Localized coherent phonon generation in monolayer MoSe2 from ultrafast exciton trapping at shallow traps.

We report spectroscopic evidence for the ultrafast trapping of band edge excitons at defects and the subsequent generation of defect-localized coherent phonons (CPs) in monolayer MoSe2. While the photoluminescence measurement provides signals of exciton recombination at both shallow and deep traps, our time-resolved pump-probe spectroscopy on the sub-picosecond time scale detects localized CPs only from the ultrafast exciton trapping at shallow traps. Based on occupation-constrained density functional calculations, we identify the Se vacancy and the oxygen molecule adsorbed on a Se vacancy as the atomistic origins of deep and shallow traps, respectively. Establishing the correlations between the defect-induced ultrafast exciton trapping and the generation of defect-localized CPs, our work could open up new avenues to engineer photoexcited carriers through lattice defects in two-dimensional materials.

K-point longitudinal acoustic phonons are responsible for ultrafast intervalley scattering in monolayer MoSe2.

In transition metal dichalcogenides, valley depolarization through intervalley carrier scattering by zone-edge phonons is often unavoidable. Although valley depolarization processes related to various acoustic phonons have been suggested, their optical verification is still vague due to nearly degenerate phonon frequencies on acoustic phonon branches at zone-edge momentums. Here we report an unambiguous phonon momentum determination of the longitudinal acoustic (LA) phonons at the K point, which are responsible for the ultrafast valley depolarization in monolayer MoSe2. Using sub-10-fs-resolution pump-probe spectroscopy, we observed coherent phonons signals at both even and odd-orders of zone-edge LA mode involved in intervalley carrier scattering process. Our phonon-symmetry analysis and first-principles calculations reveal that only the LA phonon at the K point, as opposed to the M point, can produce experimental odd-order LA phonon signals from its nonlinear optical modulation. This work will provide momentum-resolved descriptions of phonon-carrier intervalley scattering processes in valleytronic materials.

Reply to Correspondence on "Core Electron Topologies in Chemical Compounds: Case Study of Carbon versus Silicon".

In their Correspondence, von Szentpály, Schwarz, Stoll, and Werner claim that the main conclusions of our Communication previously published in this journal are based on computational artifacts and oversimplified models. We clarify the justification of our simple one-electron model to describe one-electron physics, and refute their criticism based on what they call "computational artifacts." We remind that our main conclusion on the crucial role of qualitative changes in core electron wavefunctions is evidenced not only by wavefunction topologies the complainants cling to, but also by several other physical observables, which remain unrefuted. Hence, the conclusions of our original Communication stand.

Core Electron Topologies in Chemical Compounds: Case Study of Carbon versus Silicon.

The similarities and differences between carbon and silicon have attracted the curiosity of chemists for centuries. Similarities and analogies can be found in their saturated compounds, but carbon exhibits a cornucopia of unsaturated compounds that silicon (and most other elements) cannot replicate. While this qualitative difference is empirically well known, quantum chemistry has previously only described quantitative differences related to orbital overlap, steric effects, or orbital energies. We study C2 and Si2 and their hydrides X2 H2n (X=C, Si; n=1, 2, 3) by first-principles quantum chemical calculation, and find a qualitative difference in the topologies of the core electrons: carbon has the propensity to alter its core electron topology when forming unsaturated compounds, and silicon has not. We draw a connection between the core electron topologies and ionization energies, and identify other elements we expect to have similarly flexible core topologies as carbon.

Positron Binding Properties of Glycine and Its Aqueous Complexes.

We investigate positron binding to glycine and its aqueous complexes by first-principles calculation. We show that while glycine in its ground state (Gly) does not bind positrons, several of its strongly polar conformers do, and in particular, its zwitterion form (GlyZI) binds positrons strongly. Aqueous complexes Gly·nH2O and GlyZI·nH2O also bind positrons, if their dipole moment µ > µcr. However, µ is not a sufficient quantity to describe positron binding to these complexes. We show that in addition to µ, positron binding strongly depends on the intramolecular bonding of glycine. In Gly·nH2O, positrons are weakly bound to the nitrogen in Gly, whereas in GlyZI·nH2O, the ionic oxygen in GlyZI is a strong "positron attractor".

Term rules for simple metal clusters.

Hund's term rules are only valid for isolated atoms, but have no generalization for molecules or clusters of several atoms. We present a benchmark calculation of Al2 and Al3, for which we find the high and low-spin ground states (3)Πu and , respectively. We show that the relative stabilities of all the molecular terms of Al2 and Al3 can be described by simple rules pertaining to bonding structures and symmetries, which serve as guiding principles to determine ground state terms of arbitrary multi-atom clusters.

Control of ferromagnetism via electron doping in In2O3:Cr.

Carrier-induced ferromagnetism in wide-gap transparent conductive oxides has been widely discussed and debated, leading to confusion and skepticism regarding whether dilute magnetic oxides exist at all. We show from density-functional calculations within a band-gap corrected approach that ferromagnetic Cr-Cr coupling can be switched on and off via electron doping in the wide-gap transparent n-type conductive oxide In2O3. We show that (i) Cr does not produce in In2O3 any free electrons and renders the system an insulating paramagnet. (ii) Extrinsic n-type doping of In2O3:Cr via Sn produces free electrons, whose concentration is controllable via the oxygen partial pressure. Such additional carriers stabilize a strong long-range Cr-Cr ferromagnetic coupling.

Charge self-regulation upon changing the oxidation state of transition metals in insulators.

Transition-metal atoms embedded in an ionic or semiconducting crystal can exist in various oxidation states that have distinct signatures in X-ray photoemission spectroscopy and 'ionic radii' which vary with the oxidation state of the atom. These oxidation states are often tacitly associated with a physical ionization of the transition-metal atoms--that is, a literal transfer of charge to or from the atoms. Physical models have been founded on this charge-transfer paradigm, but first-principles quantum mechanical calculations show only negligible changes in the local transition-metal charge as the oxidation state is altered. Here we explain this peculiar tendency of transition-metal atoms to maintain a constant local charge under external perturbations in terms of an inherent, homeostasis-like negative feedback. We show that signatures of oxidation states and multivalence--such as X-ray photoemission core-level shifts, ionic radii and variations in local magnetization--that have often been interpreted as literal charge transfer are instead a consequence of the negative-feedback charge regulation.

Impurity clustering and ferromagnetic interactions that are not carrier induced in dilute magnetic semiconductors: the case of Cu2O:Co.

Current models for ferromagnetism in diluted magnetic semiconductors, such as "p-d exchange" or "double-exchange", rely on the presence of partially filled gap states. We point out a new mechanism, not requiring partially filled states, in which ferromagnetic coupling arises from the occupation of previously unoccupied levels when two transition metal impurities form a close pair. We find from first-principles calculations that this mechanism explains strong ferromagnetic coupling between Co impurities in Cu2O, and at the same time gives rise to Co clustering.