153 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.
Singlet-triplet dispersion reveals additional frustration in the triangular dimer compound BaMnO
We present single crystal inelastic neutron scattering measurements of the
S=1 dimerized quasi-two-dimensional antiferromagnet BaMnO. The
singlet-triplet dispersion reveals nearest-neighbor and next-nearest-neighbor
ferromagnetic interactions between adjacent bilayers that compete against each
other. Although the inter-bilayer exchange is comparable to the intra-bilayer
exchange, this additional frustration reduces the effective coupling along the
c-axis and leads to a quasi-two dimensional behavior. In addition, the obtained
exchange values are able to reproduce the four critical fields in the phase
diagram.Comment: 4 pages, 3 color figures, submitted to an APS physical review journa
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
Tailoring Cu Electrodes for Enhanced CO 2 Electroreduction through Plasma Electrolysis in Non‐Conventional Phosphorus‐Oxoanion‐Based 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 (23 %) at −1.1 V vs. RHE, and a comparable amount of acetaldehyde (15 %) 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 vacancy–carbon 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 antisite–carbon 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
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic
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