Susceptibility and Knight-shift anomalies in cuprate superconductors.

Abstract

The unconventional temperature variation of the static susceptibility $\ensuremath{\chi}(T)$ that has been discovered in various copper oxide superconductors is explained in terms of a model density of states that has a step shape at an energy threshold ${E}_{0}$ along with a logarithmic Van Hove singularity at the same ${E}_{0}$. Calculations of $\ensuremath{\chi}(T)$ and the Knight shift above the superconducting transition temperature ${T}_{c}$ yield good fits to the YBCO, BSCCO, and LSCO data by adjusting only the Fermi energy $\ensuremath{\mu}$ in correspondence to the oxygen or Sr content, respectively. When $\ensuremath{\mu}$ is right on or slightly below the Van Hove singularity, an upturn in $\ensuremath{\chi}$ occurs as the temperature $T$ is lowered. By contrast, when $\ensuremath{\mu}$ is slightly above the threshold energy ${E}_{0}$, a downturn in $\ensuremath{\chi}$ is achieved as $T$ is lowered. A correlation of these phenomena with experimental data provides insight into the proximity of the Van Hove singularity to $\ensuremath{\mu}$ in several cuprate superconductors. The YBCO and TBCO cuprates with the higher ${T}_{c}$ values exhibit a nearly constant susceptibility that suggests a Fermi energy well removed from the Van Hove singularity. The sensitivity of ${T}_{c}$ as well as the susceptibility to chemical changes may provide tests of electronic mechanisms of electron pairing as well as the BCS theory.