Influence of localized states in a semiconducting film of critical thickness on the electrical properties of thinfilm ms' is structures

The current-voltage and high-frequency capacitance-voltage (C-V) characteristics have been used to investigate the electrical properties of thin Ga₂Ses films in Me-Ga₂Se₃-Si structures (n and p type). The features of the high-frequency C-V dependences of the structures based on p-type Si are explained in the framework of a model of the MS' IS structure with donor type centers in the semiconducting S'film (the thickness of the S' film is comparable with the generalized screening length). It is shown that in the investigated structures the charge transport mechanism is due to ionization of volume centers (the activation energy of a deep donor center is 0.72 eV) in the Ga₂Se₃ film, the mechanism being enhanced by an electric field. Analysis of the high-frequency C-V characteristics showed that the majority carriers in the Ga₂Se₃ films are electrons with concentration 10¹² cm^–3; the concentration of the deep donor centers is 5·10¹⁶ cm^–3.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 83–88, January, 1981.     

Directed polymers in Hartree-Fock approximation

A replica trick is used to map the problem of directed polymers into a quantum mechanics problem ofn-body bound states. This bound-state problem is then treated using a self-consistent method of Hartree-Fock. Ford=2, the exact result of the DP exponents is reproduced; and for complex DP, the ground-state energyE _n n² is found, confirming previous numerical results.     

Discretization of the thermal excitation in highly excited matter

In this paper the thermal energy diffusion for quantum particles is described. The quantum heat transport equation is obtained. It is shown that, for a short-time thermal excitation (of the order of the relaxation time), the excited matter response is quantized on the different levels (atomic, nuclear, quark) with quantum thermal energy equalE ^atomic 9 eV,E ^nuclear 7 MeV, andE ^quark 139 MeV.     

Spectroscopic trace-gas sensor with rapidly scanned wavelengths of a pulsed quantum cascade laser for in situ NO monitoring of industrial exhaust systems

Development of a pulsed quantum cascade laser (QCL)-based spectroscopic trace-gas sensor for sub-part-per-million detection of nitric oxide (NO) and capable of monitoring other molecular species such as CO₂, H₂O, and NH₃ in industrial combustion exhaust systems is reported. Rapid frequency modulation is applied to the QCL to minimize the influence of fluctuating non-selective absorption. A novel method utilizes only a few laser pulses within a single wavelength scan to probe an absorption spectrum at precisely selected optical frequencies. A high-temperature gas cell was used for laboratory evaluation of the NO sensor performance. A noise-equivalent sensitivity (1) of 100 ppb × m/ at room temperature and 200 ppb × m/ at 630 K was achieved by measuring the NO R(6.5) absorption doublet at 1900.075 cm^–1.     

Thermal conductivity of C60 and C70 crystals

We have performed thermal conductivity measurements on C₆₀ and C₇₀ crystals grown by sublimation. For single crystal C₆₀, the thermal conductivity k is 0.4 W/m K at room temperature and is nearly temperature independent down to 260K. We observed a sharp orientational phase transition at 260K, indicated by a 25% jump in k. Below 90K, k is time dependent, which manifests itself as a shoulder-like structure at 85K. The temperature and time dependence of k below 260K can be described by a simple model which accounts for the thermally activated hopping of C₆₀ molecules between two nearly degenerate orientations, separated by an energy barrier of 240 meV. It is found that solvents have a strong influence on the physical properties of C₇₀ crystals. For solvent-free C₇₀ crystal, k is about constant above 300K. There is a broad first-order phase transition in k at 300K with a 25% jump. We associate this transition with the aligning of the fivefold axes of the C₇₀ molecules along the c-axis of the hexagonal lattice. Upon further cooling, k increases and is time independent.     

The one dimensional anderson model with off diagonal disorder: band centre anomaly

We study theE-dependence of the Lyapounov exponent <(E)> of an electron with energyE in the one dimensional Anderson model with off diagonal disorder. In the neighbourhood of the band centre we find for nonzero disorder E)>log^–1 E0 forE0, but all even moments of Re(E) diverge logarithmically. As the probability of Re (E)=0 decreases to zero forE0 we conclude that the electron is always exponentially localised.     

Mechanochemical Transformations in α-Fe2O3 – ICEMS Study

Integral conversion electron Mössbauer spectroscopy has been used to investigate the phases and transformations which occur when -Fe₂O₃ is wet-milled in vacuum for up to 144 h. In addition to the transformation to off-stoichiometric Fe_3–x O₄ observed previously by transmission Mössbauer spectroscopy, the room temperature ICEMS spectra reveal the presence of 10–15% -Fe₂O₃ on the surfaces of particles milled for more than 24 h. Time-of-flight neutron diffraction of an -Fe₂O₃ sample wet-milled for 200 h also provides evidence of the occurrence of a small fraction (4%) of -Fe₂O₃ in the milled product. The -Fe₂O₃ is likely to occur as a surface oxide layer and does not appear to play a significant role in the mechanochemical transformation of -Fe₂O₃ to Fe_3–x O₄.     

Local magnetic moment and hyperfine field in hydrogenated iron and iron-vanadium alloy

The local magnetic moment and hyperfine fieldB _hf at Fe and V sites in hydrogenated iron and iron-vanadium were calculated using the discrete variational method. The variations in andB _hf with H occupation of the octahedral (O) site were considered. It was found that when H occupies the O site neighbouring an Fe atom, both local moment and hyperfine field at this atom decrease linearly with increasing number of H atoms. The rate of decrease is larger for Fe in iron as compared to iron in vanadium. On the other hand, when H resides at an O site next neighbouring an Fe atom, whether in iron metal or in iron-vanadium, the Fe magnetic moment increases slowly, while the hyperfine field remains almost constant. The V moment in iron, which is negative (–0.83 _B ), becomes less negative (–0.30 _B) as H occupies the neighboring O sites, whereas slight changes occur (–0.88 _B) when H is at the next neighbouring O site. The net effect of H on Fe in iron is to decrease the average magnetic moment at a rate of 1.2 _B per H/Fe for low H content. On the other hand, the average Fe moment in an iron-vanadium alloy increases if H resides at O sites which are immediate neigbours of V and next neighbours of Fe. This may explain the development of a magnetic state on hydrogenation of Fe-V alloys, which is exhibited by the specific heat and susceptibility measurements. The changes in the isomer shift were found to agree with experimental trends.     

Radiometric observations of the 752.033-GHz rotational absorption line of H2O from a laboratory jet

The intensity and lineshape of the Doppler-broadened 752.033-GHz (2₁₁ 2₀₂) rotational transition of H₂O has been studied passively using a high-resolution two-stage heterodyne radiometer with single-sideband system noise temperature of 45,000 K. The purpose of the experiments was to demonstrate the observability at submillimeter wavelengths of a high-altitude rocket plume simulated by a laboratory H₂O jet in a vacuum chamber. First-stage mixing was accomplished by means of a GaAs Schottky diode with first local-oscillator power supplied by a CO₂-laser pumped formic-acid laser (761.61 GHz), generating and X-band IF signal. Second localoscillator power was provided by a tunable C-band source. One-MHz resolution capability was obtained by means of a 3-GHz waveguide cavity filter with only 9-dB insertion loss. In the H₂O jet experiments, the center frequency of the line was determined to within 1 MHz of the previously reported value. A rotational temperature 75 K, a linewidth 5 MHz, and a Doppler shift 3 MHz (from a 45-degree rotation of the flow direction) were measured with the line-of-sight intersecting the jet axis at a distance downstream of 30 nozzle diameters. These absorption data were ogtained against continuum background radiation sources at temperatures of 1175 and 300 K.     

Optical,photoelectric and electric properties of single-crystalline Sb2Se3

Measurements of absorption coefficient in the region of the absorption edge, of spectral distribution of photoconductivity and dependence of electrical conductivity upon temperature on Sb₂Se₃ single crystals are given. The absorption of light was proved to correspond to indirect forbidden transitions. The value of optical gapE _g ^opt =(1·11±0·02) eV forE a andE c was determined. From photoconductivity and conductivity measurements the values of the gaps areE _g ^opt =1·11 eV andE _g ^el =1·04 eV. The anisotropy of the electrical conductivity parallel and perpendicular to the cleavage plane is 2·2.     

Muon Spin Dynamics and Sites in GdNi5

In GdNi₅, the muon localizes at the 3f interstitial site and below 80K a second muon site becomes populated, the so-called 6_ring site. This ring site is metastable and the muon hops to the 3f site. We determine the mean time of stay of the muon in the ring site and show that it is governed by a multi-phonon quantum diffusion process. The coincidence energy is measured to be E _a =272(10)K and the tunneling matrix element J=0.11(2)meV.     

Theory of electron escape from the surface of liquid helium

We have studied the properties of an electron bubble close to the surface of liquid³ He, by using a Density Functional approach. We find that up to an electron-surface distanced ₀ 23 Åthe bubble is stable, while at smaller distances it becomes unstable and bursts. A potential energy barrier /K _B 38°K for the thermal emission of electrons is obtained from our results, in agreement with experiments. Even when the electron-surface distance is larger thand ₀, however, tunneling through the surface layer dominates the electron escape probability. Large deviations of the electron potential energy from its ideal value are found close to the surface. These deviations have a profound effect on the calculated decay rates of the tunneling curent, which are much smaller than those obtained previously and in semi-quantitative agreement with experiments.     

Hydrogen-like recombination X-ray lasers using ps pulse drivers

We propose a scheme for producing high gain recombination X-ray lasers on hydrogen-like Balmer transitions by irradiating fibre targets with a 2 ps Chirped Pulse Amplification CPA beam of a Nd-glass laser facility. Very high gain coefficients for H-like C, N, O, F, Na Balmer transitions are predicted. The optimum electron density and temperature for maximum gain operation scale approximately asN _e 4 × 10¹³ Z ⁷ cm^–3 and T_e 7 × 10^–3 Z ⁴ eV, respectively, at the time when maximum lasing gain appears. Significant improvement in gain performance of recombination X-ray lasers is predicted by using CPA ps pulse drivers.     

Chemical Bond in Metal Azides and Their Reactivity

Results of experimental investigations into the processes of slow and explosive decomposition of metal azides and the corresponding phenomenological models are analyzed. The energy-band structure and the valence electron density for actual crystallographic planes as well as the hypothetical electron density of the first unoccupied band of KN₃ and AgN₃ are calculated by the density functional method with the use of pseudopotentials. It is demonstrated that the anion-cation chemical bond in KN₃ is predominantly ionic in character, whereas a significant covalent component in AgN₃ is caused by Ag d-states. From the character of distribution of the valence and hypothetical electron densities it follows that the exciton and vibron decomposition mechanisms are improbable. The chain process is also improbable for alkali metal azides, because the gap energy for them is E _g 8 eV, which is close to the energy of azide ion decomposition Q 9–12 eV; the chain process is probable for AgN₃, because E _g(AgN₃) 3 eV. The chain process in AgN₃ is also favored by intense delocalization of the valence electron density through the hybridization of the Ag d-state and the N p-state, which causes a relatively high hole mobility.     

Second-harmonic generation in poled films of nonlinear optical polymer composites

The second-harmonic generation (SHG) has been measured for studying the molecular ordering and its relaxation process in poled states of two different nonlinear optical (NLO) polymer composites.p-Nitroaniline (pNA) and the azo dye Disperse Orange 3 (DO3) are uniformly dispersed in photopolymer matrices followed by poling. Two different composites made-up of 1 wt% of pNA and 2.5 wt% of DO3 are poled at 200 MV m^-1 and 80 MV m^-1, respectively. One of the second-order NLO coefficients,d ₃₃, was found to be 0.27 pm V^-1 for the pNA-doped film and 0.20 pm V^-1 for the DO3-doped film. While for the latter the ratiod ₃₃/d ₃₃ (3.1) is consistent with the value of 3 in the low-field approximation, for the former the value of 3.5 indicates that higher-order effects of the poling field play a significant role in the NLO process. It is suggested that the SHG relaxation dynamics is also closely related to the UV absorption of the NLO molecules in a photopolymer matrix.     

Muon spin relaxation studies in frustrated and/or low-dimensional spin systems

An overview is given on muon spin relaxation (SR) measurements in frustrated and/or low dimensional spin systems. In the frustrated Kagomé lattice system SrCr₈Ga₄O₁₉, we observed dynamic spin fluctuations of 30 GHz, without any static frozen component even atT=0.1 K, much below the susceptibility-cusp temperatureT _g=3.5 K. This is in clear contrast with the case in dilute-alloy spin glassesCuMn andAuFe, where static order develops belowT _g. We also present the dimensionality dependence of the sub-lattice magnetization curves in 2-d Heisenberg systems, the remarkable suppression of the ordering temperature in a 1-d system Sr₂CuO₃, the observation of activation type spin dynamics in a 1-d Ising ferromagnet (DMeFc)(TCNE) aboveT _c, slow spin fluctuations ( 60 MHz) in Haldane-gap systems at low temperatures, and some results from organic 1-d and 2-d magnetic systems.     

The characteristics of multiple-photon absorption of SF6 molecules cooled in free jet expansion from a pulsed supersonic nozzle

The multiple-photon absorption of pulsed TEA CO₂ laser radiation by SF₆ molecules cooled toT _R40K andT _v160K in the free jet expansion from a pulsed supersonic nozzle has been investigated at energy fluences of 0.1 to 3.0 J·cm^–2.For practically all laser lines which coincide with the linear absorption spectrum of thev ₃ vibrational mode of SF₆ [P(12)...P(28), 10,6 m], the dependence of the absorbed energyE _ab on the exciting energy fluence was found to be steeper than linearE _abⁿ, wheren=(1.1 to 1.8). Considerable increases of the absorption cross sections with increasing energy fluence were observed.The fraction of the molecules interacting with the laser radiation is estimated.     

μSR studies in an I2-doped phenylenediamine polymer

We report SR studies of the recently synthesized I₂-doped phenylenediamine polymer. Longitudinal-field spectra exhibit typical behavior of a spin-glass with a freezing temperatureT _g 100 K and a spontaneous static local field 500 G at low temperatures. Preliminary magnetization measurements hint of antiferromagnetic spin coupling and hysteretic behavior aroundT _g. These features suggest possible spin-glass and/or antiferromagnetic spin freezing with a high transition temperature due to the doped radical spins in this organic polymer.Postdoctoral fellow of the Humboldt Foundation.     

On the interface stiffness of the random field ising model

Recent results of Grinstein, Ma, Villain and Binder on interface roughening incontinuum andlattice random field Ising models are related by introducing an effective interface stiffness function {ei247-1}. Ford3 dimensions the continuum theory is shown to be valid for non-zero random field strengthh for all temperatures and on a length scaleL>l _d (h,T) _d (h,T). Ford=2 and smallT a smeared spin-glass transition occurs at ₂(h,T)h. It is argued, that for 3<d<5 interface roughening occurs only forh larger than a critical field strengthh _R (T).