1,609 research outputs found
Two-gap superconductivity in heavily n-doped graphene: ab initio Migdal-Eliashberg theory
Graphene is the only member of the carbon family from zero- to
three-dimensional materials for which superconductivity has not been observed
yet. At this time, it is not clear whether the quest for superconducting
graphene is hindered by technical challenges, or else by the fluctuation of the
order parameter in two dimensions. In this area, ab initio calculations are
useful to guide experimental efforts by narrowing down the search space. In
this spirit, we investigate from first principles the possibility of inducing
superconductivity in doped graphene using the fully anisotropic
Migdal-Eliashberg theory powered by Wannier-Fourier interpolation. To address a
best-case scenario, we consider both electron and hole doping at high carrier
densities, so as to align the Fermi level to a van Hove singularity. In these
conditions, we find superconducting gaps of -wave symmetry, with a slight
anisotropy induced by the trigonal warping, and, in the case of -doped
graphene, an unexpected two-gap structure reminiscent of MgB. Our
Migdal-Eliashberg calculations suggest that the observation of
superconductivity at low temperature should be possible for -doped graphene
at carrier densities exceeding cm
Towards predictive many-body calculations of phonon-limited carrier mobilities in semiconductors
We probe the accuracy limit of {\it ab initio} calculations of carrier
mobilities in semiconductors, within the framework of the Boltzmann transport
equation. By focusing on the paradigmatic case of silicon, we show that fully
predictive calculations of electron and hole mobilities require many-body
quasiparticle corrections to band structures and electron-phonon matrix
elements, the inclusion of spin-orbit coupling, and an extremely fine sampling
of inelastic scattering processes in momentum space. By considering all these
factors we obtain excellent agreement with experiment, and we identify the band
effective masses as the most critical parameters to achieve predictive
accuracy. Our findings set a blueprint for future calculations of carrier
mobilities, and pave the way to engineering transport properties in
semiconductors by design.Comment: 11 pages and 8 figure
GW quasiparticle band structures of stibnite, antimonselite, bismuthinite, and guanajuatite
We present first-principles calculations of the quasiparticle band structures
of four isostructural semiconducting metal chalcogenides AB (with A =
Sb, Bi and B = S, Se) of the stibnite family within the GW approach. We
perform extensive convergence tests and identify a sensitivity of the
quasiparticle corrections to the structural parameters and to the semicore
electrons. Our calculations indicate that all four chalcogenides exhibit direct
band gaps, if we exclude some indirect transitions marginally below the direct
gap. Relativistic spin-orbit effects are evaluated for the Kohn-Sham band
structures, and included as scissor corrections in the quasiparticle band gaps.
Our calculated band gaps are 1.5 eV (SbS), 1.3 eV (SbSe), 1.4
eV (BiS) and 0.9 eV (BiSe). By comparing our calculated gaps
with the ideal Shockley-Queisser value we find that all four chalcogenides are
promising as light sensitizers for nanostructured photovoltaics.Comment: 11 pages, 5 figures. Revised manuscript - includes spin-orbit
interactio
Groups and the Entropy Floor- XMM-Newton Observations of Two Groups
Using XMM-Newton spatially resolved X-ray imaging spectroscopy we obtain the
temperature, density, entropy, gas mass, and total mass profiles for two groups
of galaxies out to ~0.3 Rvir (Rvir, the virial radius). Our density profiles
agree well with those derived previously, and the temperature data are broadly
consistent with previous results but are considerably more precise. Both of
these groups are at the mass scale of 2x10^13 Msolar but have rather different
properties. They have considerably lower gas mass fractions at r<0.3 Rvir than
the rich clusters. NGC2563, one of the least luminous groups for its X-ray
temperature, has a very low gas mass fraction of ~0.004 inside 0.1 Rvir, which
rises with radius. NGC4325, one of the most luminous groups at the same average
temperature, has a higher gas mass fraction of 0.02. The entropy profiles and
the absolute values of the entropy as a function of virial radius also differ,
with NGC4325 having a value of ~100 keV cm-2 and NGC2563 a value of ~300 keV
cm-2 at r~0.1 Rvir. For both groups the profiles rise monotonically with radius
and there is no sign of an entropy "floor". These results are inconsistent with
pre-heating scenarios which have been developed to explain the entropy floor in
groups but are broadly consistent with models of structure formation which
include the effects of heating and/or the cooling of the gas. The total entropy
in these systems provides a strong constraint on all models of galaxy and group
formation, and on the poorly defined feedback process which controls the
transformation of gas into stars and thus the formation of structure in the
universe.Comment: 22 pages, 2 figure
Steric engineering of metal-halide perovskites with tunable optical band gaps
Owing to their high energy-conversion efficiency and inexpensive fabrication
routes, solar cells based on metal-organic halide perovskites have rapidly
gained prominence as a disruptive technology. An attractive feature of
perovskite absorbers is the possibility of tailoring their properties by
changing the elemental composition through the chemical precursors. In this
context, rational in silico design represents a powerful tool for mapping the
vast materials landscape and accelerating discovery. Here we show that the
optical band gap of metal-halide perovskites, a key design parameter for solar
cells, strongly correlates with a simple structural feature, the largest
metal-halide-metal bond angle. Using this descriptor we suggest continuous
tunability of the optical gap from the mid-infrared to the visible. Precise
band gap engineering is achieved by controlling the bond angles through the
steric size of the molecular cation. Based on these design principles we
predict novel low-gap perovskites for optimum photovoltaic efficiency, and we
demonstrate the concept of band gap modulation by synthesising and
characterising novel mixed-cation perovskites.Comment: This manuscript was submitted for publication on March 6th, 2014.
Many of the results presented in this manuscript were presented at the
International Conference on Solution processed Semiconductor Solar Cells,
held in Oxford, UK, on 10-12 September 2014. The manuscript is 37 pages long
and contains 8 figure
Origin of superconductivity and latent charge density wave in NbS
We elucidate the origin of the phonon-mediated superconductivity in
2-NbS using the ab initio anisotropic Migdal-Eliashberg theory including
Coulomb interactions. We demonstrate that superconductivity is associated with
Fermi surface hot spots exhibiting an unusually strong electron-phonon
interaction. The electron-lattice coupling is dominated by low-energy
anharmonic phonons, which place the system on the verge of a charge density
wave instability. We also provide definitive evidence for two-gap
superconductivity in 2-NbS, and show that the low- and high-energy peaks
observed in tunneling spectra correspond to the - and -centered
Fermi surface pockets, respectively. The present findings call for further
efforts to determine whether our proposed mechanism underpins superconductivity
in the whole family of metallic transition metal dichalcogenides.Comment: 6 pages, 5 figures and Supplemental Materia
Band Offsets at the Si/SiO Interface from Many-Body Perturbation Theory
We use many-body perturbation theory, the state-of-the-art method for band
gap calculations, to compute the band offsets at the Si/SiO interface. We
examine the adequacy of the usual approximations in this context. We show that
(i) the separate treatment of band-structure and potential lineup
contributions, the latter being evaluated within density-functional theory, is
justified, (ii) most plasmon-pole models lead to inaccuracies in the absolute
quasiparticle corrections, (iii) vertex corrections can be neglected, (iv)
eigenenergy self-consistency is adequate. Our theoretical offsets agree with
the experimental ones within 0.3 eV
Chandra Observations of ULIRGs: Extended Hot Gas Halos in Merging Galaxies
We study the properties of hot gaseous halos in 10 nearby ultraluminous IRAS
galaxies observed with the ACIS instrument on board Chandra. For all sample
galaxies, diffuse soft X-ray emissions are found within ~10 kpc of the central
region; their spectra are well fitted by a MEKAL model plus emission lines from
alpha-elements and other ions. The temperature of the hot gas is about 0.7 keV
and metallicity is about 1 solar. Outside the central region, extended hot
gaseous halos are found for nine out of the ten ULIRGs. Most spectra of these
extended halos can be fitted with a MEKAL model with a temperature of about 0.6
keV and a low metallicity (~ 0.1 solar). We discuss the implications of our
results on the origin of X-ray halos in elliptical galaxies and the feedback
processes associated with starbursts.Comment: 31 pages, 6 figuers, ApJ in press, accepted versio
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