From 2016 to 2024 I worked at Cornell University in the laboratory of Prof. Kyle Shen and in close collaboration with the research group of Prof. Darrell Schlom on the molecular-beam epitaxy (MBE) growth of oxide thin films.  Here, much of my PhD work focused on the synthesis and measurement of the transition metal oxides, namely perovskites and related structures.   My interests mostly lie in the study of the effects of strong correlations in these materials by a variety of experimental techniques ranging from electrical transport, to angle resolved photoemission spectroscopy and resonant x-ray scattering.  These projects have ranged from heavy transition metal oxides, including the iridates SrIrO3 and Sr2IrO4  [1], where spin-orbit interactions dominate to, more recently, the perovskite and infinite-layer nickelates, RENiO3 and RENiO2, where the low energy physics are predominately controlled by the strong correlation effects.

The perovskite nickelates are negative charge-transfer insulators, where competition between the on-site coulomb repulsion of the nickel d orbitals and hybridization of the nickel 3d and oxygen 2p states renders the material tunable between metallic and insulating states.  The 3d-2p hybridization can be tuned via structural distortion or chemical doping to influence the temperature driven metal-to-insulator transition (MIT).  By changing the film structure through epitaxial strain and controlling the film thickness we can engineer interesting new states in these correlated materials.

Through topotactic removal of the apical oxygen ions, the perovskite nickelates can be transformed into the so called infinite-layer nickelates, RENiO2. The large change in the crystal field energy rearranges the energies of the nickel d orbitals, resulting in an electronic structure in the NiO4 square plaquette similar to that of the cuprate superconductors.  Also hosting superconductivity, this system provides an interesting point of comparison to the cuprate superconductors, and an opportunity to study which elements of the cuprate phase diagram are universal to high temperature superconductivity.  By synthesizing infinite-layer nickelate films using a combination of ozone assisted MBE and atomic hydrogen reduction [2] we show that the infinite-layer nickelate NdNiO2 does not show signatures of charge density wave order, only structural Bragg peaks arising from ordered interstitial oxygen atoms left over from the reduction process [3].

[1] Nelson, J. N., Parzyck, C. T., Faeth, B. D., Kawasaki, J. K., Schlom, D. G., & Shen, K. M. (2020). "Mott gap collapse in lightly hole-doped Sr2-xKxIrO4". Nature Communications, 11(1), 2597. https://doi.org/10.1038/s41467-020-16425-z

[2] Parzyck, C. T., Anil, V., Wu, Y., Goodge, B. H., Roddy, M., Kourkoutis, L. F., Schlom D. G., & Shen, K. M. (2024) Synthesis of thin film infinite-layer nickelates by atomic hydrogen reduction: clarifying the role of the capping layer. https://arxiv.org/abs/2401.07129v1 

[3] Parzyck, C. T., Gupta, N. K., Wu, Y., Anil, V., Bhatt, L., Bouliane, M., Gong, R., Gregory, B. Z., Luo, A., Sutarto, R., He, F., Chuang, Y.-D., Zhou, T., Herranz, G., Kourkoutis, L. F., Singer, A., Schlom, D. G., Hawthorn, D. G., & Shen, K. M. (2024) Absence of 3a0 charge density wave order in the infinite-layer nickelate NdNiO2. Nature Materials https://doi.org/10.1038/s41563-024-01797-0 

High brightness electron beams are an important ingredient in modern scientific applications ranging from ultrafast electron microscopes to x-ray free electron lasers.  In these applications the electron beam is generated by photoemission, and subsequent acceleration, of electrons from a source material, the photocathode, using a pulsed laser source.  The properties of the resulting electron beam depend strongly on the material from which they are emitted; chemical and physical disorder of the photocathode surface negatively impact the utility of the resulting beam - necessitating the synthesis of clean, ordered photocathode thin films.  While electron sources traditionally use metal or activated semiconductor materials, alkali antimonide photocathodes are a promising class of materials which boast high quantum efficiency (QE = # emitted electrons / # incident photons) in the visible and infrared.  However, this high QE is acompanied by significant challenges in both the synthesis and handling of these materials, owing to their alkali metal constituents.

As part of the Center for Bright Beams, I have worked on projects to both optimize the growth of existing photocathodes and discover new high QE photoemitters for use in electron source applications. Utilizing RHEED assisted MBE, we demonstrated the first ever epitaxial synthesis of the visible light photocathode Cs3Sb on a 3C-SiC (001) substrate.  Using in operando and in situ techniques, we demonstrated that these epitaxial films possess sufficient surface order that ARPES measurements reflect their intrinsic band structure, as well as demonstrating that their QE in the infrared well exceeds that of non-epitaxial films of much greater thickness [4].  We have also synthesized partially ordered (or 'fiber texture') films of the related compound, CsSb, and have demonstrated its potential as a photocathode.  While CsSb has an overall lower QE then Cs3Sb, it remains a visible light photoemitter.  We have demonstrated that it can be grown atomically smooth on a variety of substrates, again with sufficient surface order to permit ARPES measurements [5]. Additionally, this compound is found to be substantially more resistant to surface oxidation then its aforementioned cousin - providing an alternative, ordered, visible light photocathode which can be used in harsher vacuum environments then other alkali antimonides. 

[4] Parzyck, C. T.,* Galdi, A.,* Nangoi, J. K., Debenedetti, W. J. I., Balajka, J., Faeth, B. D., Paik, H., Hu, C., Arias, T. A., Hines, M. A., Schlom, D. G., Shen, K. M., & Maxson, J. M. (2022). "Single-Crystal Alkali Antimonide Photocathodes: High Efficiency in the Ultrathin Limit". Physical Review Letters, 128(11), 1–8. https://doi.org/10.1103/PhysRevLett.128.114801 

[5] Parzyck, C. T., Pennington, C. A., DeBenedetti, W. J. I., Balajka, J., Echeverria, E. M., Paik, H., Moreschini, L., Faeth, B. D., Hu, C., Nangoi, J. K., Anil, V., Arias, T. A., Hines, M. A., Schlom, D. G., Galdi, A., Shen, K. M., & Maxson, J. M. (2023). "Atomically smooth films of CsSb: A chemically robust visible light photocathode". APL Materials, 11(10). https://doi.org/10.1063/5.0166334