489 research outputs found
c-axis Josephson Tunnelling in Twinned and Untwinned YBCO-Pb Junctions
Within a microscopic two band model of planes and chains with a pairing
potential in the planes and off diagonal pairing between planes and chains we
find that the chains make the largest contribution to the Josephson tunnelling
current and that through them the d-wave part of the gap contributes to the
current. This is contrary to the usual assumption that for a d-wave tetragonal
superconductor the c-axis Josephson current for incoherent tunnelling into an
s-wave superconductor is zero while that of a d-wave orthorhombic
superconductor with a small s-wave component to its gap it is small but
non-zero. Nevertheless it has been argued that the effect of twins in YBCO
would lead to cancellation between pairs of twins and so the observation of a
current in c-axis YBCO-Pb experiments is evidence against a d-wave type order
parameter. We argue that both theory and experiment give evidence that the two
twin orientations are not necessarily equally abundant and that the ratio of
tunnelling currents in twinned and untwinned materials should be related to the
relative abundance of the two twin orientations.Comment: 6 pages, RevTeX 3.0, 15 PostScript figur
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
Fermi Liquid Properties of a Two Dimensional Electron System With the Fermi Level Near a van Hove Singularity
We use a diagrammatic approach to study low energy physics of a two
dimensional electron system where the Fermi level is near van-Hove singularies
in the energy spectrum. We find that in most regions of the
phase diagram the system behaves as a normal Fermi liquid rather than a
marginal Fermi liquid. Particularly, the imaginary part of the self energy is
much smaller than the excitation energy, which implies well defined
quasiparticle excitations, and single particle properties are only weakly
affected by the presence of the van-Hove singularities. The relevance to high
temperature superconductivity is also discussed.Comment: 10 pages, 4 postscript 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
Evidence for a Nodeless Gap from the Superfluid Density of Optimally Doped Pr_{1.855}Ce_{0.145}CuO_{4-y} Films
We present measurements of the ab-plane magnetic penetration depth,
\lambda(T), in five optimally doped Pr_{1.855}Ce_{0.145}CuO_{4-y} films for 1.6
K \leq T \leq T_c \sim 24 K. Low resistivities, high superfluid densities
n_s(T)\propto \lambda^{-2}(T), high T_c's, and small transition widths are
reproducible and indicative of excellent film quality. For all five films,
\lambda^{-2}(T)/\lambda^{-2}(0) at low T is well fitted by an exponential
temperature dependence with a gap, \Delta_{min}, of 0.85 k_B T_c. This behavior
is consistent with a nodeless gap and is incompatible with d-wave
superconductivity.Comment: 5 pages, 4 figures, reorganized for clarit
The Superconducting Condensation Energy and an Antiferromagnetic Exchange Based Pairing Mechanism
For the traditional low T_c superconductors, the superconducting condensation
energy is proportional to the change in energy of the ionic lattice between the
normal and superconducting state, providing a clear link between pairing and
the electron-ion interaction. Here, for the t-J model, we discuss an analogous
relationship between the superconducting condensation energy and the change in
the exchange energy between the normal and superconducting states. We point out
the possibility of measuring this using neutron scattering and note that such a
measurement, while certainly difficult, could provide important evidence for an
exchange interaction-based pairing mechanism.Comment: Replaced with revised versio
Bulk and surface electronic properties of SmB6: a hard x-ray photoelectron spectroscopy study
We have carried out bulk-sensitive hard x-ray photoelectron spectroscopy
(HAXPES) measurements on in-situ cleaved and ex-situ polished SmB6 single
crystals. Using the multiplet-structure in the Sm 3d core level spectra, we
determined reliably that the valence of Sm in bulk SmB6 is close to 2.55 at ~5
K. Temperature dependent measurements revealed that the Sm valence gradually
increases to 2.64 at 300 K. From a detailed line shape analysis we can clearly
observe that not only the J=0 but also the J=1 state of the Sm 4f 6
configuration becomes occupied at elevated temperatures. Making use of the
polarization dependence, we were able to identify and extract the Sm 4f
spectral weight of the bulk material. Finally, we revealed that the oxidized or
chemically damaged surface region of the ex-situ polished SmB6 single crystal
is surprisingly thin, about 1 nm only.Comment: 11 pages, 8 figure
Josephson tunneling in high- superconductors
This article describes the Josephson tunneling from time-reversal
symmetry-breaking states and compares it with that from time-reversal invariant
states for both twinned and untwinned crystals and for both -axis and
basal-plane currents, in a model for orthorhombic YBCO. A macroscopic
invariance group describing the superconducting state of a twinned crystal is
introduced and shown to provide a useful framework for the discussion of the
results for twinned crystals. In addition, a ring geometry, which allows
-wave and -wave superconductivity in a tetragonal
superconductor to be distinguished on the basis of symmetry arguments only, is
proposed and analyzed. Finally, an appendix gives details of the experimental
Josephson tunneling evidence for a superconducting state of orthorhombic
symmetry in YBCO.Comment: Latex File, 18 pages, 6 Postscript figures, submitted to Phys. Rev.
The effect of an in-plane magnetic field on the interlayer transport of quasiparticles in layered superconductors
We consider the quasiparticle c-axis conductivity in highly anisotropic
layered compounds in the presence of the magnetic field parallel to the layers.
We show that at low temperatures the quasiparticle interlayer conductivity
depends strongly on the orientation of the in-plane magnetic field if the
excitation gap has nodes on the Fermi surface. Thus measurements of the
angle-dependent c-axis (out-of-plane) magnetoresistance, as a function of the
orientation of the magnetic field in the layers, provide information on the
momentum dependence of the superconducting gap (or pseudogap) on the Fermi
surface. Clean and highly anisotropic layered superconductors seem to be the
best candidates for probing the existence and location of the nodes on the
Fermi surface.Comment: 4 pages RevTeX, including 2 PostScript figures, to appear in Phys.
Rev. Let
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