Electronic Structure in Underdoped Cuprates Due to the Emergence of a Pseudogap

The phenomenological Green’s function developed in the works of Yang, Rice, and Zhang has been very successful in understanding many of the anomalous superconducting properties of the deeply underdoped cuprates. It is based on considerations of the resonating valence bond spin liquid approximation and is designed to describe the underdoped regime of the cuprates. Here, we emphasize the region of doping, x, just below the quantum critical point at which the pseudogap develops. In addition to Luttinger hole pockets centered around the nodal direction, there are electron pockets near the antinodes which are connected to the hole pockets by gapped bridging contours. We determine the contours of nearest approach as would be measured in angular resolved photoemission experiments and emphasize signatures of the Fermi surface reconstruction from the large Fermi contour of Fermi liquid theory (which contains 1+x hole states) to the Luttinger pocket (which contains x hole states). We find that the quasiparticle effective mass renormalization increases strongly toward the edge of the Luttinger pockets beyond which it diverges.     

Effect of Normal-State Pseudogap on Physical Quantities in Hubbard Model for Underdoped High-T c Cuprates

We develop the strong-coupling theory of coexisting charge-density-wave (CDW) and superconductivityd-wave gaps within the framework of the FLEX (fluctuation exchange) approximation for the two-dimensional Hubbard model. For nested sections of the Fermi surface these equations reduce to the previous FLEX equations for superconductivity where the squared energy gap _s ² in the denominator of the Green's function is replaced by ( _s ² + _c ² ) (here _s is the superconductivity and _c the CDW gap). We solve these equations by taking for _c a phenomenologicald-wave gap. The resulting neutron scattering intensity, spin-lattice relaxation rate 1/T₁ , Knight shift, resistivity, and photoemission intensity are in qualitative agreement with the data on underdoped high-T_c cuprates. TheT_c for superconductivity decreases and the crossover temperature T_* for 1/T₁Tincreases with increasing gap amplitude of _c which is in qualitative agreement with the phase diagram for underdoped cuprates.     

Local Signatures of High-Tc Superconductivity Possible Origin of the Pseudogap

We have studied buffer ions induced holes clustering in the CuO ₂ plane by means of embedded cluster quantum chemistry calculations. This instability is suggested by Raman spectroscopy to be characteristic of the pseudo-gappedfermi surface at T     

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.     

The Pseudogaps of HTSC

The problem of the pseudogaps in oxide HTSC is analyzed by means of the off-diagonal propagators, in analogy with the method of Nambu in superconductivity. We conjecture that the small pseudogap is obtained as the Fock diagram and the large pseudogap as the total off-diagonal self-energy (which is the sum of the Hartree diagram and the Fock diagram). We obtain real gap for undoped materials, and pseudogaps (with states in the gap) for doped and conducting materials. We find that disorder is essential for the results of the doped materials.     

The Pseudogaps of HTSC

The problem of the pseudogaps in oxide HTSC is analyzed by means of the off-diagonal propagators, in analogy with the method of Nambu in superconductivity. We conjecture that the small pseudogap is obtained as the Fock diagram and the large pseudogap as the total off-diagonal self-energy (which is the sum of the Hartree diagram and the Fock diagram). We obtain real gap for undoped materials, and pseudogaps (with states in the gap) for doped and conducting materials. We find that disorder is essential for the results of the doped materials.     

The ‘Neutron’ Magnetic Resonance in the Underdoped Cuprate Superconductor

We show analytically that the magnetic neutron resonance of nonstoichiometrically doped cuprate is a sharp plasmon excitation of the dominant d-wave band at its two-channel Kondo fixed point. Despite existing throughout the ‘normal’ state, it becomes detectable when Kondo scattering is reduced strongly below the channel transition temperature. Unlike the one-off approach of the current theory of the neutron resonance as the spin exciton, all data relevant to the resonance as well as other phenomena for the crystal are amenable to explanation in the multichannel Kondo approach.     

Pseudogap and Transport in HTS

Perpendicular transport is one of the key factors to HTS superconductivity, sampling the quasi-insulating blocking layer, separating the conducting CuO-planes, and driving the metal–insulator transition (MIT) that is induced by disorder and underdoping. Various measurements have been carried out to study the transport, the MIT, and the in-plane Fermi surface especially by surface methods via the blocking layer, and these depend sensitively on surface quality. ARXPS results on UHV cleaving show that at 300 K and 10^–10 Torr, a Bi hydroxide layer occurs in 30 min, followed by H₂O or C_y H_x OH chemisorption. Consequences of this result on STS, ARPES, perpendicular transport, Coulomb charging, and pseudogap are analyzed, yielding scenario for HTS superconductivity, where static and dynamic charge exchange via and with the blocking layer initiates plaques of preformed pairs of d-wave symmetry weakening the inplane Coulomb repulsion yielding by this plasmonic mechanism, finally, HTS. Consequences of this scenario on anisotropic transport with its strong Fermi velocity v_F anisotropy and its strong in-plane scattering rate (T) const. at (, 0) in k-space with pseudo gap kT^* _{_P}/3 and superconducting gap _S 3 kT_c maxima and the strongly decreasing rate (T) T at 0.4 (, ) with pseudo gap _{_p} (k) node and superconducting gap _s (k node are given.     

Pseudogap in the Normal State of Cuprates

We consider underdoped cuprates as disordered conductors. The diffusion coefficient D can be as low as 10^–5 m² s^–1. In these conditions, Coulomb interaction between electrons must be taken into account. The main effect is to open a dip and even a gap in the density of state (DOS) near the Fermi level (FL). We show that this model explains most of the observed features of the so-called pseudogap in the normal state and in particular its value, anisotropy, and variation with doping.     

Underdoped Region of the t- J Model

We apply a spin-wave theory modified for the paramagnetic state to investigate normal-state properties of the 2D t-J model. Self-energy equations for hole and magnon Green's functions are self-consistently solved for the ranges of hole concentrations x 0.17 and temperatures T 150 K. In this region the hole spectrum differs from a conventional metallic spectrum. With decreasing x from x 0.17 hidden parts appear in the hole Fermi surface which can be interpreted as a pseudogap near (0, ). Obtained size, symmetry and concentration dependence of the pseudogap are in agreement with photoemission data in Bi2212. Calculated temperature dependencies of the spin correlation length, spin-lattice relaxation times at the Cu and O sites, and static susceptibility are typical for the quantum disordered regime with a pseudogap in the spectrum of magnetic excitations. These quantities are in agreement with experiment in underdoped YBa ₂ Cu ₃ O ₆₊.     

The mechanism of hole carrier relaxation in HTSC

The nature of hole carriers in high-T_c superconductors (HTSC) and mechanism of their relaxation are considered on the base of the HTSC electronic spectrum model suggested before. The model proposes that the distinctive feature of HTSC electronic spectrum is the pair level placed at the top of valence band. It is shown that this feature may account for the peculiarities of transport and optical properties observed in experiment.     

Fluorination of Underdoped Mercurocuprate Superconductors

We present the results of a study into the incorporation of fluorine into the mercurocuprate superconductors. In all cases, the effect of fluorination has been to increase the T_c of underdoped superconductors to the maxima reported. This has important ramifications for the role of the interstitial oxygen in the mercurocuprate superconductors. To assist us in our study, we have, additionally, developed a novel synthetic procedure for preparing HgBa₂CuO₄₊ in a highly underdoped state. Remarkably, this material shows an increase in T_c, upon fluorination, of 60K!     

Isotope Effect of Underdoped Cuprates in the Yang-Rice-Zhang Model

The underdoped region of the cuprates phase diagram displays many novel electronic phenomena both in the normal and the superconducting state. Many of these anomalous properties have found a natural explanation within the resonating valence bond spin liquid phenomenological model of Yang-Rice-Zhang (YRZ) which includes the rise of a pseudogap. This leads to Fermi surface reconstruction and profoundly changes the electronic structure. Here, we extend the previous work to consider the shift in critical temperature on ¹⁶O to ¹⁸O substitution. The isotope effect has been found experimentally to be very small at optimal doping yet to rapidly increase to very large values with underdoping. The YRZ model provides a natural explanation of this behavior and supports the idea of a pairing mechanism which is mainly spin fluctuations with a subdominant (～10%) phonon contribution.     

The Mechanism of d-Wave Superconductivity of Underdoped High-T c Cuprates

We have worked out the theory of d-wave superconductivity of the doped cuprate that is consistent with up-to-date data on the basis of the assumption of two-channel Kondo fixed point of the latter. Strong local Coulomb scattering between carriers combined with strong and weak involvement, respectively, of charge (spin) fluctuations and phonons in pairing justify the assumption of two-channel Kondo effect in the doped cuprate. This is true for diverse other data as well. The assumption explains not only the d-wave superconductivity but also all the relevant physics of this material.     

The Inward Dispersion of the Neutron Scattering Experiments in HTSC Cuprates

The theory of the high-temperature superconducting cuprates, which is based on the condensation of holes into strings in checkerboard geometry, was successful to explain the elastically scattered neutrons by spin waves. Here, it is extended to analyze the inward dispersion curve of its inelastic counterpart, up to the resonance energy (E _res). This extension is done by applying the perturbation theory of the linear response to the condensed strings. The approximated susceptibility is derived by means of the ring diagram. The dispersion relation is obtained from the dispersion of the poles of the susceptibility integral. It is found that the particle-anti-particle pair that yields the susceptibility is the time reversal pair where the particle momentum is k in phase A and the anti-particle momentum is ?k in phase B. The dispersion is found to be in agreement with experiment, subject to some suggested corrections. The weak intensity by the resonance energy, as well as the dispersion, is speculated to be modified due to interference with spin waves that are caused by direct spin flip, as in the undoped mother materials.     

Dielectricly Enhanced T c in Underdoped Cuprates

It is proposed to create a multilayer structure in which an underdoped copper-oxide high-temperature superconductor is sandwiched between high-dielectric-constant insulator layers such as ferro- or ferri-electrics, thereby reducing the Coulomb repulsion between the intrinsically present clusters or stripes in the CuO₂ layers of the pseudogap phase. This should lead to an increase in the size of such clusters, resulting in smaller distances between them and coherence at higher temperature, i.e., a higher T _c with a smaller pseudogap (T ^??T _c ).     

Double Correlations in HTSC

The Nambu–Gorkov generalized Hartree–Fock theory of superconductivity is further generalized to treat simultaneously two kinds of correlations: the correlations that lead to superconductivity, as well as the correlations that produce the pseudogap in the normal state. The treatment of these simultaneous double correlations is done by enlarging the dimension of the propagator and self-energy matrices, from two to four. The off-diagonal self-energies and the DOS are calculated. The results are compatible with tunneling and PES experiments.