Polar nanoregion vibrations control the ultrahigh piezoelectric response of relaxor-based ferroelectrics used in applications.
Lead chalcogenides have exceptional thermoelectric properties and intriguing anharmonic lattice dynamics underlying their low thermal conductivities. An ideal material for thermoelectric efficiency is the phonon glass–electron crystal, which drives research on strategies to scatter or localize phonons while minimally disrupting electronic-transport. Anharmonicity can potentially do both, even in perfect crystals, and simulations suggest that PbSe is anharmonic enough to support intrinsic localized modes that halt transport. Here, we experimentally observe high-temperature localization in PbSe using neutron scattering but find that localization is not limited to isolated modes – zero group velocity develops for a significant section of the transverse optic phonon on heating above a transition in the anharmonic dynamics. Arrest of the optic phonon propagation coincides with unusual sharpening of the longitudinal acoustic mode due to a loss of phase space for scattering. Our study shows how nonlinear physics beyond conventional anharmonic perturbations can fundamentally alter vibrational transport properties.
We present a new instrument for spin echo small angle neutron scattering (SESANS) developed at the Low Energy Neutron Source (LENS) at Indiana University. A description of the various instrument components is given along with the performance of these components. At the heart of the instrument are a series of resistive coils to encode the neutron trajectory into the neutron polarisation. These are shown to work well over a broad wavelength range of neutron wavelengths. Neutron polarisation analysis is accomplished using a continuously-operating neutron spin filter polarised by Rb-spin exchange optical pumping of 3 He. We describe the performance of the analyser along with a study of the 3 He polarisation stability and its implications for SESANS measurements. Scattering from silica Stöber particles is investigated and agrees with samples run on similar instruments.
Neutrons scattered or reflected from a diffraction grating are subject to a periodic potential analogous to the potential experienced by electrons within a crystal. Hence, the wavefunction of the neutrons can be expanded in terms of Bloch waves and a dynamical theory can be applied to interpret the scattering phenomenon. In this paper, a dynamical theory is used to calculate the results of neutron spin‐echo resolved grazing‐incidence scattering (SERGIS) from a silicon diffraction grating with a rectangular profile. The calculations are compared with SERGIS measurements made on the same grating at two neutron sources: a pulsed source and a continuous wave source. In both cases, the spin‐echo polarization, studied as a function of the spin‐echo length, peaks at integer multiples of the grating period but there are some differences between the two sets of data. The dynamical theory explains the differences and gives a good account of both sets of results.
Magnetocaloric (MC) materials present an avenue for chemical-free, solid state refrigeration through cooling via adiabatic demagnetization. We have used inelastic neutron scattering to measure the lattice dynamics in the MC material Ni45Co5Mn36.6In13.4. Upon heating across the Curie Temperature (TC), the material exhibits an anomalous increase in phonon entropy of 0.22 ± 0.04 /atom, which is ten times larger than expected from conventional thermal expansion. This transition is accompanied by an abrupt softening of the transverse optic phonon. We present first-principle calculations showing a strong coupling between lattice distortions and magnetic excitations.PACS numbers: 78.70. Nx, 75.30.Sg, 65.40.gd, 63.20.kk The coupling between magnetism and lattice excitations has been a recent topic of interest due to its importance in several solid-state systems, such as influencing decoherence in quantum magnets [1] and inducing a spin-Peierls transition in CuGeO 3 [2,3] and ZnCr 2 O 4 [4]. Studies of magnetoelastic materials have focused on metamagnetic shape memory alloys (MMSMA), which are able to reversibly deform with strains of up to ~6% [5,6] via a structural first-order phase transition (FOPT) [7][8][9]. The FOPT allows the alloy to function as magnetocaloric (MC) material, in which an adiabatic change in magnetization of the MMSMA causes an increase in its entropy, thus lowering its temperature [9][10][11][12]. While the change in entropy can be quite large, the FOPT is often accompanied by thermal and/or magnetic hysteresis, which limits the material's long term cycling efficiency. In addition, materials requiring a magnetic field of more than 2 T to induce the FOPT are not feasible for use in any magnetic refrigerator using permanent magnets [13,14].The paramagnetic-to-ferromagnetic secondorder phase transition (SOPT) offers a means to extract heat from a magnetic material using weaker applied magnetic fields and without hysteresis. This SOPT is present in any magnetic material at its Curie temperature ( ), but the change in entropy is typically significantly less than across the FOPT.In this letter, we quantify the phonon entropy and the magnon-phonon coupling in Ni45Co5Mn50-xInx (x=13.4) near the austenite using neutron scattering [15]. When annealed at temperatures below 900K, this compound forms a Huesler L21 structure that is stable down to the martensitic transition temperature , with Mn atoms occupying the 4a positions, Mn1-x/25Inx/25 the 4b, and Ni0.9Co0.1 the 8c positions (Wyckoff notation) [16]. The composition of the crystal puts it at a morphotropic phase boundary; decreasing x from 13.4 to 13.3 leads to an increase in by ~100K [17]. Ab initio calculations of similar compounds point to magnetic frustration as a possible explanation of the morphotropic phase boundary [18][19][20][21]. The magnetic exchange integral for nearest-neighbors Mn-Mn in the 4a and 4b positions -i.e. aligned along [100] -is predicted to be a large negative value (antiferromagnetic). Such an interaction competes ...
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