Polaron Coherence as Origin of the Pseudogap Phase in High Temperature Superconducting Cuprates

Within a two-component approach to high T _c copper oxides including polaronic couplings, we identify the pseudogap phase as the onset of polaron ordering. This ordering persists in the superconducting phase. A huge isotope effect on the pseudogap onset temperature T ^* is predicted and in agreement with experimental data. The anomalous temperature dependence of the mean square copper–oxygen ion displacement observed above, at and below T _c , stems from an s-wave superconducting component of the order parameter, whereas a pure d-wave order parameter alone can be excluded.     

Electronically Driven Superlattice in the Cuprates

Upon doping the Mott–Hubbard insulator, an intermediate phase may be formed before the system reaches the Fermi-liquid phase. The pseudogap state and the spin/charge stripe phase are the possible examples of such a phase. We propose a new intermediate phase that has the form of the superlattice with in the CuO₂ plane, and allows the coexistence of the Mott–Hubbard insulator and the Fermi-liquid. This hypothesis is supported by neutron diffraction data, the phonon dispersion determined by neutron scattering and the recent ARPES data. The implications of such a phase on the mechanism of high-temperature superconductivity are 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.     

Studies on (Hg0.7Mo0.3)Sr2(Sr1?xLax)Cu2Oz with a Wide Range of Doping State

The single-phased series of Sr-bearing Hg-1212 superconducting cuprate,(Hg_0.7Mo_0.3)Sr₂(Sr_1–x La _x )Cu₂O _z has been prepared. X-ray diffraction showed that, the obtained samples belong to the 1212-structure with tetragonal space group P4/mmm, similar to that of (Hg, Mo)Sr₂(Ca, Y)Cu₂O_z, and stabilized in a wide compositional range of 0.25x0.75. Refinements of the structure are carried out in which the oxygen atoms at the (Hg, Mo) layer is shifted from high-symmetry position (0.5, 0.5, 0) to (x, x, 0). Magnetization and electrical resistivity measurements show that the as-prepared samples exhibit evidence for superconductivity and their superconducting properties were improved after O₂ annealing with T ^onset _c as high as 92 K.     

Low Temperature Anharmonicity and Superconductivity in Cuprates

Temperature-dependent X-ray absorption spectra of the hole-doped La_2?x Sr _x CuO₄ and electron-doped Nd_2?x Ce _x CuO_4?δ high-temperature superconductors were investigated above the Cu K absorption edge. We observed strong anharmonicity in the superconductive CuO₂ plane. For x=0.15 it was shown that part of oxygen ions oscillate in a double-well potential and their vibrations are correlated with the local electron (hole) pair transfer. We suppose that at a low temperature the phase coherence of the local pair movement is determined by the peculiarities of the perovskite-like structure which includes the stiff CuO _n (n=4,6) complexes combined by collective rotational and breathing modes.     

Superconductivity and Critical Coupling in Non-Fermi Liquids

The critical temperature of a superconductor with a non-Fermi liquid ground state has been calculated. The density of states has two different forms and is energy dependent. The critical values of the coupling factor were calculated. The constant density of states results are obtained as particular cases. These results are an extension of the work done by Grosu et al. [Phys. Rev. B 56, 8298 (1997)].     

Effect of Pressure on the Superconducting Transition Temperature in Mercury-Based Cuprates

The pressure dependence of the transition temperature of the mercury-based cuprates is analyzed through a phenomenological model, based on the inverted parabolic relation between the critical temperature (T _c ) and the hole concentration per CuO₂ layer (n). It is found that another inverted parabolic relation between the pressure dependence of the superconducting transition temperature at the optimum hole concentration and the pressure fit the recent experimental results of the mercury-based superconductors. This relation leads to a universal relation that is obeyed not only by mercury-based cuprates but also by many other high T _c compounds. In contrast to earlier studies, the transition temperature at pressure p (T _c (p)) is always less than the transition temperature for the optimum hole concentration (T ^op _c (p)) in agreement with the experiment. The effect of the pressure-induced change in the hole concentration on the transition temperature is found to be small compared to the intrinsic effects.     

NMR Spin-Lattice Relaxation Rates in Cuprates

An analysis of nuclear spin-lattice relaxation data in the normal state of cuprates that appropriately accounts for the highly anisotropic structures shows no contrasting temperature dependence of the Cu, O, and Y relaxations, which suggests that all nuclei relax by the same mechanism of the spin liquid. To investigate the temperature dependence of this mechanism, the model of fluctuating fields is used in which the rates are expressed in terms of hyperfine interaction energies and an effective correlation time τ _eff characterizing the dynamics of the spin fluid. The former contain the effects of the antiferromagnetic static spin correlations, which cause the hyperfine field constants to be added more coherently at low temperature but incoherently at high temperature. At low temperatures, τ _eff grows linearly with temperature as in ordinary metals. At high temperatures, however, the nuclear spin-lattice relaxation rates in various cuprates unequivocally reflect local moment features. This behavior is similar to that observed for the magnetic transition metals Fe, Co, and Ni, where also some properties show a cross-over from an itinerant behavior of delocalized electrons at low to that of localized moments at high temperatures.     

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.     

The Unique Properties of Superconductivity in Cuprates

Copper oxides are the only materials that have transition temperatures, T _c, well above the boiling point of liquid nitrogen, with a maximum \(T_{\mathrm {c}}^{\mathrm {m}}\) of 162 K under pressure. Their structure is layered, with one to several CuO₂ planes, and upon hole doping, their transition temperature follows a dome-shaped curve with a maximum of \(T_{\mathrm {c}}^{\mathrm {m}}\) . In the underdoped regime, i.e., below \(T_{\mathrm {c}}^{\mathrm {m}}\) , a pseudogap Δ* ∝ T* is found, with T* always being larger than T _c, a property unique to the copper oxides. In the superconducting state, Cooper pairs (two holes with antiparallel spins) are formed that exhibit coherence lengths on the order of a lattice distance in the CuO₂ plane and one order of magnitude less perpendicular to it. Their macroscopic wave function is parallel to the CuO₂ plane near 100 % d at their surface, but only 75 % d and 25 % s in the bulk, and near 100 % s perpendicular to the plane in yttrium barium copper oxide (YBCO) [1]. There are two gaps with the same T _c [2]. As function of doping, the oxygen isotope effect is novel and can be quantitatively accounted for by a vibronic theory or by the presence of bipolarons [2, 3]. These cuprates are intrinsically heterogeneous in a dynamic way. In terms of quasiparticles, bipolarons are present at low doping and aggregate upon cooling [2] so that probably ramified clusters and/or stripes are formed, leading over to a more Fermi liquid-type behavior at large carrier concentrations.     

Superconductivity in Cuprates, the van Hove Scenario

We review the consequences of the presence of van Hove singularities close to the Fermi level in the HTCS cuprates. We show that it may explain the properties of these materials such as the high T_c, anomalous isotope effect, marginal Fermi liquid properties, gap anisotropy, etc. We show that the pseudogap observed in the normal state can be attributed to the Coulomb interaction between carriers in these disordered compounds.     

Physical Picture of High-T c Superconductivity in Cuprates

A simple physical picture of high-T _c superconductivity of CuO₂ planes is proposed. It possesses all characteristic features of HTS, such as a high superconducting transition temperature, the $d_{x^{2}-y^{2}}$ symmetry of order parameter, and the coexistence of a single-electron Fermi surface and a pseudogap in the normal state. Values of pseudogap are calculated for different doping levels.     

Oxygen Isotope Effect in Cuprates Results from Polaron-induced Superconductivity

The planar oxygen isotope effect coefficient measured as a function of hole doping in the Pr- and La-doped YBa₂Cu₃O₇ (YBCO) and the Ni-doped La_1.85Sr_0.15CuO₄ (LSCO) superconductors quantitatively and qualitatively follows the form originally proposed by Kresin and Wolf [Phys. Rev. B 49, 3652 (1994)], which was derived for polarons perpendicular to the superconducting planes. Interestingly, the inverse oxygen isotope effect coefficient at the pseudogap temperature also obeys the same formula. These findings allow the conclusion that the superconductivity in YBCO and LSCO results from polarons or rather bipolarons in the CuO₂ plane. The original formula, proposed for the perpendicular direction only, is obviously more generally valid and accounts for the superconductivity in the CuO₂ planes.     

High-Tc Superconductivity with Bipolarons and Normal State Gap in Cuprates

The BCS theory is extended to a strong-coupling regime, λ≥1 with smallpolarons and bipolarons. The normal state gap is found in this regime whichis half of the bipolaron binding energy. In the framework of the bipolarontheory we explain the magnitude of the carrier specific heat andsusceptibility as well as their universal scaling with temperature in a widerange of doped cuprates. PACS numbers: 74.20.?z,74.65.+n,74.60.Mj

     

Mechanism of Superconductivity in Graphite Intercalation Compounds Including CaC6

On the basis of the model that was successfully applied to KC₈, RbC₈, and CsC₈ about a quarter century ago, we have implemented a quantitative calculation of the superconducting transition temperature T _c to find that the observed values for T _c in both CaC₆ and YbC₆ are also reproduced in the model, indicating that the model is a unified one for superconductivity in the graphite intercalation compounds with T _c ranging three orders of magnitudes.     

Theory of High Temperature Superconductivity in Cuprates and Other Doped Polar Insulators

In the last two decades, there have been tremendous attempts to build an adequate theory of high-temperature superconductivity. Most studies used some model Hamiltonians with input parameters not directly related to the material. The dielectric response function of electrons in strongly correlated high-temperature superconductors is a priori unknown. Hence, one has to start with the generic Hamiltonian including unscreened Coulomb and Fr?hlich electron?Cphonon interactions operating on the same scale since any ad-hoc assumption on their range and relative magnitude might fail. Using such a generic Hamiltonian, the analytical theory of high-temperature superconductivity in doped polar insulators predicting the critical temperature in excess of a hundred Kelvin without any adjustable parameters has been built. The many-particle electron system is described by an analytically solvable polaronic t?CJ _p Hamiltonian with reduced hopping integral, t, allowed double on-site occupancy, large phonon-induced antiferromagnetic exchange, J _p >t, and a high-temperature superconducting state of small superlight bipolarons protected from clustering. Here, major steps of the theory are outlined suggesting that the true origin of high-temperature superconductivity is found in a proper combination of strong electron?Celectron correlations with a significant finite-range (Fr?hlich) EPI, and that the theory is fully compatible with the key experiments.     

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.