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We compute the (0-11) surface spectral function, the surface density of states (DOS), and the quasiparticle interference (QPI) patterns, both in the normal state and superconducting (SC) state of UTe_2. We consider all possible non-chiral and chiral order parameters (OPs) that could in principle describe the superconductivity in this compound. We describe the formation of surface states whose maximum intensity energy depends on the nature of the pairing. We study also the QPI patterns resulting from the scattering of these surface states. We show that the main feature distinguishing between various OPs is a QPI peak that is only observed experimentally in the superconducting state. The energy dispersion and the stability of this peak is consistent among the non-chiral OPs only with a B_3u pairing. Moreover, B_3u is the only non-chiral pairing that shows a peak at zero energy in the DOS, consistent with the experimental observations.
Lead halide perovskite nanocrystals have emerged as promising candidates for classical light-emitting devices and single-photon sources, owing to their high photoluminescence quantum yield, narrow emission line width and tunable emission. Judicious choice of ligands to passivate nanocrystal surfaces has proven to be critical to the structural stability and optoelectronic performance of such nanocrystals. While many ligands have been deployed, the resulting quality of the nanocrystal surface can be difficult to assess directly. Here, we demonstrate ultralow frequency Raman spectroscopy as a powerful tool to resolve surface-sensitive changes in size and ligand choice in perovskite nanocrystals. By investigating a size series of CsPbBr3 nanocrystals from the strong (5 nm) to the weak (28 nm) confinement range, we show that the line width of Raman-active modes provides a highly selective metric for surface disorder and quality. We further examine a series of 28 nm diameter nanocrystals with four different zwitterionic ligands, unravelling clear links between varying steric effects and surface quality evident from Raman analysis. Photoluminescence and THz photoconductivity probes reveal an evident correlation of charge-carrier dynamics and radiative emission yields with ligand chemistry and surface quality inferred from phonon broadening. We further show that surface defects preferentially trap hot charge carriers, which affects exciton stability and radiative emission yields. Overall, our approach offers powerful insights into optimizing nanocrystal-ligand boundaries to enhance the performance of nanoscale quantum light sources and optoelectronic devices.
The electronic structure of metallic Bi2Ir2O7 has been investigated by a combination of soft x-ray absorption spectroscopy, x-ray emission spectroscopy (XES), and resonant inelastic x-ray scattering (RIXS) in the vicinity of the oxygen K edge. The O K-edge RIXS spectra are found to resemble the O K-edge XES spectra with resonating features but with an absence of any Raman modes, revealing the highly itinerant nature of this compound. The O K-edge response is compared with scalar relativistic band structure calculations within the local density approximation, which captures the main spectral characteristics. The RIXS spectra either display a localized or delocalized Ir 5d response, depending on whether the photon energy is tuned to the pre-edge or above. Our results uncover a significant coupling of O K-edge RIXS via the O 2p−Ir 5d hybridization to the partial density of states of the Ir 5d states. An alternative explanation of the Ir 5d−O 2p hybridization spectral structure in the optical region of the RIXS spectra is given in terms of Ir dd excitations. Our findings allow us to infer that the relative electronic correlations in the metallic Bi2Ir2O7 compound with sizable spin-orbit coupling are 0.06≤U/W≤0.22 and thus are in the moderate-to-weak regime. We found that the Jeff model, developed to understand the existence of insulating states in various iridate perovskites, is not applicable to Bi2Ir2O7, casting doubts on its applicability to pyrochlore iridates with moderate to weak electronic correlations. We argue that the strong O 2p−Ir 5d−Bi 6s/6p hybridization is responsible for the highly itinerant nature of this material. Our results establish the capability of O K-edge RIXS to identify the electronic ground state of 5d spin-orbit transition metal compounds and to qualitatively assess their degree of localization. Published by the American Physical Society 2025
Optical excitation of large-amplitude apical oxygen phonon oscillations has been shown to renormalize the electronic properties of YB a 2 C u 3 O 6 + x , inducing a superconducting-like optical response above equilibrium T C . All of the evidence collected so far has been based on the changes of the terahertz frequency c -axis response. In these measurements, the capacitive interlayer coupling was seen to transform into a superconducting-like inductive response. This assignment was strengthened by recent measurements of ultrafast magnetic field expulsion. Here, we report an experimental determination of the transient in-plane optical properties, which has so far been elusive due to the high equilibrium reflectivity and the need to evaluate minute changes in the optical response. We report the appearance of a photoinduced in-plane optical gap 2 Δ ≃ 30 c m − 1 and a divergent imaginary conductivity, both consistent with photoinduced superconductivity. A global fit to these data suggests that in- and out-of-plane electronic properties never completely equilibrate during the dynamics.
