Past Research – George Yumnam

Nanoengineered Artificial Honeycomb Lattices

During my PhD, I patterned thin-film permalloy and Nd into artificial honeycomb lattices to study the behavior of emergent magnetic charges under nanoscale geometric control. In the Nd-based lattices, we identified a quantum-disordered state of these charges, published as the Frontispiece of Advanced Science (2021). In the permalloy networks, the same charge physics produces rectification: the lattice functions as a magnetic diode with an unusually low forward voltage, tunable through field history and temperature. The measurements relied on polarized neutron reflectometry and SANS with DWBA modeling, techniques I developed hands-on at the ORNL beamlines.

Magnetic Monopoles Spin Ice Magnetic Diode

DFT studies of thermoelectrics and thermal transport

DFT Studies of Thermoelectrics & Thermal Transport

Before moving into neutron scattering, my research focused on first-principles studies of thermoelectric materials. Using density functional theory combined with Boltzmann transport equation calculations, I investigated the electronic structure, chemical bonding, lattice dynamics, and thermal transport of transition-metal dichalcogenides, chalcopyrites, halide double perovskites, and Zintl phases. This work also produced machine-learning models that predict lattice thermal conductivity from high-throughput property maps, enabling the screening of candidate compounds before any transport calculation is performed.

DFT Boltzmann Transport Machine Learning Thermoelectrics

Miscellaneous

Alongside the main thrusts of my PhD, I studied CeAuSb2, a heavy-fermion antiferromagnet in which the interplay of Kondo screening and RKKY exchange produces a delicate magnetic ground state. Using neutron scattering together with bulk measurements, we characterized the microscopic nature of the magnetic ground state and its evolution under applied field, resolving three successive transitions and identifying the low-temperature order as a basal-plane spin density wave.

See:

Microscopic nature of magnetic ground state in CeAuSb2, Phys. Status Solidi RRL 13, 1900304 (2019)

Heavy Fermions Itinerant Magnetism Neutron Diffraction