Electronic structure of EuN: Growth, spectroscopy, and theory

Abstract

We present a detailed study of the electronic structure of europium nitride (EuN), comparing spectroscopic data to the results of advanced electronic structure calculations. We demonstrate the epitaxial growth of EuN films, and show that in contrast to other rare-earth nitrides successful growth of EuN requires an activated nitrogen source. Synchrotron-based x-ray spectroscopy shows the samples contain predominantly Eu 3+ , but with a small and varying quantity of Eu 2+ that we associate with defects, most likely nitrogen vacancies. X-ray absorption and x-ray emission spectroscopies (XAS and XES) at the nitrogen K-edge are compared to several different theoretical models, namely LSDA+U (local spin density functional theory with Hubbard U corrections), dynamic mean field theory in the Hubbard-I approximation, and QSGW (quasiparticle self-consistent GW ) calculations. The DMFT and QSGW models capture better the density of conduction band states than LSDA+U . Only the Hubbard-I model contains a correct description of the Eu 4f atomic multiplets and locates their energies relative to the band states, and we see some evidence in XAS for hybridization between the conduction band and the the lowest lying 8 S multiplet. The Hubbard-I model is also in good agreement with purely atomic mutliplet calculations for the Eu M-edge XAS. LSDA+U and DMFT find a metallic ground state, while QSGW predicts a direct band gap at X for EuN of about 0.9 eV that matches closely an absorption edge seen in optical transmittance at 0.9 eV and a smaller indirect gap. Overall, the combination of theoretical methods and spectroscopies provides insights into the complex nature of the electronic structure of this material. The results imply that EuN is a narrow band-gap semiconductor that lies close to the metal-insulator boundary, where the close proximity to the Fermi level of an empty Eu 4f multiplet raises the possibility of tuning both the magnetic and electronic states in this system.

Electronic, elastic and optical properties of zircon GdVO4 investigated from experiments and LSDA+U

“…Due to the strong on-site Coulomb repulsion between the highly localized 4f-electrons for Gd, the conventional LDA and GGA methods fail to describe the electronic localization effects and cannot yield correct ground state properties. The local spin-density approximation (LSDA) with additional Hubbard correlation terms (LSDA + U) [14][15][16][17][18][19] can properly describe well on-site electron-electron repulsion associated with the 4f narrow bands. In LSDA + U approach [14][15][16][17][18][19], the LSDA interaction is subtracted and replaced by Hartree-Fock form for the electron-electron interaction in the spherically symmetric atom.…”

“…Moreover, previous Density Function Theory (DFT) calculations with local density approximation (LDA) and generalized gradient approximation (GGA) failed to describe the strong correlation in the 4f shell of Gd. DFT with additional Hubbard correlation terms (DFT + U) can properly solve this problem [14][15][16][17][18][19]. In this paper, we present the calculation of the physical properties of zircon-type GdVO 4 with LSDA + U compared with experiments.…”

YbN: An intrinsic semiconductor with antiferromagnetic exchange

“…The nitride of the next lightest rare earth, EuN, is also a semiconductor, although the J = 0 ground state of the trivalent ion prevents ferromagnetic alignment in stoichiometric EuN [24,26]. However, nitrogen vacancies (V N ) introduce divalent Eu 2+ , with a half-filled 4f shell that then harbors the 7μ B moment characteristic of Gd 3+ [13,26,27]. EuN becomes a diluted ferromagnetic semiconductor at sufficiently high (>5%) V N concentration [13,27].…”

Beyond theGWapproximation: A second-order screened exchange correction

“…For example, in solids, the binding energies of semi-core states tend to be underestimated 17,18 whereas band gaps of polar materials are often severely overestimated 19 . Also an assessment of d-and f -electron compounds is only just emerging with mixed success [20][21][22][23] . For finite systems, G 0 W 0 based on a Perdew-Burke-Ernzerhof hybrid functional (PBE0) reference yields excellent vertical ionization energies (IEs) for molecules with an average deviation from experiment of only 3% 9,11,12,15 .…”