N. S. Sidorov

Electron transport in diborides: Observation of superconductivity in ZrB2

We report on syntheses and electron transport properties of polycrystalline samples of diborides (AB2) with different transition metals atoms (A=Zr, Nb, Ta). The temperature dependence of resistivity, ρ(T), and ac susceptibility of these samples reveal a superconducting transition of ZrB2 with Tc=5.5 K, while NbB2 and TaB2 have been observed to be nonsuperconducting up to 0.37K. Hc2(T) is linear in temperature below Tc, leading to a rather low Hc2(0)=0.1 T. At T close to Tc, Hc2(T) demonstrates a downward curvature. We conclude that these diborides, as well as MgB2 samples, behave like simple metals in the normal state with usual Bloch-Grüneisen temperature dependence of resistivity and with Debye temperatures 280, 460, and 440 K for ZrB2, NbB2, and MgB2, respectively, rather than T2 and T3, as previously reported for MgB2.

Two-gap superconductivity in ZrB12 : Temperature dependence of critical magnetic fields in single crystals

We report the measurements of the temperature dependence of the resistivity, \rho(T), magnetic penetration depth,\lambda(T) the lower, Hc1(T), and upper, Hc2(T), critical magnetic fields, for single crystals of dodecaboride ZrB12, diboride ZrB2 and thin films of diboride MgB2. We observe a number of deviations from conventional behavior in these materials. Although ZrB12 behaves like a simple metal in the normal state, the resistive Debye temperature, 300 K, is three times smaller relative to that (800-1200 K) calculated from the specific heat, C(T), data. We observe predominantly quadratic temperature behavior of resistivity in ZrB12 below 25 K, and in ZrB2 below 100 K, indicating the possible importance of the electron-electron interaction in these borides. Superfluid density of ZrB12 displays unconventional temperature dependence with pronounced shoulder at T/Tc equal to 0.65. Contrary to conventional theories we found a linear temperature dependence of Hc2(T) for ZrB12 from Tc down to 0.35 K. We suggest that both \lambda(T) and Hc2(T) dependencies in ZrB12 can be explained by two band BCS model with different superconducting gap and Tc.

Superconductivity and structure of YBa2Cu3O6 single crystals treated in halogen vapours

Abstract Dielectric tetragonal samples of Y-Ba-Cu-O single crystals have been heat treated in an atmosphere of iodine or bromine gas. After halogenation the single crystals exhibited the transition to the superconducting state with T c ≈ 60 K and 85 K, respectively.

Electron transport, penetration depth, and the upper critical magnetic field in ZrB12 and MgB2

We report on the synthesis and measurements of the temperature dependences of the resistivity ρ, the penetration depth λ, and the upper critical magnetic field Hc2, for polycrystalline samples of dodecaboride ZrB12 and diboride MgB2. We conclude that ZrB12 behaves as a simple metal in the normal state with the usual Bloch-Grüneisen temperature dependence of ρ(T) and with a rather low resistive Debye temperature TR = 280 K (to be compared to TR = 900 K for MgB2). The ρ(T) and λ(T) dependences for these samples reveal a superconducting transition in ZrB12 at Tc = 6.0 K. Although a clear exponential λ(T) dependence in MgB2 thin films and ceramic pellets was observed at low temperatures, this dependence was almost linear for ZrB12 below Tc/2. These features indicate an s-wave pairing state in MgB2, whereas a d-wave pairing state is possible in ZrB12. In disagreement with conventional theories, we found a linear temperature dependence, of Hc2(T) for ZrB12 (Hc2(0) = 0.15 T).

Influence of Rb, Cs, and Ba on the superconductivity of magnesium diboride

Magnesium diboride has been thermally treated in the presence of Rb, Cs, and Ba. The magnetic susceptibility shows the possible onsets of superconductivity in the resulting samples at 52 K (Rb), 58 K (Cs), and 45 K (Ba). Room-temperature 11B NMR indicates the cubic symmetry of the electric field gradient at the boron site for the samples reacted with Rb and Cs, in contrast to the axial symmetry in the initial MgB2 and in the sample treated with Ba.

Nonmonotonic variation of the electrical conductivity of C60 fullerene crystals dynamically compressed to 300 kbar as evidence of anomalously strong reduction of the energy barrier of C60 polymerization at high pressures

The electrical conductivity of a C60 fullerene crystal is measured under the conditions of quasi-isentropic loading by a diffuse shock wave to a pressure of 30 GPa at room temperature. Nonmonotonic behavior of the conductivity of the samples with an increase in pressure is observed: first, conductivity increases by several orders of magnitude and, then, decreases rapidly. An increase in conductivity is explained by a decrease in the band gap under pressure, whereas a decrease in conductivity may be explained under the assumption that the energy barrier of C60 polymerization decreases similarly to the band gap with an increase in pressure. As a result, the rate of C60 polymerization at high pressures increases considerably (by more than seven orders of magnitude).

Conductivity of C60 fullerene crystals under dynamic compression up to 200 kbar

The conductivity σ of C60 fullerene crystals is measured under quasi-isentropic loading by a spread shock wave to a pressure of 200 kbar at the initial temperatures 293 and 77 K. A sharp increase in σ by seven to eight orders of magnitude is detected: from 10−6−10−7 Ω−1 cm−1 at normal conditions to 5 Ω−1 cm−1 under pressure from 100 to 200 kbar. The conductivity of samples under pressure decreases with decreasing temperature, which is characteristic of semiconductors. On pressure release, σ regains its initial value.

Fluctuation conductivity in superconducting MgB2

According to the crystal structure of MgB2 and band structure calculations, quasi-two-dimensional (2D) boron planes are responsible for the superconductivity. We report on critical-field and resistance measurements of 5.6-μm-thick MgB2 films grown on a sapphire single-crystal substrate. Resistivity measurements yield a temperature dependence of the fluctuation conductivity above the critical temperature, which agrees with the Aslamazov-Larkin and Maki-Thompson theory of fluctuations in layered superconductors, indicating a quasi-two-dimensional nucleation of superconductivity in MgB2.

Specific features of NQR and EPR spectra of YBaCuO ceramics doped by iodine and bromine

Abstract Nuclear quadrupole and electron paramagnetic resonance methods have been employed to study samples of YBa 2 Cu 3 O 6 ceramics, treated in iodine or bromine vapours. The sampled singularities may be attributed to the appearance of I or Br ions in Cu(1)-O type planes.

Metastable superconductivity of the Al/Al2O3 interface

Superconductivity in the interface region between metallic Al and its oxide, Al2O3, has been detected at about 45 K in the measured dynamic magnetic susceptibility.