SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Atomic physics

149

How photoexcitations evolve into Coulomb-bound electron and hole pairs, called excitons, and unbound charge carriers is a key cross-cutting issue in photovoltaics and optoelectronics. Until now, the initial quantum dynamics following photoexcitation remains elusive in the hybrid perovskite system. Here we reveal excitonic Rydberg states with distinct formation pathways by observing the multiple resonant, internal quantum transitions using ultrafast terahertz quasi-particle transport. Nonequilibrium emergent states evolve with a complex co-existence of excitons, carriers and phonons, where a delayed buildup of excitons under on- and off-resonant pumping conditions allows us to distinguish between the loss of electronic coherence and hot state cooling processes. The nearly ∼1 ps dephasing time, efficient electron scattering with discrete terahertz phonons and intermediate binding energy of ∼13.5 meV in perovskites are distinct from conventional photovoltaic semiconductors. In addition to providing implications for coherent energy conversion, these are potentially relevant to the development of light-harvesting and electron-transport devices.

Concepts: Electron, Photon, Energy, Fundamental physics concepts, Particle physics, Condensed matter physics, Semiconductor, Atomic physics

8

As well known, Boltzmann-Gibbs statistics is the correct way of thermostatistically approaching ergodic systems. On the other hand, nontrivial ergodicity breakdown and strong correlations typically drag the system into out-of-equilibrium states where Boltzmann-Gibbs statistics fails. For a wide class of such systems, it has been shown in recent years that the correct approach is to use Tsallis statistics instead. Here we show how the dynamics of the paradigmatic conservative (area-preserving) stan-dard map exhibits, in an exceptionally clear manner, the crossing from one statistics to the other. Our results unambiguously illustrate the domains of validity of both Boltzmann-Gibbs and Tsallis statistical distributions. Since various important physical systems from particle confinement in magnetic traps to autoionization of molecular Rydberg states, through particle dynamics in accelerators and comet dynamics, can be reduced to the standard map, our results are expected to enlighten and enable an improved interpretation of diverse experimental and observational results.

Concepts: Scientific method, Statistics, Chaos theory, Probability theory, Atomic physics, Ergodic theory, Invariant measure, Measure-preserving dynamical system

5

The gas-phase optical spectra of three silicon-terminated carbon chain radicals, SiCnH (n = 3 - 5), formed in a jet-cooled discharge of silane and acetylene, have been investigated by resonant two-color two-photon ionization and laser-induced fluorescence/dispersed fluorescence. Analysis of the spectra was facilitated by calculations performed using equation-of-motion coupled cluster methods. For SiC3H and SiC5H, the observed transitions are well-described as excitations from a (2)Π ground state to a (2)Σ state, in which vibronic coupling, likely involving a higher-lying Π state with a very large predicted f-value (close to unity), is persistent. The lowest (2)Σ states of both species are characterized by a rare silicon triple bond, which was identified previously [T. C. Smith, H. Y. Li, D. J. Clouthier, C. T. Kingston, and A. J. Merer, J. Chem. Phys. 112, 3662 (2000)] in the lowest (2)Σ state of SiCH. Although a strong Π - Π transition is predicted for SiC4H, the observed spectrum near 505 nm more likely corresponds to excitation to a relatively dark Σ state which is vibronically coupled to a nearby Π state. In contrast to the chains with an odd number of carbon atoms, which exhibit relatively sharp spectral features and lifetimes in the 10-100 ns range, SiC4H shows intrinsically broadened spectral features consistent with a ∼100 fs lifetime, and a subsequent long-lived decay (>50 μs) which we ascribe to mixing with a nearby quartet state arising from the same electronic configuration. The spin-orbit coupling constants for both SiC3H and SiC5H radicals were determined to be approximately 64 cm(-1), similar to that of SiCH (69.8 cm(-1)), suggesting that the unpaired electron in these species is localized on the silicon atom. Motivated by the new optical work, the rotational spectrum of linear SiC3H was detected by cavity Fourier-transform microwave spectroscopy in the 13-34 GHz range. Each rotational transition from the [Formula: see text] ground state exhibits well-resolved Λ-doubling and hyperfine structure; the derived rotational constant of B = 2.605 GHz is in excellent agreement with our calculations.

Concepts: Spectroscopy, Carbon dioxide, Atom, Chemical bond, Carbon, Silicon, Quantum chemistry, Atomic physics

3

Recent progress in optics, atomic physics and material science has paved the way to study quantum effects in ultracold atomic alkali gases confined to non-trivial geometries. Multiply connected traps for cold atoms can be prepared by combining inhomogeneous distributions of DC and radio-frequency electromagnetic fields with optical fields that require complex systems for frequency control and stabilization. Here we propose a flexible and robust scheme that creates closed quasi-one-dimensional guides for ultracold atoms through the ‘dressing’ of hyperfine sublevels of the atomic ground state, where the dressing field is spatially modulated by inductive effects over a micro-engineered conducting loop. Remarkably, for commonly used atomic species (for example, (7)Li and (87)Rb), the guide operation relies entirely on controlling static and low-frequency fields in the regimes of radio-frequency and microwave frequencies. This novel trapping scheme can be implemented with current technology for micro-fabrication and electronic control.

Concepts: Photon, Quantum mechanics, Optics, Physics, Electromagnetic radiation, Chemistry, Radio, Atomic physics

3

The radioactive element astatine exists only in trace amounts in nature. Its properties can therefore only be explored by study of the minute quantities of artificially produced isotopes or by performing theoretical calculations. One of the most important properties influencing the chemical behaviour is the energy required to remove one electron from the valence shell, referred to as the ionization potential. Here we use laser spectroscopy to probe the optical spectrum of astatine near the ionization threshold. The observed series of Rydberg states enabled the first determination of the ionization potential of the astatine atom, 9.31751(8) eV. New ab initio calculations are performed to support the experimental result. The measured value serves as a benchmark for quantum chemistry calculations of the properties of astatine as well as for the theoretical prediction of the ionization potential of superheavy element 117, the heaviest homologue of astatine.

Concepts: Energy, Quantum mechanics, Physics, Light, Chemistry, Atom, Chemical element, Atomic physics

2

In high-intensity laser light, matter can be ionized by direct multiphoton absorption even at photon energies below the ionization threshold. However on tuning the laser to the lowest resonant transition, the system becomes multiply excited, and more efficient, indirect ionization pathways become operative. These mechanisms are known as interatomic Coulombic decay (ICD), where one of the species de-excites to its ground state, transferring its energy to ionize another excited species. Here we show that on tuning to a higher resonant transition, a previously unknown type of interatomic Coulombic decay, intra-Rydberg ICD occurs. In it, de-excitation of an atom to a close-lying Rydberg state leads to electron emission from another neighbouring Rydberg atom. Moreover, systems multiply excited to higher Rydberg states will decay by a cascade of such processes, producing even more ions. The intra-Rydberg ICD and cascades are expected to be ubiquitous in weakly-bound systems exposed to high-intensity resonant radiation.

Concepts: Electron, Photon, Energy, Quantum mechanics, Light, Atom, Excited state, Atomic physics

2

Engineering molecules with a tunable bond length and defined quantum states lies at the heart of quantum chemistry. The unconventional binding mechanism of Rydberg molecules makes them a promising candidate to implement such tunable molecules. A very peculiar type of Rydberg molecules are the so-called butterfly molecules, which are bound by a shape resonance in the electron-perturber scattering. Here we report the observation of these exotic molecules and employ their exceptional properties to engineer their bond length, vibrational state, angular momentum and orientation in a small electric field. Combining the variable bond length with their giant dipole moment of several hundred Debye, we observe counter-intuitive molecules which locate the average electron position beyond the internuclear distance.

Concepts: Electron, Quantum mechanics, Angular momentum, Fundamental physics concepts, Chemistry, Atom, Chemical bond, Atomic physics

1

The anti-perovskite superconductor MgC0.93Ni2.85 was studied using high-resolution x-ray Compton scattering combined with electronic structure calculations. Compton scattering measurements were used to determine experimentally a Fermi surface that showed good agreement with that of our supercell calculations, establishing the presence of the predicted hole and electron Fermi surface sheets. Our calculations indicate that the Fermi surface is smeared by the disorder due to the presence of vacancies on the C and Ni sites, but does not drastically change shape. The 20% reduction in the Fermi level density-of-states would lead to a significant (~70%) suppression of the superconducting T c for pair-forming electron-phonon coupling. However, we ascribe the observed much smaller T c reduction at our composition (compared to the stoichiometric compound) to the suppression of pair-breaking spin fluctuations.

Concepts: Electron, Fundamental physics concepts, Condensed matter physics, Superconductivity, Compton scattering, Absolute zero, Atomic physics, Cooper pair

1

The magnetic moment μ of a bound electron, generally expressed by the g-factor μ=-g μB s ħ(-1) with μB the Bohr magneton and s the electron’s spin, can be calculated by bound-state quantum electrodynamics (BS-QED) to very high precision. The recent ultra-precise experiment on hydrogen-like silicon determined this value to eleven significant digits, and thus allowed to rigorously probe the validity of BS-QED. Yet, the investigation of one of the most interesting contribution to the g-factor, the relativistic interaction between electron and nucleus, is limited by our knowledge of BS-QED effects. By comparing the g-factors of two isotopes, it is possible to cancel most of these contributions and sensitively probe nuclear effects. Here, we present calculations and experiments on the isotope dependence of the Zeeman effect in lithium-like calcium ions. The good agreement between the theoretical predicted recoil contribution and the high-precision g-factor measurements paves the way for a new generation of BS-QED tests.

Concepts: Electron, Fundamental physics concepts, Spin, Atom, Magnetism, Atomic physics, Electron magnetic dipole moment, Bohr magneton

1

The intrinsic properties of quantum dots (QDs) and the growing ability to interface them controllably with living cells has far-reaching potential applications in probing cellular processes such as membrane action potential. We demonstrate that an electric field typical of those found in neuronal membranes result in suppression of the QD photoluminescence (PL) and, for the first time, that QD PL is able to track the action potential profile of a firing neuron with millisecond time resolution. This effect is shown to be connected with electric-field-driven QD ionization and consequent QD PL quenching, in contradiction with conventional wisdom that suppression of the QD PL is attributable to the quantum confined Stark effect.

Concepts: Neuron, Quantum mechanics, Cell membrane, Fundamental physics concepts, Action potential, Axon, Membrane potential, Atomic physics