Planar Cu and O NMR and the Pseudogap of Cuprate Superconductors

Abstract

Recently, an analysis of all available planar oxygen shift and relaxation data for the cuprate high-temperature superconductors showed that the data can be understood with a simple spin susceptibility from a metallic density of states common to all cuprates. It carries a doping dependent but temperature independent pseudogap at the Fermi surface, which causes the deviations from normal metallic behavior, also in the specific heat. Here, a more coherent, unbiased assessment of all data, including planar Cu, is presented and consequences are discussed, since the planar Cu data were collected and analyzed prior to the O data. The main finding is that the planar Cu shifts for one direction of the external magnetic field largely follow from the same states and pseudogap. This explains the shift suppression stated more recently, which leads to the failure of the Korringa relation in contrast to an enhancement of the relaxation due to antiferromagnetic spin fluctuations originally proposed. However, there is still the need for a second spin component that appears to be associated with the Cu 3d(x2−y2) hole to explain the complex Cu shift anisotropy and family dependence. Furthermore, it is argued that the planar Cu relaxation which was reported recently to be rather ubiquitous for the cuprates, must be related to this universal density of states and the second spin component, while not being affected by the simple pseudogap. Thus, while this universal metallic density of states with a pseudogap is also found in the planar Cu data, there is still need for a more elaborate scenario that eludes planar O.

A Different NMR View of Cuprate Superconductors

“…As was noticed in the old picture, the shifts 63 K ⟂ are similar to those of planar O for all directions ( 17 K ); a recent analysis gives more details [27]. Thus, 63 K ⟂ (T, ) shows the same pseudogap behavior as O.…”

“…for which 63 K ∥ (T, ) is also temperature dependent, unlike La 2−x Sr x CuO 4 . Furthermore, if 63 K ∥ (T, ) is temperature dependent it shows the same pseudogap behavior as 63 K ⟂ and planar O [27]. Note that these large differences, between La 2−x Sr x CuO 4 and other materials, cannot be caused by changes in the hyperfine coefficients (since 63 K ⟂ (T, ) and 17 K (T, ) are not affected, as well as the universal relaxation).…”

“…Thus, 63 K ⟂ (T, ) shows the same pseudogap behavior as O. If one inspects all planar Cu data [27] one finds that there are many materials (2)…”

“…It was quite natural that, in the early days, the focus was on comparing the spin shift and relaxation of doped cuprates to those of classical metals and superconductors, given that NMR can measure the electronic spin susceptibility [6][7][8] and delivered the first proof of BCS theory [9,10] with identifying the coherence peak in 27 Al nuclear relaxation. With the cuprates being of type II, even the investigation of the condensed state is possible as the magnetic field penetrates the sample and the residual diamagnetism in the large magnetic fields used for NMR is quite small.…”

“…Very recently, all Cu and O data amassed so far have been compared [27] for an unbiased view of the data, and a number of conclusions could be derived. Foremost, the same pseudogap phenomenon governs the Cu shifts for c⊥B 0 .…”