Theory of spinconversion in XH3-systems

In a simple model of (mechanically and magnetically) isolated methyl groups with mutualintramolecular dipolar coupling between the nuclear magnetic moments of the three protons the conversion rate (i.e. for the symmetry changing transitionsE ^a,b A) is investigated. It turns out, that within the model of a fixed rotational axis only half of theE-symmetric rotor states can convert intoA-symmetric ground states.The usual coupling to the phonon bath of thermal lattice excitations is considered in lowest order perturbation theory, and used to describe the temperature dependence of shift and broadening of tunnelling-and librational lines by inelastic neutron scattering (INS). For the two possible types of this phonon coupling,breathing andshaking, a different temperature dependence of the conversion rate is obtained. The temperature dependence of the INS-spectra and of the conversion rate are predicted to be mutually interdependent.Only in case of a non zero phonon coupling of breathing type a non vanishing conversion rate at zero temperature is found; this rate is estimated to be proportional to _t ³ .     

Relaxation of hot holes in p-Ge

Saturated absorption of infrared radiation by p-doped Ge is treated theoretically, predicting that both the saturation intensity and the width of the probe-saturator spectrum oscillate as functions of the saturator photon energy, ?ω. The maxima occur for ?ω = n?ω_LO, where n is an integer and ?ω_LO is the zone-center LO phonon energy. The results may a lso provide the closest measurement of hole cascade relaxation times to date. The oscillations depend on theoretical estimates that the LO phonon emissions occur substantially more rapidly than other decay processes. Hence failure to observe the oscillations would belie these estimates.     

Raman scattering in intermediate valent SmB6

We have measured the Raman spectra of intermediate valent (IV) SmB₆ and compared it to LaB₆ and EuB₆. Beside the three high energy Raman active phonons we found additional features in the spectra. Most prominent is a peak at 172 cm^-1 for SmB₆, at 214 cm^-1 for LaB₆ and at ～220 cm^-1 for EuB₆. The spectra are analysed in terms of defect induced phonon excitations yielding a weighted phonon density of states. The softening of the line in IV SmB₆ compared to the other hexaborides is unusual since even in the trivalent MB₆ compounds the Raman active phonon modes normally are at lower frequencies than in semiconducting, divalent samples. This phonon softening is explained in analogy with the phonon anomalies in other IV compounds like Sm_1?xY_xS and TmSe.     

Tunneling-states model and the propagation characteristics of surface phonons in amorphous solids at low temperatures

The propagation characteristics of the surface phonon in an amorphous film are theoretically investigated at low temperatures based on the tunneling-states model. It is shown that the attenuation rate of the surface phonon is proportional to ω tanh ?ω/2k_BT and the relative velocity variation to ln (T/T₀) for the thickness of the amorphous film comparable to or larger than the surface phonon wavelength. Below 3K, our result can well account for recent measurements of the relative velocity variation by Hartemann et al.     

Mechanism of the ferroelastic transition of BiVO4

BiVO₄ has a pure ferroelastic transition at T_c = 528°K and atmospheric pressure. We elucidated the mechanism of this transition by studying the q≈0 soft optical phonon, with the symmetry of the ferroelastic strain, under large hydrostatic prèssures at room temperature. A free-energy analysis, including the optical-acoustical phonon couplings, shows that the transition is driven by the q≈0 soft optical phonon.     

Raman scattering from GaAs-InxGa1−xAs strained-layer superllatices

Measurements of Raman scattering were performed on GaAs-In_xGa_1?xAs strained-layer superlattices, grown by molecular beam epitaxy, with lattice periods ranging from 30 ～ 250 Å and In concentrations x, 0.22 and 0.37. Only one GaAs-like longitudinal optical phonon peak was observed in each strained-layer superlattice, in contrast to the well-known result that two peaks were observed in GaAs-Al_xGa_1?xAs superlattices. The GaAs-like phonon frequencies shifted from those of bulk GaAs to those of bulk In_xGa_1?xAs alloys as the ratio of the one-layer thickness of In_xGa_1?xAs to the lattice period increases from zero to one. We conclude that the GaAs-like phonon mode is a uniform mode of the whole strained-layer superlattice and the phonon frequency is determined by the averaged In concentration.     

The specific heat of potassium holmium double tungstate

The results of measurements of thermal properties (specific heat) of potassium holmium double tungstate KHo(WO₄)₂ as a function of temperature (from 0.5 to 300?K) and magnetic field (up to 2?T) are presented. The total specific heat without the phonon and Schottky contributions is found to have the anomaly with maximum at T _SPT?～?5?K. This anomaly is likely related with the structural phase transition (SPT) caused by the cooperative Jahn–Teller effect. The increase of specific heat at very low temperatures and its shift towards high temperatures with increasing magnetic field are observed. The origin of this behaviour can be connected with possible magnetic phase transition induced by magnetic field.     

Electronic structure for sliding charge density waves

We present a model for the electron system in NbSe₃ based on its quasi one-dimensional metallic properties. In a one-dimensional metal phonon drag of 2K_F-phonons takes place at temperatures higher than θ_D, since the phonon-electron scattering rate τ^?1_ph?el is greater than the phonon-phonon rate τ^?1_ph?ph. this situation is in contrast to the situation in three dimensional metals, where phonon drag takes place only at very low temperatures. Our model explains the transport properties of the material including the electrical conductivity anistropy, the conductivity in a strong electric field, and the Hall effect data.     

Mössbauer and magnetization studies of the U1−xNpxO2 fluorites

²³⁷Np Mössbauer effect and magnetic susceptibility measurements of the U_1?Np_xO₂ fluorite solid solution have been performed in the composition range 0.15 ? x ? 0.75. For x = 0.15 and 0.25, the Np ions order magnetically at a lower temperature T₀ than the bulk material (T_N) (T₀ ～ 19 K, T_N ～ 27 K for x = 0.15). For x = 0.50, T₀ ～ 10 K (T_N ～ 12 K from recent neutron diffraction measurements). For x = 0.75, T₀ ～ T_N ～ 9 K. The Np (induced) ordered moment is ～ 0.5 μ_B. The ²³⁷Np Mössbauer isomer shift shows that the Np ions are in a IV charge state.     

Low-temperature thermal conductivity of tellurium-doped bismuth

Phonon thermal conductivities κ₂₂ (?T ‖ C1) and κ₃₃ (? T ‖ C₃) of tellurium-doped bismuth with an electron concentration in the range 1.8 × 10¹⁹ ≤ n_L ≤ 1.4 × 10²⁰ cm^?3 were studied in the temperature interval 2 < T < 300 K. The temperature dependence of the phonon thermal conductivity obtained on doped bismuth samples of both orientations exhibits two maxima, one at a low temperature and the other at a high temperature. The effect of various phonon relaxation mechanisms on the dependence of both phonon thermal conductivity maxima on temperature, impurity concentration, and electron density is studied.     

Mode dependent scattering of phonons by domain walls in ferroelectric KDP

Thermal conductivity and ballistic phonon imaging measurements in KH₂PO₄ (KDP) at low temperature (T<3K) indicate that scattering from domain walls has a large effect on phonon transport. kDP has a ferroelectric phase transition from tetragonal to orthorhombic structure atT _c =122 K. BelowT _c domains of opposite electric polarization and crystal orientation form unless the sample is colled in an electric field. Thermal conductivity measured along the [100] (tetragonal) axis drops 30% when domain walls are present, which is independent of sample size and temperature. We attribute this decrease to phonon polarization-dependent scattering at the domain boundaries. This is verified by measurements of ballistic transport, using phonon imaging techniques, which reveal the phonon polarization and mode dependence of the scattering. The scattering is successfully modelled using continuum acoustics with simple acoustic mismatch at the domainwall. The interface scattering is found to be mode dependent: Caustic structures in the phonon images due to slow transverse phonons are most affected by the domain wall scattering, which channels these phonons along parallel planes by multiple reflections without mode conversion. Mode conversion scattering, though possible for a number of phonons, has little effect on the overall phonon transmission.     

Specific heat of Na1−x V2O5 single crystals

Temperature dependences of the specific heat C and the magnetic susceptibility χ of Na_1?x V₂O₅ single crystals (x=0, 0.01, 0.02, 0.03, and 0.04) are studied. In NaV₂O₅, the transition to the spin-gap state (T _c =34 K) is accompanied by a sharp decrease in χ, while C exhibits a λ-shaped anomaly. At low temperatures, the specific heat of NaV₂O₅ is approximated by the sum of phonon ～T ³ and magnon ～exp(?Δ/T) contributions, which makes it possible to estimate the Debye temperature Θ_D=336 K and the gap in the magnetic excitation spectrum Δ=112 K. With the departure from stoichiometry, the anomalies observed in the behavior of χ and C are spread and shifted to lower temperatures. The low-temperature specific heat of nonstoichiometric samples is determined by the sum of phonon and magnon components and the contribution due to the presence of defects. The values of magnetic entropy characterizing the phase transitions in Na_1?x V₂O₅ are calculated.     

Di-jet shape modification in heavy ion collisions

We present preliminary results from intermediate p _T(1–5 GeV/c) di-hadron azimuthal correlations induced by hadronic di-jets produced in AuAu collisions at √s _NN =200 GeV. The near-side (Δ?～0) has a typical singlepeaked structure which broadens with the centrality of the collision. A qualitatively new phenomenon shows up in the shape of the away-side (Δ?～π): it has a symmetric, double-peaked structure in central and mid-central collisions.     

Collective excitations in solid hydrogen as observed by incoherent neutron scattering

Incoherent neutron scattering experiments are reported at a number of scattering angles for solid H₂ using the time-of-flight technique. The samples had an ortho concentration ofX0.91 and the temperatures were 4.2, 2.2 and 1.2 K. The recorded scattering functions exhibit the elastic line, the ortho-para conversion line, the phonon spectrum on the energy-gain side of the neutrons and finally the phonon spectrum on the energy-loss side. Analysis of the data shows multiple scattering to be very important especially at small scattering angles. Accordingly, the results of an extensive calculation are reported that include up to four-fold scattering. The phonon spectrum that brings calculation and experiment in agreement at all angles is presented. It is concluded from the comparison between the spectra at various temperatures that unexpectedly the samples have always remained in the hexagonal close-packed phase even below the usual transition temperature into the cubic phase. Comparison is made with the spectrum (also in thehcp phase) obtained from coherent neutron scattering. The high-energy phonon tail, to be expected for quantum crystals, is observed and is qualitatively consistent with that in a similar spectral density derived theoretically from ortho-para conversion data under pressure. No evidence of a libron spectrum atT=2.2 K for a sample withX=0.91 could be found, which is consistent with the absence of the cubic orientationally ordered phase in these experiments.Supported in this research by a grant from the National Science Foundation     

Influence of a strontium-enriched intergranular cluster network on the electrical conductivity of In2O3-SrO ceramics

A metastable hexagonal R-phase is revealed in polycrystalline In₂O₃-SrO samples, which has the form of a network made up of mesoscopic clusters (60–180 Å in size). The clusters arise from strontium-enriched regions near grain boundaries in the main cubic structure of indium oxide. It is shown that annealing in oxygen at T _a ? 300°C saturates dangling bonds between the R-phase and the matrix and makes the system metastable. This state shows up in the presence of (i) solitary diffuse maxima from the R-phase imposed on Debye lines from the main phase in the X-ray diffraction pattern and (ii) the electron cyclotron resonance (ECR) line with g = 1.875. In addition, the sample in this state acquires a high resistivity (ρ ～ 10⁶ Ω cm). Relaxation at T ? 300°C after annealing at T _a > 300°C disrupts bonds between the strontium-enriched clusters of the R-phase and the indium oxide matrix. This causes spatial separation of the clusters, disruption of their coherent bonds with the matrix structure, and escape of excess oxygen from the sample along grain boundaries. As a result, a new stable state forms, which is characterized by (i) a series of diffuse maxima from the R-phase imposed on lines assigned to the main phase, (ii) the presence of the ECR line with g = 2 with the line with g = 1.875 retained, and (iii) the transition of the sample to a low-resistivity state (ρ ～ 100 Ω cm).     

Raman scattering of Zn1−xCdxS mixed crystals

The Raman spectra of Zn_1?xCd_xS crystals are studied. The narrow peak between TO (Γ) and LO (Γ) lines at x ? 0 is due to deffect-induced first order scattering. The anomalously intense peak in the same spectral region at x ? 1 is supposed to be result of resonance interaction between LO (Γ) phonon and quasilocalized vibrations.     

Inelastic neutron scattering an ab-initio calculation of negative thermal expansion in Ag2O

The compound Ag₂O undergoes large and isotropic negative thermal expansion over 0–500 K. We report temperature dependent inelastic neutron scattering measurements and ab-initio calculations of the phonon spectrum. The temperature dependence of the experimental phonon spectrum shows strong anharmonic nature of phonon modes of energy around 2.4 meV. The ab-initio calculations reveal that the maximum negative Grüneisen parameter, which is a measure of the relevant anharmonicity, occurs for the transverse phonon modes that involve bending motions of the Ag₄O tetrahedra. The thermal expansion is evaluated from the ab-initio calculation of the pressure dependence of the phonon modes, and found in good agreement with available experimental data.     

Laser characteristics of KCL:O-2

Amplified spontaneous emission and laser action have been observed at 77 K in KCL crystals containing the superoxide ion (O^-₂). The laser operates in two bands of width ～50 Å centered at 6350 Å and 5984 Å. These bands are the (lattice) phonon sidebands of vibronic transitions of the superoxide ion. At lower temperatures (～6 K) amplified spontaneous emission is observed in the zero phonon line at 6294 Å.     

Study of lattice vibrational modes in CuGaS2 by using fourier transform spectroscopy

The lattice dynamics of a single crystal of CuGaS₂, grown by iodine transport technique, have been studied by using far IR absorption spectroscopy. All the absorption maxima caused by the phonon excitation are compared with the lattice vibrational modes obtained by Raman spectroscopy and by IR reflection techniques. An absorption maximum located at 175 cm^?1 cannot be explained with the help of phonon excitation; however this peak can be attributed to the defect frequency originating from the replacement of gallium atom by sulphur in the v₁₇ mode of vibration. The frequency of this defect-induced vibrational mode is calculated by taking a modified molecular model approach, and is found to be 166.9 cm^?1, which is in reasonably good agreement with the experimentally observed value of 175 cm^?1.     

Precursor dynamics to the structural instability in SrTiO3

The structural instability of SrTiO₃ at T _S?=?105?K is reinvestigated experimentally and theoretically by birefringence measurements and within the self-consistent phonon approximation. Analogous to BaTiO₃ the phase transition shows precursor dynamics starting approximately 60?K above the phase transition temperature. These rotational dynamic clusters compete with precursor effects stemming from the polar soft mode and appear on different length scales as compared to BaTiO₃.