1,623 research outputs found
A model of large volumetric capacitance in graphene supercapacitors based on ion clustering
Electric double layer supercapacitors are promising devices for high-power
energy storage based on the reversible absorption of ions into porous,
conducting electrodes. Graphene is a particularly good candidate for the
electrode material in supercapacitors due to its high conductivity and large
surface area. In this paper we consider supercapacitor electrodes made from a
stack of graphene sheets with randomly-inserted "spacer" molecules. We show
that the large volumetric capacitances C > 100 F/cm^3 observed experimentally
can be understood as a result of collective intercalation of ions into the
graphene stack and the accompanying nonlinear screening by graphene electrons
that renormalizes the charge of the ion clusters.Comment: 13 pages, 5 figures; additional discussion and supporting
calculations adde
Thermoelectric effects in a strongly correlated model for NaCoO
Thermal response functions of strongly correlated electron systems are of
appreciable interest to the larger scientific community both theoretically and
technologically. Here we focus on the infinitely correlated t-J model on a
geometrically frustrated two-dimensional triangular lattice.
Using exact diagonalization on a finite sized system we calculate the
dynamical thermal response functions in order to determine the thermopower,
Lorenz number, and dimensionless figure of merit. The dynamical thermal
response functions is compared to the infinite frequency limit and shown to be
very weak functions of frequency, hence, establishing the validity of the high
frequency formalism recently proposed by Shastry for the thermopower, Lorenz
number, and the dimensionless figure of merit. Further, the thermopower is
demonstrated to have a low to mid temperature enhancement when the sign of the
hopping parameter is switched from positive to negative for the
geometrically frustrated lattice considered.Comment: 16 pages, 10 figures, color version available at
http://physics.ucsc.edu/~peterson/mrpeterson-condmat-NCO.pdf. V.2 has fixed
minor typos in Eq. 11, 19, 25, and 26. V.3 is a color versio
Coulomb drag as a measure of trigonal warping in doped graphene
I suggest to use the effect of Coulomb drag between two closely positioned
graphite monolayers (graphene sheets) for experimental measurement of the
strength of weak non-linearities of the spectrum in graphene. I consider
trigonal warping as a representative mechanism responsible for the drag effect.
Since graphene is relatively defect-free, I evaluate the drag conductivity in
the ballistic regime and find that it is proportional to the fourth power of
the warping strength.Comment: 4 pages, 1 figur
The development of a non-cryogenic nitrogen/oxygen supply system
A hydrazine/water electrolysis process system module design was fabricated and tested to demonstrate component and module performance. This module is capable of providing both the metabolic oxygen for crew needs and the oxygen and nitrogen for spacecraft leak makeup. The component designs evolved through previous R and D efforts, and were fabricated and tested individually and then were assembled into a complete module which was successfully tested for 1000 hours to demonstrate integration of the individual components. A survey was made of hydrazine sensor technology and a cell math model was derived
Impurity induced density of states and residual transport in nonunitary superconductors
We obtain general expressions for the residual density of states, electrical
conductivity and thermal conductivity for non-unitary superconductors due to
impurity scattering. We apply the results to the so-called `B phase' of
PrOs4Sb12, which we describe using a non-unitary gap function derived from
symmetry considerations. The conductivity tensor has inequivalent diagonal
components due to off-axis nodal positions which may be detectable in
experiments.Comment: 8 pages, 1 figur
Orbital Order and Spontaneous Orthorhombicity in Iron Pnictides
A growing list of experiments show orthorhombic electronic anisotropy in the
iron pnictides, in some cases at temperatures well above the spin density wave
transition. These experiments include neutron scattering, resistivity and
magnetoresistance measurements, and a variety of spectroscopies. We explore the
idea that these anisotropies stem from a common underlying cause: orbital order
manifest in an unequal occupation of and orbitals, arising
from the coupled spin-orbital degrees of freedom. We emphasize the distinction
between the total orbital occupation (the integrated density of states), where
the order parameter may be small, and the orbital polarization near the Fermi
level which can be more pronounced. We also discuss light-polarization studies
of angle-resolved photoemission, and demonstrate how x-ray absorption linear
dichroism may be used as a method to detect an orbital order parameter.Comment: Orig.: 4+ pages; Rev.: 4+ pages with updated content and reference
Theory of phonon-drag thermopower of extrinsic semiconducting single-wall carbon nanotubes and comparison with previous experimental data
A theoretical model for the calculation of the phonon-drag thermopower,
, in degenerately doped semiconducting single-wall carbon nanotubes
(SWCNTs) is proposed. Detailed calculations of are performed as a
function of temperature, tube radius and position of the Fermi level. We derive
a simple analytical expression for that can be utilized to determine
the free carrier density in doped nanotubes. At low temperatures shows
an activated behavior characteristic of the one-dimensional (1D) character of
carriers. Screening effects are taken into account and it is found that they
dramatically reduce the magnitude of . Our results are compared with
previous published experimental data in bulk p-doped SWCNT materials. Excellent
agreement is obtained in the temperature range 10-200 K for a consistent set of
parameters. This is a striking result in view of the complexity of these
systems.Comment: 21 pages, 6 figures. This version has been accepted for publication
in Phys. Rev.
Electronic properties of graphene multilayers
We study the effects of disorder in the electronic properties of graphene
multilayers, with special focus on the bilayer and the infinite stack. At low
energies and long wavelengths, the electronic self-energies and density of
states exhibit behavior with divergences near half-filling. As a consequence,
the spectral functions and conductivities do not follow Landau's Fermi liquid
theory. In particular, we show that the quasiparticle decay rate has a minimum
as a function of energy, there is a universal minimum value for the in-plane
conductivity of order e^2/h per plane and, unexpectedly, the c-axis
conductivity is enhanced by disorder at low doping, leading to an enormous
conductivity anisotropy at low temperatures.Comment: 4 pages, 4 figure. Reference to exciting new ARPES results on
graphite added (we thank A. Lanzara for sharing the paper prior to its
publication
Single polaron properties of the breathing-mode Hamiltonian
We investigate numerically various properties of the one-dimensional (1D)
breathing-mode polaron. We use an extension of a variational scheme to compute
the energies and wave-functions of the two lowest-energy eigenstates for any
momentum, as well as a scheme to compute directly the polaron Greens function.
We contrast these results with results for the 1D Holstein polaron. In
particular, we find that the crossover from a large to a small polaron is
significantly sharper. Unlike for the Holstein model, at moderate and large
couplings the breathing-mode polaron dispersion has non-monotonic dependence on
the polaron momentum k. Neither of these aspects is revealed by a previous
study based on the self-consistent Born approximation
Partly Occupied Wannier Functions
We introduce a scheme for constructing partly occupied, maximally localized
Wannier functions (WFs) for both molecular and periodic systems. Compared to
the traditional occupied WFs the partly occupied WFs posses improved symmetry
and localization properties achieved through a bonding-antibonding closing
procedure. We demonstrate the equivalence between bonding-antibonding closure
and the minimization of the average spread of the WFs in the case of a benzene
molecule and a linear chain of Pt atoms. The general applicability of the
method is demonstrated through the calculation of WFs for a metallic system
with an impurity: a Pt wire with a hydrogen molecular bridge.Comment: 5 pages, 4 figure
- …