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

Localization of charge carriers in crystalline solids has been the subject of numerous investigations over more than half a century. Materials that show a metal-insulator transition without a structural change are therefore of interest. Mechanisms leading to metal-insulator transition include electron correlation (Mott transition) or disorder (Anderson localization), but a clear distinction is difficult. Here we report on a metal-insulator transition on increasing annealing temperature for a group of crystalline phase-change materials, where the metal-insulator transition is due to strong disorder usually associated only with amorphous solids. With pronounced disorder but weak electron correlation, these phase-change materials form an unparalleled quantum state of matter. Their universal electronic behaviour seems to be at the origin of the remarkable reproducibility of the resistance switching that is crucial to their applications in non-volatile-memory devices. Controlling the degree of disorder in crystalline phase-change materials might enable multilevel resistance states in upcoming storage devices.

Disorder-induced localization in crystalline phase-change materials

Resistivity saturation is observed in many metallic systems with large resistivities---when the resistivity has reached a critical value, its further increase with temperature is substantially reduced. This typically happens when the apparent mean free path is comparable to the interatomic separations---the Ioffe-Regel condition. Recently, several exceptions to this rule have been found. This colloquium first reviews experimental results and early theories of resistivity saturation. It then describes more recent theoretical work, addressing cases both where the Ioffe-Regel condition is satisfied and where it is violated. In particular, the authors show how the (semiclassical) Ioffe-Regel condition can be derived quantum mechanically under certain assumptions about the system and why these assumptions are violated for high-${T}_{c}$ cuprates and alkali-doped fullerides.

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Colloquium : Saturation of electrical resistivity

This chapter aims to review our recent research results on new bulk amorphous alloys. The main topics are the following: (1) the finding of new amorphous alloys with high glass-forming ability in a number of alloy systems; (2) the mechanism for achieving high glass-forming ability; (3) the fundamental properties of the new amorphous alloys; (4) successful examples of producing bulk amorphous alloys by different techniques of water quenching, metallic mold casting, arc melting and unidirectional zone melting, etc.; (5) the high tensile strength, low Young’s modulus, and high impact fracture energy of nonferrous metal-based bulk amorphous alloys; (6) the soft magnetic properties of Fe- and Co-based bulk amorphous alloys; (7) hard magnetic properties of Nd- and Pr-based bulk amorphous alloys; (8) the viscous flow and microformability of bulk amorphous alloys in a supercooled liquid region, and (9) future aspects of applications. These new results enable eliminating of the limitation of sample shape which has prevented the development of amorphous alloys as engineering materials. They are expected to give rise to a new era of amorphous alloys.

Bulk Amorphous Alloys

The high conductivity of iron and thermal evolution of the Earth’s core

Results are presented of an investigation on the origin of the low temperature coefficient of resistance in NiCr thin films. It is shown that this low temperature coefficient is an intrinsic property of the alloy. Besides NiCr a large number of disordered alloys containing transition metals have similar conduction properties, both in bulk and as a thin film. For these materials a correlation has been found between the resistivity and its temperature coefficient. These anomalous conduction properties are probably caused by the very small electron mean free path in these materials.

J. H. Mooij

Papers

Electrical conduction in concentrated disordered transition metal alloys