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.     

Magnetism in geometrically frustrated YMnO3 under hydrostatic pressure studied with muon spin relaxation

The ferroelectromagnet YMnO3 consists of weakly coupled triangular layers of S=2 spins. Below T(N) approximately equal to 70 K muon-spin relaxation data show two oscillatory relaxing signals due to magnetic order, with no purely relaxing signals resolvable (which would require different coexisting spin distributions). The transition temperature T(N) increases with applied hydrostatic pressure, even though the ordered moment decreases. These results suggest that pressure increases both the exchange coupling between the layers and the frustration within the layers.     

Spin fluctuations in the stacked-triangular antiferromagnet YMnO<Subscript>3</Subscript>

The spectrum of spin fluctuations in the stacked-triangular antiferromagnet YMnO₃ was studied above the Néel temperature using both unpolarized and polarized inelastic neutron scattering. We find an in-plane and an out-of-plane excitation. The in-plane mode has two components just above T _N : a resolution-limited central peak and a Debye-like contribution. The quasi-elastic fluctuations have a line width that increases with q as Dq ^z and the dynamical exponent z = 2.3. The out-of-plane fluctuations have a gap at the magnetic zone center and do not show any appreciable q dependence at small wave vectors.     

On the effect of ordering on vibrational spectrum of relaxor ferroelectric PbSc1/2Ta1/2O3 in paraphase

The effect of the ordering of Sc³⁺ and Ta⁵⁺ ions on the behavior of the phonon subsystem in the relaxor ferroelectric PbSc_1/2Ta_1/2O₃ from the numerous AB′ _x B″ _x?1O₃ family has been studied. The nature of an additional contribution to the light-scattering spectra has been analyzed on the experimental data on Raman scattering in PbSc_1/2Ta_1/2O₃, SrAl_1/2Ta_1/2O₃, SrAl_1/2Nb_1/2O₃, and BaMg_1/3Ta_2/3O₃ compounds and inelastic neutron scattering in PbSc_1/2Ta_1/2O₃. It is shown that broadening of the lines in the Raman spectra of PbSc_1/2Ta_1/2O₃ crystals is caused by additional scattering by phonons from various points of the Brillouin zone.     

Geometrical aspects of QENS experiments in case of anisotropic samples

Diffusion in anisotropic samples is often studied by quasi-elastic neutron scattering at time-of-flight (t-o-f) instruments. During the data analysis the measured dynamic structure factor S(Q,E) is fitted by a model function, which - reflecting the anisotropy of the sample - depends on the sample orientation. The error introduced in this way will be discussed. Correct measurements can be performed for such cases at a triple axis spectrometer, where energy scan can be performed for a certain Q vector fixed to the sample orientation. Clays show 2D anisotropy in the structure, which is supposed to be visible also in the diffusion of water located between the clay sheets. A first experiment performed on TASP (SINQ) will be shown.     

Neutron diffraction investigation of a metamagnetic transition in the Tb<Subscript>0.1</Subscript> Tm<Subscript>0.9</Subscript>Co<Subscript>2</Subscript> compound

The Tb_0.1Tm_0.9Co₂ compound is investigated using neutron diffraction. It is shown that this compound undergoes an irreversible band metamagnetic transition induced by an external magnetic field. The magnetization of the Co sublattice increases from 0.2 to 0.6 μ_B. The critical field strength is approximately equal to 1 T at temperatures of 1.8 and 4.0 K. As the temperature increases, the effect of the magnetic field on the magnetic state of the sample weakens and, at 25 K, no noticeable changes are observed in an external field of 0.75 T. The metamagnetic transition at 1.8 K is accompanied by the disappearance of rhombohedral distortions and brings about a lattice expansion by approximately 1%.     

Spectral and thermodynamic properties of a strong-leg quantum spin ladder

The strong-leg S=1/2 Heisenberg spin ladder system (C(7)H(10)N)(2)CuBr(4) is investigated using density matrix renormalization group calculations, inelastic neutron scattering, and bulk magnetothermodynamic measurements. Measurements showed qualitative differences compared to the strong-rung case. A long-lived two-triplon bound state is confirmed to persist across most of the Brillouin zone in a zero field. In applied fields, in the Tomonaga-Luttinger spin-liquid phase, elementary excitations are attractive, rather than repulsive. In the presence of weak interladder interactions, the strong-leg system is considerably more prone to three-dimensional ordering.     

Study of phase transformation dynamics in Na1/2Bi1/2TiO3 by Mandelstam-Brillouin scattering

The results of the study of Mandelstam-Brillouin light scattering in single-crystal Na_1/2Bi_1/2TiO₃ within a broad temperature interval covering successively the cubic, tetragonal, and rhombohedral phases are reported. Measurements conducted within the range 820–300 K have revealed quasi-elastic scattering and distortion of the phonon spectra in shape, which suggests possible coupling between the low-frequency transverse optical and acoustic modes. These two observed phenomena are believed to be closely interrelated.     

Incommensurate magnetic order in TbTe3

We report a neutron diffraction study of the magnetic phase transitions in the charge-density wave (CDW) TbTe(3) compound. We discover that in the paramagnetic phase there are strong 2D-like magnetic correlations, consistent with the pronounced anisotropy of the chemical structure. A long-range incommensurate magnetic order emerges in TbTe(3) at T(mag1) = 5.78 K as a result of continuous phase transitions. We observe that near the temperature T(mag1) the magnetic Bragg peaks appear around the position (0, 0, 0.24) (or its rational multiples), that is fairly close to the propagation vector (0,0,0.29) associated with the CDW phase transition in TbTe(3). This suggests that correlations leading to the long-range magnetic order in TbTe(3) are linked to the modulations that occur in the CDW state.