Superconducting Magnesium Diboride Research Articles

Effects of disorder on the microwave properties ofMgB2polycrystalline films

The role of disorder in superconducting magnesium diboride $({\mathrm{MgB}}_{2})$ policrystalline films is investigated in the high frequency range by a coplanar microwave resonator technique. Two sources of disorder are considered, heavy-ion irradiation damage and sample ageing. Microwave measurements are analyzed in the framework of the two-gap model with strong interband scattering contribution. It turns out that disorder enhancement increases the interband scattering rate, resulting in a reduction of the surface resistance at low temperatures, due to a slight increase of the $\ensuremath{\pi}$ gap. Moreover, increasing disorder at grain boundaries induces a nonmonotonic residual surface resistance, showing the features of a resistive behavior for the highest disorder level. Finally, the effects of the different kinds of disorder on the intrinsic and on the grain-boundary properties of the ${\mathrm{MgB}}_{2}$ films are compared and discussed.

Measuring elastic constants of novel materials and thin films using small sample resonant ultrasound spectroscopy

One of the most important undertakings for materials is the measurement of the elastic constants. As derivatives of the free energy with respect to atomic displacements, the elastic constants are closely connected to the thermodynamic properties of the material, including the specific heat, the Debye temperature, the Gruneisen parameter, etc. The elastic constants are a sensitive probe of the lattice environment in which all solid state phenomena occur, particularly in the vicinity of a phase transition. Novel materials to which RUS might be applied are often available only in small samples, and for these cases we have developed a small sample RUS apparatus that can measure bulk samples only a few hundred microns in size and thin films a few hundred nanometers in thickness. Thin films are particularly interesting because substrate induced strains in the lattice may produce dramatic changes in the elastic and thermodynamic properties. In this talk, small sample RUS measurements on the new superconductor magnesium diboride and colossal magnetoresistance materials, in bulk and thin film forms, will be discussed.

Transport properties of as-grown MgB2 films using micro-bridge

The intermetallic superconductor magnesium diboride (MgB2) is a promising candidate for use in superconducting electronic devices due to its high transition temperature (Tc) and simple AlB2 type structure. Its application requires the development of a high-quality film fabrication process. We have previously reported the growth of MgB2 films deposited on MgO (100), SrTiO3 (100) and Al2O3 (0001) substrate using a co-evaporation method combined with a molecular beam epitaxy (MBE) apparatus. In this paper, we report on the details of the transport properties of these as-grown MgB2 films. These film’s basal properties were investigated by XRD, RHEED, SQUID and the dc-4 probe method. Micro-bridges made using micro-processing methods such as photolithography and dry etching were used for transport measurements. All bridges were 10μm wide×30μm long to allow estimation of their critical current density using the dc-4 probe method. We discuss the relationship between critical current densities and the substrates used.

Fabrication of as-grown MgB2 films on ZnO (0 0 0 1) substrates by molecular beam epitaxy

The intermetallic superconductor magnesium diboride (MgB2) is a promising candidate for use in superconducting electronic devices because its high transition temperature (Tc). These applications require the development of a high-quality film fabrication process. We report the first ever attempt to use ZnO (0001) substrate to fabricate as-grown MgB2 films. The lattice mismatch between ZnO and MgB2 is smaller than for previous reported substrates. The structural and physical properties of the films were investigated by RHEED, XRD, XPS, and resistivity measurements. The resistivity measurements suggest the maximum transition temperature of these films to be 35K; their resistivity at 300K is 8.9μΩcm. Our results indicate that these films, when deposited on ZnO substrates, have potential for the fabrication of higher quality MgB2 films. We also investigate the growth temperature dependence and substrate dependence for superconductivity of MgB2 films. We discuss the most important conditions and the effectiveness of ZnO substrate for fabricating as-grown MgB2 films.

CRITICAL TEMPERATURE OF LAYERED SUPERCONDUCTORS IN INTERMEDIATE e-ph COUPLING: APPLICATION TO MgB2

Critical temperature of layered superconductors is studied using anisotropic Eliashberg theory in intermediate e-ph coupling. The results are shown to be in qualitative agreement with experimental data for the superconducting magnesium diboride MgB 2.

Transmission electron microscopy study of as-grown MgB2 films

The intermetallic superconductor magnesium diboride (MgB2) is a promising candidate for use in superconducting electronic devices because of its high transition temperature (Tc). These applications require the development of a high-quality film fabrication process. We have previously reported the growth of MgB2 films deposited on MgO (100), SrTiO3 (100) and Al2O3 (0001) substrate using a co-evaporation method and molecular beam epitaxy (MBE) apparatus. In this paper, we will report the correlation between structural properties and physical properties of as-grown MgB2 films deposited on MgO (100), ZnO (0001), and Si (111) substrate. The films’ basal properties have been confirmed by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), resistivity measurements and SQUID magnetometry. The details of the interfacial structure were studied by transmission electron microscopy (TEM). We will then discuss the most important parameters for fabricating high quality as-grown MgB2 films and junctions.

Lattice dynamics study of AlB2-type 4d transition-metal diborides by extended X-ray-absorption fine structure

The recent discovery of the magnesium diboride superconductor, a system characterized by a remarkably high transition temperature ( T c = 39 K ), has renewed interest towards experimental and theoretical research of the superconductivity in isostructural systems, such as transition-metal diborides. Here we present an X-ray-absorption fine-structure investigation in AlB 2-type 4d transition-metal diborides TMB 2 (TM=Y, Zr, Nb) at the transition-metal K-edge. The vibrational behavior of these transition-metal diborides has also been investigated by analyzing the extended X-ray-absorption fine-structure (EXAFS) spectra. The change of the bond length and Debye–Waller factors of TM–B, TM–TM (in-plane) and TM–TM (out-plane) with the covalent radii of the transition-metal atoms has been shown. Data clearly demonstrate that the chemical stability is in the following order for the 4d transition-metal diborides: ZrB 2>NbB 2>YB 2. The chemical stability of YB 2 must be much less than that of the two other compounds. It suggests that the species difference of the transition metals influences the vibrational behavior of TM–B and TM–TM bonds. In particular, a clearly different behavior of the vibration is observed for the transition-metal atoms in and out of plane. The results suggest the importance of the metal atoms in the diborides for superconductivity of these compounds and an intimate relationship between the local atomic displacements and the superconductivity.

Alignment of Carbon Nanotube Additives for Improved Performance of Magnesium Diboride Superconductors

A method for aligning carbon nanotubes (CNTs) in CNT–MgB2 superconductor composite wires through a readily scalable drawing technique is described. The aligned CNT-doped MgB2 wires show an enhancement in magnetic critical current density by more than an order of magnitude in high magnetic fields compared to undoped wires. The arrows in the figure indicate CNTs in the composite.

Teoriya Ginzburga – Landau dlya dvuzonnih sverhprovodnikov

Recent studies of two-band superconductors using the Ginzburg–Landau (GL) theory are reviewed. The upper and lower critical fields [Hc2(T) and Hc1(T), respectively], thermodynamic magnetic field Hcm(T), critical current density jc(T), magnetization M(T) near the upper critical field, and the upper critical field Hc2film(T) of thin films are examined from the viewpoint of their temperature dependence at a point Tc using the two-band GL theory. The results are shown to be in good agreement with the experimental data for the bulky samples of superconducting magnesium diboride, MgB2, and nonmagnetic borocarbides LuNi2B2C and YNi2B2C. The specific heat jump turns out to be smaller than that calculated by single-band GL theory. The upper critical field of thin films of two-band superconductors is calculated and the Little–Parks effect is analyzed. It is shown that magnetic flux quantization and the relationship between the surface critical magnetic field Hc3(T) and the upper critical field Hc2(T) are the same as in the single-band GL theory. Extension of the two-band GL theory to the case of layered anisotropy is presented. The anisotropy parameter of the upper critical field Hc2 and the London penetration depth λ, calculated for MgB2 single crystals, are in good agreement with the experimental data and show opposite temperature behavior to that in single-band GL theory.

Pair-Breaking Critical Current Density of Two-Band SuperconductorMgB2

Temperature dependence of the pair-breakingcritical current densityof MgB2, jd(T), is studied using a two-band Ginzburg–Landautheory. The results are shown to be in good agreement withexperimental data for the superconducting magnesium diboride MgB2.

THE UPPER CRITICAL FIELD OF THIN FILMS OF TWO-BAND SUPERCONDUCTORS: AN APPLICATION TO MgB2

Temperature dependence of the upper critical field [Formula: see text] of thin films of two-band superconductors is studied using the two-band Ginzburg–Landau (G–L) theory. The results are shown to be in qualitative agreement with experimental data for the superconducting magnesium diboride MgB 2 films.

TEMPERATURE DEPENDENCE OF THE CRITICAL CURRENT DENSITY OF SUPERCONDUCTORS IN TWO-BAND GINZBURG–LANDAU THEORY: APPLICATION TO MgB2

Temperature dependence of the critical current density j c (T) for the superconducting magnesium diboride MgB 2 is studied in the vicinity of T c by using a two-band Ginzburg–Landau (G–L) theory. The result is in good agreement with experimental data for a bulk MgB 2. Comparision of obtained results with single-band GL theory are presented.

Advances in point-contact spectroscopy: two-band superconductor MgB2 (Review)

Analysis of the point-contact spectroscopy (PCS) data on the dramatic new high-Tc superconductor magnesium diboride MgB2 reveals quite different behavior of two disconnected σ and π electronic bands, deriving from their anisotropy, different dimensionality, and electron–phonon interaction. PCS allows direct registration of both the superconducting gaps and electron–phonon interaction spectral function of the two-dimensional σ and three-dimensional π band, establishing the correlation between the gap value and the intensity of the high-Tc driving force—the E2g boron vibrational mode. PCS data on some nonsuperconducting transition-metal diborides are surveyed for comparison.

Sample-size dependence of the magnetic critical current density inMgB2superconductors

Sample size dependent magnetic critical current density has been observed in magnesium diboride superconductors. At high fields, larger samples provide higher critical current densities, while at low fields, larger samples give rise to lower critical current densities. The explanation for this surprising result is proposed in this study based on the electric field generated in the superconductors. The dependence of the current density on the sample size has been derived as a power law $j\ensuremath{\propto}{R}^{1/n}$ $[n$ is the n factor characterizing $E\ensuremath{-}j$ curve ${E=E}_{c}{(j/j}_{c}{)}^{n}].$ This dependence provides one with a method to derive the n factor and can also be used to determine the dependence of the activation energy on the current density.

Ginzburg–Landau Equations for Two-Band Superconductors: Application to MgB2, LuNi2B2C and YNi2B2C

Temperature dependences of the upper critical field Hc2(T), lower critical field Hc1(T), thermodynamic magnetic field Hcm(T) and critical current density jc(T) are studied in the vicinity of Tc by using a two-band Ginzburg–Landau (G–L) theory. The results are shown to be in good agreement with experimental data for the superconducting magnesium diboride, MgB2, and non-magnetic borocarbides LuNi2B2C, YNi2B2C. In addition, two-band G–L theory was applied for the calculation of specific heat jump, which is smaller than in single-band G–L theory. Peculiarities of Little–Parks effect in two-band G–L theory are studied also. It is shown that quantization of the magnetic flux and relation between surface magnetic field Hc3(T) and upper critical field Hc2(T) are the same as in single band G–L theory. Generalization of two-band Ginzburg–Landau theory to the layered case is discussed.

POINT-CONTACT SPECTROSCOPY OF MAGNESIUM DIBORIDE WITH DIFFERENT COUNTER-ELECTRODES

We report on tunneling and Andreev-reflection conductance spectra of 39 K superconducting magnesium diboride, obtained with Pb and Au counter-electrodes. Two distinct steps at close to 2.7 and 7.1 meV appear in a low-resistance metallic-type Au–MgB 2 junction characteristic, whereas a tunneling-like spectrum measured for the same junction, annealed by the application of DC current, exhibits only a rounded contribution of the larger gap. Junctions with a superconducting lead counter-electrode pressed into a bulk MgB 2 sample reveal two conductance peaks that are interpreted as the result of the formation of a highly-transmitting break junctions inside the magnesium diboride ceramic. Our results strongly support the two-band model with two different gap values on quasi-two-dimensional σ (7.1 meV) and three-dimensional π (2.7 meV) Fermi surface sheets of MgB 2.

Preparation and characterization of magnesium diboride superconductor by melting process

o C for 24 hrs. The samples were characterized by using XRD, EDX, SEM, four probe AC methods and magne- tization measurements using SQUID magnetometer. The critical temperature was found to be 39 K which shifts towards lower temperature with increasing applied field (0 to 9T). The critical current density, according to Bean's critical state model was estimated and found to be ~10 5 A/cm 2 , which is comparable to the reported data.

A unique determination of boundary condition in quantitative electron diffraction: Application to accurate measurements of mean inner potentials

We combine off-axis electron holography and electron shadow imaging to accurately determine the specimen thickness and the incident electron beam direction over the illuminated area of a crystal. We, furthermore, quantify the variations in diffraction intensity with position over the same area. This unique solution to the experimental boundary condition problem enables us to make precise measurements of mean inner electrostatic potentials and structure factors that are sensitive to the bonding characteristics of materials. In this paper, we present the results of mean-inner potential determination from silicon and the newly discovered magnesiumdiboride superconductor.

Ginzburg–Landau theory for two-band s-wave superconductors: application to non-magnetic borocarbides LuNi2B2C, YNi2B2C and magnesium diboride MgB2

Abstract Temperature dependence of the upper critical field H c2 ( T ), lower critical field H c1 ( T ) and thermodynamic magnetic field H cm ( T ) are studied in the vicinity of T c by using a two-band Ginzburg–Landau (G–L) theory. The results are shown to be in a good agreement with experimental data for the superconducting magnesium diboride, MgB 2 , and non-magnetic borocarbides LuNi 2 B 2 C, YNi 2 B 2 C. In addition, two-band G–L theory was applied for the calculation of specific heat jump, which is smaller than in single-band G–L theory. Peculiarities of Little–Parks effect in two-band G–L theory are studied also. It is shown that quantization of the magnetic flux and relation between surface magnetic field H c3 ( T ) and upper critical field H c2 ( T ) are the same as in single-band G–L theory.

Ginzburg–Landau Theory for Two-Band Isotropic s-Wave Superconductors

Temperature dependence of the upper critical field Hc2(T), lower critical field Hc1(T) and thermodynamic magnetic field Hcm(T) are studied in the vicinity of Tc using a two-band Ginzburg–Landau (GL) theory. The results are shown to be in a good agreement with experimental data for the superconducting magnesium diboride (MgB2) and non-magnetic borocarbides LuNi 2 B 2 C ( YNi 2 B 2 C ). In addition, two-band GL theory was applied for the calculation of specific heat jump, which is smaller than in single-band GL theory. Peculiarities of Little–Parks effect in two-band GL theory are studied also. It is shown that the quantization of the magnetic flux and the relation between surface magnetic field Hc3(T) and upper critical field Hc2(T) are the same as in single band GL theory.