Evidence of Strong Orbital-Selective Spin-Orbital-Phonon Coupling in CrVO_{4}.

Coupling of orbital degree of freedom with a spin exchange, i.e., Kugel-Khomskii-type interaction (KK), governs a host of material properties, including colossal magnetoresistance, enhanced magnetoelectric response, and photoinduced high-temperature magnetism.

Nonthermal Bonding Origin of a Novel Photoexcited Lattice Instability in SnSe.

Lattice dynamics measurements are often crucial tools for understanding how materials transform between different structures. We report time-resolved x-ray scattering-based measurements of the nonequilibrium lattice dynamics in SnSe, a monochalcogenide reported to host a novel photoinduced lattice instability. By fitting interatomic force models to the fluence dependent excited-state dispersion, we determine the nonthermal origin of the lattice instability to be dominated by changes of interatomic interactions along a bilayer-connecting bond, rather than of an intralayer bonding network that is of primary importance to the lattice instability in thermal equilibrium.

Quasi-One-Dimensional Fermi Surface Nesting and Hidden Nesting Enable Multiple Kohn Anomalies in α-Uranium.

The topology of the Fermi surface controls the electronic response of a metal, including charge density wave (CDW) formation. A topology conducive for Fermi surface nesting (FSN) allows the electronic susceptibility χ_{0} to diverge and induce a CDW at wave vector q_{CDW}. Kohn extended the implications of FSN to show that the imaginary part of the lattice dynamical susceptibility χ_{L}^{''} also responds anomalously for all phonon branches at q_{CDW}-a phenomenon referred to as the Kohn anomaly.

Magnetoelastic coupling and spin contributions to entropy and thermal transport in biferroic yttrium orthochromite.

Direct engineering of material properties through exploitation of spin, phonon, and charge-coupled degrees of freedom is an active area of development in materials science. However, the relative contribution of the competing orders to controlling the desired behavior is challenging to decipher. In particular, the independent role of phonons, magnons, and electrons, quasiparticle coupling, and relative contributions to the phase transition free energy largely remain unexplored, especially for magnetic phase transitions.

Anharmonic Eigenvectors and Acoustic Phonon Disappearance in Quantum Paraelectric SrTiO_{3}.

Pronounced anomalies in the SrTiO_{3} dynamical structure factor, S(Q,E), including the disappearance of acoustic phonon branches at low temperatures, were uncovered with inelastic neutron scattering (INS) and simulations. The striking effect reflects anharmonic couplings between acoustic and optic phonons and the incipient ferroelectric instability near the quantum critical point. It is rationalized using a first-principles renormalized anharmonic phonon approach, pointing to nonlinear Ti-O hybridization causing unusual changes in real-space phonon eigenvectors, frequencies, group velocities, and scattering phase space.

Anharmonic lattice dynamics and superionic transition in AgCrSe.

Intrinsically low lattice thermal conductivity ([Formula: see text]) in superionic conductors is of great interest for energy conversion applications in thermoelectrics. Yet, the complex atomic dynamics leading to superionicity and ultralow thermal conductivity remain poorly understood. Here, we report a comprehensive study of the lattice dynamics and superionic diffusion in [Formula: see text] from energy- and momentum-resolved neutron and X-ray scattering techniques, combined with first-principles calculations.

Observation of low temperature metastable states in complex CaMnO.

The low temperature magnetic behaviour of the multiferroic quadruple perovskite CaMnO is investigated. The magneto-caloric effect in this material is presented for the first time. Along with the established magnetic transitions, T ~ 90 K and T ~ 45 K, an anomaly at 20 K is observed in our dc magnetization, ac susceptibility and dielectric measurements; below which, an inverse magnetocaloric effect is also observed in our  -ΔS (T) plots.

Momentum-resolved observations of the phonon instability driving geometric improper ferroelectricity in yttrium manganite.

Magnetoelectrics offer tantalizing opportunities for devices coupling ferroelectricity and magnetism but remain difficult to realize. Breakthrough strategies could circumvent the mutually exclusive origins of magnetism and ferroelectricity by exploiting the interaction of multiple phonon modes in geometric improper and hybrid improper ferroelectrics. Yet, the proposed instability of a zone-boundary phonon mode, driving the emergence of ferroelectricity via coupling to a polar mode, remains to be directly observed.

Complex optimization for big computational and experimental neutron datasets.

We present a framework to use high performance computing to determine accurate solutions to the inverse optimization problem of big experimental data against computational models. We demonstrate how image processing, mathematical regularization, and hierarchical modeling can be used to solve complex optimization problems on big data. We also demonstrate how both model and data information can be used to further increase solution accuracy of optimization by providing confidence regions for the processing and regularization algorithms.

Modeling non-harmonic behavior of materials from experimental inelastic neutron scattering and thermal expansion measurements.

Based on thermodynamic principles, we derive expressions quantifying the non-harmonic vibrational behavior of materials, which are rigorous yet easily evaluated from experimentally available data for the thermal expansion coefficient and the phonon density of states. These experimentally-derived quantities are valuable to benchmark first-principles theoretical predictions of harmonic and non-harmonic thermal behaviors using perturbation theory, ab initio molecular-dynamics, or Monte-Carlo simulations. We illustrate this analysis by computing the harmonic, dilational, and anharmonic contributions to the entropy, internal energy, and free energy of elemental aluminum and the ordered compound [Formula: see text] over a wide range of temperature.