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DDC: 530 × Subject: Condensed matter ×

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    V4 tetrahedral units in AV4X8 lacunar spinels: Near degeneracy, charge fluctuations, and configurational mixing within a valence space of up to 21 d orbitals
    (2020) Hozoi, L.; Eldeeb, M.S.; Rößler, U.K.
    All properties of a given molecule or solid are determined by the way valence electrons are distributed over single-particle energy levels. For multiple, closely spaced single-particle levels, different occupation patterns may provide many-electron quantum states that are close in energy, interact, and admix. We address such near-degeneracy electron correlation effects for V4 vanadium tetrahedral units as encountered in the lacunar spinel GaV4S8, explicitly taking into account up to 21 vanadium valence orbitals, and find effective orbital occupation numbers much different as compared to the picture previously laid out on the basis of mean-field calculations. In light of these results, a modified theoretical frame seems necessary to explain the peculiar magnetic properties of lacunar spinels and of related compounds.
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    Two-color two-dimensional terahertz spectroscopy: A new approach for exploring even-order nonlinearities in the nonperturbative regime
    (Melville, NY : American Institute of Physics, 2021) Woerner, Michael; Ghalgaoui, Ahmed; Reimann, Klaus; Elsaesser, Thomas
    Nonlinear two-dimensional terahertz (2D-THz) spectroscopy at frequencies of the emitted THz signal different from the driving frequencies allows for exploring the regime of (off-)resonant even-order nonlinearities in condensed matter. To demonstrate the potential of this method, we study two phenomena in the nonlinear THz response of bulk GaAs: (i) The nonlinear THz response to a pair of femtosecond near-infrared pulses unravels novel fourth- and sixth-order contributions involving interband shift currents, Raman-like excitations of transverse-optical phonon and intervalence-band coherences. (ii) Transient interband tunneling of electrons driven by ultrashort mid-infrared pulses can be effectively controlled by a low-frequency THz field with amplitudes below 50 kV/cm. The THz field controls the electron–hole separation modifying decoherence and the irreversibility of carrier generation.