The magnetic susceptibility of YBa$_{2}$Cu$_{3}$O$_{6+x}$ crystals: unusual Curie behavior and small contributions from charge density waves

TL;DR

This study measures the magnetic susceptibility of YBa₂Cu₃O₆₊ₓ crystals, revealing small effects from charge density waves and attributing Curie behavior mainly to oxygen disorder, with implications for understanding high-temperature superconductivity.

Contribution

The paper provides detailed susceptibility measurements across various doping levels, identifying charge density wave effects and linking Curie terms to oxygen disorder, advancing understanding of magnetic properties in cuprates.

Findings

Curie terms originate from oxygen disorder in Cu-O chains.

Charge density wave effects are observed in susceptibility derivatives.

Helmholtz free energy of CDWs is comparable to thermal energy at certain temperatures.

Abstract

We report measurements of the magnetic susceptibility of twinned single crystals of YBa$_{2}$Cu$_{3}$O$_{6+x}$ from just above their superconducting transition temperatures to 300 K with magnetic fields of up to 5 T applied parallel and perpendicular to the CuO$_2$ planes at 7 values of $x$. Appropriate analysis allows the relatively small, but still important, Curie terms to be separated from other contributions to the susceptibility. Our data support a picture in which the Curie terms arise from oxygen disorder in the Cu-O chains. This agrees with published work on polycrystalline samples where the sample cooling rate was varied, but here we show that the Curie plots flatten out above 200 K. We identify small effects of charge density wave (CDW) instabilities in the temperature ($T$) derivative of the in-plane susceptibility $d\chi_{ab}(T)/dT$ and discuss their $x$-dependence. For $x=$0.67 we make a detailed comparison with published high energy X-ray diffraction data using a minimal model involving Fermi arcs, thereby obtaining values for the CDW energy gap and the Helmholtz free energy in a coherence volume. At 80 and 100 K the latter is comparable with, or smaller than $k_BT$ respectively, highlighting the probable importance of thermal fluctuations. We note that the effect of the Lorentz force on charge carriers in the Fermi arcs could provide a simple mechanism for enhancing the CDWs in high magnetic fields, as suggested by recent experiments.