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1. Narrow-band high-lying excitons with negative-mass electrons in monolayer WSe2

   https://doi.org/10.1038/s41467-021-25499-2  

   Lin, Kai-Qiang ; Ong, Chin Shen ; Bange, Sebastian ; Faria Junior, Paulo E. ; Peng, Bo ; Ziegler, Jonas D. ; Zipfel, Jonas ; Bäuml, Christian ; Paradiso, Nicola ; Watanabe, Kenji ; et al ( December 2021 , Nature Communications)

   Abstract Monolayer transition-metal dichalcogenides (TMDCs) show a wealth of exciton physics. Here, we report the existence of a new excitonic species, the high-lying exciton (HX), in single-layer WSe 2 with an energy of ~3.4 eV, almost twice the band-edge A-exciton energy, with a linewidth as narrow as 5.8 meV. The HX is populated through momentum-selective optical excitation in the K -valleys and is identified in upconverted photoluminescence (UPL) in the UV spectral region. Strong electron-phonon coupling results in a cascaded phonon progression with equidistant peaks in the luminescence spectrum, resolvable to ninth order. Ab initio GW -BSE calculations with full electron-hole correlations explain HX formation and unmask the admixture of upper conduction-band states to this complex many-body excitation. These calculations suggest that the HX is comprised of electrons of negative mass. The coincidence of such high-lying excitonic species at around twice the energy of band-edge excitons rationalizes the excitonic quantum-interference phenomenon recently discovered in optical second-harmonic generation (SHG) and explains the efficient Auger-like annihilation of band-edge excitons. 

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2. A Laser‐ARPES View of the 2D Electron Systems at LaAlO 3 /SrTiO 3 and Al/SrTiO 3 Interfaces

   https://doi.org/10.1002/aelm.202101376  

   McKeown Walker, Siobhan ; Boselli, Margherita ; Martínez, Emanuel A. ; Gariglio, Stefano ; Bruno, Flavio Y. ; Baumberger, Felix ( March 2022 , Advanced Electronic Materials)

   The electronic structure of the two‐dimensional electron system (2DES) found at the Al/SrTiO₃(Al/STO) and LaAlO₃/SrTiO₃(LAO/STO) interfaces is measured by means of laser angle resolved photoemission spectroscopy, taking advantage of the large photoelectron escape depth at low photon energy to probe these buried interfaces. The possibility of tuning the electronic density in Al/STO by varying the Al layer thickness is demonstrated, and it is shown that the electronic structure evolution is well described by self‐consistent tight binding supercell calculations, but differs qualitatively from a rigid band shift model. It is shown that both 2DES are strongly coupled to longitudinal optical phonons, in agreement with previous reports of a polaronic ground state in similar STO based 2DESs. Tuning the electronic density in Al/STO to match that of LAO/STO and comparing both systems, it is estimated that the intrinsic LAO/STO 2DES has a bare band width of ≈60 meV and a carrier density of ≈6 × 10¹³cm⁻².

    

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3. Hot-carrier dynamics in InAs/AlAsSb multiple-quantum wells

   https://doi.org/10.1038/s41598-021-89815-y  

   Piyathilaka, Herath P. ; Sooriyagoda, Rishmali ; Esmaielpour, Hamidreza ; Whiteside, Vincent R. ; Mishima, Tetsuya D. ; Santos, Michael B. ; Sellers, Ian R. ; Bristow, Alan D. ( May 2021 , Scientific Reports)

   A type-II InAs/AlAs$$_{0.16}$$${}_{0.16}$Sb$$_{0.84}$$${}_{0.84}$multiple-quantum well sample is investigated for the photoexcited carrier dynamics as a function of excitation photon energy and lattice temperature. Time-resolved measurements are performed using a near-infrared pump pulse, with photon energies near to and above the band gap, probed with a terahertz probe pulse. The transient terahertz absorption is characterized by a multi-rise, multi-decay function that captures long-lived decay times and a metastable state for an excess-photon energy of$$>100$$$>100$meV. For sufficient excess-photon energy, excitation of the metastable state is followed by a transition to the long-lived states. Excitation dependence of the long-lived states map onto a nearly-direct band gap ($$E{_g}$$$E{}_g$) density of states with an Urbach tail below$$E{_g}$$$E{}_g$. As temperature increases, the long-lived decay times increase$$$<E{}_g$, due to the increased phonon interaction of the unintentional defect states, and by phonon stabilization of the hot carriers$$>E{_g}$$$>E{}_g$. Additionally, Auger (and/or trap-assisted Auger) scattering above the onset of the plateau may also contribute to longer hot-carrier lifetimes. Meanwhile, the initial decay component shows strong dependence on excitation energy and temperature, reflecting the complicated initial transfer of energy between valence-band and defect states, indicating methods to further prolong hot carriers for technological applications.

    

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4. Dynamical stabilization in delafossite nitrides for solar energy conversion

   https://doi.org/10.1039/c8ta07536k  

   Szymanski, N. J. ; Walters, L. N. ; Hellman, O. ; Gall, D. ; Khare, S. V. ( October 2018 , Journal of Materials Chemistry A)

   Delafossite structured ternary nitrides, ABN 2 , have been of recent experimental investigation for applications such as tandem solar and photoelectrochemical cells. We present a thorough first principles computational investigation of their stability, electronic structure, and optical properties. Nine compounds, where A = Cu, Ag, Au and B = V, Nb, Ta, were studied. For three of these compounds, CuTaN 2 , CuNbN 2 , and AgTaN 2 , our computations agree well with experimental results. Optimized lattice parameters, formation energies, and mechanical properties have been computed using the generalized gradient approximation (GGA). Phonon density of states computed at zero-temperature shows that all compounds are dynamically unstable at low temperatures. Including finite-temperature anharmonic effects stabilizes all compounds at 300 K, with the exception of AgVN 2 . Analysis of Crystal Orbital Hamiltonian Populations (COHP) provides insight into the bonding and antibonding characters of A–N and B–N pairs. Instability at low temperatures can be attributed to strong A–N antibonding character near the Fermi energy. B–N bonding is found to be crucial in maintaining stability of the structure. AgVN 2 is the only compound to display significant B–N antibonding below the Fermi energy, as well as the strongest degree of A–N antibonding, both of which provide explanation for the sustained instability of this compound up to 900 K. Hybrid functional calculations of electronic and optical properties show that real static dielectric constants in the semiconductors are related to corresponding band gaps through the Moss relation. CuTaN 2 , CuNbN 2 , AgTaN 2 , AgNbN 2 , AgVN 2 , AuTaN 2 , and AuNbN 2 exhibit indirect electronic band gaps while CuVN 2 and AuVN 2 are metallic. Imaginary parts of the dielectric function are characterized by d–d interband transitions in the semiconductors and d–d intraband transitions in the metals. Four compounds, CuTaN 2 , CuNbN 2 , AgTaN 2 , and AgNbN 2 , are predicted to exhibit large light absorption in the range of 1.0 to 1.7 eV, therefore making these materials good candidates for solar-energy conversion applications. Two compounds, AuTaN 2 and AuNbN 2 , have band gaps and absorption onsets near the ideal range for obtaining high solar-cell conversion efficiency, suggesting that these compounds could become potential candidates as absorber materials in tandem solar cells or for band-gap engineering by alloying. 

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5. Observation of chiral surface excitons in a topological insulator Bi 2 Se 3

   https://doi.org/10.1073/pnas.1813514116  

   Kung, H. -H. ; Goyal, A. P. ; Maslov, D. L. ; Wang, X. ; Lee, A. ; Kemper, A. F. ; Cheong, S. -W. ; Blumberg, G. ( February 2019 , Proceedings of the National Academy of Sciences)

   The protected electron states at the boundaries or on the surfaces of topological insulators (TIs) have been the subject of intense theoretical and experimental investigations. Such states are enforced by very strong spin–orbit interaction in solids composed of heavy elements. Here, we study the composite particles—chiral excitons—formed by the Coulomb attraction between electrons and holes residing on the surface of an archetypical 3D TI,${\mathrm{B}\mathrm{i}}_2{\mathrm{S}\mathrm{e}}_3$. Photoluminescence (PL) emission arising due to recombination of excitons in conventional semiconductors is usually unpolarized because of scattering by phonons and other degrees of freedom during exciton thermalization. On the contrary, we observe almost perfectly polarization-preserving PL emission from chiral excitons. We demonstrate that the chiral excitons can be optically oriented with circularly polarized light in a broad range of excitation energies, even when the latter deviate from the (apparent) optical band gap by hundreds of millielectronvolts, and that the orientation remains preserved even at room temperature. Based on the dependences of the PL spectra on the energy and polarization of incident photons, we propose that chiral excitons are made from massive holes and massless (Dirac) electrons, both with chiral spin textures enforced by strong spin–orbit coupling. A theoretical model based on this proposal describes quantitatively the experimental observations. The optical orientation of composite particles, the chiral excitons, emerges as a general result of strong spin–orbit coupling in a 2D electron system. Our findings can potentially expand applications of TIs in photonics and optoelectronics.

    

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