Superlattice Point Research Articles

A neutron scattering study of the quasi-one-dimensional conductor (TaSe4)2I

Neutron scattering experiments were carried out on the quasi-one-dimensional conductor (TaSe4)2I. At the superlattice point (0.05, 0.05, 0.085) a soft temperature-dependent transverse acoustic (TA) mode polarised in the c plane was observed. A 'spoon-like' anomaly of the dispersion curve of the TA branch was observed around the superlattice point. In addition, an extremely low-lying TA mode was found.

Random Anisotropy Effect in Fe0.65Co0.35TiO3

Random anisotropy effect has been observed in the neutron quasielastic scattering profile from a single crystal of Fe 0.65 Co 0.35 TiO 3 . The effect appears in magnetic diffuse scattering in the large q space. The instantaneous correlation function in small q around q around antiferromagnetic superlattice points is of the Lorentzian squared function of 1/(κ 2 + q 2 ) 2 , where κ is the temperatured dependent inverse correlation length below T N . Combined with other experimental results, we conclude that the random anisotropy field destroys the magnetic long range order in this mixture of two components with competing anisotropies.

Neutron inelastic scattering and X-ray structural study of the charge-density-wave state in K0.3MoO3

The Peierls transition in the quasi-one-dimensional conductor K0.3MoO3 has been studied to clarify its dynamical nature and the electron correlation problem. A giant Kohn anomaly and the central peak in S(q, omega ) were observed. The amplitude below the transition temperature Tc was also measured. An asymmetry of the elastic diffuse scattering above Tc around the reciprocal point which corresponds to the superlattice point can be explained by taking into account the structure factor of the charge-density wave. A detailed type of modulation of Mo atoms is presented by the X-ray study. No evidence for the correlation effect was observed and the transition of K0.3MoO3 seems to be explained by a simple band picture.

Concentration wave description of an early stage kinetics of clustering and ordering in ordered phase

Kinetic theory of early stages of coherent clustering and continuous ordering in ordered phase is developed using the concept of concentration waves on the assumption of mean field energy. Linear differential equations of atom diffusion in ordered phase are solved by diagonalizing the coefficient matrix of the equations. It is shown that at the early stages of the reaction the concentration (fluctuation) waves evolve exponentially with respect to time on the sublattices of the ordered phase. The theory is applied to the Fe-10.9 at. %Si alloy which exhibits a sequence of transitions on aging at 775K. It is proved that the first transition from the disordered to the B2 phase occurs purely due to the kinetic reason that the amplification factors of the fluctuation waves have a positive maximum value only at the superlattice point of the B2 structure in the first Brillouin zone of the disordered phase. The second transition, which is from the metastable B2 phase to the stable phase, i.e. the spinodally decomposed D03 phase, is also explained consistently by the present theory that the amplifications calculated in the B2 phase exhibit two positive maxima in the first Brillouin zone. The maximum at point (q) = ( 1 2 1 2 1 2 ) represents the amplification factor of the D03 ordering-wave in the B2 phase, while the other close to the Brillouin zone center represents the amplification factor of the fluctuation wave of the spinodal decomposition.

Neutron scattering study of the metal-semiconductor phase transition in K 2Pt(CN) 4Br 0.3 ·3D 2O under pressure

The elastic scattering associated with the 2 k F instability in the one-dimensional (1- d) conductor K 2Pt(CN) 4Br 0.3 · 3D 2O (KCP) has been studied at the low temperature super lattice point (0.5, 0.5, 3.7) as a function of temperature for different pressures. Within the investigated region between 0 and 11 kbar, we find that the value of 2 k F remains constant at 1.703 c ∗ , although the absolute value of 2 k F changes by 1.5%. At the same time the Peierls gap Δ(0) and the interchain correlation length ξ ⊥ (0) decrease. We conclude that the 1- d character of KCP is enhanced under pressure.

A comment on the phase transition in RbCaF 3

The structural phase transition in RbCaF 3 has been examined by observing super lattice points by means of neutron diffraction. There is one structural phase transition with the R 25 x soft mode phonon condensation up to 4.2 K. The rotation angle of CaF 6 octahedra, Ø, is calculated as 8.64 degrees at 60 K. The temperature dependence of Ø is also given.

Neutron scattering study of the spin dynamics and spin-wave form factor of chromium

The spin excitations in a single crystal of Cr/sub 0.98/Mn/sub 0.02/ have been studied by inelastic neutron scattering. High-resolution triple-axis spectrometer measurements have yielded a value of the spin-wave velocity at T/T/sub N/ approx. = 0.5 of (1.30 +- 0.15) x 10/sup 7/ cm/sec in good agreement with earlier measurements. The excitation strength drops extremely rapidly in the vicinity of T/sub N/ and decreases approximately linearly above T/sub N/. The spin-wave form factor has been measured below and above T/sub N/ by measuring the spin-wave intensity at various superlattice points, and is found to agree to within experimental error with the static spin form factor. The implications of these results for the theory of itinerant electron antiferromagnetism are discussed. (AIP)

Propagative spin relaxation in the Ising-like antiferromagnetic linear chain

It is shown that at low temperatures the spin dynamics of an antiferromagnetic linear chain can be governed by propagation of boundaries between antiferromagnetic one-dimensional domains. A double maximum results in the neutron-scattering function S( q, ω) if the one-dimensional momentum transfer q is not too close to a one-dimensional superlattice point. In contrast with ordinary magnons of an anisotropic system, the dispersion law corresponding to the maximum exhibits no gap. If q = π, S( q, ω) is centered at ω = 0 but has a nonlorentzian shape. At moderately low temperatures the neutron linewidth at q = π amd the spin-lattice relaxation time are both proportional to the inverse correlation length K, whereas at very low temperature T, they are proportional to K T 1 2 . Much smaller values of T 1 and of the neutron linewidth are expected if the spin dynamics is governed by thermally activated processes, as occurs if boundaries are trapped by lattice imperfection and cannot propagate. In the ferromagnetic Ising-like chain, spin dynamics is always governed by thermally activated processes.

X-Ray Critical Scattering in NH4Br

A detailed observation of X-ray diffuse scattering in CsCl-type NH 4 Br has been carried out in order to investigate the property of the microscopic interaction to cause the order-disorder phase transitions in this substance. The experimental results are summarized as follows. (i) Diffuse scattering peaked at the super lattice points has been observed up to 30 K above the critical temperature of the cubic (disordered) to tetragonal phase transition. (ii) The temperature dependence of the diffuse scattering shows a typical feature of critical scattering observed in the process of ordering of various physical quantities. In the present case, the critical scattering is attributed to the correlated fluctuation of displacements of Br - ions. (iii) The distribution of the diffuse scattering intensity in the reciprocal space is anisotropic, the intensity contours showing an elongation along [110] direction of the cubic unit cell. These experimental results are consistent with the theory 1) in which the system is treated as an Ising spin-phonon coupled system.

Magnetic Excitations in a Cr–Mn Alloy by Inelastic Neutron Scattering

The inelastic magnetic scattering from a Cr0.98Mn0.02 alloy single crystal has been studied both below and above the Néel temperature. In the spin-wave regime at 300 K, we have obtained a value of 1.29×107 cm/sec for the spin-wave velocity by using the diffraction method and suitably extracting the resolution function. At a temperature of 1.16 TN, we have observed quite strong inelastic magnetic scattering and fitted the results quite well by calculating the imaginary part of χ(Q) for paramagnetic chromium. A triple-axis spectrometer was used to study the magnetic excitations in a crystal of Cr0.99Mn0.01 below, above and close to the Néel temperature of 514 K. At low temperatures, peaks at the magnetic superlattice points were seen consistent with the above spin-wave velocities. These appear to go smoothly into the slightly broader paramagnon peaks as the temperature was raised through TN without any sudden change of intensity. The absence of Stoner excitations near TN suggests that either the neutron cross section for these is very low or else that the sample is gapless near TN due to phonon broadening of the electronic states. Full details are published elsewhere.1

Lattice-Dynamical Study of the 110°K Phase Transition in SrTiO3

The lattice-dynamical characteristic of the 110\ifmmode^\circ\else\textdegree\fi{}K transition in SrTi${\mathrm{O}}_{3}$ has been elucidated by inelastic neutron scattering measurements. The transition is caused by a ${\mathrm{T}}_{25}$ soft-phonon mode at the [111] zone boundary, confirming the model recently proposed by Fleury, Scott, and Worlock. The square of the frequency of this soft mode is proportional to $T\ensuremath{-}{T}_{0}$ above the transition temperature ${T}_{0}$. At the transition, this zone boundary becomes a superlattice point, enlarging the unit cell. The results of elastic intensity measurements at the superlattice points at 78\ifmmode^\circ\else\textdegree\fi{}K are consistent with the structure given by Unoki and Sakudo in its space group $\frac{I4}{mcm({{D}_{4h}}^{18})}$ as well as in the oxygen parameter. This structure is shown to be a logical consequence of the condensed soft mode.