Laser Pulses Create Topological State in Graphene

Topological quantum states in graphene induced by light. (Source: B. Schulte)

In topo­logical materials, electrons experience a twisted world. Instead of simply moving straight ahead when feeling a force, they may be pushed sideways. In such a material current actually flows ortho­gonally to an applied voltage. The basic model describing the effect was developed by Duncan Haldane in the late 1980s, but even its inventor was skeptical that it could ever be implemented in a real material. Never­theless, elaborate chemical synthesis eventually allowed for very similar effects to be observed, sparking a techno­logical revolution – and even­tually earning Haldane the 2016 Nobel Prize in Physics.

Topo­logical transport is usually induced in materials by applying strong magnetic fields or by crafting compounds with strong spin-orbit coupling. Researchers in Andrea Cavalleri’s group at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg have now demons­trated that a coherent inter­action with circu­larly polarized light can also induce topological electrical currents in the material graphene.

The team’s radically different approach consists of illu­minating graphene with a strong, circularly polarized laser pulse, whose electric field drives electrons in loops. When the material is illu­minated, it suddenly behaves like a topological material. It returns to its normal state once the pulse is gone. Whilst this mechanism had been tested in simulations, it was entirely unclear whether it would work in the more compli­cated context of real solids – and whether it would be possible to detect it.

To prove their disco­very, the physicists had to show currents flowing in a direction orthogonal to an applied voltage. However, there was a major challenge: “As the effect persists only for about a millionth of a millionth of a second, we had to develop a novel type of electronic circuit to measure this,” says James McIver. The result was an ultrafast opto­electronic device architecture based on photo­conductive switches. It confirmed the existence of the effect.

Moving forward, the researchers plan to use this cir­cuitry to study a variety of compelling problems in quantum materials, such as light-induced super­conductivity and photon-dressed topological edge states. “This work shows that light is capable of engineering topological properties in topo­logically trivial materials”, says Gregor Jotzu. “The ultra­fast ap­pearance of this effect holds great potential for the con­struction of extremely fast sensors or computers.” (Source: MPSD)

Reference: J. W. McIver et al.: Light-induced anomalous Hall effect in graphene, Nat. Phys., online 4. November 2019; DOI: 10.1038/s41567-019-0698-y

Link: Quantum Condensed Matter Dynamics, Max Planck Institute for the Structure and Dynamics of Matter MPSD, Hamburg, Germany

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