Showing papers on "Fermi energy published in 1984"

Magnetic ground-state of a heavy-electron system - u2zn17

Abstract: Measurements of the magnetic susceptibility, specific heat, and electrical resistivity of ${\mathrm{U}}_{2}$${\mathrm{Zn}}_{17}$ reveal a magnetic phase transition of this compound at 9.70 K. Details of the specific heat indicate that the ordering occurs among electrons whose energy spectrum shows an anomalously large density of states at the Fermi energy. This result is of interest with regard to the occurrence of superconductivity in similar systems.

Prediction of electronic surface states in layered materials: graphite

Abstract: Thin graphite films consisting of up to 25 atomic layers are studied with self-consistent local-density all-electron theory and model-potential calculations. Unoccupied surface states confined in the direction of the $c$ axis are predicted at 3.8 eV above the Fermi energy. They are split off from the bulk free-electron interlayer band. These results provide a convincing interpretation of the surface states found in recent inverse photoemission experiments by Fauster et al.

Self-consistent perturbation theory of the anderson model: Ground state properties

Abstract: The zero temperature limit of the self-consistent perturbation theory of a degenerate Anderson impurityin a metal is partially solved with analytical methods. In particular, the energy scales relevant to the system are made explicit and closed expressions are derived for the local level propagator around the Fermi energy and for the dynamical susceptibility. Results are compared with what is known from Fermi liquid theory. The argument that the theory is valid for large degeneracy is disturbed by certain pathologies.

Quantum-size effects in work functions of free-standing and adsorbed thin metal films

Abstract: Work functions versus film thickness have been calculated self-consistently for free-standing Al(111), Cr(100), Rh(111), and Au(100) films, and for one- and two-layer films of Au(100) adsorbed on Cr(100). The Al, Rh, and Au films show work-function changes of several tenths of an electron volt, between one and four layers, confirming earlier results for free-electron metals and extending them to transition and noble metals. For two- through five-layer films, the work function of Cr(100) is only weakly dependent on film thickness, owing to the existence of a prominent surface state at the Fermi energy, which stabilizes the charge-density profile in the surface region. Adsorbed films of Au on Cr show no work-function quantum-size effects (QSE), because there are no Au-localized states near the Fermi energy. The large Cr density of states at ${E}_{F}$ makes it easy for the Au and Cr states to mix. In general, metals with low state densities have low surface energies, and are thus not apt to be thermodynamically stable substrates for high-density-of-states metals. Therefore, there is no obvious candidate for a metal on a metal system in which QSE should be observed.

Magnetization of a two-dimensional electron gas

Abstract: The oscillations of magnetization of a two-dimensional free electron gas with field are derived first in the ideal situationT=0 and sharp Landau levels without electron spin and then for finiteT and for broadened levels. Explicit results are obtained in the limiting approximations that the broadening (or kT) is small or large compared with the separation of Landau levels, and the special situations of a field-independent Fermi energy and a field-independent electron density are discussed. The modifications introduced by taking account of electron spin are then considered, and finally the steady magnetic susceptibility superimposed on the oscillations is discussed.

TlCu2Se2: A p-type metal with a layer structure

Abstract: The ternary compound TlCu 2 Se 2 , which has a ThCr 2 Si 2 -type structure, was characterized by means of miscellaneous physical measurements. It is a p-type metal with a Fermi energy of 1.3 eV. There is one hole (effective mass, 1.3 m 0 ) per formula unit in the valence band. In addition to metallic conduction the delocalized holes give rise to Pauli paramagnetism. There was no conclusive evidence for a phase transition in the temperature range 25–600 K.

Electronic states at the hydrogenated amorphous silicon/silcon nitride interface

Abstract: Electronic states at the hydrogenated amorphous silicon (a‐Si:H)/silicon nitride interface are observed to depend strongly on the order of deposition. When the nitride is on top of the a‐Si:H, there is an interface charge of ∼5×1011 electrons/cm2 and the interface Fermi energy is 0.25 eV from the conduction band. The bottom nitride is also in electron accumulation but with a much smaller space charge. Our results are in marked disagreement with recent data.

Electronic energy bands of lead: Angle-resolved photoemission and band-structure calculations

Abstract: Angle-resolved photoemission with synchrotron radiation was applied to Pb(111) in order to determine the electronic band structure of Pb. The $\ensuremath{\Gamma}L$ direction was mapped out by recording normal-emission photoelectron spectra. Band energies at several symmetry points in the Brillouin zone were obtained by evaluation of off-normal spectra. An adjusted free-electron parabola describes the empty bands well when compared with the critical points resulting from our band-structure calculation using the self-consistent linear rigorous cellular method. The occupied bands close to the Fermi energy agree well with theory, particularly with respect to the magnitude of the spin-orbit splitting, whereas there are discrepancies for the lowest occupied valence band. Inelastic scattering of photoelectrons gives rise to additional peaks in the spectra which cannot be attributed to direct transitions. No surface state was observed in the $s\ensuremath{-}p$ band gap.

A study of the metal-nonmetal transition in lanthanum strontium vanadate by high-resolution electron spectroscopy

Abstract: Lanthanum strontium vanadate, La1-xSrxVO3, has been studied by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and high-resolution electron energy-loss spectroscopy (HREELS) over a composition range 0

Self-consistent electronic structure of tantalum (001): Evidence for the primary role of surface states in driving reconstructions on tungsten (001)

Abstract: The surface linearized augmented-plane-wave method is used to perform self-consistent scalar-relativistic local-density-functional calculations for the Ta(001) surface. Essentially, all of the surface states and resonances which were found on W(001) are also found on Ta(001), except that on Ta(001) most of these surface states are unoccupied; most differences between W and Ta can be described by rigid-band behavior. Unlike tungsten, where the Fermi energy falls in the center of a large surface-state peak in the surface-atom $d$-band projected density of states, this same peak in tantalum lies well above the Fermi energy. The sensitivity of surface states to the potential is demonstrated by the existence on Ta(001) of a highly localized and occupied ${\stackrel{-}{\ensuremath{\Gamma}}}_{1}$ surface state at the center of the Brillouin zone. This state is virtually identical, in all of its properties, to one found on the W(001) surface. On the basis of this calculation for Ta(001) and earlier W(001) calculations, it is concluded that surface states on W do indeed play a role in destabilizing W(001) against the observed reconstructions. The experimental observation that Ta(001) does not reconstruct is explained in this calculation by the fact that the relevant surface states are unoccupied. This is also evident in the absence of $d$-band "dangling bonds" in the contour plots of the Ta charge density near the surface. The theoretical work function for Ta(001) is 4.3 eV, in good agreement with the experimental value of 4.15 eV. The theoretical value for the surface core-level shift is 0.96 eV to greater binding energy, which is about 0.5 eV larger than the experimental value of van der Veen et al. obtained for Ta(111), but the sign of the shift agrees with experiment.

Fermi energy control of vacancy coalescence and dislocation density in melt-grown GaAs

Abstract: A striking effect of the Fermi energy on the dislocation density in melt-grown GaAs has been discovered. Thus, a shift of the Fermi energy from 0.1 eV above to 0.2 eV below its intrinsic value (at high temperature, i.e., near 1100 K) increases the dislocation density by as much as five orders of magnitude. The Fermi energy shift was brought about by n-type and p-type doping at a level of about 10 to the 17th per cu cm (under conditions of optimum partial pressure of As, i.e., under optimum melt stoichiometry). This effect must be associated with the fact that the Fermi energy controls the charge state of vacancies (i.e., the occupancy of the associated electronic states) which in turn must control their tendency to coalesce and thus the dislocation density. It appears most likely that gallium vacancies are the critical species.

High-energy spectroscopic study of the electronic-structure of ube13

Abstract: X-ray photoemission and bremsstrahlung isochromat spectroscopies have been used to probe the occupied and unoccupied states of U${\mathrm{Be}}_{13}$. Between two and three electrons are found to populate the tail of a surprisingly broad $5f$ band (\ensuremath{\sim} 5 eV) of extended states. With our resolution ($\ensuremath{\lesssim}0.5$ eV) it is not possible to observe directly at the Fermi energy any peculiarity of the density of states explaining the extraordinary properties of this compound. However, the drastic differences between the core-level spectra of U and Be indicate that the $5f$ states remain essentially confined around the U atoms and are only weakly hybridized with the $\mathrm{sp}$-band states originating from the Be atoms.

Electron-phonon effects in 4d metals: calculation of coupling constant and resistivity

Abstract: A simple technique for calculating the phonon-limited resistivity ( rho ph) of metals is described which is analogous to the Gaspari-Gyorffy method (1972) for calculating the electron-phonon coupling constant ( lambda ), rho ph and lambda are calculated for all 4d metals and silver. The HCP metals have been treated as cubic, as is common in calculations of lambda . For rho ph, however, this approach turns out to be too rough because it depends strongly on the average Fermi velocity which is very sensitive to crystal structure. In all transition metals investigated the superconducting and transport electron-phonon coupling constants are nearly equal, lambda approximately= lambda tr. Also nearly equal are their electronic parts, eta approximately= eta tr. The only exception is Pd, where eta tr is nearly 1.5 times larger than eta ; this is because the superconducting and transport properties of Pd are defined by different parts of the Fermi surface. Palladium is the only 4d metal where the Mott model of conductivity (s-d scattering) is valid.

Tunnel spectroscopy of electron sub-bands on Si surfaces

Abstract: Reports tunnelling studies of quantised electron space-charge layers on (111) and (100) Si surfaces Planar metal-SiO2-Si junctions were prepared on non-degenerate n- and p-type substrates Metals of different work functions, lower than the Si electron affinity, were used as counter electrodes, yielding surface electron densities in the range (1-20)*1012 cm2 In samples on p-type substrates the surface density of the ionised acceptors was varied by the application of a bias voltage between the substrate and the electron channel From the bias position of the structures induced by the sub-band edges the minimum energies of occupied and empty sub-bands relative to the semiconductor Fermi level are determined as functions of the depletion charge density The surface density of electrons in the nth sub-band is obtained from the difference EF-En The density of states of the lowest sub-band is obtained from a measurement of the Fermi energy in this sub-band as a function of its occupancy, which was varied by changing the depletion charge density The results correspond to a valley degeneracy factor gv=6 and a density-of-states mass mD/m0=036+or-004 for (111) Si and to gv=2 and mD/m0=020+or-002 for (100) Si, which is in accordance with predictions of the effective mass theory

The hall effect in amorphous metals

Abstract: We first derive a new linear response formula for the Hall effect (R H ) showing that R H only depends on electrons at the Fermi energy. We then calculate R H using the random phase model for ‘amorphous’ Ni and show that although d-electrons may contribute substantially to the conductivity they do not dominate in the Hall effect and the Hall coefficient is negative for this model. We also show how the Hall effect may be simulated by computer experiments on electron diffusion in a magnetic field.

The electrical conductivity and thermoelectric power of nickel ferrous ferrite Magnetic ordering effects

Abstract: The conductivity σ and thermoelectric power θ of single crystals of the ferrimagnetic system nickel ferrous ferrite (NixFe3-xO4, with 0 ≤ x ≤ 0·9) have been measured in the temperature range 300 to 1150K. The measurements are interpreted in terms of nearest-neighbour hopping between localized states distributed in energy around the Fermi energy, the finite width of the distribution (or band) arising from ionic and electronic disorder. The measured conductivity is compared with the empirical expression σ= ΓT −1exp(-W/kT), where W is the hopping energy and Γ is a constant. A cusp in d1n(σT)/d(1/T) and an associated thermopower anomaly at the Neel temperature T N are attributed to the onset of magnetic disorder. A ‘two-band’ model has been developed in which the energy levels of spin-up and spin-down electrons are split by the internal exchange field, which gives rise to magnetic ordering. The model explains the major features of the magnetic anomalies. An observed increase in Γ with temperature is ...

The conductivity near a mobility edge

Abstract: An argument is given based on the Kubo-Greenwood formula to show that the zero-temperature conductivity of a three-dimensional degenerate electron gas in the absence of a magnetic field will always tend to zero as e 2/ħξ when the Fermi energy approaches a mobility edge. Here ξ varies with energy just above the Anderson transition in the same way as the localization length just below it. This is in agreement with scaling theory and recent experimental results on Si: P and a-Si-Nb. On the other hand, many experimental results show that when the Fermi energy E F lies below the mobility edge E c, the behaviour of the conductivity is described by where [sgrave]min ∼ 0.03e 2/ha and a is the distance between potential wells. An attempt to explain this result is given. Also for metal-insulator (MI) transitions in a strong magnetic field, [sgrave]min is observed to exist in measurements on InP down to 30 mK. It is proposed that the pre-exponential factor in a field H should be 0.03e 2/hLH , with L = LH = ...

Nearly Gapless Singlet-Pairing in Pure System of Heavy Fermion Superconductor

Abstract: Heavy fermion superconductors, recently found in compounds with I-electrons (such as CeCu2Si2 and UBed, have attracted much attention.i)-6) One of the recent· issues is concerning the type of Cooper pairing; singlet (even parity) or triplet (odd parity) type? Although a nature of heavy fermion itself has not been completely clarified from a theoretical point of view, it is likely from experimental evidences that heavy fermions behave as a fermi liquid with very small Fermi energy of order TK. There was an attempt to explain, along this line, heavy fermion superconductors within the singlet BCS scheme.1l Since the motion of heavy fermion is slower than those of light-electrons and ions, the Coulomb interaction between heavy fermions is screened instantaneously by rapid motions of the latter; so that the interaction potential in k-space between heavy fermions is given by Vk=4Ke 2 /k €(k, 0»0, where €(k, 0) is the static dielectric constant. The reason why the triplet case is considered to be a likely possibility seems to be that Vk is positive for the whole regions of k-space. Indeed, several theorists have recently claimed that the triplet pairing is the most likely possibility.S),9) However, the present authors recently pointed out that s-wave pairing is still possible owing to the spatial correlations among particles, at least in principle, even if Vk is always positive. IO) The experimental results concerning the above issues are also very controversiaL For CeCu2Si., Assmus et aL 5) claimed that the Landau parameter Foa is positive which is in favour of singlet pairing_ On the other hand, Ott et aL6) recently showed that the low temperature specific heat of UBel3 exhibits T 3-dependence, which is in favour of the p-wave superconductivity of ABM-type. Moreover, Kitaoka et aL ll) recently found an unusual nuclear relaxation for CeCu2Si2 which cannot be understood by neither s-wave scheme of simple BCS-type nor p-wave scheme of ABMtype. The purpose of this paper is to point out that a singlet-pairing with nearly gapless excitations is possible for a model in which the interaction between fermions on the same site is strongly repulsive while that on the neighbour sites is appropriately attractive, and that consequences of this model are consistent with existing experiments of heavy fermion superconductors. In particular, a puzzling behavior of nuclear relaxation of CeCu2Si. found by Kitaoka et aL ll) is qualitatively explained by this model. On the basis of a quasi-particle theory of Kondo lattice,10).12) we simulate a heavy fermion system as a half-filled band of fermions with a Hamiltonian

Fermi Liquid Description of Periodic Anderson Hamiltonian

Abstract: A Fermi liquid description of a periodic Anderson Hamiltonian is presented by following Luttinger. The Fermi surface sum rule is proved. It is shown that the specific heat coefficient and the static spin susceptibility at the absolute zero temperature are related with the expansion coefficients of the self-energy of f electrons. The expansion coefficients are estimated in the single-site approximation for Coulomb correlation larger than the width of conduction bands. A possibility is pointed out that two or more than two conduction bands play a role in metallic valence fluctuating rare-earth compounds. The effective mass of quasi-particles depends on the separation between conduction bands.

Surface Effects on Particle Self-Energies

Abstract: The self-energy of a particle has a nearly singular behavior at a free surface of a Fermi liquid, as a result of the coupling to surface excitations. The imaginary part has a linear dependence on energy near the Fermi energy, whereas the effective mass shows a logarithmic divergence. The self-energy is calculated in semi-infinite nuclear matter with use of interactions constrained by self-consistency; the predictions are in qualitative agreement with experiment and more elaborate calculations for finite nuclei.

The influence of a long - ranged potential on the electronic transport properties of amorphous semiconductors

Abstract: We show that the d.c. transport properties of a wide class of amorphous semiconductors are extremely similar if the influence of the temperature dependence of the Fermi energy is taken into account. We show that a satisfactory description of the dc transport properties and of the time-of-flight experiments can be given in terms of carriers in extended states if the spatial variation of the energy of the mobility edge is taken into account properly.

Fermi energy pinning behavior and chemical reactivity of the Pd/GaAs (110) interface

Abstract: The room‐temperature growth of the Pd/n‐GaAs (110) intimate interface has been studied by soft x‐ray core level photoemission spectroscopy. It is shown that Pd reacts with the GaAs surface forming an arsenide compound in which phase segregated Ga metal is included. In addition some As is segregated on the surface of this composite. Using deconvolution of the different core level contributions, the Schottky barrier height of this system is experimentally determined to be 0.9±0.05 eV. The pinning is attributed to the deeper level of the unified defect model.