Fermi surface and helical spin ordering in Eu metal

The band structure and the Fermi surface of the b.c.c. Eu metal have been calculated by the non-relativistic KKR method. The obtained Fermi surface is grossly similar to that of Andersen and Loucks. We should like, however, to point out essential differences. Our electron surface around H is shaped like a sphere with eight low mounds in eight directions near the HP-axes (called “bumpy sphere”). Our hole surface around P is shaped like a “rounded-off cube”, as was obtained by Andersen and Loucks, but the cross-section perpendicular to the HP-axis for the tetrahedrally located wing (or island) is shaped like a “truncated-triangle”, not like an ellipse. The helical spin ordering at low temperatures can be understood on the basis of the hole Fermi surface. The electron surface around H does not seem to concern the nesting. Some other phenomena are also considered briefly.     

Inclusive projectile fragmentation in the spectator model

Grazing-angle singles spectra for projectile fragments from nuclear collisions exhibit a broad peak centered near the beam velocity, suggesting that these observed fragments play only a “spectator” role in the reaction. Using only this spectator assumption (but not DWBA), we find that a “prior form” formulation of the reaction leads, via closure, to a 〈ψ∥W∥ψ〉-type estimate of the inclusive spectator spectrum, thus relating it to the reaction cross section for the “participant” with the target. We show explicitly that this expression includes an improved multi-channel version of the Udagawa-Tamura formula for the “breakup-fusion” or incomplete-fusion cross section, and identifies it as the fluctuation part of the participant-target reaction cross section. A Glauber-type estimate of the distorted wave functions which enter clearly shows how the width of the peak in the spectator spectrum arises from the “Fermi motion” within the projectile, as in the simpler Serber model, but is modified by the “overlap geometry” of the collision.     

Electronic Structure of FeSe Monolayer Superconductors: Shallow Bands and Correlations

Electronic spectra of typical single FeSe layer superconductor—FeSe monolayer film on SrTiO₃ substrate (FeSe/STO) obtained from ARPES data reveal several puzzles: what is the origin of shallow and the so called “replica” bands near the M-point and why the hole-like Fermi surfaces near the Γ-point are absent. Our extensive LDA+DMFT calculations show that correlation effects on Fe-3d states can almost quantitatively reproduce rather complicated band structure, which is observed in ARPES, in close vicinity of the Fermi level for FeSe/STO. Rather unusual shallow electron-like bands around the M-point in the Brillouin zone are well reproduced. Detailed analysis of the theoretical and experimental quasiparticle bands with respect to their origin and orbital composition is performed. It is shown that for FeSe/STO system the LDA calculated Fe-3d _xy band, renormalized by electronic correlations within DMFT gives the quasiparticle band almost exactly in the energy region of the experimentally observed “replica” quasiparticle band at the Mpoint. However, correlation effects alone are apparently insufficient to eliminate the hole-like Fermi surfaces around the Γ-point, which are not observed in most ARPES experiments. The Fermi surfaces remain here even if Coulomb and/or Hund interaction strengths are increased while overall agreement with ARPES worsens. Increase of number of electrons also does not lead to vanishing of this Fermi surface and makes agreement of LDA+DMFT results with ARPES data much worse. We also present some simple estimates of “forward scattering” electron-optical phonon interaction at FeSe/STO interface, showing that it is apparently irrelevant for the formation of “replica” band in this system and significant increase of superconducting T _c .     

Plasma oscillations of edge Dirac fermions

The dispersion law of one-dimensional plasmons in a quasi-one-dimensional system of massless Dirac fermions has been calculated. Two model two-dimensional systems where bands of edge states filled with such Dirac fermions appear at the edge have been considered. Edge states in the first system, topological insulator, are due to topological reasons. Edge states in the second system, system of massive Dirac fermions, have Tamm origin. It has been shown that the dispersion laws of plasmons in both systems in the long-wavelength limit differ only in the definition of the parameters (velocity and localization depth of Dirac fermions). The frequency of plasmons is formally quantum (ω ∝ ? ^?1/2) and, in the case of the Coulomb interaction between electrons, depends slightly on the Fermi level E _F. The dependence on E _F is stronger in the case of short-range interaction. The quantum features of oscillations of massless one-dimensional Dirac fermions are removed by introducing the mass of Dirac fermions at the Fermi level and their density. Correspondence to the dispersion law of classical one-dimensional plasma oscillations in a narrow stripe of “Schrödinger” electrons has been revealed.     

Relativistic hartree-fock description of high-density nuclear matter

Relativistic Hartree-Fock equations are solved for an infinite system of nucleons and mesons. At high densities there exists a phase transition from a Fermi “sphere” to a Fermi “shell”, characterized by a discontinuity in the heat capacity. The precise value of the critical density is sensitive to the approximations.     

Influence of the metal nanofilm-semiconductor interface on surface properties of the nanofilm: The CO-Yb-Si(111) system

The influence of the “ytterbium nanofilm-single-crystal silicon substrate” interface on properties of the films has been investigated. It has been shown that, if the film thickness is less than 10 monolayers, the Friedel oscillations (standing waves of electron density) generated by the interface affect the work function of the films and the rate of adsorption of CO molecules on their surface. In turn, the CO molecules modify the electronic structure of ytterbium during adsorption on the surface of nanofilms by transforming ytterbium from the divalent to trivalent state. The completely filled layer of adsorbed CO molecules consists of two phases. The first phase is a two-dimensional gas whose molecules weakly interact with each other, but their lone electron pairs form a donor-acceptor bond with the Yb 5d level; as a result, this level is located below the Fermi level and the metal transforms into the trivalent state. After filling the two-dimensional phase, the second (island) phase, in which the CO molecule are bound by horizontal π-bonds, begins to grow. The formation of these bonds becomes possible due to the filling of 2π states in the molecules upon compaction of the adsorbed layer. The considered the adsorbed two-phase layer is responsible for the complete transition of ytterbium into the trivalent state.     

Luttinger theorem and imbalanced Fermi systems

The proof of the Luttinger theorem, which was originally given for a normal Fermi liquid with equal spin populations formally described by the exact many-body theory at zero temperature, is here extended to an approximate theory given in terms of a “conserving” approximation also with spin imbalanced populations. The need for this extended proof, whose underlying assumptions are here spelled out in detail, stems from the recent interest in superfluid trapped Fermi atoms with attractive inter-particle interaction, for which the difference between two spin populations can be made large enough that superfluidity is destroyed and the system remains normal even at zero temperature. In this context, we will demonstrate the validity of the Luttinger theorem separately for the two spin populations for any “Φ-derivable” approximation, and illustrate it in particular for the self-consistent t-matrix approximation.     

Gravitational phase transitions with an exclusion constraint in position space

We discuss the statistical mechanics of a system of self-gravitating particles with anexclusion constraint in position space in a space of dimension d. Theexclusion constraint puts an upper bound on the density of the system and can stabilize itagainst gravitational collapse. We plot the caloric curves giving the temperature as afunction of the energy and investigate the nature of phase transitions as a function ofthe size of the system and of the dimension of space in both microcanonical and canonicalensembles. We consider stable and metastable states and emphasize the importance of thelatter for systems with long-range interactions. For d ≤ 2, there is nophase transition. For d > 2, phase transitions can take place betweena “gaseous” phase unaffected by the exclusion constraint and a “condensed” phase dominatedby this constraint. The condensed configurations have a core-halo structure made of a“rocky core” surrounded by an “atmosphere”, similar to a giant gaseous planet. For largesystems there exist microcanonical and canonical first order phase transitions. Forintermediate systems, only canonical first order phase transitions are present. For smallsystems there is no phase transition at all. As a result, the phase diagram exhibits twocritical points, one in each ensemble. There also exist a region of negative specificheats and a situation of ensemble inequivalence for sufficiently large systems. We showthat a statistical equilibrium state exists for any values of energy and temperature inany dimension of space. This differs from the case of the self-gravitating Fermi gas forwhich there is no statistical equilibrium state at low energies and low temperatures whend ≥ 4. By a proper interpretation of the parameters, our results haveapplication for the chemotaxis of bacterial populations in biology described by ageneralized Keller-Segel model including an exclusion constraint in position space. Theyalso describe colloids at a fluid interface driven by attractive capillary interactionswhen there is an excluded volume around the particles. Connexions with two-dimensionalturbulence are also mentioned.     

The nature of the adsorption bond between graphite islands and iridium surface

To reveal the nature of adsorption bonds between two-dimensional graphite islands and iridium (111) and (100) faces, a study has been made of the adsorption of potassium and cesium atoms on the surface of these systems, using thermal desorption and Auger electron spectroscopy, as well as surface ionization and thermionic emission techniques. The graphite islands are shown to be weakly bound to the iridium substrate by Van der Waals forces. The unsaturated valence bonds at the periphery of the graphite islands are “lowered down” on to the metal. The recess between the graphite layer and the metal is filled by adsorbing particles through defects in the graphite layer. The atoms can penetrate into the recess in two ways: at T > 1000 K directly from the flux incident on the surface, and at T < 1000 K also by migration from the graphite island surface. The adsorption capacity of this state is ～ (2?3) × 10¹⁴cm^-2. Thermal destruction of the islands at T > 1900 K liberates the potassium and cesium atoms from under the graphite islands. Our study suggests that the reason for the “raised” position of the islands lies in the valence bonds of the graphite layer being saturated, the valence bonds of the metal and its crystallographic orientation being less significant. Therefore one may expect the graphite layer to be raised also above other metals as well. The filling by cesium of the recess between the graphite layer and iridium and of the adsorption phase on the graphite surface, does not change the general “graphitic” shape of the carbon Auger peak. This cesium results, however, in a pronounced splitting of the negative spike on the carbon peak (which provides information on its location relative to the graphite layer) indicating the appearance in the valence band of graphite near the Fermi level of two narrow (～ 2?3 eV) regions with an enhanced density of states originating from the presence of the alkali metal.     

On parity violation in α-decay

The Fermi liquid model of α-decay and the shell-model wave functions of Zuker, Buck and McGrory are used to calculate the “regular” and “irregular” (parity non-conserving) α-widths to some J^πT = 2±0 low-lying levels of ¹⁶O. The Pauli corrections in the α-channel wave functions are taken into account.     

Limits placed on the existence of magnetic charge in the proton by the ground-state hyperfine splitting of hydrogen

If the proton contained equal amounts of opposite magnetic charges, as is required by certain “quark” models, the Fermi contact part of the hyperfine interaction would change markedly. This in turn would affect the ground-state hyperfine splitting of atomic hydrogen. Comparison of the theoretical and experimental values of the hyperfine splitting yields δ = ?(1.6±2.7) × 10^?6, where δ is the fraction of the proton magnetic moment due to the magnetic charges. δ is well consistent with zero.     

Hidden Fermi Surface in K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>2–<Emphasis Type="Italic">y</Emphasis></Subscript>Se<Subscript>2</Subscript>: LDA + DMFT Study

In this paper we provide theoretical LDA + DMFT support of recent angle-resolved photoemission spectroscopy (ARPES) observation of the so-called hidden hole-like band and corresponding hidden Fermi surface sheet near Γ-point in the K_0.62Fe_1.7Se₂ compound. To some extent, this is a solution to the long-standing riddle of Fermi surface absence around Γ-point in the K_xFe_2–ySe₂ class of iron chalcogenide superconductors. In accordance with the experimental data, Fermi surface was found near the Γ-point within LDA + DMFT calculations. Based on the LDA + DMFT analysis in this paper it is shown that the largest of the experimental Fermi surface sheets is actually formed by a hybrid Fe-3d ( xy, xz, yz )quasiparticle band. It is also shown that the Fermi surface is not a simple circle as DFT-LDA predicts, but has (according to the LDA + DMFT) a more complicated “propeller”-like structure due to correlations and multiorbital nature of the K_xFe_2–ySe₂ materials. While the smallest experimental Fermi surface around Γ-point is in some sense fictitious, since it is formed by the summation of the intensities of the spectral function associated with “propeller” loupes and is not connected to any of quasiparticle bands.     

Structures of energy spectrum and persistent currents in mesoscopic rings with radial potential barrier in magnetic field

The energy spectrum and the persistent currents are calculated for finite-width mesoscopic annular structures with radial potential barrier in the presence of a magnetic field. The introduction of the tunneling barrier leads to the creation of extra edge states around the barrier and the occurrence of oscillatory structures superimposed on the bulk Landau level plateaus in the energy spectrum. We found that the Fermi energy E _F increases with the number of electrons N emerging many kinks. The single eigenstate persistent current exhibits complicated structures with vortex-like texture, “bifurcation”, and multiple “furcation” patterns as N is increased. The total currents versus N display wild fluctuations.     

(Non)-Abelian gauge field theories of the Fermi gas. Neutron-quark stars

It is indicated that the ground state of Fermi systems with (non)-Abelian gauge interactions has a well defined quantum theory devoid of infrared divergences and mass singularities. This is exploited to develop a systematic quantum theory of the quark gas. The equation of state of the quark gas is evaluated up to second order in the Gell-Mann-Low charge α_S(μ). The analysis based on neutron matter models suggests that the matter in the neutron stars can be in the quark phase provided the color interaction is “moderately” strong i.e. α_S (3 GeV) ? 0.3.     

Photoemission spectroscopy investigation of the electronic structure of carbidic and graphitic carbon on Ni (111)

Photoemission measurements have been carried out on different forms of carbon deposits of catalytic importance on the (111) face of nickel, and give a picture of the electronic structure of “carbidic” and “graphitic” carbon.The photoemission spectra of graphitic overlayers strongly resemble spectra taken on graphite bulk samples but show an (almost) rigid shift of the levels with respect to the Fermi level.The results we obtain for carbidic carbon can be understood on the basis of recent surface electronic structure calculations. In particular a p-like state has been observed very near the Fermi level, which is probably responsible for the high chemical reactivity of carbidic carbon.     

Chiral invariant Gross-Neveu model: Classical versus quantum

We present a unification of different and independently investigated aspects of the chiral invariant Gross-Neveu model. Special emphasis is placed on the relevance of classical (c-number, non Grassmann) spinor solutions of the G-N field equations for the construction, and thus understanding of the respective quantized Fermi model. To get an insight into the “quantum meaning of classical field theory” if specialized to the G-N case, we perform the path integral quantization procedure which first leads to the Fermi oscillator problem, and then, after appropriate generalizations, to the quantum Fermi G-N model. Path integrals are carried out with respect to c-number spinor paths only, and in fact no reference is necessary to the Grassmann algebra methods, which are conventionally used to integrate out fermions.     

A model for the electronic band structure of HMTSeF-TCNQ

An electronic band structure model is constructed for the organic charge- transfer complex HMTSeF-TCNQ, which is semi-metallic under pressure down to the lowest investigated temperatures. Some experimental data is used to estimate the various integrals involved. Admixture of molecular excited states of opposite parity to the ground state quenches the covalency gap and gives rise to a Fermi surface consisting of an ellipsoid of “electrons” and a more cylindrical FS of “holes” with long axes in the c-direction, and rather flat in the b-direction. Excellent agreement with all (independent) experimental observations is obtained.     

Theory of the Half-integer Quantum Hall Effect in Graphene

The unusual quantum Hall effect (QHE) in graphene is described in terms of the composite (c-) bosons, which move with a linear dispersion relation. The “electron” (wave packet) moves easier in the direction [1 1 0 c-axis] ≡ [1 1 0] of the honeycomb lattice than perpendicular to it, while the “hole” moves easier in [0 0 1]. Since “electrons” and “holes” move in different channels, the particle densities can be high especially when the Fermi surface has “necks”. The strong QHE arises from the phonon exchange attraction in the neighborhood of the “neck” surfaces. The plateau observed for the Hall conductivity and the accompanied resistivity drop is due to the superconducting energy gap caused by the Bose-Einstein condensation of the c-bosons, each forming from a pair of one-electron–two-fluxons c-fermions by phonon-exchange attraction. The half-integer quantization rule for the Hall conductivity: (1/2)(2P?1)(4e²/h), P=1,2,..., is derived.     

Influence of pressure on electron spectra of metals and semiconductors

Abstract

We have already reported the results of direct observations of electron-topological phase transition (ETT) in cadmium'. The appearance of new dHvA-frequencies corresponding to the Fermi surface (FS) change, i.e. restoring of folding of hole “monster” and electron “needle” appearance is observed under pressure. In t h i s report we are going to enlarge on the ETT consequences study in cadmium-on the advent of anomalous electronic features in transverse magnetoresistance and thermoelectric power.     

The transformation between the mode representation and the generalized ray representation of a sound field

The “mode generation function” φ(ν), and the “generalized ray generation function” B(u) are introduced for a stratified wave-guide. It is proved that φ(ν) and B(u) are a Fourier transform pair when lateral waves do not appear. Mode representation and the “generalized ray” representation satisfy the Poisson summation formula. Furthermore, the “local conversion” relation between the mode and the ray is considered from the point of view of “member selection”. It is found that “local conversion” is just the result of using the stationary phase approximation in the Fourier transformation of φ(ν) and B(u). This is the same result as obtained by “selective processing” in which the constructive interference between adjoining members (of the ray or mode families) is used.