38 research outputs found
Andreev Bound States in High Temperature Superconductors
Andreev bound states (ABS) at the surface of superconductors are expected for
any pair potential showing a sign change in different k-directions with their
spectral weight depending on the relative orientation of the surface and the
pair potential. We report on the observation of ABS in HTS employing tunneling
spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The
tunneling spectra were studied as a function of temperature and applied
magnetic field. The tunneling spectra of GBJ formed by YBCO, BSCCO, and LSCO
show a pronounced zero bias conductance peak that can be interpreted in terms
of Andreev bound states at zero energy that are expected at the surface of HTS
having a d-wave symmetry of the order parameter. In contrast, for the most
likely s-wave HTS NCCO no zero bias conductance peak was observed. Applying a
magnetic field results in a shift of spectral weight from zero to finite
energy. This shift is found to depend nonlinearly on the applied magnetic
field. Further consequences of the Andreev bound states are discussed and
experimental evidence for anomalous Meissner currents is presented.Comment: 17 pages, 10 figures, to appear in Eur. Phys. J.
Superconducting NdCeCuO Bicrystal Grain Boundary Josephson Junctions
We have studied the electric transport properties of symmetrical [001] tilt
NdCeCuO bicrystal grain boundary Josephson junctions (GBJs) fabricated on SrTiO
bicrystal substrates with misorientation angles of 24 and 36.8 degree. The
superconducting properties of the NdCeCuO-GBJs are similar to those of GBJs
fabricated from the hole doped high temperature superconductors (HTS). The
critical current density Jc decreases strongly with increasing misorientation
angle. The products of the critical current Ic and the normal resistance Rn
(about 0.1 mV at 4.2 K) are small compared to the gap voltage and fit well to
the universal scaling law (IcRn is proportional to the square root of Jc) found
for GBJs fabricated from the hole doped HTS. This suggests that the symmetry of
the order parameter, which most likely is different for the electron and the
hole doped HTS has little influence on the characteristic properties of
symmetrical [001] tilt GBJs.Comment: 3 pages, 4 figures, to be published in Applied Physics Letter
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
Possible pseudogap behavior of electron doped high-temperature superconductors
We have measured the low-energy quasiparticle excitation spectrum of the
electron doped high-temperature superconductors (HTS) Nd(1.85)Ce(0.15)CuO(4-y)
and Pr(1.85)Ce(0.15)CuO(4-y) as a function of temperature and applied magnetic
field using tunneling spectroscopy. At zero magnetic field, for these optimum
doped samples no excitation gap is observed in the tunneling spectra above the
transition temperature Tc. In contrast, below Tc for applied magnetic fields
well above the resistively determined upper critical field, a clear excitation
gap at the Fermi level is found which is comparable to the superconducting
energy gap below Tc. Possible interpretations of this observation are the
existence of a normal state pseudogap in the electron doped HTS or the
existence of a spatially non-uniform superconducting state.Comment: 4 pages, 4 ps-figures included, to be published in Phys. Rev. B,
Rapid Com
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Oxygen transport by oxygen potential gradient in dense ceramic oxide membranes
Numerous studies have been conducted in recent years on the partial oxidation of methane to synthesis gas (syngas: CO + H{sub 2}) with air as the oxidant. In partial oxidation, a mixed-oxide ceramic membrane selectively transports oxygen from the air; this transport is driven by the oxygen potential gradient. Of the several ceramic materials the authors have tested, a mixed oxide based on the Sr-Fe-Co-O system has been found to be very attractive. Extensive oxygen permeability data have been obtained for this material in methane conversion experiments carried out in a reactor. The data have been analyzed by a transport equation based on the phenomenological theory of diffusion under oxygen potential gradients. Thermodynamic calculations were used to estimate the driving force for the transport of oxygen ions. The results show that the transport equation deduced from the literature describes the permeability data reasonably well and can be used to determine the diffusion coefficients and the associated activation energy of oxygen ions in the ceramic membrane material
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Separation of gases with solid electrolyte ionic conductors
The authors have developed a novel method of gas separation based on electrolyte ionic membrane technology. Separation of one gas from another occurs through an ion-conducting membrane by the passage of selected ions. Most systems studied have focused on oxygen ion conduction for the separation of oxygen from air, although protonic and halide-conducting solid materials also exist. As an example of this system, this paper concentrates on a study of a membrane reactor used in the production of syngas (CO + H{sub 2}) from methane. The membrane material is a modified perovskite-type oxide exhibiting mixed (electronic/ionic) conductivity. Mixed-conductivity oxides are promising materials for oxygen-permeating membranes that can operate without electrodes or external electrical circuitry. Extruded tubes of this material have been evaluated in a reactor operating at {approx} 850 C for partial oxidation of methane into syngas in the presence of a reforming catalyst. Separated oxygen on one side of the reactor wall was obtained from air on the other side. Methane conversion efficiencies of > 99% were observed, and some of the reactor tubes have been operated for > 1,000 h. Membrane tubes were fabricated from calcined powders by a plastic extrusion technique. Characterization of the mechanical, physical, and chemical properties of this material confirmed the stability exhibited in the reactor
Observation of Bound Surface States in Grain Boundary Junctions of High Temperature Superconductors
We have performed a detailed study of the tunneling spectra of bicrystal
grain boundary junctions (GBJs) fabricated from the HTS YBCO, BSCCO, LSCO, and
NCCO. In all experiments the tunneling direction was along the CuO planes. With
the exception of NCCO, for all materials a pronounced zero bias conductance
peak was observed which decreases with increasing temperature and disappears at
the critical temperature. These results can be explained by the presence of a
dominating d-wave symmetry of the order parameter resulting in the formation of
zero energy Andreev bound states at surfaces and interfaces of HTS. The absence
of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair
potential in this material. The observed nonlinear shift of spectral weight to
finite energies by applying a magnetic field is in qualitative agreement with
recent theoretical predictions.Comment: 4 pages, 4 figures, to be published in Phys. Rev.
Nernst Effect in Electron-Doped PrCeCuO
The Nernst effect of PrCeCuO (x=0.13, 0.15, and 0.17) has
been measured on thin film samples between 5-120 K and 0-14 T. In comparison to
recent measurements on hole-doped cuprates that showed an anomalously large
Nernst effect above the resistive T and H
\cite{xu,wang1,wang2,capan}, we find a normal Nernst effect above T and
H for all dopings. The lack of an anomalous Nernst effect in the
electron-doped compounds supports the models that explain this effect in terms
of amplitude and phase fluctuations in the hole-doped cuprates. In addition,
the H(T) determined from the Nernst effect shows a conventional behavior
for all dopings. The energy gap determined from H(0) decreases as the
system goes from under-doping to over-dopingin agreement with the recent
tunnelling experiments
Breakdown of Fermi-liquid theory in a cuprate superconductor
The behaviour of electrons in solids is remarkably well described by Landau's
Fermi-liquid theory, which says that even though electrons in a metal interact
they can still be treated as well-defined fermions, called ``quasiparticles''.
At low temperature, the ability of quasiparticles to transport heat is strictly
given by their ability to transport charge, via a universal relation known as
the Wiedemann-Franz law, which no material in nature has been known to violate.
High-temperature superconductors have long been thought to fall outside the
realm of Fermi-liquid theory, as suggested by several anomalous properties, but
this has yet to be shown conclusively. Here we report on the first experimental
test of the Wiedemann-Franz law in a cuprate superconductor,
(Pr,Ce)CuO. Our study reveals a clear departure from the universal law
and provides compelling evidence for the breakdown of Fermi-liquid theory in
high-temperature superconductors.Comment: 7 pages, 3 figure
Possible Z2 phase and spin-charge separation in electron doped cuprate superconductors
The SU(2) slave-boson mean-field theory for the tt'J model is analyzed. The
role of next-nearest-neighbor hopping t' on the phase-diagram is studied. We
find a pseudogap phase in hole-doped materials (where t'<0). The pseudo-gap
phase is a U(1) spin liquid (the staggered-flux phase) with a U(1) gauge
interaction and no fractionalization. This agrees with experiments on hole
doped samples. The same calculation also indicates that a positive t' favors a
Z2 state with true spin-charge separation. The Z2 state that exists when t' >
0.5J can be a candidate for the pseudo-gap phase of electron-doped cuprates (if
such a phase exists). The experimental situation in electron-doped materials is
also addressed.Comment: 6 pages, 2 figures, RevTeX4. Homepage http://dao.mit.edu/~wen