Anomalous Noise in the Pseudogap Regime of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathbf{O}}_{7-\delta }$

An unusual noise component is found near and below about 250 K in the normal state of underdoped YBCO and Ca-YBCO films. This noise regime, unlike the more typical noise above 250 K, has features expected for a symmetry-breaking collective electronic state. These include large individual fluctuators, a magnetic sensitivity, and aging effects. A possible interpretation in terms of fluctuating charge nematic order is presented.

Figure 1

(a) Noise power $S_V$ in four consecutive octaves vs $T$ smoothed over the range of 0.3 K for the sample with $T_c=58\mathrm{K}$. Note the significant deviation of the noise power from what would be expected by extrapolating from high $T$ part (solid line). Inset: Dependence of the resistance on temperature; (b) excess noise after subtraction of high-$T$ part (see text) shown by a straight line in (a) for octave 6. The arrow marks $T_E(f)$ (see text). Inset: Dependence of $T_E$ on measurement frequency. Solid line is Arrhenius fit to the data.

Figure 2

(a) Noise power before, during and after application of 6.3 T ($H||c$). (b) Log of ratio of noise power to prefield value immediately after removal of $H$ ($dT/dt>0$) and during 2 consecutive thermal cycles ($dT/dt<0$). Ca-YBCO, smoothed over the range of 3 K. Sample $T_C=62\mathrm{K}$. (c) Persistent changes in $S_V$ that occurred in two samples (samples $A$ and $B$) from the same film with $T_C=85\mathrm{K}$. Significant changes in (b) and (c) are found only below $T_E$.

Figure 3

Boltzmann factor for the fluctuator from the inset vs inverse temperature. Solid line is a fit to a Boltzmann expression. Inset: Discrete two-level fluctuator at two temperatures. Sample: YBCO, $T_C=85\mathrm{K}$.

Figure 4

Amplitude of the excess noise at $T_E(p)$ vs $T_c$ and $\delta$ (oxygen deficiency) for each distinct sample. Circles and triangles are YBCO on LAO substrate 4 and 8—terminal geometry, respectively. Stars and hexagon are Ca-YBCO on LAO substrate with 4 and 8 terminal geometry, respectively. Squares are YBCO on STO substrate with 4 terminal geometry. The solid line is a guide to the eyes.