Properties of temperature dependence of threshold in pure and impurity-doped blue bronzes

Temperature dependence of the threshold of pure and impurity-doped blue bronzes has been studied. It is found that for pure blue bronzes the threshold field near 100K has an anomalous peak, but for impurity-doped samples no anomaly is observed in the range of liquid nitrogen temperature, and the value of the threshold field gradually increases with the decrease of temperature. The results show that in the high- quality pure blue bronzes, there may exist an incommensurate to quasi-commensurate transition near 100K. But for the impurity-doped or “dirty” pure samples, such an incommensurate to quasi-commensurate transition near 100K can be extensively restrained by impurities or defects, which leads to a pure incommensurate CDW in a wide temperature range.