Electron transport,penetration depth,and the upper critical magnetic field in ZrB<Subscript>12</Subscript> and MgB<Subscript>2</Subscript>

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 H_c2, 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 T_R = 280 K (to be compared to T_R = 900 K for MgB₂). The ρ(T) and λ(T) dependences for these samples reveal a superconducting transition in ZrB₁₂ at T_c = 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 T_c/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 H_c2(T) for ZrB₁₂ (H_c2(0) = 0.15 T).     

On electron transport in ZrB<Subscript>12</Subscript>, ZrB<Subscript>2</Subscript>, and MgB<Subscript>2</Subscript> 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, T_R=300 K, is three times smaller than T_D 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.     

Electron-phonon interaction in NbB2: a comparison with MgB2

We present a comparison of electron-phonon interaction in NbB₂ and MgB₂, calculated using full-potential, density-functional-based methods in P6/mmm crystal structure. Our results, described in terms of (i) electronic structure, (ii) phonon density of states F(ω), (iii) Eliashberg function α²F(ω), and (iv) the solutions of the isotropic Eliashberg gap equation, clearly show significant differences in the electron-phonon interaction in NbB₂ and MgB₂. We find that the average electron-phonon coupling constant λ is equal to 0.59 for MgB₂ and 0.43 for NbB₂, leading to superconducting transition temperatures T_c at around 22 K for MgB₂ and 3 K for NbB₂.     

Band structure of ZrB2, VB2, NbB2, and TaB2 hexagonal diborides: Comparison with superconducting MgB2

The band structure and Fermi surface parameters of ZrB₂, VB₂, NbB₂, and TaB₂ hexagonal diborides are considered in the framework of the self-consistent full-potential LMTO method in comparison with the relevant parameters of a MgB₂ isostructural superconductor. The factors responsible for the superconducting properties of AlB₂-like diborides are analyzed, and the results obtained are compared with previous calculations and available experimental data.     

Can the effective pinning potential of MgB2 behave logarithmically in the high temperature range?

The pinning potential barrier U_o(T,H) is the main parameter describing the flux pinning of MgB₂. Comparing the experimental results published recently by Thompson et al. and Bygoslavsky et al., one can prove theoretically that in the high temperature range, U_o(T,H) is of logarithmic form similar to that suggested for high T_c superconductors by Zeldov et al.. Meanwhile, the major pinning centers of MgB₂ employ the planar pinning mechanism. Furthermore, it is found that U_o(T,H) depends on the temperature linearly and also on the magnetic field with the form H^−1∼−2 at high temperature range.     

Effect of electron irradiation on the structure and properties of the MgB2 superconductor

This paper reports on the results of an investigation into the effect of irradiation of the Bardeen-Cooper-Schriefer superconductor MgB₂ by electrons with a mean energy ē ～ 10 MeV at low doses (0 ≤ Φt ≤ ～5 × 10¹⁶ cm^?2) on the lattice parameters, the intensity and width of diffraction lines, the superconducting transition temperature T _c , and the temperature dependence of the resistivity ρ(T) in the normal state. The results of structural investigations have revealed regularities in the defect formation in the magnesium and boron sublattices of the MgB₂ compound as a function of the electron fluence. At the initial stage, irradiation leads to the formation of vacancies, originally in the magnesium sublattice and then in the boron sublattice. For fluences Φt ≥ ～1 × 10¹⁶ cm^?2, vacancies are formed in both sublattices. The evolution of the electrical and physical properties [T _c , ρ_{273 K}, residual resistivity ratio RRR = ρ_{273 K}/ρ_{50 K}, parameters of the dependence ρ(T)] under electron irradiation is in agreement with the regularities revealed in the formation of radiation-induced defects in the crystal lattice of the MgB₂ compound.     

MgB2 coated conductors directly grown on flexible metallic Hastelloy tapes by hybrid physical–chemical vapor deposition

MgB₂ coated conductors (CCs), which can avoid the low packing density problem of powder-in-tube (PIT) processed wires, can be a realistic solution for practical engineering applications. Here we report on the superior superconducting properties of MgB₂ CCs grown directly on the flexible metallic Hastelloy tapes without any buffer layer at various deposition temperatures from 520 to 600 °C by using hybrid physical–chemical vapor deposition (HPCVD) technique. The superconducting transition temperatures (T_c) are in the range of 38.5–39.4 K, comparable to bulk samples and high quality thin films. Clear (101) and (002) reflection peaks of MgB₂ are observed in the X-ray diffraction patterns without any indication of chemical reaction between MgB₂ and Hastelloy tapes. From scanning electron microscopy, it was found that connection between MgB₂ grains and voids strongly depend on the growth temperature. A systematic increase in the flux pinning force density and thereby the critical current density with decreasing growth temperature was observed for the MgB₂ CCs. The critical current density (J_c) of J_c(5 K, 0 T) ～10⁷ A/cm² and J_c(5 K, 2.5 T) ～10⁵ A/cm² has been obtained for the sample fabricated at a low growth temperature of 520 °C. The enhanced J_c (H) behavior can be understood on the basis of the variation in the microstructure of MgB₂ CCs with growth temperature.     

Upper critical field of cellular magnesium diboride

A cellular superconducting material consisting of thin (1–20 μm) MgB_2?x layers and magnesium granules of about 100 μm has been produced. The critical temperature T _c of this superconductor decreases with the thickness of the MgB_2?x layers. In unalloyed magnesium diboride, the curvature of the temperature dependence of the upper critical field H _c2(T) changes gradually from downward to pronounced upward as the temperature T _c decreases from 38 to 36 K.     

Normal state resistivity and Tç studies of superconducting Ti1−xSbx system

The superconducting transition temperature (T_c) and the temperature dependence of the normal state resistivity of the Ti_1?xSb_x system between T_c and 300 K have been studied. The T_c values are found to depend on the heat treatment of the samples. Below 40 K, all alloys show a T² dependence of the resistivity. However, the sample with x = 0.53 is not superconducting and shows a different behaviour of the resistivity.     

Physical property characterization of bulk MgB<Subscript>2</Subscript> superconductor

We report synthesis, structure/micro-structure, resistivity under magnetic field [ρ(T)H], Raman spectra, thermoelectric power S(T), thermal conductivity κ(T), and magnetization of ambient pressure argon annealed polycrystalline bulk samples of MgB₂, processed under identical conditions. The compound crystallizes in hexagonal structure with space group P6/mmm. Transmission electron microscopy (TEM) reveals electron micrographs showing various types of defect features along with the presence of 3–4 nm thick amorphous layers forming the grain boundaries of otherwise crystalline MgB₂. Raman spectra of the compound at room temperature exhibited characteristic phonon peak at 600 cm^-1. Superconductivity is observed at 37.2 K by magnetic susceptibility χ(T), resistivity ρ(T), thermoelectric power S(T), and thermal conductivity κ(T) measurements. The power law fitting of ρ(T) give rise to Debye temperature (Θ_D) at 1400 K which is found consistent with the theoretical fitting of S(T), exhibiting Θ _D of 1410 K and carrier density of 3.81 × 10²⁸/m³. Thermal conductivity κ(T) shows a jump at 38 K, i.e., at T_c, which was missing in some earlier reports. Critical current density (J_c) of up to 10⁵ A/cm² in 1–2 T (Tesla) fields at temperatures (T) of up to 10 K is seen from magnetization measurements. The irreversibility field, defined as the field related to merging of M(H) loops is found to be 78, 68 and 42 kOe at 4, 10 and 20 K respectively. The superconducting performance parameters viz. irreversibility field (H_irr) and critical current density J_c(H) of the studied MgB₂ are improved profoundly with addition of nano-SiC and nano-diamond. The physical property parameters measured for polycrystalline MgB₂ are compared with earlier reports and a consolidated insight of various physical properties is presented.     

MgB2 coated superconducting tapes with high critical current densities fabricated by hybrid physical-chemical vapor deposition

The MgB₂ coated superconducting tapes have been fabricated on textured Cu (0 0 1) and polycrystalline Hastelloy tapes using coated conductor technique, which has been developed for the second generation high temperature superconducting wires. The MgB₂/Cu tapes were fabricated over a wide temperature range of 460-520 °C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The tapes exhibited the critical temperatures (T_c) ranging between 36 and 38 K with superconducting transition width (ΔT_c) of about 0.3-0.6 K. The highest critical current density (J_c) of 1.34 × 10⁵ A/cm² at 5 K under 3 T is obtained for the MgB₂/Cu tape grown at 460 °C. To further improve the flux pinning property of MgB₂ tapes, SiC is coated as an impurity layer on the Cu tape. In contrast to pure MgB₂/Cu tapes, the MgB₂ on SiC-coated Cu tapes exhibited opposite trend in the dependence of J_c with growth temperature. The improved flux pinning by the additional defects created by SiC-impurity layer along with the MgB₂ grain boundaries lead to strong improvement in J_c for the MgB₂/SiC/Cu tapes. The MgB₂/Hastelloy superconducting tapes fabricated at a temperature of 520 °C showed the critical temperatures ranging between 38.5 and 39.6 K. We obtained much higher J_c values over the wide field range for MgB₂/Hastelloy tapes than the previously reported data on other metallic substrates, such as Cu, SS, and Nb. The J_c values of J_c(20 K, 0 T) ∼5.8 × 10⁶ A/cm² and J_c(20 K, 1.5 T) ∼2.4 × 10⁵ A/cm² is obtained for the 2-μm-thick MgB₂/Hastelloy tape. This paper will review the merits of coated conductor approach along with the HPCVD technique to fabricate MgB₂ conductors with high T_c and J_c values which are useful for large scale applications.     

Anomalous low-temperature behavior of the thermal characteristics of MgB<Subscript>2</Subscript>

We have studied the behavior of the thermal expansion coefficient α(T) (in a zero magnetic field and at H≈4 T), the heat capacity C(T), and the thermal conductivity κ(T) of magnesium boride (MgB₂) in the vicinity of T_c and at lower temperatures. It was established that MgB₂, like oxide-based high-temperature superconductors, exhibits a negative thermal expansion coefficient at low temperatures. The anomaly of α(T) in MgB₂ is significantly affected by the magnetic field. It was established that, in addition to the well-known superconducting transition at T_c≈40 K, MgB₂ exhibits an anomalous behavior of both heat capacity and thermal conductivity in the region of T≈10–12 K. The anomalies of C(T) and κ(T) take place in the same temperature interval where the thermal expansion coefficient of MgB₂ becomes negative. The low-temperature anomalies are related to the presence of a second group of charge carriers in MgB₂ and to an increase in the density of the Bose condensate corresponding to these carriers at T_c2≈10–12 K.     

Large negative magnetoresistance in thiospinel CuCrZrS4

We report on large negative magnetoresistance observed in ferromagnetic thiospinel compound CuCrZrS₄. The electrical resistivity increased with decreasing temperature according to the exp(T₀/T)^1/2, an expression derived from variable range hopping with strong electron-electron interaction. The resistivity under a magnetic field was expressed by the same form with the characteristic temperature T₀ decreasing with increasing magnetic field. Magnetoresistance ratio ρ(T,0)/ρ(T,H) is 1.5 for H=90 kOe at 100 K and increases divergently with decreasing temperature reaching 80 at 16 K. Results of magnetization measurements are also presented. A possible mechanism of the large magnetoresistance is discussed.     

Degradation of superconducting properties in MgB2 by Cu addition

The (MgB₂)_2−xCu_x (x=0-0.5) superconducting system was prepared by a solid-state reaction technique. Microstructural evolution and transport properties including resistivity versus temperature up to a magnetic field of 6 T, activation energy, thermoelectric power and Fermi energy, E_F, and the corresponding velocity, V_F, values of the samples prepared were also investigated. The XRD analysis showed a multiphase formation and no detectable solution of Cu in MgB₂. Two different impurity phases, MgCu₂ and CuB₂₄, have been identified and their peak intensity increased when the Cu concentration increased. The temperature dependence of the resistivity of the samples showed a metallic behavior down to T_c. But, for the Cu concentrations above 0.3 the superconducting phase transition completely disappeared. The magnetic field strongly affects the electrical properties. For x=0.0 samples, the transition is found to be sharp, ΔT∼1 K, but it becomes broader with increasing magnetic field and Cu concentration. The calculated values of carrier concentration, n, of the samples are showed a sharp decrease with increasing Cu content. For x=0.0 sample the n was calculated to be 12×10²¹ cm⁻³, but for the x=0.5 sample it decreased to 1.3×10²¹ cm⁻³. We found that the activation energy, U(B), decreased sharply with increasing magnetic field. According to thermoelectric power and Fermi energy, E_F, calculations the decrease of the carrier concentration by the additions of Cu into MgB₂ gives a decrease in E_F and this could be attributed to a shift of the Fermi level towards the top of the σ-hole band.     

Effect of γ irradiation on the structural and thermal properties of [N(C2H5)4]2ZnBr4 in the vicinity of a first-order phase transition

The unit cell parameters a and c of nonirradiated [N(C₂H₅)₄]₂ZnBr₄ crystals in the temperature region 90–300 K and of samples irradiated with γ rays to doses of 10⁶ and 5 × 10⁶ R in the 270-to 300-K interval were measured using x-ray diffraction. The data obtained were used to derive the thermal expansion coefficients α_a and α_c. It is shown that the parameter a increases and the parameter c decreases with increasing temperature. In the vicinity of the phase transition (PT) at T = 285 K, the temperature dependences of a(T) and c(T) reveal anomalies in the form of jumps and the α_a(T) and α_c(T) curves have a maximum and a minimum, respectively. The heat capacity of nonirradiated and irradiated [N(C₂H₅)₄]₂ZnBr₄ samples was measured by adiabatic calorimetry. A maximum was found in the C _p(T) curve at T = 285 K. Both x-ray diffraction and heat capacity measurements showed that the PT temperature decreased after γ irradiation.     

Electron-phonon scattering in potassium at high magnetic fields

We have measured the temperature dependence of both the zero-field resistivity and the transverse magnetoresistance of polycrystalline potassium wires (?(300 K)/?(4.2 K)=140 to 6000) in fieldsH?35 kG and at temperaturesT?4.2 K. Our principal findings are: 1) The presence of a large magnetic fieldH=35 kG does not alter the temperature dependence of ? from that observed atH=0; below 4.2 K theT-dependent part of the resistivities,_?T (H=0) and_?T (H=35 kG), fit well to the function exp (?Θ*/T) with the same Θ*=23K. 2) Deviations from Matthiessen's rule are significantly reduced in a strong field so that the magnitude of_?T (H=0) approaches that of_?T (H=35 kG) as sample purity decreases. 3) The slope of the high-field linear magnetoresistance increases slightly (?8%) from 1.5 K to 4.2 K. We attribute the exponential temperature dependence of_?T (H) to the freezing out of electron-phonon umklapp processes as has been shown for the zero-field resistivity. The reduction in deviations from Matthiessen's rule at high fields can be understood within semiclassical theory, but the latter cannot explain the failure of_?T (H) to saturate at high fields. A proposal by Young that electron-phonon umklapp scattering may contribute aT-dependent high-field linear magnetoresistance in potassium is considered.     

Low-temperature anomalies in the specific heat and thermal conductivity of MgB<Subscript>2</Subscript>

The temperature dependences of the specific heat C(T) and thermal conductivity K(T) of MgB₂ were measured at low temperatures and in the neighborhood of T _c . In addition to the well-known superconducting transition at T _c ≈40 K, this compound was found to exhibit anomalous behavior of both the specific heat and thermal conductivity at lower temperatures, T≈10–12 K. Note that the anomalous behavior of C(T) and K(T) is observed in the same temperature region where MgB₂ was found to undergo negative thermal expansion. All the observed low-temperature anomalies are assigned to the existence in MgB₂ of a second group of carriers and its transition to the superconducting state at T_c2≈10?12 K.     

Superconductivity and x—ray photoemission study of MgB2 thin films

Highly c-axis oriented MgB2 thin films with Tc^onset of 39.6K were fabricated by magnesium diffusing into pulsedlaser-deposited boron precursors.The estimation of critical current density Jc,using hysteresis loops and the Bean model,has given the value of 10^7A/cm^2(15K,0T),which is one of the highest values ever reported.The x-ray photoemission study of the MgB2 thin films has revealed that the binding energies of Mg 2p and B 1s are at 49.4eV and 186.9eV,which are close to those of metallic Mg and transition-metal diborides,respectively.     

Microstructural and superconducting properties of C6H6 added bulk MgB2 superconductor

The effect of aromatic hydrocarbon (benzene, C₆H₆) addition on lattice parameters, microstructure, critical temperature (T_c), critical current density (J_c) of bulk MgB₂ has been studied. In this work only 2 mol% C₆H₆ addition was found to be very effective in increasing the J_c values, while resulting in slight reduction of the T_c. J_c values of 2 mol% C₆H₆ added MgB₂ bulks reached to 1.83×10⁶ A/cm² at 15 K and 0 T. Microstructural analyses suggest that J_c enhancement is associated with the substitution of carbon with boron and which also results in the smaller MgB₂ grain size. The change in the lattice parameters or the lattice disorder is claimed as a cause of the slight reduction in the T_c by carbon addition. We note that our results show the advantages of C₆H₆ addition include homogeneous mixing of precursor powders, avoidance of expansive nanoadditives, production of highly reactive C, and significant enhancement in J_c of MgB₂, compared to un-doped samples.     

Upper critical fields of superconducting Gd and Tm doped LaSn3: Effects of crystalline electric fields

The temperature dependence of the upper critical fields, H_c²(T), are presented for (La_1-xGd_x)Sn₃ and (La_1-xTm_x)Sn₃. For samples with nearly the same T_c, H_c²(T) of the Tm-doped LaSn₃ samples are always larger than those for the Gd-doped samples. The results are interpreted in terms of crystalline electric field splitting of magnetic levels of the Tm³⁺. Pure LaSn₃ is found to be a Type I superconductor.