2,449 research outputs found
Vanishing of the upper critical field in Bi_2Sr_2CaCu_2O_{8+\delta} from Landau-Ott scaling
We apply Landau-Ott scaling to the reversible magnetization data of
BiSrCaCuO published by Y. Wang et al. [\emph{Phys.
Rev. Lett. \textbf{95} 247002 (2005)}] and find that the extrapolation of the
Landau-Ott upper critical field line vanishes at a critical temperature
parameter, T^*_c, a few degrees above the zero resistivity critical
temperature, T_c. Only isothermal curves below and near to T_c were used to
determine this transition temperature. This temperature is associated to the
disappearance of the mixed state instead of a complete suppression of
superconductivity in the sample.Comment: 3 figure
Spectrum and diffusion for a class of tight-binding models on hypercubes
We propose a class of exactly solvable anisotropic tight-binding models on an
infinite-dimensional hypercube. The energy spectrum is analytically computed
and is shown to be fractal and/or absolutely continuous according to the value
hopping parameters. In both cases, the spectral and diffusion exponents are
derived. The main result is that, even if the spectrum is absolutely
continuous, the diffusion exponent for the wave packet may be anything between
0 and 1 depending upon the class of models.Comment: 5 pages Late
EFFECT OF SEASON ON SERUM COPPER AND ZINC CONCENTRATIONS IN CROSSBRED GOATS HAVING DIFFERENT REPRODUCTIVE STATUS UNDER SEMIARID RANGELAND CONDITIONS IN SOUTHERN MEXICO STATE
The effect of season (rainy: RS, and dry: DS) and reproductive status on copper (Cu) and zinc (Zn) concentrations in blood serum of crossbred goats (BW= 36.01 ± 1.59 kg) were studied under semiarid rangeland conditions in Southern Mexico State. Blood samples from 80 crossbred goats were taken each season (RS and DS). The goats were clustered into 10 different groups considering their reproductive status. Concentrations of Cu and Zn in serum were assayed using atomic absorption. Data were analyzed using a general linear model procedure for a completely randomized design and differences among means were examined using a Tukey test. Blood serum concentrations of Cu and Zn were affected by reproductive status and season (P<0.001). In relation to the season, Cu and Zn serum levels were lower in RS than DS (P<0.05). Overall, kidded goats had the highest values (P<0.01) for Cu than other animals in both seasons (RS or DS). Anestrous goats had the lowest concentrations (P<0.01) for Zn during RS, while all goats at their second or more kidding, rearing single or twins, showed the highest concentrations of Zn (P<0.001) in this season. Adult goats in Southern Mexico State showed a deficiency of Cu and Zn, especially during RS. As such, mineral supplements should be provided with respect to these elements in feeding systems for goats under semiarid rangeland conditions in order to evaluate their impact on health and reproduction
Thymic hyperplasia in a patient with Grave's disease
Hyperplastic changes of the thymus may be found in patients with Graves' disease. However, this rarely presents as an anterior mediastinal mass, particularly among adults. In this report, we describe a 46-year old woman with Graves' disease and thymic hyperplasia
A comparative study of high-field diamagnetic fluctuations in deoxygenated YBa2Cu3O(7-x) and polycrystalline (Bi-Pb)2Sr2Ca3O(10)
We studied three single crystals of YBa2Cu3O{7-x} with Tc= 62.5, 52, and 41
K, and a textured specimen of (Bi-Pb)2Sr2Ca2Cu3O10 with Tc=108 K, for H//c
axis. The reversible data were interpreted in terms of 2D lowest-Landau-level
fluctuation theory. The data were fit well by the 2D LLL expression for
magnetization obtained by Tesanovic etal., producing reasonable values of kappa
but larger values of dHc2/dT. Universality was studied by obtaining a
simultaneous scaling of Y123 data and Bi2223. An expression for the 2D x-axis
LLL scaling factor used to obtain the simultaneous scaling was extracted from
theory, and compared with the experimental values. The comparison between the
values of the x-axis produced a deviation of 40% which suggests that the
hypothesis of universality of the 2D-LLL fluctuations is not supported by the
studied samples. We finaly observe that Y123 magnetization data for
temperatures above obbey a universal scaling obtained for the diamagnetic
fluctuation magnetization from a theory considering non-local field effects.
The same scaling was not obbeyed by the corresponding magnetization calculated
from the two-dimensional lowest-Landau-level theory.Comment: 7 pages 5 figures, accept in Journ. Low Temp. Phy
Generalized "Quasi-classical" Ground State for an Interacting Two Level System
We treat a system (a molecule or a solid) in which electrons are coupled
linearly to any number and type of harmonic oscillators and which is further
subject to external forces of arbitrary symmetry. With the treatment restricted
to the lowest pair of electronic states, approximate "vibronic"
(vibration-electronic) ground state wave functions are constructed having the
form of simple, closed expressions. The basis of the method is to regard
electronic density operators as classical variables. It extends an earlier
"guessed solution", devised for the dynamical Jahn-Teller effect in cubic
symmetry, to situations having lower (e.g., dihedral) symmetry or without any
symmetry at all. While the proposed solution is expected to be quite close to
the exact one, its formal simplicity allows straightforward calculations of
several interesting quantities, like energies and vibronic reduction (or Ham)
factors. We calculate for dihedral symmetry two different -factors (""
and "") and a -factor. In simplified situations we obtain . The formalism enables quantitative estimates to be made for the dynamical
narrowing of hyperfine lines in the observed ESR spectrum of the dihedral
cyclobutane radical cation.Comment: 28 pages, 4 figure
Electronic Properties of Graphene in a Strong Magnetic Field
We review the basic aspects of electrons in graphene (two-dimensional
graphite) exposed to a strong perpendicular magnetic field. One of its most
salient features is the relativistic quantum Hall effect the observation of
which has been the experimental breakthrough in identifying pseudo-relativistic
massless charge carriers as the low-energy excitations in graphene. The effect
may be understood in terms of Landau quantization for massless Dirac fermions,
which is also the theoretical basis for the understanding of more involved
phenomena due to electronic interactions. We present the role of
electron-electron interactions both in the weak-coupling limit, where the
electron-hole excitations are determined by collective modes, and in the
strong-coupling regime of partially filled relativistic Landau levels. In the
latter limit, exotic ferromagnetic phases and incompressible quantum liquids
are expected to be at the origin of recently observed (fractional) quantum Hall
states. Furthermore, we discuss briefly the electron-phonon coupling in a
strong magnetic field. Although the present review has a dominating theoretical
character, a close connection with available experimental observation is
intended.Comment: 56 pages, 27 figures; published version with minor corrections and
updated reference
Sr impurity effects on the magnetic correlations of LaSrCuO
We examine the low-temperature magnetic properties of moderately doped
LaSrCuO paying particular attention to the spin-glass (SG) phase and the C-IC
transition as they are affected by Sr impurity disorder. New measurements of
the low-temperature susceptibility in the SG phase show an increase of an
anomalously small Curie constant with doping. This behaviour is explained in
terms of our theoretical work that finds small clusters of AFM correlated
regions separated by disordered domain walls. The domain walls lead to a
percolating sequence of paths connecting the impurities. We predict that for
this spin morphology the Curie constant should scale as , a
result that is quantitatively in agreement with experiment. Also, we find that
the magnetic correlations in the ground states in the SG phase are
commensurate, and that this behaviour should persist at higher temperatures
where the holes should move along the domain walls. However, our results show
that incommensurate correlations develop continuously around 5 % doping,
consistent with recent measurements by Yamada.Comment: 30 pages, revtex, 8 .ps format figures (2 meant to be in colour), to
be published in Physical Review B
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