Superconductivity in the heavy-fermion compound CeCu 2 Si 2 is a prototypical example of Cooper pairs formed by strongly correlated electrons. For more than 30 years, it has been believed to arise from nodal d-wave pairing mediated by a magnetic glue. Here, we report a detailed study of the specific heat and magnetization at low temperatures for a high-quality single crystal. Unexpectedly, the specific-heat measurements exhibit exponential decay with a two-gap feature in its temperature dependence, along with a linear dependence as a function of magnetic field and the absence of oscillations in the field angle, reminiscent of multiband full-gap superconductivity. In addition, we find anomalous behavior at high fields, attributed to a strong Pauli paramagnetic effect. A low quasiparticle density of states at low energies with a multiband Fermi-surface topology would open a new door into electron pairing in CeCu 2 Si 2 .PACS numbers: 74.70. Tx, 74.25.Bt, 74.25.Op After the first discovery of heavy-fermion superconductivity in CeCu 2 Si 2 [1], a number of unconventional superconductors, such as high-T c cuprates, iron-pnictides, organic, and heavy-fermion superconductors, have been found. Among the various issues on these novel superconductors, the identification of the superconducting gap structure is one of the most important subjects because it is closely related to the pairing mechanism. Particularly, the gap symmetry of CeCu 2 Si 2 has attracted attention because superconductivity in this compound emerges near an antiferromagnetic (AFM) quantum critical point and heavy quasiparticles (QPs) couple to quantum critical spin excitations [2].Up to now, the gap symmetry of CeCu 2 Si 2 was inferred to be an even-parity d-wave type with line nodes. The well resolved decrease in the NMR Knight shift below the transition temperature T c ≃ 0.6 K [3] is a strong evidence for the spin part of the Cooper pairs being a singlet. Indeed, the low-T saturation of the upper critical field H c2 is attributed to the Pauli paramagnetic effect due to the spin-singlet pairing. Based on the T 3 dependence of the nuclear relaxation rate 1/T 1 and the absence of a coherence peak [4][5][6], the superconducting gap was proposed to possess line nodes. Presently the debate is whether the gap symmetry is d x 2 −y 2 or d xy type [7, 8]. However, the presence of line nodes as well as the symmetry of the gap has not yet been studied precisely using low-T thermodynamic properties.To elucidate the gap structure of CeCu 2 Si 2 , the specific heat C in magnetic fields H is herein measured at temperatures down to 40 mK using a high-quality single-crystalline sample. Measurement of C probes the QP density of states (DOS) that depends on the nodal structure. An S-type single crystal (having a mass of 13.8 mg) was used that presents only a superconducting ground state without magnetic ordering, since other types of CeCu 2 Si 2 show additional contributions in C(T ) at low temperatures that make the interpretation of the data difficult. Growth and cha...

The temperature, field, and field-orientation dependences of the electronic specific heat C e of the ironpnictide superconductor KFe 2 As 2 have been investigated. Thermodynamic evidence of the presence of line nodes is obtained from the T and √ H linear dependences of C e /T in the low-T and low-H region. Under a magnetic field rotated within the tetragonal ab plane, a fourfold oscillation is observed in C e with a sign change at 0.08T c . On the basis of the Doppler-shift analysis, the observed C e minima in H [100] at low T indicate the presence of line nodes somewhere on the Fermi surface where the Fermi velocity is parallel to the [100] direction; this is consistent with the octet-line-node scenario proposed recently by a photoemission experiment. In addition, the low-T C e /T exhibits an unusual upturn on cooling at moderate fields only for H ab, which is understood in terms of the strong Pauli paramagnetic effect on multiband superconductivity.

This paper presents the results of specific-heat and magnetization measurements, in particular their field-orientation dependence, on the first discovered heavy-fermion superconductor CeCu2Si2 (Tc ∼ 0.6 K). We discuss the superconducting gap structure and the origin of the anomalous pairbreaking phenomena, leading e.g., to the suppression of the upper critical field Hc2, found in the high-field region. The data show that the anomalous pair breaking becomes prominent below about 0.15 K in any field direction, but occurs closer to Hc2 for H c. The presence of this anomaly is confirmed by the fact that the specific-heat and magnetization data satisfy standard thermodynamic relations. Concerning the gap structure, field-angle dependences of the low-temperature specific heat within the ab and ac planes do not show any evidence for gap nodes. From microscopic calculations in the framework of a two-band full-gap model, the power-law-like temperature dependences of C and 1/T1, reminiscent of nodal superconductivity, have been reproduced reasonably. These facts further support multiband full-gap superconductivity in CeCu2Si2.

A novel numerical simulation method based on finite element analysis (FEA), which can evaluate the fracture probability caused by the characteristics of flaw distribution, is considered an effective tool to facilitate and increase the use of ceramics in components and members. In this study, we propose an FEA methodology to predict the scatter of ceramic strength. Specifically, the data on the microstructure distribution (i.e., relative density, size and aspect ratio of pore, and grain size) are taken as the input values and reflected onto the parameters of a continuum damage model via a fracture mechanical model based on the circumferential circular crack emanating from an oval spherical pore. In addition, we numerically create a Weibull distribution based on multiple FEA results of a three‐point bending test. Its validity is confirmed by a quantitative comparison with the actual test results. The results suggest that the proposed FEA methodology can be applied to the analysis of the fracture probability of ceramics.

We have demonstrated the high-peak-power operation of an AlGaN-based ultraviolet laser diode (UV-LD) with a lasing wavelength of 338.6 nm. The UV-LD structure was fabricated on a bulk GaN(0001) substrate. The broad-area and vertical conductive structure of the UV-LD, whose ridge width and cavity length were 50 and 600 µm, respectively, was employed. The threshold current density and differential external quantum efficiency were estimated to be 38.9 kA/cm2 and 8.5%, respectively. The characteristic temperature of threshold current was estimated to be 119 K, and the temperature dependence of lasing wavelength was obtained to be 0.033 nm K−1. A peak power of over 1 W has been achieved in 338.6 nm under pulsed operation at room temperature, which is the highest peak power ever obtained for AlGaN-based UV-LDs.