5 research outputs found
Fractal Landau-Level Spectra in Twisted Bilayer Graphene
The Hofstadter butterfly spectrum for Landau levels in
a two-dimensional
periodic lattice is a rare example exhibiting fractal properties in
a truly quantum system. However, the observation of this physical
phenomenon in a conventional material will require a magnetic field
strength several orders of magnitude larger than what can be produced
in a modern laboratory. It turns out that for a specific range of
rotational angles twisted bilayer graphene serves as a special system
with a fractal energy spectrum under laboratory accessible magnetic
field strengths. This unique feature arises from an intriguing electronic
structure induced by the interlayer coupling. Using a recursive tight-binding
method, we systematically map out the spectra of these Landau levels
as a function of the rotational angle. Our results give a complete
description of LLs in twisted bilayer graphene for both commensurate
and incommensurate rotational angles and provide quantitative predictions
of magnetic field strengths for observing the fractal spectra in these
graphene systems
Charge Transport through Graphene Junctions with Wetting Metal Leads
Graphene is believed to be an excellent candidate material
for
next-generation electronic devices. However, one needs to take into
account the nontrivial effect of metal contacts in order to precisely
control the charge injection and extraction processes. We have performed
transport calculations for graphene junctions with wetting metal leads
(metal leads that bind covalently to graphene) using nonequilibrium
Green’s functions and density functional theory. Quantitative
information is provided on the increased resistance with respect to
ideal contacts and on the statistics of current fluctuations. We find
that charge transport through the studied two-terminal graphene junction
with Ti contacts is pseudo-diffusive up to surprisingly high energies
Fractal Landau-Level Spectra in Twisted Bilayer Graphene
The Hofstadter butterfly spectrum for Landau levels in
a two-dimensional
periodic lattice is a rare example exhibiting fractal properties in
a truly quantum system. However, the observation of this physical
phenomenon in a conventional material will require a magnetic field
strength several orders of magnitude larger than what can be produced
in a modern laboratory. It turns out that for a specific range of
rotational angles twisted bilayer graphene serves as a special system
with a fractal energy spectrum under laboratory accessible magnetic
field strengths. This unique feature arises from an intriguing electronic
structure induced by the interlayer coupling. Using a recursive tight-binding
method, we systematically map out the spectra of these Landau levels
as a function of the rotational angle. Our results give a complete
description of LLs in twisted bilayer graphene for both commensurate
and incommensurate rotational angles and provide quantitative predictions
of magnetic field strengths for observing the fractal spectra in these
graphene systems
Coupled Dirac Fermions and Neutrino-like Oscillations in Twisted Bilayer Graphene
The low-energy quasiparticles in
graphene can be described by a
Dirac–Weyl Hamiltonian for massless fermions, hence graphene
has been proposed to be an effective medium to study exotic phenomena
originally predicted for relativistic particle physics, such as Klein
tunneling and Zitterbewegung. In this work, we show that another important
particle-physics phenomenon, the neutrino oscillation, can be studied
and observed in a particular graphene system, namely, twisted bilayer
graphene. It has been found that graphene layers grown epitaxially
on SiC or by the chemical vapor deposition method on metal substrates
display a stacking pattern with adjacent layers rotated by an angle
with respect to each other. The quasiparticle states in two distinct
graphene layers act as neutrinos with two flavors, and the interlayer
interaction between them induces an appreciable coupling between these
two “flavors” of massless fermions, leading to neutrino-like
oscillations. In addition, our calculation shows that anisotropic
transport properties manifest in a specific energy window, which is
accessible experimentally in twisted bilayer graphene. Combining two
graphene layers enables us to probe the rich physics involving multiple
interacting Dirac fermions
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Dimensional Effects on the Charge Density Waves in Ultrathin Films of TiSe<sub>2</sub>
Charge
density wave (CDW) formation in solids is a critical phenomenon
involving the collective reorganization of the electrons and atoms
in the system into a wave structure, and it is expected to be sensitive
to the geometric constraint of the system at the nanoscale. Here,
we study the CDW transition in TiSe<sub>2</sub>, a quasi-two-dimensional
layered material, to determine the effects of quantum confinement
and changing dimensions in films ranging from a single layer to multilayers.
Of key interest is the characteristic length scale for the transformation
from a two-dimensional case to the three-dimensional limit. Angle-resolved
photoemission spectroscopy (ARPES) measurements of films with thicknesses
up to six layers reveal substantial
variations in the energy structure of discrete quantum well states;
however, the temperature-dependent band gap renormalization converges
at just three layers. The results indicate a layer-dependent mixture
of two transition temperatures and a very-short-range CDW interaction
within a three-dimensional framework