Specific heat ofα′−NaV2O5in magnetic fields up to 16 T

Abstract

A detailed investigation of the phase transition in chemically well characterized crystalline samples of ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}{\mathrm{Na}}_{x}{\mathrm{V}}_{2}{\mathrm{O}}_{5}(0.95lxl~1)$ by specific-heat measurements in magnetic fields up to 16 T is presented. A single sharp $\ensuremath{\lambda}$-shaped anomaly close to 33.5 K is observed in ${c}_{p}.$ Its shape and height depends sensitively on sodium deficiency in the samples. With magnetic fields along the crystallographic a and b axis the anomaly shifts to lower temperatures $\ensuremath{\propto}{H}^{2}.$ The decrease is identical for both orientations and amounts to only 20% of the decrease predicted by common theories for the spin-Peierls transition. An energy gap in the excitation spectrum of $67(5)\mathrm{K}\ensuremath{\approx}{2T}_{\mathrm{SP}}$ is obtained from the low-temperature specific heat.