Generation of coherent terahertz phonons by sharp focusing of a femtosecond laser beam in the bulk of crystalline insulators in a regime of plasma formation

The generation of coherent terahertz phonons in a regime of plasma formation by a femtosecond laser radiation with an intensity of 10¹³ W/cm² in the bulk of crystalline quartz has been detected by the method of probing by a probe pulse of the third harmonic. A smooth increase in the frequency of coherent terahertz phonons from 2.2 to 5.5 THz has been detected, along with its subsequent sharp decrease down to 2.2 THz due to an α-β phase transition in crystalline quartz. The generation of 1-THz coherent phonons has been detected in BaF₂ crystals. A smooth variation of the frequency of coherent phonons from 2 to 2.5 THz has been detected in leucosapphire. The generation of coherent phonons during local laser excitation in CaF₂ and LiF crystals develops at the frequencies of 2.3 and 0.1 THz, respectively.     

Phase transitions in RbCaF3. I: Optical studies

Phase transitions in RbCaF₃ have been investigated by optical birefringence measurements and Raman scattering experiments. A near discontinuity in the onset of spontaneous birefringence at 196 K shows that the cubic to tetragonal phase change is “slightly” first order. The Raman spectra of the tetragonal phase support a D¹⁸_4h structure in which two phonons of A_1g and E_g symmetry soften as the 196 K transition is approached from lower temperatures. A very slow transition to a lower symmetry structure was observed at about 42 K in the Raman measurements.     

Raman scattering of light in N-benzylaniline crystals

Single crystals of N-benzylaniline (C₆H₅-CH₂-NH-C₆H₅) have been grown, X-ray structural analysis of these crystals has been performed, and their symmetry has been identified as corresponding to the space group C _2h ⁵ . Raman spectra of N-benzylaniline single crystals in polarized light for different geometries of the experiment have been analyzed. Frequencies and symmetry of lattice phonons have been determined.     

Phase transition, order parameter, and their manifestation in optical spectra of Hg2Cl2

This paper reports on a high-precision study of the effects induced by the phase transition in Raman spectra of Hg₂Cl₂ crystals. The odd (acoustic and IR-active) phonons, including the soft modes, from the Brillouin zone (BZ) X points of the paraphase of these crystals, which are induced by the phase transition, unit cell doubling, and the X ?? ?? BZ folding, have been investigated both theoretically and experimentally. The temperature dependences of the intensities of these phonons, complemented by a theoretical analysis, have yielded the first information on the behavior of the transition order parameter. The corresponding critical exponents have been derived, which correlate in magnitude with the data obtained from X-ray diffraction measurements and, thus, imply the closeness of the phase transition in these crystals to the tricritical point.     

Raman scattering study of the dynamics of the pressure-induced phase transition in thallium azide (TlN3)

A Raman scattering investigation of the pressure-induced phase transition in tetragonal thallium azide (TlN₃) is reported. The most interesting features of the Raman spectrum of TlN₃ are the anti-resonant line-shapes of two E_g symmetry phonons at 35 and 50 cm^?1 superimposed on a quasi-elastic wing. The scattering data is shown to be consistent with a model in which the two phonons interact via an imaginary coupling term. The phonon at 35 cm^?1 (assigned to a translational shear mode of the Tl⁺ sublattice) softens with increasing pressure and increases in linewidth as P approaches P₀ (=8 kbar) from below. At the same time, the quasi-elastic scattering component (associated with large amplitude N₃^? librational fluctuations) becomes less damped. A displacive structural transition from tetragonal to monoclinic is indicated by the eigenvector of the soft phonon.     

Calorimetric investigation of (TEA)2MnCl4 crystals

Temperature dependencies of the specific heat in a temperature range of 14–260?K are presented for the crystals (TEA)₂MnCl₄. Two anomalies typical for the first order phase transition at T ₁?=?224?K and T ₂?=?231?K with entropy jumps of 15?J?mol^?1?K^?1 and 12?J?mol^?1?K^?1 were observed. The temperature dependence of the lattice heat was approximated by a linear combination of Debye and Einstein functions. Basing on the results an effective wave number of phonons with an essential contribution to the lattice oscillation energy was determined. Since phase transition sequence observed in (TEA)₂MnCl₄ and (TEA)₂CuCl₄ is similar and similar is the chemical composition one can suppose that the low temperature phase transition observed in (TEA)₂MnCl₄ crystals is an isostructural 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.     

Raman scattering investigation of ferroelastic Sb5O7I crystals

Sb₅O₇I undergoes a displacive phase transition at 481 K where the symmetry is changed fromC _6h ² toC _2h ⁵ . In the low temperature monoclinic phase the crystal is ferroelastic. The polarized Raman spectra of Sb₅O₇I have been measured at various temperatures below and above the phase transition. The frequencies and symmetries of most of the theoretically expected Raman active phonons in the ferroelastic phase have been determined. The observation of a soft mode in the ferroelastic phase which disappears above the phase transition together with the fact that the unit cell of the ferroelastic phase is twice as large as that of the paraelastic structure permits the conclusion that the phase transition results from a phonon instability at the Brillouin zone boundaryM-point of the hexagonal phase. The temperature dependent splittings and intensity changes of several Raman lines are discussed with respect to the ferroelastic property of the crystal and the phase transition.     

Ferroelectric properties of partly deuterated NH4HSeO4 crystals

Partly deuterated NH₄HSeO₄ crystals were grown from solutions containing 10, 20, 30, 40, 50 and 60% molar deuterium. The deuteration process leads to an increase in the phase transition temperatures. The extrapolated value of ΔT_c(∞) for completely deuterated crystals is 25 K. The I2 ? B2(C³₂) (if a axis is parallel to [110]) symmetry and the ferroelectric transition related to it is observed only in crystals growing from solution containing less than 50% deuterium. The crystals growing from solution containing more than 50% deuterium have a P2₁2₁2₁(D⁴₂) symmetry.     

EPR study of the low temperature ferroelectric phase transition in Cu doped Rb2ZnCl4 single crystals

Temperature dependent EPR measurements on copper doped Rb₂ZnCl₄ single crystals allowed us to evidence and study the P2₁cn↔C1c1 structural phase transition that takes place in this compound at 74.6 K. From the two types of Cu²⁺ centers localized at different anionic sites, called Cu²⁺(I) and Cu²⁺(II), which are formed in this compound, only the Cu²⁺(II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition.     

Spectroscopic study of the behavior of the order parameter in model ferroelastic crystals of Hg<Subscript>2</Subscript>Cl<Subscript>2</Subscript>

Information is obtained about the temperature behavior of the order parameter of a phase transition by theoretical and experimental investigation of odd (acoustic and IR-active) phonons that appear in the Raman scattering spectra from the X points of the Brillouin zone (BZ) boundary in the paraphase of Hg₂Cl₂ crystals and are induced by the phase transition, unit-cell doubling, and the X → Γ folding in the BZ. The relevant critical exponents are determined, whose values are in agreement with the results of X-ray diffraction measurements and, within the Landau phenomenological theory of phase transitions, indicate that the phase transition in these crystals is close to the tricritical point.     

Symmetry analysis in neutron diffraction studies of magnetic structures: 1. A phase transition concept to describe magnetic structures in crystals

On the basis of phase transition symmetry theory there has been developed an efficient method of calculating the possible magnetic structures liable to arise from the paramagnetic phase of the crystal. Every magnetic structure is described by the superposition of the basis functions of the irreducible representation incorporated in the magnetic representation of the crystal symmetry group. Convenient formulas are given to calculate the basis functions and composition of the magnetic representation. These formulas permit calculation by referring only to the tables of space group irreducible representations. The technique is illustrated on the example of magnetic structures in crystals with the symmetry group D⁶_3d.     

Lattice vibrations of α′-NaV2O5

We have measured far infrared reflectance and transmittance spectra as well as Raman scattering spectra of α′-NaV₂O₅ single crystals for all the principal polarizations. The temperature range above the phase transition temperature T _c =35 K was investigated, mainly. On the basis of this experimental study and of the lattice dynamics calculations we conclude that the symmetry of NaV₂O₅ in the high-temperature phase is described by the centrosymmetric D _2h ¹³ space group. This conclusion leads to important physical consequences concerning the interpretation of one-dimensional magnetic properties of NaV₂O₅ and of the phase transition at 35 K considered earlier to be an ordinary spin-Peierls transition. The assignment of the observed phonons is given. Values of dielectric constants are obtained from the infrared data. Asymmetric shapes of several infrared lines and higher-order infrared vibrational spectra are discussed. The crystal field energy levels of the 3d electron localized at the V⁴⁺ site have been calculated in the framework of the exchange charge model using the values of effective charges obtained from the lattice dynamics calculations. According to the results of these calculations, the broad optical bands observed earlier in the vinicity of 1 eV can be interpreted as phonon assisted d-d transitions. Zh. éksp. Teor. Fiz. 115, 2170–2189 (June 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

First order phase transition of Li3 ThF7 at 281 K: A comparative study between EPR and Raman scattering

Abstract

Electron Paramagnetic Resonance (EPR) and Raman Spectroscopy were used to study the low temperature phase transition of pulled Li₃ThF₇ single crystals, occurring at 281 K. In both cases, the room temperature spectra were very broad, owing to the statistical disorder and high ionic mobility of the lithium ions, which occupy only 3/4 of their structural sites. The results are compatible with a first order ferroelastic transition from the room temperature orthorhombic D_2h ²² phase to a monoclinic C_2h ^(x) one. The symmetry rules are well respected assuming a model with four lithium ions in average per chemical formula. The EPR spectra show also the appearing of additional ferroelastic domains.     

Phase situation in ferroelectric NH4HSeO4 crystals with various degrees of deuteration

NH₄HSeO₄ crystals of varying deuterium content have been studied. The high-temperature phase transition at 408 K is observed both for crystals of symmetry I2 grown from the solutions containing 0–40% D and those of symmetry P2₁2₁2₁ i.e. grown solutions containing above 50% D. This phase transition practically does not depend on %D. For the crystals of symmetry P2₁2₁2₁ an additional dielectric anomaly at 338 K (only slightly dependent on %D) has been found which most probably should be assigned to the P2₁2₁2₁-I2 transition. The results obtained and our earlier data available so far enabled to construct more detailed phase diagram for NH₄HSeO₄ with %D as a parameter.     

Doping and temperature dependence of inversion symmetry breaking in La2−xSrxCuO4

The doping dependence of the Raman spectra of high quality La_2−xSr_xCu^16,18O₄ polycrystalline compounds has been investigated at low temperatures. It is shown that symmetry forbidden bands peaked at ∼150 cm⁻¹, ∼280 cm⁻¹, and ∼370 cm⁻¹ are activated in the (xx/yy) polarization Raman spectra due to the local breaking of the inversion symmetry mainly at low temperatures and for doping concentrations for which the compound is superconducting. The apparent A₁-character of the activated modes in the symmetry reduced phase indicates a reduction from the D_2h to C_2v or D₂ crystal symmetries, which associates the observed modes to specific IR-active phonons with eigenvectors mainly along the c-axis. The temperature and doping dependence of this inversion symmetry breaking and the superconducting transition temperature are very similar, though the symmetry reduction occurs at significantly higher temperatures.     

Orientational states of C60 molecule in crystals

Local symmetry of orientational states of the C₆₀ molecule in crystals has been investigated. It was shown that the various orientational phase transitions in different crystals are related to different orientational orbits. The model of orientational phase transitions based on a sequence of orientational states with different symmetry properties has been suggested. We have found that both the local symmetry of C₆₀ molecule and the symmetry of its internal vibrations become higher after a reduction of crystal spatial symmetry at the phase transition. This effect is fairly common and can be observed in the orientational order-disorder phase transitions with wave vectors at the Brillouin zone boundary. Feasible manifestations of the predicted effect in various experiments are discussed. Zh. éksp. Teor. Fiz. 113, 1081–1093 (March 1998)     

Indirect electronic transitions in alkali halate crystals-I (sodium and potassium chlorate crystals)

The long wavelength tail of the fundamental absorption in NaClO₃ and KClO₃ crystals has been analysed based on the theory of band to band transitions of Bardeen et al.[8] developed in the case of semi-conducting crystals. Evidence of phonon involvement in the transitions giving an indirect band gap is observed. The energies of the phonons involved in the process are the same for both the crystals, and agree well with combinations of prinicple frequencies of ClO₃^? ion, their overtones and also lattice phonons. The indirect band gap in these crystals varies with temperature more or less linearly and the rate of variation is ?3·8 × 10^?4 eV/K and ?5·0 × 10^?4 eV/K for sodium chlorate and potassium chlorate respectively.     

Electronic and vibrational Raman spectroscopy of Nd0.5Sr0.5MnO3 through the phase transitions

Raman scattering experiments have been carried out on single crystals of Nd_0.5Sr_0.5MnO₃ as a function of temperature in the range of 320–50 K, covering the paramagnetic insulator-ferromagnetic metal transition at 250 K and the charge-ordering antiferromagnetic transition at 150 K. The diffusive electronic Raman scattering response is seen in the paramagnetic phase which continue to exist even in the ferromagnetic phase, eventually disappearing below 150 K. We understand the existence of diffusive response in the ferromagnetic phase to the coexistence of the different electronic phases. The frequency and linewidth of the phonons across the transitions show significant changes, which cannot be accounted for only by anharmonic interactions.     

EPR investigation of local paramagnetic centers in perovskite-like crystals

In ScF₃ single crystals (pure and doped) as well as in Rb₂KScF₆ and Rb₂KDyF₆ crystals with a perovskite-like structure, point nanodefects (vacancy in place of trivalent cations) have been found and studied. Electron paramagnetic resonance has been used to investigate local paramagnetic centers that are not detected using X-ray diffraction. The angular dependence of the spectra indicates a local distortion of the cubic symmetry of the crystals. An additional hyperfine structure in the observed spectra is due to the delocalization of electrons over six F^? ions forming the first coordination polyhedron around the vacancy. The crystals studied are characterized by a high electron mobility and a high electron velocity, which depends on the impurity. The high mobility of electrons of the cation center can be indirectly responsible for the structural phase transition occurring in the ScF₃ crystal under uniaxial pressure.