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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.

    

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2. Electronic structure in a transition metal dipnictide TaAs 2

   https://doi.org/10.1088/1361-648X/ad04fc  

   Regmi, Sabin ; Huang, Cheng-Yi ; Khan, Mojammel A. ; Wang, Baokai ; Pradhan Sakhya, Anup ; Hosen, M. Mofazzel ; Thompson, Jesse ; Singh, Bahadur ; Denlinger, Jonathan D. ; Ishigami, Masahiro ; et al ( November 2023 , Journal of Physics: Condensed Matter)

   The family of transition-metal dipnictides has been of theoretical and experimental interest because this family hosts topological states and extremely large magnetoresistance (MR). Recently,${\mathrm{T}\mathrm{a}\mathrm{A}\mathrm{s}}_2$, a member of this family, has been predicted to support a topological crystalline insulating state. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES), we reveal both closed and open pockets in the metallic Fermi surface (FS) and linearly dispersive bands on the ($\bar{2}01$) surface, along with the presence of extreme MR observed from magneto-transport measurements. A comparison of the ARPES results with first-principles computations shows that the linearly dispersive bands on the measured surface of${\mathrm{T}\mathrm{a}\mathrm{A}\mathrm{s}}_2$are trivial bulk bands. The absence of symmetry-protected surface state on the ($\bar{2}01$) surface indicates its topologically dark nature. The presence of open FS features suggests that the open-orbit fermiology could contribute to the extremely large MR of${\mathrm{T}\mathrm{a}\mathrm{A}\mathrm{s}}_2$.

    

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3. Electronic Structure of InAs and InSb Surfaces: Density Functional Theory and Angle‐Resolved Photoemission Spectroscopy

   https://doi.org/10.1002/qute.202100033  

   Yang, Shuyang ; Schröter, Niels B. M. ; Strocov, Vladimir N. ; Schuwalow, Sergej ; Rajpalk, Mohana ; Ohtani, Keita ; Krogstrup, Peter ; Winkler, Georg W. ; Gukelberger, Jan ; Gresch, Dominik ; et al ( January 2022 , Advanced Quantum Technologies)

   The electronic structure of surfaces plays a key role in the properties of quantum devices. However, surfaces are also the most challenging to simulate and engineer. Here, the electronic structure of InAs(001), InAs(111), and InSb(110) surfaces is studied using a combination of density functional theory (DFT) and angle‐resolved photoemission spectroscopy (ARPES). Large‐scale first principles simulations are enabled by using DFT calculations with a machine‐learned Hubbard U correction [npj Comput. Mater. 6, 180 (2020)]. To facilitate direct comparison with ARPES results, a “bulk unfolding” scheme is implemented by projecting the calculated band structure of a supercell surface slab model onto the bulk primitive cell. For all three surfaces, a good agreement is found between DFT calculations and ARPES. For InAs(001), the simulations clarify the effect of the surface reconstruction. Different reconstructions are found to produce distinctive surface states, which may be detected by ARPES with low photon energies. For InAs(111) and InSb(110), the simulations help elucidate the effect of oxidation. Owing to larger charge transfer from As to O than from Sb to O, oxidation of InAs(111) leads to significant band bending and produces an electron pocket, whereas oxidation of InSb(110) does not. The combined theoretical and experimental results may inform the design of quantum devices based on InAs and InSb semiconductors, for example, topological qubits utilizing the Majorana zero modes.

    

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4. Electronic stripe patterns near the fermi level of tetragonal Fe(Se,S)

   https://doi.org/10.1038/s41535-023-00592-5  

   Walker, M. ; Scott, K. ; Boyle, T. J. ; Byland, J. K. ; Bötzel, S. ; Zhao, Z. ; Day, R. P. ; Zhdanovich, S. ; Gorovikov, S. ; Pedersen, T. M. ; et al ( October 2023 , npj Quantum Materials)

   FeSe_1−xS_xremains one of the most enigmatic systems of Fe-based superconductors. While much is known about the orthorhombic parent compound, FeSe, the tetragonal samples, FeSe_1−xS_xwithx > 0.17, remain relatively unexplored. Here, we provide an in-depth investigation of the electronic states of tetragonal FeSe_0.81S_0.19, using scanning tunneling microscopy and spectroscopy (STM/S) measurements, supported by angle-resolved photoemission spectroscopy (ARPES) and theoretical modeling. We analyze modulations of the local density of states (LDOS) near and away from Fe vacancy defects separately and identify quasiparticle interference (QPI) signals originating from multiple regions of the Brillouin zone, including the bands at the zone corners. We also observe that QPI signals coexist with a much stronger LDOS modulation for states near the Fermi level whose period is independent of energy. Our measurements further reveal that this strong pattern appears in the STS measurements as short range stripe patterns that are locally two-fold symmetric. Since these stripe patterns coexist with four-fold symmetric QPI around Fe-vacancies, the origin of their local two-fold symmetry must be distinct from that of nematic states in orthorhombic samples. We explore several aspects related to the stripes, such as the role of S and Fe-vacancy defects, and whether they can be explained by QPI. We consider the possibility that the observed stripe patterns may represent incipient charge order correlations, similar to those observed in the cuprates.

    

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5. The Vacancy‐Induced Electronic Structure of the SrTiO 3−δ Surface

   https://doi.org/10.1002/aelm.201800460  

   Cook, Seyoung ; Dylla, Maxwell T. ; Rosenberg, Richard A. ; Mansley, Zachary R. ; Snyder, G. Jeffrey ; Marks, Laurence D. ; Fong, Dillon D. ( November 2018 , Advanced Electronic Materials)

   The emergence of a 2D electron gas (2DEG) on the (001) surface of oxygen‐deficient strontium titanate (SrTiO_3−δ) is investigated. Using in situ soft X‐ray spectroscopy and effective mass modeling, a series of quantitative band diagrams are developed to describe the evolution of near‐surface and bulk carrier concentrations, downward band bending, and Fermi level along a lateral gradient of oxygen vacancies formed on SrTiO_3−δby direct‐current resistive heating under ultrahigh vacuum conditions. Electrons are accumulated over a 3 nm region near the surface, confined within a potential well with saturated 300 meV downward band bending. The relation between Fermi levels and carrier concentrations near the surface suggests the density of states near the surface is much lower than the bulk density of states, which is consistent with the quantum‐confined subbands of a 2DEG. The quantitative relationship between the surface and bulk electronic structures developed in this work provides a guide for precise engineering of the oxygen‐vacancy‐induced 2DEG in SrTiO₃.

    

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