FROM MIXED-VALENCE TO PSEUDO-ONE-DIMENSIONAL SPIN CHAINS IN YB4AS3

According to a recently developed model, the Yb3+-ions in the heavy-fe rmion system Yb4As3 are positioned on parallel, quasi-one-dimensional chains at low temperatures. A small interchain coupling J' leads to a strong magnetic-field dependence of the linear specific-heat coefficie nt gamma. Within linear spin-wave approximation, it is shown that the six spin-wave excitation branches omega(x)(q,H) determine the low-temp erature thermodynamics of Yb4As3. Most importantly, field-induced gaps scaling as Delta similar or equal to root JJ' appear at q = Gamma,A. For J' = 10(-4) J, a good agreement with the experimental variation of gamma(H) is achieved.