The origin of the pseudogap phase: precursor superconductivity versus a competing energy gap scenario

Abstract

In the last few years evidence has been accumulating that there are a multiplicity of energy scales which characterize superconductivity in the underdoped cuprates. In contrast to the situation in BCS superconductors, the phase coherence temperature Tc is different from the energy gap onset temperature T * . In addition, thermodynamic and tunneling spectroscopies have led to the inference that the order parameter ∆sc is to be distinguished from the excitation gap ∆; in this way, pseudogap effects persist below Tc. It has been argued by many in the community that the presence of these distinct energy scales demonstrates that the pseudogap is unrelated to superconductivity. In this paper we show that this inference is incorrect. We demonstrate that the difference between the order parameter and excitation gap and the contrasting dependences of T * and Tc on hole concentration x and magnetic field H follow from a natural generalization of BCS theory. This simple generalized form is based on a BCS-like ground state, but with self consistently determined chemical potential in the presence of arbitrary attractive coupling g. We have applied this mean field theory with some success to tunneling, transport, thermodynamics and magnetic field effects. We contrast the present approach with the phase fluctuation scenario and discuss key features which might distinguish our precursor superconductivity picture from that involving a competing order parameter. cond-mat/0107275One of the biggest questions which faces the high temperature superconductivity community is determining the origin of the pseudogap phase. It is now becoming clear that pseudogap effects are not restricted to the normal state alone. Moreover, they appear to persist over a wide range of the phase diagram, up to and possibly above optimal doping [1]. Understanding the pseudogap phase is essential in order to find a proper replacement for BCS theory. Indeed, the failure of BCS theory is demonstrated most clearly in thermodynamical[2] and tunneling[3] data which have made it clear that the underlying normal phase below T c contains a (pseudo)gap in the excitation spectrum. The phase diagram, itself, indicates that the larger the pseudogap the lower is T c and in this way the pseudogap appears to compete with superconductivity. This competition has led many researchers to argue that T c and T * must necessarily originate from different physical mechanisms. A recent D-density wave (DDW) theory[4] presents a concrete realization of the "competing energy gap" scenario, first conjectured by Loram and co-workers [2].The present paper summarizes work from our group [5][6][7][8][9][10] which is based on a precursor superconductivity approach to understanding the pseudogap phase. We have been arguing for some time now that pseudogap effects necessarily persist below T c , and moreover, appear to compete with superconductivity -despite their common origin. Our approach is based on a simple physical picture [11] which interpolates smoothly between BCS theory and Bose Ei...

Superfluid Density in the UnderdopedYBa2Cu3O7−

“…The present paper aims at the experimental verification of the theoretical predictions 3,4,5,14,15,16,17 . To fulfil the task, we investigated λ ab (T ) dependences in YBa 2 Cu 3 O 7−x single crystal with the oxygen deficiency varied in the range 0.07 ≤ x ≤ 0.47.…”

“…The n s (T, p)/n 0 dependences calculated in Ref. 15 show that their low temperature slopes decrease with underdoping. An alternative behavior of | dn s (T )/dT | follows from magnetic precursor d-density wave (DDW) scenario of pseudogap 16 .…”

Quasiparticle spectrum of the cuprateBi2Sr2CaCu2O8+δ

“…The computation of E k (ǫ k ) is quite straightforward, leading to the complex valued Σ k (ǫ k ) and hence to the spectral function (30). Formally, the quasiparticle resonance position is given by ReE k = ǫ k + ReΣ k (ǫ k ), and its width by ImE k = ImΣ k (ǫ k ).…”

“…Although the origin is still debated, there is some strong-coupling effect on the quasiparticles : a self-energy is implied [14,15,16,17,18]. A mean-field approach is insufficient in the context of correlated electrons [19,20], coupled spin-charge degrees of freedom [21,22,23], phase fluctuations [24,25,26], or a competing order [27,28,29,30]. Thus the SC state can no longer depend on one parameter.…”

“…The challenge is to understand three contiguous phases (superconducting, pseudogap and normal). However, the T * (p) curve on the overdoped side, where the data is rare, is the subject of hot debate [6,20,26,30], in particular whether or not it crosses the T c (p) dome. The T * inferred from NMR Knight shift, resistivity and specific-heat measurements may correspond to still a higher temperature, such as the onset of anti-ferromagnetic fluctuations, distinct from the vanishing of ∆ P G as observed by ARPES [55].…”