###### Department of Physics, Massachusetts Institute of Technology

## Ultrafast Manipulation of order parameters in quantum materials

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* Denotes equal contribution ✉️ Denotes corresponding author

Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon

coupled excitonic insulator candidate Ta₂NiSe₅ and show that the order parameter can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in Ta₂NiSe₅ based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. This work expands the field of ultrafast order parameter control beyond spin and charge ordered materials.

See more details in the article: H. Ning, O. Mehio, M. Buchhold, T. Kurumaji, G. Refael, J. G. Checkelsky, D. Hsieh✉️, Signatures of ultrafast reversal of excitonic order in Ta₂NiSe₅, Phys. Rev. Lett. 125, 267602 (2020).

Ultrafast reversal of excitonic order in an excitonic insulator candidate

A variety of out-of-equilibrium protocols have been developed for rapidly switching Ising-type electronic order parameters such as ferromagnetism, ferrimagnetism, antiferromagnetism, and ferroelectricity. However, far less is understood about the mechanisms for switching more exotic order parameters that are not of magnetic and charge dipolar type. Excitonic insulator is a strongly correlated electronic phase realized through condensation of bound electron-hole pairs. In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameters of equal magnitude but opposite phase. There is currently no experimental method to switch nor directly measure the phase of an EI order parameter.

Coherent-phonon-induced switch to a hidden quadrupolar order in a multiband Mott insulator

Ultrafast laser excitation provides a means to transiently realize long-range ordered electronic states of matter that are hidden in thermal equilibrium. Recently, this approach has unveiled a variety of thermally inaccessible ordered states in strongly correlated materials, including charge density wave, ferroelectric, magnetic, and intertwined charge-orbital ordered states. More exotic symmetry broken states that exhibit ordering of higher electronic multipole moments are prevalent in strongly interacting electron systems, notably the f- and heavy d-electron based materials. However, because multipolar ordered states are challenging to directly manipulate and to detect using conventional techniques, experimental demonstrations of light-induced transitions between their equilibrium and hidden counterparts remain elusive.

Here we exploit the intrinsic coupling of a multipolar order parameter to lattice degree of freedom as a route both to impart and

to detect a dynamical transition from a thermally allowed to a thermally forbidden spin-orbit entangled quadrupolar ordered state in Ca₂RuO₄. Combining probe photon energy-resolved coherent phonon spectroscopy measurements with model Hamiltonian calculations, we show that the dynamical transition is manifested through anomalies in the temperature, pump excitation fluence, and probe photon energy dependence of the strongly coupled phonon. With this procedure, we introduce a general pathway to uncover hidden multipolar ordered states and to control their re-orientation on ultrashort timescales.

See more details in the article: H. Ning*, O. Mehio*, X. Li*, M. Buchhold, M. Driesse, H. Zhao, G. Cao, D. Hsieh✉️, A coherent phonon induced hidden quadrupolar ordered state in Ca₂RuO₄, Nat. Commun. 14, 8258 (2023).

Light-induced Weyl semiconductor-to-metal transition mediated by an inverse-Peierls distortion

Weyl nodes are topologically stable crossing points between non-degenerate bands in a crystal, which impart unconventional properties including ultrahigh charge mobility and chiral magneto-transport. The possibility to create or annihilate Weyl nodes in situ using ultrashort light pulses has been broadly explored theoretically and was recently demonstrated experimentally in Dirac and type-II Weyl semimetal materials via impulsively driven lattice symmetry changes. However, efforts have so far focused on binary switching between semi-metallic states with and without Weyl nodes.

Here, we use density functional theory calculations to show a three-state switch from Weyl semiconductor to Weyl metal to non-Weyl metal in chiral Peierls-distorted tellurium crystals as a function of the chiral chain radius. By performing time-dependent density functional theory calculations, we then demonstrate that these states can be transiently stabilized via a light-induced inverse-Peierls distortion. Predicted signatures of the inverse-Peierls structural phase transition are experimentally reproduced using time-resolved optical second harmonic generation. These results provide a pathway to multifunctional ultrafast Weyl devices and introduce Peierls systems as viable hosts of light-induced topological transitions.

See more details in the article: H. Ning*, O. Mehio*, C. Lian*, X. Li, E. Zoghlin, P. Zhou, B. Cheng, S. D. Wilson, B. M. Wong, D. Hsieh✉️, Light-induced Weyl semiconductor-to-metal transition mediated by Peierls instability, Phys. Rev. B 106, 205118 (2022).