Hari Padmanabhan

Ph.D. candidate, Penn State
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Presented: April 15 and 16, 2021

“Ultrafast optical control of magnetism in a quantum material”

‘Quantum materials’ exhibit at the macroscopic scale, the effects of microscopic quantum mechanical phenomena such as strong electronic interactions, magnetic exchange, and non-trivial band topology. These properties can have potentially transformative applications in electronics, error-tolerant quantum computing, and spintronics, the magnetic analog of conventional electronics. One of the keys to enabling new functionalities in these materials is the ability to control magnetism.

In this work, we demonstrate for the first time, the optical control of magnetism in a class of quantum materials called topological insulators. This technique allows us to modulate magnetism at speeds a thousand times faster than conventional control modalities such as applied magnetic fields and strain engineering.

This is achieved in the recently discovered quasi-two-dimensional magnetic topological insulator, MnBi2Te4. First, using cryogenic Raman spectroscopy, we find that lattice vibrational modes (phonons) strongly modulate the interaction between magnetic spins, resulting in anomalous changes to phonon energies and selection rules. We exploit this strong spin-phonon coupling to optically engineer the magnetic interactions at femtosecond timescales, through the excitation of coherent phonons by ultrafast optical pulses. Using time-resolved magneto-optic Kerr effect measurements, we show that the coherent phonons modulate the magnetization. Our work thus demonstrates proof of a ‘domino effect’ for controlling quantum materials, namely, using light to control phonons, that in turn modulate magnetism, which then modifies the desired topological properties.

Hari Padmanabhan

Hari Padmanabhan, Ph.D. candidate, Penn State

Hari Padmanabhan is currently a doctoral candidate in the Department of Materials Science and Engineering at Penn State University, advised by Prof. Venkatraman Gopalan. His research work lies in the broad area of light-matter interaction in quantum materials. The primary focus of his research is on probing and controlling these materials at their fundamental length and time scales, towards enabling new functionalities for electronic applications. To this end, he develops and employs various nonlinear and ultrafast optical spectroscopy techniques, collaborative experiments at national laboratories, and symmetry-based theoretical tools.

His research has led to 12 publications in peer-reviewed journals and 153 citations to date.

NAMCS Spring 2021 Schedule