Quantum critical points and their role in high -Tc superconductivity, stripe and pseudogap formation in hole-doped cuprates

Full text: The discovery of the new copper oxide (cuprate) high-Tc superconductors (HTSC) and the intensive studies of the physical properties of these materials have led to the revolutionary era in condensed matter physics [1]. After this discovery, it has become clear that these high-Tc cuprates deviate most strongly from conventional low-Tc superconductors, both in the normal and in the superconducting (SC) state [1, 2]. The breakdown of the well-known one-electron band theory, Fermi-liquid and BCS pairing theories (see Refs. [1, 2]) occurs in a wide range of the phase diagrams of the hole-doped cuprates, and the competitions between different ground states of the cuprates in a substantial region of the phase diagrams, lead to the occurrence of the distinct pseudogaps (PGs) and quantum critical points (QCPs) and to the stripe formation in these systems. The high-Tc superconductivity in the hole-doped cuprates occurs also in this region of their phase diagrams. Although much experimental and theoretical works have been carried out in the past two decades, they have not yet reached any conclusive understanding on the electronic phase diagrams of these high-Tc cuprates, and on the distinct QCPs and their role in high-Tc superconductivity, stripe and PG formation. The exotic properties of the high-Tc cuprates and their understanding require radically new theoretical approaches and physical concepts. In this work, we discuss the possible origins of the QCPs, metal-insulator transitions, PGs, dynamic (metallic or SC) and static (insulating) stripes, and high-Tc superconductivity in hole-doped cuprates. We develop the new and more pertinent theoretical approaches and methods for studying the quantum criticality and the existence of the distinctly different QCPs in these materials at the strong electron-phonon interactions and to examine the role played by the QCPs in high-Tc superconductivity, PG behavior and stripe formation in the cuprate HTSC. We obtain quantitatively the real and adequate phase diagrams of the hole-doped cuprates and focus on key questions related to the entire phase diagram of the different cuprates, from under doped to heavily over doped region. This should allow us to check whether the usual Fermi-liquid and BCS pairing theories valid also for the cases of the intermediate and strong electron-phonon coupling regimes or they break down. We show that the QCPs, dynamic and static stripes, PG behavior and unconventional high-Tc superconductivity are emerged in hole-doped cuprates in the strong and intermediate electron-phonon coupling regimes. We explore two types of polaronic QCPs which result from the self-trapping of hole carriers in doped polar cuprates. The first one corresponds to the carrier segregation or stripe formation and the metal-insulator crossover. We determine the existence possibility of this QCP in a wide range of the phase diagram of the hole-doped cuprates depending on material parameters and dimensionality. We argue that the second polaronic QCP lies hidden under the SC dome in the over doped region of the phase diagrams of the cuprates and it separates the usual and unusual Fermi-liquid states [3] and corresponds to the formation of large temperature independent polaronic PG. This QCP separates also two BCS and non-BCS regimes of superconductivity in the hole-doped cuprates [3]. We analyze within the large (bi)polaron model and the new Fermi-Bose-liquid model [3] the occurrence of BCS-like superconductivity in over doped cuprates and novel high-Tc superconductivity in optimally doped and under doped cuprates. Further, we show that the large bipolaron formation is manifested in hole-doped cuprates as the small temperature independent PG and the third bipolaronic QCP in their phase diagrams. We also discuss the precursor BCS-like non-SC pairing of large polarons leading to the formation of the small temperature dependent pairing PG in optimally doped and underdoped cuprates in the normal state. This pairing PG is unrelated to superconductivity and persists in the SC sta te down to T = 0. Our results are in close agreement with the existing experiments in doped cuprates. The work is supported by STCU grant U3505 (author)