3,435 research outputs found
Hall effect in superconducting Fe(Se0.5Te0.5) thin films
The Hall effect is investigated for eight superconducting Fe(Se_0.5_Te_0.5_)
thin films grown on MgO and LaSrAlO_4_ substrates with different transition
temperatures (T_c_). The normal Hall coefficients (R_H_) have positive values
with magnitude of 1 - 1.5 x 10^-3^ cm^3^/C at room temperature for the all
samples. With decreasing temperature, we find two characteristic types of
behavior in R_H_(T) depending on T_c_. For thin films with lower T_c_
(typically T_c_ < 5 K), R_H_ start decreasing approximately below T = 250 K
toward a negative side, some of which shows sign reversal at T = 50 - 60 K, but
turns positive toward T = 0 K. On the other hand for the films with higher T_c_
(typically T_c_ > 9 K), R_ H_ leaves almost unchanged down to T = 100 K, and
then starts decreasing toward a negative side. Around the temperatures when
R_H_ changes its sign from positive to negative, obvious nonlinearity is
observed in the field-dependence of Hall resistance as to keep the low-field
R_H_ positive while the high-field R_H_ negative. Thus the electronic state
just above T_c_ is characterized by n_e_ (electron density) > n_h_ (hole
density) with keeping \mu_e_ < \mu_h_. These results suggest the dominance of
electron density to the hole density is an essential factor for the occurence
of superconductivity in Fe-chalcogenide superconductors.Comment: 11 pages, 4 figures, revised version for Physical Review B. accepted
for publication in Physical Review
Polar Actions on Berger Spheres
The object of this article is to study a torus action on a so-called Berger sphere. We also make some comments on polar actions on naturally reductive homogeneous spaces. Finally, we prove a rigidity-type theorem for Riemannian manifolds carrying a polar action with a fix point
Multiscale expansion of the lattice potential KdV equation on functions of infinite slow-varyness order
We present a discrete multiscale expansion of the lattice potential
Korteweg-de Vries (lpKdV) equation on functions of infinite order of
slow-varyness. To do so we introduce a formal expansion of the shift operator
on many lattices holding at all orders. The lowest secularity condition from
the expansion of the lpKdV equation gives a nonlinear lattice equation,
depending on shifts of all orders, of the form of the nonlinear Schr\"odinger
(NLS) equationComment: 9 pages, submitted to Journ. Phys.
Break-down of the density-of-states description of scanning tunneling spectroscopy in supported metal clusters
Low-temperature scanning tunneling spectroscopy allows to probe the
electronic properties of clusters at surfaces with unprecedented accuracy. By
means of quantum transport theory, using realistic tunneling tips, we obtain
conductance curves which considerably deviate from the cluster's density of
states. Our study explains the remarkably small number of peaks in the
conductance spectra observed in recent experiments. We demonstrate that the
unambiguous characterization of the states on the supported clusters can be
achieved with energy-resolved images, obtained from a theoretical analysis
which mimics the experimental imaging procedure.Comment: 5 pages, 3 figure
Andreev bound states at a cuprate grain boundary junction: A lower bound for the upper critical field
We investigate in-plane quasiparticle tunneling across thin film grain
boundary junctions (GBJs) of the electron-doped cuprate
LaCeCuO in magnetic fields up to T, perpendicular to
the CuO layers. The differential conductance in the superconducting state
shows a zero bias conductance peak (ZBCP) due to zero energy surface Andreev
bound states. With increasing temperature , the ZBCP vanishes at the
critical temperature K if B=0, and at K for B=16 T. As
the ZBCP is related to the macroscopic phase coherence of the superconducting
state, we argue that the disappearance of the ZBCP at a field
must occur below the upper critical field of the superconductor. We
find T which is at least a factor of 2.5 higher than
previous estimates of .Comment: 4 pages, 4 figure
Reentrant Spin-Peierls Transition in Mg-Doped CuGeO_3
We report a synchrotron x-ray scattering study of the diluted spin-Peierls
(SP) material Cu_{1-x}Mg_xGeO_3. In a recent paper we have shown that the SP
dimerization attains long-range order only for x < x_c = 0.022(0.001). Here we
report that the SP transition is reentrant in the vicinity of the critical
concentration x_c. This is manifested by broadening of the SP dimerization
superlattice peaks below the reentrance temperature, T_r, which may mean either
the complete loss of the long-range SP order or the development of a
short-range ordered component within the long-range ordered SP state. Marked
hysteresis and very large relaxation times are found in the samples with Mg
concentrations in the vicinity of x_c. The reentrant transition is likely
related to the competing Neel transition which occurs at a temperature similar
to T_r. We argue that impurity-induced competing interchain interactions play
an essential role in these phenomena.Comment: 5 pages, 4 embedded eps figure
Large magneto-thermal effect and the spin-phonon coupling in a parent insulating cuprate Pr_{1.3}La_{0.7}CuO_4
The magnetic-field (H) dependence of the thermal conductivity \kappa of
Pr_{1.3}La_{0.7}CuO_4 is found to show a pronounced minimum for in-plane fields
at low temperature, which is best attributed to the scattering of phonons by
free spins that are seen by a Schottky-type specific heat and a Curie-Weiss
susceptibility. Besides pointing to a strong spin-phonon coupling in cuprates,
the present result demonstrates that the H-dependence of the phonon heat
transport should not be naively neglected when discussing the \kappa(H)
behavior of cuprates, since the Schottky anomaly is ubiquitously found in
cuprates at any doping.Comment: 6 pages, 4 figures, accepted for publication in Phys. Rev.
Theory of double resonance magnetometers based on atomic alignment
We present a theoretical study of the spectra produced by
optical-radio-frequency double resonance devices, in which resonant linearly
polarized light is used in the optical pumping and detection processes. We
extend previous work by presenting algebraic results which are valid for atomic
states with arbitrary angular momenta, arbitrary rf intensities, and arbitrary
geometries. The only restriction made is the assumption of low light intensity.
The results are discussed in view of their use in optical magnetometers
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