Discretizing Dynamics for Maximum Likelihood Constraint Inference

Maximum likelihood constraint inference is a powerful technique for identifying unmodeled constraints that affect the behavior of a demonstrator acting under a known objective function. However, it was originally formulated only for discrete state-action spaces. Continuous dynamics are more useful for modeling many real-world systems of interest, including the movements of humans and robots. We present a method to generate a tabular state-action space that approximates continuous dynamics and can be used for constraint inference on demonstrations that obey the true system dynamics. We then demonstrate accurate constraint inference on nonlinear pendulum systems with 2- and 4-dimensional state spaces, and show that performance is robust to a range of hyperparameters. The demonstrations are not required to be fully optimal with respect to the objective, and the most likely constraints can be identified even when demonstrations cover only a small portion of the state space. For these reasons, the proposed approach may be especially useful for inferring constraints on human demonstrators, which has important applications in human-robot interaction and biomechanical medicine.

Fluorescent redox-dependent labeling of lipid droplets in cultured cells by reduced phenazine methosulfate

Natural and synthetic phenazines are widely used in biomedical sciences. In dehydrogenase histochemistry, phenazine methosulfate (PMS) is applied as a redox reagent for coupling reduced coenzymes to the reduction of tetrazolium salts into colored formazans. PMS is also currently used for cytotoxicity and viability assays of cell cultures using sulfonated tetrazoliums. Under UV (340 nm) excitation, aqueous solutions of the cationic PMS show green fluorescence ({lambda}em: 526 nm), whereas the reduced hydrophobic derivative (methyl-phenazine, MPH) shows blue fluorescence ({lambda}em: 465 nm). Under UV (365 nm) excitation, cultured cells (LM2, IGROV-1, BGC-1, and 3T3-L1 adipocytes) treated with PMS (5 ug/mL, 30 min) showed cytoplasmic granules with bright blue fluorescence, which correspond to lipid droplets labeled by the lipophilic methyl-phenazine. After formaldehyde fixation blue-fluorescing droplets could be stained with oil red O. Interestingly, PMS-treated 3T3-L1 adipocytes observed under UV excitation 24 h after labeling showed large lipid droplets with a weak green emission within a diffuse pale blue-fluorescing cytoplasm, whereas a strong green emission was observed in small lipid droplets. This fluorescence change from blue to green indicates that reoxidation of methyl-phenazine to PMS can occur. Regarding cell uptake and labeling mechanisms, QSAR models predict that the hydrophilic PMS is not significantly membrane-permeant, so most PMS reduction is expected to be extracellular and associated with a plasma membrane NAD(P)H reductase. Once formed, the lipophilic and blue-fluorescing methyl-phenazine enters live cells and mainly accumulates in lipid droplets. Overall, the results reported here indicate that PMS is an excellent fluorescent probe to investigate labeling and redox dynamics of lipid droplets in cultured cells.

Van Vleck excitons in Ca$_{2}$RuO$_{4}$

A framework is presented for modeling and understanding magnetic excitations in localized, intermediate coupling magnets where the interplay between spin-orbit coupling, magnetic exchange, and crystal field effects are known to create a complex landscape of unconventional magnetic behaviors and ground states. A spin-orbit exciton approach for modeling these excitations is developed based upon a Hamiltonian which explicitly incorporates single-ion crystalline electric field and spin exchange terms. This framework is then leveraged to understand a canonical Van Vleck $jrm{_{eff}}=0$ singlet ground state whose excitations are coupled spin and crystalline electric field levels. Specifically, the anomalous Higgs mode [Jain et al. Nat. Phys. 13, 633 (2017)], spin-waves [S. Kunkem{o}ller et al. Phys. Rev. Lett. 115, 247201 (2015)], and orbital excitations [L. Das et al. Phys. Rev. X 8, 011048 (2018)] in the multiorbital Mott insulator Ca$_2$RuO$_4$ are captured and good agreement is found with previous neutron and inelastic x-ray spectroscopic measurements. Furthermore, our results illustrate how a crystalline electric field-induced singlet ground state can support coherent longitudinal, or amplitude excitations, and transverse wavelike dynamics. We use this description to discuss mechanisms for accessing a nearby critical point.

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 SrTiO$_{3}$. 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.

Spin-wave directional anisotropies in antiferromagnetic Ba$_{3}$NbFe$_{3}$Si$_{2}$O$_{14}$

Ba$_{3}$NbFe$_{3}$Si$_{2}$O$_{14}$ (langasite) is structurally and magnetically single domain chiral with the magnetic helicity induced through competing symmetric exchange interactions. Using neutron scattering, we show that the spin-waves in antiferromagnetic langasite display directional anisotropy. On applying a time reversal symmetry breaking magnetic field along the $c$-axis, the spin wave energies differ when the sign is reversed for either the momentum transfer $pm$ $vec{Q}$ or applied magnetic field $pm$ $mu_{0}$H. When the field is applied within the crystallographic $ab$-plane, the spin wave dispersion is directionally textit{isotropic} and symmetric in $pm$ $mu_{0}$H. However, a directional anisotropy is observed in the spin wave intensity. We discuss this directional anisotropy in the dispersion in langasite in terms of a field induced precession of the dynamic unit cell staggered magnetization. Directional anisotropy, or often referred to as non reciprocal responses, can occur in antiferromagnetic phases in the absence of the Dzyaloshinskii-Moriya interaction or other effects resulting from spin-orbit coupling.

From mean-field localized magnetism to itinerant spin fluctuations in the Non-metallic metal - FeCrAs

FeCrAs displays an unusual electrical response that is neither metallic in character nor divergent at low temperatures, as expected for an insulating response, and therefore it has been termed a nonmetal-metal. We carried out neutron scattering experiments on powder and single crystal samples to study the magnetic dynamics and critical fluctuations in FeCrAs. Magnetic neutron diffraction measurements find Cr3+ magnetic order setting in at 115 K with the mean-field critical exponent. Neutron spectroscopy, however, observes gapless stiff magnetic fluctuations emanating from magnetic positions with propagation wave vector q_0=(1/3,1/3), which persists up to at least 80 meV. The magnetism in FeCrAs therefore displays a response which resembles that of itinerant magnets at high energy transfers, such as chromium alloys. We suggest that the presence of stiff high-energy spin fluctuations is the origin of the unusual temperature dependence of the resistivity.

Spontaneous decay of a soft optical phonon in the relaxor ferroelectric PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$

We report the spontaneous decay of a soft, optical phonon in a solid. Using neutron spectroscopy, we find that specific phonon lifetimes in the relaxor PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$ are anomalously short within well-defined ranges of energy and momentum. This behavior is independent of ferroelectric order and occurs when the optical phonon with a specific energy and momentum can kinematically decay into two acoustic phonons with lower phase velocity. We interpret the well-known relaxor waterfall effect as a form of quasiparticle decay analogous to that previously reported in quantum spin liquids and quantum fluids.

Orphan spins and bound in-gap states in the $S={5over2}$ antiferromagnet CaFe$_{2}$O$_{4}$

CaFe$_{2}$O$_{4}$ is an anisotropic $S={5over 2}$ antiferromagnet with two competing $A$ ($uparrow uparrow downarrow downarrow$) and $B$ ($uparrow downarrow uparrow downarrow$) magnetic order parameters separated by static antiphase boundaries at low temperatures. Neutron diffraction and bulk susceptibility measurements, show that the spins near these boundaries are weakly correlated and a carry an uncompensated ferromagnetic moment that can be tuned with a magnetic field. Spectroscopic measurements find these spins are bound with excitation energies less than the bulk magnetic spin-waves and resemble the spectra from isolated spin-clusters. Localized bound orphaned spins separate the two competing magnetic order parameters in CaFe$_{2}$O$_{4}$.

Transverse acoustic phonon anomalies at intermediate wavevectors in MgV$_{2}$O$_{4}$

Magnetic spinels (with chemical formula $AX_{2}$O$_{4}$, with $X$ a 3$d$ transition metal ion) that also have an orbital degeneracy are Jahn-Teller active and hence possess a coupling between spin and lattice degrees of freedom. At high temperatures, MgV$_{2}$O$_{4}$ is a cubic spinel based on V$^{3+}$ ions with a spin $S$=1 and a triply degenerate orbital ground state. A structural transition occurs at T$_{OO}$=63 K to an orbitally ordered phase with a tetragonal unit cell followed by an antiferromagnetic transition of T$_{N}$=42 K on cooling. We apply neutron spectroscopy in single crystals of MgV$_{2}$O$_{4}$ to show an anomaly for intermediate wavevectors at T$_{OO}$ associated with the acoustic phonon sensitive to the shear elastic modulus $left(C_{11}-C_{12}right)/2$. On warming, the shear mode softens for momentum transfers near close to half the Brillouin zone boundary, but recovers near the zone centre. High resolution spin-echo measurements further illustrate a temporal broadening with increased temperature over this intermediate range of wavevectors, indicative of a reduction in phonon lifetime. A subtle shift in phonon frequencies over the same range of momentum transfers is observed with magnetic fields. We discuss this acoustic anomaly in context of coupling to orbital and charge fluctuations.

Crystal field excitations and magnons: their roles in oxyselenides Pr2O2M2OSe2 (M = Mn, Fe)

We present the results of neutron scattering experiments to study the crystal and magnetic structures of the Mott-insulating transition metal oxyselenides Pr2O2M2OSe2 (M = Mn, Fe). The structural role of the non-Kramers Pr3+ ion is investigated and analysis of Pr3+ crystal field excitations performed. Long-range order of Pr3+ moments in Pr2O2Fe2OSe2 can be induced by an applied magnetic field.