512 research outputs found
Upper bound on the Andreev states induced second harmonic in the Josephson coupling of YBa2Cu3O7-δ/Nb junctions from experiment and numerical simulations
Theory predicts that d-wave superconductivity induces a significant second harmonic J2 in the Josephson current, as a result of zero-energy Andreev states ZES formed at the junction interface. Consequently, anomalies such as half-integer Shapiro steps and signatures of period doubling of the dc Josephson current versus magnetic field should be observed. We performed experiments on junctions between untwinned d-wave YBa2Cu3O7-δ and Nb and found no trace of such anomalies although clear evidence of Andreev states formation is provided. These findings do not lead to an observable J2. This result combined with extensive numerical simulations put an upper bound on the ZES-induced J2 of about 0.1% from the first harmonic in the Josephson current for tunneling into the 010 direction and of about 2% for tunneling close to the 110 direction. Our results suggest strong J2 suppression by diffusive scattering, which is possibly due to nanoscale interface roughness. This is important for proposed quantum-electronic device concepts based on the expectance of J2
Admixtures to d-wave gap symmetry in untwinned YBa2Cu3O7 superconducting films measured by angle-resolved electron tunneling
We report on an \textit{ab}-anisotropy of and in
ramp-edge junctions between untwinned YBaCuO and % -wave
Nb. For these junctions, the angle with the YBaCuO
crystal b-axis is varied as a single parameter. The
A()-dependence presents 2-fold symmetry. The minima in
at suggest a real s-wave subdominant
component and negligible -wave or imaginary s-wave admixtures. The
()-dependence is well-fitted by 83% -, 15%
isotropic - and 2% anisotropic s-wave order parameter symmetry, consistent
with .Comment: 4 pages, 3 figures, to be published in Physical Review Letter
Observation of Andreev bound states in YBaCuO/Au/Nb ramp-type Josephson junctions
We report on Josephson and quasiparticle tunneling in YBa2Cu3O7-x(YBCO)/Au/Nb
ramp junctions of several geometries. Macroscopically, tunneling occurs in the
ab-plane of YBCO either in the (100) and (010) direction, or in the (110)
direction. These junctions have a stable and macroscopically well defined
geometry. This allows systematic investigations of both quasiparticle and
Josephson tunneling over a wide range of temperature and magnetic field. With
Nb superconducting, its gap appears in the quasiparticle conductance spectra as
Nb coherence peaks and a dip at the center of a broadened zero-bias conductance
peak (ZBCP). As we increase the temperature or an applied magnetic field both
the Nb coherence peaks and the dip get suppressed and the ZBCP fully develops,
while states are conserved. With Nb in the normal state the ZBCP is observed up
to about 77 K and is almost unaffected by an increasing field up to 7 T. The
measurements are consistent with a convolution of density of states with
broadened Andreev bound states formed at the YBCO/Au/Nb junction interfaces.
Since junctions with different geometries are fabricated on the same substrate
under the same conditions one expects to extract reliable tunneling information
that is crystallographic direction sensitive. In high contrast to Josephson
tunneling, however, the quasiparticle conductance spectra are crystallographic
orientation insensitive: independent whether the tunneling occurs in the (100)
or (110) directions, a pronounced ZBCP is always observed, consistent with
microscopic roughness of the junction interfaces. Qualitatively, all these
particularities regarding quasiparticle spectra hold regardless whether the
YBCO thin film is twinned or untwinned.Comment: 13 pages, 10 figure
Anti-ferromagnetic ordering in arrays of superconducting pi-rings
We report experiments in which one dimensional (1D) and two dimensional (2D)
arrays of YBa2Cu3O7-x-Nb pi-rings are cooled through the superconducting
transition temperature of the Nb in various magnetic fields. These pi-rings
have degenerate ground states with either clockwise or counter-clockwise
spontaneous circulating supercurrents. The final flux state of each ring in the
arrays was determined using scanning SQUID microscopy. In the 1D arrays,
fabricated as a single junction with facets alternating between alignment
parallel to a [100] axis of the YBCO and rotated 90 degrees to that axis,
half-fluxon Josephson vortices order strongly into an arrangement with
alternating signs of their magnetic flux. We demonstrate that this ordering is
driven by phase coupling and model the cooling process with a numerical
solution of the Sine-Gordon equation. The 2D ring arrays couple to each other
through the magnetic flux generated by the spontaneous supercurrents. Using
pi-rings for the 2D flux coupling experiments eliminates one source of disorder
seen in similar experiments using conventional superconducting rings, since
pi-rings have doubly degenerate ground states in the absence of an applied
field. Although anti-ferromagnetic ordering occurs, with larger negative bond
orders than previously reported for arrays of conventional rings, long-range
order is never observed, even in geometries without geometric frustration. This
may be due to dynamical effects. Monte-Carlo simulations of the 2D array
cooling process are presented and compared with experiment.Comment: 10 pages, 15 figure
Superconducting thin films of MgB2 on (001)-Si by pulsed laser deposition
Superconducting thin films have been prepared on Si-substrates, using pulsed
laser deposition from a target composed of a mixture of Mg and MgB2 powders.
The films were deposited at room temperature and post-annealed at 600 degrees
C. The zero resistance transition temperatures were 12 K, with an onset
transition temperature of 27 K. Special care has been taken to avoid oxidation
of Mg in the laser plasma and deposited film, by optimizing the background
pressure of Ar gas in the deposition chamber. For this the optical emission in
the visible range from the plasma has been used as indicator. Preventing Mg
from oxidation was found to be essential to obtain superconducting films
Static semifluxons in a long Josephson junction with π-discontinuity points
We investigate analytically a long Josephson junction with several -discontinuity points characterized by a jump of in the phase difference of the junction. The system is described by a perturbed-combined sine-Gordon equation. Via phase-portrait analysis, it is shown how the existence of static semifluxons localized around the discontinuity points is influenced by the applied bias current. In junctions with more than one corner, there is a minimum-facet-length for semifluxons to be spontaneously generated. A stability analysis is used to obtain the minimum-facet-length for multicorner junctions
Superconducting gap of overdoped Tl2Ba2CuO6+d observed by Raman scattering
We report Raman scattering spectra for single crystals of overdoped
Tl2Ba2CuO6+d (Tl-2201) at low temperatures. It was observed that the
pair-breaking peaks in A1g and B1g spectra radically shift to lower energy with
carrier doping. We interpret it as s-wave component mixing into d-wave,
although the crystal structure is tetragonal. Since similar phenomena were
observed also in YBa2Cu3Oy and Bi2Sr2CaCu2Oz, we conclude that s-wave mixing is
a common property for overdoped high-Tc superconductors.Comment: 8 pages, 3 figures, proceedings of SNS200
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