4,748 research outputs found
Magnetic Penetration Depth Measurements of PrCeCuO Films on Buffered Substrates: Evidence for a Nodeless Gap
We report measurements of the inverse squared magnetic penetration depth,
, in PrCeCuO () superconducting films grown on SrTiO (001) substrates coated with a
buffer layer of insulating PrCuO. , and
normal-state resistivities of these films indicate that they are clean and
homogeneous. Over a wide range of Ce doping, ,
at low is flat: it changes by less than 0.15% over a
factor of 3 change in , indicating a gap in the superconducting density of
states. Fits to the first 5% decrease in produce values of
the minimum superconducting gap in the range of .Comment: 4 pages 5 figure
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
Observation of Andreev bound states in bicrystal grain-boundary Josephson junctions of the electron doped superconductor LaCeCuO
We observe a zero-bias conductance peak (ZBCP) in the ab-plane quasiparticle
tunneling spectra of thin film grain-boundary Josephson junctions made of the
electron doped cuprate superconductor LaCeCuO. An applied magnetic field
reduces the spectral weight around zero energy and shifts it non-linearly to
higher energies consistent with a Doppler shift of the Andreev bound states
(ABS) energy. For all magnetic fields the ZBCP appears simultaneously with the
onset of superconductivity. These observations strongly suggest that the ZBCP
results from the formation of ABS at the junction interfaces, and,
consequently, that there is a sign change in the symmetry of the
superconducting order parameter of this compound consistent with a d-wave
symmetry.Comment: 9 pages, 7 figures; December 2004, accepted for publication in Phys.
Rev.
Anomalously Sharp Superconducting Transitions in Overdoped Films
We present measurements of -plane resistivity and
superfluid density [, = magnetic penetration
depth] in films. As Sr concentration exceeds about
0.22, the superconducting transition sharpens dramatically, becoming as narrow
as 200 mK near the super-to-normal metal quantum critical point. At the same
time, , , and transition temperature
decrease, and upward curvature develops in . Given the sharp
transitions, we interpret these results in the context of a homogeneous d-wave
superconducting state, with elastic scattering that is enhanced relative to
underdoped LSCO due to weaker electron correlations. This interpretation
conflicts with the viewpoint that the overdoped state is inhomogeneous due to
phase separation into superconducting and normal metal regions.Comment: 21 pages including 3 figures and 56 references. This version includes
responses to referees and slight correction of data on two films. Conclusions
the same as befor
Cosmic-ray Acceleration at Ultrarelativistic Shock Waves: Effects of a "Realistic" Magnetic Field Structure
First-order Fermi acceleration processes at ultrarelativistic shocks are
studied with Monte Carlo simulations. The accelerated particle spectra are
derived by integrating the exact particle trajectories in a turbulent magnetic
field near the shock. ''Realistic'' features of the field structure are
included. We show that the main acceleration process at superluminal shocks is
the particle compression at the shock. Formation of energetic spectral tails is
possible in a limited energy range only for highly perturbed magnetic fields,
with cutoffs occuring at low energies within the resonance energy range
considered. These spectral features result from the anisotropic character of
particle transport in the downstream magnetic field, where field compression
produces effectively 2D perturbations. Because of the downstream field
compression, the acceleration process is inefficient in parallel shocks for
larger turbulence amplitudes, and features observed in oblique shocks are
recovered. For small-amplitude turbulence, wide-energy range particle spectra
are formed and modifications of the process due to the existence of long-wave
perturbations are observed. In both sub- and superluminal shocks, an increase
of \gamma leads to steeper spectra with lower cut-off energies. The spectra
obtained for the ``realistic'' background conditions assumed here do not
converge to the ``universal'' spectral index claimed in the literature. Thus
the role of the first-order Fermi process in astrophysical sources hosting
relativistic shocks requires serious reanalysis.Comment: submitted to Ap
Evidence for a quantum phase transition in the electron-doped cuprate Pr2-xCexCuO4+d from Hall and resistivity measurements
The doping and temperature dependence of the Hall coefficient, RH, and
ab-plane resistivity in the normal state down to 350mK is reported for oriented
films of the electron-doped high-Tc superconductor Pr2-xCexCuO4+d. The doping
dependence of b (r=r0+AT^b) and R_sub_H (at 350 mK) suggest a quantum phase
transition at a critical doping near x=0.165.Comment: 11 pages 4 figures Phys. Rev. Lett. 92, 167001 (2004
Cosmic Ray Acceleration at Relativistic Shock Waves with a "Realistic" Magnetic Field Structure
The process of cosmic ray first-order Fermi acceleration at relativistic
shock waves is studied with the method of Monte Carlo simulations. The
simulations are based on numerical integration of particle equations of motion
in a turbulent magnetic field near the shock. In comparison to earlier studies,
a few "realistic" features of the magnetic field structure are included. The
upstream field consists of a mean field component inclined at some angle to the
shock normal with finite-amplitude sinusoidal perturbations imposed upon it.
The perturbations are assumed to be static in the local plasma rest frame.
Their flat or Kolmogorov spectra are constructed with randomly drawn wave
vectors from a wide range . The downstream field structure
is derived from the upstream one as compressed at the shock. We present
particle spectra and angular distributions obtained at mildly relativistic sub-
and superluminal shocks and also parallel shocks. We show that particle spectra
diverge from a simple power-law, the exact shape of the spectrum depends on
both the amplitude of the magnetic field perturbations and the wave power
spectrum. Features such as spectrum hardening before the cut-off at oblique
subluminal shocks and formation of power-law tails at superluminal ones are
presented and discussed. At parallel shocks, the presence of finite-amplitude
magnetic field perturbations leads to the formation of locally oblique field
configurations at the shock and the respective magnetic field compressions.
This results in the modification of the particle acceleration process,
introducing some features present in oblique shocks, e.g., particle reflections
from the shock. We demonstrate for parallel shocks a (nonmonotonic) variation
of the particle spectral index with the turbulence amplitude.Comment: revised version (37 pages, 13 figures
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