80 research outputs found
Estimation of the charge carrier localization length from Gaussian fluctuations in the magneto-thermopower of La_{0.6}Y_{0.1}Ca_{0.3}MnO_3
The magneto-thermoelectric power (TEP) of perovskite type
manganise oxide is found to exhibit a sharp peak
at some temperature . By approximating the true shape of the
measured magneto-TEP in the vicinity of by a linear triangle of the
form , we observe that . We adopt the electron localization scenario and
introduce a Ginzburg-Landau (GL) type theory which incorporates the two
concurrent phase transitions, viz., the paramagnetic-ferromagnetic transition
at the Curie point and the "metal-insulator" (M-I) transition at
. The latter is characterized by the divergence of the field-dependent
charge carrier localization length at some characteristic field
. Calculating the average and fluctuation contributions to the total
magnetization and the transport entropy related magneto-TEP
within the GL theory, we obtain a simple relationship between and the
above two critical temperatures ( and ). The observed slope
ratio is found to be governed by the competition between
the electron-spin exchange and the induced magnetic energy . The
comparison of our data with the model predictions produce ,
, , , and for the estimates of
the Curie temperature, the exchange coupling constant, the critical
magnetization, the localization length, and the free-to-localized carrier
number density ratio, respectively.Comment: 6 pages (REVTEX), 2 PS figures (epsf.sty); submitted to Phys.Rev.
Anomalous temperature behavior of resistivity in lightly doped manganites around a metal-insulator phase transition
An unusual temperature and concentration behavior of resistivity in
has been observed at slight doping
(). Namely, introduction of copper results in a splitting of
the resistivity maximum around a metal-insulator transition temperature
into two differently evolving peaks. Unlike the original -free
maximum which steadily increases with doping, the second (satellite) peak
remains virtually unchanged for , increases for and finally
disappears at with . The observed phenomenon
is thought to arise from competition between substitution induced strengthening
of potential barriers (which hamper the charge hopping between neighboring
sites) and weakening of carrier's kinetic energy. The data are well fitted
assuming a nonthermal tunneling conductivity theory with randomly distributed
hopping sites.Comment: 10 REVTEX pages, 2 PostScript figures (epsf.sty); to be published in
JETP Letter
Coexistence of pyoderma gangrenosum and sweet’s syndrome in a patient with ulcerative colitis
Pyoderma gangrenosum (PG) and Sweet's Syndrome (SS) are inflammatory skin diseases caused by the accumulation of neutrophils in the skin and, rarely, in internal organs. These neutrophilic dermatosis (NDs) are distinguished by the existence of forms of transition or overlap. They are frequently associated to systemic diseases especially hematologic and gastrointestinal ones. We report a case of a patient with ulcerative colitis (UC) who successively developed two types of NDs: PG then SS. A 66 years old patient with a history of UC consulted in July 2012 for an erythematous swelling of the back of the right hand treated with antibiotics without improvement. At that time, bacteriological samples were negative. In October 2012, he was hospitalized for polyarthralgia and impaired general condition. In physical examination, he had vesiculobullous plaque of 10 cm long of the right hand and wrist, infiltrated erythematous plaque on the right leg and another topped with a large pustule at the left ankle. Skin biopsy showed at the back of the right hand an aspect of PG and at the infiltrated plaques of the ankle an aspect of SS. Prednisone was started with improvement of the skin lesions and a recovery condition. The combination of PG and SS has already been described in cases of hematologic malignancy and rarely in UC. There is also the notion of passage from a neutrophilic dermatosis to another. Indeed, a typical lesion initially of SS can evolve to a future PG. This case demonstrates that neutrophilic dermatoses form a continous spectrum of entities that may occur in UC
Negative magnetoresistance in La(0.6)Y(0.1)Ca(0.3)MnO(3): Evidence for charge localization governed by the Curie-Weiss law
Colossal negative magnetoresistance \Delta \rho (T,B) observed in
La(0.6)Y(0.1)Ca(0.3)MnO(3) at B=1T shows a nearly perfect symmetry around
T_0=160K suggesting a universal field-induced transport mechanism in this
material. Attributing this symmetry to strong magnetic fluctuations (triggered
by the Y substitution and further enhanced by magnetic field, both above and
below the field-dependent Curie temperature T_C(B)=T_0), the data are
interpreted in terms of the nonthermal spin hopping and magnetization M
dependent charge carrier localization scenario leading to \Delta \rho (T,B)=
-\rho_s(1-exp(-\gamma M^2)) with M(T,B)=CB/|T-T_C|^n. The separate fits through
all the data points above and below T_C yield C^{+}\simeq C^{-} and n^{+}\simeq
n^{-}\simeq 1. The obtained results corroborate the importance of fluctuation
effects in this material recently found (cond-mat/9812219) to dominate its
magneto-thermopower behavior far beyond T_C.Comment: 4 pages (REVTEX), 2 PS figures (epsf.sty); submitted to Phys.Rev.
Two band gap field-dependent thermal conductivity of
The thermal conductivity of the new superconductor was
studied as a function of the temperature and a magnetic field. No anomaly in
the thermal conductivity is observed around the superconducting
transition in absence or presence of magnetic fields up to 14 Tesla; upon that
field the superconductivity of persisted. The thermal conductivity in
zero-field shows a -linear increase up to 50K. The thermal conductivity is
found to increase with increasing field at high fields. We interpret the
findings as if there are two subsystems of quasiparticles with different
field-dependent characters in a two ( and )-band superconductor reacting
differently with the vortex structure. The unusual enhancement of at low temperature but higher than a () critical field
is interpreted as a result of the overlap of the low energy states outside the
vortex cores in the -band.Comment: 6 pages,3 figure
Density functional theory calculations of the carbon ELNES of small diameter armchair and zigzag nanotubes: core-hole, curvature and momentum transfer orientation effects
We perform density functional theory calculations on a series of armchair and
zigzag nanotubes of diameters less than 1nm using the all-electron
Full-Potential(-Linearised)-Augmented-Plane-Wave (FPLAPW) method. Emphasis is
laid on the effects of curvature, the electron beam orientation and the
inclusion of the core-hole on the carbon electron energy loss K-edge. The
electron energy loss near-edge spectra of all the studied tubes show strong
curvature effects compared to that of flat graphene. The curvature induced
hybridisation is shown to have a more drastic effect on the
electronic properties of zigzag tubes than on those of armchair tubes. We show
that the core-hole effect must be accounted for in order to correctly reproduce
electron energy loss measurements. We also find that, the energy loss near edge
spectra of these carbon systems are dominantly dipole selected and that they
can be expressed simply as a proportionality with the local momentum projected
density of states, thus portraying the weak energy dependence of the transition
matrix elements. Compared to graphite, the ELNES of carbon nanotubes show a
reduced anisotropy.Comment: 25 pages, 15 figures, revtex4 submitted for publication to Phys. Rev.
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