The d1 layer metals TaS2, TaSe2, NbSe2, in all their various polytypic modifications, acquire, below some appropriate temperature, phase conditions that their electromagnetic properties have previously revealed as ‘anomalous’. Our present electron-microscope studies indicate that this anomalous behaviour usually includes the adoption, at some stage, of a superlattice. The size of superlattice adopted often is forecast in the pattern of satellite spotting and strong diffuse scattering found above the transition. Our conclusions are that charge-density waves and their concomitant periodic structural distortions occur in all these 4d1/5d1 dichalcogenides. We have related the observed periodicities of these CDW states to the theoretical form of the parent Fermi surfaces. Particularly for the 1T octahedrally coordinated polytypes the Fermi surface is very simple and markedly two-dimensional in character, with large near-parallel walls. Such a situation is known theoretically to favour the formation of...

Charge-density waves and superlattices in the metallic layered transition metal dichalcogenides

Intercalation of the layer type transition metal dichalcogenides by a variety of organic molecules, alkali metals, or ‘3d’ transition metals, provides a powerful way to finely tune the electron occupation of the relatively narrow ‘d’ bands met in these solids These transition metal dichalcogenides are highly anisotropic solids, sometimes referred to as ‘two-dimensional’ solids, and the intercalant molecules which are electron donors enter between the layers This can result in profound changes in the electronic properties of the host lattice, and these changes can be understood in terms of charge transfer and increased interlayer separation The phenomena discussed include optical properties, transport properties, super-conductivity, order-disorder phenomena and phase changes, staging, magnetic properties, metal-insulator transitions, Anderson localization, and fast-ion conduction Some possible practical applications are also considered

Electronic properties of intercalation complexes of the transition metal dichalcogenides

One of the great dream experiments in Science is to directly observe atomic motions as they occur. Femtosecond electron diffraction provided the first 'light' of sufficient intensity to achieve this goal by attaining atomic resolution to structural changes on the relevant timescales. This review covers the technical progress that made this new level of acuity possible and gives a survey of the new insights gained from an atomic level perspective of structural dynamics. Atomic level views of the simplest possible structural transition, melting, are discussed for a number of systems in which both thermal and purely electronically driven atomic displacements can be correlated with the degree of directional bonding. Optical manipulation of charge distributions and effects on interatomic forces/bonding can be directly observed through the ensuing atomic motions. New phenomena involving strongly correlated electron?lattice systems are also discussed in which optically induced changes in the potential energy landscape lead to ballistic structural changes. Concepts such as the structural order parameters are now directly observable at the atomic level of inspection to give a remarkable view of the extraordinary degree of cooperativity involved in strongly correlated electron?lattice systems. These recent examples, in combination with time-resolved real space imaging now possible with electron probes, are truly defining an emerging field that holds great promise to make a significant impact in how we understand structural dynamics.This article is dedicated to the memory of Professor David John Hugh Cockayne, a world leader in electron microscopy, who sadly passed away in December.

https://iopscience.iop.org/article/10.1088/0034-4885/74/9/096101/pdf

Femtosecond electron diffraction: heralding the era of atomically resolved dynamics

The development of tabletop femtosecond electron diffraction sources has provided an alternative way of observing atomic motions in crystalline materials. This technique has now been applied to the charge-density-wave material 1T-TaS2, in which modulation of the electron density is accompanied by a periodic lattice distortion. Previous time-resolved studies have revealed the dynamics of the electronic charge density wave, but until now the dynamics of the lattice system has been only indirectly inferred. In this new experiment, atomic motions were observed in response to a 140 femtosecond optical pulse. Periodic lattice distortion was seen to collapse on an exceptionally fast timescale (about 250 fs), indicative of an electronically driven process involving a high degree of cooperativity. The surprisingly high degree of cooperation in the observed structural dynamics between the electronic and lattice system illustrates the potential for the technique in studies of strongly correlated systems.

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Snapshots of cooperative atomic motions in the optical suppression of charge density waves

Electron and X-ray diffraction studies have identified three metastable phases in 1T TaS2. Extra reflexions are interpreted as arising from charge density waves coupled to periodic distortions modulating a trigonal lattice. These waves are incommensurate with the matrix in 1T1 and 1T2, but form a commensurate super-lattice in 1T3. Within 1T2, the temperature dependence of the wave vector is correlated with a change in the cross-section of the Fermi surface, while the diffuse scattering which accompanies the discrete reflexions in 1T1 is discussed as an image of this surface in the phonon spectrum. Changes in the stacking of distortions and in their amplitude are also reported.

The role of charge density waves in structural transformations of 1T TaS2

Diffraction studies have enabled the deformation structure of 1T TaS2to be interpreted in terms of charge density waves arising from a Kohn anomaly. There is good agreement between the observed periodicity of the lattice distortion and the value predicted from a Fermi surface based on recent band structure calculations.

G. S. Parry

Papers

Diffraction evidence for the Kohn anomaly in 1T TaS2