High field electron spin resonance experiments on spin-Peierls compounds

The spin-Peierls (SP) transition is still one of the most challenging effects in quasi-one-dimensional magnetism. A few years ago the first inorganic spin-Peierls compound CuGeO3 with T-SP = 14.3 K was discovered, and recently alpha'-NaV2O5 was found to be another inorganic SP system with the highest transition temperature so far observed: T-SP = 35 K. Electron spin resonance (ESR) is the only direct way to probe electron spin dynamics in magnetic fields higher than 12 T, where a transition to an incommensurate magnetic phase can occur. We present ESR results on single crystals of pure and Si-doped CuGeO3 and pure alpha'-NaV2O5. Our experiments were done in a wide frequency range 35-440 GHz in magnetic fields up to 16 T, covering a large temperature range 1.5-100 K. The temperature dependence of the ESR absorption in the D-phase in alpha'-NaV2O5 points to transitions among triplet states, which are separated from the singlet ground state by an energy gap Delta approximate to 85 K for T --> 0. In contrast to (chi)(T) the ESR absorption does not stay finite for T --> 0(.) In the incommensurate phase of slightly Si-doped CuGeO3, (0.2% Si) ESR signals were observed, but their behaviour is much different from the ones in the pure compound. (C) 1998 Elsevier Science B.V. All rights reserved.