Research

"Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world." -Albert Einstein

"Physics is like sex: sure, it may give some practical results, but that's not why we do it." -Richard Feynman*

Research Techniques and Expertise

Actinides: sample handling, security/safety, and containment
Sample Synthesis: single crystals, polycrystals, Czochralski, molten-metal flux, solid-solid/solid-gas reactions, actinide-bearing materials
High Pressures: Diamond Anvil Cell (including designer diamond anvils), Paris-Edinburgh Cell, Piston-Cylinder Cell techniques
User Facilities: synchrotron, reactor neutron, spallation neutron sources
X-ray and Neutron Scattering: inelastic scattering, diffraction, spectroscopy
Characterization: magnetic, electronic, structural, thermodynamic measurements

Research Topics

Additive Manufacturing: liquid metal jetting, structure-property relationships
Pressure-Induced Instabilities: magnetic, electronic, structural, valence
Superconductivity: heavy fermions, cuprates, ferropnictides
Topological Insulators
Actinide-Based Materials Synthesis
Plutonium Science and Aging

HIGHLIGHTS

The Pressure of Freedoms

We have performed high-pressure magnetotransport and x-ray diffraction measurements on ferromagnetic LaCo5 , confirming the theoretically predicted electronic topological transition driving the magnetoelastic collapse seen in the related compound YCo5. Our x-ray diffraction results show an anisotropic lattice collapse of the c axis near 10 GPa that is also commensurate with a change in the majority charge carriers evident from high-pressure Hall effect measurements. In addition to the conventional Hall channel, we have probed the anomalous Hall component, which is proportional to the ferromagnetic magnetization of the sample. This anomalous Hall measurement represents a novel technique for probing the magnitude of magnetization without resorting to synchrotron x-ray or high-pressure neutron methods. The coupling of the electronic, magnetic, and lattice degrees of freedom is further substantiated by the evolution of the anomalous Hall effect, which couples to the magnetization of the ordered state of LaCo5.This work appeared in Physical Review B 92, 174421 (2015).

Uncovering Symmetries within Hidden Order

We experimentally investigate the symmetry in the hidden order (HO) phase of intermetallic URu2Si2 by mapping the lattice and magnetic excitations via inelastic neutron and x-ray scattering measurements in the HO and high-temperature paramagnetic phases. At all temperatures, the excitations respect the zone edges of the body-centered tetragonal paramagnetic phase, showing no signs of reduced spatial symmetry, even in the HO phase. The magnetic excitations originate from transitions between hybridized bands and track the Fermi surface, whose features are corroborated by the phonon measurements. Due to a large hybridization energy scale, a full uranium moment persists in the HO phase, consistent with a lack of observed crystal-field-split states. Our results are inconsistent with local order-parameter models and the behavior of typical density waves. We suggest that an order parameter that does not break spatial symmetry would naturally explain these characteristics.This work appeared in Physical Review B 91, 035128 (2015).

Finding The Right Moment (for Tc)

Using non-resonant Fe Kβ x-ray emission spectroscopy, we reveal that Sr substitution into CaFe2As2 decouples the Fe moment from the volume collapse transition, yielding a collapsed-tetragonal, paramagnetic normal state out of which superconductivity develops. X-ray diffraction measurements implicate the c-axis lattice parameter as the controlling criterion for the Fe moment, promoting a generic description for the appearance of pressure- induced superconductivity in the alkaline-earth-based 122 ferropnictides (AFe2As2). The evolution of Tc with pressure lends support to theories for superconductivity involving unconventional pairing mediated by magnetic fluctuations.

This work appeared in Physical Review B 90, 144506 (2014).

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