Skip to main content
2D materials

2D materials

Chiral metamaterial enables scatter-free propagation

24 Jul 2017 Michela Picardi 
Fermi arcs experimentally observed with near field scanning at 5.46 GHz. Middle solid circle represents air-light cone, while the other white curves are the simulated dispersion curves for the bulk material.

A team of scientists from the UK and China has for the first time observed Fermi arcs – a distinct signature of the presence of topological properties – in a microwave metamaterial. Recent experiments have revealed Fermi arcs in quantum matter, but this is the first time they have been seen in a classical 3D system. The finding paves the way towards the study of a new class of topological optical materials, which could have important applications in communications because of their promise to send signals around corners or over defects without any loss of signal strength from scattering.

Fermi arcs are known to provide the connection between two topologically different surfaces in a quantum material called a Weyl semimetal. The electronic band structure in this material features so-called Weyl points, the 3D version of the Dirac points observed in graphene and other two-dimensional materials, where the dispersion is linear and the electronic bands cross each other. These Weyl points have a definite chirality, which can be understood as topological “charges”.

The researchers, led by Shuang Zhang from the University of Birmingham, therefore exploited chirality in the design of their topological metamaterial, along with hyperbolicity, to engineer the required dispersion. The material comprises a stacking of multiple tri-layers. The bottom layer possesses hyperbolic dispersion, which results from 200 μm-wide metallic wires running across its top surface, with metallic crosses superimposed on the wires to increase the capacitance and suppress non-local effects. The middle layer is a thin dielectric spacer that prevents electrical contact between top and bottom layers, while the top layer introduces chirality through the presence of metallic helices, each having 2.5 turns.

The researchers exploited a near-field scanning technique using microwave antannas to observe the Fermi arcs on both the top and side surfaces of their chiral hyperbolic metamaterial. The near-field distribution, once Fourier transformed into the frequency domain, clearly reveals the presence of Fermi arcs on the surface between the bulk states.

To further investigate the topological nature of the system, the group stacked several layers of the chiral hyperbolic metamaterial together to form a step. In this arrangement the topological protection of the surface state forces the surface wave excited at the top layer to bend around the step and to propagate forwards without any reflections from the edges. The absence of scattering as the surface wave propagates across the step confirms the topological nature of the chiral hyperbolic metamaterial.

One key feature of topologically protected states is that certain bands in the dispersion relation cannot interact, which means that a wave travelling in one band is not allowed to jump into another band. In this experiment, the researchers explain, the surface wave travels over the corner without any scattering because there is a topological charge difference between the Weyl points connected by the Fermi arc.

Next the researchers intend to investigate other systems that support topologically protected waves and find ways to more accurately steer waves on surfaces. Using simpler geometries, the group will be able to miniaturize these materials so that they can work at THz, infrared and optical frequencies.

The research has been published in Nature Communications.

Copyright © 2024 by IOP Publishing Ltd and individual contributors