IMPURITY-INDUCED MODULATIONS IN NBSE3 DETECTED BY ATOMIC-FORCE MICROSCOPY

Long-range modulations at room temperature in NbSe3 doped with interstitial impurities have been observed using both atomic-force microscopy and scanning tunneling microscopy. The wavelength of these modulations is a function of the impurity concentration starting as high as ten unit cells in wavelength and changing through a sequence of wavelengths to a wavelength of two unit cells. These modulations are closely connected to the existence of a strong susceptibility anomaly which induces two charge-density waves (CDWs) with onset temperatures of 144 and 59 K, respectively. If the CDWs are quenched above certain critical concentrations, then the room-temperature modulations are also quenched. This general behavior has been observed for eight different interstitial impurities: V, Mn, Cr, Gd, Pd, Fe, Co, and Ni. The impurity-induced modulations are not observed for substitutional impurities such as Ti. In a number of cases interstitial impurities at certain critical concentrations can introduce crystal phases which exhibit a range of electronic properties such as a metal-insulator transition. The presence of the high-temperature modulations has only small effects on CDW formation until the concentration approaches critical values. These effects have been monitored by measuring the temperature dependence of resistance in the range 3004.2 K. The high-temperature modulations result from long-range impurity screening effects and preferred interstitial impurity sites, in the presence of a complex susceptibility response of the electron gas. © 1995 The American Physical Society.