Congratulations to cohort 2 Student Aitor Garcia has had his paper published – in Physical Review B.
Graphene is a layer of carbon atoms arranged in regular hexagons and the first of atomically thin materials known as two-dimensional crystals. Initially obtained by mechanical exfoliation from graphite (which can be thought of as a thick stack of graphene layers), it is, amongst many superlatives, the strongest material ever measured and conducts electricity better than copper.
One of the most commonly used methods to characterise graphene materials is Raman spectroscopy which involves illuminating the sample with intense light of a certain frequency and detecting scattered light with a different frequency. The frequency shift arises because some of the energy of the incident light is passed to the material. By measuring the scattered light, we can deduce both how much energy was passed (by looking at the Raman shift – frequency difference between the incident and detected light) and how “efficient” this passing process was (by looking at how intense the scattered light at a given frequency is), hence learning something about the properties of the material. In fact, Raman spectroscopy is routinely used to determine the number of layers in a thin graphene fill, the density of defects in the material, doping or presence of strain.
In our paper, we have investigated theoretically whether Raman spectroscopy could be used to determine if the graphene sample is superconducting. Superconductivity is a consequence of an effective attraction between any two electrons with opposite momentum and spin, commonly known as Cooper pairing. When the temperature is low enough, such attraction induces a phase transition in the system that lowers its energy by a certain amount, often referred to as the superconducting gap. We found that if graphene is superconducting, a new peak appears in its Raman spectrum. The position of this peak (its Raman shift) is equal to the magnitude of the superconducting gap and its shape is related to the density of Cooper pairs in the material. Our work is especially timely given the recent discovery of superconductivity in twisted bilayer graphene.
Figure 1: The low-energy electronic contribution to the Raman spectrum of superconducting graphene with chemical potential 150 meV, for incoming photon energy 1 eV and (a) circular and (b) linear polarization of the incoming/scattered light. The solid and dashed lines correspond to superconducting gaps of 1 meV and 4 meV, respectively.