95 research outputs found
Large Deployable Reflector (LDR) feasibility study update
In 1982 a workshop was held to refine the science rationale for large deployable reflectors (LDR) and develop technology requirements that support the science rationale. At the end of the workshop, a set of LDR consensus systems requirements was established. The subject study was undertaken to update the initial LDR study using the new systems requirements. The study included mirror materials selection and configuration, thermal analysis, structural concept definition and analysis, dynamic control analysis and recommendations for further study. The primary emphasis was on the dynamic controls requirements and the sophistication of the controls system needed to meet LDR performance goals
Planet geometric center tracker, volume 1 Final report, Oct. 1964 - Jul. 1967
Tracker for locating geometric centers of Mars, Venus, or Jupite
Anomalous Low Temperature Behavior of Superconducting Nd(1.85)Ce(0.15)CuO(4-y)
We have measured the temperature dependence of the in-plane London
penetration depth lambda(T) and the maximum Josephson current Ic(T) using
bicrystal grain boundary Josephson junctions of the electron-doped cuprate
superconductor Nd(1.85)Ce(0.15)CuO(4-y). Both quantities reveal an anomalous
temperature dependence below about 4 K. In contrast to the usual monotonous
decrease (increase) of lambda(T) (Ic(T)) with decreasing temperature, lambda(T)
and Ic(T) are found to increase and decrease, respectively, with decreasing
temperature below 4 K resulting in a non-monotonous overall temperature
dependence. This anomalous behavior was found to be absent in analogous
measurements performed on Pr(1.85)Ce(0.15)CuO(4-y). From this we conclude that
the anomalous behavior of Nd(1.85)Ce(0.15)CuO(4-y) is caused by the presence of
the Nd3+ paramagnetic moments. Correcting the measured lambda(T) dependence of
Nd(1.85)Ce(0.15)CuO(4-y) for the temperature dependent susceptibility due to
the Nd moments, an exponential dependence is obtained indicating isotropic
s-wave pairing. This result is fully consistent with the lambda(T) dependence
measured for Pr(1.85)Ce(0.15)CuO(4-y).Comment: 4 pages including 4 figures, to appear in Phys. Rev. Let
Electronic structure of d-wave superconducting quantum wires
We present analytical and numerical results for the electronic spectra of
wires of a d-wave superconductor on a square lattice. The spectra of Andreev
and other quasiparticle states, as well as the spatial and particle-hole
structures of their wave functions, depend on interference effects caused by
the presence of the surfaces and are qualitatively different for half-filled
wires with even or odd number of chains. For half-filled wires with an odd
number of chains N at (110) orientation, spectra consist of N doubly degenerate
branches. By contrast, for even N wires, these levels are split, and all
quasiparticle states, even the ones lying above the maximal gap, have the
characteristic properties of Andreev bound states. These Andreev states above
the gap can be interpreted as a consequence of an infinite sequence of Andreev
reflections experienced by quasiparticles along their trajectories bounded by
the surfaces of the wire. Our microscopic results for the local density of
states display atomic-scale Friedel oscillations due to the presence of the
surfaces, which should be observable by scanning tunneling microscopy. For
narrow wires the self-consistent treatment of the order parameter is found to
play a crucial role. In particular, we find that for small wire widths the
finite geometry may drive strong fluctuations or even stablilize exotic
quasi-1D pair states with spin triplet character.Comment: 21 pages, 20 figures. Slightly modified version as published in PR
Two regimes for effects of surface disorder on the zero-bias conductance peak of tunnel junctions involving d-wave superconductors
Impurity-induced quasiparticle bound states on a pair-breaking surface of a
d-wave superconductor are theoretically described, taking into account
hybridization of impurity- and surface-induced Andreev states. Further a theory
for effects of surface disorder (of thin impurity surface layer) on the
low-bias conductance of tunnel junctions is developed. We find a threshold
for surface impurity concentration , which separates the two regimes
for surface impurity effects on the zero-bias conductance peak (ZBCP). Below
the threshold, surface impurities do not broaden the ZBCP, but effectively
reduce its weight and generate impurity bands. For low impurity bands can
be, in principle, resolved experimentally, being centered at energies of bound
states induced by an isolated impurity on the surface. For larger
impurity bands are distorted, move to lower energies and, beginning with the
threshold concentration , become centered at zero energy. With
increasing above the threshold, the ZBCP is quickly destroyed in the case
of strong scatterers, while it is gradually suppressed and broaden in the
presence of weak impurity potentials. More realistic cases, taking into account
additional broadening, not related to the surface disorder, are also
considered.Comment: 9 pages, 7 figure
Inhomogeneous electronic structure probed by spin-echo experiments in the electron doped high-Tc superconductor Pr_{1.85}Ce_{0.15}CuO_{4-y}
63Cu nuclear magnetic resonance (NMR) spin-echo decay rate (T_2^{-1})
measurements are reported for the normal and superconducting states of a single
crystal of Pr_{1.85}Ce_{0.15}CuO_{4-y} (PCCO) in a magnetic field B_0=9T over
the temperature range 2K<T<200K. The spin-echo decay rate is
temperature-dependent for T<55K, and has a substantial dependence on the radio
frequency (rf) pulse parameters below T~25K. This dependence indicates that
T_2^{-1} is strongly effected by a local magnetic field distribution that can
be modified by the rf pulses, including ones that are not at the nuclear Larmor
frequency. The low-temperature results are consistent with the formation of a
static inhomogeneous electronic structure that couples to the rf fields of the
pulses.Comment: 4 pages, 4 figure
Evidence for Surface Andreev Bound states in Cuprate Superconductors from Penetration Depth Measurements
Tunneling and theoretical studies have suggested that Andreev bound states
form at certain surfaces of unconventional superconductors. Through studies of
the temperature and field dependence of the in-plane magnetic penetration depth
lambda_ab at low temperature, we have found strong evidence for the presence of
these states in clean single crystal YBCO and BSCCO. Crystals cut to expose a
[110] interface show a strong upturn in lambda_ab at around 7K, when the field
is oriented so that the supercurrents flow around this surface. In YBCO this
upturn is completely suppressed by a field of ~0.1 T.Comment: 4 pages 2 column revtex + 4 postscript figures. Submitted to PR
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
Josephson Current between Triplet and Singlet Superconductors
The Josephson effect between triplet and singlet superconductors is studied.
Josephson current can flow between triplet and singlet superconductors due to
the spin-orbit coupling in the spin-triplet superconductor but it is finite
only when triplet superconductor has , where and
are the perpendicular components of orbital angular momentum and spin angular
momentum of the triplet Cooper pairs, respectively. The recently observed
temperature and orientational dependence of the critical current through a
Josephson junction between UPt and Nb is investigated by considering a
non-unitary triplet state.Comment: 4 pages, no figure
ac Josephson effect in superconducting d-wave junctions
We study theoretically the ac Josephson effect in superconducting planar
d-wave junctions. The insulating barrier assumed to be present between the two
superconductors may have arbitrary strength. Many properties of this system
depend on the orientation of the d-wave superconductor: we calculate the ac
components of the Josephson current. In some arrangements there is substantial
negative differential conductance due to the presence of mid-gap states. We
study how robust these features are to finite temperature and also comment on
how the calculated current-voltage curves compare with experiments. For some
other configurations (for small barrier strength) we find zero-bias conductance
peaks due to multiple Andreev reflections through midgap states. Moreover, the
odd ac components are strongly suppressed and even absent in some arrangements.
This absence will lead to a doubling of the Josephson frequency. All these
features are due to the d-wave order parameter changing sign when rotated
. Recently, there have been several theoretical reports on parallel
current in the d-wave case for both the stationary Josephson junction and for
the normal metal-superconductor junction. Also in our case there may appear
current density parallel to the junction, and we present a few examples when
this takes place. Finally, we give a fairly complete account of the method used
and also discuss how numerical calculations should be performed in order to
produce current-voltage curves
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