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) $\Delta S(T,H)$ of perovskite type manganise oxide $La_{0.6}Y_{0.1}Ca_{0.3}MnO_3$ is found to exhibit a sharp peak at some temperature $T^{*}=170K$. By approximating the true shape of the measured magneto-TEP in the vicinity of $T^{*}$ by a linear triangle of the form $\Delta S(T,H)\simeq S_p(H)\pm B^{\pm}(H)(T^{*}-T)$, we observe that $B ^{-}(H)\simeq 2B ^{+}(H)$. 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 $T_C$ and the "metal-insulator" (M-I) transition at $T_{MI}$. The latter is characterized by the divergence of the field-dependent charge carrier localization length $\xi (T,H)$ at some characteristic field $H_0$. Calculating the average and fluctuation contributions to the total magnetization and the transport entropy related magneto-TEP $\Delta S(T,H)$ within the GL theory, we obtain a simple relationship between $T^{*}$ and the above two critical temperatures ($T_{C}$ and $T_{MI}$). The observed slope ratio $B ^{-}(H)/B ^{+}(H)$ is found to be governed by the competition between the electron-spin exchange $JS$ and the induced magnetic energy $M_sH_0$. The comparison of our data with the model predictions produce $T_{C}=195K$, $JS=40meV$, $M_0=0.4M_s$, $\xi_0=10\AA$, and $n_e/n_i=2/3$ 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 $La_{0.7}Ca_{0.3}Mn_{1-x}Cu_xO_3$ has been observed at slight $Cu$ doping ($0\leq x \leq 0.05$). Namely, introduction of copper results in a splitting of the resistivity maximum around a metal-insulator transition temperature $T_0(x)$ into two differently evolving peaks. Unlike the original $Cu$-free maximum which steadily increases with doping, the second (satellite) peak remains virtually unchanged for $x<x_c$, increases for $x\ge x_c$ and finally disappears at $x_m\simeq 2x_c$ with $x_c\simeq 0.03$. The observed phenomenon is thought to arise from competition between substitution induced strengthening of potential barriers (which hamper the charge hopping between neighboring $Mn$ 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 MgB2MgB_2

The thermal conductivity $\kappa (H,T)$ of the new superconductor $MgB_2$ was studied as a function of the temperature and a magnetic field. No anomaly in the thermal conductivity $\kappa (H,T)$ is observed around the superconducting transition in absence or presence of magnetic fields up to 14 Tesla; upon that field the superconductivity of $MgB_2$ persisted. The thermal conductivity in zero-field shows a $T$-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 ($L$ and $S$)-band superconductor reacting differently with the vortex structure. The unusual enhancement of $\kappa (H ,T)$ at low temperature but higher than a ($H_{c2S}\simeq 3T$) critical field is interpreted as a result of the overlap of the low energy states outside the vortex cores in the $S$-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 $\pi-\sigma$ 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.