Gossamer superconductivity

A new superconducting Hamiltonian is introduced for which the exact ground state is the Anderson resonating valence bond. It differs from the t–J and Hubbard Hamiltonians in possessing a powerful attractive force. Its superconducting state is characterized by a full and intact d-wave tunnelling gap, quasiparticle photoemission intensities that are strongly suppressed, a suppressed superfluid density, and an incipient Mott–Hubbard gap.     

Gossamer superconductivity near antiferromagnetic Mott insulator in layered organic conductors

Layered organic superconductors are on the verge of the Mott insulator. We use the Gutzwiller variational method to study a two-dimensional Hubbard model including a spin exchange coupling term as a minimal model for the compounds. The ground state is found to be a Gossamer superconductor at small on-site Coulomb repulsion U and an antiferromagnetic Mott insulator at large U, separated by a first order phase transition. Our theory is qualitatively consistent with major experiments reported in organic superconductors.     

On Gossamer metals and insulating behaviour

We extend the Gossamer technique recently proposed to describe superconducting ground states to metallic ground states. The Gossamer metal in a single band model will describe a metallic phase that becomes arbitrarily hard to differentiate from an insulator as one turns the Coulomb correlations up. We were motivated by the phase diagram of V₂O₃ and f-electron systems which have phase diagrams in which a line of first-order metal–insulator transition ends at a critical point above which the two phases are indistinguishable. This means that one can go continuously from the metal to the ‘insulator’, suggesting that they might be the same phase.     

Effects of the Next Nearest Neighbor Hopping on Superconductivity and Antiferromagnetism of Gossamer Superconductivity

The two dimensions hole-doped t-t '-J-U model was studied based on the Gutzwiller approach and the renormalized mean-field theory.The phase diagrams of gossamer superconductors and the effects of the next-nearestneighbor hopping(t ') on superconductivity and antiferromagnetism based on the t-t '-J-U model were investigated.The results show that the qualitative feature of the phase diagrams in the t-t '-J-U model is the same as in the case of the t-J-U model.The antiferromagnetic order coexists with the d-wave superconductivity(dSC) in the underdoped region below the doping δ≈ 0.1 and is enhanced by the t '.The dSC order is slightly suppressed by t ' in the underdoped region and greatly enhanced in the overdoped region.The dSC order is pushed to a larger doping region and the coexistence region of the AF and dSC extends to higher doping.     

Surface superconductivity and twinning-plane superconductivity in aluminum

The critical supercooling field H _sc is measured in aluminum single crystals and twinned bicrystals in a temperature range slightly below T _c0 (T _c0 ? 0.055 K < T < T _c0), where T _c0 is the critical superconducting transition temperature. It is found that, even in this small temperature range, the H _sc(H _c) dependence, which is considered to be identical to the H _c3(H _c) dependence for single crystals, is substantially nonlinear. The H _sc(H _c) dependences of the twinned bicrystals and single crystals are shown to be significantly different. The qualitative features of the phase diagram of the twinned aluminum bicrystals coincide with those of the phase diagram of twinning-plane superconductivity obtained earlier for tin in [1]. These findings allow the conclusion that the phenomenon of twinning-plane superconductivity also exists in face-centered cubic crystal lattices.     

Photo-induced superconductivity

Recent advances in laser technology have made it possible to generate of precisely shaped strong-field pulses at terahertz frequencies. These pulses are especially useful to selectively drive collective modes of solids, for example, to drive materials in a fashion similar to what done in the synthetic environment of optical lattices. One of the most interesting applications involves the creation of non-equilibrium phases with new emergent properties. Here, I discuss coherent control of the lattice to favour superconductivity at ‘ultra-high’ temperatures, sometimes far above the thermodynamic critical temperature T_c.     

Unconventional superconductivity

‘Conventional’ superconductivity, as used in this review, refers to electron–phonon-coupled superconducting electron pairs described by BCS theory. Unconventional superconductivity refers to superconductors where the Cooper pairs are not bound together by phonon exchange but instead by exchange of some other kind, e.g. spin fluctuations in a superconductor with magnetic order either coexistent or nearby in the phase diagram. Such unconventional superconductivity has been known experimentally since heavy fermion CeCu₂Si₂, with its strongly correlated 4f electrons, was discovered to superconduct below 0.6?K in 1979. Since the discovery of unconventional superconductivity in the layered cuprates in 1986, the study of these materials saw T_c jump to 164?K by 1994. Further progress in high-temperature superconductivity would be aided by understanding the cause of such unconventional pairing. This review compares the fundamental properties of 9 unconventional superconducting classes of materials – from 4f-electron heavy fermions to organic superconductors to classes where only three known members exist to the cuprates with over 200 examples – with the hope that common features will emerge to help theory explain (and predict!) these phenomena. In addition, three new emerging classes of superconductors (topological, interfacial – e.g. FeSe on SrTiO₃, and H₂S under high pressure) are briefly covered, even though their ‘conventionality’ is not yet fully determined.     

p-wave superconductivity

Spin triplet, p-wave superfluidity was discovered in ³He a quarter of a century ago, and unconventional spin singlet d-wave superconductivity is now known to exist in the high-temperature superconducting cuprates. An established example of a spin triplet superconductor has still, however, been lacking. In the past few years, evidence suggesting the existence of triplet pairing has been reported in several correlated electron compounds, with perhaps the most consistent picture emerging in the layered perovskite oxide Sr₂RuO₄. A brief review is given of the recent developments, stressing the important role played by high-quality samples with long mean free paths.     

Twinning-plane superconductivity

A new phenomenon has been discovered recently: twinning-plane superconductivity (TPS). The present paper is a survey of experimental and theoretical work on TPS. TPS arises at temperatures higher than the bulk critical temperature of a single crystal, and has a localized character. This phenomenon is caused by Cooper-pairing enhancement near the twinning plane. TPS is characterized by anomalously strong diamagnetism above the critical point. The phase (H, T) diagram of TPS is very specific and differs noticeably for type-I and type-II superconductors (e.g. tin and niobium). The interaction of closely spaced twinning planes is discussed, as well as the case of TPS in microscopic particles. The proximity effect is shown to be reduced under these conditions, and the critical temperature noticeably increased (by a factor of two to three times for tin). The prospects for further investigations of TPS are considered.     

Gapless color superconductivity

We present the dispersion relations for quasiparticle excitations about the color-flavor locked ground state of QCD at high baryon density. In the presence of condensates which pair light and strange quarks there need not be an energy gap in the quasiparticle spectrum. This raises the possibility of gapless color superconductivity, with a Meissner effect but no minimum excitation energy. Analysis within a toy model suggests that gapless color superconductivity may occur only as a metastable phase.     

-wave nonadiabatic superconductivity

The inclusion of nonadiabatic corrections to the electron-phonon interaction leads to a strong momentum dependence in the generalized Eliashberg equations beyond Migdal's limit. For a s-wave symmetry of the order parameter, this induced momentum dependence leads to an enhancement of when small momentum transfer is dominant. Here we study how the d-wave symmetry affects the above behavior. We find that the nonadiabatic corrections depend only weakly on the symmetry of the order parameter provided that only small momentum scatterings are allowed for the electron-phonon interaction. In this situation, We show that also for a d-wave symmetry of the order parameter, the nonadiabatic corrections enhance . We also discuss the possible interplay and crossover between s- and d-wave depending on the material's parameters. Received 12 May 2000     

Nanoengineered magnetic-field-induced superconductivity

The perpendicular critical fields of a superconducting film have been strongly enhanced by using a nanoengineered lattice of magnetic dots (dipoles) on top of the film. Magnetic-field-induced superconductivity is observed in these hybrid superconductor/ferromagnet systems due to the compensation of the applied field between the dots by the stray field of the dipole array. By switching between different magnetic states of the nanoengineered field compensator, the critical parameters of the superconductor can be effectively controlled.     

Space-time aspects of superconductivity

A set of electromagnetic responses of superconductivity, [A] the zero-resistivity and [B] the perfect diamagnetism, has been reconsidered both electrodynamically and quantum mechanically. We point out an equivalence between the London gauge with the boundary condition or his “rigidity” of wave function and the phase locking in the BCS-type macroscopic wave function Φ_macro or the Ginzburg- Landau order parameter at T= 0 K. By reexamining the macroscopic wave function of condensed pairs Φ_macro at the ground state with the Josephson concept of coherent phase locking and the energy spectrum of quasi-particle excitation, we realize that the electric response [A] and magnetic response [B] can be decoupled and separable as independent with each other in the limits of an electrostatic field and of a uniform magnetic field rather than in an electrodynami-cal consideration. They are equally substantial in the physics of superconductivity. We recognize first a proper view to simultaneously account for [A] the zero-resistivity and [B] the perfect diamagnetism in the space-time aspects. By supplementing these space-time aspects with a concept of the gauge field theory, we can reach a further consistent and finally complete understanding of the theory of superconductivity.     

Electronegativity and superconductivity

This article gives a new criterion for superconductivity. It can be applied to all the elements in the periodic table. The criterion shows that the values of electronegativity of all superconducting elements concentrate in a narrow range from 1.3–1.9 and that elements with values outside this region would be non-superconductive.Further study of the relation between electronegativity and superconductivity of elements will contribute to understanding the connection between the electrostatic action of the electron to the crystal lattice and superconductivity, which may further elucidate the mechanism of superconductivity.     

Heavy fermion superconductivity

The phenomenon of Heavy Fermion Superconductivity in Kondo lattice systems is investigated via renormalized perturbation theory for the Anderson lattice model with inclusion of phonons. It is demonstrated how the conventional theory of superconductivity can be modified to take into account the coupling mediated by breathing of Rare Earth ions and the effect of strong local Coulomb correlations on these ions. The results give support to a recent theory of Razafimandimby, Fulde and Keller, which is based on an intermediate Fermi liquid picture.