Thermodynamics of the frustrated ferromagnetic spin-1/2 Heisenberg chain

We studied the thermodynamics of the one-dimensional J1-J2 spin-1/2 Heisenberg chain for ferromagnetic nearest-neighbor bonds J1 < 0 and frustrating antiferromagnetic next- nearest-neighbor bonds J2 > 0 using full diagonalization of finite rings and a second-order Green-function formalism. Thereby we focus on J2 < |J1|/4 where the ground state is still ferromagnetic, but the frustration influences the thermodynamic properties. We found that their critical indices are not changed by J2. The analysis of the low-temperature behavior of the susceptibility χ leads to the conclusion that this behavior changes from χ ∝ T-2 at J2 < |J1|/4 to χ ∝ T-3/2 at the quantum-critical point J2 |J1|/4. Another effect of the frustration is the appearance of an extra low-T maximum in the specific heat Cv(T) for J2 & |J1|/8, indicating its strong influence on the low-energy spectrum.