We report a synchrotron x-ray scattering study of the diluted spin-Peierls (SP) material Cu 12x Mg x GeO 3 . We find that for x . 0 the temperature T m at which the spin gap is established is significantly higher than the temperature T s at which the SP dimerization attains long-range order. The latter is observed only for x , x c ϳ 0.021, while for x . x c the SP correlation length quickly decreases with increasing x. We argue that impurity-induced competing interchain interactions play a central role in these phenomena. PACS numbers: 75.30.Kz, 75.10.Jm, 75.40.Cx, 75.80. + q Low-dimensional quantum spin systems exhibit a variety of intriguing and often counterintuitive properties. A prominent example of such a material is the spin-Peierls (SP) system [1] which consists of an array of one-dimensional (1D) antiferromagnetic spin chains with S
We report a synchrotron x-ray scattering study of the magnetoresistive manganite La0.875Sr0.125MnO3. At low temperatures, this material undergoes an x-ray induced structural transition at which charge ordering of Mn 3+ and Mn 4+ ions characteristic to the low-temperature state of this compound is destroyed. The transition is persistent but the charge-ordered state can be restored by heating above the charge-ordering transition temperature and subsequently cooling. The charge-ordering diffraction peaks, which are broadened at all temperatures, broaden more upon x-ray irradiation, indicating the finite correlation length of the charge-ordered state. Together with the recent reports on x-ray induced transitions in Pr1−xCaxMnO3, our results demonstrate that the photoinduced structural change is a common property of the charge-ordered perovskite manganites.
We report a synchrotron x-ray-scattering study of the diluted spin-Peierls ͑SP͒ material Cu 1Ϫx Mg x GeO 3 . In a recent paper we have shown that the SP dimerization attains long-range order only for xϽx c ϳ0.022 Ϯ0.001. Here we report that the SP transition is reentrant in the vicinity of the critical concentration x c . This is manifested by broadening of the SP dimerization superlattice peaks below the reentrance temperature T r , which may mean either the complete loss of the long-range SP order or the development of a short-range ordered component within the long-range ordered SP state. Very long relaxation times and related marked hysteresis are found in samples with Mg concentrations in the vicinity of x c . The reentrant SP transition is likely related to the competing Néel transition which occurs at a temperature similar to T r . We argue that impurity-induced competing interchain interactions play an essential role in these phenomena.
The critical fluctuations of CuGeO3 have been measured by synchrotron x-ray scattering, and two length scales are clearly observed. The ratio between the two length scales is found to be significantly different along the a axis, with the a axis along the surface normal direction. We believe that such a directional preference is a clear sign that surface random strains, especially those caused by dislocations, are the origin of the long length scale fluctuations.
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