984 research outputs found
Electron-electron interaction and charging effects in graphene quantum dots
We analyze charging effects in graphene quantum dots. Using a simple model,
we show that, when the Fermi level is far from the neutrality point, charging
effects lead to a shift in the electrostatic potential and the dot shows
standard Coulomb blockade features. Near the neutrality point, surface states
are partially occupied and the Coulomb interaction leads to a strongly
correlated ground state which can be approximated by either a Wigner crystal or
a Laughlin like wave function. The existence of strong correlations modify the
transport properties which show non equilibrium effects, similar to those
predicted for tunneling into other strongly correlated systems.Comment: Extended version accepted for publication at Phys. Rev.
Flow equations for Hamiltonians: Contrasting different approaches by using a numerically solvable model
To contrast different generators for flow equations for Hamiltonians and to
discuss the dependence of physical quantities on unitarily equivalent, but
effectively different initial Hamiltonians, a numerically solvable model is
considered which is structurally similar to impurity models. By this we discuss
the question of optimization for the first time. A general truncation scheme is
established that produces good results for the Hamiltonian flow as well as for
the operator flow. Nevertheless, it is also pointed out that a systematic and
feasible scheme for the operator flow on the operator level is missing. For
this, an explicit analysis of the operator flow is given for the first time. We
observe that truncation of the series of the observable flow after the linear
or bilinear terms does not yield satisfactory results for the entire parameter
regime as - especially close to resonances - even high orders of the exact
series expansion carry considerable weight.Comment: 25 pages, 10 figure
Thermoluminescent dosimetry for LDEF experiment M0006
Experiment M0006 on the Long Duration Exposure Facility had as its objective the investigation of space radiation effects on various electronic and optical components, as well as on seed germination. The Grumman Corporate Research Center provided the radiation dosimetric measurements for M0006, comprising the preparation of thermoluminescent dosimeters (TLD) and the subsequent measurement and analysis of flight exposed and control samples. In addition, various laboratory exposures of TLD's with gamma rays and protons were performed to obtain a better understanding of the flight exposures
Chirality probe of twisted bilayer graphene in the linear transport regime
We propose a minimal transport experiment in the linear regime that can probe
the chirality of twisted moir\'e structures. First, we point out that usual
two-terminal conductance measurements cannot access the chirality of a system.
Only with a third contact and in the presence of an in-plane magnetic field, a
chiral system displays non-reciprocal transport even if all contacts are
symmetric. We thus propose to use the third lead as a voltage probe and show
that opposite enantiomers give rise to different voltage drops on the third
lead. The third lead can also be used as a current probe in the case of
layer-discriminating contacts that can detect different handedness even in the
absence of a magnetic field. Our exact symmetry considerations are supported by
numerical calculations that confirm our conclusions and also demonstrate that
there is a change of chirality around the magic angle.Comment: 13 pages, 6 figure
Electronic transport in graphene: A semi-classical approach including midgap states
Using the semi-classical Boltzmann theory, we calculate the conductivity as
function of the carrier density. As usually, we include the scattering from
charged impurities, but conclude that the estimated impurity density is too low
in order to explain the experimentally observed mobilities. We thus propose an
additional scattering mechanism involving midgap states which leads to a
similar k-dependence of the relaxation time as charged impurities. The new
scattering mechanism can account for the experimental findings such as the
sublinear behavior of the conductivity versus gate voltage and the increase of
the minimal conductivity for clean samples. We also discuss temperature
dependent scattering due to acoustic phonons.Comment: 10 pages, 4 figure
Zigzag materials: selective interchain couplings control the coexistence of one-dimensional physics and deviations from it
The coexistence in the low-temperature spin-conducting phases of the zigzag
materials BaCo2V2O8 and SrCo2V2O8 of one-dimensional (1D) physics with
important deviations from it is not well understood. The studies of this paper
account for an important selection rule that follows from interchain spin
states being coupled more strongly within the spin dynamical structure factor
of such zigzag materials whenever they are connected by a specific symmetry
operation of the underlying lattice. In the case of excited states, this
symmetry operation is only a symmetry in spin-space ifno electronic spin flip
is performed within the generation of such states. Our results on both the role
of selective interchain couplings in protecting the 1D physics and being behind
deviations from it and on the dynamical properties being controlled by
scattering of singlet pairs of physical spins 1/2 open the door to a key
advance in the understanding of the physics of the spin chains in BaCo2V2O8 and
SrCo2V2O8.Comment: 24 pages, 13 figure
Effect of Holstein phonons on the optical conductivity of gapped graphene
We study the optical conductivity of a doped graphene when a sublattice
symmetry breaking is occurred in the presence of the electron-phonon
interaction. Our study is based on the Kubo formula that is established upon
the retarded self-energy. We report new features of both the real and imaginary
parts of the quasiparticle self-energy in the presence of a gap opening. We
find an analytical expression for the renormalized Fermi velocity of massive
Dirac Fermions over broad ranges of electron densities, gap values and the
electron-phonon coupling constants. Finally we conclude that the inclusion of
the renormalized Fermi energy and the band gap effects are indeed crucial to
get reasonable feature for the optical conductivity.Comment: 12 pages, 4 figures. To appear in Eur. Phys. J.
Bacterial Contamination on Household Toys and Association with Water, Sanitation and Hygiene Conditions in Honduras
There is growing evidence that household water treatment interventions improve microbiological water quality and reduce diarrheal disease risk. Few studies have examined, however, the impact of water treatment interventions on household-level hygiene and sanitation. This study examined the association of four water and sanitation conditions (access to latrines, improved sanitation, improved water and the plastic biosand filter) on the levels of total coliforms and E. coli on existing and introduced toys during an on-going randomized controlled trial of the plastic biosand filter (plastic BSF). The following conditions were associated with decreased bacterial contamination on children’s toys: access to a latrine, access to improved sanitation and access to the plastic BSF. Overall, compared to existing toys, introduced toys had significantly lower levels of both E. coliand total coliforms. Results suggest that levels of fecal indicator bacteria contamination on children’s toys may be associated with access to improved water and sanitation conditions in the home. In addition, the fecal indicator bacteria levels on toys probably vary with duration in the household. Additional information on how these toys become contaminated is needed to determine the usefulness of toys as indicators or sentinels of water, sanitation and hygiene conditions, behaviors and risks
Tomonaga-Luttinger model with an impurity for a weak two-body interaction
The Tomonaga-Luttinger model with impurity is studied by means of flow
equations for Hamiltonians. The system is formulated within collective density
fluctuations but no use of the bosonization formula is made. The truncation
scheme includes operators consisting of up to four fermion operators and is
valid for small electron-electron interactions. In this regime, the exact
expression for the anomalous dimension is recovered. Furthermore, we verify the
phase diagram of Kane and Fisher also for intermediate impurity strength. The
approach can be extended to more general one-body potentials.Comment: 10 pages, 1 figur
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