Dielectric Anomalies in Cuprates and Nickelates

Low frequency measurements of the dielectric response of hole-doped insulators La$_{2}$Cu$_{1-x}$Li$_{x}$O$_{4}$ and La$_{2-x}$Sr$_{x}$NiO$_{4}$ find a two order of magnitude decrease in the real part of the dielectric constant $\varepsilon $' as these crystals are cooled below a material- and frequency-dependent temperature T$_{f}$. The real and imaginary parts of $\varepsilon $ exhibit universal scaling in Cole-Cole plots, which are used commonly to analyze classical structural glasses. The relaxation rate $\tau $, obtained from the frequency dependence of T$_{f}$, is best described by $\tau $(T) = a(T-T$_{0})^{-n}$, where T$_{0}$ is the zero-frequency limit of T$_{f}$. This power-law form, consistent with critical scaling, is followed over four decades in frequency. Together, these observations are consistent with glassy charge dynamics arising from collective behavior of doped holes in both the cuprate and nickelate. Intrinsic non-uniformities (stripes in the nickelate and stripes/potential non-stripe patterns in the cuprate) are common to both systems and drive the glassy dielectric behaviors observed. The temperature scale on which charge glassiness develops correlates with the onset of glassy spin dynamics found in NMR, $\mu $SR and susceptibility measurements, which suggests their interrelationship.