Possibility of electrical conduction in filled bands

It is shown that a moving neutral particle interacting with electrons may cause an “electron drag” within a filled band. The calculation uses perturbation theory and periodic boundary conditions and is based on the one-electron model. WithN being the number and ¯v the average velocity of the electrons, one finds that for largeN the electronic velocity sumN¯v induced by the motion of the neutral particle is independent ofN, i.e. of the size of the system. The lowest-order contributions toN¯v that do not necessarily vanish are seen to be those of second order in the interaction potential. These second-order contributions are studied. In a simple one-dimensional model they are found to be, in fact, not necessarily zero and to be proportional to the velocity of the neutral particle. An order-of-magnitude formula forN¯v is derived for this case. The calculation suggests that mobile neutral particles may act as charge carriers, their effective charge possibly being much smaller than the elementary charge. In real systems, neutral particles which interact with electrons might be represented by phonons and excitons.     

Competing orders in one-dimensional spin-3/2 fermionic systems

Novel competing orders are found in spin-3/2 cold atomic systems in one-dimensional optical traps and lattices. In particular, the quartetting phase, a four-fermion counterpart of Cooper pairing, exists in a large portion of the phase diagram. The transition between the quartetting and singlet Cooper pairing phases is controlled by an Ising symmetry breaking in one of the spin channels. The singlet Cooper pairing phase also survives in the purely repulsive interaction regime. In addition, various charge and bond ordered phases are identified at commensurate fillings in lattice systems.     

Omni-directional reflection bands in one-dimensional plasma dielectric photonic crystals

In this paper the omni-directional reflection bands in one-dimensional plasma photonic crystal (PPC) have been studied theoretically. We present the study of plasma photonic crystal, having alternate regions of plasma?dielectric (Al₂O₃ or ZnS). Reflectances from this periodic multilayered structure in TE- and TM-modes are calculated for different angles of incidence in microwave region for omni-directional reflection bands. The reflectance is obtained by solving a Maxwell's equation using a translational matrix method. In addition to this, we have also studied the effect of variation of plasma width as well as plasma density on the reflection properties of plasma dielectric photonic crystal in TE- and TM-modes. The study of reflectance bands of such plasma photonic crystals shows that it can be used as omni-directional reflector.     

Many-body effects in the electron spectroscopies of incompletely filled bands

Summary Photoemission and Auger line shapes from almost completely filled bands have been widely discussed in recent years within a simplified model based on an Anderson Hamiltonian in which the virtual level shift due to the interactions is suitably compensated for. Up to now, the theory has been much more successful with XPS than with AES, and the reason for this was obscured by the lack of an exact solution and by the difficulty to assess the degree of validity of various approximate treatments that have been proposed. Here we present a Green’s function formalism that allows us to extend the closed band solution to the partially occupied case and lends itself to the exact numerical treatment of finite systems. By applying the theory to 27 and 125 atom clusters, we analyse the dependence of the spectra on hole-hole repulsionU with a degree of unfillingn _h≤0.25. We also consider the case when one of the spin subbands is full as a rough model for ferromagnetic metals. Correlation effects on the one-hole density of states produce a narrowing of the band region, while a split-off structure develops below the band forU comparable to the band width. The low-density approximation is good agreement with the exact results forn _h=0.1 and also forn _h=0.25 for small and moderateU. Our results on the Auger line shapes justify somewhat the suggestion by Haak and Bennetet al. that split-off states observed in photoemission must be discarded before computing the two-hole spectrum. Indeed self-energy corrections must be excluded also in bandlike cases, when the simple procedure of cutting off the unwanted structure is not applicable. This arises because, in a wide range of physical situations, the Auger line shape reflects the mutual scattering of undressed final-state holes.

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Riassunto Il modello teorico piú usato negli ultimi anni per le forme di riga di fotoemissione ed Auger di bande quasi completamente occupate si basa su un hamiltoniano di Anderson in cui lo spostamento del livello virtuale dovuto alle interazioni è opportunamente compensato. Finora la teoria ha dato risultati molto migliori per la fotoemissione che per lo spettro Auger. Le ragioni di ciò non sono chiare poiché non si dispone di una soluzione esatta ed è difficile valutare l’attendibilità delle varie approssimazioni note. Pertanto noi proponiamo un formalismo basato sulle funzioni di Green che permette, nel caso di aggregati finiti di atomi, di estendere la soluzione esatta, nota nel caso di bande chiuse, al caso di bande parzialmente occupate. Applicando il metodo a sistemi di 27 e 125 atomi, analizziamo la dipendenza degli spettri dalla repulsioneU tra lacune, per una concentrazione di lacunen _h≤0.25. Abbiamo considerato anche il caso in cui una delle sottobande di spin è completamente occupata come una prima approssimazione al problema di metalli ferromagnetici. Gli effetti di correlazione restringono il continuo nella densità di stati ad una particella e producono una struttura discreta sotto la banda seU è confrontabile con la larghezza della stessa. L’approssimazione di bassa densità è in buon accordo coi risultati esatti pern _h=0.1 ed anche pern _h=0.25 seU è piccolo o moderato. I nostri risultati sulla forma di riga Auger legittimano in qualche modo quanto suggerito da Haak e Bennetet al., e cioè che il picco satellite di fotoemissione deve essere soppresso nel calcolo della forma di riga Auger. Anzi, tutte le correzioni al propagatore ad un corpo, dovute agli effetti di correlazione, debbono essere escluse anche nel caso di spettri di tipo banda, dove, per l’assenza della struttura satellite in fotoemissione, il procedimento di Bennettet al. non è piú applicabile: infatti nella maggior parte dei casi d’interesse fisico, la forma di riga Auger riflette la dinamica di lacune non rinormalizzate.

     

Experimental evidence for spin-split bands in a one-dimensional chain structure

Gold atom chains on vicinal Si(111) surfaces exhibit an unusual doublet of half-filled bands, whose origin has remained uncertain. The splitting is identified by angle-resolved photoemission as a spin splitting induced by the spin-orbit interaction (Rashba effect), in agreement with a theoretical prediction by Sánchez-Portal, Riikonen, and Martin. This interaction leads to a characteristic pattern of avoided band crossings at a superlattice zone boundary. Two out of four crossings are avoided, with a minigap E_{G}=85 meV and a k offset of 0.05 A;{-1}.     

Frequency bands of negative refraction in finite one-dimensional photonic crystals

We have discussed theoretically the negative refraction in finite one-dimensional (1D) photonic crystals (PCs) composed of alternative layers with high index contrast. The frequency bands of negative refraction are obtained with the help of the photonic band structure, the group velocity and the power transmittance, which are all obtained in analytical expression. There shows negative transverse position shift at the endface when negative refraction occurs, which is analysed in detail.     

Multiple narrow bandpass optical filters based on one-dimensional rugate photonic structures of two periodicities

A theoretical analysis of a design of multiple narrow bandpass filters based on one-dimensional (1D) rugate photonic structures with a period jump defect is presented. The optical properties, including transmittance and energy density distributions, are numerically calculated using the propagation matrix method. Our results show that multiple resonance transmission modes are produced when the period jump defect is introduced into the 1D rugate film. Both blueshift and redshift of the stop band of the rugate structure and wavelengths of resonant modes are observed, depending on the change of period jump. The number, the wavelengths, the band intervals, and the intensities of multiple resonance transmission modes are tunable by adjusting structure parameters of the rugate structure. Experimental feasibility of the proposed multiple narrow bandpass optical filters using the technique of glancing angle deposition is also discussed.     

TE and TM defective bands splitting in one-dimensional coupled cavity waveguides

Transverse magnetic (TM) and transverse electric (TE) defective bands will split with different incident angles (wave-vectors) in the one-dimensional coupled cavity waveguide. Different defective band properties are shown, including the shift of pass-band frequency and the variation of defective band width resulting from different localization properties of the electric field in the defective layer. The critical splitting angle, at which splitting of the TE and TM defective bands emerges, will decrease with increasing dielectric index of the defective layer.     

Spectroscopy of stop bands of globular photonic crystals filled with water

Reflectance spectra from the surface of artificial opals are measured at various globule diameters. A method for calculating the spectra of globular photonic crystals taking into account the globule size dispersion is proposed. The effect of the filling of opal with water on the spectral position of stop bands is revealed. Satisfactory agreement of theoretical results with experimental data was found.     

Competing orders in one-dimensional half-integer fermionic cold atoms: A conformal field theory approach

The physical properties of arbitrary half-integer spins F=N−1/2$F=N-1/2$ fermionic cold atoms loaded into a one-dimensional optical lattice are investigated by means of a conformal field theory approach. We show that for attractive interactions two different superfluid phases emerge for F?3/2$F?3/2$: A BCS pairing phase, and a molecular superfluid phase which is formed from bound-states made of 2N   fermions. In the low-energy approach, the competition between these instabilities and charge-density waves is described in terms of ZN${\mathbb{Z}}_N$ parafermionic degrees of freedom. The quantum phase transition for F=3/2,5/2$F=3/2,5/2$ is universal and shown to belong to the Ising and three-state Potts universality classes respectively. In contrast, for F?7/2$F?7/2$, the transition is non-universal. For a filling of one atom per site, a Mott transition occurs and the nature of the possible Mott-insulating phases are determined.     

Can surface states of a one-dimensional model lie in the allowed energy bands?

The existence conditions for surface states of a semi-infinite Kronig-Penney model, recently reported by Roy and Tripathy, are analyzed critically and it is shown that the surface state energies must always lie in the forbidden gaps.     

Photoemission and Auger CVV spectra of partially filled bands: A cluster approach

A theory of XPS and Auger lineshapes from almost completely filled bands, that had previously been proposed in the context of a graph-theoretical expansion, is applied to a finite cluster and solved exactly. The new method avoids the calculation of complex vertex corrections and reveals the full potential of the theory. We find that while the holes observed in photoemission are dressed quasi-holes, those observed in Auger CVV spectra are essentially bare. This explains certain discrepancies between calculated and experimental spectra, that had been attributed to intrinsic limitations of the theoretical set-up, but actually were due to the mathematical approximations. A simple analytic approach based on the use of bare propagators is found to agree closely with the exact numerical results.     

Spectroscopy of stop bands in artificial opals filled with an alcohol solution of potassium iodide

The spectral position of the stop bands in photonic crystals based on artificial opals filled with an alcohol solution of potassium iodide is investigated. The energy-band structure of samples with quartz globules 230 nm in diameter is modeled based on the dispersion equation. The spectral position of the stop bands in the [111] direction at different solution concentrations is determined. The conditions for forbidden-band ??collapse?? are established. The possibility of applying artificial opals in optical cavities of lasers of different types is analyzed.     

Theory of Auger XVV spectra of solids: Many body effects in incompletely filled bands

The availability of empty electron states above the Fermi level and the presence of strong intra-atomic correlations may be expected to lead to new features in the Auger XVV spectra of conduction bands. Here, the qualitative aspects of the problem are studied within a simplified model. The case of a low equilibrium hole density n_h in the band is considered in detail and the various propagators are calculated in a low density approximation. It is found that correlation and shake up effects can be approximately factored out and the singularity exponent relevant to the problem is linear in n_h. In the single particle density of states we find a peak in the high binding energy side, which appears to be consistent with the experimental XPS spectra of Fe, Co and Ni. The Auger XVV spectra are obtained in the same approximation. As in the case of closed bands (n_h = 0) two-hole resonances appear in the spectra provided that intra-atomic correlations are strong enough (quasiatomic case), but the condition for their occurrence is found to involve the total band width, rather than the width of the occupied portion of the band only. Moreover, even in the quasiatomic case, the resonances have a width that is related to the energy difference between the top of the band and the Fermi level.     

Deconfined fractionally charged excitation in any dimensions

An exact incompressible quantum liquid is constructed at the filling factor 1/m²$1/m^2$ in the square lattice. It supports deconfined fractionally charged excitation. At the filling factor 1/m²$1/m^2$, the excitation has fractional charge e/m²$e/m^2$, where e$e$ is the electric charge. This model can be easily generalized to the n$n$-dimensional square lattice (integer lattice), where the charge of excitations becomes e/mⁿ$e/m^n$.     

Rational numbers of the fractionally quantized new states and their observation in fractionally quantized hall effect

We have found energy states at a fraction of the frequency of the Landau levels. The fraction is in accord with the experimental measurements of fractionally quantized conductance.     

Electronegativity of fractionally charged atoms in the Density Functional Theory

The electronegativity (identified with the negative of the chemical potential) of atoms and ions has been calculated in several isoelectronic series using the Density Functional Theory. Then, the electronegativities of atoms and ions with fractional nuclear charge have been obtained by interpolation in each isoelectronic series. Similar interpolations have been performed, starting with approximate electronegativities obtained by Mulliken's finite difference approximation. Both sets of results have been compared.