Magnetocaloric effect and Grüneisen parameter of quantum magnets with a spin gap

Abstract

We obtained, within a path-integral formalism (mean-field variational Gaussian approximation), analytical expressions for thermodynamic quantities such as magnetization, heat capacity, and the magnetic Grüneisen parameter ΓH of the system of triplons in spin gapped quantum magnets. Near the critical temperature ΓH is discontinuous and changes its sign upon the Bose-Einstein condensation (BEC) of triplons. We predict that in the limit of low temperature T and near the critical magnetic field Hc,ΓH diverges as ΓH∼1/T2, while it scales as ΓH∼1/(H-Hc) as the magnetic field approaches Hc.