Ideas of topology have been widely applied to electronic band structures. We have been exploring the application these ideas to phonon and other excitations.
Figure 1: A cartoon of double Weyl phonons as we reported in Phys. Rev. Lett. 121, 035302 (2018).
Example papers
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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
Phys. Rev. Lett.
123,
245302
(2019)
[Abs]
[PDF]
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[Supp]
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.
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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
Phys. Rev. Lett.
121,
035302
(2018)
[Abs]
[PDF]
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[Supp]
[ANL Highlight]
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.