Narrow Results

Using the Stuttgart Full-potential Linearized Muffin-tin Orbital method, we study a crossover of the electronic structure of MgB₂ from three-dimensional (3D) to 2D by artificially increasing the lattice constant along the c-axis. With the inter-layer coupling decreased by increasing c, the density of states (DOS) at E_F has a considerable growth; at the same time, the charge transfer from Mg plane to B plane decreases so that the hole density in the B plane increases. As c is increased to 4c₀, the electronic structure exhibits 2D characteristic and a van Hove peak in the DOS appears. The E_2g phonon frequency at Γ point and the electron–phonon coupling constant λ are estimated by using the frozen phonon method. The calculated results indicate that T_c in MgB₂ could be further increased if the inter-layer coupling might be reduced.

The self-energy analysis of the frequency-dependent infrared conductivity σ(ω) of MgB₂ in normal state is presented. Experimentally obtained σ(ω) is inverted to yield the self-energy of electrons. From the extracted self-energy, other physical properties such as the effective interaction between electrons can also be computed. We suggest from the self-energy analysis that the small electron-phonon coupling constant of MgB₂ obtained previously can be attributed to an underestimate of the plasma frequency.

We investigated the transport properties of MgB₂ while changing the contents of excess Mg. The samples containing almost no excess Mg showed the highest T_c and the sharpest transition width (ΔT_c). A residual resistivity ratio (RRR) of ~ 5.8, and a magnetoresistance (MR) of 12%, at 40 K, were obtained for this stoichiometric sample. Moreover, no upturn of resistivity in a low temperature region at 10 Tesla was observed. However, the samples containing appreciable amounts of excess Mg showed quite different behaviors; the values of ΔT_c, RRR and MR were much larger. Surprisingly, big upturn appeared in this Mg-excess MgB₂

We present the first direct studies of photoinduced Cooper-pair breaking and subsequent superconducting state recovery dynamics by means of femtosecond time-resolved optical-pump terahertz probe spectroscopy focusing on the superconducting state recovery dynamics in MgB₂. The superconducting state recovery proceeds on the timescale of several hundred picoseconds and shows a strong temperature dependence. In particular, upon increasing the temperature the recovery time first decreases, reaches a minimum, followed by a quasi-divergence upon further increasing the temperature towards T_c. Moreover, the recovery time shows virtually no intensity dependence, even though the photoexcitation fluence is changed by over an order of magnitude. This suggests that pair recovery is goverend by a phonon-bottleneck mechanism, where the lifetime of high energy phonons (ℏω > 2Δ) is governed by their anharmonicity.

We performed ab initio, local density functional calculations for the electronic structure of MgB₂. The Fermi level of MgB₂ cuts through relatively narrow electron bands which have a dominant contribution from B(2p) states. The above metallic behavior of boron layers can also be seen from the calculated partial density of states. The metallic nature, strong covalent bonds, and the light atomic mass in the boron layers could lead to a strong electron-phonon (e–ph) interaction. The strong e–ph interaction is particularly important for the high energy phonons that are associated with boron layers in MgB₂.

Based on a viewpoint of chemical pairwise interactions between electrons, a Hamiltonian was proposed for the "flat/steep band" scenario. This model has been studied analytically, and numerically with the first-principles method. With Hg and MgB₂ as examples, we have explained the characteristics of this model and observed peak-like structures of the electron-phonon coupling constants λ(q) in q space. The strong coupling of the "flat band" electrons with phonons has been corroborated by developing a new functional Psib(Φ), through which we can quantitatively compare different electronic states in coupling to a specific phonon. The relevance of our model to an electronic inhomogeneity is also discussed. Investigations on experimental and theoretical low-energy electronic structures of superconductors support our flat/steep scenario.

The effect of YBCO nanoparticles added into MgB₂ on T_c, J_c, and flux pinning was studied for MgB₂(YBCO)_x with x=0, 5, 10, 15 wt%. Phase analysis shows that none of elements are doped into the MgB₂ lattice in the samples with YBCO addition. For the samples with YBCO addition, the J_c-H characteristics behave poorly in comparison with the pure sample. Our experimental results show that the nanoscale size of addition dosen't comprise the only condition for its effectiveness as pinning centers.

The review of works about the study of the thermodynamic properties of the superconductors with energy bands overlapping on Fermi surface is done. The base of the review is Moscalenco's model¹ with the formation of cooper pairs of electrons inside each energy band and their transition as a whole entity from one band to another. The model was generalized in order to consider the interband pairings of electrons in addition to the pairings that have been taken into account in Ref. 1. The main system of equations in this theory of superconductivity is derived for the two-band systems at arbitrary density of charge carriers (including the very low densities). The detailed studies of dependences of the temperature of superconducting transition T_C, the jump of heat capacity (C_S-C_N), as well as the chemical potential μ on the density of charge carriers are made. Singularities in the behavior of the two-band superconductors at low densities of charge carriers (μ~T_C) are revealed. There has been studies on the influence of the additional pairings of electrons that result in the formation of the cooper pairs of electrons from different energy bands on the thermodynamic properties of the system. Both phonon and non-phonon mechanisms of superconductivity in the BCS scenario of the formation of superconducting pairs (μ>0) have been taken into account. The theory of superconductivity in the mean-field approximation at T = 0 in the picture of Bose condensation of localized pairs (Schaffroth's scenario μ<0) is built. The technique of functional integration with regard to the two-band system is developed and the crossover from Fermi to Bose picture of elementary excitations at T≠0 with the two-particle bound state in the system has been shown. The temperature of Bose condensation T_K is determined and the influence of the overlapping of energy bands on Fermi surface onto Bose condensation of localized pairs is studied.

Thermal conductivity of MgB₂ at room temperature is measured from the photoacoustic technique and analyzed for electron and lattice contribution to the thermal conductivity in the normal state. These results are compared with the flow cryostat measurements.

Thermal expansion of MgB₂ at normal state is measured experimentally in the temperature range 300–400 K. Lattice and electronic contribution to the thermal expansion is determined from the Gruneisen relation using electronic and lattice specific heat. This is compared with the low temperature measurements for predicting superconducting transition.

We measured the magneto-optical images (MOIs) and the magnetic hysteresis (M – H) curves of c-axis-oriented MgB₂ thin films to investigate the flux penetration in the form of dendritic avalanches. In order to understand the role of the thermal effects, we prepared Au-coated MgB₂ thin films with different thicknesses of gold. While the MOI provides a spatially resolved flux pattern, the M – H curve presents global and average information about the flux noise associated with avalanches. These two types of measurements complement each other. The upper threshold field, above which the flux noise disappears, was determined from the M-H curves while the lower threshold field was determined from both the M – H curves and the MO images. The field range where the flux penetrates via avalanches is found to be smaller for thicker gold layers. These results are important for many superconducting applications.

We have investigated the two-gap nature of bulk (Mg_1-xAl_x)B₂ for 0 ≤ x ≤ 0.07 by studying the upper critical field H_c2(T). The H_c2(T) is analyzed based on the dirty-limit two-gap model. While the effective diffusivities of the π-band decreased gradually, those of the σ-band were relatively unchanged, which indicated that unlike the 2 dimensional σ-band, the isotropic π-band is seriously affected by the Al doping. The upward curvature of H_c2(T) near T_c is also explained by the dirty-limit two-gap model.

The time evolution of the voltage response (V(t)) of the zero-field-cooled (ZFC) vortex matter for MgB₂ single crystals while applying a constant current application was recorded. Near the peak effect (PE) region, we observed a specific pattern of the V(t), that of a random telegraph signal for which the voltage mainly remained at two different levels. The origin of this random telegraph signal is closely related to the motional ordering of vortices resulting from the competition between the pinning and the Lorentz forces.

To get some information about the interaction of two totally different kinds of superconductors in a mixture, we prepared the samples of mixture of two kinds of superconductors (YBa₂Cu₃O₇₊MgB₂), using normal annealing method at low temperature (400 °C ~ 600 °C). The variations of the transition temperature (T_c) and critical current (J_c) of the mixtures are investigated. We also prepared a series of samples of MgB₂ with small ratios of YBCO or Bi-2223 as addition. Carefully comparison shows that the J_c drops when the ratio of YBCO additions increases, but in the Bi-2223 additions the trend goes just the opposite.

In this work we describe the effects of adding different amount of single-wall (sw) carbon nanotubes (CNT) on the superconducting properties of MgB₂. We observed that the amount of swCNT that partially dilute into the MgB₂ matrix is lower than for double-wall (dw) CNT samples for the same nominal content. This C incorporation leads to an increase of the upper critical field (H_c2), with a maximum H_c2(0) extrapolated value of 33.5 T, expected for a two-gap superconductor in the dirty limit. At the same time, the fraction of swCNT that has not been incorporated into MgB₂ lattice may retain their structural integrity acting as strong vortex pinning centers. In this case the optimum nominal swCNT content for increasing critical current density (J_c) is in the range 7.5-10%at.

We studied the effect of sintering time on microstrain, critical current density (J_c), and resistivity (ρ) in MgB₂ bulks. All samples were fabricated by solid state reaction and sintered within a time range of 30 minutes – 60 hours. It has been found that microstrain increases as the sintering time increases, while the connectivity decreases with sintering time from 30 minutes to 20 hours, and then increases to 60 hours. Both the irreversibility field, H_irr, and the upper critical field, H_c2, were increased with microstrain. This indicates that the disorder significantly affects H_irr and H_c2. The sample with the sintering time of 30 minutes exhibited the highest J_c at all fields, due to its having the highest connectivity. When the sample is sintered for a very long time (60 hours), J_c at high fields can be improved. This is mainly attributed to increased H_c2.

The superconductivity of nano-SiC doped MgB₂ sintered in pulsed magnetic field (PMF) was investigated with Raman scattering measurements and Raman spectral fit analysis. The critical transition temperature, T_c, for the sample sintered in 5T PMF is improved compared with that of the sample sintered without PMF. The high T_c is attributed to the strengthening of the electron-phonon coupling (EPC) in MgB₂, as reflected by the broadened E_2g mode in the Raman spectra. The EPC constants are estimated as 0.876 and 0.874, with the electron-E_2g coupling contribution 2.30 and 2.25, respectively. Magnetic field processing technology has been proved to be a powerful tool to improve the superconducting properties of SiC-doped MgB₂ superconductor.

The microstructure of the MgB₂ wires with hydrocarbon, pyrene (C₁₆H₁₀) addition has been investigated by transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDX). In our study, MgB₂ wires have been fabricated through powder-in-tube (PIT) process using pure Fe as sheath material and sintered at 600°C ~800°C for 30min to 4 hrs under high purity argon gas. TEM and EDS analyses were conducted on the specimens prepared by tripod polishing and ion milling method. The structural features such as second phases, interface and defects were investigated by electron microscopy.

We investigated the effects of different operating temperatures on the performance of transport critical current density, J_c, for MgB₂ + 10 wt%C₄H₆O₅MgB₂/Fe wires. It was shown that the J_c values of the malic acid doped wires sintered at 900°C reached 10⁴ Acm^-2 at 20 K and 5 T. The J_c value extrapolated to 2 T and 20 K exceeds the practical level of 10⁵ Acm^-2. According to the Kramer plots, the pinning force, F_K = J_c^1/2 x B^1/4, is expected to be a linear function of magnetic field B. The irreversibility field, B_irr, at which extrapolated F_K reaches zero, was 1.8 T at 32.8 K, 2.8 T at 30 K, 5.7 T at 25 K, 8.6 T at 20 K, and 12.5 T at 15 K, respectively.

The feasibility of the infiltration of molten magnesium into porous boron skeleton was examined. The particle size and crystalline state of starting boron powders as well as the confinement of Mg vapors play an important role in successful infiltration. Magnesium penetrates more readily into porous skeletons formed by large crystalline boron particles than those formed by fine amorphous powders. The infiltration of magnesium mainly occurs via capillary condensation rather than liquid flow. Although the infiltrated microstructures are seemingly very dense, nano-scale structures contain many tiny inter-particle voids due to random packing of MgB₂ grains. This is due to the absence of a liquid phase in final microstructures as a result of transient liquid-phase infiltration.