Free-electron Picture Research Articles

Low-energy electronic recoil in xenon detectors by solar neutrinos

Low-energy electronic recoil caused by solar neutrinos in multi-ton xenon detectors is an important subject not only because it is a source of the irreducible background for direct searches of weakly-interacting massive particles (WIMPs), but also because it provides a viable way to measure the solar pp and 7Be neutrinos at the precision level of current standard solar model predictions. In this work we perform ab initio many-body calculations for the structure, photoionization, and neutrino-ionization of xenon. It is found that the atomic binding effect yields a sizable suppression to the neutrino-electron scattering cross section at low recoil energies. Compared with the previous calculation based on the free electron picture, our calculated event rate of electronic recoil in the same detector configuration is reduced by about 23%. We present in this paper the electronic recoil rate spectrum in the energy window of 100 eV to 30 keV with the standard per ton per year normalization for xenon detectors, and discuss its implication for low energy solar neutrino detection as the signal and WIMP search as a source of background.

Dirac Node Lines in Pure Alkali Earth Metals.

Beryllium is a simple alkali earth metal, but has been the target of intensive studies for decades because of its unusual electron behavior at surfaces. The puzzling aspects include (i)severe deviations from the description of the nearly free-electron picture, (ii)an anomalously large electron-phonon coupling effect, and (iii)giant Friedel oscillations. The underlying origins for such anomalous surface electron behavior have been under active debate, but with no consensus. Here, by means of first-principles calculations, we discover that this pure metal system, surprisingly, harbors the Dirac node line (DNL) that in turn helps to rationalize many of the existing puzzles. The DNL is featured by a closed line consisting of linear band crossings, and its induced topological surface band agrees well with previous photoemission spectroscopy observations on the Be (0001) surface. We further reveal that each of the elemental alkali earth metals of Mg, Ca, and Sr also harbors the DNL and speculate that the fascinating topological property of the DNL might naturally exist in other elemental metals as well.

Specific heat and thermal conductivity of UCu4+x Al8−x compounds

We report on thermal conductivity and specific heat measurements for eight UCu4+x Al8−x compounds (0 ≤ x ≤ 2.0) as a function of temperature and magnetic field. For this series of compounds, previous magnetic and transport studies indicated a transition from magnetic to a non-magnetic heavy fermion state near x cr ≈ 1.15. This paper presents supplementary specific heat and thermal conductivity studies. The ratio of the specific heat over temperature C/T data on the non magnetic compound with x cr ≈ 1.15 show logarithmic dependence with T, a hallmark of non-Fermi liquid (NFL) behavior due to the proximity of a quantum critical point. Compounds with higher Cu content (x > x cr ) exhibit unusual temperature scaling in the specific heat possibly due to an increase in disorder between Cu and Al. Thermal conductivity data show stark contrast in the behaviors between the magnetic (x = 0.5) and non-magnetic compound (x = 1.75). Our results confirm that a simple free-electron picture is inadequate for the description of the low-temperature thermal conductivity properties in non-magnetic UCu4+x Al8−x compounds.

Physical properties of the V-Al5Cu6Mg2 complex intermetallic phase

The cubic V-Al5Cu6Mg2 phase with 39 atoms in the unit cell is an intermetallic phase with intermediate structural complexity between the simple Laves phase and the complex Bergman phase. Using 27Al NMR spectroscopy, we have determined the electric-field-gradient tensors at the positions of three crystallographically nonequivalent Al sites in the unit cell and confirmed the local site symmetries of these sites, as predicted by the Samson structural model of the V-phase from 1949. The influence of structural complexity on the physical properties of a solid was studied by determining bulk electrical and thermal properties (electrical resistivity, thermoelectric power, Hall coefficient, thermal conductivity and specific heat) and local electronic properties of the V-Al5Cu6Mg2 monocrystal by studying the 27Al NMR Knight shift and the spin-lattice relaxation rate. The experiments reveal that free-electron picture is good approximation to the V-Al5Cu6Mg2 electronic structure, despite the structural complexity of the lattice. The positive thermopower and Hall coefficient reveal that V-Al5Cu6Mg2 is a hole-type electrical conductor. Electrical resistivity shows linear temperature dependence with a positive temperature coefficient, typical of regular metals and alloys. The relatively large T→0 residual resistivity and the low thermal conductivity suggest the presence of quenched structural disorder, very likely intrinsic to the V-Al5Cu6Mg2 structure. We did not find any experimental evidence of a pseudogap close to the Fermi energy in the electronic density of states that could contribute to the electronic stabilization of the structure.

Infrared and microwave shielding of transparent Al-doped ZnO superlattice grown via atomic layer deposition

The system of atomic-layer Al-doped ZnO (AZO) superlattice is rather interesting in that it exhibits a “bipolar transparency,” as it is transparent in the visible and opaque in the infrared to microwave. Here, we report on our measurements of the infrared and microwave shielding properties of thin films (~150 nm) of an Al-doped ZnO superlattice grown via atomic layer deposition (ALD). These optically transparent conductive oxide thin films have large DC electrical conductivity (>50,000 S/m) which increases with Al doping. Their infrared optical properties are well described by a free-electron (Drude) model, which results from the very large carrier concentrations (>3 × 1020 cm−3) resulting from AlOx-heterolayer-doping. It is found that increasing Al concentrations lead to an increase in the relaxation energy which, however, is not strong enough to cause the plasma frequency to red-shift. Microwave shielding properties were investigated in the frequency range from 1 to 30 GHz, and shielding efficiencies as high as ~22 dB were observed, confirming that the free-electron picture extends into the microwave regime. The dynamic conductivity in the microwave range was found to correspond well to the measured DC values. Due to their high electrical conductivity and high microwave shielding efficiency in thin film format, these materials may be desirable for applications in transparent electronics, optically transparent EMI shielding coatings, and heat/microwave reflecting coatings for windows.

Plasmon features in electron energy loss spectra from carbon materials

A phenomenological approach is proposed to derive plasmon energies for C materials as a function of mass density ρ and sp 2 fraction f. It is shown that the energy of the graphite in-plane (π and π + σ) and out-of-plane plasmons, as well as the energy of the diamond bulk (σ) plasmon are correctly reproduced by the model. Crucial factors in this respect – and responsible for the deviation from a free electron picture – are the energy of interband transitions associated with plasmon excitation and the screening of π electrons due to σ electron polarization. Plasmon energies, derived as a function of ρ and f, are discussed.

Ab initiosimulations in liquid caesium at high pressure and temperature

The structure of liquid caesium undergoes a sharp change at around $4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ from a simple liquid to a low-coordination complex structure. This mirrors a similar change in the crystal structure at this pressure. Here, we show that both changes are accurately described in good agreement with experiment by density functional theory calculations, which shed light on the nature of the liquid structure and its electronic origins. Analysis of the wave function character shows $s\text{\ensuremath{-}}d$ hybridization, but not $s\text{\ensuremath{-}}d$ transfer, in both solid and liquid at the pressure of the complex Cs-III phase. This implies that a nearly free electron picture is more appropriate than one based on the atomic orbitals. The similarity of $s\text{\ensuremath{-}}d$ hybridization in crystal and liquid phases indicates that hybridization is a general consequence of densification, rather than being induced by a particular crystal structure. The free-electron picture predicts that stable structures will have diffraction peaks associated with the Fermi vector, and this is borne out by comparison with experiment.

Green functions in the renormalized many-body perturbation theory for correlated and disordered electrons

The ways of introducing and handling renormalizations in the many-body perturbation theory are reviewed. We stress the indispensable role the technique of Green functions plays in extrapolating the weak-coupling perturbative approaches to intermediate and strong couplings. We separately discuss mass and charge renor- malizations. The former is incorporated in a self-consistent equation for the self-energy derived explicitly from generating Feynman diagrams within the Baym and Kadanoff approach. The latter amounts to self-consistent equations for two-particle irreducible vertices. We analyze the charge renormalization initiated by De Domi- nicis and Martin and demonstrate that its realization via the parquet approach may become a powerful and viable way of using the many-body diagrammatic approach reliably in non-perturbative regimes with coopera- tive phenomena induced by either strong interaction or strong static randomness. Electric and magnetic properties of solids are determined by the behavior of valence electrons weakly bound to the atoms forming the crystalline lattice. The behavior of the gas of valence electrons in solids is most profound in metals with an open Fermi surface. Conduction electrons in metals can be described in a number of situations and for various purposes quite accurately by Bloch waves, eigenstates of the Fermi-gas Hamiltonian. We know, however, that electrons even in metals are not a noninteracting gas. They are generally exposed to forces driving them out of the equilibrium state of the Fermi gas. Firstly, no metal is a pure crystal and hence the electrons do not move freely in space. They are scattered on randomly distributed impurities and lattice defects. Secondly, the electrons are neither free and their mutual repulsion cannot be neglected particularly for transition and heavy metals. It is the most important and long standing task of condensed-matter theorists to comprehend and qualitatively capture the deviations in the behavior of electrons in real metals from their idealized representation via Bloch waves. The problem with quantification of the behavior of electrons beyond the almost free-electron picture is in paucity of available tools for accomplishing this task. Even if we can reduce the description of the electron gas in metals to rather simple and generic models, we are unable to solve them exactly, except for a few limiting cases that mostly do not correspond to situations of physical interest. With the increasing power of modern computers the role of numerical solutions and numerical "brute force" approaches has increased. Numerical solutions are usually unbiased and can reach a rather high level of quantitative precision. In some aspects they substitute the missing exact analytic solutions. Numerical solutions are suitable and even indispensable in showing and displaying the trends and some global features of the models studied. They, however, fail in critical regions of phase transitions in the vicinity of singularities in correlation functions. In these

The Molecular Designing of Materials and Devices

AbstractArthur von Hippel, a pioneer in the emergence of modern materials science, had a great goal: “the molecular designing of materials and devices.” In this article, I describe how computational materials theory has evolved over the last half century, helping to transform that goal from dream to reality. I start with two great puzzles of the 1950s: why band theory and the nearly free electron picture work. These were resolved by Landau's quasiparticle theory and by pseudopotential theory, respectively.Together with the creation and development of density functional theory, key methodological advances, and the rapid evolution of computer hardware and software, these two insights have resulted in the achievement of the quantitative prediction of the structures and properties of complex materials. Bandgapengineering and design of multilayer multifunctional materials are given as examples of “molecular design.”

Origin of the complex crystal structures of elements at intermediate pressure

We present a unifying theory for the observed complex structures of sp-bonded elements under pressure on the basis of nearly free electron picture. In the intermediate pressure regime, the dominant contribution to crystal structure arises from Fermi-surface Brillouin zone interactions—structures which allow this are favoured. This simple theory explains the observed crystal structures, transport properties and the evolution of internal and unit cell parameters with pressure and appears to hold for elements in groups I–VI. We illustrate it with experimental data for these elements and ab initio calculations for Li.

Validity of the Julliere model of spin-dependent tunneling

We consider spin-dependent tunneling between two ferromagnets separated by a simple step barrier, and examine four models for the magnetoconductance ratio $\ensuremath{\Delta}G/G:$ A model due to Julliere which characterizes the magnetoconductance solely in terms of the tunneling spin polarization, a model due to Slonczewski which provides an approximate expression for the magnetoconductance of free electrons tunneling through a barrier, the exact expression for the magnetoconductance of free electrons tunneling through a barrier, and the numerical calculation of the magnetoconductance of band electrons in iron tunneling through a barrier. We find that the Julliere model does not accurately describe the magnetoconductance of free electrons tunneling through a barrier. Although Slonczewski's model provides a good approximation to the exact expression for free electrons in the limit of thick barriers, we find that the tunneling of band electrons shows features that are not described well by any free electron picture and which reflect the details of the band structure of iron at the Fermi energy.

The valence of small fullerenes

The production and isolation of small fullerenes and of their stable compounds and the knowledge of their chemistry should pave the way to the syntesis of novel carbon-based cluster-assembled materials like carbon clathrates, hollow diamonds and diamond-like thin films. In this quest, the knowledge of the valence of the small fullerenes is essential. We report here that the small fullerenes C n (20 ≤ n ≤ 32), aside from the well known values associated with the local one electron picture of dangling bonds, display hidden valences connected with the free electron picture of the shell structure of π-electrons.

Thermodynamic properties of the organic superconductor κL-(ET) 2Ag(CF 3) 4·TCE

We present specific-heat, magnetization, and ac-susceptibility measurements of the recently discovered organic superconductor κ L-(ET) 2Ag(CF 3) 4·TCE, where ET (= BEDT-TTF) stands for bisethylenedithio-tetrathiafulvalene and TCE is the neutral solvent 1,1,2-trichloroethane. We observed a very large anisotropy of the critical field B ∗ extracted from ac-susceptibility data in accord with the enhanced ET-layer separation due to the large anions. The strong frequency dependence of B ∗ reveals the weak pinning forces in the material. Magnetization and specific-heat, C, measurements prove the bulk effect of superconductivity. The linear coefficient of C is γ ≈ 50 mJ/molK 2 which is a factor 9 larger than expected from a free-electron picture. Indications for strong coupling are found.

Triplet states and optical absorptions in finite polyenes and conjugated polymers

We study the nature of triplet states in correlated quasi-one-dimensional bands, with emphasis on the half-filled case relevant to conducting polymers. To incorporate both electron-phonon and electron-electron interaction effects, we use a Peierls-extended Hubbard Hamiltonian, which we solve for finite systems via (numerically) exact diagonalization (Lanczös) techniques. We extend our results to polymers using both standard finite-size extrapolation techniques and a novel boundary-condition averaging scheme. First, we examine the nature of the lowest triplet state, focusing on the crossover from the free-electron picture of neutral soliton pairs to the strongly correlated limit of spin excitation. We then examine the optical absorption spectra corresponding to (allowed) triplet-triplet transitions; for this purpose it is important that our novel boundary-condition averaging method maintains sharp gap-edge features while reducing spurious rapid frequency variations produced by the finite-size approximations to the band-to-band continuum. We compare our results to data on triplet transitions in finite polyenes and discuss possible additional experimental implications, including the interpretation of recent spin-dependent photomodulation (SDPM) experiments in both trans-(CH) ∋ and non-degenerate ground-state conjugated polymers.

Perspectives on the Theory of the New High Tc Superconducting Oxides

There is a widespread feeling that the discovery of high temperature superconductors will force us to change our way of thinking about superconductivity in solids. It has steadily emerged that the simple free-electron picture is inadequate, that a new mechanism of superconductivity is most likely at work, and that modifications (if not outright revisions of the many-body theory) are needed. The situation is not entirely without precedent: much the same could have been said of superfluidity in liquid He. That, however, did not cause such a stir: it had been anticipated some 12 years before its eventual discovery, and the transition temperature of liquid He is so low that experiments have been confined to the few laboratories around the world with a milli-kelvin capability. Finally, the normal state of He was already wellunderstood, so that theorists were poised and ready to tackle the problems posed by the superfluid.Contrast the oxides. Even the rather extensive earlier studies of Ba1-x PbxBiO3 and other superconducting oxides did not prepare us for the advances of the past two years. Laboratories all over the world have been able to prepare and study the new superconductors rather easily, although well-characterized samples and incisive experiments have not been so easy to come by. The flood of new information poses a particular challenge for condensed matter theory — to distil the essence of these complicated multicomponent materials and to explain how the genie of high temperature superconductivity has escaped after so many years. Despite the existing understanding of the properties of oxides, much work remains to be done before we have a good grip on the many-body theory of these strongly correlated systems.

The development of the quantum mechanical electron theory of metals: 1900-28

The development of the electron theory of metals from Drude’s free electron picture to Bloch’s quantum mechanical treatment of electrons in crystal lattices reflects in structure the evolution of quantum mechanics itself. As in that development, the steps leading to the quantum theory of metals may be divided into three periods: classical, 1900-26; semi-classical, 1926-8; and modern, late 1928 onwards. The classical period was dominated by the model of Drude and Lorentz in which a metal contained an ideal gas of conduction electrons governed by kinetic theory. Although the failures and contradictions of the model were strikingly apparent by World War I, few useful new concepts were added until Pauli’s crucial application in 1926 of Fermi-Dirac statistics to metals opened up the semi-classical period. In the following two years Sommerfeld, and others in his circle, by further application of the new statistics within the framework of the classical Drude-Lorentz theory, were able to resolve most of that theory’s outstanding difficulties. But it was not until Bloch’s paper in August 1928 that the full machinery of quantum mechanics, developed in 1925-6, was brought to bear on solids, thereby spearheading the creation between 1928 and 1933, by the first generation of theoretical solid-state physicists including Peierls, Wilson, Mott and others, of the modern quantum theory of solids.

Conduction- and localized-electron spin resonance in the lithium–methylamine system: Inferences for the existence of the metallic compound tetramethylaminelithium(zero), Li(CH3NH2)4

Electron spin resonance (ESR) studies of fluid and frozen solutions of lithium in anhydrous methylamine are reported. The composition and temperature dependence of the ESR properties (electron spin relaxation times, electronic ge factor) are indicative of metallic compound formation in the lithium–methylamine system at low temperatures. Marked changes in both the electron spin relaxation properties and the ESR line shapes in the temperature interval 156–158 K are consistent with those expected for a metallic system moving through the melting point. Electron spin relaxation characteristics of the corresponding metal–ammonia compounds Li(NH3)4 and Ca(NH3)6 have been adequately interpreted in terms of a nearly free-electron (NFE) picture for these low electron density materials. In contrast, electronic properties of the compound tetramethylaminelithium(zero), although nominally metallic (σ∼400 Ω−1 cm−1 for a 22 mole% metal fluid solution), cannot be described in the context of a NFE material. In particular, conduction electron spin relaxation properties reveal a breakdown in the Elliott treatment of spin relaxation in pure metals. We take this as evidence for incipient localization in the very strong scattering electronic regime. It is shown that an enhancement of the electron spin relaxation time relative to the predicted Elliott value in Li(CH3NH2)4 arises quite naturally if one considers the increasing importance of electron–electron and electron–phonon interaction in the approach to the metal nonmetal transition. This changeover in electron spin relaxation characteristics in lithium–methylamine solutions is correlated with the corresponding changes in the electronic conductivity. We suggest that the enhancement in the electron spin relaxation time can be used as an indicator, albeit qualitative, of the degree of localization of the electronic wave function in a nominally metallic system. This behavior contrasts markedly with that of nuclear spin relaxation in the lithium–methylamine system, where incipient localization gives rise to an enhancement in the nuclear relaxation rate relative to that predicted for a metallic system.

Embedded cluster calculations for hydrogen on Ni, Pd and Pt

We have developed an Embedded Cluster Model for treating H chemisorption on Ni, Pd and Pt, which is also applicable to other systems. Our model tries to embody in the simplest way the essential physics, believed to be as follows: (i) the d-component of the bond will be local so we take into account the H atom and only its three (or four) nearest neighbours via muffin tins surrounding the atoms; (ii) the sp component is best described in a free-electron picture requiring that the cluster be embedded in the electron gas; (iii) the rise in potential from inner to vacuum potential. The model can be transformed into a novel Anderson-like model. Results show a chemisorption-induced resonance below the d-band in approximately the position observed in UPS measurements. Consequences for the chemisorption energy and (for Ni) magnetization are pointed out. The results imply hybridization between the H 1s and transition metal d and s orbitals.

ON THE STATISTICAL PROPERTIES OF THE ELECTRONIC LEVELS OF SMALL METALLIC PARTICLES

The arguments of Kubo and Gor'kov-Eliashberg concerning the level fluctuations of small metallic particles (s.m.p.) are reviewed. We make plausible that Random Matrix Theory is not applicable to be s.m.p. problem within the free electron picture and when Dirichlet boundary conditions are used. Next, in the frame of an exactly soluble two-dimensional model, we determine the level-spacing-distribution which shows no level repulsion. Finally we calculate numerically the specific heat C, at finite temperature for an assembly of spectra following the random distribution law.

Atomic-size effect in binary alloys

The free-electron model is applied in the form given by Brooks to calculate atomic dimensions in binary alloys. Results are in good agreement with experiments for mono- and divalent components. This may be understood as a confirmation that, at least for these cases, the free-electron picture is quite realistic.