To observe an electronic motion has always been a difficult task. For example, the sliding motion of a charge density wave (CDW) under an electric current has been observed only by a few experiments. Resistivity measurements were the first techniques able to observe this phenomenon [1]. In the sliding regime, the Ohm law is no longer fulfilled and a characteristic electronic noise appears. However, CDW systems, such as blue bronze, are rarely homogeneous and such macroscopic measurements fail to give information at the atomic scale.

Fig. 87: 3D diffraction pattern of the 2kF satellite reflection associated with the CDW. The central peak corresponds to the 2kF satellite reflection. Periodic sequence of satellites flanking the 2kF reflection appear in the sliding regime (indicated by arrows).

X-rays could in principle bring new information since the periodic displacement associated with the CDW can be easily measured by diffraction, leading to 2kF satellite reflections, where kf represents the Fermi wave vector. Unfortunately, the loss of the phase prevents the observation of translation of a whole system by diffraction. Nevertheless, a few consequences of the sliding phenomenon have been measured. When the wave slides, the domain size decreases along the direction transverse to the direction of sliding and the 2kF wave vector changes close to the electric contacts. By using coherent X-ray diffraction at beamline ID01, we have observed another consequence. In the sliding regime, secondary satellites appear in the close vicinity of the 2kF satellite reflection (Figure 87). The period of the corresponding modulation is particularly impressive since it is larger than a micrometre, i.e. 1000 times larger than the period of the charge density wave itself (2/2kF = 10 Å). Such long-range electronic correlations are an unprecedented feature in electronic systems. The existence of this super long-range order could be a direct consequence of the incommensurability of the CDW. In blue bronze, the 2kF wave vector is incommensurate along the chain axis b*, but it is close to the commensurate value 0.750 b*. Thus, the proximity to the commensurability point could result in a lattice of discommensurations, or soliton lattice with a periodic phase increment = 3/4, leading to the secondary satellites.

 

Principal publication and authors

D. Le Bolloc’h (a), V.L. Jacques (a,b), N. Kirova (a), J. Dumas (b), S. Ravy (c), J. Marcus (b), and F. Livet (d), Phys. Rev. Lett. 100, 096403 (2008).
(a) Laboratoire de Physique des Solides bât 510, CNRS, Orsay (France)
(b) Institut Néel, CNRS Grenoble (France)
(c) Synchrotron SOLEIL, L’Orme des merisiers, Saint-Aubin (France)
(d) INP Grenoble CNRS St Martin d’Hères (France)

References

[1] P. Monceau, N.P. Ong, A.M. Portis, A. Meerschaut and J. Rouxel, Phys. Rev. Lett 37, 602 (1976).