Collective Excitations at the FeSe(001) Surface and in Ultrathin FeSe Films
Final Report Abstract
The origin of high-temperature superconductivity in a monolayer (ML) of FeSe on Nb-doped SrTiO3 (or Nb-STO) belongs to one of the unresolved mysteries in condensed-matter physics. Within this project the collective excitations in high-quality epitaxial FeSe MLs on Nb-STO(001) were probed by means of spin-polarized high-resolution electron energy-loss spectroscopy (SPHREELS) over a wide range of momentum and energy. The experiments were carried out over a wide range of incident electron energy and temperature. The results were compared to those recorded at the (001) surface of the β–FeSe bulk crystals. In the following we summarize the main results of our studies. (i) By probing the dynamic charge response of the FeSe ML on Nb-STO, we identified a charge depletion layer at the interface. The formation of the depletion layer explains the long-standing question regarding the origin of the large charge density in FeSe ML when it is grown on STO(001). The presence of the depletion layer has several consequences on the electronic properties of the system, including an interfacial band bending. The observed phenomenon is general and exists also at the interface of other superconducting MLs with many other oxide substrates, allowing an interfacial engineering of the superconducting states by growing MLs of high-temperature superconducting materials on dielectric oxides. Moreover, the observed effect is also of great importance for all van der Waals MLs put in contact with oxide or semiconducting substrates. In a similar way, the exotic properties of these MLs can be altered/tuned through interfacing with dielectrics. (ii) While probing the dynamic charge response of the FeSe superconducting ML on Nb-STO by means of spin-polarized electrons, we observed that the scattering cross section is strongly spin dependent. The observed spin asymmetry is attributed to a large spin-orbit coupling (SOC) at and near the surface region. This large SOC, which is intimately connected to the dielectric depletion layer below the FeSe ML, has several consequences on the properties of the system. One of the possible consequences is the formation of topological states. Moreover, it might be decisive for the mechanism of superconductivity in this system. (iii) Aiming at a detailed understanding of the impact of SOC on the scattering cross section, the theory of low-energy electron scattering was extended by including the SOC term in the formalism. It was shown that, under some assumptions, the spin asymmetry is independent of the electron energy-loss and the momentum transfer. Assuming a Hermitian reflection matrix, one can derive an expression for the scattering cross section, useful for the numerical calculation of the spin-polarized spectra recorded in SPHREELS experiments. (iv) Inspired by the discovery of a large SOC in the FeSe ML on STO and its influence on the scattering cross section, we could demonstrate that in the presence of a large SOC the magnons excited in an ultrathin ferromagnetic film may exhibit nonreciprocal amplitudes. (v) Probing the dynamic charge response of the FeSe ML on STO over a wide range of temperature between 15 and 300 K revealed that, unlike its bulk counterpart, FeSe ML exhibits a nearly temperature independent Fermi surface. This indicates that any temperature-induced change of the chemical potential in FeSe ML is much smaller than that of its bulk counter part. The size of Fermi surface is decisive for several key properties of the system e.g., the value of superconducting transition temperature, the nesting characteristics of the electronic bands, as well as the nature of superconductivity. Our results provide critical information on the evolution of Fermi surface with temperature and are important for a microscopic understanding of high-temperature superconductivity and the associated phenomena in FeSe ML. Moreover, the results may provide guidelines for a possible tuning of superconductivity in Fe-chalcogenide monolayers on oxide surfaces.
Publications
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Generation of spin-polarized hot electrons at topological insulators surfaces by scattering from collective charge excitations. Communications Physics, 4(1).
Zakeri, Khalil; Wettstein, Janek & Sürgers, Christoph
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Theory of spin-polarized high-resolution electron energy loss spectroscopy from nonmagnetic surfaces with a large spin-orbit coupling. Physical Review B, 106(23).
Zakeri, Khalil & Berthod, Christophe
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Direct evidence of a charge depletion region at the interface of van der Waals monolayers and dielectric oxides: The case of superconducting FeSe/STO. Physical Review B, 107(18).
Zakeri, Khalil; Rau, Dominik; Wettstein, Janek; Döttling, Markus; Jandke, Jasmin; Yang, Fang; Wulfhekel, Wulf & Schmalian, Jörg
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Direct Probing of a Large Spin–Orbit Coupling in the FeSe Superconducting Monolayer on STO. ACS Nano, 17(10), 9575-9585.
Zakeri, Khalil; Rau, Dominik; Jandke, Jasmin; Yang, Fang; Wulfhekel, Wulf & Berthod, Christophe
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Giant Spin-Orbit Induced Magnon Nonreciprocity in Ultrathin Ferromagnets. Physical Review Letters, 132(12).
Zakeri, Khalil & von, Faber Albrecht
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Temperature evolution of the Fermi surface of the FeSe monolayer on SrTiO3. Physical Review B.
Zakeri, Khalil; Roemer, Ryan & Zou, Ke
