The effect of gap state density on the photoconductivity and photoluminescence of a-Si:H

This paper discusses experimental evidence that a fixed amount of hydrogen introduced into amorphous silicon at different hydrogen partial pressures during sputtering may have quite different results for the total density and distribution in energy of states in the pseudogap. Whereas the expected effect is the removal of gap states through the formation of strong Si-H bonds, we show evidence that the gap state densities in some films increase by an order of magnitude over other similar hydrogen content samples. Moreover, such material has much increased photoconductivity response and additional photoluminescence features. We speculate briefly on the possible origin of this higher density of gap states in terms of either natural defects of a-Si or states associated with the Si-H complex.     

Limiting stable states of high-Tc superconductors in the alternating current modes

The limiting current-carrying capacity of high-T _c superconductor and superconducting tape has been studied in the alternating current states. The features that are responsible for their stable formation have been investigated under the conduction-cooled conditions when the operating peak values of the electric field and the current may essentially exceed the corresponding critical values of superconductor. Besides, it has been proved that these peak values are higher than the values of the electric field and the current, which lead to the thermal runaway phenomenon when the current instability onset occurs in the operating modes with direct current. As a result, the stable extremely high heat generation exists in these operating states, which can be called as overloaded states. The limiting stable peak values of charged currents and stability conditions have been determined taking into account the flux creep states of superconductors. The analysis performed has revealed that there exist characteristic times defining the corresponding time windows in the stable development of overloaded states of the alternating current. In order to explain their existence, the basic thermo-electrodynamics mechanisms have been formulated, which have allowed to explain the high stable values of the temperature and the induced electric field before the onset of alternating current instability. In general, it has been shown that the high-T _c superconductors may stably operate in the overloaded alternating current states even under the not intensive cooling conditions at a very high level of heat generation, which is not considered in the existing theory of losses.     

Behavior of the effective interaction through third order in G

To investigate the usual assumption of rapid perturbative convergence, the (sd)-shell effective interaction is calculated including effects through third order for mass-18. All JT states are considered. Third order effects on the T=1 states tend to be small compared to corresponding second order effects. Some evidence for non-convergent behavior is found in T=0 states.     

Low-lying excitations in the S = 1 antiferromagnetic Heisenberg chain

In order to confirm the picture of domain-wall excitations in the hidden antiferromagnetic order of the Haldane phase, the structure of the low-lying excitations in the S = 1 antiferromagnetic Heisenberg chain is studied by a quantum Monte Carlo method. It is confirmed that there exists a finite energy gap between the first- and the second-excited states at k = π as well as between the ground state and the first-excited state at k = π. In the thermodynamic limit, the second-excited state at k = π is separated from the ground state by the gap which is three times as large as the Haldane gap. From the size dependences of the low-lying-excitation energies, the interactions between the elementary excitations in the excited states are concluded to be repulsive.     

The unstable system withN states and the propagator matrix with a pole of higher order

Starting from the general quantum-mechanical laws, the general form of the mass-matrix for a special unstable system withN states and of the propagator with higher order pole is derived.     

Cluster decay of the high-lying excited states in 14C

A cluster-transfer experiment of ⁹Be(⁹Be,¹⁴C→α+¹⁰Be)α at an incident energy of 45 MeV was carried out in order to investigate the molecular structure in high-lying resonant states in ¹⁴C. This reaction is of extremely large Q-value, making it an excellent case to select the reaction mechanism and the final states in outgoing nuclei. The high-lying resonances in ¹⁴C are reconstructed for three sets of well discriminated final states in ¹⁰Be. The results confirm the previous decay measurements with clearly improved decay-channel selections and also show a new state at 23.5(1) MeV. The resonant states at 22.4(3) and 24.0(3) MeV decay primarily into the typical molecular states at about 6 MeV in ¹⁰Be, indicating a well developed cluster structure in these high-lying states in ¹⁴C. Further measurements of more states of this kind are suggested.     

Nitric oxide chemisorption on Re(0001) at 100 and 300 K: Evolution of the adsorbed layer during annealing from 100 to 700 K; XPS,UPS, TDS and work function measurements

At 100 K NO is molecularly adsorbed on Re(0001). Bridge bonded and linear species have been identified by XPS and UPS measurements. Moreover a weakly bonded species reversibly adsorbed at 100 K has been found, but not precisely identified. As the temperature of the surface is increased a complex transformation of the layer occurs: the weakly bonded molecules are probably transformed into a more strongly bonded state and desorb between 100 and 300 K. One part of the linear species desorbs between 300 and 500 K giving the α₂ molecular state, the other part dissociates and desorbs between 600 and 700 K giving the β₁ nitrogen molecules. In the same temperature range the bridge bonded molecules dissociate into nitrogen and oxygen atoms, but nitrogen desorbs into the gas phase between 700 and 1100 K as β₂ and β₃ states with a second order process. Oxygen is adsorbed as atoms and desorbs at higher temperature. If adsorption takes place at room temperature, NO is mainly dissociated and nitrogen desorbs as β₂ and β₃ states with a second order process.     

Geometry effect on the magnetic ordering of geometrically frustrated rectangular and triangular magnets

We investigate the effect of geometry on the ground-state ordering of artificially frustrated magnetic rectangular and triangular lattices by Monte Carlo method. By varying the vertical lattice spacing while keeping the horizontal one fixed, we show that when the ratio of vertical to horizontal lattice spacing, labeled by η, is less than 1.82, the ground state of the rectangular lattice presents long-range antiferromagnetic order and for η?1.82 the ground state changes to long-range mixed ferromagnetic and antiferromagnetic order. For the frustrated triangular lattice, the short-range ordered state as well as two long-range ordered ground states occurs transiently at η=0.87 and 2, where the energies of the two ground states with long-range order are approximately equal. In addition, the level of frustration of both frustrated lattices is found to be largely relevant to the ratio η.     

Frequency and intensity measurements in the fundamental infrared band of HD

The fundamental band of HD has been studied at room temperature and 0.6 atm pressure with a 48-m absorption path and a resolution of 0.01 cm^?1. The frequencies of nine electric dipole and three electric quadrupole transitions were measured with an accuracy of 0.001 cm^?1, and their analysis gives improved molecular parameters for the v = 0 and 1 states of HD. The intensities of the dipole transitions were measured in order to determine the v = 1-0 electric dipole transition moment. These measurements extend earlier experiments to higher J values, thus refining the determination of the rotational dependence of the transition moment.     

Factorization and the couplings of the vacuum trajectory

The decoupling theorems associated with an isolated factorizable pomeron pole of unit intercept are re-examined. It is found that the coupling of three such poles, Γ(t, t, 0), need not vanish, precisely at the point t = 0. This is demonstrated by summing only over states in the appropriate unitarity sum, and sum rule, which are consistent with the M², s/M² → ∞ limit. The triple-Regge region then makes a constant contribution to σ_total, insteadsb of the ln lns result obtained if the isolated pole is assumed to couple also to states such that s/M² = constant. The physical implications regarding factorization and the pole-cut relationship are discussed. The relationship between higher order optical theorems (Mueller discontinuities) and particular terms in the unitarity sum for the two → two absorptive part A₂₂ is exploited. Consistent contributions to the triple-Regge region contribute constant vertex corrections to pure pole behaviour in A₂₂. There is no cut contribution and the magnitude of the vertex corrections reflects the relative amount of diffractive production. The analysis is extended to multiple fireball production where pure multipole structures emerge. The series naturally terminates if the diffractive component is sufficiently small. The implications for the behaviour of the total cross section at machine energies are discussed.     

The effect of the spatial variation of the order parameter on the tunneling density of states of superconducting alloys

In the first part of the paper we derive expressions of the Ginzburg-Landau (GL) type for the local tunneling density of states of superconducting alloys. These expressions are quite generally applicable at high excitation energies. One can see immediately that the density of states,N(r, ω), at any positionr and high energiesω is always larger than the local BCS density of states if the space dependence of the order parameter is governed by the GL-equation. This effect is largest for long mean free pathsl. In the second part of the paper we calculate the spatial average of the density of states,¯N, at all energiesω for a lattice of vortex lines in a magnetic field slightly below the upper critical field. The resulting curve of [¯N? N(0)]/N(0) versus co shows no gap and has a zero at about the gap value in zero field. Its value at ω=0 depends onl like ln(ξ₀/l) for l?ξ₀ [N(0) denotes the normal density of states, and ξ₀ is the BCS coherence length].     

Role of magnetism in the formation of a short-range order in an Fe-Ga alloy

The formation of a short-range order in an Fe-Ga bcc alloy has been studied by Monte Carlo simulation with the use of effective interaction potentials calculated within the density functional theory for the ferromagnetic and paramagnetic states. It has been found that the pronounced short-range order of the D0₃ type is formed at Ga concentrations close to the boundary of the two-phase region at T < T _c, whereas no short-range order is observed at T < T _c. The results obtained are in agreement with the experimental X-ray diffraction analysis. The relation of the features of the short-range order in the Fe-Ga alloy to the magnetostriction value has been discussed.     

Pressure dependence of the absorption edge for Cd1-xMnxTe

The effect of pressure on the optical absorption edge of mixed crystals Cd_1-xMn_xTe with different manganese concentrations is reported. The observed absorption edge shifts to higher energy with increasing pressure at a rate of α=7?8×10^?3 eV/kbar and a second order coefficient of β=-4×10^?5 eV/kbar² for x<0.5, to lower energy with increasing pressure at a rate of α=-5.0 ×10^?3 eV/kbar for x?0.5. A phase transition occurs for all the samples studied. The absorption edge of the new phase is outside the wavenumber range of the instrument. The physical origins of different pressure coefficients are discussed in the light of the deformation potentials of energy band states and the hybridization of the Mn²⁺ 3d levels with the p-like states in the valence band.     

Electronic structure of LiGaS2

X-ray photoelectron spectroscopy (XPS) measurement has been performed to determine the valence band structure of LiGaS₂ crystals. The experimental measurement is compared with the electronic structure obtained from the density functional calculations. It is found that the Ga 3d states in the XPS spectrum are much higher than the calculated results. In order to eliminate this discrepancy, the LDA+U method is employed and reasonable agreement is achieved. Further calculations show that the difference of the linear and nonlinear optical coefficients between LDA and LDA+U calculations is negligibly small, indicating that the Ga 3d states are actually independent of the excited properties of LiGaS₂ crystals since they are located at a very deep position in the valence bands.     

A DLTS study of the effects of boron counterdoping on the gap states in n-type hydrogenated amorphous silicon

The technique of Deep Level Transient Spectroscopy (DLTS) and related space charge measurement techniques have been applied to a series of 300 vppm phosphine (PH₃) doped, 0–44 vppm diborane (B₂H₆) counterdoped samples of hydrogenated amorphous silicon (a-Si:H). The trends in the resulting density of states, g(E), show: (1) A large increase of g(E) at ≈0.4 to 0.6 eV from the valence band, comparable in magnitude to the total number of B atoms introduced; (2) A smaller decrease in the midgap dangling bond states on the order of ≈2.5% of the added B; (3) A drop in the number of occupied shallow states ≈0.5%; and (4) A quantitative assessment of the degree of autocompensation which shows roughly five compensating midgap states removed for every six shallow acceptor states introduced by B.     

Influence of nonmagnetic impurities on the kondo effect

The influence of scattering by nonmagnetic impurities is studied in perturbation theory. While the finite lifetime of the electrons in intermediate states due to scattering by nonmagnetic impurities does not lead to a change in the logT-behaviour of the third-order self-energy, certain vertex-corrections give rise to an additional term which varies like 1/√T at low temperatures. Similar correction terms are found to occur in the higher order self-energy contributions. Although these terms diverge more strongly atT=0 than the logarithmic contributions they are quite small at finite temperatures since they depend on the lifetime τ of the electrons through a factor of (? _F τ)^?5/2 (? _F Fermi energy). The possibility of observing these interference effects experimentally is discussed.     

Ritz assignment and Watson fits of the high-resolution ring-puckering spectrum of oxetane

Oxetane is a four-membered ring molecule exhibiting a large-amplitude ring-puckering motion. In order to analyze this vibration we recorded a rotationally resolved far-infrared spectrum between 50 and 145 cm⁻¹. The analysis of the ring-puckering fundamental band with the assignment of 1108 lines, has been presented in a previous paper. In the present work we present a list of further 6531 assigned transitions between the five lowest excited ring-puckering states. The 4983 term values involved in the transitions assigned in this and in the preceding work have been evaluated by the “Ritz” program, and are now available. An A-reduced Watson Hamiltonian in any of the three representations I^r, II^r, and III^r was used to perform a fit of the assigned transitions. Precise rotational constants and quartic as well as a full set of sextic centrifugal distortion constants were obtained for the investigated ring-puckering states. For the first time, high-resolution values for the vibrational G_v parameters have been obtained, and we have added terms in x⁶ and x⁸ to the double minimum-potential well describing the ring-puckering motion, in order to reproduce their values within the experimental accuracy. The same potential still reproduces the lower resolution values of the Q-branch origins involving higher ring-puckering states up to v_rp=14 found in the previous literature.     

Effect of correlation on electronic properties of NiO: A study from dynamical mean field theory

In order to describe a typical strongly correlated insulator NiO at electronic level, we perform a first principles calculation for temperature effect on electronic properties of NiO using a many-body method merging local density approximation (LDA) with dynamical mean field theory, so called the LDA+DMFT scheme. Band gap and density of states (DOS) are in good agreement with available experimental data and theoretical calculations, and Ni d-e_g and d-t_2g components both exhibit insulating character. Calculated hybridization functions indicate that Ni d-e_g states strongly hybrid with O p states at T = 58 K, 116 K, 145 K, 232 K and 464 K. In order to compare with experimental angle-resolved photoemission spectrum (ARPES), we also calculate momentum-resolved electronic spectrum function, which is established that obvious electronic excitation mainly arises from Ni d-t_2g states at temperature T = 232 K, and the spectrum functions between −0.5 eV and 0.0 eV are almost symmetric about certain k points. Finally, we analyze the effect of temperature on electronic properties of NiO by carrying out LDA+DMFT calculations at T = 58 K, 116 K, 145 K, 232 K and 464 K, respectively. Results show that temperature mainly influences the valence states of spectrum function and hybridization function, in particular high-lying states close to Fermi level. Electronic excitation distributions and spectrum characters in electronic spectrum function are also discussed.     

Two-dimensional superconductive state near the intermediate plane

The superconductive layer near a plane surface arises due to the electronic states (Tamm levels) localized on the surface at temperature T₀, higher than the bulk critical temperature.     

High temperature superconductivity in the mixed state and the dynamics of charge carriers

A model to calculate the temperature and magnetic field dependence of the scattering time τ_ν(H, T) of quasiparticles by bound electron states in a vortex in high-temperature superconductors is proposed. In this model, the hydrodynamic interaction of a moving gas of quasiparticles with the discrete states of the vortex velocity field is regarded as quasielastic scattering and the resulting scattering time of quasiparticles is different from scattering of individual vortices. The normalized scattering time was found to decrease exponentially with increasing temperature. This behavior is due to the suppression of the amplitude of the superconducting order parameter as the temperature increases. This model accounts for the observed temperature and field dependence of the scattering time particularly at low-field regime.