top of page

Emergent phase and symmetry engineering through collective mode excitation

HN_icons-01.png
HN_icons-02.png
HN_icons-03.png
HN_icons-04.png

​​​↘ Research Directions

* Denotes equal contribution  ✉️ Denotes corresponding author

Hubbard exciton fluid in a photodoped antiferromagnetic Mott insulator

3.1

The undoped antiferromagnetic Mott insulator naturally has one charge carrier per lattice site. When it is doped with additional carriers, they are unstable to spin-fluctuation-mediated Cooper pairing as well as other unconventional types of charge, spin and orbital current ordering. Photo-excitation can produce charge carriers in the form of empty (holons) and doubly occupied (doublons) sites that may also exhibit charge instabilities. There is evidence that antiferromagnetic correlations enhance attractive interactions between holons and doublons, which can then form bound pairs known as Hubbard excitons, and that these might self-organize into an insulating Hubbard exciton fluid. However, this out-of-equilibrium phenomenon has not been experimentally detected.

Here we report the transient formation of a Hubbard exciton fluid in the antiferromagnetic Mott insulator Sr₂IrO₄ using ultrafast terahertz conductivity. Following photo-excitation, we observe rapid spectral-weight transfer from a Drude metallic response to an insulating response. The latter is characterized by a finite-energy peak originating from intraexcitonic transitions, whose assignment is corroborated by our numerical simulations of an extended Hubbard model. The lifetime of the peak is short (approximately one picosecond) and scales exponentially with the Mott gap size, implying extremely strong coupling to magnon modes. These results suggest that 2D magnetic Mott insulators, which host myriad ordered and quantum disordered phases, are a promising platform for discovering novel excitonic states.

See more details in the article: O. Mehio*, X. Li*, H. Ning*, Z. Lenarčič, Y. Han, M. Buchhold,  Z. Porter, N. J. Laurita, S. D. Wilson, D. Hsieh✉️, A Hubbard exciton fluid in a photo-doped antiferromagnetic Mott insulator, Nat. Phys. 19, 1876 (2023).

Differential impact of photo-excited free and bound carriers on ultrafast demagnetization

Photo-doped antiferromagnetic Mott insulators are predicted to exhibit exotic non-thermal magnetic phenomena. Theoretical models typically introduce photo-dopants as free carriers with particle-hole symmetry. However, in real materials, the effects of particle-hole asymmetry and attraction must be taken into account.

 

Here we use time-resolved second harmonic generation and terahertz spectroscopy to resolve the differential impact of free and bound photo-excited holon-doublon pairs on magnetic order in the single band Mott antiferromagnet Sr₂IrO₄. By tuning the relative population of these quasiparticles via the excitation wavelength, we find the demagnetization efficiency to be higher for free doublons compared to free holons, and to be greatly diminished for bound holon-doublon pairs, consistent with a picture of string-defect mediated demagnetization. Our work highlights the importance of quasiparticle species-dependent models of photo-doping, which can serve as a novel tuning parameter for out-of-equilibrium magnetic phenomena.

See more details in the article: H. Ning*, O. Mehio*, Y. Han, X. Li, K. L. Seyler, Z. Porter, S. D. Wilson, D. Hsieh✉️, Differential impact of photo-excited free and bound carriers on ultrafast demagnetization, in prep (2024)

Terahertz Control of Linear and Nonlinear Magno-Phononics

Fig3-3.png

Coherent manipulation of magnetism through the lattice provides unprecedented opportunities for controlling spintronic functionalities on the ultrafast timescale. Such nonthermal control conventionally involves nonlinear excitation of Raman-active phonons which are coupled to the magnetic order. Linear excitation, in contrast, holds potential for more efficient and selective modulation of magnetic properties. However, the linear channel remains uncharted, since it is conventionally considered forbidden in inversion symmetric quantum materials.

 

Here, we harness strong coupling between magnons and Raman-active phonons to achieve both linear and quadratic excitation regimes of magnon-polarons, magnon-phonon hybrid quasiparticles. We demonstrate this by driving magnon-polarons with an intense terahertz pulse in the van der Waals antiferromagnet FePS₃. Such excitation behavior enables a unique way to coherently control the amplitude of magnon-polaron oscillations by tuning the terahertz field strength and its polarization. The polarimetry of the resulting coherent oscillation amplitude breaks the crystallographic C₂ symmetry due to strong interference between different excitation channels. Our findings unlock a wide range of possibilities to manipulate material properties, including modulation of exchange interactions by phonon-Floquet engineering.

Fig3-2.png

Spontaneous emergence of phonon chirality through hybridization with magnons

Chirality, the breaking of improper rotational symmetry, is
a fundamental concept spanning diverse scientific domains. Asymmetric population of opposite chiralities is fundamentally prevalent across a vast array of systems, ranging from parity violation in weak nuclear interactions to the homochirality of biomolecules like DNA. In condensed matter physics, chiral phonons, representing circular atomic motions, have aroused intense interest due to their coupling to magnetic degrees of freedom, enabling potential phonon-controlled spintronics. However, selective excitation of single-handed chiral phonons has primarily required external stimuli to break the degeneracy of chiral counterparts. Whether energetically nondegenerate chiral phonons can appear spontaneously without structural or external symmetry breaking remains an open question.

Fig3-4.png

Here, we demonstrate that nondegenerate enantiomeric pairs can be inducedby coupling to chiral magnons in the van der Waals antiferromagnet FePSe₃. We confirm the presence of magnon-phonon hybrids, dubbed magnon polarons, which exhibit inherent elliptical polarization with opposite chiralities and distinct energies. This nondegeneracy enables their coherent excitation with linearly polarized terahertz pulses. By tuning the terahertz drive polarization and measuring phase-resolved polarimetry of the resultingcoherent oscillations, we determine the ellipticity and map the trajectory of these hybrid quasiparticles. Our findings establish a general approach to search for intrinsically nondegenerate chiral phonons and introduce a new methodology for characterizing their ellipticity, outlining a novel roadmap towards chiral-phonon-controlled spintronic functionalities.

See more details in the article: H. Ning*, T. Luo*, B. Ilyas*, E. Viñas Boström, J. Park, J. Kim, J.-G. Park, D. M. Juraschek, A. Rubio, N. Gedik✉️, Spontaneous emergence of phonon chirality through hybridization with magnons, in prep (2024).

See more details in the article: T. Luo*, H. Ning*, B. Ilyas*, A. von Hoegen*, E. Viñas Boström, J. Park, J. Kim, J.-G. Park, D. M. Juraschek, A. Rubio, N. Gedik✉️, Terahertz control of linear and nonlinear magno-phononics, under review (2024).

bottom of page