Effects of Magnetic Impurities on Two-Band Superconductor

The calculation was made of the effect of magnetic impurities on the normal and superconductive properties of a multiband s-wave superconductor by direct solution of the two-band Eliashberg equations. In this way, the critical temperature was determined, as well as the values of the superconductive gaps, and the shape of the superconductive density of states and other physical quantities that depend on the concentration of magnetic impurities. It was found that the gaps and the penetration lengths display an unusual behaviour as a function of temperature. The possibility is examined that the presence of a negative induced gap raises the critical temperature.     

Point-Contact Spectroscopy in Mn-Doped MgB2 Single Crystals: Effects of Magnetic Impurities in a Two-Band Superconductor

We performed point-contact spectroscopy (PCS) measurements in Mg_1−x Mn _x B₂ single crystals, with x≤0.015 and bulk T _c down to 13.3 K. The gaps Δ _σ and Δ _π were obtained by fitting the conductance curves of the point contacts with the two-band Blonder–Tinkham–Klapwijk (BTK) model. Both Δ _σ and Δ _π decrease with the critical temperature of the junctions T _c ^A , but remain clearly distinct down to the lowest critical temperature (T _c ^A ≅9 K). Once analyzed within the Eliashberg theory, the gap trends as a function of T _c ^A can be explained by a doping-induced increase in the pair-breaking scattering within the σ band, with smaller contributions from the π-π or the σ-π channels.     

On Some Factors Affecting the Critical Temperature in a Two-Band Model of Superconductivity

The influence of a few factors on the critical temperature in a two-band superconducting system is investigated. The system contains conventional Cooper pairs from a wider band (the d-band) as well as local pairs (local bipolarons) from a narrower band (the f-band), which are induced via a pair-exchange potential. These factors are the Coulomb repulsion between f-electrons, the position of the f-band with respect to the bottom of the d-band and two kinds of hopping in the f-band: a single-polaron hopping and a pair hopping. The Coulomb potential turns out to lower the critical temperature from higher values to the pure BCS one. Each of the kinds of hopping is treated by making use of perturbation theory. Pair hopping is incorporated as the first order correction but the hopping of single polarons enters as the second order one. Each of them increases this temperature on its own, however, the hopping of single polarons makes it stronger. The position of the f-band that corresponds to the site energy of a f-electron poses a very interesting case. There appear two peaks at two values of that energy meaning a rapid increase of the critical temperature due to the strong effect of the presence of local electron pairs. In this case one has to do with a purely chemical mechanism of the increase of the critical temperature.     

Metastability and Phase Separation in a Simple Model of a Superconductor with Extremely Short Coherence Length

We present studies of the atomic limit of the extended Hubbard model with pair hopping for arbitrary electron density and arbitrary chemical potential. The Hamiltonian consists of (i) the effective on-site interaction U and (ii) the intersite charge exchange term I, determining the hopping of electron pairs between nearest-neighbour sites. In the analysis of the phase diagrams and thermodynamic properties of this model we treat the intersite interactions within the mean-field approximation. In this report we focus on metastable phases and determine their ranges of occurrence. Our investigations in the absence of the external magnetic field show that the system analysed exhibits tricritical behaviour. Two metastable phases (superconducting and nonordered) can exist inside the regions of the phase separated state stability and a first-order transition occurs between these metastable phases.     

Vortex Matter in a Two-Band SQUID-Shaped Superconducting film

In the present work, we studied the magnetization, vorticity, Cooper pairs density, and the spatial distribution of the local magnetic field in a three-dimensional superconductor with a SQUID geometry (a square with a central hole connected to the outside vacuum through a very thin slit). Our investigation was carried out in both the Meissner-Ochsenfeld and the Abrikosov state solving the two-band Ginzburg-Landau equations considering a Josephson coupling between the bands. We found a non-monotonic vortex behavior and the respective generation of vortex clusters due to the Josephson coupling used between condensates.

     

Critical Current Measurements in the Ring-Shaped Configuration Magnet–Superconductor

Critical current measurements in superconducting rings were made in the configuration superconductor – permanent magnet. Results show as the magnetic losses of the inducting core can be compensated with the magnets, an increase in the critical current is achieved. For this purpose, field of the magnets has opposite direction to the core losses and approximately the same magnitude. On the other hand, the critical current is always lower when the field is applied in the same direction to the core losses. It can be observed as both measurements converge with the increase of the field of the magnet.     

Theoretical Study on the Upper Critical Field of a Layered Superconductor NbSe_2

Based on two-band Ginzburg-Landau theory, we study the temperature and angular dependences of the upper critical field for superconducting NbSe \(_2\) crystals. We introduce two variational parameters in our approach, and provide the explicit equations to determine the upper critical field in arbitrary direction. Our analyses strongly support previous scanning tunneling microscopy results and specific heat data, pointing to the existence of two energy gaps in this compound. Our results thus suggest that besides MgB \(_{2}\) , NbSe \({_2}\) is another example of two-gap superconductors.     

提高MgB2超导体临界电流密度的研究进展

临界电流是超导体载流能力大小的重要表征,论述了制备工艺、元素掺杂和物理场对MgB2超导体临界电流密度的影响.使用非晶态高纯硼粉制备致密的MgB2试样有利于样品临界电流密度的提高,添加SiC或者Ti也能起到提高临界电流密度的作用,质子照射使MgB2试样在高磁场条件下的临界电流密度增加,强磁场下烧结则有利于提高MgB2样品临界的传输电流密度.     

Simulators of Superconductor Critical Current: Design,Characteristics, and Applications

The superconductor simulator is an electronic circuit that emulates the extremely nonlinear voltage-current characteristic (the basis of a critical-current measurement) of a superconductor along with its other major electrical properties. Three different types of simulators have been constructed: the passive, active, and hybrid simulator. The passive simulator has the fewest circuit components and offers the least amount of versatility, while the active and hybrid simulators offer more versatility and consequently have more components. Design, characteristics, and applications of the superconductor simulator along with a summary of features are presented. These simulators are high precision instruments, and are thus useful for establishing the integrity of part of a superconductor measurement system. They are potentially useful for testing the measurement method and data acquisition and analysis routines. The 50 A simulator provides critical-current precision of 0.1% at a 1 μV signal. This is significantly higher than the precision of a superconducting standard reference material. The superconductor simulator could significantly benefit superconductor measurement applications that require high-precision quality assurance.     

Ferromagnetism in a Realistic Two-Band Model: A Slave Boson Study

Using a slave boson representation of multi-band Hubbard models, we investigate a two-band model relevant to layered perovskites in the vicinity of half-filling. Beside the strong influence of the Hund's rule coupling, we obtain that the phase diagram separates into two regions: a weak to moderate coupling region where the effective mass is weakly renormalized, and a strong coupling regime where it is strongly renormalized. The transition between these two regimes is very sharp. It takes place in a (vanishingly) small domain. A ferromagnetic instability in only found in the strongly correlated regime, and is triggered by the Hund's rule coupling. The results are compared to La-doped layered ruthenates.     

Fluctuation of Specific Heat in Two-Band Superconductors

Analytical calculations of fluctuation part of specific heat conducted using two-band Ginzburg?CLandau equations. Results applied to MgB₂, comparison with available experimental data, and theoretical calculations are conducted and agreement achieved.     

Enhanced Critical Current Density in MgB2 Superconductor via Si and C Coadditions

In this study, nanosize Silicon and Carbon (Si+C) were reacted with MgB₂ in order to enhance the critical current density. The polycrystalline bulks were synthesized by the direct in situ reaction method and their phase formation, crystal structure, and superconducting properties were evaluated. The enhanced relative peak intensity of Mg₂Si and MgB₄ indicates the formation of a large volume fraction of these two phases with increasing (Si+C) additions. The a-axis lattice parameter shrinks significantly while c-axis increases slightly. The estimated C doping level at B site increases, leading to a degradation of the superconducting transition temperature with increasing (Si+C) additions. By a reaction with (Si+C), the field dependence of critical current density is shown to enhance at both 5 K and 20 K.     

Pair-Breaking Effect in a van Hove Superconductor

he pair breaking effect in the critical temperature has been considered. We showed that the decrease in the critical temperature can be described by an Abrikosov-Gorkov-like formula which contains the correction due to the van Hove density of states.     

Design for a Room-Temperature Superconductor

The vision of ”room-temperature superconductivity“ has appeared intermittently but prominently in the literature since 1964, when W. A. Little and V. L. Ginzburg began working on the problem of high-temperature superconductivity around the same time. Since that time the prospects for room-temperature superconductivity have varied from gloom (around 1980) to glee (the years immediately after the discovery of HTS), to wait-and-see (the current feeling). Recent discoveries have clarified old issues, making it possible to construct the blueprint for a viable room-temperature superconductor.     

Power-Law Relationship Between Critical Current Density,Microstructure, and the n-Value in MgB2 Superconductor Wires

Dissipation-free MgB₂ superconducting wires are valuable in terms of practical applications. Herein, we have found a strong correlation between critical current density (J _c ) and the n-value extracted from the electric field versus current density characteristic. The power-law relationship (m) between the J _c and the n-value, \(n \propto J_{c}^{m}\) , represents a critical index which is strongly dependent on operating temperatures.     

Vortex Avalanches in a Type II Superconductor

We report on a study of the spatiotemporal variation of magnetic induction in a superconducting niobium sample during a slow sweep of external magnetic field. A sizable fraction of the increase in the local vortex population occurs in abrupt jumps. We compare the size distribution of these avalanches with the predictions of self-organized-criticality models for vortex dynamics.