A detailed lattice-dynamic investigation on the relaxor ferroelectric PMN was started to search for "additional" modes (dispersion curves). But the results are complex with 4 branches observed below 6.5 THz, a broad unresolved continuum between 6.5 and 12 THz, a well defined excitation at 13.4 THz with longitudinal and transverse character and further weak signals above 16 THz. Possible effects caused by disorder are discussed.
We report an x-ray diffraction study on the charge-density-wave (CDW) LaTe$_3$ and CeTe$_3$ compounds as a function of pressure. We extract the lattice constants and the CDW modulation wave-vector, and provide direct evidence for a pressure-induced quenching of the CDW phase. We observe subtle differences between the chemical and mechanical compression of the lattice. We account for these with a scenario where the effective dimensionality in these CDW systems is dependent on the type of lattice compression and has a direct impact on the degree of Fermi surface nesting and on the strength of fluctuation effects.
Magnetization, magnetic torque, neutron diffraction and NMR experiments are used to map out the $H\ensuremath{-}T$ phase diagram of the prototypical quasi-two-dimensional ferro-antiferromagnet ${\mathrm{Pb}}_{2}\mathrm{VO}{({\mathrm{PO}}_{4})}_{2}$ in magnetic fields up to 27 T. When the field is applied perpendicularly to the axis of magnetic anisotropy, a new magnetic state emerges through a discontinuous transition and persists in a narrow field range just below saturation. The measured NMR spectra suggest a complex and possibly incommensurate magnetic order in that regime.
The low-energy part of the vibration spectrum in PbMg$_{1/3}$Nb$_{2/3}$O$_3$ (PMN) relaxor ferroelectric has been studied by neutron scattering above and below the Burns temperature, T$_d$. The transverse acoustic and the lowest transverse optic phonons are strongly coupled and we have obtained a model for this coupling. We observe that the lowest optic branch is always underdamped. A resolution-limited central peak and quasi-elastic scattering appear in the vicinity of the Burns temperature. It is shown that it is unlikely that the quasi-elastic scattering originates from the combined effects of coupling between TA and TO phonons with an increase of the damping of the TO phonon below T$_d$. The quasi-elastic scattering has a peak as a function of temperature close to the peak in the dielectric constant while the intensity of the central peak scattering increases strongly below this temperature. These results are discussed in terms of a random field model for relaxors.
Relaxor ferroelectrics are difficult to study and understand. The experiment shows that at low energy scattering there is an acoustic mode, an optic mode, dynamic quasi-elastic and strictly elastic scattering as well as Bragg peaks at the zone centre. We have studied the scattering using the TASP spectrometer at PSI and have analysed the data using a model with interactions between the different components particularly to determine the properties of the elastic scattering. The quasi-elastic scattering begins to become significant at the Burns temperature of 620 K. It steadily increases in intensity on cooling reaching a maximum at ∼ 400 K. Below this temperature the strictly elastic scattering begins to increase and shows a broadened line shape characteristic of crystals in a random applied field. We show that all the results obtained from PMN for the elastic scattering are consistent with the crystal having a random field transition at ∼ 400 K. We have obtained similar results for PMN-PT and PZN-PT suggesting that random fields also play an important role in these materials.
This paper presents studies of lattice dynamics of a crystal of Cs 5 H 3 (SO 4 ) 4 x H 2 O (PCHS) and its deuterated analog Cs 5 D 3 (SO 4 ) 4 x D 2 O (DPCHS) in wide temperature and frequency ranges by dielectric spectroscopy and modulated temperature differential scanning calorimetry. It is shown that deuteration of PCHS gives rise to an additional structural phase transition at T c3 , 250 K. The temperature of the isostrctural phase transition T c2 (372 K for PCHS and 370 K for DPCHS) has been determined more accurately. It has been found that deuteration does not suppress phase transition at T c2 . The temperature dependence of dielectric response is analyzed.
We report a neutron scattering study of the ferroelectric phase transition in Sr$_{0.585}$Ce$_{0.025}$Ba$_{0.39}$Nb$_2$O$_6$ (SBN-61:Ce). We find no evidence for a soft transverse optic phonon. We do, however, observe anisotropic diffuse scattering. This scattering has inelastic and elastic contributions. In the paraelectric phase the susceptibility associated with the elastic diffuse scattering follows well the anomaly of the dielectric susceptibility of SBN-61:Ce. In the ferroelectric phase the lineshape of the elastic scattering is consistent with the form expected for the ferroelectric domain walls. In contrast to the macroscopic observations, the scattering properties of Ce-doped crystal do not exhibit important changes with respect to those of pure Sr$_{0.61}$Ba$_{0.39}$Nb$_2$O$_6$.
The relaxor ferroelectric PbMg 1/3 Ta 2/3 O 3 was studied by single-crystal neutron and synchrotron X-ray diffraction, and its detailed atomic structure modelled in terms of static Pb displacements that lead to the formation of polar nanoregions. Similar to the other members of the Pb-based relaxor family like PbMg 1/3 Nb 2/3 O 3 or PbZn 1/3 Nb 2/3 O 3 the diffuse scattering in the [ H 00]/[0 K 0] scattering plane has a butterfly shape around the h 00 Bragg reflections and is orthogonal to the scattering vector for h h 0 peaks. In the [ H H 0]/[00 L ] plane the diffuse scattering is elongated along the 〈112〉 directions and is orthogonal to the scattering vector for h h h reflections. It is found that a model consisting of correlated Pb displacements along the 〈111〉 directions reproduces adequately the main features of the diffuse scattering in PbMg 1/3 Ta 2/3 O 3 when the correlation lengths between the Pb-ion displacement vectors are longest along the 〈111〉 and shortest along the 〈11{\overline 2}〉 and 〈1{\overline 1}0〉 directions.
Magnetic, thermodynamic, neutron diffraction and inelastic neutron scattering are used to study spin correlations in the easy-axis XXZ triangular lattice magnet K_{2}Co(SeO_{3})_{2}. Despite the presence of quasi-2D "supersolid" magnetic order, the low-energy excitation spectrum contains no sharp modes and is instead a broad and structured multiparticle continuum. Applying a weak magnetic field drives the system into an m=1/3 fractional magnetization plateau phase and restores sharp spin wave modes. To some extent, the behavior at zero field can be understood in terms of spin wave decay. However, the presence of clear excitation minima at the M points of the Brillouin zone suggest that the spinon language may provide a more adequate description, and signals a possible proximity to a Dirac spin liquid state.
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