Showing papers by "Alain Junod published in 2006"
TL;DR: The second highest critical temperature in the boride family was reported in this article, where the specific heat, resistivity, magnetic susceptibility, and thermal expansion from 2πphantom{\rule{0.3em}{0ex}}\text{to}ϵϵγγγδδ ε(ϵ, 0.3m), ϵ, 1.5m, ε, 2.2m, and ϵγε (ϵ) were measured on a single crystal with ϵ = 7.7.2
Abstract: The superconductor $\mathrm{Y}{\mathrm{B}}_{6}$ has the second highest critical temperature ${T}_{c}$ among the boride family $M{\mathrm{B}}_{\mathrm{n}}$. We report measurements of the specific heat, resistivity, magnetic susceptibility, and thermal expansion from $2\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, using a single crystal with ${T}_{c}=7.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. 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 $F(\ensuremath{\omega})$, the spectral electron-phonon scattering function ${\ensuremath{\alpha}}_{\mathit{tr}}^{2}F(\ensuremath{\omega})$, and the phonon density of states weighted by the frequency-dependent Gr\"uneisen parameter ${\ensuremath{\gamma}}_{G}(\ensuremath{\omega})F(\ensuremath{\omega})$, respectively. Lattice vibrations extend to high frequencies $g100\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, but a dominant Einstein-like mode at $\ensuremath{\sim}8\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, 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 ${H}_{\mathrm{c}3}$ is also observed.
78 citations
TL;DR: In this paper, the superconducting transition in a single overdoped, detwinned YBa 2 Cu 3 O x (YBCO) crystal was studied using four different probes.
Abstract: The superconducting transition in a single overdoped, detwinned YBa 2 Cu 3 O x (YBCO) crystal is studied using four different probes. Whereas the AC and DC magnetic susceptibilities find a dominant transition at 88 K with a smaller effect near 92 K, the specific heat and electrical resistivity reveal only a single transition at 88 K and 92 K, respectively. Under hydrostatic pressures to 0.60 GPa these two transitions shift in opposite directions, their separation increasing. The present experiments clearly show that the bulk transition lies at 88 K and originates from fully oxygenated YBCO; the 92 K transition likely arises from filamentary superconductivity in a minority optimally doped phase (
31 citations
TL;DR: In this article, a new type of calorimeter was used to resolve a small lambda anomaly within a narrow temperature range around the ${H}_{c2}$ line, and the evolution of the anomaly as a function of magnetic field follows scaling laws expected in the presence of critical fluctuations.
Abstract: The range of critical thermal fluctuations in ``classical'' bulk superconductors is extremely small and especially in low fields hardly experimentally accessible. With a new type of calorimeter we have been able to resolve a small lambda anomaly within a narrow temperature range around the ${H}_{c2}$ line. We show that the evolution of the anomaly as a function of magnetic field follows scaling laws expected in the presence of critical fluctuations. The lower onset of the fluctuation regime shows many characteristics of a continuous solid-to-liquid transition in the vortex matter. It can be driven into a first-order vortex melting transition by a small ac field which helps the vortex matter to reach equilibrium.
26 citations
TL;DR: In this article, the optical properties of high-quality single crystals of low temperature superconductors ZrB12 (Tc = 5.95 K) and Yttrium hexaboride YB6 (tc = 7.15 K) were reported.
Abstract: We report the optical properties of high-quality single crystals of low temperature superconductors zirconiumdodecaboride ZrB12 (Tc = 5.95 K) and yttrium hexaboride YB6 (Tc = 7.15 K) in the range 6 meV - 4.6 eV at room temperature. The experimental optical conductivity was extracted from the analysis of the reflectivity in the infrared range and ellipsometry measurement of the dielectric function in the visible range. The electronic band structure of these compounds was calculated by the self-consistent full-potential LMTO method and used to compute the interband part of the optical conductivity and the plasma frequency Ωp. A good agreement was observed between the interband part of the experimental optical conductivities and the band structure calculations. Different methods combining optical spectroscopy, resistivity, specific heat measurements and results of band structure calculations are used to determine the electron-phonon coupling constant. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
6 citations