Temperature dependence of the Knight shift for cadmium in palladium

The Knight shift and its temperature dependence for a Cd impurity in palladium metal were measured by means of DPAD- and DPAC-methods utilizing the well known 5/2⁺, 247-keV state in¹¹¹Cd. The shift at 80 K was found to be KS (CdPd, 80 K)=?0.8(2)%. The observed variation of the KS in the temperature range from 80 K up to 1400 K is 0.5%. For calibration purposes an accurate remeasurement of the magnetic moment of the 5/2⁺ state in¹¹¹Cd was necessary and yieldedμ(¹¹¹Cd, 5/2⁺, 247 keV)=?0.7697(20) n.m.     

On classical rotational rainbow structures for initially rotating molecules

The specific mass shift (SMS) dominates the isotope shift for a light atom like Na. It arises from a correlation between the electronic momenta and is very sensitive to pair correlation effects in the wavefunction. We here extend earlier work on the SMS of Li and K and evaluate the second-order pair correlation contributions for the 3s, 3p and 3d levels of Na. For the resonance line, 3s→3p, the inclusion of the lowest-order correlation effects changes the theoretical result from 16% to about 80% of the experimental value. We also demonstrate that for scalar operators, like the SMS and also the field shift, diagrams involving only single excitations are included in a restricted Hartree-Fock calculation if the valence electron is included in the potential. The analogy is illustrated by numerical results both for the specific mass shifts and the field shifts for Na and K.     

Temperature and magnetic field dependence of the absorption edge and the conduction band width of ferromagnetic semiconductors

We investigate the shift of the absorption edge and the behaviour of the conduction band width of ferromagnetic semiconductors as functions of temperature T and an external field B, respectively. Numerical results are given for EuO and EuS. As a consequence of electron-magnon scattering processes the band width of EuO is enhanced by more than 14% in the temperature region: 40 K…T…80 K. The external field tries to surpress this effect.     

161Dy Mössbauer study of the endohedral metallofullerenes Dy@C n (n = 80, 82, 84)

Mössbauer studies of Dy@C _n (n = 80, 82, 84) metallofullerenes were performed at 4.2, 9.6 and 78 K with the ¹⁶¹Dy (25.6 keV) resonance. The observed spectra consist of two subspectra splitted by magnetic hyperfine fields near to the full moment value of trivalent Dy. Paramagnetic relaxation is observed even at 4.2 K. The observed isomer shift is consistent with the Dy³⁺ state and indicates a full charge transfer to the fullerene cage.     

Low-temperature electron transport in GaAs

For temperatures between 5 and 80 K, electron mobilities in n-type GaAs have been calculated using ionized impurity cross sections derived from the partial wave phase shift method. The mobilities obtained differ significantly from those calculated previously using Brooks-Herring theory. Although agreement with data from a number of experimental samples is good above 30 K, the theoretical mobilities tend to overestimate experiment at lower temperatures. In order to improve the agreement it may be necessary to incorporate the effects of multi-ion scattering and collision-broadening, and to generalize the screened Coulomb scattering potential.     

Pressure and temperature induced modification of luminescence from tetracene single crystals

Fluorescence spectra of crystalline tetracene have been recorded in the temperature range 120 to 300 K under hydrostatic pressure up to 600 MPa. From discontinuities in both emission spectra and spectral intensities it is concluded that two phase transitions occur. The room temperature phase is transformed to a low temperature phase/high pressure phase I at T^I_t (p = 0) = 182 K, the temperature coefficient being dT^I_t/dp = 0.395 K/MPa. The phase transition is induced by a decrease of the specific volume under pressure and/or upon cooling. Lack of a significant shift of the origin of the fluorescence band near T^I_t at constant pressure is an artifact resulting from the neglect of reabsorption effects. The Stokes shift is 260 cm^-1, independent of temperature and crystal modification. In accord with previous Raman data a second phase transition occurs at T^II_t (p) = 143 K, the pressure shift being dT^II_t/dp = 0.088 K/MPa.In addition, the shift of the triplet energy as a function of pressure as well as the pressure-dependence of the rate constants governing fission of a singlet exciton into a pair of triplets is discussed utilizing their magnetic field dependences.     

The adsorption of CO on Pd(100) studied by IR reflection absorption spectroscopy

High resolution vibrational spectra of the C-O stretching region have been measured for the system CO/Pd(100) at temperatures between 80 and 340 K. LEED and flash desorption studies enabled the absolute coverage for each spectrum to be determined. The C-0 stretching frequency was found to shift linearly with coverage for the three adsorption temperatures 80, 200 and 300 K. At saturation coverage (θ ~ 0.8) the total shift is of the order of 100 cm⁻¹. Experiments with ¹²CO /¹³CO mixtures showed that the contribution of vibrational coupling is ~ 35 cm⁻¹ the rest being due to a static, partly chemical, lateral interaction. The apparent commensurate-incommensurate transition at θ = 0.5 has no significant influence on shift, width or intensity of the absorption band.     

Mobility of positrons in silicon

The drift velocity v₊ of positrons in Si has been measured by observing the Doppler shift of the annihilation γ's. The electric field dependence of v₊ yields the positron mobility μ₊: at 80 K μ₊=460±20 cm²V^-1 sec^-1 and at 184 K μ₊=173±15 cm²V^-1 sec^-1.     

Das Emissionsspektrum gebundener Excitonen in Zinkoxydkristallen in Abhängigkeit von Temperatur und uniaxialer Verspannung

The recombination radiation of bound excitons in ZnO-crystals (spectral range 3,38–3,35 eV at 4.2 °K) and the adjacent phonon-assisted spectrum have been studied at temperatures between 4.2 °K and 90 °K. The phonon-assisted spectrum changes its structure at about 30 °K. With increasing temperature the exciton lines shift toward smaller quantum energies (ΔE～T ²) and their half widthH increases (H～T ²). The line shift is explained by a band gap variation due to deformation potential coupling of the holes toTA phonons. The line width is explained by broadening of an infinitely sharp line under the influence of the mean square fluctuation of thermally generated crystal deformations. The influence of uniaxial stress on the spectral positions of the lines has been studied.     

Spin gaps and spin dynamics in superconducting cuprates

Spin gap effects, consisting of a declining uniform susceptibility and spin paramagnetic NMR shift at low temperaturesin the normal state and associatedT ₁ behavior, are discussed and documented in several cuprate superconductors. Dynamic spin magnetism in these systems is further reviewed in the light of mean-field models, where we note that detailed results from the model by Millis, Monien, and Pines are not borne out in recent neutron data on YBa₂Cu₃O_6.92.T ₁ data on¹⁷O in La_1.85Sr_0.15CuO₄ are presented, showing consistency with neutron dynamic susceptibility data forT≧80 K, but exhibiting a strong spin gap character below 80 K which is not present in the neutron data. Data for Zn-doped YBCO withT _c≈60 K are also presented, showing strong RKKY broadening from localized moments in the planes, but no spin gap effect such as that found in theT _c=60 K oxygen-deficient phase.     

Optical absorption in MnIn2S4 single crystals

Optical absorption in MnIn₂S₄ single crystals has been studied. Direct and indirect optical transitions are found to occur at photon energies of 1.90?C2.16 eV in the temperature range of 80?C342 K. The temperature dependence of the band gap is determined; its temperature coefficients E _gd and E _gi are found to be ?4.84 × 10^?4 and ?6.33 × 10^?4 eV/K, respectively. The electron-phonon interaction is the main mechanism of the temperature shift of the intrinsic-absorption edge. MnIn₂S₄ single crystals exhibit anisotropy in polarized light at the absorption edge in the temperature range of 90?C190 K; the nature of this anisotropy is explained.     

Properties of low-temperature spectral relaxation of eosin phosphorescence in a glycerol-water mixture

The low-temperature dynamics of the Stokes shift of the instantaneous phosphorescence spectra of eosin in 94% glycerol-water solution is studied upon pulsed excitation. At temperatures T<T _tg , where T _g =183–187 K is the glass transition temperature of the solution, the blue shift of the instantaneous phosphorescence spectra is observed. In the vicinity of T _g , the spectral dynamics undergoes inversion, and the red shift of the instantaneous spectra is observed at temperatures between 188 and 210 K, which is caused by freezing out the orientation mobility of the solvent molecules in the first solvate shells of the chromophore molecule. In the temperature range from 230 to 273 K, the enhancement effect was observed at the initial parts of phosphorescence kinetic curves, which is probably explained by cooperative phenomena in the solvent.     

Magnetic phase transitions in Fe80-xNixCr20 (14 ⩽ x ⩽ 30) alloy studied by using 57Fe Mössbauer spectroscopy

This paper reports ⁵⁷Fe Mössbauer spectroscopic studies of the polycrystalline samples of the substitutionally disordered, isostructural (fcc), ternary alloy system Fe_80-xNi_xCr₂₀ for x = 30, 26, 19 and 14 in the temperature range of 10–295 K. The data have been analyzed in terms of the magnetic phase transitions occurring in these alloys by examining the temperature dependence of the various Mössbauer parameters like line‐width, center shift, resonance area, distribution of hyperfine field, P(H), and the average hyperfine field 〈H〉. An estimate of the magnetic transition temperature T _C is made for the alloys with x = 30 and 26 and these results are compared with those previously obtained by magnetic measurements and neutron diffraction experiments. The data for the second order Doppler shift have been analyzed to estimate the Debye temperature ΘD for alloys with x = 30 and 26.     

Anomalous temperature dependence of the energy gap of AgGaS2

The lowest energy gap Eg of AgGaS₂ in the temperature range from 4.2 to 300° K was determined from the reflectivity, photoluminescence and absorption measurements. Below ～ 80° K the temperature coefficient of the energy gap is +6 × 10^-5eV/°K. Above ～80° K the sign of the coefficient reverses and the value is -1.8 × 10^-4eV/°K. The positive value is explained with the lattice dilation effect being the dominant mechanism for the energy gap variation at lower temperatures than ～80°K.     

Optical band-gap of Zn3As2

Absorption measurements of single Zn₃As₂ crystals were made at temperatures 5, 80 and 300 K. Free-carrier absorption is interpreted in the simple classical model. Interband absorption shows contributions from Urbach-like excitations. The direct optical gap has been estimated as 0.99 eV at 300 K, 1.09 eV at 80 K and 1.11 eV at 5 K. The linear dependence of band-gap on temperature was found in the range 80–300 K with dE_g/dT = ? 4.55 × 10^?4eVK^?1.     

Temperature behavior of the 6.05-eV band in optical absorption spectra of oxygen-deficient corundum

The effect of temperature on the 6.05-eV absorption band in α-Al₂O₃ has been studied in the 80–515 K region. The data obtained are analyzed in terms of a one-coordinate model with strong electron-phonon coupling. This band is shown to be formed by two peaks at 5.91 and 6.22 eV (T=293 K) originating from absorption at the F ⁺ and F centers, respectively. An analysis of the experimental temperature dependences has allowed us to calculate the energies of effective phonons responsible for the broadening and shift of the peaks. The energies calculated agree with the data obtained in other studies and lie in the region of corundum acoustic-vibration frequencies. The Huang-Rhys factors have been evaluated for both centers and found to be close to the estimates made by other authors. The results are discussed in detail and compared with independent data on optical absorption and luminescence of anion centers in colored and irradiated α-Al₂O₃ single crystals.     

Mössbauer study of Fe57 and Co57 in Co1 + xV2−xO4

Mössbauer sources and absorbers, prepared by doping Fe⁵⁷ and radioactive Co⁵⁷ into samples of Co_1?xV_2?xO₄ (0 ? x ? 1), were studied in the temperature range of 80–500 K. The source and absorber spectra are very similar. The absence of any Fe²⁺ at the A site can be understood by partial covalent bond formation with an anion. However, the predominance of Fe³⁺ at the B site (with some Fe²⁺ for x = 1) cannot be explained by simple crystal-field or molecular-orbital theories. The x dependence of the isomer shift and of the Fe³⁺B-site quadrupole interaction can be related to changes in the lattice constant and the oxygen parameter. The temperature dependence of the Fe²⁺B-site quadrupole interaction can be fitted in the motional-averaging model. In the range of 0 ? x ? 0.5 the temperature dependence of the isomer shift shows effects of chemical bonding beyond the second-order Doppler shift.     

Further experiments on energy transfer from bom barding ions to Na2O containing glasses

Abstract

The kinetics of short-range atomic ordering in austenitic Fe-17 Cr-xNi (x=10-25 wt%) alloys and in Sc-doped alloys has been studied by residual resistivity measurements during isochronal annealing in the temperature range 300-815 K. Dynamics of residual resistivity variations has shown that (i) diffusion in these alloys is observable at temperature above 700 K, (ii) preliminary cold-work deformation as well as doping by Sc atoms forms the disorder atomic structure in the austenitic matrix. It was found that Sc Atoms retard the atomic ordering process and shift it to higher temperatures. The value of temperature shift is increased from 25 to 80 K when the nickel concentration grows up to 25 wt%.     

Observation of transverse spin freezing by TDPAC

Mössbauer spectroscopy investigation of superconducting Sm_0.85Ba_0.15FeAsO_0.7F_0.3 (T _c = 37 K) has been performed. The spectra appear to be a singlet pattern throughout the temperature range of 20–290 K. The value of Debye temperature is obtained (θ _D ～ 390 K). Unusual behavior of the central shift as a function of temperature is observed below 60 K. Unlike cuprate superconductors, phonon softening is not observed near T _c of the iron pnictide.     

The intrinsic thermal conductivity of AIN

The thermal conductivity, λ, of high purity single crystals of AIN has been measured from 0.4 to 1800 K. These results show that the heat conduction is by phonons and that at the lowest temperatures the phonon mean-free-path, l, is limited by boundary scattering. The maximum λ = 23 W/cmK is at 52 K. Above 150 K the value of l is determined by phonon-phonon scattering and at 300 K the value of λ for pure AIN is 3.19 W/cmK (80% of that of pure copper). Above 500 K the value of λ decreases as T^−1.25.