Charge Glassiness and Magnetotransport in Lightly Doped Cuprates

Magnetotransport and resistance noise have been studied in high quality singlecrystals of La2Cu1−xLixO4 and La2−xSrxCuO4 with x=0.03. The detailed measurements are performed at temperatures ranging from 0.03 to 190K and in magnetic fields of up to 18T parallel and perpendicular to the c axis of the crystals. Two specific problems have been addressed: the emergence of both the charge glassiness and positive magnetoresistance at very low temperatures in both materials, and the formation of skyrmions at moderate temperatures in Li doped La2CuO4. The signatures of charge glassiness have been observed for the first time in lightly doped cuprates in this study. It is shown that the magnetoresistance exhibits features of glassy systems such as hysteresis, history dependence and memory effects at temperatures far below the spin-glass transition temperature. The resistance noise spectroscopy has also revealed that the charge dynamics becomes increasingly slow and correlated, i.e. glassy, as temperature goes to 0. All our results support the picture of the spatial segregation of holes into the hole-rich regions in the presence of the hole-poor antiferromagnetic (AF) domains in CuO2 planes, as inferred from other studies and different experimental techniques. However, for the first time, we have disclosed that these hole-rich regions are dynamic at such low temperatures. Furthermore, in the same temperature regime, we have observed that the magnetoresistance becomes positive. Our data strongly indicate that the onset of this positive sign strongly correlates to the emergence of the charge glass dynamics. The complicated behavior of the cuprates, as strongly correlated electron systems, is once again manifested through our observation of the emergence of the skyrmion texture in La2Cu0.97Li0.03O4. Our work, described in Chapter 5, presents the first experimental evidence for quantum (T = 0) skyrmions in the ground state of a doped antiferromagnet. The skyrmions are topologically nontrivial magnetic excitations that can be detected using the magnetoresistance measurements. In particular, we demonstrate that the skyrmions make a clear signature in the magnetotransport, since their formation is responsible for the change in the monotonic increase of the magnetoresistance step size with decreasing temperature.