Contrast-enhanced ultrasonography for the management of portal hypertension in cirrhosis

Portal hypertension is a major pathophysiological condition in patients with cirrhosis. This accounts for the occurrence and severity of the various manifestations. The degree is determined by the portal pressure or hepatic venous pressure gradients, both of which are obtained by invasive interventional radiological procedures. Ultrasound (US) is a simple and minimally invasive imaging modality for the diagnosis of liver diseases. Owing to the availability of microbubble-based contrast agents and the development of imaging modes corresponding to contrast effects, contrast-enhanced US (CEUS) has become popular worldwide for the detailed evaluation of hepatic hemodynamics, diffuse liver disease, and focal hepatic lesions. Recent advancements in digital technology have enabled contrast-based demonstrations with improved resolution, leading to a wider range of applications. This review article describes the current role, benefits, and limitations of CEUS in the management of portal hypertension

V-shaped dislocations in a GaN epitaxial layer on GaN substrate

In this study, V-shaped dislocations in a GaN epitaxial layer on a free-standing GaN substrate were observed. Our investigation further revealed that the V-shaped dislocations were newly generated at the interface in the epilayer rather than propagated from the GaN substrate. V-shaped dislocations consist of two straight parts. The straight parts of the V-shaped dislocations were separated from each other in the m-direction and tilted toward the step-flow direction of the GaN epitaxial layer. The V-shaped dislocations are continuous single dislocations having a Burgers vector component of 1a and an intrinsic stacking fault between their straight parts

Superconductivity in the Mo_2Re_3B compound

We report on the synthesis and on basic superconducting properties of a completely new Mo_2Re_3B ternary boride. The crystal structure of the Mo_2Re_3B compound is characterised by Pmmm space group and the cell parameters: a=11.626 A, b=8.465 A and c=8.026 A. The critical temperature is Tc=8.5 K, whereas the lower and the upper-critical fields at zero temperature are equal to Hc1(0)=19.2 mT and to Hc2(0)=3.7 T, respectively. The corresponding Ginzburg-Landau parameter is equal to k=16.5 and the superconducting gap is estimated to be 2delta/kTc=3.2.Comment: 17 pages, 5 figure

Anisotropic s-wave superconductivity in MgB_2

It has recently been observed that MgB_2 is a superconductor with a high transition temperature. Here we propose a model of anisotropic s-wave superconductivity which consistently describes the observed properties of this compound, including the thermodynamic and optical response in sintered MgB_2 wires. We also determine the shape of the quasiparticle density of states and the anisotropy of the upper critical field and the superfluid density which should be detectable once single-crystal samples become available.Comment: RevTex, 10 pages with 4 eps figure

Ansiotropic s- wave superconductivity in borocarbides LuNi2B2C and YNi2B2C

The symmetry of superconductivity in borocarbides LuNi2B2C and YNi2B2C is an outstanding issue. Here an anisotropic s- wave order parameter (or s+g model) is proposed for these borocarbides. In spite of the dominant s- wave component the proposed superconducting order parameter has nodes and gives rise to the H^(1/2) dependent specific heat in the vortex state (the Volovik effect). This model predicts the fourfold symmetry both in the angular dependent thermal conductivity and in the excess Dingle temperature in the vortex state, which should be readily accessible experimentally.Comment: 4 pages, 3 figure

Surface effects in multiband superconductors. Application to MgB2_2

Metals with many bands at the Fermi level can have different band dependent gaps in the superconducting state. The absence of translational symmetry at an interface can induce interband scattering and modify the superconducting properties. We dicuss the relevance of these effects to recent experiments in MgB$_2$

Mixed state properties of superconducting MgB2 single crystals

We report on measurements of the magnetic moment in superconducting MgB2 single crystals. We find \mu_0H_{c2}^c(0) = 3.2 T, \mu_0H_{c2}^{ab}(0) = 14.5 T, \gamma = 4.6, \mu_0H_c(0) = 0.28 T, and \kappa(T_c) = 4.7. The standard Ginzburg-Landau and London model relations lead to a consistent data set and indicate that MgB2 is a clean limit superconductor of intermediate coupling strength with very pronounced anisotropy effects

Upper critical field in dirty two-band superconductors: breakdown of the anisotropic Ginzburg-Landau theory

We investigate the upper critical field in a dirty two-band superconductor within quasiclassical Usadel equations. The regime of very high anisotropy in the quasi-2D band, relevant for MgB$_{2}$, is considered. We show that strong disparities in pairing interactions and diffusion constant anisotropies for two bands influence the in-plane $H_{c2}$ in a different way at high and low temperatures. This causes temperature-dependent $H_{c2}$ anisotropy, in accordance with recent experimental data in MgB$_{2}$. The three-dimensional band most strongly influences the in-plane $H_{c2}$ near $T_{c}$, in the Ginzburg-Landau (GL) region. However, due to a very large difference between the c-axis coherence lengths in the two bands, the GL theory is applicable only in an extremely narrow temperature range near $T_c$. The angular dependence of $H_{c2}$ deviates from a simple effective-mass law even near $T_c$.Comment: 12 pages, 5 figures, submitted to Phys.Rev.

Superconducting properties and c-axis superstructure of Mg1-xAlxB2

The superconducting and structural properties of a series of Mg1-xAlxB2 samples have been investigated. X-ray diffraction results confirmed the existence of a structural transition associated with the significant change in inter-boron layer distance as reported previously by Slusky et al. Moreover,transmission-electron-microscopy observations revealed the existence of a superstructure with doubled lattice constant along the c-axis direction. We propose that this superstructure is essentially related to the structural transition. The modifications of superconducting transition temperature Tc, the normal state resistivity, and the upper critical field Bc2 by Al doping are discussed in terms of Al-substitution induced changes in the electronic structure at the Fermi energy.Comment: 15 pages, 7 figure

Limits of the upper critical field in dirty two-gap superconductors

An overview of the theory of the upper critical field in dirty two-gap superconductors, with a particular emphasis on MgB$_2$ is given. We focus here on the maximum $H_{c2}$ which may be achieved by increasing intraband scattering, and on the limitations imposed by weak interband scattering and paramagnetic effects. In particular, we discuss recent experiments which have recently demonstrated ten-fold increase of $H_{c2}$ in dirty carbon-doped films as compared to single crystals, so that the $H_{c2}(0)$ parallel to the ab planes may approach the BCS paramagnetic limit, $H_p[T] = 1.84T_c[K] \simeq 60-70T$. New effects produced by weak interband scattering in the two-gap Ginzburg-Landau equations and $H_{c2}(T)$ in ultrathin MgB$_2$ films are addressed.Comment: Submitted to the special issue of Physica C on MgB$_2$ on Dec. 3, 200