**Decoupling interactions in complex matter**

I am interested in **complex electronic materials** that exhibit intriguing behavior originating in microscopic quantum phenomena. These ‘**quantum materials**‘ typically possess multiple microscopic degrees of freedom – electronic, spin, orbital, lattice – intertwined by interactions at the atomic scale. Decoupling these relationships is essential to enabling new functionalities in these materials, with applications in next-generation information storage, computing, and energy technology.

In my research, I achieve this by probing these materials at their **fundamental length-scales **(~10^{-12} m)** and time-scales** (~10^{-15} s) using a variety of experimental and theoretical tools. The techniques I use include **nonlinear optics, magneto-Raman spectroscopy, and ultrafast optical pump-probe spectroscopy**, as well as **pump-probe X-ray diffraction and ultrafast electron scattering** in collaboration with national labs. I complement these efforts with **density functional theory simulations**, leveraging **symmetry and group theory** as powerful tools in interrogating physical phenomena.

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

The emergence of magnetism in quantum 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, optical pump-probe experiments, and MeV ultrafast electron diffraction measurements at the SLAC National Accelerator Laboratory, we discover a sub-picosecond coherent magnetophononic coupling in the layered magnetic topological insulator, MnBi_{2}Te_{4}, enabling magnetization modulation speeds orders of magnitude greater than traditional techniques.

Terahertz oscillations in the reflectivity (top) and magneto-optic Kerr rotation (bottom) after ultrafast optical excitation, demonstrating a coherent coupling between optical phonons and magnetization in MnBi_{2}Te_{4}.

You can find here a recent invited talk I presented at the North American Materials Colloquium Series on this work.

Selected publications**Sub-picosecond coherent magnetophononic coupling in MnBi _{2}Te_{4}**.

H. Padmanabhan*, M. Poore*, P. 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

*arXiv*

*2104.08356.*

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

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

*Physical Review B 2016.*

###### Ultrafast dynamics of strongly correlated materials

Electrons in matter typically behave as independent particles, appearing ‘invisible’ to each other. However, in some materials, the Coulomb interactions between electrons cannot be neglected, and this ‘strong electronic correlation’ gives rise to a range of unexpected properties. In the family of ruthenate oxides, lattice distortions are strongly coupled to a metal-to-insulator phase transition driven by strong electronic correlation. Using time-resolved X-ray diffraction on thin films of Ca_{2}RuO_{4}, we find that strain stabilizes the coexistence of metallic and insulating phases in the form of nanoscale stripes. Upon pumping with an ultrafast optical pulse, we observe the transient melting and recovery of this striped phase at a nanosecond timescale. This work was done in collaboration with beamline scientists at the Advanced Photon Source in Argonne National Lab.

The transient reciprocal space map of strained Ca_{2}RuO_{4}, 150 ps after optical excitation. While the central lattice peak remains unchanged, the satellite peaks corresponding to nanoscale metal/insulator stripes melt with increasing optical excitation power.

You can find here a recent invited talk I presented at the SPIE Optics + Photonics conference on ultrafast quasiparticle dynamics in Ca_{3}Ru_{2}O_{7}, where I describe how optical pump-probe reflectivity can be used to sensitively probe the pseudogap phase at the metal-to-insulator transition.

###### 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. In particular, 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. The white scale bar is 10 microns.

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 switching of magnetization using an electric field.

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.*