Asymmetric field dependence of specific heat in two-leg Heisenberg antiferromagnetic spin ladders with asymmetric leg interactions

Two-leg Heisenberg antiferromagnetic spin-1/2 ladders with asymmetric leg interactions (J1≠J2) are studied using the bond-mean field method. With different J1−J2−J⊥ couplings, magnetization process exhibits distinct properties. For J1:J2:J⊥=4:2:1 and 4:1:2, M-h curves exhibit cusp-type singularities which do not appear for J1:J2:J⊥=2:1:4. Field dependence of specific heat (Cv/T-h curve) has been found to be asymmetric about hm=(h1+h2)/2, h1 and h2 are two critical fields. The cusp in M-h curves and the asymmetric structure of Cv/T-h curves are closely related to their energy spectrums. The linearity of specific heat in the gapless Luttinger-liquid phase and the underlying asymmetric ladder structure can be characterized by Cv/T-h curves.