193 research outputs found
Structure and vibrational spectra of carbon clusters in SiC
The electronic, structural and vibrational properties of small carbon
interstitial and antisite clusters are investigated by ab initio methods in 3C
and 4H-SiC. The defects possess sizable dissociation energies and may be formed
via condensation of carbon interstitials, e.g. generated in the course of ion
implantation. All considered defect complexes possess localized vibrational
modes (LVM's) well above the SiC bulk phonon spectrum. In particular, the
compact antisite clusters exhibit high-frequency LVM's up to 250meV. The
isotope shifts resulting from a_{13}C enrichment are analyzed. In the light of
these results, the photoluminescence centers D_{II} and P-U are discussed. The
dicarbon antisite is identified as a plausible key ingredient of the
D_{II}-center, whereas the carbon split-interstitial is a likely origin of the
P-T centers. The comparison of the calculated and observed high-frequency modes
suggests that the U-center is also a carbon-antisite based defect.Comment: 15 pages, 6 figures, accepted by Phys. Rev.
Carbon antisite clusters in SiC: a possible pathway to the D_{II} center
The photoluminescence center D_{II} is a persistent intrinsic defect which is
common in all SiC polytypes. Its fingerprints are the characteristic phonon
replicas in luminescence spectra. We perform ab-initio calculations of
vibrational spectra for various defect complexes and find that carbon antisite
clusters exhibit vibrational modes in the frequency range of the D_{II}
spectrum. The clusters possess very high binding energies which guarantee their
thermal stability--a known feature of the D_{II} center. The di-carbon antisite
(C_{2})_{Si} (two carbon atoms sharing a silicon site) is an important building
block of these clusters.Comment: RevTeX 4, 6 pages, 3 figures Changes in version 2: Section headings,
footnote included in text, vibrational data now given for neutral
split-interstitial, extended discussion of the [(C_2)_Si]_2 defect incl.
figure Changes version 3: Correction of binding energy for 3rd and 4th carbon
atom at antisite; correction of typo
Infrared phonons and specific heat in Ba3Cr2O8
We report on the phonon spectrum of Ba3Cr2O8 determined by infrared
spectroscopy, and on specific heat measurements across the Jahn-Teller
transition in magnetic fields up to 9 T. Phonon modes split below the
Jahn-Teller transition, which occurs at T_{JT} = 70 K as detected by specific
heat measurements. The field-dependent specific heat data is analyzed in terms
of the contributions from lattice, magnetic and orbital degrees of freedom. In
contrast to the isostructural compound Sr3Cr2O8 our analysis does not indicate
the existence of orbital fluctuations below the Jahn-Teller transition in
Ba3Cr2O8.Comment: 5 pages, 4 figure
Boundary States in Graphene Heterojunctions
A new type of states in graphene-based planar heterojunctions has been
studied in the envelope wave function approximation. The condition for the
formation of these states is the intersection between the dispersion curves of
graphene and its gap modification. This type of states can also occur in smooth
graphene-based heterojunctions.Comment: 5 pages, 3 figure
The growth exponent for planar loop-erased random walk
We give a new proof of a result of Kenyon that the growth exponent for
loop-erased random walks in two dimensions is 5/4. The proof uses the
convergence of LERW to Schramm-Loewner evolution with parameter 2, and is valid
for irreducible bounded symmetric random walks on any two-dimensional discrete
lattice.Comment: 62 pages, 7 figures; fixed typos, added reference
Tailoring Cu Electrodes for Enhanced CO 2 Electroreduction through Plasma Electrolysis in NonâConventional PhosphorusâOxoanionâBased Electrolytes
This study presents a green, ultra-fast, and facile technique for the fabrication of micro/nano-structured and porous Cu electrodes through in-liquid plasma electrolysis using phosphorous-oxoanion-based electrolytes. Besides the preferential surface faceting, the Cu electrodes exhibit unique surface structures, including octahedral nanocrystals besides nanoporous and microporous structures, depending on the employed electrolyte. The incorporation of P-atoms into the Cu surfaces is observed. The modified Cu electrodes display increased roughness, leading to higher current densities for CO2 electroreduction reaction. The selectivity of the modified Cu electrodes towards C2 products is highest for the Cu electrodes treated in Na2HPO3 and Na3PO4 electrolytes, whereas those treated in Na2H2PO2 produce the most H2. The Cu electrode treated in Na3PO4 produces ethylene (23â%) at â1.1â
V vs. RHE, and a comparable amount of acetaldehyde (15â%) that is typically observed for Cu(110) single crystals. The enhanced selectivity is attributed to several factors, including the surface morphology, the incorporation of phosphorus into the Cu structure, and the formation of Cu(110) facets. Our results not only advance our understanding of the influence of the electrolyte\u27s nature on the plasma electrolysis of Cu electrodes, but also underscores the potential of in-liquid plasma treatment for developing efficient Cu electrocatalysts for sustainable CO2 conversion
Angle-resolved photoemission spectra of graphene from first-principles calculations
Angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental
technique for directly probing electron dynamics in solids. The energy vs.
momentum dispersion relations and the associated spectral broadenings measured
by ARPES provide a wealth of information on quantum many-body interaction
effects. In particular, ARPES allows studies of the Coulomb interaction among
electrons (electron-electron interactions) and the interaction between
electrons and lattice vibrations (electron-phonon interactions). Here, we
report ab initio simulations of the ARPES spectra of graphene including both
electron-electron and electron-phonon interactions on the same footing. Our
calculations reproduce some of the key experimental observations related to
many-body effects, including the indication of a mismatch between the upper and
lower halves of the Dirac cone
Structure of the silicon vacancy in 6H-SiC after annealing identified as the carbon vacancyâcarbon antisite pair
We investigated radiation-induced defects in neutron-irradiated and subsequently annealed 6H-silicon carbide (SiC) with electron paramagnetic resonance (EPR), the magnetic circular dichroism of the absorption (MCDA), and MCDA-detected EPR (MCDA-EPR). In samples annealed beyond the annealing temperature of the isolated silicon vacancy we observed photoinduced EPR spectra of spin S=1 centers that occur in orientations expected for nearest neighbor pair defects. EPR spectra of the defect on the three inequivalent lattice sites were resolved and attributed to optical transitions between photon energies of 999 and 1075 meV by MCDA-EPR. The resolved hyperfine structure indicates the presence of one single carbon nucleus and several silicon ligand nuclei. These experimental findings are interpreted with help of total energy and spin density data obtained from the standard local-spin density approximation of the density-functional theory, using relaxed defect geometries obtained from the self-consistent charge density-functional theory based tight binding scheme. We have checked several defect models of which only the photoexcited spin triplet state of the carbon antisiteâcarbon vacancy pair (CSi-VC) in the doubly positive charge state can explain all experimental findings. We propose that the (CSi-VC) defect is formed from the isolated silicon vacancy as an annealing product by the movement of a carbon neighbor into the vacancy
Symmetry Breaking in Few Layer Graphene Films
Recently, it was demonstrated that the quasiparticle dynamics, the
layer-dependent charge and potential, and the c-axis screening coefficient
could be extracted from measurements of the spectral function of few layer
graphene films grown epitaxially on SiC using angle-resolved photoemission
spectroscopy (ARPES). In this article we review these findings, and present
detailed methodology for extracting such parameters from ARPES. We also present
detailed arguments against the possibility of an energy gap at the Dirac
crossing ED.Comment: 23 pages, 13 figures, Conference Proceedings of DPG Meeting Mar 2007
Regensburg Submitted to New Journal of Physic
Superlattice Based on Graphene on a Strip Substrate
A graphene-based superlattice formed due to the periodic modulation of the
band gap has been investigated. Such a modulation is possible in graphene
deposited on a strip substrate made of silicon oxide and hexagonal boron
nitride. The advantages and some possible problems in the superlattice under
consideration are discussed. A model describing such a superlattice is proposed
and the dispersion relation between the energy and momentum of carriers has
been obtained using the transfer matrix method within this model.Comment: 6 pages, 3 figure
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