Quasiparticle lifetime broadening in resonant x-ray scattering ofNH4NO3

Abstract

It has been previously shown that two effects cause dramatic changes in the x-ray absorption and emission spectra from the N K edge of the insulating crystal ammonium nitrate. First, vibrational disorder causes major changes in the absorption spectrum, originating not only from the thermal population of phonons, but, significantly, from zero-point motion as well. Second, the anomalously large broadening (~ 4 eV) of the emission originating from nitrate σ states is due to unusually short lifetimes of quasiparticles in an otherwise extremely narrow band. In this work we investigate the coupling of these effects to core and valence excitons that are created as the initial x-ray excitation energy is progressively reduced toward the N edge. Using a GW/Bethe-Salpeter approach, we show the extent to which this anomalous broadening is captured by the GW approximation. The data and calculations demonstrate the importance that the complex self-energies (finite lifetimes) of valence bands have on the interpretation of emission spectra. We produce a scheme to explain why extreme lifetimes should appear in σ states of other similar compounds.

Resonant x-ray emission and valence-band lifetime broadening in LiNO3

“…Each structure in the ensemble is a possible configuration of a subset of the atoms within the crystal. To generate the ensemble of disordered structures we make use of the phonon-mode approach introduced previously [2,3], and expanded on in [13] and separately in [14]. Briefly, the phonon modes of a structure are assumed to be harmonic.…”

“…An additional, Gaussian broadening of 0.25 eV (0.50 eV) was applied to the calculated absorption (emission) spectra before averaging. The alignment of the XAS energy scale of the ocean calculations follows previous work [3]. The emission energy of the calculations is fixed relative to the absorption alignment, including the band-gap correction from the GW.…”

Resonant x-ray emission of hexagonal boron nitride

“…This effect can be modeled by summing calculated absorption and RIXS spectra over a representative ensemble of structures. Previously it was shown that such structures can be generated by considering the phonon modes of the system within a harmonic approximation [27, 28], and this technique has since been independently adopted elsewhere [29]. The advantage of the phononmode approach is that the quantum nature of the nuclear wave functions, i.e., zero-point motion, is included automatically, whereas in standard molecular dynamics simulations the nuclei are classical particles.…”

“…The energy scale of the spectrometer was calibrated using elastic scattering of the excitation radiation. The details of the monochromator and spectrometer energy calibration have been previously described [28]. …”

“…Further details on the extension to RIXS with OCEAN can be found in Ref. 28. Norm-conserving pseudopotentials were generated for both boron and nitrogen using the OPIUM code [48].…”