##### Shining light on complex matter

I am interested in **complex materials** that exhibit intriguing behavior originating in microscopic quantum phenomena. These ‘**quantum materials**‘ possess microscopic traits – electronic, spin, orbital, lattice – intertwined by interactions at the atomic scale. Disentangling these interactions is essential to discovering new **emergent** behavior in these materials, with applications in next-generation information processing, computing, and energy technology.

I achieve this by probing these materials at their **fundamental length- and time-scales** using a variety of experimental and theoretical tools. The techniques I use are primarily **optical spectroscopy** and **ultrafast X-ray and electron scattering** at large national facilities. I complement this with **density functional theory**, leveraging **symmetry** as a powerful tool in interrogating physical phenomena.

###### Spin-lattice coupling pathways in layered magnets

The emergence of magnetism in topological materials creates a platform to realize spin-based phenomena with applications in spintronics, magnetic memory, and quantum information science. A key to unlocking new functionalities in these materials is the identification of enhanced coupling routes between spins and other microscopic degrees of freedom. Bringing together insights from magneto-Raman spectroscopy and optical pump-probe experiments, we discover that phonons can act as a ‘knob’ to control interlayer exchange interaction in the antiferromagnetic topological insulator MnBi_{2}Te_{4}.

Magnetic field-induced interlayer magnetic phase transitions in MnBi2Te4 result in the modulation of *A*_{1g} Raman phonon scattering intensities, evidence of an interlayer magnetophononic coupling.

Selected publications**Large Itinerant Electron Exchange Coupling in the Magnetic Topological Insulator MnBi _{2}Te_{4}.**H. Padmanabhan, V. A. Stoica, P. K. Kim, M. Poore, T. Yang, X. Shen, A. H. Reid, M-F. Lin, S. Park, J. Yang, H. Wang, N. Z. Koocher, D. Puggioni, L. Min, S. H. Lee, Z. Mao, J. M. Rondinelli, A. M. Lindenberg, L-Q. Chen, X. Wang, R. D. Averitt, J. W. Freeland, V. Gopalan.

*arXiv 2022*.

**Interlayer magnetophononic coupling in MnBi _{2}Te_{4}**.

H. Padmanabhan*, M. Poore*, P. K. Kim*, N. Z. Koocher, V. A. Stoica, D. Puggioni, H. Wang, X. Shen, A. H. Reid, M. Gu, M. Wetherington, S. H. Lee, R. Schaller, Z. Mao, A. M. Lindenberg, X. Wang, J. M. Rondinelli, R. D. Averitt, V. Gopalan.

*Nature Communications*

*2022.*

**Intertwined lattice deformation and magnetism in monovacancy graphene.**

H. Padmanabhan, B. R. K. Nanda.

*Physical Review B 2016.*

###### Nonlinear optical probes of unconventional ferroelectrics

Ferroelectric materials are characterized by long-range ordering of electric dipoles, realized by spontaneous symmetry breaking across structural phase transitions. I am interested in understanding the mechanisms that stabilize unconventional polar phases in these materials. Nonlinear optics, as a sensitive probe of symmetry breaking, is well suited to the study of such materials and their phase transitions. For example, using nonlinear optical anisotropy and Raman spectroscopy, we find that the seemingly incompatible properties of polar order and metallic conduction are stabilized in LiOsO_{3} through a decoupling of charge carriers from the polar distortion. In another collaborative study, our nonlinear optical measurements helped design a lead-free ferroelectric material stabilized by rotational distortions.

An image of polar, ferroelastic domains in LiOsO3, measured using a home-built scanning second harmonic generation microscope. The domains are a result of breaking of three-fold rotational symmetry and inversion symmetry across structural transitions.

Selected publications**Linear and nonlinear optical probe of the ferroelectric-like phase transition in a polar metal, LiOsO _{3}.**H. Padmanabhan, Y. Park, D. Puggioni, Y. Yuan, Y. Cao, L. Gasparov, Y. Shi, J. Chakhalian, J. M. Rondinelli, V. Gopalan.

*Applied Physics Letters 2018.*

**Hybrid Improper Ferroelectricity in (Sr,Ca)**.

_{3}Sn_{2}O_{7}and Beyond: Universal Relationship between Ferroelectric Transition Temperature and Tolerance Factor in*n*= 2 Ruddlesden–Popper Phases**S. Yoshida, H. Akamatsu, R. Tsuji, O. Hernandez, H. Padmanabhan, A. S. Gupta, A. S. Gibbs, K. Mibu, S. Murai, J. M. Rondinelli, V. Gopalan, K. Tanaka, K. Fujita.**

*Journal of the American Chemical Society*

*2018*.

###### Symmetry-based approach to transition state theory

Symmetry is a singularly powerful concept in physics, and lays the foundation for the study of everything from fundamental particles to atomic wavefunctions and crystals. In materials science, point group and space group symmetries are ubiquitous in the description of physical properties. To help extend these ideas to dynamical processes, we formulate the concept of ‘distortion symmetry’. By incorporating distortion symmetry into transition state tools in density functional theory codes, we systematically explore the energy landscape of various processes in materials, revealing new and unexpected dynamical pathways. For example, we apply this to the multiferroic BiFeO_{3}, to help address a physical problem of great interest – switching magnetic moments using an electric field. Using our calculations, we predict competing transient switching pathways that prohibit the deterministic switching of magnetism through an electric field, in agreement with experimental results.

Possible ferroelectric switching pathways in BiFeO_{3} – one that simultaneously switches the magnetization (right), and one that does not (left). Our calculations show that the two pathways have comparable energy barriers, *inhibiting* deterministic electric switching of magnetization.

Selected publications**Antisymmetry: Fundamentals and Applications. **H. Padmanabhan*, J. M. Munro*, I. Dabo, V. Gopalan.

*Annual Review of Materials Research 2020.*

**Discovering minimum energy pathways via distortion symmetry groups.**

J. M. Munro, H. Akamatsu, H. Padmanabhan, V. S. Liu, Y. Shi, L. Q. Chen, B. VanLeeuwen, I. Dabo, V. Gopalan.

*Physical Review B 2018.*

**Spatio-Temporal Symmetry—Point Groups with Time Translations.**

H. Padmanabhan, M. L. Kingsland, J. M. Munro, D. B. Litvin, V. Gopalan.

*Symmetry 2017.*