422 research outputs found
c-axis Josephson Tunneling in Twinned YBCO Crystals
Josephson tunneling between YBCO and Pb with the current flowing along the
c-axis of the YBCO is persumed to come from an s-wave component of the
superconductivity of the YBCO. Experiments on multi-twin samples are not
entirely consistent with this hypothesis. The sign change of the s-wave order
parameter across the N_T twin boundaries should give cancelations, resulting in
a small tunneling current. The actual current is larger than this.
We present a theory of this unexpectedly large current based upon a surface
effect: disorder-induced supression of the d-wave component at the (001)
surface leads to s-wave coherence across the twin boundaries and a non-random
tunneling current. We solve the case of an ordered array of d+s and d-s twins,
and estimate that the twin size at which s-wave surface coherence occurs is
consistent with typical sizes observed in experiments. In this picture, there
is a phase difference of between different surfaces of the material. We
propose a corner junction experiment to test this picture.Comment: 5 pages, 4 eps figure
Phase-sensitive Evidence for d-wave Pairing Symmetry in Electron-doped Cuprate Superconductors
We present phase-sensitive evidence that the electron-doped cuprates
Nd1.85Ce0.15CuO4-y (NCCO) and Pr1.85Ce0.15CuO4-y (PCCO) have d-wave pairing
symmetry. This evidence was obtained by observing the half-flux quantum effect,
using a scanning SQUID microscope, in c-axis oriented films of NCCO or PCCO
epitaxially grown on tricrystal [100] SrTiO3 substrates designed to be
frustrated for a d(x2-y2) order parameter. Samples with two other
configurations, designed to b unfrustrated for a d-wave superconductor, do not
show the half-flux quantum effect.Comment: 4 pages, Latex, 4 figure
Ground state and bias current induced rearrangement of semifluxons in 0-pi long Josephson junctions
We investigate numerically a long Josephson junction with several phase
pi-discontinuity points. Such junctions are usually fabricated as a ramp
between an anisotropic cuprate superconductor like YBCO and an isotropic metal
superconductor like Nb. From the top, they look like zigzags with pi-jumps of
the Josephson phase at the corners. These pi-jumps, at certain conditions, lead
to the formation of half-integer flux quanta, which we call semifluxons (SF),
pinned at the corners. We show (a) that the spontaneous formation of SFs
depends on the junction length, (b) that the ground state without SFs can be
converted to a state with SFs by applying a bias current, (c) that the SF
configuration can be rearranged by the bias current. All these effects can be
observed using a SQUID microscope.Comment: ~8 pages, 6 figures, submitted to PR
Spontaneous Flux and Magnetic Interference Patterns in 0-pi Josephson Junctions
The spontaneous flux generation and magnetic field modulation of the critical
current in a 0-pi Josephson junction are calculated for different ratios of the
junction length to the Josephson penetration depth, and different ratios of the
0-junction length to the pi-junction length. These calculations apply to a
Pb-YBCO c-axis oriented junction with one YBCO twin boundary, as well as other
experimental systems. Measurements of such a junction can provide information
on the nature of the c-axis Josephson coupling and the symmetry of the order
parameter in YBCO. We find spontaneous flux even for very short symmetric 0-pi
junctions, but asymmetric junctions have qualitatively different behavior.Comment: 13 pages, TEX,+ 7 figures, postscrip
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
Intrinsic frustration effects in anisotropic superconductors
Lattice distortions in which the axes are locally rotated provide an
intrinsic source of frustration in anisotropic superconductors. A general
framework to study this effect is presented. The influence of lattice defects
and phonons in and layered superconductors is studied.Comment: enlarged versio
Resolving the order parameter of High-T Superconductors through quantum pumping spectroscopy
The order parameter of High-T superconductors through a series of
experiments has been quite conclusively demonstrated to not be of the normal
type. It is either a pure -wave type or a mixture of
a with a small imaginary or
component. In this work a distinction is brought out among the four types,
i.e., , , and
types with the help of quantum pumping
spectroscopy. This involves a normal metal double barrier structure in contact
with a High-T superconductor. The pumped current, heat and noise show
different characteristics with change in order parameter revealing quite easily
the differences among these.Comment: 11 pages, 3 figures, 1 table, Manuscript revised with new material on
d+id' cas
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
Upper limit on spontaneous supercurrents in SrRuO
It is widely believed that the perovskite SrRuO is an unconventional
superconductor with broken time reversal symmetry. It has been predicted that
superconductors with broken time reversal symmetry should have spontaneously
generated supercurrents at edges and domain walls. We have done careful imaging
of the magnetic fields above SrRuO single crystals using scanning Hall
bar and SQUID microscopies, and see no evidence for such spontaneously
generated supercurrents. We use the results from our magnetic imaging to place
upper limits on the spontaneously generated supercurrents at edges and domain
walls as a function of domain size. For a single domain, this upper limit is
below the predicted signal by two orders of magnitude. We speculate on the
causes and implications of the lack of large spontaneous supercurrents in this
very interesting superconducting system.Comment: 9 page
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