Diamagnetism and some peculiarities of interatomic interactions in Cu2Sb

Results are reported for magnetic susceptibility measurements of high purity Cu₂Sb which characterize it as a diamagnetic. The observed anomalies of magnetic susceptibility, electrical resistivity and linear expansion coefficient in the neighbourhood of 370 K verify the existence of a phase transition of nonmagnetic origin in this temperature range. The analysis of the peculiarities of interatomic interaction in Cu₂Sb indicates the covalent character of the bonding. The value of the strength of the Cu Cu and Cu Sb bond is evaluated. It is shown that the Cu Cu bond in the Cu₂Sb compound is the most strong one.     

Local quasiparticle density of states of superconducting SmFeAsO(1-x)F(x) single crystals: evidence for spin-mediated pairing

We probe the local quasiparticles density of states in micron-sized SmFeAsO(1-x)F(x) single crystals by means of scanning tunnelling spectroscopy. Spectral features resemble those of cuprates, particularly a dip-hump-like structure developed at energies larger than the gap that can be ascribed to the coupling of quasiparticles to a collective mode, quite likely a resonant spin mode. The energy of the collective mode revealed in our study decreases when the pairing strength increases. Our findings support spin-fluctuation-mediated pairing in pnictides.     

Single crystals of LnFeAsO1−xFx (Ln = La,Pr, Nd,Sm, Gd) and Ba1−xRbxFe2As2: Growth,structure and superconducting properties

A review of our investigations on single crystals of LnFeAsO_1?xF_x (Ln = La, Pr, Nd, Sm, Gd) and Ba_1?xRb_xFe₂As₂ is presented. A high-pressure technique has been applied for the growth of LnFeAsO_1?xF_x crystals, while Ba_1?xRb_xFe₂As₂ crystals were grown using a quartz ampoule method. Single crystals were used for electrical transport, structure, magnetic torque and spectroscopic studies. Investigations of the crystal structure confirmed high structural perfection and show incomplete occupation of the (O, F) position in superconducting LnFeAsO_1?xF_x crystals. Resistivity measurements on LnFeAsO_1?xF_x crystals show a significant broadening of the transition in high magnetic fields, whereas the resistive transition in Ba_1?xRb_xFe₂As₂ simply shifts to lower temperature. The critical current density for both compounds is relatively high and exceeds 2 × 10⁹ A/m² at 15 K in 7 T. The anisotropy of magnetic penetration depth, measured on LnFeAsO_1?xF_x crystals by torque magnetometry is temperature dependent and apparently larger than the anisotropy of the upper critical field. Ba_1?xRb_xFe₂As₂ crystals are electronically significantly less anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the existence of two energy gaps in LnFeAsO_1?xF_x. Scanning Tunneling Spectroscopy reveals in addition to a superconducting gap, also some feature at high energy (～20 meV).     

Point-contact Andreev-reflection spectroscopy in ReFeAsO1−xFx (Re = La,Sm): Possible evidence for two nodeless gaps

A deep understanding of the character of superconductivity in the recently discovered Fe-based oxypnictides ReFeAsO_1?xF_x (Re = rare-earth) necessarily requires the determination of the number of the gaps and their symmetry in k space, which are fundamental ingredients of any model for the pairing mechanism in these new superconductors. In the present paper, we show that point-contact Andreev-reflection spectroscopy experiments performed on LaFeAsO_1?xF_x (La-1111) polycrystals with T_c ～ 27 K and SmFeAsO_0.8F_0.2 (Sm-1111) polycrystals with T_c ～ 53 K gave differential conductance curves exhibiting two peaks at low bias and two additional structures (peaks or shoulders) at higher bias voltages, an experimental situation quite similar to that observed by the same technique in pure and doped MgB₂. The single-band Blonder–Tinkham–Klapwijk model is totally unable to properly fit the conductance curves, while the two-gap one accounts remarkably well for the shape of the whole experimental dI/dV vs. V curves. These results give direct evidence of two nodeless gaps in the superconducting state of ReFeAsO_1?xF_x (Re = La, Sm): a small gap, Δ₁, smaller than the BCS value (2Δ₁/k_BT_c ～ 2.2–3.2) and a much larger gap Δ₂ which gives a ratio 2Δ₂/k_BT_c ～ 6.5–9.0. In Sm-1111 both gaps close at the same temperature, very similar to the bulk T_c, and follow a BCS-like behaviour, while in La-1111 the situation is more complex, the temperature dependence of the gaps showing remarkable deviations from the BCS behaviour at T close to T_c.The normal-state conductance reproducibly shows an unusual, but different, shape in La-1111 and Sm-1111 with a depression or a hump at zero bias, respectively. These structures survive in the normal state up to T¹ ～ 140 K, close to the temperatures at which structural and magnetic transitions occur in the parent, undoped compound.     

Transport properties of MgB2 single crystals doped with electrons and holes

The thermoelectric power of C, Mn, C:Li, and Al:Li substituted MgB₂ single crystals has been investigated in the temperature range 10-300 K. Both the in-plane (S_ab) and the out-of-plane (S_c) thermopowers are positive for the non-substituted crystal and both S_ab and S_c change a sign for crystals doped with electrons where C is substituted for B in the amount larger than 5 at%. When Li is substituted for Mg, the π band rather than the σ band is doped with holes and the doping effects are much more subtle. The anisotropy ratio of the non-substituted crystal S_ab/S_c≈3 and this ratio is strongly reduced by the substitution of C. Isovalent magnetic Mn ions, which substitute for Mg with a drastic reduction of T_c, do not influence the values and temperature dependences of both S_ab and S_c.     

Discussion on a possible neutrino detector located in India

We have identified some important and worthwhile physics opportunities with a possible neutrino detector located in India. Particular emphasis is placed on the geographical advantage with a stress on the complimentary aspects with respect to other neutrino detectors already in operation.     

A new triclinic modification of the pyrochlore-type KOs2O6 superconductor

A new modification of KOs₂O₆, the representative of a new structural type (Pearson symbol aP18, a=5.5668(1) Å, b=6.4519(2) Å, c=7.2356(2) Å, α=65.377(3)°, β=70.572(3)°, γ=75.613(2)° space group P−1, no. 2 was synthesized employing high pressure technique. Its structure was determined by single-crystal X-ray diffraction. The structure can be described as two OsO₆ octahedral chains relating to each other through inversion and forming big voids with K atoms inside. Quantum chemical calculations were performed on the novel compound and structurally related cubic compound. High-pressure X-ray study showed that cubic KOs₂O₆ phase was stable up to 32.5(2) GPa at room temperature.     

Effect of magnetic impurities in a two-band superconductor: a point-contact study of mn-substituted single crystals

We present the first results of directional point-contact measurements in Mg1-xMnxB2 single crystals, with x up to 0.015 and bulk Tc down to 13.3 K. The order parameters Deltasigma and Deltapi were obtained by fitting the conductance curves with the two-band Blonder-Tinkham-Klapwijk model. BothDeltapi and Deltasigma decrease with the critical temperature of the junctions TAC, but remain clearly distinct up to the highest Mn content. Once analyzed within the Eliashberg theory, the results indicate that spin-flip scattering is dominant in the sigma band, as also confirmed by first-principles band-structure calculations.     

Light element analysis in oxycarbonate superconductors using EELS

Elemental analysis in an oxycarbonate superconductor ((Cu,N,C)Sr2CaCu2Oy) is conducted using transmission electron microscope-electron energy-loss spectroscopy with detector-gain correction. The gain correction enables highly sensitive elemental analysis and precise measurement of energy-loss near edge structures (ELNESs). It is found that carbon is included as a CO3 group, because the carbon K-edge in the oxycarbonate shows the same ELNES observed from CaCO3. Nitrogen ELNES is similar to that of Sr(NO3)2, so nitrogen is contained as a NO3 group. Although both CO3(2-) and NO3- have similar planar atomic arrangements, the nitrogen ELNES observed is different from that of carbon. EEL spectrum simulation based on DV-Xalpha method is used to interpret the difference.