62 research outputs found
Real-time observation of epitaxial graphene domain reorientation.
Graphene films grown by vapour deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, that is, 'ripening'; domain boundary motion within islands; and continuous lattice rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1 meV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moiré corrugation of the graphene sheet due to local variations in the graphene-substrate interaction reproduces the results. This work suggests new strategies for improving the van der Waals epitaxy of 2D materials
Thermionic Emission as a tool to study transport in undoped nFinFETs
Thermally activated sub-threshold transport has been investigated in undoped
triple gate MOSFETs. The evolution of the barrier height and of the active
cross-section area of the channel as a function of gate voltage has been
determined. The results of our experiments and of the Tight Binding simulations
we have developed are both in good agreement with previous analytical
calculations, confirming the validity of thermionic approach to investigate
transport in FETs. This method provides an important tool for the improvement
of devices characteristics.Comment: 3 pages, 3 figure, 1 tabl
Inverted orbital polarization in strained correlated oxide films
Manipulating the orbital occupation of valence electrons via epitaxial strain
in an effort to induce new functional properties requires considerations of how
changes in the local bonding environment affect the band structure at the Fermi
level. Using synchrotron radiation to measure the x-ray linear dichroism of
epitaxially strained films of the correlated oxide CaFeO3, we demonstrate that
the orbital polarization of the Fe valence electrons is opposite from
conventional understanding. Although the energetic ordering of the Fe 3d
orbitals is confirmed by multiplet ligand field theory analysis to be
consistent with previously reported strain-induced behavior, we find that the
nominally higher energy orbital is more populated than the lower. We ascribe
this inverted orbital polarization to an anisotropic bandwidth response to
strain in a compound with nearly filled bands. These findings provide an
important counterexample to the traditional understanding of strain-induced
orbital polarization and reveal a new method to engineer otherwise unachievable
orbital occupations in correlated oxides
Interface trap density metrology from sub-threshold transport in highly scaled undoped Si n-FinFETs
Channel conductance measurements can be used as a tool to study thermally
activated electron transport in the sub-threshold region of state-of-art
FinFETs. Together with theoretical Tight-Binding (TB) calculations, this
technique can be used to understand the evolution of source-to-channel barrier
height (Eb) and of active channel area (S) with gate bias (Vgs). The
quantitative difference between experimental and theoretical values that we
observe can be attributed to the interface traps present in these FinFETs.
Therefore, based on the difference between measured and calculated values of
(i) S and (ii) |dEb/dVgs| (channel to gate coupling), two new methods of
interface trap density (Dit) metrology are outlined. These two methods are
shown to be very consistent and reliable, thereby opening new ways of analyzing
in situ state-of-the-art multi-gate FETs down to the few nm width limit.
Furthermore, theoretical investigation of the spatial current density reveal
volume inversion in thinner FinFETs near the threshold voltage.Comment: 12 figures, 13 pages, Submitted to Journal of Applied Physic
Electronic structure of negative charge transfer CaFeO3 across the metal-insulator transition
We investigated the metal-insulator transition for epitaxial thin films of
the perovskite CaFeO3, a material with a significant oxygen ligand hole
contribution to its electronic structure. We find that biaxial tensile and
compressive strain suppress the metal-insulator transition temperature. By
combining hard X-ray photoelectron spectroscopy, soft X-ray absorption
spectroscopy, and density functional calculations, we resolve the
element-specific changes to the electronic structure across the metal-insulator
transition. We demonstrate that the Fe electron valence undergoes no observable
change between the metallic and insulating states, whereas the O electronic
configuration undergoes significant changes. This strongly supports the
bond-disproportionation model of the metal-insulator transition for CaFeO3 and
highlights the importance of ligand holes in its electronic structure. By
sensitively measuring the ligand hole density, however, we find that it
increases by ~5-10% in the insulating state, which we ascribe to a further
localization of electron charge on the Fe sites. These results provide detailed
insight into the metal-insulator transition of negative charge transfer
compounds and should prove instructive for understanding metal-insulator
transitions in other late transition metal compounds such as the nickelates.Comment: Minor typographic changes mad
Measurement of the Eta Production in Proton Proton Collisions with the COSY Time of Flight Spectrometer
The reaction pp -> pp eta was measured at excess energies of 15 and 41 MeV at
an external target of the Juelich Cooler Synchrotron COSY with the Time of
Flight Spectrometer. About 25000 events were measured for the excess energy of
15 MeV and about 8000 for 41 MeV. Both protons of the process pp eta were
detected with an acceptance of nearly 100% and the eta was reconstructed by the
missing mass technique. For both excess energies the angular distributions are
found to be nearly isotropic. In the invariant mass distributions strong
deviations from the pure phase space distributions are seen.Comment: 15 pages, 14 figures, 4 table
Distinguishing electronic contributions of surface and sub-surface transition metal atoms in Ti-based MXenes
MXenes are a rapidly-expanding family of 2D transition metal carbides and nitrides that have attracted attention due to their excellent performance in applications ranging from energy storage to electromagnetic interference shielding. Numerous other electronic and magnetic properties have been computationally predicted, but not yet realized due to the experimental difficulty in obtaining uniform surface terminations (Tx), necessitating new design approaches for MXenes that are independent of surface terminations. In this study, we distinguished the contributions of surface and sub-surface Ti atoms to the electronic structure of four Ti-containing MXenes (Ti2CTx, Ti3C2Tx, Cr2TiC2Tx, and Mo2TiC2Tx) using soft x-ray absorption spectroscopy. For MXenes with no Ti atoms on the surface transition metal layers, such as Mo2TiC2Tx and Cr2TiC2Tx, our results show minimal changes in the spectral features between the parent MAX phase and its MXene. In contrast, for MXenes with surface Ti atoms, here Ti3C2Tx and Ti2CTx, the Ti L-edge spectra are significantly modified compared to their parent MAX phase compounds. First principles calculations provide similar trends in the partial density of states derived from surface and sub-surface Ti atoms, corroborating the spectroscopic measurements. These results reveal that electronic states derived from sub-surface M-site layers are largely unperturbed by the surface terminations, indicating a relatively short length scale over which the Tx terminations alter the nominal electron count associated with Ti atoms and suggesting that desired band features should be hosted by sub-surface M-sites that are electronically more robust than their surface M-site counterparts
Unemployment and Smoking: Causation, Selection, or Common Cause? - Evidence from Longitudinal Data
Machine learning uncovers the most robust self-report predictors of relationship quality across 43 longitudinal couples studies
Given the powerful implications of relationship quality for health and well-being, a central mission of relationship science is explaining why some romantic relationships thrive more than others. This large-scale project used machine learning (i.e., Random Forests) to 1) quantify the extent to which relationship quality is predictable and 2) identify which constructs reliably predict relationship quality. Across 43 dyadic longitudinal datasets from 29 laboratories, the top relationship-specific predictors of relationship quality were perceived-partner commitment, appreciation, sexual satisfaction, perceived-partner satisfaction, and conflict. The top individual-difference predictors were life satisfaction, negative affect, depression, attachment avoidance, and attachment anxiety. Overall, relationship-specific variables predicted up to 45% of variance at baseline, and up to 18% of variance at the end of each study. Individual differences also performed well (21% and 12%, respectively). Actor-reported variables (i.e., own relationship-specific and individual-difference variables) predicted two to four times more variance than partner-reported variables (i.e., the partner’s ratings on those variables). Importantly, individual differences and partner reports had no predictive effects beyond actor-reported relationship-specific variables alone. These findings imply that the sum of all individual differences and partner experiences exert their influence on relationship quality via a person’s own relationship-specific experiences, and effects due to moderation by individual differences and moderation by partner-reports may be quite small. Finally, relationship-quality change (i.e., increases or decreases in relationship quality over the course of a study) was largely unpredictable from any combination of self-report variables. This collective effort should guide future models of relationships
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