Characteristics of surface states at the insulator-semiconductor interface of ZnS:Mn thin-film electroluminescent emitters

The energy distribution of the density of occupied surface states (N _ss) at the cathode insulatorphosphor interface in ZnS:Mn electroluminescent thin-film (ELTF) emitters has been modeled on the basis of experimental data. Changes in this distribution depending on the parameters of exciting voltage pulses have been studied. It is established that the energy distribution of N _ss shifts toward deeper levels upon a decrease in the frequency of the exciting signal and the resulting increase in the pause between the adjacent switch-on states. This behavior corresponds to a cascade relaxation mechanism of electrons trapped on the surface states. Maximum values of the N _ss (∼2.5 × 10¹³ cm⁻²) and the specific density of surface states per unit energy (2 × 10¹⁴–10¹⁵ cm⁻² eV⁻¹) are determined for the cathode insulator-phosphor interface from which electrons are tunneling. Positions of the equilibrium (∼1.25 eV below the conduction-band bottom) and the quasi-equilibrium (0.6–1.25 eV) Fermi levels during the ELTF emitter operation are estimated.     

Evidence of Electron–Phonon Interaction in Al-Substituted Mg1?xAlxB2

Phonon density of states in Al substituted Mg_1–x Al _x B₂ has been studied by means of inelastic neutron scattering experiments with pulsed neutron time-of-flight technique. Phonon dynamics have a significant role for realizing superconductivity in the recent discovered superconductor MgB₂. We present a systematic investigation on generalized phonon density of states (PDOS) up to E = 120 meV on Al content of x = 0.1, 0.25, and 0.5. We have clearly observed a large effect on the PDOS by Al substitution; a large energy shift occurs around at 70 meV and 89 meV. We have also observed a visible increase of PDOS at a particular energy range below T _c. Those phonon states are related with E _2g mode to be expected to have a strong coupling with the electron states at the –A region.     

Theoretical Study of Specific Heat, Density of States and Free Energy of Itinerant Ferromagnetic Superconductor URhGe

Following the equation of motion method and Green’s function technique, the coexistence of itinerant ferromagnetism (FM) and superconductivity (SC) is investigated in a single band homogeneous system. Self-consistent equations for superconducting order parameter (Δ) and magnetic order parameter (Δ_FM) are derived. It is shown that there generally exists a coexistent (Δ≠0 and Δ_FM≠0) solutions to the coupled equations of the order parameter in the temperature range 0<T<min (T _C,T _FM) where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. Expressions for the specific heat, density of states and free energy are derived. The specific heat has a linear temperature dependence at low temperatures as opposed to the exponential decrease in the BCS theory. The density of states for a finite Δ_FM increases as opposed to that of a standard ferromagnetic metal. The free energy shows that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and therefore is realized at low enough temperature. The theory is applied to explain the observations of URhGe. The agreement between theory and experimental results is quite satisfactory.      

Theoretical Study of Interplay of Superconductivity and Ferromagnetism in Hybrid Rutheno–Cuprate Superconductors RuSr2RCu2O8

We consider the extended two-band s–f model with additional terms, describing intersite Cooper pairs’ interaction between 4f (5f) and conduction electrons. Following Green’s function technique and equation of motion method, self-consistent equations for superconducting order parameter (Δ) and magnetic order parameter (m _f ) are derived. The expressions for specific heat, density of states, and free energy are also derived. The theory has been applied to explain the coexistence of superconductivity and ferromagnetism in hybrid rutheno-cuprate superconductors RuSr₂RECu₂O₈ (RE = Gd, Eu). The theory shows that it is possible to become superconducting if the system is already ferromagnetic. A study of specific heat, density of states and free energy is also presented. The agreement between theory and experimental observations is quite satisfactory.      

Deep surface states on the interface between SiC and its native thermal oxide

Deep surface states are discovered on the interface between 6H-SiC and its native thermal oxide by analyzing the C-V characteristics of metal-oxide-semiconductor structures measured at a high temperature (600 K). The maximum of the density of states distributed according to energy (D _tm=2×10¹² cm⁻²· eV⁻¹) is at an energy about 1.2 eV below the bottom of the conduction band of SiC. It is postulated that the states discovered are similar in nature to the P _b centers observed in the SiO₂/Si system. Pis’ma Zh. Tekh. Fiz. 23, 55–60 (October 26, 1997)     

Correlating yield response with molecular architecture in polymer glasses

The yield response of nine architecturally different glassy networks is investigated under several stress states, strain rates, and temperatures, and correlations are made among them. Differences in molecular architecture are quantified through two proposed governing parameters; the glass transition temperature, T _g, capturing network stiffness and the cohesive energy density, E _c, reflecting network strength. Cohesive energy density is estimated using molecular modeling techniques and supported by solvent swelling experiments. The limits of the correlations made between molecular architecture and yield behavior are further studied with attempts to relate yielding in thermoplastic glasses and heterogeneous networks.     

Electronic Structure of Non-oxide Perovskite Superconductor MgCNi3

The electronic structure of new superconducting perovskite MgCNi₃ has been studied using pure Hartree-Fock and density functional theory methods. The main peak of density of states is located below the Fermi level and it is dominated by Ni d. The results of total-energy calculations and electronic density calculations show that MgCNi₃ is not energy favorable in ferromagnetic state. Also we found that the bonding feature between C orbitals and Ni orbitals is covalent, but the bonding between Mg and Ni atoms is ionic or metallic.      

Quantum Dot in the Pseudo-gap Sate with Spin–Orbit Interaction

We study the linear conductance in quantum dot with spin–orbit interaction coupled to Fermi liquid leads with a power-low density of states. The conductance at zero temperature is calculated as a function of the power exponent from the density of state ρ(ω)∼|ω−E _F | ^r at the Fermi energy E _F and the different energy rates. The phase shift of the conduction electrons is also r-dependent. The model can be used in the study of the quantum phase transition.     

Generalized McMillan tunneling model of the superconducting proximity effect

We have extended the McMillan tunneling model (MTM) of the superconducting proximity effect to the case of strongly coupled normal-superconducting composites. The generalization has been carried out by treating the tunneling Hamiltonian to all orders of self-consistent perturbation theory. The electronic density of states, the real and imaginary parts of the order parameter, the energy gap, and the transition temperatureT _c calculated in the generalized model are significantly different from the MTM results. Generally, the peak inN _N (E) is broader, shifted, and damped. The energy gap is lower and the depression inT _c is softened.Work supported in part by a grant from the National Research Council of Canada.     

Critical magnetic fields of a Kondo superconductor:ZnMn

The superconducting critical fields of pure Zn and of a series ofZnMn alloys have been measured as a function of temperatureT down to 0.06 K. The critical fields have been used to calculate the entropy difference between the normal and superconducting states. For all the alloys studied, the entropy in the superconducting state is proportional toT asT 0, indicating a finite density of states at the Fermi energy. We interpret this result as evidence for the existence of localized bound states within the energy gap centered at the Mn impurities. Results obtained for the depression ofT _c , for the critical field atT=0, for the jump in specific heat atT _c , and for the law of corresponding states are also presented and compared with the predictions of the Abrikosov-Gor'kov theory. A simple expression relating the critical fields of these alloys to the Mn concentration has been found to be in very good agreement with experiment.Research supported by the National Science Foundation.     

Nature of chemical bonding in ThF<Subscript>4</Subscript>

The fine structure of the X-ray photoelectron spectrum of ThF₄ in the range of valence electrons (binding energy from 0 to 35 eV) is examined. The analysis takes into account the results of theoretical calculation of the electronic structure of the ThF₈⁴⁻ cluster (point group C ₂) as a model of the nearest surrounding of the Th atom in ThF₄, performed by the relativistic discrete variation method. It is demonstrated theoretically and confirmed experimentally that formation of a chemical bond gives rise to filled states of Th5f electrons (∼0.5 Th5f electron) in the energy range of electrons of outer valence molecular orbitals (valence band). The Th6p electrons noticeably participate in formation not only of inner valence but also of outer valence (∼0.4 Th6p electron) molecular orbitals. The composition and order of inner valence molecular orbitals in the energy range from 13 to 35 eV is determined, and the density of states of valence electrons in the range from 0 to 35 eV in ThF₄ is calculated. The results obtained allowed the fine structure of the high-resolution O _4,5(Th)-emission spectrum of thorium in ThF₄ in the photon energy range from ∼60 to ∼85 eV, associated with the formation of outer and inner valence molecular orbitals, to be interpreted for the first time.     

Resistivity and T c in disordered superconductors

The universal depression of the superconducting transition temperature T _cin disordered A-15 compounds is examined. It is found that their anomalous behavior can be explained by a simple model for the density of states, which is enhanced by disorder in some cases. The dramatic drop in T _cin constant-density-of-states A-15 compounds like Nb₃Ge or Nb₃Al at a critical value of the resistivity can be attributed to overdamping of acoustic plasmons, which decreases the electron pairing interaction despite relatively small changes in the density of states. Agreement for T _cand susceptibility with previous calculations is found if the position of the Fermi energy is near a peak. Possible experiments are proposed to check the above models.This work was supported in part by NSF Grant No. DMR77-13167.     

The flux-flow hall effect in type II superconductors. An explanation of the sign reversal

We consider a time-dependent Ginzburg-Landau (TDGL) model modified to take into account two mechanisms responsible for the Hall voltage in superconductors: the usual effect of the magnetic field on the normal current, and the vortex traction by the superflow. For the BCS model of superconductivity, the contribution of the vortex traction is proportional to the energy derivative of the quasiparticle density of states. Our theory gives the correct order of magnitude for the Hall angle in the mixed state. It predicts that the vortex-traction mechanism results in a negative Hall angle for the quasiparticle spectrum with a positive energy derivative of the density of states averaged over the Fermi surface, and vice versa. For the Fermi surface with a complicated shape, the sign of the Hall effect in the mixed state can be different from that in the normal state. If the signs are opposite, the Hall angle changes its sign as a function of the magnetic field belowH _c2 .     

Te concentration dependent photoemission and inverse-photoemission study of FeSe1−xTex

Abstract

We have characterized the electronic structure of FeSe_1?xTe_x for various x values using soft x-ray photoemission spectroscopy (SXPES), high-resolution photoemission spectroscopy (HRPES) and inverse photoemission spectroscopy (IPES). The SXPES valence band spectral shape shows that the 2 eV feature in FeSe, which was ascribed to the lower Hubbard band in previous theoretical studies, becomes less prominent with increasing x. HRPES exhibits systematic x dependence of the structure near the Fermi level (E_F): its splitting near E_F and filling of the pseudogap in FeSe. IPES shows two features, near E_F and approximately 6 eV above E_F; the former may be related to the Fe 3d states hybridized with chalcogenide p states, while the latter may consist of plane-wave-like and Se d components. In the incident electron energy dependence of IPES, the density of states near E_F for FeSe and FeTe has the Fano lineshape characteristic of resonant behavior. These compounds exhibit different resonance profiles, which may reflect the differences in their electronic structures. By combining the PES and IPES data the on-site Coulomb energy was estimated at 3.5 eV for FeSe.     

Density of States in Superconductor-Normal Metal-Superconductor Junctions

We consider the ₀ -dependence of the density of states inside the normal metal of a superconductor-normal metal-superconductor (SNS) junction. Here ₀ is the phase difference of two superconductors of the junction. It is shown that in the absence of electron-electron interaction the energy dependence of the density of states has a gap which decreases as ₀ increases and closes at ₀ = . Both the analytical expressions for the ₀ -dependence of the density of states and the results of numerical simulations are presented.     

Interplay of Superconductivity and Ferromagnetism in UGe<Subscript>2</Subscript>

The emergence of pressure induced superconductivity (SC) under the background of ferromagnetic state in 5f-electron based itinerant ferromagnetic superconductor UGe₂ is studied in the single band model by using a mean-field approximation. The solutions to the coupled equations of superconducting gap (Δ) and magnetization (m) are obtained using Green’s function technique and equation of motion method. It is shown that there generally exists a coexistent (Δ≠0, m≠0) solution to the coupled equations of the order parameters in the temperature range 0<T<min (T _C,T _FM), where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. The study of electronic specific heat (C/T), density of states, free energy, etc. are also presented. The specific heat capacity at low temperature shows linear temperature dependence as opposed to the activated behavior. Density of states increases as opposed to the case of a standard ferromagnetic metal. Free energy study reveals that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and, therefore, realized at low enough temperature. The agreement between theory and experimental results for UGe₂ is quite satisfactory.      

Coexistence of Superconductivity and Itinerant Ferromagnetism in Ferromagnetic Metals UCoGe and UIr

A microscopic coexistence of itinerant ferromagnetism (FM) and superconductivity (SC) is studied in a single band homogenous system, following an equation of motion method and Green’s function technique. Self-consistent equations for superconducting order parameter (Δ) and magnetization parameter (M) are derived. It is shown that there generally exists a coexistent (Δ≠0, M≠0) solution to the coupled equations of the order parameters in the temperature range 0<T<min (T _C,T _FM), where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. The expressions for electronic specific heat (C/T), density of states, free energy, transition probabilities, ultrasonic attenuation, and nuclear relaxation are also derived. The theory is applied to explain the observations in UCoGe and UIr. The specific heat capacity at low temperature shows linear temperature dependence as opposed to the activated behavior. Density of states increases as opposed to the case of a standard ferromagnetic metal. Free energy study reveals that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and, therefore, coexistence of FM and SC realized at a low enough temperature. The agreement between theory and experimental results for UCoGe and UIr is quite encouraging.     

Adsorption of hydrogen atom on graphene

The adsorption of atomic hydrogen on graphene monolayer is considered using the model density of states (M model). Analytical expressions for the density of states of H adatom and its occupation number n _a are obtained. The main (local) contribution n _l to the total occupation number n _a is due to a local level of the adatom, which occurs below the edge of the continuous spectrum. A variant of the surface molecule approximation in the case of n _l ≫ n _a − n _l is proposed.