Degenerate four-wave-mixing spectroscopy in NaH

We performed DFWM spectroscopy on X ^–1+–A ¹⁺ transitions in NaH produced in an indirect photochemical reaction between Na(3p) and H₂ and detected v=1, 2 and 3 ground state vibrational levels of NaH molecules, whereas with resonance enhanced CARS, we observed v=0 levels only. This different sensitivity can be explained by considering the Franck-Condon-factors and the relevant damping coefficients for the corresponding transitions in the NaH molecule. Time resolved DFWM spectroscopy showed that NaH(v=1) molecules effectively live much longer than Na(3p) atoms which merely follow the laser excitation pulse.     

The effects of collisional quenching on degenerate four-wave mixing

We report a theoretical and experimental investigation of the effects of collisional quenching on resonant degenerate four-wave mixing (DFWM). Using single-mode laser radiation, peak signal intensity measurements were performed on an isolated line in the A – X transition of NO. By using appropriate mixtures of N₂ and CO₂ as buffer gases, we varied the collisional quenching rate over several orders of magnitude while maintaining a fixed total collisional dephasing rate. The mixtures had approximately 100 Torr total pressure and were at room temperature. For I/I _sat approximately equal to 0.02, DFWM intensities were found to be less affected by variations in quench rate than were laser-induced fluorescence (LIF) intensities (I and I _sat are the pump laser and one-photon saturation intensities, respectively). Moreover, for I/I _sat roughly equal to 0.5, DFWM intensities were observed to be nearly independent of quench rate. The results are compared to two theoretical predictions, with good agreement observed. Both theories indicate that the minimum sensitivity of DFWM to quenching occurs near I/I _sat1.     

Critical currents in polycrystalline thin films of high-T c superconductors

Summary It is shown that the behaviour of the temperature dependence of the critical current in polycrystalline thin films of high-T _c superconductors depends crucially on the assumption made concerning the nature of the intergranular material. The usual assumption of a superconductor-insulator-superconductor (=SIS) ?sandwich? between each grain leads to a crossover fromI _c∼(1−T/T _c) toI _c∼(1−T/T _c)^3/2, for temperatures nearT _c (whereI _c is the critical current,T the absolute temperature, andT _c the superconducting transition temperature). Instead, for a superconductor-normal metal-superconductor (=SNS) sandwich the dependenceI _c∼(1−T/T _c)² is found for all temperatures. Consideration is given to the effect of self-magnetic field on the analysis. The comparison between expressions for continuous and granular systems is extended. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

Transport critical-current and voltage-current characteristics of Bi(2223) silver-clamped thick films

The transport critical current properties of Bi(2223) silver-clamped thick films are studied by the measurement of its dependence on magnetic field and temperature close to T _c. It is found that the transport critical current follows a power law J _c(1–T/T _c)^3/2 for the sample with J _c>2.0×10⁴ A/cm² (77K, zero field) and that J _c(H) is basically reversible for increasing and decreasing magnetic field. After the transport current exceeds the critical current, the voltage-current (V-I) characteristics show a flux-creep-like behaviour until they smoothly join the flux-flow state. From the measurement of V-I curves, the pinning property of the sample may be estimated using the flux-creep mode. The flux-flow resistance is found to have a nonlinear magnetic field dependence.     

Impact of the side-path model on the muon cycling rate density dependence in D-T mixtures

The side-path model previously suggested by us allows for muon transferfrom tritiumto deuterium via intermediate dtµ^* resonances formed in tµ(2s)-D₂ collisions. Taking this effect into account, the density dependence of the muon cycling rate in D-T mixtures at low temperatures is analysed. Compared with cascade models where only one-sided transfer is present, the predicted density dependence of _c is in better agreement with measurements.     

Preparation and transport properties of Ba1−x Sr x CuO2+δ infinite layer films grown by molecular-beam epitaxy

Thin films of Ba_1–x Sr _x CuO₂₊ in the infinite layer structure were prepared by molecular beam epitaxy on SrTiO₃ substrates. Excellent in-plane order during growth was shown by RHEED. The lattice constant inc-direction was determined by x-ray diffraction. It changed from 0.404 nm to 0.345 nm whenx increased from 0 to 1. The film surfaces were smooth with some outgrowths as revealed by atomic force microscopy. Excellent crystal structure and epitaxy of the films was demonstrated by high resolution transmission electron microscopy. The room temperature dc resistivities of the films varied from 10^–3 cm to 10² cm, depending onx and on the oxidation conditions during growth. The resistivities of most films showed negative temperature coefficients and obeyed the conduction model of variable range hopping at low temperatures. In the composition rangex=0.5–0.8, however, an anomalous resistance dependence on temperature was observed in many samples. The resistivities started to deviate from the monotonic behaviour just below 200 K and in some cases dropped remarkably at temperatures below 140 K.     

Role of interstitial “caged” Fe in the superconductivity of FeTe1/2Se1/2

All samples are synthesized through a standard solid state reaction route and are quenched to room temperature systematically at 700 °C, 500 °C, 300 °C, and room temperature (RT); the samples are denoted 700Q, 500Q, 300Q, and RTQ, respectively. The structural, and magnetic properties are studied. Careful Rietveld analysis of the X-ray diffraction patterns revealed that all samples except 700Q crystallized in a single phase with space group P4/nmm; the amount of interstitial Fe (Fe_int) at the 2c site increased from 5% for RTQ to 8% for 500Q. Sample 700Q crystallized in the Fe₇Se₈ phase. The magnetization result revealed that RTQ and 300Q are superconducting at 10 K and 13 K, respectively, while 500Q and 700Q are not superconducting. Magnetic ordering was observed at around 125 K for all the samples. The prominence of in terms of effective moment is sufficiently higher for 500Q and 700Q than for RTQ and 300Q. Summarily, it is found that quenching-induced disorder affects the occupancy of interstitial Fe in FeTe_1/2Se_1/2 and thus both its superconducting and magnetic properties. Further, it clear that limited presence of interstitial Fe at 2c site is not completely contrary to the observation of superconductivity, because the 300Q sample possesses higher T_c (13 K) for higher Fe_int (6%) than the RTQ sample with relatively lower T_c (10 K) having lower Fe_int (5%). Further, the 500Q sample, with much higher Fe_int (8%), is non-superconducting.     

Critical current density of a YBa2Cu3O7 film: Comparison between experiment and collective pinning theory

With the vibrating reed technique we have measured the critical current densityJ _c of a Y 123 200 nm thin film as a function of magnetic field perpendicular to the CuO₂ planes (O T<B8 T) and temperature (20KT60 K). At fieldsB<0.1 TJ _c is magnetic field independent and decreases at higher fields. A comparison with theory indicates that a crossover from a single pinning to a small bundle collective pinning regime may explain the observed behaviour. According to our estimate the main pinning centers are weak point pins due to oxygen vacancies. From the temperature dependence ofJ _c atB0 we obtain a temperature dependence of the thermodynamical critical fieldB _c (1–T/T _c )² forT20 K which agrees with the anomalous temperature dependence ofB _c2 observed recently in highly anisotropic high temperature superconductors.     

Temperature measurement using degenerate four-wave mixing with non-saturating laser powers

We report the use of Degenerate Four-Wave Mixing (DFWM) in the OHA ^{2 +} X ² ( = 0 = 0) band for temperature determination in a propane/air flame using laser powers below the saturation level. We show that at these low power levels the dependence of the signal on the dipole moment for the transition has to be established before meaningful temperature data can be obtained. This presents a paradox in that the temperature has to be known before the form of the dependence on the dipole moment can be determined. Solutions to this paradox are presented. We also show that absorption of the laser beams in this OH band system cannot be neglected and that failure to correctly account for the absorption leads to a large over estimate of the flame temperature. Furthermore, we show that the accuracy of the absorption-corrected temperature is critically dependent on the accuracy with which the measurement position within the flame is known. Finally, the temperature calculated from DFWM spectra using the correct dipole moment power and absorption is compared to the temperatures obtained using Laser-Induced Fluorescence (LIF) and Coherent Anti-Stokes Raman Spectroscopy (CARS).     

Thermometry of an oxy-acetylene flame using multiplex degenerate four-wave mixing of C2

Thermometry of an oxy-acetylene flame using multiplex Degenerate Four-Wave Mixing (DFWM) of C₂ is demonstrated. More than 100 rotational transitions in thed ³ _g a ³ _u (0,0) Swan band of C₂ could be recorded simultaneously by use of a pulsed, broad bandwidth modeless laser. Temperatures were inferred by fitting temperature-dependent synthetic spectra of single- or multiple-shot averaged spectra. The strength and reliability of recorded signals together with the large number of rotational lines observed suggest that multiplex DFWM is a promising technique for minor species detection and for temporally resolved temperature measurements in luminous environments. Factors influencing the accuracy and precision of single-shot thermometry using the technique are discussed.     

Fluorescence and attenuation properties of Er3+ -doped phosphate-glass fibers and efficient infrared-to-visible up-conversion

Fluorescence spectra of phosphate glasses with different erbium doping have been measured. The flourescence intensity reaches its maximum at an Er³⁺-doping concentration of 0.75 mol%. When the Er³⁺ doping exceeds 0.75 mol%, the fluorescence intensity decreases due to concentration quenching. The attenuation at 1.53 µm of the fiber is 12.8 db/m. The fluorescence up-conversion of 1.064 µm Nd:YAG laser pulses into intense green 547 and 667 nm light in the fiber has been measured. The fluorescence output power of green (547 nm) and red (667 nm) light is 178 and 42 W, respectively with an excitation power of 1 W. The two signals are referred to as⁴ S _3/2 ⁴ I _{1 5/2} and⁴ F _9/2 ⁴ I _{1 5/2} transitions through two-photon absorption fluorescence.     

Elastic constants of the alloys Cd1−x Zn x (x<0.3%) and their temperature dependence

The temperature dependence of elastic constants of Cd_1–x Zn _x alloys, havingx=0.021, 0·042 and 0·233 at.%, has been measured in the temperature range from 4·2 to 300 K by the pulse-echo-overlap ultrasonic method. The adiabatic compressibilities and the Debye temperatures have been calculated. All elastic constants with the exception ofc ₄₄ increase slightly with the growing concentration of Zn.The summary of this paper was presented on the 7^th Conference on Ultrasonic Methods in ilina, September 11^th–13^th, 1980.     

Analysis of degenerate four-wave mixing spectra of NO in a CH4/N2/O2 flame

4 /N₂/O₂ flame to spectral simulations based on a two-level theory for stationary, saturable absorbers by Abrams et al. Temperatures determined from least-squares fits of simulations to experimental spectra in the A²Σ⁺?X²Π⁺(0,0) band are compared to temperatures obtained from OH absorption spectroscopy and a radiation-corrected thermocouple. We find that DFWM rotational temperatures derived from Q-branch spectra agree with thermocouple and are independent of pump laser intensity for low to moderate saturation (I≈I_sat). However, the temperatures are systematically low and depend on pump intensity if the analysis neglects saturation effects. We demonstrate a method for obtaining an effective pump saturation intensity for use with the two-level model. This approach for analyzing saturated DFWM line intensities differs from previous work in that the use of the theory of Abrams et al. rather than a transition-dipole-moment power law allows treatment of a much wider range of saturation. Based on the observed signal-to-noise ratio an NO detection sensitivity of 25 ppm is projected, limited by a DFWM background interference specific to hydrocarbon flames. Received: 15 September 1998 / Revised version: 18 November 1998 / Published online: 24 February 1999     

Unusual behaviour of conductivity and superconductivity in solid Sn−Xe mixtures

We have measured the normal statedc conductivity , the superconducting transition temperatureT _c and the critical superconducting currentI _c0 of frozen Sn–Xe mixtures in dependence of the Sn atomic fractionc. A new, high-T _c phase is observed forc between 0.58±0.02 and 0.74±0.02. This phase has aT _c-value of about 5.7 K, it shows a strong decrease in and inI _c0 and transforms into disordered Sn during annealing between 15 K and 50 K.It is a pleasure for us to dedicate this paper to Prof. Dr. S. Methfessel on the occasion of his 60^th birthday.This work was supported by the Deutsche Forschungs-gemeinschaft     

Inhomogeneous muonium diffusion in solid nitrogen

The spin dynamics of the muonium (Mu) atom diffusing quantum mechanically in solid nitrogen (s-¹⁴N₂) has been studied using the technique of Mu spin relaxation. A strong relationship between longitudinal (T ₁ ^–1 ) and transverse (T ₂ ^–1 ) relaxation rates (familiar in NMR) has been experimentally demonstrated for the first time for muonium relaxation. At low temperatures the results are inconsistent with diffusion models using a single correlation time _c; this is taken as evidence for the intrinsic inhomogeneity of the problem. The temperature dependence of theaverage Mu hop rate _c ^–1 gives clear evidence that Mu quantum diffusion ins-N₂ is governed by the two-phonon interaction.     

Temperature dependence of the critical current density in Y-Ba-Cu-O thin films

We present a study of the temperature dependence of the critical currentJ _c of several dc magnetron sputtered thin Y-Ba-Cu-O films on single crystalline SrTiO₃, ZrO₂ and Al₂O₃ substrates. Near the critical temperature T_c it is found thatJ _c(1–T/T_c)ⁿ withn=3 for the SrTiO₃ and ZrO₂ substrates, whilen=1·3 for the Al₂O₃ substrate. The temperature dependence in our samples approximately agrees with standard theories for weak links or with the Ambegaokar-Baratoff equation.     

Raman-spectroscopy study of PbTiO3 thin films grown on Si substrates by metalorganic chemical vapor deposition

Raman spectra have been investigated in PbTiO₃ thin films grown on Si by metalorganic chemical vapor deposition. A large grazing-angle scattering technique was taken to measure the temperature dependence of Raman spectra below room temperature. All Raman modes in the thin films are assigned and compared with those in the bulk single crystal, a newA ₁(TO) soft mode at 104 cm^–1 was recorded which satisfies the Curie-Weiss relation ² =A(T _c –T). Intensities of theA ₁(1TO) andE(1TO) modes were anomalously strengthened with increasing temperature. Raman modes for the thin films exhibit remarkable frequency downshift and upshift which is related to the effect of internal stress.     

Anisotropic optical properties of single-crystal GdBa2Cu3O7−δ

The optical spectrum of reduced-T _c GdBa₂Cu₃O_7– has been measured for polarizations parallel and perpendicular to theab plane. The sample was an oxygen-deficient single crystal with a large face containing thec axis. The polarized reflectance from this face was measured from 20–300 K in the spectral region from 30–3000 cm^–1, with 300 K data to 30000 cm^–1. Kramers-Kronig analysis was used to determine the spectral dependence of theab and thec components of the dielectric tensor. The optical properties are strongly anisotropic. Theab-plane response resembles that of other reduced-T _c materials whereas thec axis, in contrast, shows only the presence of several phonons. There is a complete absence of charge carrier response alongc aboveand belowT _c. This observation allows us to set an upper limit to the free-carrier spectral weight for transport perpendicular to the CuO₂ planes.Permanent address: Institute of Physics, CSAV, Prague, Czechoslovakia     

High-resolution laser spectroscopy on H2 at 97–98 nm

A narrowband tunable eXtreme UltraViolet (XUV) laser source is used for a high resolution study of the Lyman (B ¹ _u ⁺ X ¹ _g ⁺ ) band system of molecular hydrogen. Seven rotational transitions of two vibrational bands, (10,0) and (11,0), in the wavelength range from 97.2–98.3 nm have been investigated for the first time under sub-Doppler molecular beam conditions. A calibration procedure using the I₂ standard in the visible yielded an absolute frequency accuracy of 0.02 cm^–1. The obtained H₂ transition frequencies provide a calibration standard in the extreme ultraviolet wavelength region.     

Magnetic order in a Kondo lattice: A neutron scattering study of CeCu2Ge2

Elastic and inelastic neutron scattering studies of the Kondo lattice CeCu₂Ge₂ were performed. AtT _N=4.1 K an incommensurate magnetic order develops with an ordering wave vectorq ₀=(0.28, 0.28, 0.54) and an ordered moment µ _s =0.74 µB. The crystalline electric field splits the 4f ¹-J-multiplet of the Ce ion into a ground state doublet and a quartet at 191 K. The wave function of the ground state yields an ordered moment of 1.54µB. Thus, due to the onset of the formation of a Kondo singlet the magnetic moment is considerably reduced. The magnetic relaxation rate was investigated via quasielastic neutron scattering. The temperature dependence of (T) is characteristic of heavy-fermion systems with a high temperature square root dependence and a limiting low temperature value, yielding a Kondo temperatureT _K10K. The quasielastic component of the scattered neutron intensities persists down to the lowest temperatures, well belowT _N. This quasielastic line is regarded as a characteristic feature of heavy-fermion systems and corresponds to the enhanced value of the linear term of the specific heat.