Phase transitions in a MgB<Subscript>2</Subscript> granular BCS superconductor in weak magnetic fields

The character of temperature dependences of the electric conductivity of MgB₂ granular BCS superconductors at temperatures of ~35–45 K in external magnetic fields H _ext of up to ~2 kOe is studied. An increase in the superconducting transition width ΔT _c with an increase in Hext is found. The presence of a system of weak links in MgB₂-based granular superconductors is established. On the basis of experimental data, MgB₂ granular superconductor is assigned to two-level superconducting systems and the H–T phase diagram is constructed.     

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.     

Preparation of MgB<Subscript>2</Subscript>/Fe superconductor wire by electrical self-heating

MgB₂/Fe wires were prepared by electrical self-heating of in situ powder-in-tube wires for the first time at ambient conditions. Characterization of the wires processed at 750 °C, 800 °C and 850 °C for 15 min by XRD, SEM, ϱ–T, susceptibility and J_C measurements shows that the MgB₂ formed is of high quality particularly with respect to phase purity and transport J_C. The method considerably reduces the overall energy consumption vis-à-vis the production cost, simplifies the complexity of the fabrication procedure and is promising for manufacture of high-quality MgB₂ superconducting wires. PACS 74.70.Ad; 74.62.Bf; 74.25.Fy; 74.25.Ha; 81.20.Hy     

Evolution of the structure and properties of the MgB<Subscript>2</Subscript> superconductor under isothermal annealing

The effect of comparatively weak actions on the structure of the two-gap BCS superconductor MgB₂ was studied. The MgB₂ samples studied differed in terms of the annealing time at 900°C. It was found that the lattice parameters, residual resistivity, and critical temperature depend only weakly on the annealing time, whereas the electrical resistivity decreases by a few times when the annealing time is increased from 2 to 10 h. It is assumed that the observed effects may be caused by the influence of Mg and B atom ordering in the MgB₂ lattice on charge transfer over the two-dimensional B-B σ bonds.     

X-ray photoemission study of MgB<Subscript>2</Subscript> films synthesized from in-situ annealed MgB<Subscript>2</Subscript>/Mg multilayers

Superconducting MgB₂ films were obtained by in-situ annealing of precursor multilayers deposited at low substrate temperature by sputtering from a MgB₂ stoichiometric target and by thermal evaporation of pure Mg. After an in-situ annealing at 500–600 °C, the films showed a zero resistance critical temperature up to 31 K. The as-obtained MgB₂ films were investigated by X-ray photoelectron spectroscopy (XPS) and X-ray auger electron spectroscopy (XAES). The electronic structure was studied by monitoring the B 1s, Mg 2p, O 1s core-levels and the Mg KL₂L₃ Auger line. For comparison, the electronic structure of an MgB₂ commercial superconducting sputtering target, of a not-annealed precursor film and of a sample obtained by direct sputtering from the MgB₂ target have also been investigated. Electron spectroscopy showed that in the superconducting systems the Mg KL₂L₃ Auger line kinetic energy position is always higher by about 0.9 eV with respect to the energy position of the same Auger line measured in the non-superconducting samples. PACS 74.25Jb; 74.78.Bz; 74.70.Ad     

Nonstoichiometry and critical temperature of MgB<Subscript>2</Subscript>

The influence of nonstoichiometry of the new high-temperature superconductor MgB₂ on its critical temperature was studied by the direct magnetooptical observations of the penetration and trapping of magnetic flux. To preclude the possible influence of accidental factors, a special sample with transition from pure boron to the MgB₂ with an excess of Mg was synthesized. In a narrow region near the unreacted boron, the magnetic-field trapping and screening disappear at a temperature 1.5 K higher than in the dominant stoichiometric region of the sample.     

Low-temperature anomalies in MgB<Subscript>2</Subscript> thermal expansion

The thermal expansion coefficient α(T) of MgB₂ was measured at low temperatures both in a zero magnetic field and at H=36 kOe. As in the oxide HTSCs, a region of anomalous (negative) thermal expansion and a strong effect of magnetic field on α(T) were revealed. The results obtained indicate the anomalous properties of MgB₂ and the oxide HTSCs to follow a common pattern.     

Surface critical magnetic field H<Subscript>c3</Subscript>(T) of a bulk superconductor MgB<Subscript>2</Subscript> using two-band Ginzburg-Landau theory

Two-band Ginzburg-Landau (TB G–L) equations for a bulk MgB₂ were solved analytically to determine the temperature dependence of surface critical magnetic fieldH _c3 (T). It is shown thatH _c3 (T) has the same temperature dependence with H_c2 (T), similar to the case of a single-band superconductor,H _c3 (T) = 166H _c2 (T). We use an elimination procedure for the decoupling of G–L equations of two-band superconductivity, which eases the calculations. It is expected that the temperature dependence forH _c3 (T) gives positive curvature nearT _c .     

Modification of the Al<Subscript>2</Subscript>O<Subscript>3</Subscript> surface by high-energy bismuth ions

This paper reports on an atomic-force microscopy study of the surface of α-Al₂O₃ single crystals irradiated by Bi ions with energies of 710, 557, 269, and 151 MeV. The shape of the radiation defects produced by single ions was established to depend on the ionization energy loss. The threshold ionization density above which the surface topography is observed to change lies in the 27–35 keV/nm interval. Possible mechanisms of defect formation in the thermal-spike model, namely, a phase transition and the creation of thermoelastic stresses in the high-energy ion track, are considered.     

Electron transport properties of MgB<Subscript>2</Subscript> in the normal state

We report measurements of the resistivity, ρ, and the Seebeck coefficient, S , of a MgB₂ sintered sample, and compare S with theoretical calculations based on precise electronic structure calculations. ρ is fitted well by a generalized Bloch-Grüneisen equation with a Debye temperature Θ _R of 1050 K. S is given by the sum of a diffusive and a phonon drag term and the behavior in the temperature region T _c < T < 0.1Θ _R follows the relationship AT+BT³. The phonon drag term indicates a strong electron-phonon interaction. The diffusive term, compared with calculations, suggests that σ bands give the main contribution to the Seebeck effect. Received 16 November 2001 and Received in final form 21 December 2001     

Electronic structure of novel multiple-band superconductor SrPt<Subscript>2</Subscript>As<Subscript>2</Subscript>

The electronic structure of the recently discovered superconductor SrPt₂As₂ with T _c = 5.2 K has been calculated in the local-density approximation. Despite its chemical composition and crystal structure are somehow similar to FeAs-based high-temperature superconductors, the electronic structure of SrPt₂As₂ is very much different. The crystal structure is orthorhombic (or tetragonal if idealized) and has layered nature with alternating PtAs₄ and AsPt₄ tetrahedra slabs sandwiched with Sr ions. The Fermi level is crossed by Pt-5d states with rather strong admixture of As-4p states. Fermi surface of SrPt₂As₂ is essentially three-dimensional, with complicated sheets corresponding to multiple bands. We compare SrPt₂As₂ with 1111 and 122 representatives of FeAs-class of superconductors, as well as with isovalent (Ba,Sr)Ni₂As₂ superconductors. Brief discussion of superconductivity in SrPt₂As₂ is also presented.     

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.     

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 tunneling spectroscopy of the phonon spectrum of MgB<Subscript>2</Subscript>

The specific features of the phonon spectrum of the MgB₂ compound (T _c = 38 K) are investigated by tunneling spectroscopy. It is demonstrated that both the position and the energy width of the fundamental optical mode E _2g in the phonon spectrum are in good agreement with inelastic X-ray spectroscopy data but differ substantially from Raman spectroscopy results. Among possible factors responsible for this discrepancy, the anharmonic and nonadiabatic effects that are characteristic of the MgB₂ system are discussed.     

Theoretical analysis of tunneling experiments in the MgB<Subscript>2</Subscript> system

A detailed theoretical analysis of the experimental data obtained earlier in the studies of the tunneling spectra in the MgB₂ two-band superconducting system has been performed. It is shown that most these data are well described in the generalized two-band Bardeen-Cooper-Schrieffer theory with the constants of the intraband electron-phonon interaction that reasonably coincide with the ab initio calculation results. It is shown that the existence of specific collective excitation in this system induced by oscillations of the relative phase of two superconducting condensates (the Leggett mode) indicates the overestimation of the constants of the interband electron-phonon interaction in the ab initio calculations. The dependences of the superconducting gaps and the Leggett mode frequency on the temperature and the disorder degree in the Mg_{1 − x} Al _x B₂ system have been thoroughly studied.     

Superconducting weak bonds at grain boundaries in MgB<Subscript>2</Subscript>

The possibility of preparing bicrystalline Josephson junctions and bolometers based on superconducting MgB₂ on specially prepared bicrystalline MgO substrates is investigated. Microbridges 0.85–6.00 μm in width, intersecting the bicrystalline interface, are formed in epitaxial bicrystalline MgB₂ films grown on these substrates. It is found that annealing of bicrystalline samples in oxygen leads to a systematic decrease in the critical current, an increase in the temperature width of the superconducting transition region, and to an improvement of the current-voltage (IV) characteristic, which becomes close in shape to the IV characteristic of a Josephson junction. The response of such a junction to radiation at a frequency of 110 GHz with an amplitude attaining 0.5 mV is measured.     

Two regimes in the magnetic field response of superconducting MgB<Subscript>2</Subscript>

Using Scanning Tunneling Microscope at low temperature we explore the superconducting phase diagram in the π-band of the two-band superconductor MgB₂. In this band the peculiar shape of the local tunneling spectra and their dynamics in the magnetic field reveal the complex character of the quasiparticle density of states (DOS). The gap in the DOS is first rapidly filled with states in raising the magnetic field up to 0.5 T and then slowly approaches the normal state value: the gap is observed up to 2 T. Such a change in the DOS dynamics suggests the existence of two terms in the DOS of the π-band: a first one, reflecting an intrinsic superconductivity in the band and a second one, originating from an inter-band coupling to the σ-band. Our findings allow a deeper understanding of the unique phase diagram of MgB₂.     

Gain in the stability of NbTi composite superconductor doped by large-heat-capacity substance Gd<Subscript>2</Subscript>O<Subscript>2</Subscript>S

The effect of internal doping of a NbTi composite wire by a large-heat-capacity substance (Gd₂O₂S ceramics) on the critical currents and stability against short thermal disturbances (with a typical time on the order of 1 ms) is studied experimentally and theoretically. The composite wire studied in this work is similar in design to conductors used in the international thermonuclear experimental reactor (ITER). The additive introduced into the wire in an amount of 5 vol % raises its specific heat ninefold at 4.2 K. It is found that the critical current of the (NbTi + Gd₂O₂S) wire increases by 8–11% in comparison with a reference NbTi wire depending on the external magnetic field varying between 5 and 7 T. Although the potential of high specific heat is not utilized completely, the critical thermal energies of the doped wires are three to four times higher than those of the undoped (reference) wires at near-critical currents.     

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.     

Band structure of a new layered La<Subscript>3</Subscript>Ni<Subscript>4</Subscript>P<Subscript>4</Subscript>O<Subscript>2</Subscript> superconductor

The ab initio FLAPW-GGA calculations of the band structure of a new layered low-temperature (T _C ～ 2.2 K) La₃Ni₄P₄O₂ superconductor are presented. The energy bands, distributions of the densities of electron states, charge states of the atomic layers, low-temperature electron specific heat, and molar Pauli paramagnetic susceptibility for La₃Ni₄P₄O₂ have been determined. They are discussed compared to the existing experimental data.