V. A. Gasparov

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.

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).

Electron transport and anisotropy of the upper critical magnetic field in Ba0.68K0.32Fe2As2 single crystals

Early work on the iron-arsenide compounds supported the view, that a reduced dimensionality might be a necessary prerequisite for high-Tc superconductivity. Later, however, it was found that the zero-temperature upper critical magnetic field, Hc2(0), for the 122 iron pnictides is in fact rather isotropic. Here, we report measurements of the temperature dependence of the electrical resistivity, ρ(T), in Ba0.5K0.5Fe2As2 and Ba0.68K0.32Fe2As2 single crystals in zero magnetic field and in Ba0.68K0.32Fe2As2 in static and pulsed magnetic fields up to 60 T. We find that the resistivity of both compounds in zero field is well described by an exponential term due to inter-sheet umklapp electron-phonon scattering between light electrons around the M point to heavy hole sheets at the Γ point in reciprocal space. From our data, we construct an H-T phase diagram for the inter-plane (H | c) and in-plane (H | ab) directions for Ba0.68K0.32Fe2As2. Contrary to published data for 122 underdoped FeAs compounds, we find that Hc2(T) is in fact anisotropic in optimally doped samples down to low temperatures. The anisotropy parameter, γ = Hc2ab/Hc2c, is about 2.2 at Tc. For both field orientations we find a concave curvature of the Hc2 lines with decreasing anisotropy and saturation towards lower temperature. Taking into account Pauli spin paramagnetism, we perfectly can describe Hc2 and its anisotropy.

Upper critical magnetic field in Ba0.68K0.32Fe2As2 and Ba(Fe0.93Co0.07)2As2

We report measurements of the temperature dependence of the radio frequency magnetic penetration depth in Ba0.68K0.32Fe2As2 and Ba(Fe0.93Co0.07)2As2 single crystals in pulsed magnetic fields up to 60 T. From our data, we construct an H-T-phase diagram for the inter-plane (H ‖ c) and in-plane (H ‖ ab) directions for both compounds. For both field orientations in Ba0.68K0.32Fe2As2 we find a concave curvature of the Hc2(T) lines with decreasing anisotropy and saturation towards lower temperature. Taking into account Pauli spin paramagnetism we can describe Hc2(T) and its anisotropy. In contrast, we find that Pauli paramagnetic pair breaking is not essential for Ba(Fe0.93Co0.07)2As2. For this electron-doped compound, the data support a Hc2(T) dependence that can be described by the Werthamer-Helfand-Hohenberg model for H ‖ ab and a two-gap behavior for H ‖ c.

Superconducting, surface and anomalous electron transport properties of BaNbO3−x films

Abstract Review of superconducting ( T c , H c 2 (T), J c ), electron transport R s ( T ), surface (XPS, UPS) and structural (XRD, RBS) properties of thin films of a novel superconductor BaNbO 3− x on different substrates is presented. Superconducting films have been obtained when grown on the Al 2 O 3 with T c =14 K , J c ≈1×10 4 A cm −2 and large H c 2 (0)=28 T . At the same time, films on NdGaO 3 exhibit behavior typical to granular superconductors.

Iron pages of HTSC

Experimental data are presented on the superconducting and electronic properties of iron-based high-temperature superconductors in the normal and superconducting states. The following topics are discussed: lattice structure; structure of magnetic vortices; magnetic penetration depth; Fermi surface; isotope effect; and critical magnetic fields both in oxide compounds of 1111 type and oxide-free compounds of 122, 111, and 011 types as a function of the doping level, temperature, and external pressure.

On electron transport in ZrB12, ZrB2, and MgB2 in normal state

We report on measurements of the temperature dependence of resistivity, ρ(T), for single-crystal samples of ZrB12, ZrB2, and polycrystalline samples of MgB2. It is shown that the cluster compound ZrB12 behaves as a simple metal in the normal state, with a typical Bloch-Grüneisen ρ(T) dependence. However, the resistive Debye temperature, TR=300 K, is three times smaller than TD obtained from specific heat data. We observe the T2 term in ρ(T) of all these borides, which could be interpreted as an indication of strong electron-electron interaction.

Normal-state electrical resistivity and superconducting magnetic penetration depth in Eu0.5K0.5Fe2As2 polycrystals

We report measurements of the temperature dependence of the electrical resistivity, ρ(T), and magnetic pen-etration depth, λ(T), for polycrystalline samples of Eu0.5K0.5Fe2As2 with Tc = 31 K. ρ(T) follows a linear temperature dependence above Tc and bends over to a weaker temperature dependence around 150 K. The magnetic penetration depth, determined by radio frequency technique displays an unusual minimum around 4 K which is associated with short-range ordering of localized Eu3+ moments.

Scanning tunneling microscope induced nanostructuring of a Si(111)/Ag(3×3)R30° surface

An atomically flat Si(111)/Ag(3×3)R30° surface has been modified using a scanning tunneling microscope in ultrahigh vacuum. Mesoscopic pits have been created by applying negative voltage pulses to the sample, while at opposite voltage polarity mounds were formed. Moreover, lines could be written by moving the scanner at elevated voltages. The threshold voltage for pit formation increases almost linearly with the distance of the tip to the surface and drops to a value below 2 V for the closest approach. At sufficiently high voltages the depth extends beyond the silver layer height. The lateral pit size is well below 8 nm and can be reduced to values between 2 nm and 5 nm for voltages slightly above the threshold. Even selective top layer Ag atom removal has been achieved.