F. Hensel

The applicability of the transport-energy concept to various disordered materials

It is known that in disordered semiconductors with purely exponential energy distribution of localized band-tail states, as in amorphous semiconductors, all transport phenomena at low temperatures are determined by hopping of electrons in the vicinity of a particular energy level, called the transport energy. We analyse whether such a transport level exists also in materials with densities of localized states (DOSs) different from the purely exponential one. We consider two DOS functions with , typical for polymers, heavily doped semiconductors, and, probably, liquid semiconductors and , typical for mixed crystals. It is shown that in both cases the transport energy exists, implying that it also exists for all intermediate forms of the DOS. Special attention is paid to the dependences of the transport level and of its width on the DOS parameters and temperature.

Electrical properties of unusual ionic melts

Abstract The paper reviews the advances that have been made in recent years in the understanding of electrical transport in fully ionized molten salts, partly dissociated molecular liquids and liquid ionic stoichiometric alloys like CsAu. Special emphasis is placed on the recently observed temperature and pressure induced gradual transition between the limiting cases of molecular insulators and ionic melts. At supercritical temperatures salts undergo a continuous transition from an insulating vapour to a highly conducting ionic fluid if the density is increased sufficiently. This transition is due to a shift of the ionization equilibrium between molecules and ions, in favour of the ions, with increasing density. Poorly conducting molten salts and polar substances like water and ammonia also become more ionic, and consequently better conductors, at very high pressures. Recent thermodynamic, magnetic and electrical measurements on liquid alloys which are composed of two metallic elements and which are non-m...

The potential energy curve and dipole polarizability tensor of mercury dimer

Scalar relativistic coupled cluster calculations for the potential energy curve and the distance dependence of the static dipole polarizability tensor of Hg2 are presented and compared with current experimental work. The role of the basis set superposition error for the potential energy curve and the dipole polarizability is discussed in detail. Our recently optimized correlation consistent valence basis sets together with energy adjusted pseudopotentials are well suited to accurately describe the van der Waals system Hg2. The vibrational–rotational analysis of the best spin–orbit corrected potential energy curve yields re=3.74 A, D0=328 cm−1, ωe=18.4 cm−1, and ωexe=0.28 cm−1 in reasonable agreement with experimental data (re=3.69±0.01 A, De=380±25 cm−1, ωe=19.6±0.3 cm−1 and ωexe=0.25±0.05 cm−1). We finally present a scaled potential energy curve of the form ∑ja2jr−2j which fits the experimental fundamental vibrational transition of 19.1 cm−1 and the form of our calculated potential energy curve best (re=...

The electrical conductivity of fluid selenium up to supercritical temperatures and pressures

Abstract The electrical conductivity of fluid selenium has been measured as a function of temperature and pressure to 1750°C and 1200 bars, respectively. The conductivity isobars exhibit strong increases to nearly-metallic behaviour in selenium above 1300°C at supercritical pressures. As the fluid is heated above 1500-1600°C the conductivity isobars drop sharply toward more insulating behaviour in the pressure range investigated here.

Electrical and thermodynamic properties of mercury in the metal-semiconductor transition range

Abstract The paper presents accurate experimental results for the electrical conductivity σ, the equation of state and its derivatives such as isothermal compressibility, thermal expansion coefficient and thermal pressure coefficient, as a function of pressure and temperature to 2500 bar and 1580°C, respectively. Special consideration is given to the range of densities d smaller than 9 g cm−3. The results give considerable evidence, that a temperature-dependent mobility gap (E c − E f) occurs for d < 9 g cm−3. The variation of (E c − E f) with d and T, which is determined from a logarithmic plot of σ versus 1/T and the thermoelectric power S, suggests that (E c − E f) vanishes at d about 8.8 g cm−3. The value of σ0 is about 140–200 ohm−1 cm−1 and in excellent agreement with the prediction of Mott (1971). An attempt is made to analyse the equation-of-state data in terms of a rigid-sphere model. The results indicate the first evidence for a change from metallic cohesion to another interatomic force law in t...

The liquid-vapour phase transition in fluid mercury

Abstract The paper discusses recent experimental results in the liquid-vapour critical region of metals which show that the existence of the metal-non-metal transition noticeably influences the electronic, thermodynamic, structural and interfacial features of the fluid. The main emphasis is on surface-induced phenomena. Reflectivity experiments on mercury against an optically transparent sapphire window close to the vapour-liquid coexistence curve reveal clearly the existence of a prewetting transition of mercury on the sapphire substrate. The transition line, which terminates at high temperatures at a prewetting critical temperature T pwc lying below the bulk critical temperature T c and at low temperatures at the wetting transition temperature T w lies close to the bulk vapour-liquid coexistence curve.

The metal-insulator transition: a perspective

The metal–insulator transition, a quantum phase transition signifying the natural transformation of a metallic conductor to an insulator, continues to be the focus of intense inquiry and debate. The first discussion of the heuristic differences between metals and insulators, and implicitly the critical conditions for the transition between these canonical electronic regimes, dates back to the dawn of the twentieth century. As we approach the end of the century, the precise nature of the metal–insulator transition remains one of the major intellectual challenges in condensed matter science. In this article we present a brief introduction to just some of the key underlying features of this enduring physical phenomenon. The following articles and discussion present a detailed current account of the many facets of the science of the metal–insulator transition.

Will solid hydrogen ever be a metal

At ultra-high pressures, liquid hydrogen becomes metallic. So should solid hydrogen, yet it stubbornly resists. A newly predicted spontaneous asymmetry of molecules in the solid may be the reason.

Equation of state of liquid sulfur and selenium

Abstract Densities of liquid sulfur and selenium have been determined up to supercritical conditions in a newly developed high temperature, high pressure apparatus. The results allow a comparison between the two elements with respect to their critical properties and their molecular composition. The densities of sulfur in the critical region can be described by critical exponents as other molecular fluids. Liquid selenium shows a tendency towards higher coordination numbers at temperatures above 1000°C and pressures above 400 bars.

THE LIQUID-VAPOUR PHASE TRANSITION IN FLUID METALS

Fluid metals are typical examples of materials whose electronic structures depend strongly on the thermodynamic state of the system. The most striking manifestation of this state dependence is the metal–non–metal transition which occurs when the dense liquid evaporates to the dilute vapour or when the fluid is expanded by heating to its liquid–vapour critical point. The main emphasis of the paper is on the intimate interplay between the changes in interparticle forces and the changes in the electronic structure associated with the metal–non–metal transition. The most significant experiments relevant to this question are those on the liquid–vapour coexistence curves, the static– and dynamic–structure factors S(Q) and S(Q,ω), the electrical properties and interfacial features in the liquid vapour–critical region. For example, reflectivity experiments on mercury against sapphire reveal clearly the existence of a prewetting transition. The transition line starts at the wetting transition temperature Tw. Tw lies in the metal–non–metal transition region.