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1. Evolution of the Kondo lattice electronic structure above the transport coherence temperature

   https://doi.org/10.1073/pnas.2001778117  

   Jang, Sooyoung ; Denlinger, J. D. ; Allen, J. W. ; Zapf, V. S. ; Maple, M. B. ; Kim, Jae Nyeong ; Jang, Bo Gyu ; Shim, Ji Hoon ( September 2020 , Proceedings of the National Academy of Sciences)

   The temperature-dependent evolution of the Kondo lattice is a long-standing topic of theoretical and experimental investigation and yet it lacks a truly microscopic description of the relation of the basic f-c hybridization processes to the fundamental temperature scales of Kondo screening and Fermi-liquid lattice coherence. Here, the temperature dependence of f-c hybridized band dispersions and Fermi-energy f spectral weight in the Kondo lattice system CeCoIn₅is investigated using f-resonant angle-resolved photoemission spectroscopy (ARPES) with sufficient detail to allow direct comparison to first-principles dynamical mean-field theory (DMFT) calculations containing full realism of crystalline electric-field states. The ARPES results, for two orthogonal (001) and (100) cleaved surfaces and three different f-c hybridization configurations, with additional microscopic insight provided by DMFT, reveal f participation in the Fermi surface at temperatures much higher than the lattice coherence temperature,$T^{*}\approx 45$K, commonly believed to be the onset for such behavior. The DMFT results show the role of crystalline electric-field (CEF) splittings in this behavior and a T-dependent CEF degeneracy crossover below$T^{*}$is specifically highlighted. A recent ARPES report of low T Luttinger theorem failure for CeCoIn₅is shown to be unjustified by current ARPES data and is not found in the theory.
2. Momentum-resolved electronic structure and band offsets in an epitaxial NbN/GaN superconductor/semiconductor heterojunction

   https://doi.org/10.1126/sciadv.abi5833  

   Yu, Tianlun ; Wright, John ; Khalsa, Guru ; Pamuk, Betül ; Chang, Celesta S. ; Matveyev, Yury ; Wang, Xiaoqiang ; Schmitt, Thorsten ; Feng, Donglai ; Muller, David A. ; et al ( December 2021 , Science Advances)

   The electronic structure of heterointerfaces is a pivotal factor for their device functionality. We use soft x-ray angle-resolved photoelectron spectroscopy to directly measure the momentum-resolved electronic band structures on both sides of the Schottky heterointerface formed by epitaxial films of the superconducting NbN on semiconducting GaN, and determine their momentum-dependent interfacial band offset as well as the band-bending profile. We find, in particular, that the Fermi states in NbN are well separated in energy and momentum from the states in GaN, excluding any notable electronic cross-talk of the superconducting states in NbN to GaN. We support the experimental findings with first-principles calculations for bulk NbN and GaN. The Schottky barrier height obtained from photoemission is corroborated by electronic transport and optical measurements. The momentum-resolved understanding of electronic properties of interfaces elucidated in our work opens up new frontiers for the quantum materials where interfacial states play a defining role.
3. Growth of PdCoO ₂ films with controlled termination by molecular-beam epitaxy and determination of their electronic structure by angle-resolved photoemission spectroscopy

   https://doi.org/10.1063/5.0101837  

   Song, Qi ; Sun, Jiaxin ; Parzyck, Christopher T. ; Miao, Ludi ; Xu, Qing ; Hensling, Felix V. ; Barone, Matthew R. ; Hu, Cheng ; Kim, Jinkwon ; Faeth, Brendan D. ; et al ( September 2022 , APL Materials)

   Utilizing the powerful combination of molecular-beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we produce and study the effect of different terminating layers on the electronic structure of the metallic delafossite PdCoO 2 . Attempts to introduce unpaired electrons and synthesize new antiferromagnetic metals akin to the isostructural compound PdCrO 2 have been made by replacing cobalt with iron in PdCoO 2 films grown by MBE. Using ARPES, we observe similar bulk bands in these PdCoO 2 films with Pd-, CoO 2 -, and FeO 2 -termination. Nevertheless, Pd- and CoO 2 -terminated films show a reduced intensity of surface states. Additionally, we are able to epitaxially stabilize PdFe x Co 1− x O 2 films that show an anomaly in the derivative of the electrical resistance with respect to temperature at 20 K, but do not display pronounced magnetic order.
4. Electronic correlation in nearly free electron metals with beyond-DFT methods

   https://doi.org/10.1038/s41524-022-00867-8  

   Mandal, Subhasish ; Haule, Kristjan ; Rabe, Karin M. ; Vanderbilt, David ( August 2022 , npj Computational Materials)

   For more than three decades, nearly free-electron elemental metals have been a topic of debate because the computed bandwidths are significantly wider in the local density approximation to density-functional theory (DFT) than indicated by angle-resolved photoemission (ARPES) experiments. Here, we systematically investigate this using first principles calculations for alkali and alkaline-earth metals using DFT and various beyond-DFT methods such as meta-GGA, G₀W₀, hybrid functionals (YS-PBE0, B3LYP), and LDA + eDMFT. We find that the static non-local exchange, as partly included in the hybrid functionals, significantly increase the bandwidths even compared to LDA, while the G₀W₀bands are only slightly narrower than in LDA. The agreement with the ARPES is best when the local approximation to the self-energy is used in the LDA + eDMFT method. We infer that even moderately correlated systems with partially occupiedsorbitals, which were assumed to approximate the uniform electron gas, are very well described in terms of short-range dynamical correlations that are only local to an atom.
5. Chemical and Electronic Structures of Cobalt Oxynitride Films Deposited by NH3 vs. N2 Plasma: Theory vs. Experiment

   https://doi.org/10.1039/D0CP04168H  

   Osonkie, Adaeze ; Lee, Veronica ; Oyelade, Adeola ; Mrozek-McCourt, Maximillian ; Chukwunenye, Precious ; Golden, Teresa ; Cundari, Thomas R. ; Kelber, Jeffry A ( January 2020 , Physical Chemistry Chemical Physics)

   The chemical structures of Co oxynitrides – in particular, interactions among N and O atoms bonded to the same cobalt – are of great importance for an array of catalytic and materials applications. X-ray diffraction (XRD), core and valence band X-ray photoelectron spectroscopy (XPS) and plane wave density functional theory (DFT) calculations are used to probe chemical and electronic interactions of nitrogen-rich CoO1-xNx (x > 0.7) films deposited on Si(100) using NH3 or N2 plasma-based sputter deposition or surface nitridation. Total energy calculations indicate that the zincblende (ZB) structure is energetically favored over the rocksalt (RS) structure for x > ~ 0.2, with an energy minimum observed in the ZB structure for x ~ 0.8 - 0.9. This is in close agreement with XPS-derived film compositions when corrected for surface oxide/hydroxide layers. XRD data indicate that films deposited on Si (100) at room temperature display either a preferred (220) orientation or no diffraction pattern, and are consistent with either rocksalt (RS) or zincblende (ZB) structure. Comparison between experimental and calculated X-ray excited valence band densities of states – also similar for all films synthesized herein – demonstrates a close agreement with a ZB, but not an RS structure. Core levelmore »XPS spectra exhibit systematic differences between films deposited in NH3 vs N2 plasma environments. Films deposited by N2 plasma magnetron sputtering exhibit greater O content as evidenced by systematic shifts in N 1s binding energies. Excellent agreement with experiment for core level binding energies is obtained for DFT calculations based on the ZB structure, but not for the RS structure. The agreement between theory and experiment demonstrates that these N-rich Co oxynitride films exhibit the ZB structure, and forms the basis of a predictive model for understanding how N and O interactions impact the electronic, magnetic and catalytic properties of these materials.« less