Specific heat of single crystal MgB_2: a two-band superconductor with two different anisotropies

Heat-capacity measurements of a 39 microgramm MgB_2 single crystal in fields up to 14 T and below 3 K allow the determination of the low-temperature linear term of the specific heat, its field dependence and its anisotropy. Our results are compatible with two-band superconductivity, the band carrying the small gap being isotropic, that carrying the large gap having an anisotropy of ~ 5. Three different upper critical fields are thus needed to describe the superconducting state of MgB2.Comment: 4 pages, 4 figures - V2: Bibliography updated and some typo corrected. One reference added - V3: version accepted for publication in PRL, changes made in the tex

Inequivalent representations of commutator or anticommutator rings of field operators and their applications

Hamiltonian of a system in quantum field theory can give rise to infinitely many partition functions which correspond to infinitely many inequivalent representations of the canonical commutator or anticommutator rings of field operators. This implies that the system can theoretically exist in infinitely many Gibbs states. The system resides in the Gibbs state which corresponds to its minimal Helmholtz free energy at a given range of the thermodynamic variables. Individual inequivalent representations are associated with different thermodynamic phases of the system. The BCS Hamiltonian of superconductivity is chosen to be an explicit example for the demonstration of the important role of inequivalent representations in practical applications. Its analysis from the inequivalent representations' point of view has led to a recognition of a novel type of the superconducting phase transition.Comment: 25 pages, 6 figure

Observation of Magnetic Flux Generated Spontaneously During a Rapid Quench of Superconducting Films

We report observations of spontaneous formation of magnetic flux lines during a rapid quench of YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ films through T$_{c}$. This effect is predicted according to the Kibble-Zurek mechanism of creation of topological defects of the order parameter during a symmetry-breaking phase transition. Our previous experiment, at a quench rate of 20K/sec, gave null results. In the present experiment, the quench rate was increased to \TEXTsymbol{>} 10$^{8}$ K/sec. Within experimental resolution, the dependence of the measured flux on the cooling rate is consistent with the prediction

Superconductivity mediated by a soft phonon mode: specific heat, resistivity, thermal expansion and magnetization of YB6

The superconductor YB6 has the second highest critical temperature Tc among the boride family MBn. We report measurements of the specific heat, resistivity, magnetic susceptibility and thermal expansion from 2 to 300 K, using a single crystal with Tc = 7.2 K. The superconducting gap is characteristic of medium-strong coupling. The specific heat, resistivity and expansivity curves are deconvolved to yield approximations of the phonon density of states, the spectral electron-phonon scattering function and the phonon density of states weighted by the frequency-dependent Grueneisen parameter respectively. Lattice vibrations extend to high frequencies >100 meV, but a dominant Einstein-like mode at ~8 meV, associated with the vibrations of yttrium ions in oversized boron cages, appears to provide most of the superconducting coupling and gives rise to an unusual temperature behavior of several observable quantities. A surface critical field Hc3 is also observed.Comment: 29 pages, 5 tables, 17 figures. Accepted for publication in Phys. Rev.

SO(5) superconductor in a Zeeman magnetic field: Phase diagram and thermodynamic properties

In this paper we present calculations of the SO(5) quantum rotor theory of high-T$_{c}$ superconductivity in Zeeman magnetic field. We use the spherical approach for five-component quantum rotors in three-dimensional lattice to obtain formulas for critical lines, free energy, entropy and specific heat and present temperature dependences of these quantities for different values of magnetic field. Our results are in qualitative agreement with relevant experiments on high-T$_{c}$ cuprates.Comment: 4 pages, 2 figures, to appear in Phys. Rev. B, see http://prb.aps.or

Dense breast on screening mammography: utility and futility of additional ultrasound.

High breast density decreases the sensitivity of mammography. Regardless of masking effect, it is also a stronger predictor for breast cancer than most other risk factors, including family history. Up to 50% of women have dense breast (categories C+D). The relative risk is 2.1 to 2.3 in women with extremely dense breast (category d). There is little consensus on the potential need of additional measures. Breast Ultrasound (US) may be proposed as an adjunctive test, as it is non-invasive and non-irradiating. Since 2012, our program for breast cancer screening propose to realise an additional US to the women with category d breasts. An evaluation of the data of the first year showed that less 50% of the women act upon the recommendation. In spite of the low compliance supplementary cancers were found. Since, no more evaluation was performed. The aim of this study was: To evaluate the effect of the recommendation in term of additional cancer's identification, additional biopsies and additional investigations. To identify the reasons of non compliance

Low energy physical properties of high-Tc superconducting Cu oxides: A comparison between the resonating valence bond and experiments

In a recent review by Anderson and coworkers\cite{Vanilla}, it was pointed out that an early resonating valence bond (RVB) theory is able to explain a number of unusual properties of high temperature superconducting (SC) Cu-oxides. Here we extend previous calculations \cite{anderson87,FC Zhang,Randeria} to study more systematically low energy physical properties of the plain vanilla d-wave RVB state, and to compare results with the available experiments. We use a renormalized mean field theory combined with variational Monte Carlo and power Lanczos methods to study the RVB state of an extended $t-J$ model in a square lattice with parameters suitable for the hole doped Cu-oxides. The physical observable quantities we study include the specific heat, the linear residual thermal conductivity, the in-plane magnetic penetration depth, the quasiparticle energy at the antinode $(\pi, 0)$, the superconducting energy gap, the quasiparticle spectra and the Drude weight. The traits of nodes (including $k_{F}$, the Fermi velocity $v_{F}$ and the velocity along Fermi surface $v_{2}$), as well as the SC order parameter are also studied. Comparisons of the theory and the experiments in cuprates show an overall qualitative agreement, especially on their doping dependences.Comment: 12 pages, 14 figures, 1 tabl

Direct Observation and Anisotropy of the Contribution of Gap nodes in the Low Temperature Specific Heat of YBa_2Cu_3O_7

The specific heat due to line nodes in the superconducting gap of YBa2Cu3O7 has been obscured up to now by magnetic terms of extrinsic origin, even for high quality crystals. We report the specific heat of a new single crystal grown in a non-corrosive BaZrO3 crucible, for which paramagnetic terms are reduced to less than one spin-1/2 center for 20'000 Cu atoms. The contribution of line nodes shows up directly in the difference C(B,T) - C(0,T) at fixed temperatures (T < 5 K) as a function of the magnetic field parallel to the c-axis (B<=14 T). These data illustrate the smooth crossover from C propotional to T^2 at low fields to C propotional to TB^1/2 at high fields, and provide new values for gap parameters which are quantitatively consistent with tunneling spectroscopy and thermal conductivity in the framework of dx^2-y^2 pairing symmetry. Data for B along the nodal and antinodal directions in the ab-plane are also provided. The in-plane anisotropy predicted in the clean limit is not observed.Comment: 29 pages(using Revtex style), 14 postscript figures, submitted to Phys. Rev. B Content of the file changed after replacin

Phase fluctuations and the pseudogap in YBa2Cu3Ox

The thermodynamics of the superconducting transition is studied as a function of doping using high-resolution expansivity data of YBa2Cu3Ox single crystals and Monte-Carlo simulations of the anisotropic 3D-XY model. We directly show that Tc of underdoped YBa2Cu3Ox is strongly suppressed from its mean-field value (Tc-MF) by phase fluctuations of the superconducting order parameter. For overdoped YBa2Cu3Ox fluctuation effects are greatly reduced and Tc ~ Tc-MF . We find that Tc-MF exhibits a similar doping dependence as the pseudogap energy, naturally suggesting that the pseudogap arises from phase-incoherent Cooper pairing.Comment: 9 pages, 3 Figure