Instability of the electron gas in an expanding metal

Structure and Electronic State in a Fluid Alkali Metal: Synchrotron Radiation Studies

Structure and electronic state in a fluid alkali metal have been investigated by using synchrotron radiation. X-ray diffraction and small angle X-ray scattering measurements of fluid rubidium (Rb) were carried out for observing local and intermediate atomic arrangements, and X-ray Compton scattering measurements were carried out for observing electronic state in the fluids. Variation in physical quantities obtained by these measurements has revealed a particular density range of the fluid, where the stability problem of the electron gas becomes an issue.

Experimental observation of density fluctuations in liquid metals associated with liquid-liquid, liquid-gas and metal-nonmetal transitions

We have developed a special technique and succeeded to carry out small-angle x-ray scattering measurements for some liquid metal systems. The purpose is to investigate effects of transitions such as liquid-liquid (LLT), liquid-gas (LGT) and metal-nonmetal (MNMT) transitions on mesoscopic density fluctuations in liquids. In liquid Te systems (Se-Te and Ge-Te mixtures), which show continuous LLT accompanying MNMT, parameters of density fluctuations show maxima almost in the middle of the transition, both in strength and spatial size. This work (and Kajihara et al 2012 Phys. Rev. B86 214202) was the first direct observation that density fluctuations exhibit maximum corresponding to LLT. However in this study, we could not clearly separate the effects of LLT and MNMT on the observed density fluctuations. Thus, we also investigated fluid Hg under high pressure and high temperature conditions, which shows MNMT near a critical point of LGT, to investigate how MNMT affects them. We observed distinct density fluctuations; a strength and a correlation length of them show maxima at around a critical isochore of LGT, and the former is basically consistent with a phase diagram (compressibility) of LGT; they do not show any peaks at MNMT region. Precise analysis revealed that MNMT only affects a shift of another parameter, a short-range correlation length. These results in fluid Hg indicate that the density fluctuations are mainly derived from a critical phenomena of LGT and MNMT does not play any critical role on them. We believe that the latter conclusion also holds true for liquid Te systems; MNMT plays no important role on the density fluctuations in liquid Te systems and LLT is the main origin of them.

Plasmons in Liquid Metals Studied by Inelastic X-ray Scattering

Plasmon is a collective excitation of electrons in materials. Since plasmon can be observed in a wide range of the phase diagram including the solid, liquid, and classical plasma phases, the investigation of the electronic states through the plasmon is of great significance in order to obtain a unified insight into the electronic states in various phases of matter. Inelastic X-ray scattering (IXS) is an ideal tool for such an investigation, because it can be applied to the samples in the liquid state and those in an extreme conditions. In this review, we discuss IXS results on the plasmons in liquid metals, and also describe a formulation to predict the plasmon energy and the lifetime in liquid metals. The formulation takes into account the effect of the ionic structure within the nearly free electron approximation, and reproduces well the experimental results.

The calculation of vapor-liquid coexistence curve of Morse fluid: application to iron

The vapor-liquid coexistence curve of Morse fluid was calculated within the integral equations approach. The critical point coordinates were estimated. The parameters of Morse potential, fitted for elastic constants in solid phase, were used here to apply the results of present calculations to the determination of iron binodal. The properties of copper and sodium were considered in an analogous way. The calculations of pair correlation functions and isobars at liquid phase have shown that only for sodium these potential parameters allow one to obtain agreement with the measurements data. For iron another parameters are necessary to get this agreement in liquid phase. However, they give rise to very low critical temperature and pressure with respect to the estimates of other authors. Consequently, one can suppose that Morse potential is possibly inapplicable to the calculation of high temperature properties of non-alkali metals in disordered phases.

'... a metal conducts and a non-metal doesn't'

In a letter to one of the authors, Sir Nevill Mott, then in his tenth decade, highlighted the fact that the statement '... a metal conducts, and a non-metal doesn't' can be true only at the absolute zero of temperature, T=0 K. But, of course, experimental studies of metals, non-metals and, indeed, the electronic and thermodynamic transition between these canonical states of matter must always occur above T=0 K, and, in many important cases, for temperatures far above the absolute zero. Here, we review the issues-theoretical and experimental-attendant on studies of the metal to non-metal transition in doped semiconductors at temperatures close to absolute zero (T=0.03 K) and fluid chemical elements at temperatures far above absolute zero (T>1000 K). We attempt to illustrate Mott's insights for delving into such complex phenomena and experimental systems, finding intuitively the dominant features of the science, and developing a coherent picture of the different competing electronic processes. A particular emphasis is placed on the idea of a 'Mott metal to non-metal transition' in the nominally metallic chemical elements rubidium, caesium and mercury, and the converse metallization transition in the nominally non-metal elements hydrogen and oxygen. We also review major innovations by D. A. Goldhammer (Goldhammer 1913 Dispersion und absorption des lichtes) and K. F. Herzfeld (Herzfeld 1927 Phys. Rev. 29, 701-705. (doi:10.1103/PhysRev.29.701)) in a pre-quantum theory description of the metal-non-metal transition, which emphasize the pivotal role of atomic properties in dictating the metallic or non-metallic status of the chemical elements of the periodic table under ambient and extreme conditions; a link with Pauling's 'metallic orbital' is also established here.

Towards first-principles understanding of the metal-insulator transition in fluid alkali metals

By treating the electron-ion interaction as a perturbation in the first-principles Hamiltonian, we have calculated the density response functions of a fluid alkali metal to find an interesting charge instability due to anomalous electronic density fluctuations occurring at some finite wavevector Q in a dilute fluid phase above the liquid-gas critical point. Since |Q| is smaller than the diameter of the Fermi surface, this instability necessarily impedes the electric conduction, implying its close relevance to the metal-insulator transition in fluid alkali metals.

Structural and electronic properties of expanding fluid metals

It has been theoretically predicted that interacting electron gas suffers a negative compressibility when the electron density is sufficiently reduced. Dielectric anomaly is another expression of compressional instability in which the static dielectric function of electron gas changes sign from positive to negative. In a medium with a negative dielectric function, like charges can attract. Expanding fluid alkali metals are the ideal system in which such instability of electron gas can be probed via the structural response of ions. We have performed x-ray diffraction and small angle x-ray scattering measurements using synchrotron radiation for expanding fluid rubidium and cesium. On the basis of the experimental results we discuss the existence of compressional instability and dielectric anomaly in three-dimensional electron gas.

Analysis of the electronic structure of liquid rubidium by the methods of ab initio molecular dynamics, linear muffin-tin orbitals and recursion

It is well known that liquid rubidium shows some unusual properties at low densities. The ab initio SIESTA package and the supercell technique within the linear muffin-tin orbital method were used to investigate this phenomenon. Electronic structures of liquid rubidium at different temperatures from the melting point up to the critical point were obtained. The atomic structure for the supercell technique was simulated for a cluster of 4000 atoms by the Schommers method on the basis of experimental structure factors of Rb obtained by Tamura and co-workers at different temperatures (from 373 up to 1973 K). The Kubo-Greenwood formula was applied for the calculations of the melt conductivity. The results obtained indicate that the metal-nonmetal transition in liquid rubidium is not connected to the gap at the Fermi energy in the density of electronic states, but, more likely, with electron localization on some kind of atomic cluster.