361 research outputs found
Induced paramagnetic states by localized -loops in grain boundaries
Recent experiments on high-temperature superconductors show paramagnetic
behavior localized at grain boundaries (GB). This paramagnetism can be
attributed to the presence unconventional d-wave induced -junctions. By
modeling the GB as an array of and conventional Josephson junction we
determine the conditions of the occurrence of the paramagnetic behavior.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Paramagnetic effect in YBaCuO grain boundary junctions
A detailed investigation of the magnetic response of YBaCuO grain boundary
Josephson junctions has been carried out using both radio-frequency
measurements and Scanning SQUID Microscopy. In a nominally zero-field-cooled
regime we observed a paramagnetic response at low external fields for 45 degree
asymmetric grain boundaries. We argue that the observed phenomenology results
from the d-wave order parameter symmetry and depends on Andreev bound states.Comment: To be published in Phys. Rev.
Limits on Superconductivity-Related Magnetization in SrRuO and PrOsSb from Scanning SQUID Microscopy
We present scanning SQUID microscopy data on the superconductors Sr2RuO4 (Tc
= 1.5 K) and PrOsSb (Tc = 1.8 K). In both of these materials,
superconductivity-related time-reversal symmetry-breaking fields have been
observed by muon spin rotation; our aim was to visualize the structure of these
fields. However in neither SrRuO nor PrOsSb do we observe
spontaneous superconductivity-related magnetization. In SrRuO, many
experimental results have been interpreted on the basis of a
superconducting order parameter. This order parameter is expected to give
spontaneous magnetic induction at sample edges and order parameter domain
walls. Supposing large domains, our data restrict domain wall and edge fields
to no more than ~0.1% and ~0.2% of the expected magnitude, respectively.
Alternatively, if the magnetization is of the expected order, the typical
domain size is limited to ~30 nm for random domains, or ~500 nm for periodic
domains.Comment: 8 pages, 7 figures. Submitted to Phys. Rev.
Silicon Superconducting Quantum Interference Device
We have studied a Superconducting Quantum Interference SQUID device made from
a single layer thin film of superconducting silicon. The superconducting layer
is obtained by heavily doping a silicon wafer with boron atoms using the Gas
Immersion Laser Doping (GILD) technique. The SQUID device is composed of two
nano-bridges (Dayem bridges) in a loop and shows magnetic flux modulation at
low temperature and low magnetic field. The overall behavior shows very good
agreement with numerical simulations based on the Ginzburg-Landau equations.Comment: Published in Applied Physics Letters (August 2015
Limits on the Superconducting Order Parameter in NdFeAsOF from Scanning SQUID Microscopy
Identifying the symmetry of the superconducting order parameter in the
recently-discovered ferro-oxypnictide family of superconductors,
RFeAsOF, where is a rare earth, is a high priority. Many of
the proposed order parameters have internal phase shifts, like the d-wave
order found in the cuprates, which would result in direction-dependent phase
shifts in tunnelling. In dense polycrystalline samples, these phase shifts in
turn would result in spontaneous orbital currents and magnetization in the
superconducting state. We perform scanning SQUID microscopy on a dense
polycrystalline sample of \NdFeAsOF with K and find
no such spontaneous currents, ruling out many of the proposed order parameters.Comment: 10 pages, 5 figures; to appear in JPS
Fermions on half-quantum vortex
The spectrum of the fermion zero modes in the vicinity of the vortex with
fractional winding number is discussed. This is inspired by the observation of
the 1/2 vortex in high-temperature superconductors (Kirtley, et al, Phys. Rev.
Lett. 76 (1996) 1336). The fractional value of the winding number leads to the
fractional value of the invariant, which describes the topology of the energy
spectrum of fermions. This results in the phenomenon of the "half-crossing":
the spectrum approaches zero but does not cross it, being captured at the zero
energy level. The similarity with the phenomenon of the fermion condensation is
discussed.Comment: In revised version the discussion is extended and 4 references are
added. The paper is accepted for publication in JETP Letters. 10 pages, LaTeX
file, 3 figures are available at
ftp://boojum.hut.fi/pub/publications/lowtemp/LTL-96004.p
How to create Alice string (half-quantum vortex) in a vector Bose-Einstein condensate
We suggest a procedure how to prepare the vortex with N=1/2 winding number --
the counterpart of the Alice string -- in a Bose--Einstein condensate with
hyperfine spin F=1. Other possible vortices in Bose-condensates are also
discussed.Comment: RevTex file, 3 pages, no figures, extended version submitted to JETP
Letter
Ground state properties and dynamics of the bilayer t-J model
We present an exact diagonalization study of bilayer clusters of t-J model.
Our results indicate a crossover between two markedly different regimes which
occurs when the ratio J_perp/J between inter-layer and intra-layer exchange
constants increases: for small J_perp/J the data suggest the development of 3D
antiferromagnetic correlations without appreciable degradation of the
intra-layer spin order and the d_(x2-y2) hole pairs within the planes persist.
For larger values of J_perp/J local singlets along the inter-layer bonds
dominate, leading to an almost complete suppression of the intra-layer spin
correlation and the breaking of the intra-layer pairs. The ground state with
two holes in this regime has s-like symmetry. The data suggest that the
crossover may occur for values of J_perp/J as small as 0.2. We present data for
static spin correlations, spin gap, and electron momentum distribution and
spectral function of the `inter-layer RVB state' realized for large J_perp/J.
The latter deviates from the single layer ground state, making it an
implausible candidate for modelling high-temperature superconductors.Comment: Revtex-file, 6 PRB pages, figures appended as uu-encoded postscript.
Hardcopies of figures (or the entire manuscript) can be obtained by e-mailing
to: [email protected]
Sub-Dominant Pairing Channels in Unconventional Superconductors: Ginzburg-Landau Theory
A Ginzburg-Landau theory is developed for unconventional superconductors with
the three relevant singlet pairing channels. Various consequences of the
sub-dominant channels (i.e., s- and d_{xy}-channels) are examined in detail.
(1) In the case of a d_{x^2-y^2}+is-wave superconductor, The structure of a
single vortex above and below T_{DS} is four-fold and two-fold symmetric,
respectively. (2) In the case of a d_{x^2-y^2}+id_{xy}-wave superconductor,
there is also a second order zero-field phase transition from the pure
d_{x^2-y^2}-phase to the Time-reversal-symmetry-breaking
d_{x^2-y^2}+id_{xy}-wave phase at the temperature T_{DD'}. But the subdominant
phase can (not) be induced by vortices above T_{DD'}. Below the time-reversal-
symmetry-breaking transition, the sub-dominant phase in the mixed state is
nontrivial: it survives at low fields, but may disappear above a field
(increasing with decreasing temperature) presumably via a first-order
transition. (3)By including the strong coupling effects, a
time-reversal-symmetry -breaking coupling term between the d_{x^2-y^2}- and
d_{xy}-waves is found to have significant effects on the low temperature
behavior of d_{x^2-y^2}+id_{xy} superconductors. In a magnetic field, a
d_{x^2-y^2}+id_{xy} state is always established, but the field-dependence of
d_{xy}-amplitude above T_{DD'} is different from that below T_{DD'}. Above but
not very close to T_{DD'}, the induced minimum gap Delta_0 proportional to
B/(T-T_{DD'}).Comment: updated, 7 two-column pages with one embedded figure, one formula
corrected, to appear in Phys. Rev. B 6
Strong interconversion of non-polar phonons and Josephson plasma oscillations induced by equilibrium Josephson currents in high T_c superconductors
We analyze consequences of dynamical modulations of Josephson current by
non-polar lattice mode in the Josephson junction barrier. In the high
junctions, the effect of such modulations can be anomalously strong due to the
proximity of the insulating barrier to the superconducting state. Accordingly,
the interconversion of sound (as well as other non-polar phonons) and the
Josephson plasma oscillations mediated by stationary Josephson currents, which
may be present in the junction due to various reasons, becomes possible. We
suggest that this effect can be employed for imaging of the stationary
Josephson currents. Estimates of the effect are given.Comment: 11 RevTeX pages, no figure
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