The list below is only occasionally updated.
For the most up-to-date list of my publications, please see my Google scholar or ORCID profiles:
2023
The nature of dynamic local order in CH3NH3PbI3 and CH3NH3PbBr3
Hybrid lead-halide perovskites (LHPs) are semiconductors with novel properties that are distinctively governed by structural fluctuations. Diffraction experiments sensitive to long-range order reveal a cubic structure in the device-relevant, high-temperature phase. Local probes find additional short-range order with lower symmetry that may govern structure-function relationships. However, our understanding is impeded by unresolved dimensionality, participating atoms, and dynamics of short-range order. Here, we determine the true structure of two hybrid LHPs, CH3NH3PbI3 and CH3NH3PbBr3, using a combination of single-crystal diffuse scattering, neutron inelastic spectroscopy, and molecular dynamics simulations. The remarkable collective dynamics, not observed in previous studies, consist of a network of local, two-dimensional, circular regions of dynamically tilting lead-halide octahedra (lower symmetry) that induce longer-range intermolecular correlations. The dynamic local structure may introduce transient ferroelectric or antiferroelectric domains that increase charge carrier lifetimes and strongly affect halide migration, a poorly understood degradation mechanism.
In the search for high-performance thermoelectrics, materials such as clathrates have drawn attention due to having both glasslike low phonon thermal conductivity and crystal-like high electrical conductivity. Ba8Ga16Ge30 (BGG) has a loosely bound guest Ba atom trapped inside rigid Ga-Ge cage structures. Avoided crossings between acoustic phonons and the flat guest atom branches have been proposed to be the source of the low lattice thermal conductivity of BGG. Ga-Ge site disorder with Ga and Ge exchanging places in different unit cells has also been reported. We used time-of-flight neutron scattering to measure the complete phonon spectrum in a large single crystal of BGG and compared these results with predictions of density functional theory to elucidate the effect of the disorder on heat-carrying phonons. Experimental results agreed much better with the calculation assuming the disorder than with the calculation assuming the ordered configuration. Although the atomic masses of Ga and Ge are nearly identical, we found that disorder strongly reduces phonon group velocities, which significantly reduces thermal conductivity. Our work points to a path towards optimizing thermoelectrics.
2021
Effect of the electronic charge gap on LO bond-stretching phonons in La2CuO4 calculated using LDA+U
Typical density-functional theory calculations that wrongly predict undoped cuprates to be metallic also predict Cu-O half- and full-breathing phonon energies that are significantly softer than observed, presumably because of weak on-site Coulomb repulsion on the Cu 3d orbitals. We used DFT+U calculations with antiferromagnetic supercells of La2CuO4 to establish correlation between the on-site repulsion strength, tuned via adjusting the value of U, and phonon dispersions. We find that breathing and half-breathing phonons reach experimental values when U is tuned to obtain the correct optical gap and magnetic moments. We demonstrate that using distorted supercells within DFT+U is a promising framework to model phonons in undoped cuprates and other perovskite oxides with complex, interrelated structural and electronic degrees of freedom.
Reinvestigation of crystal symmetry and fluctuations in La2CuO4
Undoped La2CuO4 (LCO), is the parent compound of the high-temperature superconductor La2Sr2-xCuO4+delta (LSCO). Due to technological interest in the superconductivity, LSCO is much more widely studied. However, LCO is independently interesting: there are several structural transitions, rich magnetic behavior, and recently a large thermal Hall effect has been observed (a temperature gradient is applied along one direction, a magnetic field is applied perpendicular to that, and a temperature gradient is observed along the magnetic field direction). There is evidence that the thermal Hall effect could be due to coupling between the magnetic field and 'chiral' phonons. In this work, our collaborators measured the lattice dynamics in a high quality single crystal of LCO and found soft phonons indicating a nearby transition toward a different space group than previously thought. The soft modes could be the chiral phonon. I calculated the inelastic neutron spectra using density-functional-theory+U (DFT+U) calculations in the antiferromagnetic phase and found spectacular agreement with the experimental data. Actually the structural phase I modeled is nearly the same as the one the soft modes would lead to. Analysis of the eigenvectors shows that the acoustic modes from the DFT calculations have chiral-like character and could potentially lead to novel lattice dynamics in a magnetic field.
2020
FeP is a double-helix antiferromagnet. The non-collinear magnetic ordering results in coupling of the spin and charge channels to the lattice dynamics. Our collaborators measured the phonon spectrum of FeP using time-of-flight neutron scattering and I carried out the data analysis using the Phonon Explorer software developed in the Raman Spectroscopy and Neutron Scattering Lab led by Prof. Dmitry Reznik at the University of Colorado Boulder. We fit the dispersions of some of the low energy excitations and found good agreement with density functional theory calculations for all but one of the branches studied, which was softer in the experiment that in the calculations.
Phonon spectrum of underdoped HgBa2CuO4+delta investigated by neutron scattering
We carried out time-of-flight (TOF) inelastic neutron scattering measurements on an underdoped sample of HgBa2CuO4+delta (HBCO). Similar to the other cuprates, HBCO hosts charge density ordering that couples to the lattice dynamics. Signatures of the charge density order are observed in the form a 'dip' in the dispersion of the longitudinal Cu-O bond stretching phonon. The dispersions in the overdoped and optimally doped compounds have previously been measured, but the underdoped compounds are difficult to make, resulting in small crystals and poor statistics measured by neutron scattering. However, TOF measurements enable simultaneously measuring a large volume of reciprocal space. Using the Phonon Explorer software developed in the Raman Spectroscopy and Neutron Scattering Lab, led by Prof. Dmitry Reznik at the University of Colorado Boulder, we were able to simultaneously analyze many Brillouin zones at once, overcoming the poor statistics by fitting the phonons in multiple zones simultaneously (the so called 'multi-zone fitting' technique). I participated in the data analysis and calculated the phonon dispersions using density functional theory.
2019
Heat and charge transport in bulk semiconductors with interstitial defects
We calculated the thermoelectric properties of silicon with interstitial defects. Thermoelectrics are promising candidates for space technologies, but the effects of radiation induced defects on transport properties are not really understood; e.g. silicon atoms kicked off a lattice site into an interstitial position will scatter both phonons and electrons. My collaborators calculated the Seebeck coefficients with various defect concentrations (Si, Ge, C, and Li were studied) and the lattice thermal conductivity for the same configurations to determine the thermoelectric figure of merit. I computed the frequency resolved contributions to the thermal conductivity to study how the scattering effects different phonons.
2016
Robotic welding parameter optimization based on weld quality evaluation
Work from when I was an undergrad in manufacturing engineering.
2015
Welding parameter optimization based on Gaussian process regression Bayesian optimization algorithm
Work from when I was an undergrad in manufacturing engineering.