Petre Badica

S-wave spin-triplet order in superconductors without inversion symmetry: Li2Pd3B and Li2Pt3B.

We investigate the order parameter of noncentrosymmetric superconductors Li2Pd3B and Li2Pt3B via the behavior of the penetration depth lambda(T). The low-temperature penetration depth shows BCS-like behavior in Li2Pd3B, while in Li2Pt3B it follows a linear temperature dependence. We propose that broken inversion symmetry and the accompanying antisymmetric spin-orbit coupling, which admix spin-singlet and spin-triplet pairing, are responsible for this behavior. The triplet contribution is weak in Li2Pd3B, leading to a wholly open but anisotropic gap. The significantly larger spin-orbit coupling in Li2Pt3B allows the spin-triplet component to be larger in Li2Pt3B, producing line nodes in the energy gap as evidenced by the linear temperature dependence of lambda(T). The experimental data are in quantitative agreement with theory.

Superconductivity in a New Pseudo-Binary Li2B(Pd1-xPtx)3 (x=0–1) Boride System

Recently we have found superconductivity in a cubic antiperovskite-like compound Li 2 BPd 3 . A new pseudo-binary complete solid solution Li 2 B(Pd 1- x Pt x ) 3 , x =0–1 with similar structure has been synthesized and observation of superconductivity in the entire x -range is reported. Our results strongly suggest that superconductivity is of bulk type. Critical temperature T c is decreasing approximately linearly with amount ( x ) of Pt from 7.2–8 K for Li 2 BPd 3 to 2.2–2.8 K for Li 2 BPt 3 . From isothermal magnetization ( M – H ) measurements, lower critical fields H c1 (138 Oe/ x =0 , 38 Oe/ x =1 ), upper critical fields H c2 WHH (3.4 T/ x =0 , 1 T/ x =1 ), coherence length ξ(0) (9.8 nm/ x =0 , 17.9 nm/ x =1 ) and penetration depth λ(0) (190 nm/ x =0 , 364 nm x =1 ) were estimated and shown to follow approximately linear dependencies with x , either. Structure and superconducting similarities with MgCNi 3 , viewed as a bridge between low and high T c superconductors are increasing the expectations t...

Review on Bi-Sr-Ca-Cu-O whiskers

We review the current status of the growth, characterization and applications of Bi-Sr-Ca-Cu-Ο (BSCCO) whiskers from the materials science point of view. Our analysis leads to the conclusion that reconsideration of the importance of the whisker studies is necessary; whiskers can play a major role in studies of the growth mechanism and defects, properties control and their understanding, and new devices and applications. Arguments favourable to the idea that BSCCO whiskers can possibly have a significant impact on future synthesis of nano-objects in this system or other high-T c superconducting systems, as well as in investigation and understanding of the growth processes under elevated magnetic fields, are discussed. Emphasis is made on superconducting whiskers, but non-superconducting ones are also briefly addressed.

Room and low temperature direct three-dimensional-strain measurements by neutron diffraction on as-reacted and prebent CuNb∕Nb3Sn wire

We measured directly by neutron diffraction the axial and lateral residual strains for the prebent and the as-reacted CuNb∕Nb3Sn wires at room temperature and at 7K, in order to investigate the change of the residual strain with prebending treatment. In the axial direction of the wire, the residual strain was changed with 0.20% to the tensile side when measured at both temperatures, while in the lateral direction, the change was 0.08% and 0.03% to the compressive side for the measurements at RT and 7K, respectively. From the obtained data, we estimated the deviatoric strain. At 7K, the value is 0.40% for the as-reacted wires and it reduces to 0.19% when the prebending is applied with a strain epb=0.8%. These results suggest that the reduction of the residual strain in the axial direction as well as in the lateral direction, i.e., of the deviatoric strain is responsible for the observed enhancement in the superconducting properties of the prebent Nb3Sn wires. In addition, we succeeded in the quantitative e...

Magnetization measurements on Li2Pd3B superconductor

Magnetization in dc magnetic fields and at different temperatures has been measured on the antiperovskite Li2Pd3B with a cubic structure composed of distorted Pd6B octahedrons. This material was recently found to exhibit superconductivity at 7–8K. The critical fields Hc1(0) and Hc2(0) are determined to be 135Oe and 4T, respectively. Critical current density, scaling of the pinning force within the Kramer model, and irreversibility field data are presented. Several superconductivity parameters were deduced: Coherence length ξ=9.1nm, penetration depth λ=194nm, and Ginzburg–Landau parameter κ=21. The material resembles other boride superconductors from the investigated points of view.

Superconducting properties under magnetic field in Na0.35CoO2.1.3H2O single crystal

We report the in-plane resistivity and magnetic susceptibility of the layered cobalt oxide Na 0.35 CoO 2 ·1.3H 2 O single crystal. The temperature dependence of the resistivity shows metallic behavior from room temperature to the superconducting transition temperature T c of 4.5 K. A sharp resistive transition, zero resistivity and almost perfect superconducting volume fraction below T c indicate the good quality and bulk superconductivity of the single crystal. The upper critical field H c2 and coherence length ξ are obtained from the resistive transitions in the magnetic fields parallel to the c -axis and a b -plane. The anisotropy of ξ, ξ a b /ξ c = 12 nm/1.3 nm ≃9.2, suggests that this material is an anisotropic three-dimensional superconductor. In the field parallel to the a b -plane, H c2 seems to be suppressed to the Pauli paramagnetic limit. This suggests spin singlet superconductivity in the cobalt oxide.

Nanodots-induced pinning centers in thin films: effects on critical current density, activation energy and flux jump rate

By very short time rf sputtering in certain deposition conditions we have grown three-dimensional Ag nanodots on the substrate prior to the growth of Tl-based superconducting films. These nanodots create pinning centers, leading to an increase in the critical current density about 10 times. From the frequency dependence of the critical current density we estimated also the activation energy for the flux jumps, which resulted to be several times higher. The rate of the thermally-activated flux jumps decreases several orders of magnitude. We suggest that our method can be used for the reduction of thermal noise in high-T/sub c/ dc SQUIDs.

Transport properties of superconducting MgB2 composites with carbon-encapsulated Fe nanospheres

We present the magnetic and transport properties of superconducting composites fabricated by admixing carbon-encapsulated Fe nanospheres and MgB2 powder. The addition of nanoparticles is expected to enhance the critical current density by carbon-doping the MgB2 matrix and by providing artificial pinning sites. Three samples with estimated amounts of 0.35, 0.6, and 1.0 wt. % metallic Fe were prepared using the spark plasma sintering technique. The average size of these nanoparticles is comparable to the superconducting coherence length of MgB2 at approximately 5 nm. We found that the additions do not significantly alter the critical temperature which is very high, close to that of the pure MgB2 samples. We have also observed improved current densities, as high as 1100 kA/cm2 for the samples with 0.35 wt. % metallic Fe at 5 K and 1 T. A core-shell model for explaining the transport data is presented. The field and temperature dependence of the reduced pinning force is described in terms of pinning on grain ...

Anisotropy of Na0.35CoO2·1.3H2O superconductor

Quasi-single crystals (up to 2x2x1 mm3) of Na0.35CoO2.1.3H2O-superconductor have been grown. Magnetization M(H, T) and M(T, H) curves with magnetic field approximately parallel and perpendicular to c-axis indicates on large anisotropy, comparable with Bi-based high-temperature superconducting (HTS) phases.

Preparation of pure and doped MgB2 by the field-assisted sintering technique and superconducting properties

High density (more than 90% of the theoretical density) bulk samples with relatively large size (2.0 cm in diameter) of MgB2, pristine and doped with 5 mol% SiC and B4C were obtained by the field-assisted sintering technique (FAST) from commercial MgB2 powder. The superconducting properties in the magnetic field of the doped samples show depressed upper critical field Hc2 and irreversibility field Hirr when compared with pristine MgB2. The reason for better Hc2 and Hirr in the undoped sample is not clear. Most probable is that this is due to the limited diffusion time which restricts the substitution of carbon for boron in this short-time processing method as well as due to the way grain boundaries are formed in the FAST process. However, all our samples have shown higher values of Hc2 and Hirr than the values reported previously for FAST-MgB2 samples. The data were further investigated through scaling analysis. Data suggest that both additions of SiC and B4C stabilize two well defined pinning regimes in FAST-processed MgB2. At low temperatures, the pinning occurs at the grain border whereas at high temperatures, T≥24 K, there is mixed pinning (H≤5 T).