113 research outputs found
Resistive and rectifying effects of pulling gold atoms at thiol-gold nano-contacts
We investigate, by means of first-principles calculations, structural and
transport properties of junctions made of symmetric dithiolated molecules
placed between Au electrodes. As the electrodes are pulled apart, we find that
it becomes energetically favorable that Au atoms migrate to positions between
the electrode surface and thiol terminations, with junction structures
alternating between symmetric and asymmetric. As a result, the calculated
\emph{IV} curves alternate between rectifying and non-rectifying behaviors as
the electrodes are pulled apart, which is consistent with recent experimental
results
Manganese 3×3 and √3 × √3-R30º structures and structural phase transition on w-GaN(0001̄) studied by scanning tunneling microscopy and first-principles theory
et al.Manganese deposited on the N-polar face of wurtzite gallium nitride [GaN (0001̄)] results in two unique surface reconstructions, depending on the deposition temperature. At lower temperature (less than 105ºC), it is found that a metastable 3×3 structure forms. Mild annealing of this Mn 3×3 structure leads to an irreversible phase transition to a different, much more stable √3×√3-R30º structure which can withstand high-temperature annealing. Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction data are compared with results from first-principles theoretical calculations. Theory finds a lowest-energy model for the 3×3 structure consisting of Mn trimers bonded to the Ga adlayer atoms but not with N atoms. The lowest-energy model for the more stable √3×√3-R30º structure involves Mn atoms substituting for Ga within the Ga adlayer and thus bonding with N atoms. Tersoff-Hamman simulations of the resulting lowest-energy structural models are found to be in very good agreement with the experimental STM images.Research supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317 (STM studies of nanoscale spintronic nitride systems) and by the National Science Foundation under Award No. 0730257 (advancing nanospintronics through
international collaboration). V.F. and M.A.B. would like to acknowledge support from CONICET (PIP0038) and ANPCyT (PICT1857) as well as the Ohio Supercomputing Center for computer time. P.O. was supported by Spanish MICINN (FIS2009-12721-C04-01, FIS2012-37549-C05-02 and CSD2007-00050).Peer reviewe
Magnetoresistance and Magnetic Ordering Fingerprints in Hydrogenated Graphene
Spin-dependent features in the conductivity of graphene, chemically modified
by a random distribution of hydrogen adatoms, are explored theoretically. The
spin effects are taken into account using a mean-field self-consistent Hubbard
model derived from first-principles calculations. A Kubo-Greenwood transport
methodology is used to compute the spin-dependent transport fingerprints of
weakly hydrogenated graphene-based systems with realistic sizes. Conductivity
responses are obtained for paramagnetic, antiferromagnetic, or ferromagnetic
macroscopic states, constructed from the mean-field solutions obtained for
small graphene supercells. Magnetoresistance signals up to are
calculated for hydrogen densities around 0.25%. These theoretical results could
serve as guidance for experimental observation of induced magnetism in
graphene.Comment: 4 pages, 4 figure
Colossal phonon drag enhanced thermopower in lightly doped diamond
Diamond is one of the most studied materials because of its unique
combination of remarkable electrical, mechanical, thermal and optical
properties. Using a fully self-consistent ab initio theory of coupled
electron-phonon transport, we reveal another striking behavior: a huge drag
enhancement of the thermopower of lightly doped diamond. Thermopower values of
around 100,000 microvolts per Kelvin are found at 100 K, significantly
exceeding the highest previously measured value in the correlated metal FeSb2,
and occurring at much higher temperatures. The enormous thermopower in diamond
arises primarily from exceptionally weak anharmonic phonon decay around and
below 100 K that facilitates efficient momentum exchange between charge
carriers and phonons through electron-phonon interactions. Exceedingly large
thermoelectric power factors are also identified. This work gives insights into
the physics of the coupled electron-phonon system in solids and advances our
understanding of thermoelectric transport in the regime of strong drag
Band selection and disentanglement using maximally-localized Wannier functions: the cases of Co impurities in bulk copper and the Cu (111) surface
We have adapted the maximally-localized Wannier function approach of [I.
Souza, N. Marzari and D. Vanderbilt, Phys. Rev. B 65, 035109 (2002)] to the
density functional theory based Siesta method [J. M. Soler et al., J. Phys.:
Cond. Mat. 14, 2745 (2002)] and applied it to the study of Co substitutional
impurities in bulk copper as well as to the Cu (111) surface. In the Co
impurity case, we have reduced the problem to the Co d-electrons and the Cu
sp-band, permitting us to obtain an Anderson-like Hamiltonian from well defined
density functional parameters in a fully orthonormal basis set. In order to
test the quality of the Wannier approach to surfaces, we have studied the
electronic structure of the Cu (111) surface by again transforming the density
functional problem into the Wannier representation. An excellent description of
the Shockley surface state is attained, permitting us to be confident in the
application of this method to future studies of magnetic adsorbates in the
presence of an extended surface state
Tuning the topological band gap of bismuthene with silicon-based substrates
Some metastable polymorphs of bismuth monolayers (bismuthene) can host non-trivial topological phases. However, it remains unclear whether these polymorphs can become stable through interaction with a substrate, whether their topological properties are preserved, and how to design an optimal substrate to make the topological phase more robust. Using first-principles techniques, we demonstrate that bismuthene polymorphs can become stable over silicon carbide (SiC), silicon (Si), and silicon dioxide (SiO2) and that proximity interaction in these heterostructures has a significant effect on the electronic structure of the monolayer, even when bonding is weak. We show that van der Waals interactions and the breaking of the sublattice symmetry are the main factors driving changes in the electronic structure in non-covalently binding heterostructures. Our work demonstrates that substrate interaction can strengthen the topological properties of bismuthene polymorphs and make them accessible for experimental investigations and technological applications
Dielectric screening in extended systems using the self-consistent Sternheimer equation and localized basis sets
We develop a first-principles computational method for investigating the
dielectric screening in extended systems using the self-consistent Sternheimer
equation and localized non-orthogonal basis sets. Our approach does not require
the explicit calculation of unoccupied electronic states, only uses two-center
integrals, and has a theoretical scaling of order O(N^3). We demonstrate this
method by comparing our calculations for silicon, germanium, diamond, and LiCl
with reference planewaves calculations. We show that accuracy comparable to
planewaves calculations can be achieved via a systematic optimization of the
basis set.Comment: 6 pages, 3 figure
Carbon nanotubes as substrates for molecular spiropyran-based switches
We present a joint theory–experiment study investigating the excitonic
absorption of spiropyran-functionalized carbon nanotubes. The
functionalization is promising for engineering switches on a molecular level,
since spiropyrans can be reversibly switched between two different
conformations, inducing a distinguishable and measurable change of optical
transition energies in the substrate nanotube. Here, we address the question
of whether an optical read-out of such a molecular switch is possible.
Combining density matrix and density functional theory, we first calculate the
excitonic absorption of pristine and functionalized nanotubes. Depending on
the switching state of the attached molecule, we observe a red-shift of
transition energies by about 15 meV due to the coupling of excitons with the
molecular dipole moment. Then we perform experiments measuring the absorption
spectrum of functionalized carbon nanotubes for both conformations of the
spiropyran molecule. We find good qualitative agreement between the
theoretically predicted and experimentally measured red-shift, confirming the
possibility for an optical read-out of the nanotube-based molecular switch
Density-wave instability in alpha-(BEDT-TTF)2KHg(SCN)4 studied by x-ray diffuse scattering and by first-principles calculations
α−(BEDT-TTF)2KHg(SCN)4 develops a density wave ground state below 8 K whose origin is still debated. Here we report a combined x-ray diffuse scattering and first-principles density functional theory study supporting the charge density wave (CDW) scenario. In particular, we observe a triply incommensurate anharmonic lattice modulation with intralayer wave vector components which coincide within experimental errors to the maximum of the calculated Lindhard response function. A detailed study of the structural aspects of the modulation shows that the CDW instability in α−(BEDT-TTF)2KHg(SCN)4 is considerably more involved than those following a standard Peierls mechanism. We thus propose a microscopic mechanism where the CDW instability of the BEDT-TTF layer is triggered by the anion sublattice. Our mechanism also emphasizes the key role of the coupling of the BEDT-TTF and anion layers via the hydrogen bond network to set the global modulation.Peer reviewe
Composition-dependent structural properties in ScGaN alloy films: A combined experimental and theoretical study
Experimental and theoretical results are presented regarding the incorporation of scandium into wurtzite GaN. Variation of the aa and cc lattice constants with Sc fraction in the low Sc concentration regime (0%–17%) are found that can be well explained by the predictions of first-principles theory. The calculations allow a statistical analysis of the variations of the bond lengths and bond angles as functions of Sc concentration. The results are compared to predictions from both a prior experimental study [Constantin et al., Phys. Rev. B 70, 193309 (2004)] and a prior theoretical study [Farrer and Bellaiche et al. Phys. Rev. B 66, 201203(R) (2002)]. It is found that the ScGaN lattice can be very well modeled as being wurtzitelike but with local lattice distortions arising from the incorporation of the Sc atoms. Effects of the addition of Sc on the stacking order for a large Sc fraction is also studied by high resolution transmission electron microscopy. The results show the existence of stacking faults, and induced stacking disorder. The explanation for the lattice constant variations is based on the effects of local lattice distortions and not related to the stacking faults.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87322/2/123501_1.pd
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