A quantized microwave quadrupole insulator with topologically protected corner states

By Christopher W. Peterson, Wladimir A. Benalcazar, T. Hughes, Gaurav Bahl. Nature, 2017

756 citations recorded by the source corpus.

The theory of electric polarization in crystals defines the dipole moment of an insulator in terms of a Berry phase (geometric phase) associated with its electronic ground state. This concept not only solves the long-standing puzzle of how to calculate dipole moments in crystals, but also explains topological band structures in insulators and superconductors, including the quantum anomalous Hall insulator and the quantum spin Hall insulator, as well as quantized adiabatic pumping processes. A recent theoretical study has extended the Berry phase framework to also account for higher electric multipole moments, revealing the existence of higher-order topological phases that have not previously been observed. Here we demonstrate experimentally a member of this predicted class of materials—a quantized quadrupole topological insulator—produced using a gigahertz-frequency reconfigurable microwave circuit. We confirm the non-trivial topological phase using spectroscopic measurements and by identifying corner states that result from the bulk topology. In addition, we test the critical prediction that these corner states are protected by the topology of the bulk, and are not due to surface artefacts, by deforming the edges of the crystal lattice from the topological to the trivial regime. Our results provide conclusive evidence of a unique form of robustness against disorder and deformation, which is characteristic of higher-order topological insulators.

Subfields: Atomic and Molecular Physics, and Optics, Condensed Matter Physics, Materials Chemistry

View Paper: https://www.semanticscholar.org/paper/9f51eecc326df20e11cd880ffc4de25af66e12b4

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In Atomic and Molecular Physics, and Optics

In Condensed Matter Physics

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Citations in the corpus

Corner states of light in photonic waveguides (2018, background): “The first experimental realisations of such higher-order boundary modes have very recently been achieved: the quadrupole topological insulator [11] has been realised in topolectric circuits [19], mechanical metamaterials [20], and microwave circuits [21] while corner modes predicted to appear in bre”
Higher-order non-Hermitian skin effect (2020, background): “Higher-order topology was observed in various experiments [20–28], and may lead to unique phenomena and functionalities due to its boundary physics.”
Observation of quadrupole transitions and edge mode topology in an LC circuit network (2018, background): “Shortly after their theoretical proposal, several experimental observations of higher-order topological effects have been reported in phononic [17], electronic [18], microwave [19] and circuit”