Observation of excitonic N-body bound states: polyexcitons in diamond

Self-trapped excitons in diamond: a Δ-SCF approach

This paper reports the first variationally-based predictions of the lowest excited state in diamond (Γ25՛ → Γ15) in the optical and thermal configurations, from direct Δ-SCF calculations based on B3LYP, PBE0, HSE06 and GGA functionals. For the B3LYP functional the energy of the optical state, 7.27 eV, is within the observed range of (7.2-7.4) eV and is predicted to be insulating, with indirect band gaps of (5.6-5.8) eV. Mulliken analyses of the excited state wavefunction indicate extensive redistributions of charge and spin resulting in a strongly excitonic state with a central charge of -0.8 ǀeǀ surrounded by charges of +0.12 ǀeǀ at the four nearest neighbour sites. Calculations of the ground and excited state relaxations lead to a Stokes' shift of 0.47 eV and predicted Γ-point luminescence energy of 6.89 eV. Assuming a similar shift at the band edge (X1), an estimate of 5.29 eV is predicted for the luminescence energy, which compares with the observed value of 5.27 eV. Excited state vibrational spectra show marked differences from the ground state, with the introduction of an infra-red peak at 1150 cm-1 and a modest shift of 2 cm -1 in the TO(X) Raman mode at 1340 cm-1. Similar calculations of the lowest energy bi- and tri-excitons predict these to be bound states in both optical and thermal configurations and plausible precursors to exciton condensation. Estimates of bi- and triexciton luminescence energies predict red shifts with respect to the single exciton line which are compared to the recently reported values.

Anomalous Blue Shift of Exciton Luminescence in Diamond

Generally speaking, for a semiconductor, the temperature dependence of excitonic emission corresponds to that of its band gap. However, an anomalous behavior is exhibited by the excitonic luminescence of diamond where as the temperature increases (from 10 to 300 K), its indirect exciton luminescence peak displays a spectral-distinguishable blue shift, whereas the indirect band-gap absorption shows a weak red shift. According to experimental high-resolution deep-ultraviolet spectra and theoretical analysis, the weak red shift of its indirect band gap is ascribed to its large Debye temperature (Θ_D ≈ 2220 K), which makes the lattice constant change comparatively little in a large temperature range, so the change of its band gap is relatively small; in this case, as the temperature rises, the thermal population of valence-band holes that moves to a high-energy state far away from the Fermi surface contributes to the macroscopic blue shift of its excitonic emission.

Probing Metastable Space-Charge Potentials in a Wide Band Gap Semiconductor

While the study of space-charge potentials has a long history, present models are largely based on the notion of steady state equilibrium, ill-suited to describe wide band gap semiconductors with moderate to low concentrations of defects. Here we build on color centers in diamond both to locally inject carriers into the crystal and probe their evolution as they propagate in the presence of external and internal potentials. We witness the formation of metastable charge patterns whose shape-and concomitant field-can be engineered through the timing of carrier injection and applied voltages. With the help of previously crafted charge patterns, we unveil a rich interplay between local and extended sources of space-charge field, which we then exploit to show space-charge-induced carrier guiding.

Giant Exciton Mott Density in Anatase TiO_{2}

Elucidating the carrier density at which strongly bound excitons dissociate into a plasma of uncorrelated electron-hole pairs is a central topic in the many-body physics of semiconductors. However, there is a lack of information on the high-density response of excitons absorbing in the near-to-mid ultraviolet, due to the absence of suitable experimental probes in this elusive spectral range. Here, we present a unique combination of many-body perturbation theory and state-of-the-art ultrafast broadband ultraviolet spectroscopy to unveil the interplay between the ultraviolet-absorbing two-dimensional excitons of anatase TiO_{2} and a sea of electron-hole pairs. We discover that the critical density for the exciton Mott transition in this material is the highest ever reported in semiconductors. These results deepen our knowledge of the exciton Mott transition and pave the route toward the investigation of the exciton phase diagram in a variety of wide-gap insulators.

Bright Luminescence from Indirect and Strongly Bound Excitons in h-BN

A quantitative analysis of the excitonic luminescence efficiency in hexagonal boron nitride (h-BN) is carried out by cathodoluminescence in the ultraviolet range and compared with zinc oxide and diamond single crystals. A high quantum yield value of ∼50% is found for h-BN at 10 K comparable to that of direct band-gap semiconductors. This bright luminescence at 215 nm remains stable up to room temperature, evidencing the strongly bound character of excitons in bulk h-BN. Ab initio calculations of the exciton dispersion confirm the indirect nature of the lowest-energy exciton whose binding energy is found equal to 300±50  meV, in agreement with the thermal stability observed in luminescence. The direct exciton is found at a higher energy but very close to the indirect one, which solves the long debated Stokes shift in bulk h-BN.

Efimov physics: a review

This article reviews theoretical and experimental advances in Efimov physics, an array of quantum few-body and many-body phenomena arising for particles interacting via short-range resonant interactions, that is based on the appearance of a scale-invariant three-body attraction theoretically discovered by Vitaly Efimov in 1970. This three-body effect was originally proposed to explain the binding of nuclei such as the triton and the Hoyle state of carbon-12, and later considered as a simple explanation for the existence of some halo nuclei. It was subsequently evidenced in trapped ultra-cold atomic clouds and in diffracted molecular beams of gaseous helium. These experiments revealed that the previously undetermined three-body parameter introduced in the Efimov theory to stabilise the three-body attraction typically scales with the range of atomic interactions. The few- and many-body consequences of the Efimov attraction have been since investigated theoretically, and are expected to be observed in a broader spectrum of physical systems.

Microscopic origin and universality classes of the Efimov three-body parameter

The low-energy spectrum of three particles interacting via nearly resonant two-body interactions in the Efimov regime is set by the so-called three-body parameter. We show that the three-body parameter is essentially determined by the zero-energy two-body correlation. As a result, we identify two classes of two-body interactions for which the three-body parameter has a universal value in units of their effective range. One class involves the universality of the three-body parameter recently found in ultracold atom systems. The other is relevant to short-range interactions that can be found in nuclear physics and solid-state physics.