5 research outputs found
One-dimensional moir\'e superlattices and flat bands in collapsed chiral carbon nanotubes
We demonstrate that one-dimensional moir\'e patterns, analogous to those
found in twisted bilayer graphene, can arise in collapsed chiral carbon
nanotubes. Resorting to a combination of approaches, namely, molecular dynamics
to obtain the relaxed geometries and tight-binding calculations validated
against ab initio modeling, we find that magic angle physics occur in collapsed
carbon nanotubes. Velocity reduction, flat bands and localization in AA regions
with diminishing moir\'e angle are revealed, showing a magic angle close to
1. From the spatial extension of the AA regions and the width of the
flat bands, we estimate that many-body interactions in these systems are
stronger than in twisted bilayer graphene. Chiral collapsed carbon nanotubes
stand out as promising candidates to explore many-body effects and
superconductivity in low dimensions, emerging as the one-dimensional analogues
of twisted bilayer graphene
Universality of moir\'e physics in collapsed chiral carbon nanotubes
We report the existence of moir\'e patterns and magic angle physics in all
families of chiral collapsed carbon nanotubes. A detailed study of the
electronic structure of all types of chiral nanotubes, previously collapsed via
molecular dynamics, has been performed. We find that each family possesses a
unique geometry and moir\'e disposition, as well as a characteristic number of
flat bands. Remarkably, all kinds of nanotubes behave the same with respect to
magic angle tuning, showing a monotonic behavior that gives rise to magic
angles in full agreement with those of twisted bilayer graphene. Therefore,
magic angle behavior is universally found in chiral collapsed nanotubes with a
small chiral angle, giving rise to moir\'e patterns. Our approach comprises
first-principles and semi-empirical calculations of the band structure, density
of states and spatial distribution of the localized states signaled by flat
bands
Persistence of symmetry-protected Dirac points at the surface of the topological crystalline insulator SnTe upon impurity doping
We investigate the effect of a non-magnetic donor impurity located at the surface of the SnTe topological crystalline insulator. In particular, the changes on the surface states due to a Sb impurity atom are analyzed by means of ab initio simulations of pristine and impurity-doped SnTe. Both semi-infinite and slab geometries are considered within the first-principles approach. Furthermore, minimal and Green's function continuum models are proposed with the same goal. We find that the Dirac cones are shifted down in energy upon doping; this shift strongly depends on the position of the impurity with respect to the surface. In addition, we observe that the width of the impurity band presents an even-odd behavior by varying the position of the impurity. This behavior is related to the position of the nodes of the wave function with respect to the surface, and hence it is a manifestation of confinement effects. We compare slab and semi-infinite geometries within the ab initio approach, demonstrating that the surface states remain gapless and their spin textures are unaltered in the doped semi-infinite system. In the slab geometry, a gap opens due to hybridization of the states localized at opposite surfaces. Finally, by means of a continuum model, we extrapolate our results to arbitrary positions of the impurity, clearly showing a non-monotonic behavior of the Dirac cone
Polarization-tuneable excitonic spectral features in the optoelectronic response of atomically thin ReS2
The low crystal symmetry of rhenium disulphide (ReS2) leads to the emergence
of dichroic optical and optoelectronic response, absent in other layered
transition metal dichalcogenides, which could be exploited for device
applications requiring polarization resolution. To date, spectroscopy studies
on the optical response of ReS2 have relied almost exclusively in
characterization techniques involving optical detection, such as
photoluminescence, absorbance, or reflectance spectroscopy. However, to realize
the full potential of this material, it is necessary to develop knowledge on
its optoelectronic response with spectral resolution. In this work, we study
the polarization-dependent photocurrent spectra of few-layer ReS2
photodetectors, both in room conditions and at cryogenic temperature. Our
spectral measurements reveal two main exciton lines at energies matching those
reported for optical spectroscopy measurements, as well as their excited
states. Moreover, we also observe an additional exciton-like spectral feature
with a photoresponse intensity comparable to the two main exciton lines. We
attribute this feature, not observed in earlier photoluminescence measurements,
to a non-radiative exciton transition. The intensities of the three main
exciton features, as well as their excited states, modulate with linear
polarization of light, each one acquiring maximal strength at a different
polarization angle. We have performed first-principles exciton calculations
employing the Bethe-Salpeter formalism, which corroborate our experimental
findings. Our results bring new perspectives for the development of ReS2-based
nanodevices
Correction: persistence of symmetry-protected Dirac points at the surface of the topological crystalline insulator SnTe upon impurity doping
Depto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu