Ideas of topology have been widely applied to electronic band structures, yielding Weyl and Dirac semimetals and topological insulators. We explore the extension of topological concepts to phonons and other bosonic excitations, having identified topologically protected nodal-line and Weyl phonons in several materials. We also study candidate Kitaev quantum spin liquid materials such as α-RuCl₃, where bond-directional spin interactions give rise to fractionalized Majorana excitations that are relevant to topological quantum computation.
Figure 1: A cartoon of double Weyl phonons as we reported in Phys. Rev. Lett. 121, 035302 (2018).
References
2021
Giant phonon anomalies in the proximate Kitaev quantum spin liquid α-RuCl3
Haoxiang Li, T. T. Zhang, A. Said, G. Fabbris, D. G. Mazzone, J. Q. Yan, D. Mandrus, Gábor B. Halász, S. Okamoto, S. Murakami, M. P. M. Dean, H. N. Lee, and H. Miao
The Kitaev quantum spin liquid epitomizes an entangled topological state, for which two flavors of fractionalized low-energy excitations are predicted: the itinerant Majorana fermion and the Z2 gauge flux. It was proposed recently that fingerprints of fractional excitations are encoded in the phonon spectra of Kitaev quantum spin liquids through a novel fractional-excitation-phonon coupling. Here, we detect anomalous phonon effects in α-RuCl3 using inelastic X-ray scattering with meV resolution. At high temperature, we discover interlaced optical phonons intercepting a transverse acoustic phonon between 3 and 7 meV. Upon decreasing temperature, the optical phonons display a large intensity enhancement near the Kitaev energy, JK 8 meV, that coincides with a giant acoustic phonon softening near the Z2 gauge flux energy scale. These phonon anomalies signify the coupling of phonon and Kitaev magnetic excitations in α-RuCl3 and demonstrates a proof-of-principle method to detect anomalous excitations in topological quantum materials.
@article{li2021fractional,dimensions={true},author={Li, Haoxiang and Zhang, T. T. and Said, A. and Fabbris, G. and Mazzone, D. G. and Yan, J. Q. and Mandrus, D. and Hal{\'a}sz, G{\'a}bor B. and Okamoto, S. and Murakami, S. and Dean, M. P. M. and Lee, H. N. and Miao, H.},title={<strong>Giant phonon anomalies in the proximate Kitaev quantum spin liquid $\alpha$-RuCl<sub>3</sub></strong>},journal={Nature Communications},year={2021},month=jun,day={10},volume={12},number={1},pages={3513},issn={2041-1723},doi={10.1038/s41467-021-23826-1},url={https://doi.org/10.1038/s41467-021-23826-1}}
2019
Phononic Helical Nodal Lines with PT Protection in MoB2
T. T. Zhang, H. Miao, Q. Wang, J. Q. Lin, Y. Cao, G. Fabbris, A. H. Said, X. Liu, H. C. Lei, Z. Fang, H. M. Weng, and M. P. M. Dean
While condensed matter systems host both fermionic and bosonic quasiparticles, reliably predicting and empirically verifying topological states is only mature for Fermionic electronic structures, leaving topological Bosonic excitations sporadically explored. This is unfortunate, as Bosonic systems such as phonons offer the opportunity to assess spinless band structures where nodal lines can be realized without invoking special additional symetries to protect against spin-orbit coupling. Here we combine firstprinciples calculations and meV-resolution inelastic x-ray scattering to demonstrate the first realization of parity-time reversal symmetry protected helical nodal lines in the phonon spectrum of MoB2. This structure is unique to phononic systems as the spin-orbit coupling present in electronic systems tends to lift the degeneracy away from high-symmetry locations. Our study establishes a protocol to accurately identify topological Bosonic excitations, opening a new route to explore exotic topological states in crystalline materials.
@article{zhang2019phononic,dimensions={true},title={<strong>Phononic Helical Nodal Lines with PT Protection in MoB<sub>2</sub></strong>},author={Zhang, T. T. and Miao, H. and Wang, Q. and Lin, J. Q. and Cao, Y. and Fabbris, G. and Said, A. H. and Liu, X. and Lei, H. C. and Fang, Z. and Weng, H. M. and Dean, M. P. M.},year={2019},month=dec,journal={Phys. Rev. Lett.},volume={123},pages={245302},doi={10.1103/PhysRevLett.123.245302}}
2018
Observation of double Weyl phonons in parity-breaking FeSi
Hu Miao, TT Zhang, Le Wang, Derek Meyers, AH Said, YL Wang, YG Shi, HM Weng, Zhong Fang, and MPM Dean
Condensed matter systems have now become a fertile ground to discover emerging topological quasiparticles with symmetry protected modes. While many studies have focused on fermionic excitations, the same conceptual framework can also be applied to bosons yielding new types of topological states. Motivated by Zhang et al.’s recent theoretical prediction of double Weyl phonons in transition metal monosilicides [Phys. Rev. Lett. 120, 016401 (2018)], we directly measure the phonon dispersion in parity-breaking FeSi using inelastic x-ray scattering. By comparing the experimental data with theoretical calculations, we make the first observation of double Weyl points in FeSi, which will be an ideal material to explore emerging bosonic excitations and its topologically nontrivial properties.
@article{miao2018observation,dimensions={true},title={<strong>Observation of double Weyl phonons in parity-breaking FeSi</strong>},author={Miao, Hu and Zhang, TT and Wang, Le and Meyers, Derek and Said, AH and Wang, YL and Shi, YG and Weng, HM and Fang, Zhong and Dean, MPM},journal={Phys. Rev. Lett.},volume={121},number={3},pages={035302},note={[<a href="https://www.aps.anl.gov/APS-Science-Highlight/2018-08-15/topological-excitations-emerge-from-a-vibrating-crystal-lattice" target="_blank">ANL Highlight</a>]},year={2018},month=jul,doi={10.1103/PhysRevLett.121.035302},publisher={American Physical Society}}
