Thermodynamic studies of the field-induced gap in the quasi- one-dimensional S=1/2 antiferromagnet Yb4As3

The quasi-onedimensional S = (1)/(2) antiferromagnet Yb4As3 is studied by using low-temperature measurements of the specific heat C(T,B), thermal expansion alpha(T,B) and longitudinal elastic mode c(11)(T,B). As has been previously shown [M. Koppen et al., Phys. Rev. Lett. 82, 4548 (1999)], finite magnetic fields perpendicular to the spin chains induce a gap in the spin-excitation spectrum (reminiscent of massive, soliton-like excitations) which manifests itself in distinct anomalies in the specific heat and thermal expansion. In this paper, we present an extension of the above work placing special emphasis on the lattice response and the evolution of the gap at higher fields. The main observations are; (i) the field-induced gap causes a minimum in the ell elastic constant both as a function of temperature and field. Applying a simple two-level model allows for a determination of the gap value Delta(B) as well as the constant G(B) = partial derivativeDelta/partial derivativeepsilon introduced to account for the spin-lattice coupling. (ii) At B less than or equal to 9 T, the Delta(B) values derived from the various quantities are consistent with Delta(B) proportional to B-2/3 as predicted by the quantum sine-Gordon model. (iii) Measurements of C(T,B = const) in dc-fields up to 18 T and of c(11)(T = const, B) in pulsed fields up to 50 T, however, reveal deviations from this behavior at higher fields. (iv) Isothermal measurements of c(11)(B) show a sharp increase above 35 T which is almost T- independent for T less than or equal to 10 K and whose origin is unknown