Normal-state magnetic susceptibility in a bilayer cuprate

Abstract

The magnetic susceptibility of high-Tc superconductors is investigated in the normal state using a coupled bilayer model. While this model describes in a natural way the normal-state pseudogaps seen in c-axis optical conductivity on underdoped samples, it predicts a weakly increasing susceptibility with decreasing temperature and cannot explain the magnetic pseudogaps exhibited in NMR measurements. Our result, together with some experimental evidence suggest that the mechanism governing the c-axis optical pseudogap is different from that for the a-b plane magnetic pseudogap.PACS numbers: 74.80. Dm, 74.25.Jb, Normal-state pseudogaps of underdoped samples have been one of the important subjects in the study of high-T c cuprates in the past few years [1]. Experimentally strong decrease of NMR Knight shift and spin-lattice relaxation rate with decreasing temperature has been observed in various normal-state underdoped samples below some cross over temperature T * [2-6]. These decreases have often been considered in terms of the opening of a "spin gap" due to the antiferromagnetic (AF) spin fluctuations [7][8][9] or, alternatively, the notion of "preformed Cooper pairs" in the case of significant order parameter phase fluctuations [10]. Measurements on dc resistivity for underdoped samples also reveal that the inplane resistivity ρ ab (T ) deviates from a linear in temperature behavior at some temperature consistent with the crossover temperatures T * in NMR experiments [11]. On the other hand, the c-axis optical conductivity σ c (ω) [12][13][14][15] exhibits a striking gap-like depression at low frequency and the c-axis resistivity ρ c (T ) displays a upturn semiconducting feature [11] in the normal-state underdoped samples, characteristic of the formation of some kinds of gap.A central question in the study of pseudogaps is -are the pseudogaps seen in the c-axis optical conductivity related at all to the magnetic pseudogaps seen in NMR experiments? Or, could there be two different mechanisms? While there is currently no consensus on this point, we begin by comparing first some of the properties of these two pseudogaps seen in experiments. Consider the magnitude and anisotropy of these two pseudogaps: (i) Magnitude -Although one does not measure the magnetic pseudogap directly, an analysis done by Williams et al.[16] of Knight-shift data based on quasiparticle excitation spectrum given by1/2 where ∆ k is the normal-state pseudogap shows that the magnitude of magnetic pseudogap is strongly dependent on doping or carrier density (larger gap with lower doping) and scales with the crossover temperature T * . In contrast, the c-axis optical conductivity measured by Homes et al.[13] on YB 2 C 3 O 6+x (YBCO) at different doping x indicates that the frequency ranges where conductivity is suppressed (direct measurement for the size of the gap) are almost independent of doping. Moreover, a compari- [17] result), while the flat spectral weight seen within the pseudogap region in c-axis optical data implies that the optical ps...

s−s*−d-wave superconductor on a square lattice and its BCS phase diagram

“…16 It should also be a useful tool for studies of transport, magnetic or optic properties of the cuprates, where bulk or surface impurities and other inhomogeneities must be taken into account. [17][18][19] We present in this article a detailed study of an effective pairing model, where the kinetic energy term comes from a tight-binding fit to the ARPES band structure of Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212), 20 and we use either an attractive or repulsive on-site interaction plus a nearestneighbors attraction term. In other words: we view the ARPES band structure of Ref.…”