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- ItemEvidence of two-dimensional flat band at the surface of antiferromagnetic kagome metal FeSn([London] : Nature Publishing Group UK, 2021) Han, Minyong; Inoue, Hisashi; Fang, Shiang; John, Caolan; Ye, Linda; Chan, Mun K.; Graf, David; Suzuki, Takehito; Ghimire, Madhav Prasad; Cho, Won Joon; Kaxiras, Efthimios; Checkelsky, Joseph G.The kagome lattice has long been regarded as a theoretical framework that connects lattice geometry to unusual singularities in electronic structure. Transition metal kagome compounds have been recently identified as a promising material platform to investigate the long-sought electronic flat band. Here we report the signature of a two-dimensional flat band at the surface of antiferromagnetic kagome metal FeSn by means of planar tunneling spectroscopy. Employing a Schottky heterointerface of FeSn and an n-type semiconductor Nb-doped SrTiO3, we observe an anomalous enhancement in tunneling conductance within a finite energy range of FeSn. Our first-principles calculations show this is consistent with a spin-polarized flat band localized at the ferromagnetic kagome layer at the Schottky interface. The spectroscopic capability to characterize the electronic structure of a kagome compound at a thin film heterointerface will provide a unique opportunity to probe flat band induced phenomena in an energy-resolved fashion with simultaneous electrical tuning of its properties. Furthermore, the exotic surface state discussed herein is expected to manifest as peculiar spin-orbit torque signals in heterostructure-based spintronic devices.
- ItemPlethora of tunable Weyl fermions in kagome magnet Fe3Sn2 thin films([London] : Nature Publishing Group, 2022) Ren, Zheng; Li, Hong; Sharma, Shrinkhala; Bhattarai, Dipak; Zhao, He; Rachmilowitz, Bryan; Bahrami, Faranak; Tafti, Fazel; Fang, Shiang; Ghimire, Madhav Prasad; Wang, Ziqiang; Zeljkovic, IlijaInterplay of magnetism and electronic band topology in unconventional magnets enables the creation and fine control of novel electronic phenomena. In this work, we use scanning tunneling microscopy and spectroscopy to study thin films of a prototypical kagome magnet Fe3Sn2. Our experiments reveal an unusually large number of densely-spaced spectroscopic features straddling the Fermi level. These are consistent with signatures of low-energy Weyl fermions and associated topological Fermi arc surface states predicted by theory. By measuring their response as a function of magnetic field, we discover a pronounced evolution in energy tied to the magnetization direction. Electron scattering and interference imaging further demonstrates the tunable nature of a subset of related electronic states. Our experiments provide a direct visualization of how in-situ spin reorientation drives changes in the electronic density of states of the Weyl fermion band structure. Combined with previous reports of massive Dirac fermions, flat bands, and electronic nematicity, our work establishes Fe3Sn2 as an interesting platform that harbors an extraordinarily wide array of topological and correlated electron phenomena.
- ItemIntertwined electronic and magnetic structure of the van-der-Waals antiferromagnet Fe2P2S6([London] : Nature Publishing Group, 2023) Koitzsch, A.; Klaproth, T.; Selter, S.; Shemerliuk, Y.; Aswartham, S.; Janson, O.; Büchner, B.; Knupfer, M.Many unusual and promising properties have been reported recently for the transition metal trichalcogenides of the type MPS3 (M = V, Mn, Fe, Ni..), such as maintaining magnetic order to the atomically thin limit, ultra-sharp many-body excitons, metal-insulator transitions and, especially for Fe2P2S6, giant linear dichroism among others. Here we conduct a detailed investigation of the electronic structure of Fe2P2S6 using angle-resolved photoemission spectroscopy, q-dependent electron energy loss spectroscopy, optical spectroscopies and density functional theory. Fe2P2S6 is a Mott insulator with a gap of E gap ≈ 1.4 eV and zigzag antiferromagnetism below T N = 119 K. The low energy excitations are dominated by Fe 3d states. Large and sign-changing linear dichroism is observed. We provide a microscopic mechanism explaining key properties of the linear dichroism based on the correlated character of the electronic structure, thereby elucidating the nature of the spin-charge coupling in Fe2P2S6 and related materials.
- ItemValence-state reflectometry of complex oxide heterointerfaces(London : Nature Publishing Group, 2016) Hamann-Borrero, Jorge E.; Macke, Sebastian; Choi, Woo Seok; Sutarto, Ronny; He, Feizhou; Radi, Abdullah; Elfimov, Ilya; Green, Robert J.; Haverkort, Maurits W.; Zabolotnyy, Volodymyr B.; Lee, Ho Nyung; Sawatzky, George A.; Hinkov, VladimirEmergent phenomena in transition-metal-oxide heterostructures such as interface superconductivity and magnetism have been attributed to electronic reconstruction, which, however, is difficult to detect and characterise. Here we overcome the associated difficulties to simultaneously address the electronic degrees of freedom and distinguish interface from bulk effects by implementing a novel approach to resonant X-ray reflectivity (RXR). Our RXR study of the chemical and valance profiles along the polar (001) direction of a LaCoO3 film on NdGaO3 reveals a pronounced valence-state reconstruction from Co3+ in the bulk to Co2+ at the surface, with an areal density close to 0.5 Co2+ ions per unit cell. An identical film capped with polar (001) LaAlO3 maintains the Co3+ valence over its entire thickness. We interpret this as evidence for electronic reconstruction in the uncapped film, involving the transfer of 0.5e− per unit cell to the subsurface CoO2 layer at its LaO-terminated polar surface.
- ItemStructure-property relationship of Co 2 MnSi thin films in response to He + -irradiation([London] : Macmillan Publishers Limited, part of Springer Nature, 2019) Hammerath, Franziska; Bali, Rantej; Hübner, René; Brandt, Mira R. D.; Rodan, Steven; Potzger, Kay; Böttger, Roman; Sakuraba, Yuya; Wurmehl, SabineWe investigated the structure-property relationship of Co2MnSi Heusler thin films upon the irradiation with He+ ions. The variation of the crystal structure with increasing ion fluence has been probed using nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM), and associated with the corresponding changes of the magnetic behavior. A decrease of both the structural order and the moment in saturation is observed. Specifically, we detect a direct transition from a highly L21-ordered to a fully A2-disordered structure type and quantify the evolution of the A2 structural contribution as a function of ion fluence. Complementary TEM analysis reveals a spatially-resolved distribution of the L21 and A2 phases showing that the A2 disorder starts at the upper part of the films. The structural degradation in turn leads to a decreasing magnetic moment in saturation in response to the increasing fluence.
- ItemMagnetic patterning of Co/Ni layered systems by plasma oxidation([London] : Macmillan Publishers Limited, part of Springer Nature, 2022) Anastaziak, Błażej; Andrzejewska, Weronika; Schmidt, Marek; Matczak, Michał; Soldatov, Ivan; Schäfer, Rudolf; Lewandowski, Mikołaj; Stobiecki, Feliks; Janzen, Christian; Ehresmann, Arno; Kuświk, PiotrWe studied the structural, chemical, and magnetic properties of Ti/Au/Co/Ni layered systems subjected to plasma oxidation. The process results in the formation of NiO at the expense of metallic Ni, as clearly evidenced by X-ray photoelectron spectroscopy, while not affecting the surface roughness and grain size of the Co/Ni bilayers. Since the decrease of the thickness of the Ni layer and the formation of NiO increase the perpendicular magnetic anisotropy, oxidation may be locally applied for magnetic patterning. Using this approach, we created 2D heterostructures characterized by different combinations of magnetic properties in areas modified by plasma oxidation and in the regions protected from oxidation. As plasma oxidation is an easy to use, low cost, and commonly utilized technique in industrial applications, it may constitute an improvement over other magnetic patterning methods.
- ItemNew magneto-polaron resonances in a monolayer of a transition metal dichalcogenide([London] : Macmillan Publishers Limited, part of Springer Nature, 2023) Trallero-Giner, Carlos; Santiago-Pérez, Darío G.; Fomin, Vladimir M.Transition metal dichalcogenide (TMD) semiconductors are two-dimensional materials with great potential for the future of nano-optics and nano-optoelectronics as well as the rich and exciting development of basic research. The influence of an external magnetic field on a TMD monolayer raises a new question: to unveil the behavior of the magneto-polaron resonances (MPRs) associated with the phonon symmetry inherent in the system. It is shown that the renormalized Landau energy levels are modified by the interplay of the long-range Pekar–Fröhlich (PF) and short-range deformation potential (DP) interactions. This leads to a new series of MPRs involving the optical phonons at the center of the Brillouin zone. The coupling of the two Landau levels with the LO and A1 optical phonon modes provokes resonant splittings of double avoided-crossing levels giving rise to three excitation branches. This effect appears as bigger energy gaps at the anticrossing points in the renormalized Landau levels. To explore the interplay between the MPRs, the electron-phonon interactions (PF and DP) and the couplings between adjacent Landau levels, a full Green’s function treatment for the evaluation of the energy and its life-time broadening is developed. A generalization of the two-level approach is performed for the description of the new MPR branches. The obtained results are a guideline for the magneto-optical experiments in TMDs, where three MPR peaks should be observable.