Analysis of a theoretical model for the incommensurate-to-incommensurate phase transition in NbTe4.

Abstract

We use a theoretical model for ${\mathrm{NbTe}}_{4}$ introduced previously, to examine the stability of the \ensuremath{\surd}2 a\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 a incommensurate state with respect to domain-wall distortions that correspond to all possible wave vectors q in the basal plane, and to homogeneous phason strains, at low temperatures in the incommensurate phase. We find that for certain values of the model parameters either the previously discussed single-q, q=(\ensuremath{\pi}/a)x^, state or a homogeneous phason strain state is stable in the low-temperature incommensurate phase relative to the \ensuremath{\surd}2 a\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 a state of the high-temperature incommensurate phase. We determine (for a certain range of the model parameters) the phase boundary between the \ensuremath{\surd}2 a\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 a state and the single-q, q=(\ensuremath{\pi}/a)x^, state; we show that the phase transition corresponding to this boundary is second order.