Anomalous Voltage Noise at the Superconducting Transition of Tin Films

We report voltage noise studies in the superconducting transition of thin Tin (Sn) films. Voltage noises are measured as a function of temperature and ac current. The noise spectral power S ^1/2 strongly depends on the temperature and current, with the peak temperature in S ^1/2 shifted down from that of dR/dT. Comparison with the dc noise measurement shows the S ^1/2 is much larger with ac current than dc. I–V characteristics and voltage noises are measured simultaneously to reveal the nature of the excess noises. The noise dependence on voltage strongly suggests the nature of vortex shot noise with a characteristic dependence for small V and reduction of S ^1/2 due to strong correlation of vortices at large V.      

Two-Dimensional Superconducting Fluctuations in MgB2 Films

The origin of the resistive transition broadening for superconducting MgB₂ films is investigated experimentally. The crucial role of two-dimensional weak and critical fluctuations is demonstrated.     

MgB2超导膜制备技术

介绍了自MgB2超导电性被发现以来,用于制备MgB2薄膜/厚膜的各种基片、主要生长方法以及退火工艺,简单概述了薄膜制作超导隧道结方面的研究状况.     

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.     

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.     

Magnetization Loss of MgB2 Superconducting Wire at Various Temperatures

The magnetization loss of MgB₂ wire was investigated using numerical calculations based on the finite element method. Various superconducting properties of MgB₂ wire such as nonlinearity and the field dependence of the critical current were considered in the numerical formulation. An analysis of magnetization loss was carried out as a function of the external magnetic field for a wide range of operating temperatures. The numerical results were compared with conventional theories and were found to be in relatively good agreement. An alternate approach based on a normalization method using critical current data was also employed as a simple method for predicting magnetization loss. The effectiveness of the simple equation for predicting loss was verified by comparisons of both values for various temperatures.     

Direct Numerical Simulations for Non-equilibrium Superconducting Dynamics at the Transition Edge: Simulation for MgB2 Neutron Detectors

We perform large-scale numerical simulations on the non-equilibrium superconducting dynamics after a neutron capture at the superconducting transition edge in MgB₂ by solving the time-dependent Ginzburg-Landau equation coupled with the Maxwell and the heat diffusion equations. The simulations are carried out under the current-biased condition in order to explain experimental results made in the JAEA reactor JRR-3, and the time scale of the obtained voltage signal is found to be almost consistent with the experiments. Moreover, the time evolution of the voltage signal is connected with that of the spatial profile of the superconducting order parameter.      

Effects of Small Fields on the Magnetic Response of Superconducting Thin Films of MgB2

Under the influence of a magnetic field applied perpendicularly to its plane, a superconducting thin film develops dendritic patterns of penetrated regions that coexist with Meissner areas. For a thin film of MgB₂ submitted to an alternate field of moderate amplitude, the AC susceptibility measured while cooling the sample exhibits a quite unusual behavior, reentering and fluctuating with temperature. The effect is more pronounced at frequencies around 1 kHz. Two plausible explanations for the effect are discussed.     

Superconducting Gap in the Multi-band System MgB2

Based on the angle-resolved photoemission spectra of single crystals, it is demonstrated that a couple of bands cross the Fermi energy in MgB₂, which is in good agreement with band theory. The superconducting gap in this multiband system is carefully examined by Raman scattering spectroscopy with various polarizations. It has been revealed that the large gap (24k _B T _c) that is typical for a clean limit s-wave superconductor is restricted to the -bands, while the gap on the -bands is much smaller (21.1k _B T _c) and strongly affected by the impurity scattering, which gives a dirty limit behavior. This unusual two-gap behavior might be caused by the lack of interband scattering due to special separation of the - and -bands, as predicted by Mazin et al.     

High Critical Current Density of c-Axis-Oriented MgB2 Thin Films

From the magnetic hysteresis loops (M-H) and the current-voltage (I-V) relations, we obtained the critical current density (J _c) for c-axis-oriented MgB₂ thin films with a transition temperature of 39 K. The temperature dependence of J _c obtained from the M-H and the I-V curves coincide, which indicates the strongly linked nature of the intergrains in these thin films. And, the high value of J _c was estimated to be 4×10⁷ A/cm² at 5 K and 1×10⁵ A/cm² at 37 K under a self field, indicating a promising candidate for technological applications.     

Vortex Contribution to the Excess Noise in Superconducting Transition

We report voltage noise studies on thin tin films in the superconducting transition region to understand the mechanism of the excess noises observed in the transition. Temperature and I–V characteristics studies suggest evidence of a Kosterlitz–Thouless transition for the thin Sn films studied. A correlation of the onset of rapid increases of noise with the power law in I–V points to the dominant vortex contribution to the excess noises, consistent with the characteristic \(\sqrt{V}\) dependence of the spectral power of the noise.     

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 α-alumina container closed with a cup and fired under high purity argon gas. The other series of samples was placed in an α-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.     

Effect of Substrates on Superconducting Properties of MgB2 Thick Films Fabricated by Spraying Method

MgB₂ thick films have been prepared on Al₂O₃, MgO and SrTiO₃ ceramic substrates using the spray method with two different heat-treatment cycles, 925???C with 10?min (Group?A) and 610???C with 24?h (Group?B). The structural/microstructural (XRD, SEM) and transport (R?CT, MR?CT) properties of the films prepared were investigated. XRD analysis showed that the films in Groups?A and?B consisted mainly of MgB₂, but the peaks originating from the substrates were also observed in films fabricated on the Al₂O₃ substrate. From scanning electron microscopy, it was seen that the surface of MgB₂ thick films prepared is not sufficiently homogeneous. Some cracks and heaps with different sizes were observed. The best electrical results were obtained for films in Group?A on Al₂O₃ substrate. The T _c value for films in Group?A was found to be 36.1?K, 36?K and 35?K, for Al₂O₃, MgO and SrTiO₃ substrates, respectively. No superconducting state was reached for films in Group?B on the SrTiO₃ substrate. In all the films which showed a superconducting state, the magnetic field strongly affected T _c . A?tail in the resistance curves was observed with increasing magnetic field for films in Group?A on the MgO and SrTiO₃ substrates. At magnetic fields above 1?T for films in Group?A on the SrTiO₃ substrate and above 2?T for films in Group?A on the MgO substrate, the zero-resistance temperature, T ₀, was not obtained.     

Fabrication of Superconducting MgB2 Thin Films by Magnetron co-Sputtering on (001) MgO Substrates

We fabricated superconducting MgB₂ thin films on (001) MgO substrates by magnetron rf and dc co-sputtering on heated substrates. We annealed the samples ex-situ and in-situ at temperatures between 450 ^°C and 750 ^°C. The substrate temperature during the sputtering process and the post annealing temperatures play a crucial role in forming MgB₂ superconducting thin films. We achieved a critical onset temperature of up to 27.1 K for the ex-situ and 25.6 K for the in-situ annealed samples at a film thickness of 30 nm. The samples shows an out of plane (0002)-Peak which was determined by x-ray diffraction.     

Thickness Dependence of Critical Current Density in MgB2 Films Prepared by Thermal Evaporation Method

MgB₂ films with the thickness of 350 to 1150 nm have been prepared on the Al₂O₃ (001) single crystal substrates from high purity B and Mg powder by the thermal evaporation method. Films were then heat treated ex-situ under Mg vapor at 950?°C to achieve actual MgB₂ stoichiometry. Thickness of the films, so the deposition time, was varied to investigate its influence on critical current density of the films. The films fabricated were analyzed by means of microstructural, transport, and magnetic properties. The best T _c and T _zero values were obtained to be 39.5 K and 38 K, respectively, and decreased with increasing the thickness. We found that the critical current density of the films prepared is highly thickness dependent. The maximum $J_{c}^{\mathrm{mag}}$ value was calculated to be 3.18×10⁶ A?cm^?2 at 10 K and zero field for 1150 nm thick films but dropped drastically by thickness.     

Observation of Hall Scaling in MgB2 Thin Films

We have measured the Hall resistivity, _xy , and the longitudinal resistivity, _xx , in superconducting MgB₂ thin films in a mixed-state while changing the magnetic field and the current density. A Hall scaling behavior without the anomalous Hall effect was observed with a exponent of 2.0±0.1 in _xy =A _xx . This exponent is observed to be constant, i.e., independent of magnetic field, temperature, and current density.     

Strong Renormalization of Phonon Frequencies in Superconducting MgB2 and Related Compounds

We report on recent Raman- and inelastic neutron scattering measurements on Mg_1–x Al _x B₂, LiBC and TMB₂ (TM=transition metal). All materials crystallize in the AlB₂ structure and show in particular for MgB₂ a pronounced anharmonic behavior for the high energy in-plane boron mode. Part of our analysis are ab initio calculations.     

Properties of MgB2 Thin Films Deposited on Different Substrates Prepared by Ex-Situ Annealing Process

MgB₂ thin films were deposited on MgO (100) substrate and r-plane Al₂O₃ $(1\bar{1}02)$ substrate by ex-situ annealing of boron film in magnesium vapor. The thickness of ex-situ annealed MgB₂ films is approximately 600 nm according to data observation by ellipsometer. The magnetic properties of samples were determined using a vibrating sample magnetometer. The magnetic field dependence of the critical current density J _c was calculated from M–H loops and also the magnetic field dependence of F _p was compared for the different temperature ranges from 5 to 25 K. The critical current density J _c was found to be around 1.0×10⁶ A/cm² and 1.7×10⁶ A/cm² in zero field at 5 K for MgB₂ films deposited on MgO and r-plane Al₂O₃ substrates, respectively. It was found that the critical current density of the film deposited on MgO became stronger than that of r-plane Al₂O₃ in the magnetic field. The superconducting transition temperature was determined by ac susceptibility measurement using physical properties measurement system. ac susceptibility measurements for MgB₂ films deposited on MgO and r-plane Al₂O₃ substrates were performed as a function of temperatures at constant frequency and ac field amplitude in the absence of dc bias field. The critical current densities as a function of temperature were estimated from the ac susceptibility data.     

Nonlocal in-plane Resistance in High-T c Superconducting Films in the Vicinity of a Kosterlitz-Thouless-Type Transition

We have measured the nonlocal in-plane resistance in high-T _c superconducting Yba ₂ Cu ₃ O _7–d films. The results obtained in zero magnetic field show a negative nonlocal resistance in the vicinity of the Kosterlitz-Thouless-type vortex-antivortex unbinding transition, as it was predicted recently by Wortis and Huse [Phys. Rev. B 54 , 12413 (1996)]. We also found that the negative nonlocal resistance takes place within a certain domain of the current-temperature diagram. Our findings indicate that nonlocal measurements in planar geometry can be a new method to study vortex dynamics in superconductors.     

Thermal Conductivity of n-Aluminium Nitride-Added MgB2 Superconductor in Normal and Superconducting State

Thermal conductivity (k) of 0, 5, 10, and 15 wt.% aluminium nitride (n-AlN)-added polycrystalline MgB ₂ superconductors, synthesized by solid reaction is discussed both in the normal and superconducting states between 20 and 300 K. The prepared samples are characterized using X-ray diffraction (XRD) and field electron gun scanning electron microscope (FEG–SEM). Resistivity measurement confirms a decrease in superconducting transition temperature of MgB ₂ (T _c=38.5 K) with n-AlN addition and decreases to ～35 K in case of 15 wt.% n-AlN-added MgB ₂ sample. Thermal conductivity of both MgB ₂ and n-AlN-added MgB ₂ pellets does not show any hump around T _c, and the absolute values of k decrease with increasing n-AlN in MgB ₂. Temperature dependence of the thermal conductivity of MgB ₂ and n-AlN-added MgB ₂ has been analyzed, assuming the role of both electrons and phonons. The Wiedemann–Franz law does not work well for the present samples, which indicates inelastic scattering (L _eff < L ₀). Thermal conductivity of MgB ₂ and n-AlN-added MgB ₂ pellets is explained by assuming effective Lorentz number, L _eff= 0.1 L ₀. Electronic thermal conductivity in superconducting state ( \(k_{\text {el}}^{\mathrm {s}} )\) follows “two-gap model” and has been used to estimate the values of band gaps, relative contribution of each band in thermal transport, and intraband scattering relaxation time. The estimated values are fairly consistent with the previously reported results for MgB ₂. We further confirm that n-AlN addition in MgB ₂ introduces disorders in π bands, which reduce the π band gapsand intraband relaxation time ( \(\tau _{\pi }^{\text {im}})\) . The lattice contribution of thermal conductivity in both normal and superconducting states is analyzed in the terms of Callaway’s model, assuming various phonon scatterings. Our analysis indicates that the lattice thermal conductivity of MgB ₂ is dominated by phonon-sheet-like fault scattering. Addition of n-AlN in MgB ₂ enhances the phonon scattering from sheet-like faults, and dislocations induced strain field scattering by >7 times compared to that for pure MgB ₂ pellets.