Origin Of Pseudogap Research Articles

Euclidean Q-balls of electronic spin/charge densities confining superconducting condensates as the origin of pseudogap and high-Tc superconducting behaviours

Euclidean Q-ball type solutions carrying Cooper/local-pair condensates inside finite volume filled with oscillating in Matsubara time semiclassical electronic spin/charge densities are found in the ‘nested’ repulsive Hubbard model of high-Tc superconductors. Euclidean Q-ball solutions, analogous to famous Q-balls of squarks in the supersymmetric standard model, arise due to global invariance of the effective theory under the phase rotation of the Fourier amplitudes of spin/charge density wave-like short-range fluctuations, leading to conservation of the ‘Noether charge’ Q along the Matsubara time axis. It is demonstrated analytically that condensation of Cooper/local pairs self-consistently creates local minimum at finite amplitude in the effective potential energy of the spin/charge density wave fluctuations. Exchange with density fluctuations of finite amplitude provides greater binding energy of fermions into local/Cooper pairs inside the Q-balls than exchange with infinitesimal lattice/charge/spin quasiparticles in the standard Fröhlich mechanism. The Q-balls arise with a finite density of superconducting condensate inside them below temperature T∗. Self-consistent solutions are found from the Eliashberg like equations with the ‘nesting’ wave vector of the spin-/charge fluctuations connecting ‘antinodal’ points of the Brillouin zone, combined with semiclassical equation for the minimum of Euclidean action of the fluctuations field. Fermionic spectral gap arises inside the Q-balls in the vicinity of the ‘antinodal’ points. The ‘breathing modes’ of the Q-balls in Matsubara time, as well as sharp maximum of specific heat across transition temperature T∗ into Q-balls gas phase are predicted.

Euclidean Q-Balls of Fluctuating SDW/CDW in the ‘Nested’ Hubbard Model of High-Tc Superconductors as the Origin of Pseudogap and Superconducting Behaviors

The origin of the pseudogap and superconducting behaviors in high-Tc superconductors is proposed, based on the picture of Euclidean Q-balls formation that carry Cooper/local-pair condensates inside their volumes. Euclidean Q-balls that describe bubbles of collective spin-/charge density fluctuations (SDW/CDW) oscillating in Matsubara time are found as a new self-consistent solution of the Eliashberg equations in the ‘nested’ repulsive Hubbard model of high-Tc superconductors. The Q-balls arise due to global invariance of the effective theory under the phase rotation of the Fourier amplitudes of SDW/CDW fluctuations, leading to conservation of the ‘Noether charge’ Q in Matsubara time. Due to self-consistently arising local minimum of their potential energy at finite amplitude of the density fluctuations, the Q-balls provide greater binding energy of fermions into local/Cooper pairs relative to the usual Frohlich mechanism of exchange with infinitesimal lattice/charge/spin quasiparticles. We show that around some temperature T* the Q-balls arise with a finite density of superconducting condensate inside them. The Q-balls expand their sizes to infinity at superconducting transition temperature Tc. The fermionic spectral gap inside the Q-balls arises in the vicinity of the ‘nested’ regions of the bare Fermi surface. Solutions are found analytically from the Eliashberg equations with the ‘nesting’ wave vectors connecting ‘hot spots’ in the Brillouin zone. The experimental ‘Uemura plot’ of the linear dependence of Tc on superconducting density ns in high-Tc superconducting compounds follows naturally from the proposed theory.

Origin of pseudo gap and thermoelectric signatures of semimetallic Ru2TaGa: Structural stability from phonon dynamics, mechanical, and thermodynamic predictions

We have studied the origin of the semimetallic pseudo gap and the role of site preference in the L21-type Heusler compound Ru2TaGa. A detailed account of the structural stability, phonon-dependent properties, electronic structure, and thermoelectric properties of the host compound is presented herein. Stability assessments have been made on the basis of phonon band structure, formation energy, mechanical criteria, and hull energy calculations. From the calculated density of states (DOS) and band structure, a semimetallic scenario is evident, with the Fermi level trapped in the pseudo gap. HSE calculations provide a decisive picture, with an indirect overlap between electron and hole pockets near the Fermi level of Ru2TaGa being responsible for the formation of a negative or pseudo gap. Comparison of the Ru eg and Ta t2g states reveals the role of d-d hybridization in pseudo gap formation in Ru2TaGa-like compounds. Additionally, we endeavor to discuss the transport coefficients of Ru2TaGa. The room temperature Seebeck coefficient calculated for Ru2TaGa is 66.42 μV/K, considerably larger than those of other semimetallic compounds of the same family. Considering the lattice thermal conductivity (κL = 2.5 W/mK) at 800 K, we obtain a significant figure of merit of ZT = 0.025 in Ru2TaGa. Although the absolute thermoelectric figure of merit (0.002) is relatively low in Ru2TaGa, it is still the highest among the non-doped full Heuslers of the same family hitherto reported.

First-Principles Calculation of Copper Oxide Superconductors That Supports the Kamimura-Suwa Model

In 1986 Bednorz and Műller discovered high temperature superconductivity in copper oxides by chemically doping holes into La2CuO4 (LCO), the antiferromagnetic insulator. Despite intense experimental and theoretical research during the past 34 years, no general consensus on the electronic-spin structures and the origin of pseudogap has been obtained. In this circumstance, we performed a first-principles calculation of underdoped cuprate superconductors La2-xSrxCuO4 (LSCO) within the meta-generalized gradient approximation of the density functional theory. Our calculations clarify first the important role of the anti Jahn-Teller (JT) effect, the backward deformation against the JT distortion in La2CuO4 by doping extra holes. The resulting electronic structure agrees with the two-component theory provided by the tight-binding model of Kamimura and Suwa (K-S), which has been also used to elucidate the d-wave superconductivity. Our first-principles calculation thus justifies the K-S model and demonstrates advanced understanding of cuprates. For example, the remarkable feature of our calculations is the appearance of the spin-polarized band with a nearly flat-band character, showing the peaky nature in the density of states at the Fermi level.

Origin of pseudo gap in high-TC and Iron based superconductors

Many high-Tc superconductors, Iron based superconductors and other recently discovered superconducting materials shows the signature of existence of a strange Pseudo Gap above the superconducting transition temperature. Various experimental techniques such as NMR, Tunneling and ARPES suggest that pseudo gap does not change with temperature. As evidenced from these experimental results, the Pseudo gap possess the d-wave pairing symmetry similar to superconducting energy gap. The physical properties shows unusual behavior in the non-superconducting state where pseudo gap exist. Based on experimental evidences we argue about the origin of pseudo gap in these superconductors. Although a complete understanding of mechanism is lacking, the existence of preformed cooper indicated by various experimental probes pairs seems to be responsible for the origin of Pseudo gap.

Preformed Cooper Pairs as Origin of Pseudogap in Cuprates

A weak-coupling scenario wherein bosonic preformed electron pairs emerge upon cooling from two-electron correlations can explain the pseudogap phase consisting of segments where a Bogoliubov-like energy-momentum relation gapped spectrum alternates with a normal ungapped one. Bose-Einstein condensation (BEC) of preformed pairs interacting with the background fermions leads to either d- or s wave-like superconducting gaps, the result being sensitive to the magnitude of the total number density of pairs nB at which BEC occurs and becomes possible already for a moderately anisotropic s wave pairing of fermions repelling each other via isotropic coulombic forces. The present model is compatible with the coexistence of both pseudogap and of superconductivity phenomena.

Lattice Instability in High-Temperature Superconducting Cuprates: Polarons Probed by EXAFS

Lattice instability in the planar Cu–O bond in high-temperature superconducting cuprates is probed by the Cu K-edge extended X-ray absorption fine structure (EXAFS). Refined temperature-dependent polarized EXAFS data for high-quality La1.85Sr0.15CuO4 (LSCO) single crystals grown by MBE and TSFZ methods are analyzed and compared with the transport properties. Temperature-dependent oxygen displacement shows a signature of bond splitting at T * and a sharp drop due to superconductivity coherence at T c. Detailed analysis of the anomalous oxygen displacement demonstrates the Cu–O bond splitting, i.e., long and short bonds (ΔR≈0.12 A), which is in good agreement with the recent cluster calculation. The results indicate that the origin of pseudogap is related to the formation of charged dynamical lattice distortions (polarons) that are coexistent with metallic (superconducting) domains.

On the interplay of Jahn-Teller physics and Mott physics in cuprates

The extended two-story house model which is now called the Kamimura-Suwa (K-S) model has clarified how the interplay of Mott physics and Jahn-Teller physics plays an important role in determining the superconducting as well as metallic state of underdoped cuprates. In this paper it is first pointed out for underdoped cuprates that Mott physics leads to the existence of local antiferromagnetic order constructed from the localized spins while that the anti-Jahn-Teller effect as a central issue of Jahn-Teller physics leads to the existence of two kinds of orbitals parallel and perpendicular to a CuO2 plane whose states have nearly the same energy. As a result of the interplay of both physics the K-S model has shown that the exchange interactions between the spins of a localized hole and of a carrier hole play an important role in producing the coexistence of superconductivity and antiferromagnetism in underdoped cuprates. The appearance of d-wave superconductivity even in the phonon-involved mechanism is also shown to be due to the interplay of Jahn-Teller physics and Mott Physics. Brief review of these facts as well as the K-S model is given in this paper. More outstanding result in this paper is that the origin of pseudogap in the deeply underdoped regime has been clarified. In this paper it is shown theoretically for the first time that the so-called T* pseudogap observed in ARPES, STM and tunneling experiments below Tc in underdoped cuprates corresponds to the real transition of photo-excited electrons from the occupied states in the originally conduction band below the superconducting gap to a free-electron state above the vacuum level. Thus we conclude that the T* pseudogap in the underdoped cuprates which increases with decreasing the hole concentration is not ‘pseudo’, but a real gap which exists even below Tc.

Doping Dependence of Pseudogap in LaFeAsO1-xFx

We have performed systematic ultrahigh-resolution photoemission spectroscopy on the iron-based superconductor LaFeAsO 1- x F x ( x =0-14%) to elucidate the evolution of electronic states with doping. We found that the density of states (DOS) around the Fermi level of the superconducting sample is markedly smaller than that of the undoped sample in contrast to conventional phonon-mediated superconductors. We observed a pseudogap in the DOS above the superconducting transition temperature ( T c ) which opens/closes at a temperature ( T * ) well above T c . The energy scale of pseudogap and the T * value are gradually reduced upon fluorine substitution. The origin of pseudogap is discussed in terms of the partial nesting of Fermi surfaces due to the spin-density-wave formation or the short-range spin correlations.