Twists and The Electronic Structure of Graphitic Materials

We analyze the effect of twists on the electronic structure of configurations of infinite stacks of graphene layers. We focus on three different cases: an infinite stack where each layer is rotated with respect to the previous one by a fixed angle, two pieces of semi-infinite graphite rotated with respect to each other, and finally a single layer of graphene rotated with respect to a graphite surface. In all three cases we find a rich structure, with sharp resonances and flat bands for small twist angles. The method used can be easily generalized to more complex arrangements and stacking sequences.Comment: 18 pages, 14 figure

Higgs-mode radiance and charge-density-wave order in 2H-NbSe2_2

Despite being usually considered two competing phenomena, charge-density-wave and superconductivity coexist in few systems, the most emblematic one being the transition metal dichalcogenide 2H-NbSe$_2$. This unusual condition is responsible for specific Raman signatures across the two phase transitions in this compound. While the appearance of a soft phonon mode is a well-established fingerprint of the charge-density-wave order, the nature of the sharp sub-gap mode emerging below the superconducting temperature is still under debate. In this work we use the external pressure as a knob to unveil the delicate interplay between the two orders, and consequently the nature of the superconducting mode. Thanks to an advanced extreme-conditions Raman technique we are able to follow the pressure evolution and the simultaneous collapse of the two intertwined charge density wave and superconducting modes. The comparison with microscopic calculations in a model system supports the Higgs-type nature of the superconducting mode and suggests that charge-density-wave and superconductivity in 2H-NbSe$_2$ involve mutual electronic degrees of freedom. These findings fill knowledge gap on the electronic mechanisms at play in transition metal dichalcogenides, a crucial step to fully exploit their properties in few-layers systems optimized for devices applications

Narrow bands and electrostatic interactions in graphene stacks

Superconducting and insulating phases are well-established in twisted graphene bilayers, and they have also been reported in other arrangements of graphene layers. We investigate three such situations, (untwisted) bilayer graphene on hBN, two graphene bilayers twisted with respect to each other, and a single ABC stacked graphene trilayer on hBN. In all these cases, narrow bands emerge. The resulting high density of states enhances the role of interactions. We study the effect of the long-range electron-electron interaction on these narrow bands. A self consistent electrostatic potential does not modify significantly the shape and width of the bands in the three cases considered here, in contrast to the effect that such a potential has in twisted bilayer graphene.Comment: 18 pages, 12 figure

Optical signatures of the superconducting Goldstone mode in granular aluminum: experiments and theory

Recent advances in the experimental growth and control of disordered thin films, heterostructures, and interfaces provide a fertile ground for the observation and characterisation of the collective superconducting excitations emerging below $T_c$ after breaking the $U(1)$ gauge symmetry. Here we combine THz experiments in a nano-structured granular Al thin film and theoretical calculations to demonstrate the existence of optically-active phase modes, which represent the Goldstone excitations of the broken gauge symmetry. By measuring the complex transmission trough the sample we identify a sizeable and temperature-dependent optical sub-gap absorption, which cannot be ascribed to quasiparticle excitations. A quantitative modelling of this material as a disordered Josephson array of nano-grains allows us to determine, with no free parameters, the structure of the spatial inhomogeneities induced by shell effects. Besides being responsible for the enhancement of the critical temperature with respect to bulk Al, already observed in the past, this spatial inhomogeneity provides a mechanism for the optical visibility of the Goldstone mode. By computing explicitly the optical spectrum of the superconducting phase fluctuations we obtain a good quantitative description of the experimental data. Our results demonstrate that nanograins arrays are a promising setting to study and control the collective superconducting excitations via optical means

Superconductivity and correlated phases in bilayer, trilayer graphene and related structures

The discovery of a very rich phase diagram in twisted bilayer graphene [1,2] renewed the interest into the properties of other systems based on graphene. An unexpected finding has been the observation of superconductivity in non-twisted graphene bilayers and trilayers [3-5]. In this perspective, we give an overview of the search for uncommon phases in non-twisted graphene systems. We first describe results related to the topic before the aforementioned experiments [3-5] were published. Then, we address the new experimental findings which have triggered the recent interest in the problem. Lastly, we analyze the already numerous theory works studying the underlying physical processes [6].Comment: 10 Pages, 6 figures, 2 tables. Comments are very welcome. Invited Review Nature Physics Perspective