34 research outputs found
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-NbSe
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. 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 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 after breaking the 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