Search results for "Soft modes"
showing 10 items of 29 documents
X-ray thermal-diffuse-scattering study of soft modes in paraelectricBaTiO3
1995
Anomalous x-ray thermal diffuse scattering (TDS) from the paraelectric ${\mathrm{BaTiO}}_{3}$ has been measured as a function of temperature using synchrotron radiation. Sheets of intensities that were previously reported have been confirmed. The origin of the observed TDS intensities is attributed to the soft on-(100) TA modes and overdamped [010${]}_{\mathrm{TO}}$ mode for reduced wave vector qg0.25 and 0.25, respectively. In agreement with the previous neutron inelastic scattering experiment, our results support the notion of the one-dimensionally correlated optic motion of atoms.
Phase Behavior of TmVO4 under Hydrostatic Compression: An Experimental and Theoretical Study
2020
We present a structural and optical characterization of magnetoelastic zircon-type TmVO4 at ambient pressure and under high pressure. The properties under high pressure have been determined experimentally under hydrostatic conditions and theoretically using density functional theory. By powder X-ray diffraction we show that TmVO4 undergoes a first-order irreversible phase transition to a scheelite structure above 6 GPa. We have also determined (from powder and single-crystal X-ray diffraction) the bulk moduli of both phases and found that their compressibilities are anisotropic. The band gap of TmVO4 is found to be Eg = 3.7(2) eV. Under compression the band gap opens linearly, until it unde…
Critical effects in optical response due to charge transfer vibronic excitons and their structure in perovskite-like systems
1999
Abstract A mechanism for bilinear interaction between high-frequency light-induced electronic polarization and low-frequency soft lattice polarization is proposed. It is based on the fluctuations of the charge transfer connected with charge transfer vibronic excitons (CTVE). This bilinear mechanism leads to the appearance of the critical peculiarities of the absolute diffraction efficiency of transient gratings near the ferroelectric phase transition. A semi-empirical Hartree–Fock INDO method was used for the evaluation of the energy parameters and the equilibrium displacements for the CTVE in KTaO 3 . This numerical study did confirm the proposed CTVE-model. It was shown that the CTVE-phas…
Mössbauer study of properties and phase transitions of Cd2Nb2–2xSn2xO7–2xF2x
1973
With the aid of Mossbauer spectroscopy the solid solution series Cd2Nb2–2xSn2xO7–2xF2x are investigated in the temperature range 4.2 to 400 K. The crystal structure belongs to the cubic pyrochlore type. The BY6 octahedra (B = Nb, Sn; Y = O, F) are linked three-dimensionally through their corners, forming groups of four octahedra which are extended or compressed in the (111)-direction for x ≤ 0.375 or x ≥ 0.400 respectively. The dependence of the quadrupole splitting ΔEQ on x shows a small but significant discontinuity in the region 0.375 < x < 0.400. This can be attributed to a first order phase-transition which depends on concentration, but not on temperature over the temperature region st…
High-Pressure Raman Study of Fe(IO3)3: Soft-Mode Behavior Driven by Coordination Changes of Iodine Atoms
2020
[EN] We report high-pressure Raman spectroscopy studies of Fe(IO3)(3) up to nearly 21 GPa that have been interpreted with the help of density functional theory calculations, which include the calculation of phonon dispersion curves and elastic constants at different pressures. Zero-pressure Raman-active mode frequencies and their pressure dependences have been determined. Modes have been assigned and correlated to atomic movements with the help of calculations. Interestingly, in the high-frequency region, there are several modes that soften under compression. These modes have been identified as internal vibrations of the IO3 coordination polyhedron. Their unusual behavior is a consequence o…
Broadband dielectric spectroscopy of PbMg1/3Nb2/3O3–PbSc1/2Nb1/2O3 ceramics
2010
Abstract Broadband dielectric spectroscopy results of various ordered and disordered (1 − x)Pb(Mg1/3Nb2/3)O3–(x)Pb(Sc1/2Nb1/2)O3 (PMN–PSN) ceramics are investigated in the temperature range from 80 K to 300 K and frequency range from 20 Hz to 2 THz. Dielectric dispersion is very broad and in the ferroelectrics case (x = 1, 0.95) consists of two parts: low-frequency part caused by ferroelectric domains and higher frequency part caused by soft mode. The relaxational soft mode exhibits pronounced softening close to phase transition temperature, as it is typical for order–disorder phase transitions. By substituting Sc3+ by Mg2+ in PMN–PSN ceramics relaxation slows down, and for relaxors (x = 0.…
Broadband Dielectric Spectroscopy of Ferroelectric Phase Transitions in PbSc1/2Nb1/2O3Ordered Ceramics
2008
Broadband dielectric spectroscopy of PbSc 1/2 Nb 1/2 O 3 (PSN) ordered ceramics are reported within the range of 20 Hz ≤ ν ≤ 2 THz in 80 K ≤ T ≤ 300 K temperatures. Spectrally very broad dielectric dispersion consisting of two parts − at lower frequencies, ferroelectric domains cause a dispersion and at higher frequencies, higher temperatures the relaxation soft mode is responsible for the dispersion. The relaxation soft mode exhibit pronounced hardening on cooling, whilst the ferroelectric phase transition is connected with an abrupt freezing and rise of polar nanoregions into ferroelectric domains.
Confinement effects on phase behavior of soft matter systems.
2008
When systems that can undergo phase separation between two coexisting phases in the bulk are confined in thin film geometry between parallel walls, the phase behavior can be profoundly modified. These phenomena shall be described and exemplified by computer simulations of the Asakura-Oosawa model for colloid-polymer mixtures, but applications to other soft matter systems (e.g. confined polymer blends) will also be mentioned. Typically a wall will prefer one of the phases, and hence the composition of the system in the direction perpendicular to the walls will not be homogeneous. If both walls are of the same kind, this effect leads to a distortion of the phase diagram of the system in thin …
LaAg x In1?x
1980
The phonon dispersion of LaAg x In1−x (x=1, 0.89, 0.8) has been studied by inelastic neutron scattering in the cubic high temperature phase. A soft mode behaviour was observed at theM-point. The doubling and the cubic to tetragonal deformation of the elementary cell was observed through the phase transition. The measurements of the elastic constants were extended to 450 K and their magnetic field dependence was investigated.
Properties of dirty two-band superconductors with repulsive interband interaction: Normal modes, length scales, vortices, and magnetic response
2018
Disorder in two-band superconductors with repulsive interband interaction induces a frustrated competition between the phase-locking preferences of the various potential and kinetic terms. This frustrated interaction can result in the formation of an $s+is$ superconducting state, that breaks the time-reversal symmetry. In this paper we study the normal modes and their associated coherence lengths in such materials. We especially focus on the consequences of the soft modes stemming from the frustration and time-reversal-symmetry breakdown. We find that two-bands superconductors with such impurity-induced frustrated interactions display a rich spectrum of physical properties that are absent i…