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Abstract:
A detailed investigation of the phase transition in chemically well characterized crystalline samples of α′−NaxV2O5(0.95<x<~1) by specific-heat measurements in magnetic fields up to 16 T is presented. A single sharp λ-shaped anomaly close to 33.5 K is observed in cp. 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 ∝H2. 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)K≈2TSP is obtained from the low-temperature specific heat.
