A modified Ashcroft pseudopotential: application to aluminium

Unified study of the different properties of metals clearly reveals the inadequacy of the empty-core Ashcroft pseudopotential even in the case of simple metals. In the present paper we propose a modification of the one-parameter Ashcroft pseudopotential by assuming the parameterr _c to be wave vector-dependent. This introduces a simple modification of the electron-ion pseudopotential in the reciprocal space. The corresponding potential in the configuration space shows that the abrupt change in the Coulomb potential atr=r _c is replaced by a continuous change spread over a small region near the core boundary. The present model has been used to make a unified study of Al and is found to be a significant improvement over the simple Ashcroft model. The agreement between the calculated and experimental values is found to be quite satisfactory.     

Hall-Effekt und spezifischer elektrischer Widerstand flüssiger Übergangsmetalle

The Hall-coefficient and the electrical resistivity of liquid transition metals and their alloys with mono- and polyvalent simple metals have been measured with a sensitiveac current-ac magnetic field method. The transport properties of liquid transition metals are quite different from the well known behavior of liquid simple metals. The pure liquid metals La, Ce, Pr, Nd and U and also a great number of alloys of transition metals with simple metals show a positive sign of the Hall-coefficient. For alloys of transition metals with polyvalent simple metals we observed negative temperature coefficients of the electrical resistivity over large concentration ranges. This behavior can be understood by a modified form of the Faber-Ziman formula for the electrical resistivity of liquid metals.     

High temperature paramagnon modifications in nearly magnetic metals

The standard paramagnon structure in nearly magnetic metals is shown to be drastically modified at high temperatures. Qualitative consequences for the electron paramagnon temperature dependence of the electrical resistivity are considered. Behavior like that exhibited in αPu resistivity might be expected.     

Pressure-induced superconductivity in CaLi(2)

A search for superconductivity has been carried out on the hexagonal polymorph of Laves-phase CaLi(2), a compound for which Feng, Ashcroft, and Hoffmann predict highly anomalous behavior under pressure. No superconductivity is observed above 1.10 K at ambient pressure. However, high-pressure ac susceptibility and electrical resistivity studies to 81 GPa reveal bulk superconductivity in CaLi(2) at temperatures as high as 13 K. The normal-state resistivity displays a dramatic increase with pressure.     

Structure of liquid metals

A brief review is given of the structure of liquid metals. The structure is examined from the viewpoints of how the pair potential in metals gives rise to various structural properties and how the structure of liquid metals effects other properties. Use is made of the Paskin and Rahman molecular dynamic calculations on liquid sodium to illustrate the insensitive nature of the structure to details in the pair potential. Diffraction data are used to demonstrate that a law of corresponding states exists, at the melting point, for simple liquids, metals and insulators. The molecular dynamic calculations of sodium are also used to demonstrate that some time dependent correlation functions may be more sensitive to the form of the pair potential than the structure factor is to the potential. Some analysis is also made of the role of structure in the electrical resistivity. It is noted that more accurate structure data are needed to answer some of the questions raised in recent attempts to use the structure to extract pair potentials and in detailed correlations of resistivity and structure data.     

Activation energy of protons in copper

The movement of protons in copper is studied using Ashcroft model potential and modified Hartree dielectric function. Octahedral position is found to be more favourable. The calculated activation energy is in good agreement with experiment for 0.31 e effective charge of the proton.     

Calculation of the temperature-dependent energy characteristics of adsorption systems based on transition metals

Self-consistent calculations of the temperature-dependent absorption energy and electron work function on the surface of absorption systems based on transition metals have been performed via the electron-density functional method. The influence of substrate surface relaxation on the energy characteristics of adsorption systems is investigated. Gradient corrections to kinetic and exchange-correlation energies are estimated to describe a strong inhomogeneity arising in the electron system of the subsurface region. The influence of electron-ion interaction on energy characteristics is calculated with the use of the Ashcroft pseudopotential. The temperature-dependent formation of various adsorption structures from transition metal atoms is discussed.     

Bestimmung der statischen Strukturfunktion von Natrium bei 102 und 149°C

X-ray measurements of the structure functionS(q) on high purity sodium (99.99%) are reported. Data were taken at 102 and 149 °C. An analysis of that part of the curve, in the regionq≦2k _F shows some deviations from the neutron measurements of Gingrich and Heaton. Better agreement is obtained with the experimental values of Orton, Shaw and Williams and with the theoretical “hard core” curve of Ashcroft and Lekner. Using two temperature-dependent pseudopotentials (Schneider and Stoll, Ashcroft) the electrical resistivity is calculated using the conductivity formula of Ziman. These results are generally about 2/3 of the experimental value. This is attributed either to an incorrect atomic form factor of sodium or to an inaccurately known pseudopotential. The measurements confirm Greenfield's determination of the ratio Γ_{146°c/102°c}(q). As a test of the general form ofS(q) curves the radial distribution function is calculated and discussed.     

The effect of segregated transition metal ions on the grain boundary resistivity of gadolinium doped ceria: Alteration of the space charge potential

The effect of segregated transition metals on the grain boundary resistivity of 1 mol% Gd-doped ceria has been investigated. The main focus of interest is whether the space charge potential that causes the blocking effect of the grain boundaries of the ceria can be extrinsically modified. The introduction of a small amount (< 0.5 mol%) of transition metals (Fe, Co, Mn and Cu) to 1 mol% Gd-doped ceria results in significant reductions in only the grain boundary resistivities of the samples attributed to exclusive segregation of the transition metals into the grain boundary core. In the case of Co- and Fe-doped samples, the grain boundary resistivity is lowered by an order of magnitude. EELS line scans across the grain boundaries of the Fe-doped sample have indicated that the grain boundaries are free of a secondary phase of transition metal oxide and that the Fe in the grain boundary likely exists as point defects. These results strongly suggest that it is indeed possible to reduce the excess positive charge in the grain boundary core, and thus the grain boundary resistivity in a ceria electrolyte, extrinsically as initially postulated. A defect chemistry model which explains partial counterbalance to the positive grain boundary core charge has been suggested. The resistivity minimum shown for the samples with different Fe concentration indicates that there is an optimum concentration of transition metal in the grain boundary core of the ceria necessary for such a countbalance.     

Electrons in non-crystalline metals —Still a challenging problem

The following aspects of the electronic properties of liquid and amorphous metals are studied: (i) density of states of polyvalent liquid metals by means of finite cluster calculations, (ii) the Hall coefficient of simple metals by the use of a generalised transport equation, (iii) the effect of multiple scattering on the electrical resistivity.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Model pseudopotentials and electronic properties of non-transition metals

Two new model pseudopotentials for electron-ion interaction in metals are proposed which contain a single adjustable parameter r_c, the radius of the ion core. The models are used in the evaluation of several properties like electrical resistivity (liquid and solid metals), thermoelectric power, electron dispersion, Fermienergy, density of states and electronic susceptibilities of certain non-transition liquid metals. The obtained results are largely satisfactory with available theoretical and experimental values. The study reveals that the present model pseudopotentials have improved in most of the cases the previous results of other one parameter models.     

Theory of the interface between a classical plasma and a hard wall

The interfacial density profile of a classical one-component plasma confined by a hard wall is studied in planar and spherical geometries. The approach adapts to interfacial problems a modified hypernetted-chain approximation developed by Lado and by Rosenfeld and Ashcroft for the bulk structure of simple liquids. The specific new aim is to embody self-consistently into the theory a “contact theorem”, fixing the plasma density at the wall through an equilibrium condition which involves the electrical potential drop across the interface and the bulk pressure. The theory is brought into fully quantitative contact with computer simulation data for a plasma confined in a spherical cavity of large but finite radius. It is also shown that the interfacial potential at the point of zero charge is accurately reproduced by suitably combining the contact theorem with relevant bulk properties in a simple, approximate representation of the interfacial charge density profile.     

Excitonic insulator: dependence of energy gap width on carrier density

On the basis of the Ashcroft empty core model potential, the equation for the constant of Coulomb interaction in the theory of excitonic insulator is modified. It is shown that in this case the dependence of the energy gap width on the charge carrier density obeys the Mott criterion in the limit of low densities. The conformity of the theory with some experimental data concerning metal–insulator phase transitions in doped semiconductors and transition metal compounds is discussed.     

电磁感应式液固态金属电阻率定性测量装置及应用

介绍了一种自主研制的非接触式电阻率测量装置,该装置基于电磁感应原理,可实现降温过程中金属从液态到固态各阶段电阻率的定性测量,为实时表征金属材料的结构变化提供了一种有效的测量工具.详细介绍了该装置的设计原理及其结构,同时利用该装置,测量了金属Zn,Sb以及过共晶Zn-70 wt.% Sb合金降温过程中电阻率随温度的变化情况,验证了该装置的可靠性. 关键词： 电阻率 电磁感应 液态金属 结构转变     

Application of a structure factor-potential relationship to the electrical resistivity of the liquid alkali metals

In the Ziman formulation for the electrical properties of liquid metals, the resistivity depends on an average of the product of the structure factor and the pseudopotential. Ascarelli, Harrison and Paskin have derived a relationship for small wave vector between the structure factor and the pseudopotential for liquid metals such as the alkali metals. This formulation has been used over the entire range of wave vector (k = 0 to 2k _F). The resistivities of Na, K, Rb and Cs calculated with no adjustable parameters are within 25% of the observed values, while Li is underestimated by about a factor of five. The temperature dependencies of all but Li (which is anomalously non-linear) are in similar agreement with experiments made at constant volume.     

Raman fingerprint of doping due to metal adsorbates on graphene

The properties of single-layer graphene are strongly affected by metal adsorbates and clusters on graphene. Here, we study the effect of a thin layer of chromium (Cr) and titanium (Ti) metals on chemical vapor deposition (CVD)-grown graphene by using Raman spectroscopy and transport measurements. The Raman spectra and transport measurements show that both Cr and Ti metals affect the structure as well as the electronic properties of the CVD-grown graphene. The shift of peak frequencies, intensities and widths of the Raman bands are analyzed after the deposition of metal films of different thickness on CVD-grown graphene. The shifts in G and 2D peak positions indicate the doping effect of graphene by Cr and Ti metals. While p-type doping was observed for Cr-coated graphene, n-type doping was observed for Ti-coated graphene. The doping effect is also confirmed by measuring the gate voltage dependent resistivity of graphene. We have also found that annealing in Ar atmosphere induces a p-type doping effect on Cr- or Ti-coated CVD-grown graphene.     

Nature of the quantum critical point as disclosed by extraordinary behavior of magnetotransport and the lorentz number in the heavy-fermion metal YbRh2Si2

Physicists are engaged in vigorous debate on the nature of the quantum critical points (QCP) governing the low-temperature properties of heavy-fermion metals. Recent experimental observations of the much-studied compound YbRh₂Si₂ in the regime of vanishing temperature incisively probe the nature of its magnetic-field-tuned QCP. The jumps revealed both in the residual resistivity ??₀ and the Hall resistivity R _H, along with violation of the Wiedemann-Franz law, provide vital clues to the origin of such non-Fermi-liquid behavior. The empirical facts point unambiguously to association of the observed QCP with a fermion-condensation phase transition. Based on this insight, the resistivities ??₀ and R _H are predicted to show jumps at the crossing of the QCP produced by application of a magnetic field, with attendant violation of the Wiedemann-Franz law. It is further demonstrated that experimentally identifiable multiple energy scales are related to the scaling behavior of the effective mass of the quasiparticles responsible for the low-temperature properties of such heavy-fermion metals.     

“Phonon ineffectiveness” and the influence of Coulomb interactions on the phonon controlled conductivity in high resistivity metals

The theory of electronic transport for high resistivity metals developed recently by Belitz and Schirmacher which provides an explanation for the Mooij resistivity anomalies observed in such materials is extended to include the electron-electron Coulomb interaction explicitly. This makes it possible to quantify previous speculations about “phonon ineffectiveness” in disordered metals. It is shown that the weakening of the electron phonon coupling applies only to current relaxation (scattering) but not to density relaxation (phonon-induced tunnelling) processes. It is argued that the presence of density relaxation and not that of the Coulomb induced “phonon ineffectiveness” is the main reason for the occurrence of the Mooij anomalies in high resistivity metals.     

Spin fluctuations in metals: Application to the actinides

We present here a review of the spin fluctuation theory and of its applications to transition and actinide systems, with a particular emphasis to the latter where some very anomalous properties find an explanation in terms of spin fluctuation effects. Firstly, we summarize the development of the spin fluctuation model which had been initially applied to transition metals and alloys such as palladium or Pd–Ni alloys. Then, we present the extension of the paramagnon model to nearly magnetic actinide systems by taking into account explicitly the temperature dependence of the Stoner susceptibility, because the 5f-band of actinides is much narrower than the d-band of transition metals. As a result the paramagnon contribution to the resistivity departs from the usual T ² and T power laws at temperatures higher than the spin fluctuation one and saturates at high temperatures, with eventually the presence of a maximum at intermediate temperatures. We present also the calculation of the other properties of actinide systems, namely the thermal resistivity, the thermoelectric power, the magnetic susceptibility, the specific heat capacity and the NMR relaxation rate, which are generally enhanced by the presence of paramagnons. Finally, we have introduced the concept of ‘antiferromagnetic-like’ spin fluctuations which have a maximum of the q-dependent susceptibility _χ(q) at a q value different from q =0, in contrast to the regular ferromagnetic spin fluctuations; both types of spin fluctuation give the same resistivity behaviour, while they yield a markedly different behaviour of the magnetic susceptibility, in agreement with experiment. The spin fluctuation theory is applied successfully to the different properties of neptunium and plutonium metals and of many nearly magnetic compounds such as UAl₂.     

Temperature dependence of the electrical resistivity in amorphous metals

In amorphous metals the electrical resistivity increases linearly in the temperature range from 2 to 40 K. This result differs fundamentally from the nonlinear behaviour known for crystalline metals and it suggests the conduction electrons not to be scattered by the vibrations of the amorphous point lattice. The temperature dependent part of the resistivity in amorphous metals is explained with scattering of conduction electrons by fluctuations ofp-electrons.