Superconductivity Within the Pair-Tunneling Model Close to the Metal-Insulator Transition

We have considered the problem of s-wave and d-wave superconductivity in the two-layer Hubbard model close to the metal-insulator transition. To mimic the formation of the insulating gap, the Coulomb correlations have been taken into account within the Hubbard I approximation. The interlayer momentum-conserving Josephson tunneling between the layers has been included on the mean-field level. We demonstrate that the interlayer tunneling may contribute to the occurrence of mixed d + s wave symmetry of the superconducting state with a dominating d-wave component at a low concentration of holes. The problem of the validity of the pair-tunneling model is also briefly discussed.     

Stripe-Induced High-Temperature Superconductivity in Cuprates

The single-particle spectral function of the strongly correlated electron system is composed of a coherent peak and incoherent hills on both sides. The non-resonant electronic scattering of the incoherent spectral function becomes the same in the $B_{\rm 1g}$ and $B_{\rm 2g}$ channels and also correlates to the optical conductivity. The mid-infrared hill in the optical conductivity has the same origin as the electronic Raman susceptibility induced by the hole hopping perpendicular to the stripe. The wide-energy Raman spectra in the hole-doped and electron-doped cuprates are different, because the hole-doped cuprates are in the stripe phase while the electron-doped ones are not.     

Correct Nature of High-Temperature Superconductivity

The correct nature of high-T _C superconductivity is outlined: (i) the cuprates do not superconduct in their cuprate-planes, but in their BaO, SrO, or interstitial-oxygen regions; (ii) doped Ba₂YRuO₆ and Sr₂YRuO₆, ruthenates without cuprate-planes, superconduct in their BaO or SrO layers; (iii) the rutheno-cuprates GdSr₂Cu₂RuO₈ and Gd_2−z Ce _z Sr₂Cu₂RuO₁₀ have cuprate-planes which do not superconduct, but superconduct in their SrO layers; (iv) LaFeAsO_1−x F _x superconducts via its oxygen ions, as do related compounds; and (v) the organic compound κ-[BEDT-TTF]₂Cu[NCS]₂ superconducts along the S chains of the molecule. In YBa₂Cu₃O₇, the superconductivity is consistent with observations by positive muon spin rotation and with analyses indicating an absence of Cu d-band contribution to the superconductivity. Hence only, the BaO layer superconducts. The superconductivity is s-wave in character (not d-wave), once fluxon depinning has been properly accounted for. The superconducting BaO layers are p-type and adjacent to the n-type cuprate-planes. The hole-pairing is not phononic, but Coulombic. Many experiments can be explained by understanding the characteristics of the high-T _C mechanism, which contradicts the theories with d-wave superconductivity in the cuprate-planes.     

Vitaly Ginzburg and High-Temperature Superconductivity

Vitaly L. Ginzburg has contributed in many areas of physics. One of these is the field of high-temperature superconductivity (HTSC). I have benefited from his insight in this field and have enjoyed his friendship and support over a period of almost 40 years. I present some personal reminiscences of these interactions and discuss recent progress towards the understanding of the mechanism responsible for the superconductivity of the high T _c cuprate superconductors, its relation to our earlier work, and what lies ahead.     

High-Temperature Superconductivity Without Cuprate Planes

Sr₂YRuO₆ and Ba₂YRuO₆, both doped with Cu on Ru sites, superconduct at onset temperatures of ∼49 and ∼93 K, despite having no cuprate planes. Sr₂YRuO₆ has two sister cuprate compounds, GdSr₂Cu₂RuO₈ and Gd_2-z Ce _z Sr₂Cu₂RuO₁₀, that begin superconducting at nearly the Sr₂YRuO₆ temperature, which suggests that all of these materials superconduct not in their cuprate planes (which do not exist in Sr₂YRuO₆ or Ba₂YRuO₆), but in their SrO layers (BaO layers for Ba₂YRuO₆). Generalization of this result to all high-T _c superconductors implies that the cuprate planes do not superconduct, and that the SrO layers (or equivalent layers) do.Sr₂YRuO₆ superconducts with less than 1% Cu, too few Cu atoms to form a significant number of CuO₂ planes [1].Cu-doped Sr₂YRuO₆ becomes fully superconducting at 23 K. See Ref. [2].     

Rare-Earths as Probes of High-Temperature Superconductivity

Using only two principles: (i) high-temperature superconductivity requires hypercharged oxygen, and (ii) the superconducting condensates are located in those parts of the crystal structures where they are unaffected by magnetic pair breaking, we are able to explain why certain rare-earth ions R are compatible with superconductivity and others are not, in the compounds RBa₂Cu₃O₇, RBa₂Cu₄O₈, RBa₂Cu₂NbO₈, R_{2 – z} Ce _z CuO₄, and R_{2 – z} Ce _z Sr₂Cu₂NbO₁₀. Various defects are proposed as having central roles in the superconductivity or the suppression of superconductivity in these compounds. Many experiments for testing this physical picture are suggested.     

High-Temperature Superconductivity in Sulfur-Doped Amorphous Carbon Systems

Magnetization measurements performed on amorphous carbon (aC) powder that contained a small amount of sulfur revealed traces of inhomogeneous superconductivity (SC) at T _c=65 K. Thin films of granular aC-W composite obtained by Electron Beam Induced Deposition showed no sign of SC. However, SC at T _c≈40 K was induced upon treating this film with sulfur at 250 ^∘C for 24 hours. Although the superconducting volume fraction in both cases is very low, our results prove the necessity of sulfur for inducing SC in aC, and open new pathways to achieve high-temperature SC in the unique system of aC-based materials.     

High-Temperature Superconductivity: A Macroscopic View

Over the last 12 years, impressive progress has been made in characterizingthe high temperature superconducting phenomenon and in discovering trendsabout its occurrence. As a result, many models have been advanced to accountfor the observations. In this presentation, I would like to discuss a fewissues that may be important to the understanding of high temperaturesuperconductivity but have not yet been given the attention they deserve inthe development of theoretical models.     

Probing High-Temperature Superconductivity with Positive Muons

Muon-spin rotation (SR) is a unique tool to investigate internal magnetic fields in superconductors on a microscopic scale. In particular, the complex vortex structure in high-temperature superconductors can be explored with this technique. As an example, SR experiments on the vortex phase diagram of single-crystal Bi₂Sr₂CaCu₂O₈₊ are briefly described. A novel SR technique using low-energy muons allows the measurement of internal magnetic fields near the surface of a superconductor with a few nanometers' resolution. First results obtained with this technique on a thin YBa₂Cu₃O_7- film are presented.     

Fermi Glass Versus High-Temperature Superconductivity Behavior

Very recently BaSrNiO₄ was reported to be a Fermi glass (Schilling et al. in J. Phys., Condens. Matter 21:015701, 2009). Its structure is essentially the one of K₂NiF₄ as is that of La₂CuO₄, in which the occurrence of high-temperature superconductivity (HTS) upon hole doping was first reported (Bednorz and Müller in Z. Phys. B 64:189, 1986; Adv. Chem. 100:757, 1988). The carriers in both have mainly e_g character, and move in a stochastic potential as documented by a number of experiments. The difference of the two behaviors is mainly ascribed to the formation of intersite bipolarons (Kabanov and Mihailovic in J. Supercond. 13:950, 2000) , which is estimated to be up to two orders of magnitude larger in La₂CuO₄ than in BaSrNiO₄. From this it follows that for HTS to occur, a large bipolaron formation energy in layered structures is required.     

Dual Fermion Approach to High-Temperature Superconductivity

We present an extension of the recently proposed dual fermion approach to investigate the superconducting properties of the Hubbard model in two dimensions. From the spin–spin susceptibility, we find a reduction of the Néel temperature compared to results from dynamical mean-field theory (DMFT) calculations due to the incorporation of spatial correlations. We present results for the temperature dependence of the leading eigenvalue of the Bethe–Salpeter equation for the particle–particle channel. In agreement with previous studies, we find singlet d-wave pairing to be the dominant contribution to pairing. We further present first results for the finite-doping phase diagram obtained from the leading eigenvalue of the Bethe–Salpeter equations for the particle–hole and particle–particle channels.     

Interlayer Tunneling in High-Temperature Superconductors Beyond the Mean-Field Approximation

The interlayer tunneling in high-temperature superconductors has been reconsidered beyond the mean-field approximation. The modification of the quasiparticle spectrum originating from the momentum-conserving interlayer transportation of Cooper pairs has been taken into account exactly. The pronounced reduction of the superconducting transition temperature when compared to the mean-field solution can be observed. Our self-consistent Green's functions approach allows for incorporation of Coulomb correlation and gives support for the mixed s- and d-wave scenario of high-temperature superconductivity.     

High-Temperature Superconductivity: The Attractive Up Regime

We present the results of an infinite order unitary transformation applied to a multiband Hubbard Hamiltonian by which it can be shown rigorously that an attractive interaction term appears at the oxygen ion sites as a result of oxygen–copper virtual charge excitations. This exact result yields convincing evidence that the pairing mechanism in two and/or three band Hubbard models results precisely from this attraction.     

High-Temperature Superconductivity in Hydrides: Experimental Evidence and Details

Journal of Superconductivity and Novel Magnetism - Since the discovery of superconductivity at?~?200 K in H3S [1], similar or higher transition temperatures, Tcs, have been...     

Superconductivity in the Repulsive Hubbard Model

The two-dimensional repulsive Hubbard model has been investigated by a variety of methods, from small to large U. Superconductivity with d-wave symmetry is consistently found close to half filling. After a brief review of the various methods a variational many-electron state is discussed in more detail. This trial state is a natural extension of the Gutzwiller ansatz and provides a substantial improvement thereof.     

Synergetic Phonon–Spin–Charge Mechanism of High-Temperature Superconductivity

For a long time the majority opinion in the field of high-temperature superconductivity (HTSC) has been that it is purely an electronic phenomenon involving spin, and could be explained by a t–J Hamiltonian. Phonons and local distortion were regarded as irrelevant or harmful to HTSC. However, various experimental results indicate strong phonon involvement and ubiquitous presence of local spin–charge–lattice inhomogeneity. We suggest that the electron–phonon (e–p) coupling in the cuprate is unconventional, and a synergetic coupling of spin, charge, and phonon could explain the HTSC phenomenon. In our view the spin–charge–lattice inhomogeneity is a signature of such a coupling and an important component of the HTSC mechanism.