This work investigates the relation between superconductivity and correlations. A simple calculation shows that the appropriate spin-charge correlation is the key role to any superconductivity, and this calculation is consistent with the analyses of unusual properties of superconductors. (Note: the Tc of this model is not given clearly in this work, but we have advanced this mechanism to a t-x model which includes various superconductivities and magnetisms (please see arXiv:0707.3660 and following works).)
Fluctuating local diamagnetic pairs of electrons, embedded in a Fermi sea, are candidates for non-phonon-mediated superconductors without the stringent conditions on Tc which arise in phonon-mediated BCS classical low-Tc superconductors. The local accumulations of charge, from which such diamagnetic fluctuations originate, are irrevocably coupled to local dynamical lattice instabilities and form composite charge-lattice excitations of the system. For a superconducting phase to be realized, such excitations must be itinerant spatially phase-coherent modes. This can be achieved by resonant pair …
The nonadiabatic Heisenberg model predicts a structural distortion in LiFeAs below a temperature higher than (or at least equal to) the superconducting transition temperature. Within this group-theoretical model, the reduction of the symmetry caused by the distortion is a prerequisite for the superconducting state in this compound and can be realized by a mere displacement of the iron atoms from their positions in the space group P4/nmm.
The formation of electron pairs is a prerequisite of superconductivity. The fermionic nature of electrons yields four classes of superconducting correlations with definite symmetry in spin, space and time. Here, we suggest double quantum dots coupled to conventional s-wave superconductors in the presence of inhomogeneous magnetic fields as a model system exhibiting unconventional pairing. Due to their small number of degrees of freedom, tunable by gate voltages, quantum-dot systems provide an ideal tool to gain fundamental insight in unconventional pairing. We propose two detection schemes for…
We investigate the interplay of the electron-phonon and the spin fluctuation interaction for the superconducting state of YBa$_2$Cu$_3$O$_{7}$. The spin fluctuations are described within the nearly antiferromagnetic Fermi liquid theory, whereas the phonons are treated using a shell model calculation of all phonon branches. The electron-phonon coupling is calculated using rigidly displaced ionic potentials screened by a background dielectric constant $ε_\infty$ and by holes within the CuO$_2$ planes. Taking into account both interactions we get a superconducting state with $d_{x^2-y^2}$-symmetr…
R2-xCexRuSr2Cu2O10-d(Ru-2122) is the first Cu-O based system in which superconductivity (SC) in the CuO2 planes and weak-ferromagnetism (W-FM) in the Ru sublattice coexists. The hole doping in the CuO2 planes, is controlled by appropriate variation of the Ce concentration and/or increasing the oxygen concentration. In Eu2-xCexRuSr2Cu2O10, SC occurs for Ce contents of 0.4-0.8, with the highest TC=35 K for Ce=0.6. The as-prepared non-SC EuCeRuSr2Cu2O10 sample exhibits magnetic irreversibility below Tirr=125 K and long range o anti-ferromagnetic (AFM) order at TM =165 K. The saturation moment at …
The idea of raising Tc in the spin-fluctuation mediated superconductivity on disconnected Fermi surfaces with the gap function changing sign across but not within the Fermi pockets, proposed by Kuroki and Arita for two dimensions (2D), is here extended to three-dimensional (3D) systems. Two typical cases of 3D disconnected Fermi surfaces (stacked bond-alternating lattice and stacked ladder layers) are considered. By solving Eliashberg's equation for Green's function obtained with the fluctuation exchange approximation (FLEX) for the repulsive Hubbard model on these structures, we have shown th…
Non-equilibrium studies of two-dimensional (2D) superconductors (SCs) with Ising spin-orbit coupling are prerequisite for their successful application to equilibrium spin-triplet Cooper pairs and, potentially, Majorana fermions. By taking advantage of the recent discoveries of 2D SCs and their compatibility with any other materials, we fabricate here non-local magnon devices to examine how such 2D Ising superconductivity affects the conversion efficiency of magnon spin to quasiparticle charge in superconducting flakes of 2H-NbSe2 transferred onto ferrimagnetic insulating Y3Fe5O12. Comparison w…
Simple cubic phosphorus exhibits superconductivity with a maximum $T_c$ of up to 12 K under pressure. The pressure dependence of $T_c$ cannot be consistently explained with a simple electron-phonon mechanism, which has stimulated investigations into the role of electronic correlations and plasmonic contributions. Here, we solve the gap equation of density functional theory for superconductors using different electron-electron and electron-phonon contributions to the kernel. We find that the phonon contribution alone yields an overestimation of $T_c$, while the addition of the static electronic…
The distinction between type I and type II superconductivity is re-examined in the context of the SO(5) model recently put forth by Zhang. Whereas in conventional superconductivity only one parameter (the Ginzburg-Landau parameter $κ$) characterizes the model, in the SO(5) model there are two essential parameters. These can be chosen to be $κ$ and another parameter, $β$, related to the doping. There is a more complicated relation between $κ$ and the behaviour of a superconductor in a magnetic field. In particular, one can find type I superconductivity even when $κ$ is large, for appropriate va…
We study theoretically the simultaneous influence of spin accumulation potential $eV_\uparrow-eV_\downarrow$ and the Zeeman exchange field on singlet superconductivity. It is shown that the pair-breaking effect of the Zeeman field can be fully compensated by creation of the appropriate spin accumulation potential in the superconductor. Moreover, superconductivity can be recovered for exchange fields well exceeding the Pauli limiting field. It is proposed that the effect can be experimentally realized on the basis of voltage biased junction consisting of a thin superconducting film sandwiched b…
We study the spin-fluctuation-mediated $s\pm$-wave superconductivity in the bilayer Hubbard model with vertical and diagonal interlayer hoppings. As in the two-leg ladder model with diagonal hoppings, studied previously by the present authors, superconductivity is strongly enhanced when one of the bands lies just below (or touches) the Fermi level, that is, when the band is incipient. The strong enhancement of superconductivity is because large weight of the spin fluctuations lies in an appropriate energy range, whereas the low energy, pair-breaking spin fluctuations are suppressed. The optimi…
Based on recent magnetic-quantum-oscillation, ARPES, neutron-scattering and other data, we propose that superconductivity in the cuprates occurs via a convenient matching of the spatial distribution of incommensurate spin fluctuations to the amplitude and phase of the $d_{x^2-y^2}$ Cooper-pair wavefunction; this establishes a robust causal relationship between the lengthscale of the fluctuations and the superconducting coherence length. It is suggested that the spin fluctuations are driven by the Fermi surface, which is prone to nesting; they couple to the itinerant holes via the on-site Coulo…
Doped Hubbard model is a simple model for the high-Tc cuprate superconductors, while its ground state remains a challenge. Here, by performing state-of-the-art variational Monte Carlo calculations for the strong-coupling Hubbard model, we find evidences that the d-wave superconducting phase emerges always near the phase separation region and the superconducting order has one-to-one correspondence with the enhancement of charge compressibility. The order as well as the phase separation are vulnerable to realistic intersite Coulomb interaction while the superexchange interaction enhances both. A…
The problems of participation of crystal chemical factors in creating of conditions to occurrence of a superconducting state in layered superconductors are considered. On the basis of the experimental data, given in the literature, new correlations of Tc with relation of crystal chemical parameters in high-Tc cuprates, diborides and borocarbides are established, the critical crystal chemical parameters controlling Tc are selected, and their functions increating of prerequisites to the occurrence of superconductivity are determined. The similar character of the correlations allows to make concl…
The combination of spin-orbit coupling with interactions results in many exotic phases of matter. In this Letter, we investigate the superconducting pairing instability of the two-dimensional extended Hubbard model with both Rashba and Dresselhaus spin-orbit coupling within the mean-field level at both zero and finite temperature. We find that both first- and second-order time-reversal symmetry breaking topological gapped phases can be achieved under appropriate parameters and temperature regimes due to the presence of a favored even-parity $s+id$-wave pairing even in the absence of an externa…
The $d$-$wave$ superconductivity is analyzed within the three-band $d$-$p$ model with the use of the diagrammatic expansion of the Guztwiller wave function method (DE-GWF). The determined stability regime of the superconducting state appears in the range of hole doping $δ\lesssim 0.35$, with the optimal doping close to $δ\approx 0.19$. The pairing amplitudes between the $d$-orbitals due to copper and $p_x/p_y$ orbitals due to oxygen are analyzed together with the hybrid $d$-$p$ pairing. The $d$-$d$ pairing between the nearest neighboring atomic sites leads to the dominant contribution to the S…
We preform a multi-orbital analysis on the novel superconductivity in Na_{x}CoO_{2} \cdot yH_{2}O. We construct a three-orbital model which reproduces the band structure expected from the LDA calculation. The effective interaction leading to the pairing is estimated by means of the perturbation theory. It is shown that the spin triplet superconductivity is stabilized in the wide parameter region. This is basically owing to the ferromagnetic character of spin fluctuation. The p-wave and f-wave superconductivity are nearly degenerate. The former is realized when the Hund's rule coupling is large…
Recently, topological superconductors based on Josephson junctions in two-dimensional electron gases with strong Rashba spin-orbit coupling have been proposed as attractive alternatives to wire-based setups. Here, we elucidate how phase-controlled Josephson junctions based on quantum wells with [001] growth direction and an arbitrary combination of Rashba and Dresselhaus spin-orbit coupling can also host Majorana bound states for a wide range of parameters as long as the magnetic field is oriented appropriately. Hence, Majorana bound states based on Josephson junctions can appear in a wide cla…
We show that the interplay between antiferromagnetic interaction and hole motion gives rise to a topological superconducting phase. This is captured by the one dimensional anisotropic $t-J$ model which can be experimentally achieved with ultracold polar molecules trapped onto an optical lattice. As a function of the anisotropy strength we find that different quantum phases appear, ranging from a gapless Luttinger liquid to spin gapped conducting and superconducting regimes. In presence of appropriate $z$-anisotropy, we also prove that a phase characterized by non-trivial topological order take…
We investigate the competition between acoustic-phonon-mediated superconductivity and the long-range Coulomb interaction in moiréless graphene multilayers, specifically, Bernal bilayer graphene, rhombohedral trilayer graphene, and ABCA-stacked tetralayer graphene. In these graphene multilayers, the acoustic phonons can realize, through electron-phonon coupling, both spin-singlet and spin-triplet pairings, and the intra-sublattice pairings ($s$-wave spin-singlet and $f$-wave spin-triplet) are the dominant channels. Our theory naturally explains the distinct recent experimental findings in Berna…
Cold dense quark matter is in a crystalline color superconducting phase wherever pairing occurs between species of quarks with chemical potentials whose difference δμlies within an appropriate window. If the interaction between quarks is modeled as point-like, this window is rather narrow. We show that when the interaction between quarks is modeled as single-gluon exchange, the window widens by about a factor of ten at accessible densities and by much larger factors at higher density. This striking enhancement reflects the increasingly (1+1)-dimensional nature of the physics at weaker and weak…
It is assumed that U atoms in $UGe_2$ have a number of $f$ electrons appropriate to give them each a spin $s=1$ as well as one extra itinerant electron which may equally well be on one or other U atom. The dynamical degrees of freedom are spin-s operators of localized spins and spin-1/2 fermi operators of itinerant electrons. Applying hydrostatic pressure changes the bandwidths of spin-up and spin-down itinerant electrons in different way, which leads to decreasing of the contribution of the fermions to the magnetization keeping the spin-fermion interaction unchanged. In turn the local spin-fe…
Hund's pairing refers to Cooper pairing generated by onsite interactions that become attractive due to large Hund's exchange $J$. This is possible in multiorbital systems even when all local bare interactions are repulsive, since attraction in specific channels are given by certain linear combinations of interaction parameters. On the other hand, pairing processes such as the exchange of spin fluctuations, are also present. We compare these two mechanisms on an equal footing using electronic bands appropriate for different classes of multiorbital systems over a wide range of interaction parame…
Boundary conditions in quasiclassical theory of superconductivity are of crucial importance for describing proximity effects in heterostructures between different materials. Although they have been derived for the ballistic case in full generality, corresponding boundary conditions for the diffusive limit, described by Usadel theory, have been lacking for interfaces involving strongly spin-polarized materials, such as e.g. half-metallic ferromagnets. Given the current intense research in the emerging field of superconducting spintronics, the formulation of appropriate boundary conditions for t…
This ``Perspective'' briefly reviews recent work showing that a class of organic superconductors have important similarities to the cuprate superconductors: (i) There is competition betweeen superconductivity and antiferromagnetism. (ii) Uncoventional metallic behavior is observed near the metal-insulator transition. A more detailed review and discussion of the appropriate strongly correlated electron model can be found in cond-mat/9802198.
The recent discovery of an incipient charge density wave (CDW) instability competing with superconductivity in a class of high temperature cuprate superconductors has brought the role of charge order in the cuprate phase diagram under renewed focus. Here we take a mean field (Q = 2pi/3,2pi/3) bi-axial CDW state and calculate the Fermi surface topology and the resulting Hall and Seebeck coefficients as a function of temperature and hole doping. We show that, in the appropriate doping ranges where the low temperature state (in the absence of superconductivity) is a bi-directional CDW, the Fermi …
Theoretical proposal of superconductivity in hole-doped reduced bilayer nickelate La3Ni2O6: a manifestation of orbital-space bilayer model with incipient bands
The University of Osaka（大阪大学）；Tottori University（鸟取大学）
