Selecting between two transition states by which water oxidation intermediates on an oxide surface decay

While catalytic mechanisms on electrode surfaces have been proposed for decades, the pathways by which the product's chemical bonds evolve from the initial charge-trapping intermediates have not been resolved in time. Here, we discover a reactive population of charge-trapping intermediates with states in the middle of a semiconductor's band-gap to reveal the dynamics of two parallel transition state pathways for their decay. Upon photo-triggering the water oxidation reaction from the n-SrTiO3 surface with band-gap, pulsed excitation, the intermediates' microsecond decay reflects transition state theory (TST) through: (1) two distinct and reaction dependent (pH, T, Ionic Strength, and H/D exchange) time constants, (2) a primary kinetic salt effect on each activation barrier and an H/D kinetic isotope effect on one, and (3) realistic activation barrier heights (0.4 - 0.5 eV) and TST pre-factors (10^11 - 10^12 Hz). A photoluminescence from midgap states in n-SrTiO3 reveals the reaction dependent decay; the same spectrum was previously assigned by us to hole-trapping at parallel Ti-O(dot)-Ti (bridge) and perpendicular Ti-O(dot) (oxyl) O-sites using in situ ultrafast vibrational and optical spectroscopy. Therefore, the two transition states are naturally associated with the decay of these respective intermediates. Furthermore, we show that reaction conditions select between the two pathways, one of which reflects a labile intermediate facing the electrolyte (the oxyl) and the other a lattice oxygen (the bridge). Altogether, we experimentally isolate an important activation barrier for water oxidation, which is necessary for designing water oxidation catalysts with high O2 turn over. Moreover, in isolating it, we identify competing mechanisms for O2 evolution at surfaces and show how to use reaction conditions to select between them

Signatures of pressure induced superconductivity in insulating Bi2212

We have performed several high pressure electrical resistance experiments on Bi1.98Sr2.06Y0.68Cu2O8, an insulating parent compound of the high-Tc Bi2212 family of copper oxide superconductors. We find a resistive anomaly, a downturn at low temperature, that onsets with applied pressure in the 20-40 kbar range. Through both resistance and magnetoresistance measurements, we identify this anomaly as a signature of induced superconductivity. Resistance to higher pressures decreases Tc, giving a maximum of 10 K. The higher pressure measurements exhibit a strong sensitivity to the hydrostaticity of the pressure environment. We make comparisons to the pressure induced superconductivity now ubiquitous in the iron arsenides.Comment: 5 pages, 4 figures, submitted to Phys. Rev.

Single polaron properties of the breathing-mode Hamiltonian

We investigate numerically various properties of the one-dimensional (1D) breathing-mode polaron. We use an extension of a variational scheme to compute the energies and wave-functions of the two lowest-energy eigenstates for any momentum, as well as a scheme to compute directly the polaron Greens function. We contrast these results with results for the 1D Holstein polaron. In particular, we find that the crossover from a large to a small polaron is significantly sharper. Unlike for the Holstein model, at moderate and large couplings the breathing-mode polaron dispersion has non-monotonic dependence on the polaron momentum k. Neither of these aspects is revealed by a previous study based on the self-consistent Born approximation

ARPES kink is a "smoking gun" for the theory of high-Tc superconductors: dominance of the electron-phonon interaction with forward scattering peak

The ARPES spectra in high-Tc superconductors show four distinctive features in the quasiparticle self-energy. All of them can be explained consistently by the theory in which the electron phonon interaction (EPI) with the forward scattering peak dominates over the Coulomb scattering. In particular, this theory explains why there is no shift of the nodal kink at 70 meV in the superconducting state, contrary to the clear shift of the anti-nodal singularity at 40 meV. The theory predicts a ``knee''-like structure of the imaginary part of the self-energy, which is phonon dominated for $\omega \approx \omega^{(70)}_{ph}$, and shows linear behavior for $\omega > \omega^{(70)}_{ph}$ - due to the Coulomb scattering. Recent ARPES spectra give that the EPI coupling constant is much larger than the Coulomb one. The dip-hump structure in the spectral function comes out naturally from the proposed theory.Comment: 5 pages, 3 figure

Momentum average approximation for models with electron-phonon coupling dependent on the phonon momentum

We generalize the momentum average (MA) approximation to study the properties of models with momentum-dependent electron-phonon coupling. As in the case of the application of the original MA to the Holstein model, the results are analytical, numerically trivial to evaluate, exact for both zero bandwidth and for zero electron-phonon coupling, and are accurate everywhere in parameter space. Comparison with available numerical data confirms this accuracy. We then show that further improvements can be obtained based on variational considerations, using the one-dimensional breathing-mode Hamiltonian as a specific example. For example, by using this variational MA, we obtain ground state energies within at most 0.3% error of the numerical data.Comment: 15 pages, 10 figure

Tidal Evolution of the Earth–Moon System with a High Initial Obliquity

A giant impact origin for the Moon is generally accepted, but many aspects of lunar formation remain poorly understood and debated. \'Cuk et al. (2016) proposed that an impact that left the Earth-Moon system with high obliquity and angular momentum could explain the Moon's orbital inclination and isotopic similarity to Earth. In this scenario, instability during the Laplace Plane transition, when the Moon's orbit transitions from the gravitational influence of Earth's figure to that of the Sun, would both lower the system's angular momentum to its present-day value and generate the Moon's orbital inclination. Recently, Tian and Wisdom (2020) discovered new dynamical constraints on the Laplace Plane transition and concluded that the Earth-Moon system could not have evolved from an initial state with high obliquity. Here we demonstrate that the Earth-Moon system with an initially high obliquity can evolve into the present state, and we identify a spin-orbit secular resonance as a key dynamical mechanism in the later stages of the Laplace Plane transition. Some of the simulations by Tian and Wisdom (2020) did not encounter this late secular resonance, as their model suppressed obliquity tides and the resulting inclination damping. Our results demonstrate that a giant impact that left Earth with high angular momentum and high obliquity ($\theta > 61^{\circ}$) is a promising scenario for explaining many properties of the Earth-Moon system, including its angular momentum and obliquity, the geochemistry of Earth and the Moon, and the lunar inclination.Comment: Accepted for the Planetary Science Journa

Uncovering a pressure-tuned electronic transition in BiSrYCu2O8 using Raman scattering and x-ray diffraction

We report pressure tuned Raman and x-ray diffraction data of Bi1.98Sr2.06Y0.68Cu2O8 revealing a critical pressure at 21 GPa with anomalies in six physical quantities: electronic Raman background, electron-phonon coupling, spectral weight transfer from high to low frequency, density dependent behaviour of phonon and magnon frequencies, and a compressibility change in the c-axis. For the first time in a cuprate, mobile charge carriers, lattice, and magnetism all show anomalies at a distinct critical pressure in the same experimental setting. Furthermore, the Raman spectral changes are similar to that seen traversing the superconducting dome with doping, suggesting that the critical pressure at 21 GPa is related to the much discussed critical point at optimal doping.Comment: 5 pages, 4 figures, submitted to PR