Degradation of Superconducting Properties in MgB2 by Formation of the MgB4 Phase

Superconducting MgB₂ polycrystalline samples have been fabricated under two different conditions in order to determine the effect of MgB₄ phase. A series of samples was placed in an -alumina container closed with a cup and fired under high purity argon gas. The other series of samples was placed in an -alumina boot without any lid and fired under similar conditions. For the first series of samples, we have found pure MgB₂ phase formation and a narrow transition width at 0.4 K. For the second series of samples, significant amount of MgB₄ phase were formed and the T _zero was decreased to 27 K. For both the group of samples magnetization hysteresis loops obtained at various temperature range and applied field up to 2 T. The best J _cmag for the first series of samples was 1.9 × 10⁵ A/cm² at 10 K and 0 T, and for the second series of samples was 0.7 × 10⁴ A/cm² at 10 K and 0 T.     

Deposition and Properties of Superconducting MgB2 Thin Films

The recently discovered superconductor MgB₂ with T _c at 39 K has great potential in superconducting electronics. In this paper, we review the deposition techniques used for MgB₂ thin films in the light of a thermodynamic study of the Mg-B system with the calculation of phase diagrams (CALPHAD) modeling technique. This thermodynamic study identifies a growth window in the pressure–temperature phase diagram, in which the magnesium pressure is very high for likely in situ growth temperatures. A Hybrid Physical–Chemical Vapor Deposition (HPCVD) technique that successfully achieves such a high Mg pressure is shown to produce in situ epitaxial MgB₂ thin films with bulk superconducting properties.     

Transport current densities of MgB2 wire, cable and continually transposed conductor

Multi-core MgB₂ wire, cable and continually transposed conductor (CTC) have been assembled from a single-core in situ made Ti/Cu sheathed composite. It was shown that the filament’s current densities J_c of composed wire, cable and CTC are comparable, but the engineering current densities J_e and the window current densities J_w are much different. MgB₂ cable shows apparently lower sensitivity to bending strain than monolithic wire. Instead of the highest J_w for CTC, it offer also high surface to volume ratio, which is important parameter for efficient cooling and thermal stability. The measurement of the resistance to tensile strain has shown the best performance for monolithic wire and the lowest irreversible strain for CTC.     

A New Method for the Estimation of Hc2 Anisotropy in Polycrystalline MgB2 Samples

By using isothermal magnetization measurements in polycrystalline MgB₂ samples, we estimate the H ^c _c2 in the interval [0, T _c]. By combining these measurements to the estimated H ^ab _c2 from the onset of the diamagnetic transition in isofield and isothermal magnetic measurements, an estimation of the anisotropy parameter can be achieved. The H ^c _c2 values coming from high quality polycrystalline samples agrees nicely to those obtained on single crystals. Our results show a temperature variation of the (T ) = H ^ab _c2/H^c2 with (T _c) 3.     

Effect of Silver Admixture on Transport Properties in Polycrystalline MgB2

The normal-state and superconducting properties of polycrystalline MgB₂ samples including various amounts of Ag powder were investigated by measurements of electrical resistivity. The temperature-dependent electrical resistivity roughly obeys the power law (₀ + aT ⁿ with n > 1) in the normal state and becomes linear as we increase the Silver content. The critical superconducting temperatures remain almost unchanged. Silver, not reacting with MgB₂, reacts with excess magnesium to form intermetallic layers of AgMg that cluster at crystallite boundaries.     

Microwave Properties of In Situ Grown MgB2 Superconducting Thin Films

The electrodynamic response at 20 GHz of c-axis oriented MgB₂ superconducting thin films is reported. Mg-rich Mg-B precursor samples were grown on MgO and Al₂O₃ single crystal substrates by a d.c. planar magnetron sputtering technique, and subsequently annealed in situ at 800°C for 10 min in a In-sealed Nb box in the presence of saturated Mg vapor. The films were characterized by a variety of structural and electronic techniques including XRD, EDS, STM-AFM analyses, and transport measurements. The dependence of the surface impedance from temperature and radiofrequency (r.f.) field amplitude was measured via a dielectric resonator technique. Temperature data clearly confirm the s-wave nature of the newly discovered superconductor, even if the value of the energy gap is smaller than BCS prediction. An effective two-band model can be applied to quantitatively explain the experimental results. In spite of previous reports claiming the absence of weak link behavior in MgB₂, the power dependence show that granularity governs the performance of these films in the microwave region.     

Low Temperature Elastic Constants of Polycrystalline MgB2

We have used a resonant ultrasound spectroscopy technique to measure the bulk and shear modulus of fully dense, polycrystalline MgB₂ between 4 and 300 K. Both moduli show good agreement with published first principle calculations. The internal friction shows a broad maximum around 40 K.     

Design, development and experiment of a 1.5 T MgB2 superconducting test magnet

Superconducting magnets using MgB₂ tapes are potentially applicable in many areas, such as medical magnetic resonance imaging and fault current limiting. Under conduction cooling environments, the magnets can work at 15-20 K. In this work, a solenoid structured magnet with ∅ 100 mm bore is designed, built and tested. The maximum field at its center is up to 1.5 T. The field homogeneity, the thermal stability and the quench characteristics in the magnet are also investigated.     

Weak Links and Phase Slip Centers in Superconducting MgB2 Wires

MgB₂ superconducting wires were produced by the Mg diffusion method. Scanning electron microscopy (SEM), optical microscopy, dispersive X-ray analysis (EDS), and XRD diffraction were used to study the physical structure and content of the wires. Magnetic properties (T _c ^m, H _c1, H _c2, J _c by the Bean model) were obtained with a SQUID magnetometer, and transport properties (T _c ^r , H _c2, resistivity and residual resistivity ratio) were measured using a standard four-lead configuration. The V-I characteristics of the wires close to the critical temperature showed a staircase response, which was attributed to the presence of weak links, creating phase slip centers. The origin of those weak links is discussed in relation to their formation and structure.     

Normal-State Optical Response Functions of MgB2 Superconductor

The reflectivity of superconducting MgB₂ (T _c = 39 K) has been measured on a randomly oriented thin film at room temperature over a wide-range of frequencies, 20 < 100000 cm^–1. The conductivity shows highly metallic behavior but cannot be explained with a simple Drude model alone. The electronic contribution is analyzed by a generalized Drude model. The scattering rate 1/() and the mass renormalization ratio m*()/m = 1 + () exhibit clear frequency dependence. The electron–phonon coupling strength is estimated to be 1.5 ± 0.5 while the plasma frequency _p is 2.4 eV.     

Thermally stabilized MgB2 composite wires with different barriers

Glidcop sheathed in situ MgB₂ wires with Nb and Ti barriers and Cu and Al inner cores have been made and tested. It was found that the inner aluminium stabilization is not appropriate, due to a reaction with Nb during the heat treatment at 630 °C. Using Ti barriers, much higher J_c values were obtained when compared to Nb barriers. The enhancement by 1.5 to 2.1 times is explained by the absorption of impurities from MgB₂ filaments by titanium. On the other hand, Ti reacts with copper and reduces the conductivity of metallic sheath (RRR ratio), which affects the thermal stabilization of the MgB₂ wire.     

Fabrication and Transport Properties of MgB2 Thin Films and Tunnel Junctions

We report on fabrication and characterization of MgB₂ thin films and tunnel junction structures. The MgB₂ films were prepared on Al₂O₃, Si, glass, and plastic foil substrates by either vacuum codeposition of boron and magnesium, or high-temperature magnesium annealing of boron films. The crystalline structure of our films depended directly on the method of preparation. The films prepared by codeposition and postannealed in Ar atmosphere were amorphous with nanocrystal inclusions and were characterized by very smooth surfaces. On the other hand, the boron-precursor films annealed in magnesium vapor were rough, polycrystalline with approximately 1-m-diameter single-crystal blocks. Because of their surface quality, the amorphous films were used for preparation of point contact junctions and for optical characterization. The point-contact spectra of tested junctions exhibited a two-gap structure. The MgB₂ polycrystalline films was used for bulk transport studies. The best films were characterized by the critical temperature T _c of up to 39 K and the current density j _c at 4.2 K of about 10⁷ A/cm².     

Generalized Multiple Gap Model for the Superconductivity in MgB2

The free energy of the two-band superconductor is obtained taking into account all actual electron–phonon and Coulomb interactions. The temperature dependences of the superconducting gaps, the specific heat with its jump at T = T _c (T _c is the critical temperature) are calculated minimizing the free energy. The theory is applied to the MgB₂ superconductor.     

Superconductivity in High-Pressure Synthesized Pure and Doped MgB2 Compounds

Dense pure and doped (Mg_{1 – x} A _x )B₂ samples with A = Na, Ca, Cu, Ag, Zn, and Al were synthesized at high pressure–high temperature in a multi-anvils press (3.5–6 GPa, 900–1000°C) for 0 < x 0.20. They were studied by X-ray diffraction, scanning electron microscopy and their superconducting properties were investigated by AC susceptibility, magnetization, and transport measurements. Only Al is really substituted on the Mg site. The other elements form secondary phases with B or Mg. No large effect is observed on the superconducting properties T _c, j _c critical current, H _irr, and H _c2.     

Magnetic flux pinning and flux jumps in polycrystalline MgB2

MgB₂ polycrystalline samples were fabricated under varying conditions of isostatic pressing in argon gas. The critical current densities (J_C) were determined through measurements of hysteresis loops, and the highest value of J_C at 10 K was 1.9 × 10⁴ A/cm² at 4.8 T. The depinning temperatures were measured at various magnetic fields using the vibrating reed technique. Flux jumps appeared below 7.4 K. The hysteresis loops were carefully examined to determine the temperature and magnetic field range where flux jumps appeared.     

MgB2 Superconducting Tips for Scanning Tunneling Microscopy Study

We report a simple method for the fabrication of reproducible, clean, and stable MgB₂ superconducting tips. The quality of these tips has been verified by imaging the surface of a thin Au(111) film sample, using a low temperature scanning tunneling microscopy (STM). Using the MgB₂ superconducting tip, high-quality semiatomically resolved STM surface images of the thin Au(111) film sample have been observed, which unambiguously indicates that the fabrication of relatively superconducting MgB₂, suitable for use as STM tips, is feasible.     

Comparative Study of Dense Bulk MgB2 Materials Prepared by Different Methods

We report on the results of a comparative investigation of highly dense bulk MgB₂ samples prepared by three methods: (i) hot deformation; (ii) high pressure sintering; and (iii) mechanical alloying of Mg and B powders with subsequent hot compaction. All types of samples were studied by AC susceptibility, DC magnetization, and resistivity measurements in magnetic fields up to ₀ H = 160 kOe. A small but distinct anisotropy of the upper critical field connected with some texture of MgB₂ grains was found for the hot deformed samples. The samples prepared by high pressure sintering as well as by mechanical alloying show improved superconducting properties, including high upper critical fields H _c2 (₀ H _c2 (0) 23 T), irreversibility fields H _irr which are strongly shifted towards higher values H _irr(T) 0.8 H _c2(T) and high critical current J _c (J _c = 10⁵ A/cm² at 20 K and 1 T).     

Mild solvothermal synthesis of complex fluorides Li2BeF4 and LiSrAlF6

The complex fluorides Li₂BeF₄ and LiSrAlF₆ were synthesized solvothermally at 180–240 °C and characterized by means of X-ray powder diffraction (XRD), Scanning electron microscopy (SEM), thermogravimetric analysis (TGA), infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). The different influence factors such as solvents, molar ratios of initial mixtures, reaction temperature and reaction time were investigated. The experimental results indicated that Li₂BeF₄ and LiSrAlF₆ powders could be controllably synthesized in the solvothermal process.     

One-pot synthesis of intestine-like SnO2/TiO2 hollow nanostructures

In the present study the intestine-like binary SnO₂/TiO₂ hollow nanostructures are one-pot synthesized in aqueous phase at room temperature via a colloid seeded deposition process in which the intestine-like hollow SnO₂ spheres and Ti(SO₄)₂ are used as colloid seeds and Ti-source, respectively. The novel core (SnO₂ hollow sphere)-shell (TiO₂) nanostructures possess a large surface area of 122 m²/g (calcined at 350 °C) and a high exposure of TiO₂ surface. The structural change of TiO₂ shell at different temperatures was investigated by means of X-ray diffraction and Raman spectroscopy. It was observed that the rutile TiO₂ could form even at room temperature due to the presence of SnO₂ core and the unique core-shell interaction.     

Nonequilibrium Superconductivity Probed by Time-Resolved Far-Infrared Conductivity Dynamics: Comparison Between MgB2 and YBa2Cu3O7

We present studies of superconducting state recovery dynamics in superconductors probed by time-resolved optical-pump terahertz-probe spectroscopy. In these experiments femtosecond optical pulses excite the superconductor out of equilibrium, while the condensate and quasiparticle dynamics are probed by measuring the associated changes in the far-infrared conductivity as a function of time delay with respect to the excitation pulse. Recent results on MgB₂ and YBa₂Cu₃O₇ are presented, with main differences and similarities between the two pointed out and discussed.