We report low-temperature measurements of the specific heat, thermal expansion, and thermal conductivity on the quasi-one-dimensional, effective S = 1/2 Heisenberg antiferromagnet Yb4As3. Distinct field-induced anomalies were found in the above quantities which are well described by the classical sine-Gordon soliton solution for an easy-plane Heisenberg antiferromagnet. Our findings strongly suggest that Yb4As3 represents the first example of an antiferromagnetic S = 1/2 spin-chain system where this type of nonlinear excitation could be identified. [S0031-9007(99)09295-9].

We present results of high-resolution thermal-expansion and specific-heat measurements on single crystalline alpha'-NaV2O5. We find clear evidence for two almost degenerate phase transitions associated with the formation of the dimerized state around 33K: A sharp first-order transition at T1=(33+-0.1)K slightly below the onset of a second-order transition at T2onset around (34+-0.1)K. The latter is accompanied by pronounced spontaneous strains. Our results are consistent with a structural transformation at T1 induced by the incipient spin-Peierls (SP) order parameter above T2=TSP.

We present results of high-resolution thermal-expansion and specific-heat measurements on single crystalline alpha'-NaV2O5. We find clear evidence for two almost degenerate phase transitions associated with the formation of the dimerized state around 33K: A sharp first-order transition at T1=(33+-0.1)K slightly below the onset of a second-order transition at T2onset around (34+-0.1)K. The latter is accompanied by pronounced spontaneous strains. Our results are consistent with a structural transformation at T1 induced by the incipient spin-Peierls (SP) order parameter above T2=TSP.