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The vibrational excitations of crystalline solids corresponding to acoustic or optic one phonon modes appear as sharp features in measurements such as neutron spectroscopy. In contrast, many-phonon excitations generally produce a complicated, weak, and featureless response. Here we present time-of-flight neutron scattering measurements for the binary solid uranium nitride (UN), showing well-defined, equally-spaced, high energy vibrational modes in addition to the usual phonons. The spectrum is that of a single atom, isotropic quantum harmonic oscillator and characterizes independent motions of light nitrogen atoms, each found in an octahedral cage of heavy uranium atoms. This is an unexpected and beautiful experimental realization of one of the fundamental, exactly-solvable problems in quantum mechanics. There are also practical implications, as the oscillator modes must be accounted for in the design of generation IV nuclear reactors that plan to use UN as a fuel.
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Uranium mononitride, UN, is considered a potential accident tolerant fuel due to its high uranium density, high thermal conductivity, and high melting point. Compared with the relatively inert UO2, UN has a high reactivity in water, however, studies have not considered the significant effect of radiation, which is known to cause corrosion of UO2. This study uses 0.1 M H2O2 to simulate the effects of water radiolysis in order to compare the radiolytic corrosion rates of UO2, UN, and U2N3 thin films at room temperature. X-ray reflectivity was used to investigate the changes in film morphology as a function of H2O2 exposure time, allowing changes in film thickness and roughness to be observed on the Angstrom length-scale. Results showed significant differences between UO2, UN, and U2N3, with corrosion rates of 0.083(3), 0.020(4), and 0.47(8) A/s, respectively, showing that UN corrodes more slowly than UO2 in 0.1 M H2O2.
We carried out Heyd-Scuseria-Ernzerhof hybrid density functional theory plane wave supercell calculations in wurtzite aluminum nitride in order to characterize the geometry, formation energies, transition levels and hyperfine tensors of the nitrogen split interstitial defect. The calculated hyperfine tensors may provide useful fingerprint of this defect for electron paramagnetic resonance measurement.
Nitrogen interstitials (N$_mathrm{i}$) have the lowest formation energy among intrinsic defects of hexagonal boron nitride (hBN) under n-type and N-rich conditions. Using an optimized hybrid functional, which reproduces the gap and satisfies the generalized Koopmans condition, an N$_mathrm{i}$ configuration is found which is lower in energy than the ones reported so far. The (0/-) charge transition level is also much deeper, so N$_mathrm{i}$ acts as a very efficient compensating center in n-type samples. Its calculated photoluminescence (PL) at 3.0 eV agrees well with the position of an N-sensitive band measured at 3.1 eV. It has been also found that the nitrogen vacancy (V$_mathrm{N}$) cannot be the origin of the three boron electron (TBC) electron paramagnetic resonance (EPR) center and in thermal equilibrium it cannot even exist in n-type samples.
Quantum emitters in hexagonal boron nitride (hBN) are promising building blocks for the realization of integrated quantum photonic systems. However, their spectral inhomogeneity currently limits their potential applications. Here, we apply tensile strain to quantum emitters embedded in few-layer hBN films and realize both red and blue spectral shifts with tuning magnitudes up to 65 meV, a record for any two-dimensional quantum source. We demonstrate reversible tuning of the emission and related photophysical properties. We also observe rotation of the optical dipole in response to strain, suggesting the presence of a second excited state. We derive a theoretical model to describe strain-based tuning in hBN, and the rotation of the optical dipole. Our work demonstrates the immense potential for strain tuning of quantum emitters in layered materials to enable their employment in scalable quantum photonic networks.
Quantum emitters in layered hexagonal boron nitride (hBN) have recently attracted a great attention as promising single photon sources. In this work, we demonstrate resonant excitation of a single defect center in hBN, one of the most important prerequisites for employment of optical sources in quantum information application. We observe spectral linewidths of hBN emitter narrower than 1 GHz while the emitter experiences spectral diffusion. Temporal photoluminescence measurements reveals an average spectral diffusion time of around 100 ms. On-resonance photon antibunching measurement is also realized. Our results shed light on the potential use of quantum emitters from hBN in nanophotonics and quantum information.
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