4,305 research outputs found
Electron Interactions and Scaling Relations for Optical Excitations in Carbon Nanotubes
Recent fluorescence spectroscopy experiments on single wall carbon nanotubes
reveal substantial deviations of observed absorption and emission energies from
predictions of noninteracting models of the electronic structure. Nonetheless,
the data for nearly armchair nanotubes obey a nonlinear scaling relation as a
function the tube radius . We show that these effects can be understood in a
theory of large radius tubes, derived from the theory of two dimensional
graphene where the coulomb interaction leads to a logarithmic correction to the
electronic self energy and marginal Fermi liquid behavior. Interactions on
length scales larger than the tube circumference lead to strong self energy and
excitonic effects that compete and nearly cancel so that the observed optical
transitions are dominated by the graphene self energy effects.Comment: 4 page
Surface States of Topological Insulators
We develop an effective bulk model with a topological boundary condition to
study the surface states of topological insulators. We find that the Dirac
point energy, the band curvature and the spin texture of surface states are
crystal face-dependent. For a given face on a sphere, the Dirac point energy is
determined by the bulk physics that breaks p-h symmetry in the surface normal
direction and is tunable by surface potentials that preserve T symmetry.
Constant energy contours near the Dirac point are ellipses with spin textures
that are helical on the S/N pole, collapsed to one dimension on any side face,
and tilted out-of-plane otherwise. Our findings identify a route to engineering
the Dirac point physics on the surfaces of real materials.Comment: 4.1 pages, 2 figures and 1 tabl
Dwelling on Earth by learning from Nature. Urban and building systems more sustainable and resilient through the use of Nature Based Solutions and Biomimicry
In the last two and a half centuries the human species has compromised every ecological niche on the planet, upsetting the delicate balances of homeostasis that regulate the biosphere. In 2020, the mass of all man-made artefacts made by humanity, over one trillion tonnes, exceeded the mass of all living organisms. The impact of human activities on the planet has reached levels that go beyond the safety thresholds of the different planetary systems on which the biosphere is based. In particular, the impact of the construction sector and built-up areas is extremely problematic due to the intensive use of resources and energy. A paradigm shift in the way of building and living is urgently needed so that the human species can continue to inhabit the earth
Quantum Spin Hall Effect in Graphene
We study the effects of spin orbit interactions on the low energy electronic
structure of a single plane of graphene. We find that in an experimentally
accessible low temperature regime the symmetry allowed spin orbit potential
converts graphene from an ideal two dimensional semimetallic state to a quantum
spin Hall insulator. This novel electronic state of matter is gapped in the
bulk and supports the quantized transport of spin and charge in gapless edge
states that propagate at the sample boundaries. The edge states are non chiral,
but they are insensitive to disorder because their directionality is correlated
with spin. The spin and charge conductances in these edge states are calculated
and the effects of temperature, chemical potential, Rashba coupling, disorder
and symmetry breaking fields are discussed.Comment: 4 pages, published versio
Switchable valley filter based on a graphene - junction in a magnetic field
Low-energy excitations in graphene exhibit relativistic properties due to the
linear dispersion relation close to the Dirac points in the first Brillouin
zone. Two of the Dirac points located at opposite corners of the first
Brillouin zone can be chosen as inequivalent, representing a new valley degree
of freedom, in addition to the charge and spin of an electron. Using the valley
degree of freedom to encode information has attracted significant interest,
both theoretically and experimentally, and gave rise to the field of
valleytronics. We study a graphene - junction in a uniform out-of-plane
magnetic field as a platform to generate and controllably manipulate the valley
polarization of electrons. We show that by tuning the external potential giving
rise to the - junction we can switch the current from one valley
polarization to the other. We also consider the effect of different types of
edge terminations and present a setup, where we can partition an incoming
valley-unpolarized current into two branches of valley-polarized currents. The
branching ratio can be chosen by changing the location of the - junction
using a gate.Comment: 8 pages, 7 figure
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