Using graphene to speak to superconductors
Nuno Peres tells us how graphene can shed light on the elusive behaviour of superconducting materials
Nuno Peres and his team at Graphene Flagship Partner University of Minho, Portugal, in collaboration with other Graphene Flagship scientists at Partners ICFO (Spain) and the Technical University of Denmark, as well as Graphene Flagship Associated Member Université de Montpellier (France), and other institutions in the US, have used graphene to bridge two seemingly unrelated topics: superconductivity and plasmonics.
Peres joined the Graphene Flagship during the ramp-up phase. Within the Graphene Flagship Work Package Photonics and Optoelectronics, his team modelled the electronic and optical properties of layered materials and devices, and proposed new lines of research for experimental groups. In this interview, we explore this vibrant research field.
What are superconductors?
The electrical resistance of conducting materials, such as copper, dissipates part of the electrical current in the form of heat. On the contrary, superconductors allow current to flow through without electrical resistance and losses. They are also good magnetic shields that squeeze magnetic fields out of their interior, and find applications in high-tech sectors, including levitating high-speed trains, high-energy physics and quantum computer prototypes.
What makes these superconducting materials so special is their behaviour at the subatomic level: electrons move collectively in pairs and without hindrance. However, several physical phenomena of superconductors are still elusive and not easily observable, because they are invisible to our traditional optical devices. One of these “invisible behaviours” is the Higgs mode, roughly speaking an oscillation in the density of the electron pairs, or more precisely, an oscillation of the superconductor order parameter.
What made you think about combining graphene with superconductors?
Graphene is a wonderful tool to study superconductors. It can help us to unveil superconductors’ modes, such as the Higgs mode.
To understand these particular oscillations happening in the superconductor, we thought, based on a hunch of Dmitri Basov (Columbia University) and Frank Koppens (Graphene Flagship Partner ICFO), to take advantage of electrons’ collective oscillations in graphene. These are called graphene plasmons and, in contrast to the Higgs mode, they do interact with electromagnetic radiations ranging from Terahertz to mid-infrared, so they are easier to detect for us.
My interest in graphene plasmons dates back to 2011, when our group unveiled a mechanism for a graphene-plasmon-based optoelectronic switch, using the experimental technique of total attenuated reflection. In 2018, our team together with researchers from Graphene Flagship Partners ICFO and the Technical University of Denmark realised that graphene plasmons could be used to detect nonlocal properties of metals, in other words, an “action at a distance” effect, where the electromagnetic properties of the metal at one point in space depend on the properties of all the other points in the metal. This is critical to understand how devices made of graphene and hexagonal boron nitride (hBN) can confine light in tiny dimensions, much smaller than the wavelength of the light itself, and be applied to nanophotonics.
A natural extension was to see whether graphene plasmons could be used to study superconductors, as superconductors are intrinsically nonlocal. It turned out that there is a strong interaction between the Higgs mode and plasmons in graphene. In this way, we could combine two apparently disjoint fields: superconductivity and plasmonics.
Please tell us all about your recent findings!
Our work with Graphene Flagship Partner ICFO and other international collaborators led to a publication in PNAS. We considered a graphene–superconductor hybrid device with a superconducting substrate and a single sheet of graphene sandwiched between a few atomic layers of hexagonal boron nitride. We showed that graphene plasmons interact with the Higgs mode of a superconductor placed a few nanometres away.
The process of probing the Higgs mode requires the following steps: we first excited graphene plasmons with traditional optical instruments or with a SNOM (scanning near optical microscope) tip. This, in turn, triggered the Higgs mode in the superconductor. The superconductor underwent intense fluctuations in the density of its order parameter, which altered the way graphene’s electrons behave collectively. Using an analogy, the oscillations in the superconductor and the ones in graphene “speak to each other”. We cannot hear the superconductor directly, but we can communicate with graphene and it tells us what the superconductor is doing. In this way, we showed that graphene provides the missing link between the superconductor’s Higgs mode and our measuring instruments.
What's next for your research? What's your vision?
In the Science paper we published in 2018, we collaborated with Graphene Flagship Partner ICFO Graphene Flagship Associated Member Université de Montpellier, and MIT researchers to develop a metallic grating covered with a graphene sheet for probing the ultimate graphene plasmons’ confinement: one atom. We showed that graphene plasmons can confine light into an atom-thick channel between a layer of graphene and a metal, with very low losses. A slight modification of that grating system allows the formation of graphene plasmons that behave topologically, which means they can propagate along interfaces without suffering scattering from the materials defects. This light confinement and high intensity of the electromagnetic fields allow us to sense minute quantities of a given analyte, both in solution and in gases.
This research can help us understand the fundamental physics behind quantum phenomena and is indispensable for the development of new quantum technologies and nanophotonics platforms. Graphene plasmons coupled to other well-studied materials pave the way to new physics, creating new avenues for devices and applications.
What kinds of technology do you dream of?
At the moment, the most common quantum technologies are based on quantum encryption and quantum computers; the latter using Josephson-junctions – devices made of two superconducting electrodes separated by a barrier – working at extremely low temperatures. It would be desirable to have quantum technologies based on solid-state devices working at moderate temperatures. We are very far from that now. I would like to see plasmons guided at the surface of graphene enter the realm of quantum computation. Will this happen? Some theory papers are suggesting this route may be possible, but this is still a mixture of prospective and dream. However, dreaming can propel out thinking towards new physical paradigms.
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