Nuclear Spectroscopy of Charge Density Waves in Molybdenum Bronzes

Abstract

The transition metal bronzes are ternary compounds of generic formula AxTyOz, where A is usually an alkali metal. The bronzes have been of interest for a number of research fields, ranging from intercalation chemistry, fast ionic transport, to physics of low dimensional conductors. Those of them which exhibit low dimensional electronic properties have been recently reviewed by M. Greenblatt [1]. The existence of charge density wave (CDW) instabilities has been firmly demonstrated for the molybdenum bronzes [2] which include the quasi two dimensional purple bronzes A0.9M06O17 with A = Li, Na, K, Tl and the quasi one dimensional blue bronzes A0.3M0O3 with A = K, Rb, Tl. These latter are maybe the best studied among quasi one dimensional systems in which the charge density waves are depinned by a small electric field [3]. Compared to other systems, large crystals have been grown [4], enabling the determination of the crystalline structure and its deformations below the Peierls transition in detail [5]. The dynamic response to an electric field both at low and high frequencies has been thoroughly investigated [6]. The crystals grow with high purity and the CDW may be depinned at low electric fields so that various properties have been studied in the depinned state also. The microscopic methods, especially nuclear magnetic resonance, contributed considerably to the understanding of both the static and dynamic aspects. As we shall detail in this chapter, it provided an unambiguous test for the incommensurate plane wave nature of the CDW in a broad temperature range [7, 8, 9] and a direct proof for the collective motion [10, 11]. The phase velocity of the depinned wave was measured with high precision [12, 13]. Temporal fluctuations of the CDW phase have also been studied [13] but here a lot remains to be done.