Specific heat of MgB_2 after irradiation

We studied the effect of disorder on the superconducting properties of polycrystalline MgB_2 by specific-heat measurements. In the pristine state, these measurements give a bulk confirmation of the presence of two superconducting gaps with 2 Delta 0 / k_B T_c = 1.3 and 3.9 with nearly equal weights. The scattering introduced by irradiation suppresses T_c and tends to average the two gaps although less than predicted by theory. We also found that by a suitable irradiation process by fast neutrons, a substantial bulk increase of dH_{c2}/dT at T_c can be obtained without sacrificing more than a few degrees in T_c. The upper critical field of the sample after irradiation exceeds 28 T at T goes to 0 K.Comment: 11 pages text, 6 figures, accepted by Journal of Physics: Condensed Matte

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

Specific heat of heavy fermion CePd2Si2 in high magnetic fields

We report specific heat measurements on the heavy fermion compound CePd2Si2 in magnetic fields up to 16 T and in the temperature range 1.4-16 K. A sharp peak in the specific heat signals the antiferromagnetic transition at T_N ~ 9.3 K in zero field. The transition is found to shift to lower temperatures when a magnetic field is applied along the crystallographic a-axis, while a field applied parallel to the tetragonal c-axis does not affect the transition. The magnetic contribution to the specific heat below T_N is well described by a sum of a linear electronic term and an antiferromagnetic spin wave contribution. Just below T_N, an additional positive curvature, especially at high fields, arises most probably due to thermal fluctuations. The field dependence of the coefficient of the low temperature linear term, gamma_0, extracted from the fits shows a maximum at about 6 T, at the point where an anomaly was detected in susceptibility measurements. The relative field dependence of both T_N and the magnetic entropy at T_N scales as [1-(B/B_0)^2] for B // a, suggesting the disappearance of antiferromagnetism at B_0 ~ 42 T. The expected suppression of the antiferromagnetic transition temperature to zero makes the existence of a magnetic quantum critical point possible.Comment: to be published in Journal of Physics: Condensed Matte

Effect of Nanoscale Confinement on the \beta-\alpha Phase Transition in Ag2Se

The confinement of silver selenide was investigated using mesoporous silica. Results from x-ray diffraction and electron microscopy show that the confined material still exhibits a \beta to \alpha transition similar to the one that takes place in the bulk crystalline state but with a transition temperature that depends significantly on the confinement conditions. Decreasing the pore size leads to an increase of the transition temperature, opposite to the behavior of the melting point observed in several metallic and organic materials. In the free particles, on the other hand, no size dependence is observed with particle sizes down to 4 nm

Thermodynamic transitions in inhomogeneous d-wave superconductors

We study the spectral and thermodynamic properties of inhomogeneous d-wave superconductors within a model where the inhomogeneity originates from atomic scale pair disorder. This assumption has been shown to be consistent with the small charge and large gap modulations observed by scanning tunnelling spectroscopy (STS) on BSCCO. Here we calculate the specific heat within the same model, and show that it gives a semi-quantitative description of the transition width in this material. This model therefore provides a consistent picture of both surface sensitive spectroscopy and bulk thermodynamic properties.Comment: 4 pages, 4 figure

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

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

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.

Peculiarities of electronic heat capacity of thulium cuprates in pseudogap state

Precise calorimetric measurements have been carried out in the 7 - 300 K temperature range on two ceramic samples of thulium 123 cuprates TmBa2Cu3O6.92 and TmBa2Cu3O6.70. The temperature dependence of the heat capacity was analyzed in the region where the pseudogap state (PGS) takes place. The lattice contribution was subtracted from the experimental data. The PGS component has been obtained by comparing electronic heat capacities of two investigated samples because the PGS contribution for the 6.92 sample is negligible. The anomalous behavior of the electronic heat capacity near the temperature boundary of PGS was found. It is supposed that this anomaly is due to peculiarities in N(E) function where N is the density of electronic states and E is the energy of carriers of charge.Comment: 12 pages, 3 Postscript figure