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Journal Article
A quantized microwave quadrupole insulator with topologically protected corner states
2018
Overview
A quantized quadrupole topological insulator composed of capacitively coupled microwave resonators has corner states that are protected by bulk topology and exhibit exceptional robustness against edge deformation.
Topological corner states
The properties of many materials with topological band structures can be understood in terms of a quantization of the electric polarization. A new class of higher-order topological insulators has recently been predicted by considering a quantization of higher-order polarizations. Christopher Peterson
et al.
now use a metamaterial composed of coupled microwave resonators to demonstrate such a system experimentally: a quantized quadrupole topological insulator that has corner states that are protected by topology. By deforming one of the edges from the topological to the trivial regime, they show that these corner states move inward to the corners of the newly generated boundaries, confirming that they are protected by the topology of the bulk. Such demonstrations not only provide evidence of a unique form of robustness, but also show that reconfigurable microwave circuits are a promising platform for exploring exotic topological phases of matter.
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
1
,
2
. 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
3
,
4
and the quantum spin Hall insulator
5
,
6
,
7
, as well as quantized adiabatic pumping processes
8
,
9
,
10
. A recent theoretical study has extended the Berry phase framework to also account for higher electric multipole moments
11
, 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.
Publisher
Nature Publishing Group UK,Nature Publishing Group
Subject
