Spin‐Gapless Semiconductors

The spin‐gapless semiconductors (SGSs) are a new class of zero‐gap materials which have fully spin polarized electrons and holes. They bridge the zero‐gap materials and the half‐metals. The band structures of the SGSs can have two types of energy dispersion: Dirac linear dispersion and parabolic dispersion. The Dirac‐type SGSs exhibit fully spin polarized Dirac cones, and offer a platform for massless and fully spin polarized spintronics as well as dissipationless edge states via the quantum anomalous Hall effect. With fascinating spin and charge states, they hold great potential for spintronics. There have been tremendous efforts worldwide to find suitable candidates for SGSs. In particular, there is an increasing interest in searching for Dirac type SGSs. In the past decade, a large number of Dirac or parabolic type SGSs have been predicted by density functional theory, and some parabolic SGSs have been experimentally demonstrated. The SGSs hold great potential for spintronics, electronics, and optoelectronics with high speed and low‐energy consumption. Here, both the Dirac and the parabolic types of SGSs in different material systems are reviewed and the concepts of the SGS, novel spin and charge states, and the potential applications of SGSs in next‐generation spintronic devices are outlined.     

Theoretical study of superconductivity in MgB2 and its alloys

Using density-functional-based methods, we have studied the fully-relaxed, fulltronic structure of the newly discovered superconductor, MgB ₂, and BeB₂, NaB₂ and AlB₂. Our results, described in terms of (i) total density of states (DOS) and (ii) the partial DOS around the Fermi energy, E_F, clearly show the importance of B p-electrons for superconductivity. For BeB₂ and NaB₂, our results indicate qualitative similarities but significant quantitative differences in their electronic structure due to differences in the number of valence electrons and the lattice constantsa andc. We have also studied Mg _1-xM_xB₂ (M = Al, Li or Zn) alloys using coherent-potential to describe disorder, Gaspari-Gyorffy approach to evaluate electron-phonon coupling, and Allen-Dynes equation to calculate the superconducting transition temperature, T_c. We find that in Mg_1-xM_xB₂ alloys (i) the way Tc changes depends on the location of the added/modified k-resolved states on the Fermi surface and (ii) the variation of Tc as a function of concentration is dictated by the Bp DOS.     

Neutron crystal-field spectroscopy in underdoped and overdoped copper oxide superconductors

In many cuprate superconductors rare-earth (R) ions can be placed close to the superconducting copper oxide planes; thus, the crystal-field interaction at the R site constitutes an ideal probe of the local symmetry as well as the local charge distribution and thereby monitors directly changes of the carrier concentration induced by doping. For several compounds the crystal-field spectra observed by inelastic neutron scattering separate into different local components whose spectral weights distinctly depend on the doping level, i.e., there is clear experimental evidence for the formation of clusters which make the systems inhomogeneous. Moreover, it is found that the intrinsic linewidths of crystal-field transitions vary with temperature, which is essentially a reflection of the density of states associated with the charge carriers at the Fermi energy. Linewidth studies can therefore reveal information about the energy gap. For underdoped systems there is evidence for the formation of a pseudogap atT* >T _c .     

Progress in superconductivity

Superconductors are in a quantum state, extending over macroscopic distances. Consequently, they trap magnetic flux in multiples of the flux quantum. This point of view will be taken for a discussion of some recent developments related mainly to induction phenomena in superconductors. 1) In superconductors of the second kind magnetic flux penetrates at external magnetic field values within a certain interval (mixed state). This state violates the traditional characteristics of superconductivity. Not only does it fail to show the perfect Meissner diamagnetism, but it is also, in principle, a resistive state. The latter feature is correlated with the instability of the magnetic structure in the mixed state when a current is injected, the induced flux flow phenomenon being responsible for a variety of effects recently studied, including the Hall effect and a mode of magnetic coupling which makes a dc transformer feasible. 2) It is well known by now that superconductors of the second kind may obtain excellent current-carrying capacities if the mixed state magnetic pattern is stabilized, or pinned. This process is still not fully understood. The recent manufacture of some interesting and practicable wires and cables and the problems of coil construction and their stabilization will be briefly surveyed. 3) By proper manipulation of magnetic flux in superconducting systems, electromagnetic induction of heavy direct currents may be achieved. Fully superconducting dc generators or dynamos have recently been developed, the qualities of which will be discussed. The results of some of the author's recent investigations will be given, including those on multikiloampere dynamos for energizing supermagnets.     

Molecular bond in superconductivity

In the presence of a superconducting current an additional intermolecular bond appears, whose value determines the critical parameters.Belarusian Technological Institute, Minsk, Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 67, Nos. 3–4, pp. 184–189, September–October, 1994.     

Research opportunities in superconductivity

Opportunities for research in the field of superconductivity are identifield in this report of a ‘Workshop on problems in superconductivity’ held at Copper Mountain, Colorado, August 22–23, 1983. Key problems in superconductivity, high payoff areas of research, barriers to progress, and the need for new facilities are outlined in the three areas of basic physics, materials, and devices.     

Domain-wall superconductivity in superconductor-ferromagnet hybrids

Superconductivity and magnetism are two antagonistic cooperative phenomena, and the intriguing problem of their coexistence has been studied for several decades. Recently, artificial hybrid superconductor-ferromagnet systems have been commonly used as model systems to reveal the interplay between competing superconducting and magnetic order parameters, and to verify the existence of new physical phenomena, including the predicted domain-wall superconductivity (DWS). Here we report the experimental observation of DWS in superconductor-ferromagnet hybrids using a niobium film on a BaFe(12)O(19) single crystal. We found that the critical temperature T(c) of the superconductivity nucleation in niobium increases with increasing field until it reaches the saturation field of BaFe(12)O(19). In accordance with the field-shift of the maximum value of T(c), pronounced hysteresis effects have been found in resistive transitions. We argue that the compensation of the applied field by the stray fields of the magnetic domains as well as the change in the domain structure is responsible for the appearance of the DWS and the coexistence of superconductivity and magnetism in the superconductor-ferromagnet hybrids.     

Collaborative superconductivity research in Europe

The collaboration in Europe for research in superconductivity, especially for large Scale applications, has a long tradition. In the early seventies already a so-called "GESSS group" (Group on European Superconducting System Studies) became active for accelerator and detector magnet development. In the second half of the seventies the fusion technology programme in Europe called for an increasing effort in magnet development. Due to the fact that the fusion work is at all a collaborative European effort and the earlier GESSS laboratories got involved in that area too, these activities were carried out jointly from the beginning. Participation in the IEA-Large Coil Task by two European groups also proved the capability of an even broader international collaboration. Based on the different management schemes for the collaborations lessons have been learned and are discussed, which might be valuable for foreseeable large European projects such as NET (Next European Torus) and LHC (Large Hadron Collider) in the next decade. The role of industry as a collaboration partner in the different areas is discussed, too.     

Electric-field-induced superconductivity in an insulator

Electric field control of charge carrier density has long been a key technology to tune the physical properties of condensed matter, exploring the modern semiconductor industry. One of the big challenges is to increase the maximum attainable carrier density so that we can induce superconductivity in field-effect-transistor geometry. However, such experiments have so far been limited to modulation of the critical temperature in originally conducting samples because of dielectric breakdown. Here we report electric-field-induced superconductivity in an insulator by using an electric-double-layer gating in an organic electrolyte. Sheet carrier density was enhanced from zero to 10(14) cm(-2) by applying a gate voltage of up to 3.5 V to a pristine SrTiO(3) single-crystal channel. A two-dimensional superconducting state emerged below a critical temperature of 0.4 K, comparable to the maximum value for chemically doped bulk crystals, indicating this method as promising for searching for unprecedented superconducting states.     

Numerical effects in the simulation of Ginzburg–Landau models for superconductivity

Inadequate spatial mesh resolution for simulation of the time‐dependent Ginzburg–Landau equations is shown to give rise to spurious solutions. Phenomenological studies to examine the effect of the physical parameters and boundary conditions in 2D and 3D are presented to illustrate the solution structure and to highlight non‐linear effects related to evolving vortex patterns. We illustrate and explore this issue further by considering a related simplified model in which the number of vortices is equal to the ‘winding number’ that is associated with the applied boundary conditions. Using this model we demonstrate that the solution structure is non‐unique for several values of winding number. Copyright © 2004 John Wiley & Sons, Ltd.     

Possible high-temperature superconductivity in fluoride materials

The results are presented of a study of fluorine-containing materials, the chemical compositions of which were chosen from calculations of the valence electron concentration in a single bond. In the resistance-temperature curves of the specimens, a narrow minimum was observed at T 40°C.Translated from Izmeritel'naya Tekhnika, No. 6, pp. 43–45, June, 1993.     

Theory of superconductivity in heavy-fermion compounds

A single-order-parameter and two-gap model (a modified Suhl two-gap model) is proposed for heavy-fermion superconductors. A simple relation between two gaps is found. The thermodynamic quantities in the superconducting state are calculated in terms of this model. It is shown that the critical values of the physical quantities are quantitatively in agreement with experiment. The behavior of the specific heat, of the ultrasonic attenuation, and of the NMR rate are also in good agreement with those of CeCu₂Si₂ and UBe₁₃ at all temperatures.     

New research opportunities in superconductivity IV

This report surveys the considerable progress made over the last five years—such as the marketing of superconducting quantum interference devices (SQUIDs), cellular wireless filter systems, and high current leads—and assesses needs and opportunities in the areas of fundamental science, materials development, thin film and device applications, and wire and bulk applications. It examines the challenges facing high-temperature superconductivity: from the need to understand the mechanism of high-temperature superconductivity and the unusual “normal” state to the need for new instrumentation for material characterization. Advances in thin film and bulk materials are reviewed, and obstacles impeding the commercialization of HTS materials are examined. A report on the workshop on research needs and opportunities in superconductivity, held in Monterey, California, February 10–12, 1997.     

Resonance-valence-bond superconductivity in CuO2 planes

We find the exact ground state of a system of 4 × 4 unit cells described by an effective model for cuprate superconductors. The model has the form of the t - J one, but includes a three-site term t like that derived from the Hubbard model in the large U limit, but of opposite sign for realistic or large O-O hopping. For t > t_c (where t_c/t 0.1 for realistic J and doping) the system undergoes a transition to a phase in which the ground state has a significant overlap with a very simple superconducting resonance-valence-bond (RVB) wave function. In the RVB phase there is a large increase of the d-wave superconducting susceptibility. We calculate the boundary of the RVB phase.     

Josephson effect in high-T c superconductivity

In this report we present the most important results of our recent analysis (Kupriyanov and Likharev 1990) of the Josephson effect in both the natural (intergrain) and artificial junctions using high-T _c superconductors (HTS). A comparison of the experimental data with the BCS-based theories of the Josephson effect in various tunnel-junction-type and weak-link-type structures has been carried out. The main conclusion is that the data presently available do not enable one to either confirm or reject the theories, and thus to reveal possible deviations of the real microscopic mechanism of the high-T _c superconductivity from the BCS mechanism. We suggest several experiments which would be more fruitful for this purpose, as well as for finding ways of reproducible fabrication of practically useful Josephson junctions. This work was supported by the Soviet Scientific Council on the high-Tc superconductivity problem (Grant No.42). Invited talk at the International Conference on Superconductivity, Bangalore, January 1990.     

Anisotropic superconductivity in the Emery model

We have investigated anisotropic superconductivity originating from intersite pairing between holes in nearest and next nearest neighboring sites in the Emery model. Strong local Coulomb correlations among holes in copper orbitals have been taken into account within the Hubbard I approximation scheme. The superconducting transition temperature has been evaluated as a function of the hole concentration. It has been shown that with the onset of superconductivity, pairing among oxygen-like quasiparticles in the mixeds-wave+d-wave channel plays the dominating role, being replaced by pairing in the extendeds-wave channel for higher concentration of holes. Superconducting correlations are mostly effective for a rather narrow range of the model parameter values, close to values derived from band structure calculations. Therefore, the coupling betweens-wave andd-wave channels seems to be a general feature of superconductivity in CuO₂ planes.     

An important parameter in high-temperature superconductivity

It is widely held that the single most important parameter describing the strength of the electron-phonon interaction in superconductivity is the mass enhancement factor λ, and that the maximum possible transition temperature for a given class of materials (e.g., the polyvalent metals) is attained for a very large value, λ≈3, of the parameter. We show that it is not likely that the highest attainable values ofT _c will reflect such large values of λ. Instead, we expect that they will be associated with values of λ that do not differ greatly from a value of about 1.5, and with very large values of the parameterA, whereA is the area under the electron-phonon interaction spectrum α² F(v). This is certainly the case forNb ₃ Sn, which has aT _c of 18 K.