The quasi-onedimensional S = 1/2 antiferromagnet Yb4As3 is studied by using low-temperature measurements of the specific heat C(T,B), thermal expansion α(T,B) and longitudinal elastic mode c11(T,B). As has been previously shown [M. Köppen 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 c11 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) = ∂Δ/∂ε introduced to account for the spin-lattice coupling. (ii) At B ≤ 9 T, the Δ(B) values derived from the various quantities are consistent with Δ(B) ∝ B2/3 as predicted by the quantum sine-Gordon model. (iii) Measurements of C(T,B = const) in dc-fields up to 18 T and of c11(T = const, B) in pulsed fields up to 50 T, however, reveal deviations from this behavior at higher fields. (iv) Isothermal measurements of C11(B) show a sharp increase above 35 T which is almost T-independent for T ≤ 10 K and whose origin is unknown.
ASJC Scopus subject areas
- Statistical and Nonlinear Physics
- Condensed Matter Physics