62 research outputs found
Scaling of the superfluid density in severely underdoped YBa2Cu3O6+y
Recent measurements on extremely-underdoped YBa2Cu3O6+y [Phys. Rev. Lett. 99,
237003 (2007)] have allowed the critical temperature (T_c), superfluid density
[rho_0 (T << T_c)] and dc conductivity [sigma_dc (T ~ T_c)] to be determined
for a series of electronic dopings for T_c ~ 3 - 17 K. The general scaling
relation rho_0/8 ~ 4.4 sigma_dc T_c is observed, extending the validity of both
the ab-plane and c-axis scaling an order of magnitude and creating a region of
overlap. This suggests that severely underdoped materials may constitute a
Josephson phase; as the electronic doping is increased a more uniform
superconducting state emerges.Comment: 5 pages, 3 figure
Flux quantization for a vortex in two-gap superconductor
Contrary to recent theoretical prediction, we show that the magnetic flux of
a vortex in SU(2) model of two-gap superconductor is quantized in units of
2\pi/g, not 4 \pi/g. For the U(1) version of this model, the flux is quantized
in units of 2 \pi\alpha/g where 0 < \alpha < 1. The parameter \alpha depends on
the masses and concentrations of the Cooper pairs in the two condensates.Comment: 7 page
Gravitational Field of a Spinning Cosmic String
We study the effect of internal space rotation on the gravitational
properties of infinite straight and stationary cosmic string. From the
approximate solution of Einsten equations for the spinning Q-lump string we
obtain long range gravitational accelleration resembling that of a rotating
massive cylindrical shell. We also compute the angular velocity of the inertial
frame dragging and the angle of light deflection by the Q-lump string. Matter
accretion on to spinning strings can play a role in galaxy formation when the
angular velocity times the string width is comparable to the speed of light.Comment: 14 page
Superconductivity in silicon nanostructures
We present the findings of the superconductivity observed in the silicon
nanostructures prepared by short time diffusion of boron on the n-type Si(100)
surface. These Si-based nanostructures represent the p-type ultra-narrow
self-assembled silicon quantum wells, 2nm, confined by the delta - barriers
heavily doped with boron, 3nm. The EPR and the thermo-emf studies show that the
delta - barriers appear to consist of the trigonal dipole centres, which are
caused by the negative-U reconstruction of the shallow boron acceptors. Using
the CV and thermo-emf techniques, the transport of two-dimensional holes inside
SQW is demonstrated to be accompanied by single-hole tunneling through these
negative-U centres that results in the superconductivity of the delta -
barriers. The values of the correlation gaps obtained from these measurements
are in a good agreement with the data derived from the temperature and magnetic
field dependencies of the magnetic susceptibility, which reveal a strong
diamagnetism and additionally identify the superconductor gap value.Comment: 4 pages, 6 figures, presented at the 4th International Conference on
Vortex Matter in Superconductors, Crete, Greece, September 3-9, 200
Effective Vortex Mass from Microscopic Theory
We calculate the effective mass of a single quantized vortex in the BCS
superconductor at finite temperature. Based on effective action approach, we
arrive at the effective mass of a vortex as integral of the spectral function
divided by over frequency. The spectral function is
given in terms of the quantum-mechanical transition elements of the gradient of
the Hamiltonian between two Bogoliubov-deGennes (BdG) eigenstates. Based on
self-consistent numerical diagonalization of the BdG equation we find that the
effective mass per unit length of vortex at zero temperature is of order (=Fermi momentum, =coherence length), essentially
equaling the electron mass displaced within the coherence length from the
vortex core. Transitions between the core states are responsible for most of
the mass. The mass reaches a maximum value at and decreases
continuously to zero at .Comment: Supercedes prior version, cond-mat/990312
Growth and electronic and magnetic structure of iron oxide films on Pt(111)
Ultrathin (111)-oriented polar iron oxide films were grown on a Pt(111)
single crystal either by the reactive deposition of iron or oxidation of
metallic iron monolayers. These films were characterized using low energy
electron diffraction, scanning tunneling microscopy and conversion electron
Mossbauer spectroscopy. The reactive deposition of Fe led to the island growth
of Fe3O4, in which the electronic and magnetic properties of the bulk material
were modulated by superparamagnetic size effects for thicknesses below 2 nm,
revealing specific surface and interface features. In contrast, the oxide films
with FeO stoichiometry, which could be stabilized as thick as 4 nm under
special preparation conditions, had electronic and magnetic properties that
were very different from their bulk counterpart, w\"ustite. Unusual long range
magnetic order appeared at room temperature for thicknesses between three and
ten monolayers, the appearance of which requires severe structural modification
from the rock-salt structure.Comment: 17 pages, 6 figures, 50 reference
Spin rotation for ballistic electron transmission induced by spin-orbit interaction
We study spin dependent electron transmission through one- and
two-dimensional curved waveguides and quantum dots with account of spin-orbit
interaction. We prove that for a transmission through arbitrary structure there
is no spin polarization provided that electron transmits in isolated energy
subband and only two leads are attached to the structure. In particular there
is no spin polarization in the one-dimensional wire for which spin dependent
solution is found analytically. The solution demonstrates spin evolution as
dependent on a length of wire. Numerical solution for transmission of electrons
through the two-dimensional curved waveguides coincides with the solution for
the one-dimensional wire if the energy of electron is within the first energy
subband. In the vicinity of edges of the energy subbands there are sharp
anomalies of spin flipping.Comment: 9 oages, 7 figure
Shifts of the nuclear resonance in the vortex lattice in YBaCuO
The NMR and NQR spectra of Cu in the CuO plane of
YBaCuO in the superconducting state are discussed in terms of the
phenomenological theory of Ginzburg-Landau type extended to lower temperatures.
We show that the observed spectra, Kumagai {\em et al.}, PRB {\bf 63}, 144502
(2001), can be explained by a standard theory of the Bernoulli potential with
the charge transfer between CuO planes and CuO chains assumed.Comment: 11 pages 7 figure
Quantum depinning of a pancake-vortex from a columnar defect
We consider the problem of the depinning of a weakly driven ()
pancake vortex from a columnar defect in a Josephson-coupled superconductor,
where denotes the force acting on the vortex ( is the critical
force).
The dynamics of the vortex is supposed to be of the Hall type. The Euclidean
action is calculated in the entire temperature range; the result
is universal and does not depend on the detailed form of the pinning potential.
We show that the transition from quantum to classical behavior is second-order
like with the temperature of the transition scaling like
Special attention is paid to the regime of applicability of our results, in
particular, the influence of the large vortex mass appearing in the superclean
limit is discussed.Comment: 11 pages, RevTeX, 4 figures inserte
Dynamic vortex mass in clean Fermi superfluids and superconductors
We calculate the dynamic vortex mass for clean Fermi superfluids including
both s- and d-wave superconductors as a response to a vortex acceleration.
Assuming a finite quasiparticle mean free time, the vortex mass appears to be a
tensor. The diagonal component dominates in the limit of long mean free time
while the off-diagonal mass takes over in the moderately clean regime.Comment: 4 pages, no figures, typeset using RevTe
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