Acousto-induced structural change in amorphous As2S3

The acoustic flux injection method was applied to amorphous As₂S₃. The velocity and the attenuation constant for shear wave at ～ 100MHz were measured through the observation of the Brillouin scattering of the light beam from a He-Ne laser. The optical transmission around absorption edge was found to change after the injection of acoustic fluxes. It was also found that the sound velocity decreased with increasing injection time of acoustic fluxes and the amount of the change of the sound velocity reached ～ 20% after 10⁵ injections. The changes in the optical transmission and the sound velocity tended to be erased by annealing below the glass transition temperature. These phenomena are considered to be due to structural changes caused by the injection of strong acoustic fluxes.     

Excitation migration among inhomogeneously broadened levels of Eu3+ ions

Excitation migration between ⁵D₀ states of Eu³⁺ ions in Ca(PO₃)₂ glass is studied by using the technique of the selective excitation of the inhomogeneously broadened ⁵D₀ levels. In time-resolved emission spectra for ⁵D₀→⁷F₁ transition after the pulse excitation due to ⁷F₀→⁵D₀ transition by the use of a tunable dye laser at 77°K, a subpeak is observed to appear and to grow in intensity with increasing delay time, besides the main peak excited directly by the excitation light. The observed facts are explained by considering the phonon-assisted energy transfer between ⁵D₀ states involving an acoustic phonon. From the analysis of the results, the probability of the excitation migration due to the phonon-assisted process is estimated to be ≌ 10 A ms^-1. From the theoretical consideration, the rate of the linewidth change and the probability of the excitation migration due to the resonant transfer process are obtained. These results indicate that the phonon-assisted process is more dominant than the resonant process for the excitation migration in the system studied.     

Inelastic neutron scattering from transverse acoustic modes in K2SnCl6

In cubic K₂SnCl₆ the dispersion curve für q q; [110] of [1$\text{1}$0] polarized acoustic phonons has been measured at three temperatures near the phase transition temperature T_c1. The acoustic branch shows no temperature dependence in the low wave vector region and stiffens slightly near the X-point of the fcc-Brillouin zone as the temperature approaches the phase transition T_c1= 262 K from above. The results support a previously developed model on acoustic anomalies in this compound.     

Ultrasound anomaly at Tc in YBa2Cu3Oy

We measured the fundamental longitudinal acoustic resonant frequency f_r of polycrystalline samples of YBa₂Cu₃O_y. The frequency increases with decreasing temperature and displays a sharp rise as we cool below T_c. The fractional change in frequency at T_c, Δf_r/f_r, is 4.5 × 10^-3 and is opposite in sign to that expected at the usual second-order super-conducting transition.     

High-frequency ultrasonic studies of the structural phase transition in an La<Subscript>0.875</Subscript>Sr<Subscript>0.125</Subscript>MnO<Subscript>3</Subscript> single crystal

The specific features of a phase transition from a disordered orbital state to an ordered orbital state in an La_0.875Sr_0.125MnO₃ single crystal are investigated using acoustic methods at a frequency f = 500 MHz. The phase transition is accompanied by a distortion of MnO₆ octahedra due to the cooperative Jahn-Teller effect and is a first-order phase transition, as judged from the sharp change observed in the damping of acoustic pulses, the acoustic wave velocity, and the temperature hysteresis. It is revealed that the parameters of the acoustic waves change significantly throughout the temperature range of existence of the cooperatively distorted structure. In an external magnetic field, the structural phase transition is shifted toward lower temperatures.     

Soft modes and phonon interactions in studied by neutron scattering

The low frequency lattice dynamics and its relationship to the second order paraelectric-to-ferroelectric transition in Sn₂P₂S₆ is studied. The dispersion branches of the acoustic and lowest lying optical phonons in the a^*-c^* plane have been obtained in the ferroelectric phase, for x-polarized phonons. Close to the phase transition a considerable softening is found for the lowest optical mode (P_x), comparable to the behaviour observed in previous Raman investigations. As found previously in Sn₂P₂Se₆, a strong coupling between the TO(P_x) and TA(u_xz) phonons is observed, although, apparently, not strong enough to lead to an incommensurate phase. The soft TO(P_x) mode at the zone center is observed. The temperature dependence of its frequency and damping shows that the transition is not entirely displacive. At low temperatures an unusual apparent negative LO-TO splitting is observed which is shown to arise from the coupling of the x-polarized soft mode to the nearby z-polarized optical phonon. For comparison, the soft TO(P_x) dispersion in the a^*-b^* plane is measured in both the paraelectric and ferroelectric phases. Consistent frequency changes and LO-TO splitting are observed, revealing a significant interaction between the TA(u_yx) and LA(u_xx) acoustics branches and the TO and LO soft optic branches, respectively. In contrast, the nearby y-polarized optic branch shows almost no temperature dependence. Finally, the influence of piezoelectric effects on the limiting acoustic slopes in the ferroelectric phase is discussed. Received: 11 May 1998 / Revised and Accepted: 15 June 1998     

Hydrostatic pressure effect on commensurate—incommensurate transition in K2SeO4-Elastic anomalies studied by Brillouin scattering

A Brillouin scattering study of the commensurate—incommensurate transition in K₂SeO₄ submitted to a hydrostatic pressure was performed. It was found that an increasing pressure lowers the transition temperature (dT_i/dP_i = ?59 K/GPa). The strong anomalies of velocity and damping of the longitudinal acoustic wave propagating along [0 0 1] are weakly affected when pressure acts. The effect of pressure on these anomalies has been calculated on the basis of the lowest order couplings eQ² and e²Q² between strain e and order parameter Q.     

Debye-Waller factor through the glass transition temperature in a-selenium,by incoherent inelastic neutron scattering

We report, in this work, Incoherent Inelastic Neutron Scattering (IINS) measurements in amorphous bulk selenium. Taking into account the low frequency Vibrational Density-of-States(VDOS), we study the modification introduced in the Debye-Waller factor by increasing temperature through the glass transition temperature (T_g). The occurrence, in the vibrational density-of-states, of a ω² dependence in the acoustic region, allowed us to apply the Debye theory from which the variations of the Deybe-Waller factor are calculated in addition. It is shown that the main contribution to it is given by the acoustic region of the vibrational density of states and has a faster increase for temperatures above T_g.     

Effect of Sn substitution on the elastic anomalies and phase transition behaviors of antiferroelectric PbHfO3 single crystals studied by Brillouin scattering

The phase transition sequence of a PbHf_0.7Sn_0.3O₃ single crystal was investigated by using Brillouin spectroscopy. The longitudinal acoustic mode showed three distinct changes at ∼370 K, ∼455 K and ∼495 K which corresponded to the antiferroelectric 1 → antiferroelectric 2 → intermediate → paraelectric phase transitions upon heating. The paraelectric phase was characterized by softening acoustic mode, increasing acoustic damping and the appearance of quasielastic central peaks which were observed upon approaching the phase transition temperature. The relaxation times derived from the acoustic mode anomalies and the central peak were consistent with each other and showed a slowing-down behavior. These precursor phenomena were attributed to the polar clusters having broken inversion symmetry. The substitution of Hf with Sn induced a very soft intermediate phase between the paraelectric and the antiferroelectric phases, where the longitudinal acoustic mode exhibited the lowest mode frequency along with substantially high acoustic damping. The transverse acoustic mode, which was observed in the two antiferroelectric phases, did not appear in the intermediate phase. These results indicated that the average symmetry of the intermediate phase may be cubic consisting of fine ferroelastic domains.     

Phase transition in a NaBi(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal: Acoustic investigations

This paper reports on the results of acoustic investigations of a NaBi(MoO₄)₂ crystal in the temperature range from 20 to 70°C. The temperature dependences of the velocity of longitudinal ultrasonic waves propagating along the crystallographic axes z and x are measured at a frequency of 4 MHz. The results obtained demonstrate that a structural phase transition occurs in the NaBi(MoO₄)₂ crystal at a temperature of 309 K. The experimental findings are consistent with the assumption that the observed phase transition is either a second-order ferroelastic transition or a first-order ferroelastic transition that is very close to being a second-order phase transition.     

Acoustic properties of the disordered relaxor ferroelectric PbSc1/2Ta1/2O3

The behavior of acoustic phonons in crystals of a relaxor ferroelectric, namely, the lead scandium tantalate PbSc_1/2Ta_1/2O₃ (PST), is studied in the vicinity of the diffuse phase transition. The behavior of longitudinal and transverse acoustic phonons in a PST single crystal is examined using Brillouin scattering. The phonon subsystem is found to behave anomalously in the vicinity of T = 297 K, which can probably be assigned to the existence of a phase transition. Analysis of the results obtained yields the values of the elastic moduli C₁₁, C₁₂, and C₄₄ for the cubic phase of the crystal over a wide temperature range.     

A high pressure Raman study of TeO2 to 30 GPa and pressure-induced phase changes

The effect of pressure on the Raman modes in TeO₂ (paratellurite) has been investigated to 30GPa, using the diamond cell and argon as pressure medium. The pressure dependence of the Raman modes indicates four pressure-induced phase transitions near 1 GPa, 4.5 GPa, 11 GPa and 22 GPa. Of these the first is the well studied second-order transition fromD ₄ ⁴ symmetry toD ₂ ⁴ symmetry, driven by a soft acoustic shear mode instability. The remarkable similarity in the Raman spectra of phases I to IV suggest that only subtle changes in the structure are involved in these phase transitions. The totally different Raman spectral features of phase V indicate major structural changes at the 22GPa transition. It is suggested that this high pressure-phase is similar to PbCl₂-type, from high pressure crystal chemical considerations. The need for a high pressure X-ray diffraction study on TeO₂ is emphasized, to unravel the structure of the various high pressure phases in the system.     

Acoustical investigations of a La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> single crystal

The acoustical, resistive, and magnetic properties of a La_0.75Sr_0.25MnO₃ lanthanum manganite single crystal are investigated in the temperature range involving the second-order magnetic phase transition. The acoustical measurements are performed by the pulse-echo method in the frequency range 14–90 MHz. It is found that, as the temperature decreases, the velocity of a longitudinal acoustic wave propagating along the [111] axis in the single crystal drastically increases at temperatures below the critical point of the magnetic phase transition. No dispersion of the acoustic velocity is revealed. A sharp increase in the acoustic velocity is accompanied by the appearance of an acoustical absorption peak. The observed effects are discussed with due regard for the interaction of acoustic waves with the magnetic moments of the manganese ions.     

Propagation of high-frequency acoustic waves through the structure of Jahn-Teller ions in lithium niobate with iron

This paper reports on the results of an investigation into the specific features of the propagation of acoustic waves in lithium niobate LiNbO₃ with iron ions under laser irradiation of regions with increased and decreased concentrations of Fe²⁺ ions. It is shown that the observed effects of local variations in the damping and velocity of acoustic waves are associated with the strong electron-phonon interaction of Jahn-Teller Fe²⁺ ions.     

Acousto-optical tweezers for stretch of DNA molecule

The appearance of microlenses in the amorphous crystal Ge₃₃As₁₂Se₃₃ modulated by the acoustic waves with intensity of 10⁷ W/m² and frequency of (200–400) MHz is presented. The flexibility of microlens, i.e. the dependence of its focal point in 3D space on the parameters of acoustic wave is studied. Basing on flexibility of microlens, a model of acousto-optical tweezers to stretch the λ-phage WLC DNA molecule is proposed. And then the control process of stretched length of DNA molecule in 3D space of the water (fluid) by calibration of the acoustic frequency is numerically observed and discussed. The obtained results show the possibility to use the acousto-optical tweezers to control the stretched length of WLC DNA molecule with high fineness.     

Study of alkyl chain length dependent characteristics of imidazolium based ionic liquids [CnMIM]+[TFSA]− by Brillouin and dielectric loss spectroscopy

This study examined the acoustic phonon mode of ionic liquids consisting of 1-alkyl-3-methyl-imidazolium family (C_nMIM) cations with n values ranging from 2 to 10 and bis(trifluoromethylsulfonyl)amide (TFSA) anion in the temperature range from 300 K to 100 K. [C_nMIM]⁺[TFSA]^? showed depolarized (VH) components of Brillouin peaks at temperatures below the glass transition temperature when n is larger than 4. On the other hand, in the case of ionic liquids with different anions, such as [C₄MIM]⁺[BF₄]^?, [C₄MIM]⁺[PF₆]^? and [C₈MIM]⁺[BF₄]^?, the VH component of Brillouin peaks was not observed in the temperature range investigated. The dielectric loss spectra showed that the temperature dependence of alkyl chain domain relaxation of all ionic liquids followed the Arrhenius law and showed an increase in activation energy at the temperature where the VH component of Brillouin peak appeared. These results suggest that the observed depolarized component of Brillouin peak might originate from uniquely induced polarization in the 2nd domain composed of head groups of cations and anions.     

Brillouin scattering investigation of paraelectric KH2PO4 under applied uniaxial stress

The Brillouin scattering spectra of KH₂PO₄ under an applied uniaxial stress have been studied in the neighborhood of the ferroelectric transition. Dependence of shear acoustic phonon on the uniaxial stress is similar to dependence on electric field. The acoustic phonon is overdamped near T_c, but become underdamped under the applied shear stress. In the case of ΔT(= T — T_c) < ～ 0.05 K, application of stress caused a transition not to have any soft acoustic phonon.     

The hydrostatic pressure derivatives of the elastic constants of indium and an indium cadmium alloy

The hydrostatic pressure derivatives of the elastic stiffness constants of indium and indium-3.4 at.% cadmium alloy single crystals have been obtained from pulse echo overlap measurements of the dependence of ultrasonic wave velocities upon pressure. The softest zone centre acoustic phonon mode in indium is a shear mode propagating k along the [101] direction rather than that (k[110], e[11?0]) which drives the ferroelastic phase transition in the indium-cadmium alloys. The derivative δ((C₁₁ – C₁₂)/2)/δP is positive, accounting for the stability of the fct structure of indium under high pressure. Using the quasiharmonic, anisotropic continuum model the acoustic mode Grüneisen parameters have been calculated and are discussed in terms of mode softening. The high temperature limiting value$\text{\$}\text{?}$γ_H (= 2.56) of the mean acoustic mode Grüneisen parameter is found to be close to the thermodynamic Grüneisen parameter γ^th (=2.5).     

Coupling of orientational and translational modes in solid C60 and C70

The orientational phase transitions in solid C₆₀ and C₇₀ are accompanied by quite different anomalies in the crystalline strains. In solid C₆₀ the phase transition Fm3m→Pa3 is primarily an orientational effect (antiferro-rotational), which is driven by the condensation of orientational modes belonging to X₅ ⁺ irreducible representation (irreps) of Fm3m. These modes are the primary order parameters (oops) and their number is equal to the number of irreps of T_2g and T_1g symmetry within the manifolds under consideration. Taking into account irreps up to the manifold 1=12, we have studied the rotation-rotation-translation (RRT) coupling between the oops and the lattice displacements. We have investigated the resulting lattice contraction and the change of the elastic constant c₁₁ at the phase transition. In solid C₇₀ (fcc-phase) we investigate the bilinear coupling of orientational fluctuations of T_2g symmetry to transverse acoustic lattice displacements. This coupling is the driving mechanism for the ferroelastic phase transition Fm3m → R3m. Finally we investigate the transition from the rhombohedral phase to a low temperature monoclinic phase. This transition in antiferro-rotational.     

Zero- and first-sound damping in NaF observed by inelastic neutron scattering

The frequency and temperature dependence of the damping of transverse acoustic modes with 0.06 ? q/q_max ? 0.25 in NaF was determined by means of inelastic neutron scattering. The results clearly show a transition from the Akhiezer to the Landau-Rumer regime.