Double-resonant structure in the high-frequency transresistance of coupled electron-hole systems

The linear dc and high-frequency transresistivity of coupled electron-hole systems are investigated using the Lei-Ting balance equations approach extended to include many-body corrections. A possible indirect method of experimentally measuring the dynamical transresistivity in the high frequency (terahertz) regime is designed basing on the detailed analysis on the relationship between the directly measurable resistivities in the electron- and hole-layer and the dynamical transresistance. The theoretically predicted dc transresistance is in good agreement with the experimental data for the given electron-hole system experimentally investigated. The calculated dynamical transresistance exhibits pronounced double-resonant structure, which can be attributed to the cooperation and competition between the two plasmon modes. It is pointed out that the behavior of the frequency-dependent transresistance is temperature-sensitive and the dynamical transport properties are essentially influenced by the short range correlations. Received: 1st April 1998 / Revised: 22 June 1998 / Accepted: 6 August 1998     

Spectroscopic studies of laser-produced plasmas formed in CO and CO2 using 193, 266, 355, 532 and 1064 nm laser radiation

The wavelength dependence of laser-produced breakdown in air, CO and CO₂ has been studied using the four Nd:YAG harmonics (266 nm, 355 nm, 532 nm and 1064 nm) and the ArF-excimer laser (193 nm). Breakdown thresholds at these wavelengths are reported for air, CO and CO₂. A significant reduction in the breakdown thresholds for both CO and CO₂ is apparent when comparing 193 nm with the four Nd:YAG harmonics. This reduction is attributed to the resonance-enhanced two-photon ionization of metastable carbon atoms generated in the laser focus at the ArF-laser wavelength. In addition to reporting breakdown thresholds, the laser-produced plasmas in CO and CO₂ are characterized in terms of plasma temperatures and electron densities which are measured by time-resolved emission spectroscopy. Electron densities range from 9 × 10¹⁷ cm^–3 to 1 × 10₁₇ cm^–3. Excitation temperatures range from 22 000 K at 0.2 µs to 11 000 K at 2 µs. Ionization temperatures range from 22 000 K at 0.1 µs to 16 000 K at 2 µs. Evidence is presented to indicate that, like ArF-laser-produced plasmas, Nd:YAG-laser-produced plasmas formed in CO and CO₂ are in or near a state of Local Thermodynamic Equilibrium (LTE) soon after their formation.     

Effect of topological thresholds on thermal behaviour of germanium telluride glasses containing metallic additive

Differential Scanning Calorimetric (DSC) studies on Ag_xGe₁₅Te_85-x glasses have been undertaken over a wide range of compositions, to understand the effect of topological thresholds on thermal properties. It is found that the compositional dependence of glass transition temperature (T _g ), crystallization temperature (T _c ), activation energy for crystallization and thermal stability show anomalies at the rigidity percolation threshold. Unusual variations also observed in different thermal properties at the composition x = 20, clearly establishes the occurrence of chemical threshold in these glasses. Received: 27 January 1998 / Revised: 12 June 1998 / Accepted: 3 July 1998     

Dynamic hysteresis in ferroelectric systems: experiment and Monte Carlo simulation

A Monte Carlo algorithm for dynamic hysteresis simulation in ferroelectric spin systems is developed based on a DIFFOUR model in which the local spontaneous polarization is defined by the double-well potential energy and the nearest-neighbor spin interaction as well as an external electrical field of variable amplitude and frequency. A direct measurement of the hysteresis loop for ferroelectric Pb(Zr_0.52Ti_0.48)O₃ thin film capacitors using the Sawyer–Tower technique is performed. Significant dependence of the hysteresis shape and pattern on the external field is revealed. Direct imaging of the simulated domain pattern indicates serious suppression of the domain switching over the high-frequency range. The evaluated scaling relations from the simulation for remanence, coercivity, and the area of the hysteresis over the low-frequency range are supported by theoretical predictions and experiments, but the high-frequency scaling behaviors as derived are different from one and another. Received: 23 January 2001 / Accepted: 17 August 2001 / Published online: 20 December 2001     

Correlation among thresholds of evoked otoacoustic emissions,behavioral responses and auditory brainstem responses

In 35 adult human subjects(58 ears)thresholds of rarefactlon clickevoked otoacoustic emission(EOAE),behavioral response(BR)and auditorybrainstem response(ABR)were measured and compared,and correlationcoefficients(r)among them calculated.The results revealed that 86％ of themeasured thresholds were in the range from 10 to 45 dB(nHL)for EOAE,88％ from 10 to 25 dB for BR and 88％ from 10 to 35 for ABR.The correlationcoefficients for the whole threshold sample were 0.415(p<0.002)for EOAE vs.BR,0.501(p<0.001)for EOAE vs.ABR and 0.702(p<0.001)for ABR vs.BR,all indicating highly significant correlation.However,for those ears whoseBR,ABR or EOAE thresholds were elevated,equaling to or exceeding 25 dB,there is no significant correlation between thresholds of EOAE and BR and ofEOAE and ABR(r range:0.176-0.310,p>0.05).In contrast,significantcorrelation between BR and ABR thresholds in the same conditions still re-mained(r:0.533-0.720,p<0.05).The experimental results indicate that thecorrelation between EOAE and hea     

Total cross-sections for positron scattering by a series of molecules

The additivity rule is employed to obtain the total (elastic+inelastic) cross-sections for positron scattering from molecules including a number of diatomic, polyatomic molecules (H₂, N₂, HCl, CO₂, NH₃, SF₆, CH₄, C₂H₄ and C₃H₈) over an incident energy range of 10-1000 eV. The total cross-sections (TCS) of the constituent atoms of molecules are obtained by employing a complex optical model potential (composed of static, polarization and absorption potential). The present results are compared with experimental data and other theoretical calculations, good agreement is obtained in intermediate- and high-energy region. Received: 11 November 1997 / Revised: 23 March 1998 / Accepted: 16 June 1998     

Guiding and high-harmonic generation of sub-10-fs pulses in hollow-core fibers at 1015 W/cm2

15 W/cm². The high resistance of fused silica to damage in the sub-10-fs regime allows stable reproducible operation without degradation of the capillary waveguide. In preliminary experiments, we demonstrate kHz-repetition-rate guided-wave high-harmonic generation in helium down to the 10-nm range. The reported experiments open up the way to realizing high-field interactions with plane-wave excitation at intensity levels in excess of 10¹⁵ W/cm² under well-controlled conditions. Received: 28 May 1998     

Low frequency dielectric permittivity of quasi-one-dimensional conductor (TMTTF)2Br

Conductivity and permittivity of the organic transfer salt (TMTTF)₂Br have been measured at low frequencies (10²-10⁷ Hz) between room temperature down to 4 K. The real part of the permittivity, , is shown to grow below the temperature at which the conductivity is maximum due to charge localization of Mott-Hubbard type. reaches a maximum of 10⁵-10⁶ at 35 K-50 K depending on the samples. Decreasing temperature below , sharply decreases down to helium temperature through the antiferromagnetic phase transition at T _N = 15 K. We explain the magnitude, the temperature and frequency dependence of as resulting from short range charge density wave states in the temperature range where charge localization occurs. This interpretation is supported by recent X-ray scattering measurements. Received: 10 October 1997 / Revised: 28 February 1998 / Accepted: 3 March 1998     

Expansions for the eigenvalues of three-term recurrence relations. Two applications in molecular physics

Approximate expressions for the eigenvalue of a three-term recurrence relation with a general form describing various physical problems are proposed. Their range of availability is examined by comparison with exact values for two different problems: the bound and continuum states of monoelectronic diatomic ions and the Schr?dinger equation describing molecular alignment in intense laser fields. For each case, very good predictions have been obtained, which may be useful as initial values in iterative procedures for deriving exact solutions. Received: 30 January 1998 / Received in final form: 10 April 1998 / Accepted: 25 May 1998     

High dynamic range measurement of temporal shape and contrast of ultrashort UV pulses

This paper presents an experimental technique for measurement of the contrast ratio of ultrashort UV pulses. As a multiple-shot device based on the scheme of difference frequency generation this is, to our knowledge, the only cross correlator in the UV so far, which offers a dynamic range of 10⁷ and operates with input pulse energies as low as 5 μJ. By changing the cross correlator into single-shot mode, the temporal shape of the UV pulse can be measured. Received: 26 October 1998 / Revised version: 13 January 1998 / Published online: 28 April 1999     

Calculation of X-ray natural circular dichroism

A method is presented to calculate the natural circular dichroism recently discovered in the X-ray range (XNCD). The basic formula represents XNCD as an odd second-rank tensor and leads to a sum rule that relates XNCD to the mixing of odd and even orbitals in the ground state. A multiple-scattering theory of XNCD is presented, and calculated spectra for the L-edges of iodine in LiIO₃ compare favorably with the experiments. Received: 20 October 1997 / Revised: 18 February 1998 / Accepted: 31 March 1998     

Size-dependent light emission from mass-selected clusters

The emission of photons in the visible wavelength range from mass-selected Ag⁺ _n, Cu⁺ _n, Pt⁺ _n and Pd⁺ _n () clusters is observed. Photons are detected 10^-4 s after the cluster generation in a sputter source. The emission intensities display distinct variations with cluster size and material. The observations are interpreted in terms of the decay of metastable states which are excited during the high-energy sputtering process used for the generation of these clusters. Received: 28 October 1997 / Revised: 5 January 1998 / Accepted: 30 January 1998     

High-E_T inclusive jet cross sections in photoproduction at HERA

Inclusive jet differential cross sections for the reaction with quasi-real photons have been measured with the ZEUS detector at HERA. These cross sections are given for the photon-proton centre-of-mass energy interval 134 277 GeV and jet pseudorapidity in the range in the laboratory frame. The results are presented for three cone radii in the plane, . Measurements of above various jet-transverse-energy thresholds up to 25 GeV and in three ranges of W are presented and compared to next-to-leading order (NLO) QCD calculations. For jets defined with differences between data and NLO calculations are seen at high and low . The measured cross sections for jets defined with are well described by the calculations in the entire measured range of and . The inclusive jet cross section for GeV is consistent with an approximately linear variation with the cone radius R in the range between 0.5 and 1.0, and with NLO calculations. Received: 11 February 1998 / Published online: 2 July 1998     

Effect of He+ and H+ ion irradiation on polystyrene films probed by NIR photothermal deflection spectroscopy

+ and H⁺ ions. Photothermal deflection spectroscopy (PDS) technique was used to record the absorption spectrum in the wavelength range 1.20 μm to 2.0 μm. The evolution of the various overtone and combination bands occurring in this range has been related to the changes taking place in the polystyrene structure. Received: 1 September 1998 / Accepted: 20 November 1998 / Published online: 24 February 1999     

Nanosecond laser damage resistance of differently prepared semi-finished parts of optical multimode fibers

Optical multimode fibers are applied in materials processing (e.g. automotive industry), defense, aviation technology, medicine and biotechnology. One challenging task concerning the production of multimode fibers is the enhancement of laser-induced damage thresholds. A higher damage threshold enables a higher transmitted average power at a given fiber diameter or the same power inside a thinner fiber to obtain smaller focus spots.In principle, different material parameters affect the damage threshold. Besides the quality of the preform bulk material itself, the drawing process during the production of the fiber and the preparation of the fiber end surfaces influence the resistance. Therefore, the change of the laser-induced damage threshold of preform materials was investigated in dependence on a varying thermal treatment and preparation procedure.Single and multi-pulse laser-induced damage thresholds of preforms (F300, Heraeus) were measured using a Q-switched Nd:YAG laser at 1064 nm wavelength emitting pulses with a duration of 15 ns, a pulse energy of 12 mJ and a repetition rate of 10 Hz. The temporal and spatial shape of the laser pulses were controlled accurately.Laser-induced damage thresholds in a range from 150 J cm⁻² to 350 J cm⁻² were determined depending on the number of pulses applied to the same spot, the thermal history and the polishing quality of the samples, respectively.     

Diffractive effects in singly resonant continuous-wave parametric oscillators

I /k_P=0.33and0.50. It is found that the ratio of pump depletion to maximal depletion as a function of the ratio of pump power to threshold power agrees with the plane-wave prediction to within 5%, for a wide range of focusing conditions. The observed trends are explained as resulting from intensity- and phase-dependent mechanisms. Received: 19 January 1998/Revised version: 13 March 1998     

High-frequency audiometry: test reliability and procedural considerations

This study compared the reliability of a recently developed high-frequency audiometer (HFA) [Stevens et al., J. Acoust. Soc. Am. 81, 470-484 (1987)] with a less complicated system that uses supraaural earphones (Koss system). The new approach permits calibration on an individual basis, making it possible to express thresholds at high frequencies in dB SPL. Data obtained from 50 normal-hearing subjects, ranging in age from 10-60 years, were used to evaluate the effects on reliability of threshold variance, earpiece/earphone fitting variance, and the variance associated with the HFA calibration process. Without earpiece/earphone replacement, the reliability of thresholds for the two systems is similar. With replacement, the HFA showed poorer reliability than the Koss system above 11 kHz, largely due to errors in estimating the calibration function. HFA reliability is greater for subjects with valid calibration functions over the entire frequency range. When average correction factors are applied to the Koss data in an effort to convert threshold estimates to dB SPL, individual transfer functions are not represented accurately. Thus the benefit of being able to express thresholds at high frequencies in dB SPL must be weighed against the additional source of variability introduced by the HFA calibration process.     

Growth and absorption properties of Dy-doped and undoped p-type TlGaSe2

2 and Dy-doped p-TlGaSe₂ (p-TlGaSe₂:Dy)single crystals were grown by the Bridgman–Stockbarger method. Absorption spectra were measured on freshly cleaved (001) surfaces. The freshly cleaved crystals had a mirror-like surface and there was no need for mechanical treatment. The absorption measurements were carried out in p-TlGaSe₂ and p-TlGaSe₂:Dy samples in the temperature range 10–320 K with a step of 10 K. The phonon energies calculated in p-TlGaSe₂ and p-TlGaSe₂:Dy are 23.0 meV and 21.0 meV, respectively. The direct band gaps of p-TlGaSe₂ and p-TlGaSe₂:Dy are 2.279 eV and 2.294 eV at 10 K, respectively. There is an abrupt change for the energy peak for p-TlGaSe₂ in the temperature ranges 105–120 K, 240–250 K, and for p-TlGaSe₂:Dy in the temperature ranges 100–110 K, 240–260 K. Received: 3 December 1997 / Accepted: 5 October 1998     

Pulsed laser-induced ablation of absorbing liquids and acoustic-transient generation

2 CrO₄ are irradiated by a KrF excimer laser (λ=248 nm, FWHM=24 ns) with moderate energy density (up to 100 MW/cm²) below the plasma-formation threshold. The ablation process, including the vapor-cavity formation and the acoustic-wave propagation is visualized by laser-flash photography. The ablation thresholds are determined by measuring the generated pressure transients and vapor-phase kinetics using a broadband piezoelectric pressure transducer and a simultaneous optical-transmission probe, respectively. The mechanisms of liquid ablation and acoustic-pulse generation are investigated based on the thermoelastic behavior of the liquid medium and the evaporation dynamics. A numerical model is proposed to describe the explosive-vaporization process at high laser fluences. The computation results are compared with the experiment. In short-pulse heating, ablation can be initiated at low laser fluences by the tensile component of the thermoelastic stress without a significant increase in the liquid temperature. On the other hand, if the heating rate is rapid enough to achieve a high degree of superheating of the liquid, the abrupt increase of the homogeneous-bubble-nucleation rate leads to explosive vaporization, which then plays the major role in the ablation dynamics. The pressure transient in the liquid is generated thermoelastically at low laser fluences, but the contribution of the vapor-phase expansion and/or the recoil momentum exerted by the ablation plume becomes significant at high laser fluences. Shock waves are formed in the ambient air in the case of explosive vaporization. The propagation of these wave fronts is in good agreement with the numerical-computation results. Received: 8 February 1998/Accepted: 10 February 1998