Softening of charge density wave excitations at the superstructure transition in 2H-TaSe2

Raman scattering measurements performed between 5 K and 300 K on 2H-TaSe₂ reveal new modes which are assigned to the modes of the charge density wave, observed in light scattering due to the Fermi surface induced distortion. The mode at 49 cm^?1 of E_2g symmetry softens (with concurrent line-width broadening) towards 122 K, the transition temperature from the incommensurate distorted to the undistorted phase. The mode at 82 cm^?1 of A_1g symmetry appears to be connected with the transition at 90 K from the commensurate to the incommensurate superstructure. The mode at 24.5 cm^?1 of E_2g shows no temperature dependence and is clearly due to the rigid-layer vibration.     

Room-temperature electrosynthesized ZnO thin film with strong (0 0 2) orientation and its optical properties

ZnO thin film with strong orientation (0 0 2) and smooth surface morphology was electrosynthesized on ITO-coated glass substrate at room temperature under pulsed voltage. Photoluminescence (PL) shows two obvious peaks: violet band and strong green band. The former is due to the free-excitonic transition and the latter is believed to arise from the single ionized oxygen vacancy (V_O⁺). Raman scattering reveals that the 580 cm⁻¹ mode and the shoulder peak mode at 550 cm⁻¹ originate from the N-related local vibration mode (LVM) and E₁ (LO) mode, respectively.     

Optical mode softening in the incommensurate phase of Sr2Nb2O7

In the incommensurate phase below about 220°C of Sr₂Nb₂O₇, the soft mode, of which frequency decreases toward 215°C has been found by the Raman scattering measurements in the $\text{b(cc)}\text{b}$ scattering geometry. The level repulsion and the intensity transfer between the soft mode and another low frequency mode are clearly observed. The uncoupled soft mode frequency ω_s has been found to be expressed as ω_s =A(T_tr?T)^β, where β= 0.38 ± 0.02. Pressure dependence of the soft mode has also been measured up to 24kbar. No remarkable pressure dependence has been observed.     

The soft mode properties in Raman spectra of improper ferroelastics Hg2Cl2 and Hg2Br2

The temperature dependence of the frequency and intensity of the soft mode line appearing in the Raman spectra of Hg₂Cl₂, Hg₂Br₂ below the critical point T_c was studied. A proportionality between the scattering cross section of the soft mode and the square of its frequency v_sm was established. A strong stress-induced increase of the intensity and frequency of the soft mode line was observed near T_c, the relative shift of v_sm being ～30% at ～0.5 kg/mm² stress and T_c?T=5°K.     

Raman scattering in K2Pt(CN)4Br0.3 · 3H2O

Raman scattering experiments on K₂Pt(CN)₄Br_0.3 · 3H₂O are reported between 5 and 300 K as a function of temperature. A line of A₁ symmetry detected at 44 cm^?1 shows interesting temperature dependent properties. It is concluded from a comparison of the frequency, symmetry, and scattering intensity of this line with theoretical predictions that the excitation concerned represents the amplitude mode of the charge density wave (the line observed in infrared absorption being the phase mode). No Peierls transition is observed, but the results are consistent with a Peierls distortion present at all temperatures. The findings are correlated with inelastic neutron scattering and infrared studies. Finally, the CN stretching modes at 2189 and 2173 cm^?1 and the water mode at 3490 cm^?1 are studied as a function of temperature.     

Electron hopping and optic phonons in Eu3S4

Raman scattering on single crystals of Eu₃S₄ does not show the allowed q=o phonon modes in the cubic phase and exhibits no new modes in the distorted low temperature phase (T<186 K). Above the Curie temperature T_c=3.8 K the scattering is dominated by a spin-disorder induced one-phonon density of states allowing for the observation of the zone boundary phonon breathing mode of the S^2?ions. This mode does not show any anomaly near the charge order -disorder phase transition T_t=186 K. Temperature tunable spin fluctuations associated with the temperature activated Eu²⁺→Eu³⁺ electron hopping are detected in the scattering intensity, superimposed on the usual thermal spin disorder.     

Vibrationally inelastic scattering of H∼ ions from H2, N2, O2 and CO2

State-resolved measurements are presented for vibrational excitation of H₂, N₂, O₂ and CO₂ by H^? impact in the collision energy rangeE _cm=20–180 eV and for scattering in the forward direction (0±0.5°). The data obtained from the measurements are the relative intensities and differential cross sections for vibrational excitation up toν′=4, the transition probabilitiesP _0→ν′ and the vibrational energy transferΔE _vib. For the systems H^?-H₂, N₂, O₂ the vibrational inelasticity increases in the order H₂-N₂-O₂. The mechanism for vibrational excitation in these systems is due to transient charge transfer from the H^? ion into antibonding orbitals of the target molecule which provides a bond stretching force during the collision. For H₂ and N₂, the results are compared with corresponding measurements for H⁺ scattering where the interaction mechanism is quite different. In the case of CO₂, vibrational excitation in forward scattering is caused primarily by the long-range dipole interaction. The spectra are very similar for H^?-CO₂ and H⁺-CO₂. Finer details are attributed to the influence of transient charge transfer and valence interactions.     

氧离子辐照二氧化钛单晶后的结构与磁性

氧离子辐照二氧化钛单晶可以诱发其铁磁性．辐照后在室温下也能观察到二氧化钛的铁磁性,且对温度依赖性较小．结合X射线衍射实验、卢瑟福背散射/沟道实验、拉曼散射实验谱、电子自旋共振实验谱、超导量子干涉仪实验、单位原子随沟道位移实验,测定了晶格的损伤随辐照流强的增加而增加．发现在氧离子辐照二氧化钛时出现了Ti³⁺替代氧空位(O_V)的缺陷复合体,即形成Ti³⁺-OV复合体．这种缺陷复合体导致了局部(TiO_6-x)的拉曼模式的伸展．说明了Ti³⁺结合一个未成对的3d电子是二氧化钛局部铁磁性的起源.     

Infrared lattice vibration spectra of tetragonal ZnP2

Infrared reflectivity spectra of tetragonal ZnP₂ are measured in the frequency range from 40 to 600 cm^-1 for both polarization directions E ⊥ c and E 6 c. The parameters of 9 E modes and 4 A₂ modes are determined by a dispersion analysis of the spectra. Three additional A₂ modes are detected by infrared transmission measurements. The results obtained are compared with previous Raman scattering and two-phonon combination mode spectra.     

Coupling of the CDW and the water mode in K2Pt(CN)4Br0.3⋯3.2H2O

In previous work we have observed the amplitude mode of the charge density wave (CDW) in K₂Pt(CN)₄Br_0.3?3.2H₂O (KCP) by means of Raman scattering. New measurements made on deuterated material, K₂Pt(CN)₄Br_0.3?3.2D₂O (KCP1), show the same mode but shifted from 44 to 38 cm_?1, maintaining the symmetry properties and temperature dependence of frequency and linewidth. This considerable isotope effect is interpreted in terms of a coupling of the CDW with the water stretching mode, which by the deuteration is shifted from 3494 cm^?1 in KCP to 2560 cm^?1 in KCP1 according to the change in atomic mass. Both of these modes exhibit A₁(z) symmetry. At 5 K the resulting decoupled frequency of the CDW amplitude mode is 57 cm^?1, and the coupling energy about 140 cm^?1. A discussion of the temperature dependence of various important quantities is given. The present results show that the water molecules, which are located in between the Pt chains are strongly involved in the eigenvector of the CDW amplitude mode.     

Resonant Raman scattering from FeS2 (pyrite)

We have observed resonant Raman scattering from the A_g mode of FeS₂ (pyrite) for incident laser energies between 2.41 and 2.73 eV, these energies being well above the fundamental gap at 0.9 eV. In contrast to the behavior of the A_g mode intensity, the E_g and T_g modes do not show resonant behavior. A simple two band model which predicts the Raman cross section proportional to |?χ/?ω|² is found to be inadequate to describe the observed intensity variation with the incident laser energy.     

Asymptotic restrictions and ππ scattering lengths in non-linear chiral SU(2) × SU(2) dynamics

The most general effective lagrangian for ππ scattering, that takes into account the exchange of scalar (0⁺), tensor (2⁺) and ? mesons, is constructed within the framework of non-linear chiral SU(2) × SU(2) dynamics. Asymptotic restrictions proposed by Weinberg are imposed on the amplitude of ππ scattering. These restrictions allow one to obtain four independent sum rules containing particle masses, coupling constants and the coefficient A₂ in an expansion of the symmetry-breaking term in a power series of π². The use of only the f⁰ meson in the obtained sum rules leads to the following values of the ππ scattering lengths, the width of ?(700) → 2π decay, and the parameter A₂: α₀⁰ = 0.92m_π^?1, α₀² = 0.27 m_π^?1, α₁¹ = 0.033 m_π^?3, Γ[ε(700) → 2π] = 390 MeV, A₂ = + 1.6. It is shown that the inclusion of heavier mesons results in some increase of the ππ scattering lengths and the parameter A₂, but a decrease of the value of Γ(? → 2π). The field-algebra requirements lead to negligible changes in the results. Analysis of the sum rules shows the incompatability of the asymptotic restrictions with a symmetry breaking which has the transformation properties of the fourth component of a chiral vector.     

Giant M2 and transversal E1 resonances in light nuclei

The cross section for inelastic backward electron scattering on 1p shell nuclei at incident energy E_e = 70 MeV is calculated in the shell model. Comparison is made with radiative pion capture and muon capture. It is shown that the T_> branch of the M2 resonance in (e, e') and the main maxima in the (π^?, γ) response function are formed by identical partial transitions. We consider the basic features of the M2 resonance excitation in 1p shell nuclei and predict configurational and isospin splitting of this mode.     

Electronic structure and lattice dynamics of Li2Ni(WO4)2

We combined spectroscopic ellipsometry and Raman scattering measurements to explore the electronic structure and lattice dynamics in Li₂Ni(WO₄)₂. The optical absorption spectrum of Li₂Ni(WO₄)₂ measured at room temperature presents a direct optical band gap at 2.25 eV and two bands near 5.2 and 6.0 eV, which are attributed to charge-transfer transitions from oxygen 2p states to nickel 3d or tungsten 5p states. The Raman scattering spectrum of Li₂Ni(WO₄)₂ measured at room temperature presents seventeen phonon modes at approximately 112, 143, 193, 222, 267, 283, 312, 352, 387, 418, 451, 476, 554, 617, 754, 792, and 914 cm⁻¹. When the temperature is decreased to 20 K, the frequency, linewidth, and normalized intensity of all phonon modes exhibited almost no temperature dependence. Upon cooling across 13 K, which is the antiferromagnetic phase transition temperature, the oxygen octahedra stretching mode at 914 cm⁻¹ exhibited a softening and an increase in intensity, thus suggesting a coupling between the magnetic and lattice degrees of freedom. The spin-phonon coupling constant was estimated to be 0.94 mRy/Ų, indicating a weak spin-phonon interaction in Li₂Ni(WO₄)₂.     

Levelcrossing Experimente im Tm I-Spektrum

A semi phenomenological dynamical theory for the rotations of the BO₆ octahedra in ABO₃ perovskites is proposed, which accounts for the soft mode and the central peak forT≧T _c observed by neutron scattering in SrTiO₃. The neutron scattering cross section is discussed. The EPR line width is predicted to diverge as ?^?ν(1–2η) and as ?^?ν(2–2η) for three- and two dimensional correlations with ?=(T-T _c )/T _c . The theory is generalized to other structural transitions.     

Three-body scattering in configuration space

The asymptotic forms of the wavefunction and Faddeev components in configuration space are shown to determine uniquely the solutions of the Schrödinger or Faddeev differential equations for 2 → (2, 3) and 3 → (2, 3) processes. An antisymmetrized form of the Faddeev differential equation for three equivalent fermions is given and its angular analysis is performed in the general case of local potentials with tensor interaction for neutron-deuteron scattering. We describe a numerical method for solving the corresponding boundary value problem and apply it to scattering and break-up at E_n^1ab = 14.4 MeV in the doublet S state for the four local potentials of Malfliet and Tjon, Reid, de Tourreil and Sprung, and de Tourreil, Rouben and Sprung. For the three realistic potentials, elastic scattering amplitudes differ by 5%, and amplitudes for break-up in the two-neutron state ¹S₀ differ by less than 4%.     

Study of the heavy-fermion compound CeRu<Subscript>2</Subscript>Si<Subscript>2</Subscript> by the small-angle neutron scattering method

In order to directly observe neutron scattering by heavy fermion quasiparticles at low temperatures, a CeRu₂Si₂ single crystal has been studied by the small-angle neutron scattering method. In the experiment, neutron scattering is observed at T = 0.85 K for momentum transfers q ≤ 0.04 Å^?1, which is treated as the orbital component of magnetic scattering by heavy fermion quasiparticles. It has been found that the application of a magnetic field H = 1 T leads to both an increase in the observed scattering and its anisotropy with respect to the field direction. Moreover, measurements in the magnetic field reveal additional scattering for q > 0.04 Å^?1, which is well described by a Lorentzian and is interpreted as neutron magnetic scattering by spin-density fluctuations with a correlation radius R_c ≈ 30 Å.     

Fano line shape and anti-crossing of Raman active E2g peaks in the charge density wave state of NbSe2

Low energy Raman scattering from the ab-plane of the 2H polytype single crystal NbSe₂ has been investigated in the normal (N), incommensurate charge density wave (ICDW) and superconducting (SC) phases. The temperature dependence of the polarization resolved Raman response has been obtained for the excitation wavelength of 647 nm and fitted to phenomenological models for the E_2g and A_1g symmetry channels. The A_1g response can be fitted by a simple damped oscillator peak superimposed on continuous background. The E_2g response displays an anti-resonance interference pattern between the inter-layer phonon and the CDW-induced mode such that a hybridized configuration (Fano line shape [1]) is required for modelling. The polarization specific peak maxima positions and line widths as a function of temperature, deduced in this manner, are presented. Partial suppression of the electronic continuum scattering in the Raman shift range up to 110 cm⁻¹ in the A_1g symmetry channel and beyond 300 cm⁻¹ in the E_2g symmetry channel is indicative of high energy electronic states far away from the Fermi surface participating in the ICDW formation.     

Evolution of molecular states studied in collisions of ions with laser excited atoms by a crossed beam experiment

Differential scattering cross sections in the energy rangeE _CM=25–50 eV have been measured in a crossed beam machine for the scattering of Na⁺ by laser excited Na(3² P _3/2). The superelastically scattered ions resulting from the collisional deexcitation process 3² P _3/2-² S _1/2 are sharply peaked at a constant reduced scattering angle of 180eV degrees. Through the choice of either linearly or circularly polarized light, one can selectively excite the magnetic sublevels of the Na atoms. This ability to align or orient the Na^* allows us to discuss in considerable detail both the transition from the space-fixed (atomic) to the body-fixed (molecular) system and the dynamics in the merging region of the two frames. We find that the symmetry axis of the excited electron charge cloud is locked to the internuclear axis and hence to the body-fixed reference frame at an internuclear separation of about 35 a.u. A pronounced asymmetry of the scattering from atoms excited with LHC or RHC polarized light is discussed in terms of simple models.     

Some thermomagnetic transport effects of Cd3P2

In this paper for the first time measurements are presented of the transverse Nernst effect, the Righi-Leduc effect and the Maggi-Righi-Leduc effect in cadmium phosphide. The measurements were performed on unoriented Cd₃P₂ single crystals, with electron concentrations in the range 0.05–1.7 × 10²⁴m^?, at 110 and 300 K in magnetic fields up to 1.8 $\text{Wb}\text{m}{}^2$. We also measured the room temperature dependence of the zero-field Seebeck coefficient on electron concentration of a large number of undoped and Cu-doped samples. A reversal of sign of the transverse Nernst effect was observed at an electron concentration of about 1.2?1.5 × 10²⁴ m^?3. This reversal of sign and the zero-field Seebeck coefficient vs electron concentration for undoped material can be quantitatively described by a Kane-type conduction band with ?_g = 0.50 eV, m¹_e = 0.040 m_o and a scattering parameter r = ?1. (Optical phonon scattering, if interpreted in a single mode.) This strongly confirms the results of Radautsan et al. that Cd₃P₂ has a non-parabolic Kane-type conduction band. The Righi-Leduc and Maggi-Righi-Leduc effect measurements at 110 K on samples with high electron concentrations, yielded Lorenz numbers only half their theoretical values, indicating that the electron scattering contains an inelastic contribution.