3,941 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
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
Heavy Quark Fragmentation into Heavy Mesons
We present a QCD based interpretation of heavy quark fragmentation which
utilizes the heavy quark mass expansion. By distinguishing between perturbative
and non-perturbative QCD effects, we show how to reliably extract mass
independent parameters characterizing the fragmentation function. Because these
parameters are quark mass independent, this procedure should permit tests of
heavy quark symmetry. Furthermore, we show that heavy quark mass corrections
vanish at order in QCD. There also exist higher twist corrections of
order and
which we relate to the leading twist fragmentation function.Comment: 36 pages (Plain TeX with a PostScript figure appended at end), MIT
CTP #218
Topological Order and the Quantum Spin Hall Effect
The quantum spin Hall (QSH) phase is a time reversal invariant electronic
state with a bulk electronic band gap that supports the transport of charge and
spin in gapless edge states. We show that this phase is associated with a novel
topological invariant, which distinguishes it from an ordinary insulator.
The classification, which is defined for time reversal invariant
Hamiltonians, is analogous to the Chern number classification of the quantum
Hall effect. We establish the order of the QSH phase in the two band
model of graphene and propose a generalization of the formalism applicable to
multi band and interacting systems.Comment: 4 pages RevTeX. Added reference, minor correction
General model and equivalent circuit for the chemical noise spectrum associated to surface charge fluctuation in potentiometric sensors
This paper firstly reports a general and powerful approach to evaluate the power spectral density (PSD) of the surface charge fluctuations, so-called \u201cchemical noise\u201d, from a generic set of reactions at the sensing surface of potentiometric sensors such as, for instance, Ion-Sensitive Field Effect Transistors (ISFETs). Starting from the master equation, the spectral noise signature of a reaction set is derived as a function of the reaction kinetic parameters and of the interface concentration of the ionic species. Secondly, we derive an equivalent surface admittance, whose thermal noise PSD produces a noise PSD equal to that of the surface charge fluctuations. We also show how to expand this surface admittance into stair-case RC networks, with a number of elementary cells equal to the number of surface reactions involved. This admittance can be included in circuit simulations coupled with a SPICE compact model of the underlying FET, to enable the physically based modelling of frequency dispersion and noise of the sensing layer when simulating the sensor and the read-out. Validation with existing models and literature results as well as new application examples are provided. The proposed methodology to compute the PSD from rate equations is amenable to use in different contexts where fluctuations are generated by random transitions between discrete states with given exchange rates
General Approach to Model the Surface Charge Induced by Multiple Surface Chemical Reactions in Potentiometric FET Sensors
We propose a general methodology to calculate the individual sensitivity and the cross-sensitivities of potentiometric sensor devices (e.g., ion sensitive FETs (ISFETs), CHEMFETs) with an arbitrary number of non-interacting receptors binding to ionic species or analytes in the electrolyte. The surface charge generated at the (bare or functionalized) interface with the electrolyte is described by the Poisson equation coupled to a linear system of equations for each type of receptor, where the unknowns are the fractions of sites binding with a given ion/analyte. Our general model encompasses in a unique framework a few simple special cases so far separately reported in the literature and provides for them closed-form expressions of the average site occupation probability. Detailed procedural description of the usage and benefits of the model is shown for specific cases with concurring surface chemical reactions
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