From One- to Two-Magnon Excitations in the S=3/2 Magnet ß-CaCr_{2}O_{4}.

We apply neutron spectroscopy to measure the magnetic dynamics in the S=3/2 magnet ß-CaCr_{2}O_{4} (T_{N}=21 K). The low-energy fluctuations, in the ordered state, resemble large-S linear spin waves from the incommensurate ground state. However, at higher energy transfers, these semiclassical and harmonic dynamics are replaced by an energy and momentum broadened continuum of excitations. Applying kinematic constraints required for energy and momentum conservation, sum rules of neutron scattering, and comparison against exact diagonalization calculations, we show that the dynamics at high-energy transfers resemble low-S one-dimensional quantum fluctuations. ß-CaCr_{2}O_{4} represents an example of a magnet at the border between classical Néel and quantum phases, displaying dual characteristics.

Anharmonic Magnon Excitations in Noncollinear and Charge-Ordered RbFe

RbFe^{2+}Fe^{3+}F_{6} is an example of a charge ordered antiferromagnet where iron sites, with differing valences, are structurally separated into two interpenetrating sublattices. The low temperature magnetically ordered Fe^{2+} (S=2) and Fe^{3+} (S=5/2) moments form a noncollinear orthogonal structure with the Fe^{3+} site displaying a reduced static ordered moment. Neutron spectroscopy on single crystals finds two distinct spin wave branches with a dominant coupling along the Fe^{3+} chain axis (b axis). High resolution spectroscopic measurements find an intense energy and momentum broadened magnetic band of scattering bracketing a momentum-energy region where two magnon processes are kinematically allowed. These anharmonic excitations are enhanced in this noncollinear magnet owing to the orthogonal spin arrangement.

Broadband critical dynamics in disordered lead-based perovskites.

Materials based on the cubic perovskite unit cell continue to provide the basis for technologically important materials with two notable recent examples being lead-based relaxor piezoelectrics and lead-based organic-inorganic halide photovoltaics. These materials carry considerable disorder, arising from site substitution in relaxors and molecular vibrations in the organic-inorganics, yet much of our understanding of these systems derives from the initial classic work of Prof. Roger A Cowley, who applied both theory and neutron scattering methods while at Chalk River Laboratories to the study of lattice vibrations in SrTiO3. Neutron scattering continues to play a vital role in characterizing lattice vibrations in perovskites owing to the simple cross section and the wide range of energy resolutions achievable with current neutron instrumentation. We discuss the dynamics that drive the phase transitions in the relaxors and organic-inorganic lead-halides in terms of neutron scattering and compare them to those in phase transitions associated with a 'central peak' and also a soft mode. We review some of the past experimental work on these materials and present new data from high-resolution time-of-flight backscattering spectroscopy taken on organic-inorganic perovskites. We will show that the structural transitions in disordered lead-based perovskites are driven by a broad frequency band of excitations.

Decoupled molecular and inorganic framework dynamics in CH3NH3PbCl3.

The organic-inorganic lead-halide perovskites are composed of organic molecules imbedded in an inorganic framework. The compounds with general formula CH3NH3PbX 3 (MAPbX 3) display large photovoltaic efficiencies for halogens X = Cl, Br, and I in a wide variety of sample geometries and preparation methods. The organic cation and inorganic framework are bound by hydrogen bonds that tether the molecules to the halide anions, and this has been suggested to be important to the optoelectronic properties. We have studied the effects of this bonding using time-of-flight neutron spectroscopy to measure the molecular dynamics in CH3NH3PbCl3 (MAPbCl3). Low-energy/high-resolution neutron backscattering reveals thermally activated molecular dynamics with a characteristic temperature of ~95 K. At this same temperature, higher-energy neutron spectroscopy indicates the presence of an anomalous broadening in energy (reduced lifetime) associated with the molecular vibrations. By contrast, neutron powder diffraction shows that a spatially long-range structural phase transitions occurs at 178 K (cubic → tetragonal) and 173 K (tetragonal → orthorhombic). The large difference between these two temperature scales suggests that the molecular and inorganic lattice dynamics in MAPbCl3 are actually decoupled. With the assumption that underlying physical mechanisms do not change with differing halogens in the organic-inorganic perovskites, we speculate that the energy scale most relevant to the photovoltaic properties of the lead-halogen perovskites is set by the lead-halide bond, not by the hydrogen bond.

Common acoustic phonon lifetimes in inorganic and hybrid lead halide perovskites.

The acoustic phonons in the organic-inorganic lead halide perovskites have been reported to have anomalously short lifetimes over a large part of the Brillouin zone. The resulting shortened mean free paths of the phonons have been implicated as the origin of the low thermal conductivity. We apply neutron spectroscopy to show that the same acoustic phonon energy linewidth broadening (corresponding to shortened lifetimes) occurs in the fully inorganic CsPbBr3 by comparing the results on the organic-inorganic CH3NH3PbCl3. We investigate the critical dynamics near the three zone boundaries of the cubic P m 3 ¯ m Brillouin zone of CsPbBr3 and find energy and momentum broadened dynamics at momentum points where the Cs-site (A-site) motions contribute to the cross section. Neutron diffraction is used to confirm that both the Cs and Br sites have unusually large thermal displacements with an anisotropy that mirrors the low temperature structural distortions. The presence of an organic molecule is not necessary to disrupt the low-energy acoustic phonons at momentum transfers located away from the zone center in the lead halide perovskites and such damping may be driven by the large displacements or possibly disorder on the A site.

Lifetime-shortened acoustic phonons and static order at the Brillouin zone boundary in the organic-inorganic perovskite CH3NH3PbCl3.

Lead halide hybrid perovskites consist of an inorganic framework hosting a molecular cation located in the interstitial space. These compounds have been extensively studied as they have been identified as promising materials for photovoltaic applications with the interaction between the molecular cation and the inorganic framework implicated as influential for the electronic properties. CH3NH3PbCl3 undergoes two structural transitions from a high temperature cubic unit cell to a tetragonal phase at 177 K and then a subsequent orthorhombic transition at 170 K. We have measured the low-frequency lattice dynamics using neutron spectroscopy and observe an energy broadening in the acoustic phonon linewidth towards the high-symmetry point Q X = ( 2 , 1 2 , 0 ) when approaching the transitions. Concomitant with these zone boundary anomalies is a hardening of the entire acoustic phonon branch measured in the q → 0 limit near the (2, 0, 0) Bragg position with decreasing temperature. Measurements of the elastic scattering at the Brillouin zone edges Q X = ( 2 , 1 2 , 0 ) , Q M = ( 3 2 , 1 2 , 0 ) , and Q R = ( 3 2 , 3 2 , 5 2 ) show Bragg peaks appearing below these structural transitions. Based on selection rules of neutron scattering, we suggest that the higher 177 K transition is displacive with a distortion of the local octahedral environment and the lower transition is a rigid tilt transition of the octahedra. We do not observe any critical broadening in energy or momentum, beyond resolution, of these peaks near the transitions. We compare these results to the critical properties reported near the structural transitions in other perovskites and particularly CsPbCl3 [Y. Fujii, S. Hoshino, Y. Yamada, and G. Shirane, Phys. Rev. B 9, 4549 (1974)]. We suggest that the simultaneous onset of static resolution-limited Bragg peaks at the zone boundaries and the changes in acoustic phonon energies near the zone center is evidence of a coupling between the inorganic framework and the molecular cation. The results also highlight the importance of displacive transitions in organic-inorganic hybrid perovskites.