Short-range spin correlations and pseudogap in underdoped cuprates

Abstract

In this paper we show that local spin-singlet amplitude with d-wave symmetry can be induced by short-range spin correlations even in the absence of pairing interactions. In the present scenario for the pseudogap, the normal state pseudogap is caused by the induced local spin-singlet amplitude due to short-range spin correlations, which compete in the low energy sector with superconducting correlations to make Tc go to zero near half-filling.PACS numbers: 71.10. Fd, 71.27.+a The recent discovery of pseudogap in underdoped high T c cuprates has challenged condensed matter physicists for several years. The pseudogap behavior [1] is observed as strong suppression of low frequency spectral weight below some characteristic temperature T * higher than transition temperature T c . This anomalous phenomenon has been observed in angle resolved photoemission spectroscopy (ARPES), [2,3] One of the most puzzling questions in pseudogap phenomena is why T * has a completely different doping dependence from T c , in spite of their possibly close relation. In this paper we demonstrate that induced local spin-singlet amplitude due to short-range spin correlations can cause a normal state pseudogap with d-wave symmetry even in the absence of pairing interactions.First of all we argue that there are two energy scales in the problem, because the pseudogap appears as a crossover phenomenon according to experiments. The low energy (or long distance) physics of antiferromagnetic (AF) and superconducting (SC) correlations is well captured by a static mean-field approach, while the relatively high energy (or short distance) physics of the pseudogap is invisible in such a study. Thus we resort to fluctuation theory in order to describe the dynamical nature of the pseudogap, and to determine T * and pseudogap size ∆ pg . The mean-field result of the t − J model will be used below solely to find the onset of leading correlations, and to compute mean-field AF and SC order parameters for the calculation of local spin-singlet amplitude.The mean-field t − J Hamiltonian readswhere ε( k) ≃ −2tx(cos k x + cos k y ) − µ with x the hole density,m = 1/ (2N φ d ( k) = cos k x − cos k y and Q is the (commensurate) AF wave vector (π, π) in two dimensions. In this paper we restrict ourselves to a uniform solution which is just enough for our purpose. In a mean-field approximation, meanfield order already sets in when the correlation length reaches roughly one lattice spacing. This forces the above mean-field phase line ( Fig. 1(a)) to be interpreted as the onset of the corresponding short-range correlations. We identify T MF N with another crossover temperature T 0 at which some magnetic experiments such as Knight shift show their maximum. For the parameter (t/J = 3.0) used in this paper, short-range spin correlations disappear at x = x c ≃ 0.19 − 0.20 at low temperature.We introduce spin-singlet [8] correlation function1