasing T_c. For n>11 the artificial structures
are no longer superconducting, as expected, for a uniformly distributed charge
carriers density inside the conducting block layer. The sheet resistance of
such artificial structures (n nearly equal to 11) turns out to be quite
temperature independent and close to the 2D quantum resistance 26 kOhm. A
further increase of the number of CuO_2 planes results in an insulator-type
dependence of R(T) in the wide range of temperatures from room temperature to 1
K. The value of the sheet resistance separating the Superconducting and the
Insulating regimes supports the fermionic scenario of the
Superconductor-Insulator transition in these systems.Comment: 12 pages, 5 figures. Corresponding author: [email protected]
Effects of confinement and surface enhancement on superconductivity
Within the Ginzburg-Landau approach a theoretical study is performed of the
effects of confinement on the transition to superconductivity for type-I and
type-II materials with surface enhancement. The superconducting order parameter
is characterized by a negative surface extrapolation length . This leads to
an increase of the critical field and to a surface critical
temperature in zero field, , which exceeds the bulk . When the
sample is {\em mesoscopic} of linear size the surface induces
superconductivity in the interior for .
In analogy with adsorbed fluids, superconductivity in thin films of type-I
materials is akin to {\em capillary condensation} and competes with the
interface delocalization or "wetting" transition. The finite-size scaling
properties of capillary condensation in superconductors are scrutinized in the
limit that the ratio of magnetic penetration depth to superconducting coherence
length, , goes to zero, using analytic
calculations. While standard finite-size scaling holds for the transition in
non-zero magnetic field , an anomalous critical-point shift is found for
H=0. The increase of for H=0 is calculated for mesoscopic films,
cylindrical wires, and spherical grains of type-I and type-II materials.
Surface curvature is shown to induce a significant increase of ,
characterized by a shift inversely proportional to the
radius .Comment: 37 pages, 5 figures, accepted for PR
Vortex Plastic Motion in Twinned Superconductors
We present simulations, without electrodynamical assumptions, of
, and , in hard superconductors, for a variety
of twin-boundary pinning potential parameters, and for a range of values of the
density and strength of the pinning sites. We numerically solve the overdamped
equations of motion of up to 10^4 flux-gradient-driven vortices which can be
temporarily trapped at pinning centers. These simulations relate
macroscopic measurements (e.g., M(H), ``flame'' shaped profiles) with
the underlying microscopic pinning landscape and the plastic dynamics of
individual vortices
STM revealing of twin microlayers with quantized width on cleaved bismuth surface
Scanning tunnelling microscopy of cleaved bismuth surfaces at low temperatures
revealed the existence of identical thin twin interlayers, embedded in the
main crystal. The interlayers were strictly ordered along the surface atomic
rows. They were of the macroscopic lengths of the order of one micrometre. The
width of interlayers was about . It corresponds nearly to the
distance at which the height gained due to the tilt of twin interlayer atomic
lattice with respect to the main crystal surface reaches the interplanar
spacing in the [0001] direction, i.e. . Hence the width
"quantization" is connected with the matching of atomic planes at both
sides of twin interlayer