Critical properties of the unconventional spin-Peierls system TiOBr

We have performed detailed x-ray scattering measurements on single crystals of the spin-Peierls compound TiOBr in order to study the critical properties of the transition between the incommensurate spin-Peierls state and the paramagnetic state at TC2∼48 K. We have determined a value of the critical exponent β which is consistent with the conventional three-dimensional universality classes, in contrast with earlier results reported for TiOBr and TiOCl. Using a simple power-law-fit function we demonstrate that the asymptotic critical regime in TiOBr is quite narrow, and obtain a value of βasy=0.32±0.03 in the asymptotic limit. A power-law-fit function which includes the first-order correction-to-scaling confluent singularity term can be used to account for data outside the asymptotic regime, yielding a more robust value of βavg=0.39±0.05. We observe no evidence of commensurate fluctuations above TC1 in TiOBr, unlike its isostructural sister compound TiOCl. In addition, we find that the incommensurate structure between TC1 and TC2 is shifted in Q space relative to the commensurate structure below TC1.