Properties of high temperature superconductors in states of mixed symmetry (original) (raw)

Evolution with hole doping of the electronic excitation spectrum in the cuprate superconductors

Journal of Physics: Condensed Matter, 2008

The recent scanning tunnelling results of Alldredge et al on Bi-2212 and of Hanaguri et al on Na-CCOC are examined from the perspective of the BCS/BEC boson-fermion resonant crossover model for the mixed-valent HTSC cuprates. The model specifies the two energy scales controlling the development of HTSC behaviour and the dichotomy often now alluded to between nodal and antinodal phenomena in the HTSC cuprates. Indication is extracted from the data as to how the choice of the particular HTSC system sees these two basic energy scales (U, the local pair binding energy and, Δ sc , the nodal BCS-like gap parameter) evolve with doping and change in degree of metallization of the structurally and electronically perturbed mixed-valent environment.

Doping-induced inhomogeneity in high-Tc superconductors

Physica C: Superconductivity, 2001

Doping and disorder are inseparable in the superconducting cuprates. Assuming the simplest possible disordered doping, we construct a semiphenomenological model and analyze its experimental consequences. Among the affected experimental quantities are the ARPES spectra and thermodynamic properties. From our model we make a prediction for the width of the local superconducting gap distribution with the only experimentally unknown parameter being the superconducting correlation length. Thus, our model provides a direct way of determining the superconducting correlation length from a known experimental gap distribution.

Doping Dependence of the Electronic-Structure and Magnetic Order in High-T(C) Superconductors

Physical Review B, 1994

The local densities of states of an extended Hubbard model describing the Cu02 planes of superconducting cuprates are calculated by means of an approximate treatment that divides the lattice into Cu02 clusters. The exact diagonalization of the Hamiltonian on these trimers is utilized to solve the lattice problem, where the hopping between different trimers is treated as a perturbation. The hole concentrations on both orbitais and the amplitude of the staggered magnetization are obtained as a function of the total number of holes. The overall sbape of the band structure is in good agreement with exact diagonalization on larger clusters. The stoichiometric compound is found to be metallic in the paramagnetic phase, but becomes a charge-transfer insulator in the antiferromagnetic phase. Electron and hole doping introduce a new band at the bottom or at the top of the charge-transfer gap, respectively. Magnetic order is destroyed when the antiferromagnetic phase becomes unstable against tbe paramagnetic pbase.

Electron-hole coupling in high-Tc cuprate superconductors

Physica C: Superconductivity, 2005

From recent Hall effect measurements and angle-resolved photo-emission spectroscopy the interesting picture emerges of co-existing hole-and electron-like quasiparticle bands, both in electron-and hole-doped superconducting cuprates. We reflect on the idea that bosonic electron-hole pairs may be formed in the cuprates and on the possibility that these pairs undergo Bose-Einstein condensation. The relevance to high-T c superconductivity in the cuprates will be discussed. High critical temperature (T c) superconductors are commonly categorized into two groups: hole-doped (p-type) and electron-doped (n-type) cuprates. To the former category belong, for example, the first high-T c material La 2-x Ba x CuO 4 [1], YBa 2 Cu 3 O 7 [2], Bi 2 Sr 2 Ca n-1 Cu n O 2n+4 [3] and the superconductor with the highest critical temperature HgBa 2 Ca n-1 Cu n O 2n+2 [4, 5]. The latter category is formed by the materials Ln 2-x Ce x CuO 4 , with Ln = La, Nd, Pr, Eu, or Sm [6]. In line with the classical BCS concepts, the superconductivity in these materials is considered to result from pairing of two holes into Cooperpairs with a charge of 2e for the p-type compounds and of two electrons into pairs of-2e charge for the n-type materials. The formation of bosonic pairs of two coupled fermionic particles has indeed been evidenced in the high-T c cuprates from the quantization of magnetic flux in a superconducting ring in units of the flux quantum Φ 0 = h/2e [7]. The mechanism of the pair formation in the high-T c cuprates is however still elusive and one of the unresolved questions is whether the mechanism depends principally on the sign of the charges constituting the pairs.

Electronic valence band structure of high-Tc superconductors

Physica C: Superconductivity, 1991

The distribution of the electronic density of 0 2p and Cu 3d states in a series of bismuthates and cuprates has been studied by X-ray emission spectroscopy (XES). It has been found that in BaPb, _xBi,09, La2_,Sr,Cu04 and YBa2Cu@_n a high-energy shift of the 0 Ku spectrum in both photon energy and Fermi level-related scales occurs during the transition from semiconducting to superconducting phase. This shift is explained by the redistribution of 2p states of apical oxygen atoms. The binding energy of Cu 3d states for YBa2Cug06.2 is rather low in contrast to that for YBaZCu306,9 because of the presence of monovalent copper atoms in the semiconductor as a result of oxygen depletion of Cu-0 chains. A correlation between Z', and the energy localization of the 0 2p band relative to EF has been observed in a series of cuprates, the superconducting transition temperature of which varies from 35 to 12.5 K. On increasing T,, 0 2p states shift towards the Fermi level while the binding energy of Cu 3d states insignificantly rises.

Properties of normal and superconducting phases in the low-energy models of high-$T_c$ cuprates

2004

In the framework of the effective low-energy model for High-$T_c$ cuprates with account for three-centers interaction terms and spin fluctuations the properties of normal and superconducting phases of p- and n-type cuprates are investigated. Microscopic model parameters were obtained from ARPES data on undoped compounds. Obtained evolution of the chemical potential with doping, Fermi Surface at optimal doping, and Tc(x)T_c(x)Tc(x) phase diagram for n-type cuprates are in remarkably good agreement with the experiment. It is shown that the spin-exciton mechanism due to singlet-triplet hybridization takes place in p-type, although it is too small to reproduce observed qualitative difference between p- and n-type cuprates.

A model for high Tc superconductors with energy band overlapping

The correlation between pressure and the critical temperature of high-Tc superconductors is studied. The specific case of the Hg cuprates is considered in detail. In a simple model in which overlapping of bands increases the electronic density of states, a direct description of the increase and saturation of Tc with pressure is obtained. Low values of the isotope coefficient are taken into account and can be partially explained in this framework.

Multiband Responses in High-T c Cuprate Superconductors

Journal of Superconductivity and Novel Magnetism, 2013

We report on the interplay of localized and extended degrees of freedom in the metallic state of high-temperature superconductors in a multiband setting. Various ways in which the bare magnetic response may become incommensurate are measured against both phenomenological and theoretical requirements. In particular, the pseudogap temperature is typically much higher than the incommensurability temperature. When microscopic strong-coupling effects with real-time dynamics between copper and oxygen sites are included, they tend to restore commensurability. Quantum transport equations for low-dimensional multiband electronic systems are used to explain the linear doping dependence of the dc conductivity and the doping and temperature dependence of the Hall number in the underdoped LSCO compounds. Coulomb effects of dopands are inferred from the doping evolution of the Hartree-Fock model parameters.

Pair–pair interactions as a mechanism for high- T c superconductivity

Superconductor Science and Technology, 2015

The mutual interaction between Cooper pairs is proposed as a mechanism for the superconducting state. Above Tc, pre-existing but fluctuating Cooper pairs give rise to the unconventional pseudogap (PG) state, well-characterized by experiment. At the critical temperature, the pair-pair interaction induces a Bose-like condensation of these preformed pairs leading to the superconducting (SC) state. Below Tc, both the condensation energy and the pair-pair interaction β are proportional to the condensate density Noc(T), whereas the usual Fermi-level spectral gap ∆p is independent of temperature. The new order parameter β(T), can be followed as a function of temperature, carrier concentration and disorder-i.e. the phase diagrams. The complexity of the cuprates, revealed by the large number of parameters, is a consequence of the coupling of quasiparticles to Cooper-pair excitations. The latter interpretation is strongly supported by the observed quasiparticle spectral function.