K. Uozato

Two Types of Multistack Structures in MgB2-Type Superconductor CaAlSi

Crystal structure analysis of an AlB 2 -type superconductor, CaAlSi, has been carried out using synchrotron X-rays and neutron diffraction measurements. Two different stackings along the c -axis–5-...

Anisotropic s-wave superconductivity in CaAlSi single crystals from penetration depth measurements

In- and out-of-plane London penetration depths were measured in single crystals CaAlSi (T_{c}=6.2 K and 7.3 K) using a tunnel-diode resonator. A full 3D BCS analysis of the superfluid density is consistent with a prolate spheroidal gap, with a weak-coupling BCS value in the ab-plane and stronger coupling along the c-axis. The gap anisotropy was found to significantly decrease for higher T_{c} samples.

ANISOTROPIC SUPERCONDUCTING PROPERTIES OF CaAlSi AND CaGaSi

Anisotropic superconducting properties of new layered intermetallic compounds CaAlSi and CaGaSi with AlB2 structure are studied. These superconductors show moderate anisotropy with anisotropy parameter determined by both magnetic and transport measurements. Despite the small value of γ, angular dependence of Hc2 in CaAlSi shows cusp-like feature for fields parallel to the superconducting plane, suggesting the presence of two-dimensional superconducting units. By contrast, Hc2(θ) in CaGaSi can be reasonably well explained by the anisotropic GL-model. X-ray diffraction measurements reveal the presence of superstructure along the c-axis in CaAlSi but not in CaGaSi. These findings suggest a close relationship between the anomalous angular dependence of Hc2 and the superstructure. Possible origins of the two-dimensional superconducting units in CaAlSi are discussed.

Anisotropic superconductivity in layered silicides

Abstract Microcrystalline powders of boron-doped diamond were produced in the C–H–B system under a pressure of 8 GPa and at a temperature of more than 2000 K. The presence of boron in the C–B–H system was shown to decrease the temperature–pressure parameters for diamond synthesis compared with those for the binary C–H system (naphthalene). A decrease in the parameters for synthesis in the system with boron may be due to the formation of graphite with less perfect crystal structure during an intermediate stage of diamond formation. Superconducting diamond microcrystals are synthesized in the C–H–B system with boron content of about 5–10 at% in a mixture with naphthalene. Superconductivity below 3.5 K in boron-doped diamond powder is detected in AC magnetic susceptibility measurements.

Search for Superconductivity in Layered Silicides

Several layered silicide systems are studied in order to search for superconductivity. CaAl2Si2 and YAl2Si2, both having puckered double Si‐Al layers, are found to be a semimetal and a typical metal, respectively. Despite the fact that these two compounds have the same crystal structure, it is difficult to obtain a homogeneous solid solution Y1−xCaxAl2Si2. None of them show superconductivity down to 0.3 K. LaAlSi2 is composed of alternate stacking of LaSi2 and LaAl2Si2 layers. LaAlSi2 polycrystalline sample with noticeable amount of impurities shows a trace of superconductivity starting from 0.7 K. However, it is found that LaAlSi2 is not a superconductor, but the superconductivity originates from an unidentified impurity phase containing a part or all of La, Al, and Si.

Origin of Two‐Dimensional Superconductivity in CaAlSi

CaAlSi crystallized in AlB2 structure and shows superconductivity at about 8 K. Although the anisotropy of the upper critical field (Hc2) is weak, its angular dependence shows an anomalous cusp‐like feature when the field is applied parallel to the plane. Such a cusp‐like angular dependence is predicted in the case of thin a film superconductor, when the coherence length is larger than the film thickness. It suggests that the superconductivity in CaAlSi is decoupled into two‐dimensional superconducting units. The size of the two‐dimensional superconducting unit is estimated from the temperature dependence of Hc2(θ). We also examined a possible defect structure using magneto‐optical imaging.