Anomalous effect of magnetic field on the low-temperature thermal expansion of the Bi2Sr2−xLaxCuO6+δ and Nd2−xCexCuO4−δ HTSC systems in the non-superconducting phase

The thermal expansion of single crystals of the Bi2Sr2−xLaxCuO6+δ high-temperature superconducting (HTSC) system in the insulating phase with compositions having no superconducting transition to a temperature of 1.8 K (x ≥ 0.8) is measured in an arbitrary direction in the (ab) plane in the temperature range of 7–50 K. Temperature regions of material compression upon heating are found. The study of anomalies in magnetic fields of 3 and 6 T, parallel and perpendicular to the c-axis revealed an anisotropic and nonmonotonic effect of the field on thermal expansion. Such anomalies for the n-type Nd2−xCexCuO4−δ HTSC sample also having no superconducting transition are detected for the first time. The results show that the anomaly nature is caused by anisotropic electronic ordering, probably, by the charge density wave in the CuO2 plane and superconductivity fluctuations in the insulating phase.