Seminars Archive

k-resolved electronic structure by soft-X-ray ARPES: From 3D systems to buried interfaces and impurities

Abstract
The spectroscopic power of soft-X-ray ARPES (SX-ARPES), employing photon energies hv around 1 keV, arises from enhancement of the photoelectron escape depth by a factor of 3-5 compared to the conventional VUV-ARPES, concomitant sharp definition of the 3D electron momentum k, and resonant photoexcitation delivering elemental and chemical state specificity. The advanced instrumentation at the Swiss Light Source has allowed SX ARPES to develop from the traditional applications to 3D bulk crystals to the most photon-hungry cases of buried interfaces and impurities. In this talk, I unfold this development vector with a series of recent highlights [1]. 3D materials. Applications to 3D materials are illustrated, for example, by the Fermi surface (FS) of VSe₂ measured at hv around 1 keV. Improved definition of 3D momentum results in textbook clarity of the experimental FS. Its autocorrelation analysis reveals an out-of-plane nesting which acts as the precursor for the exotic 3D charge density waves. Other examples include bulk Rashba splitting in non-centrosymmetric topological insulator BiTeI, bulk FS of the pseudocubic perovskite La_1-xSr_xMnO₃ with characteristic shadow features resembling cuprates and manifesting the rhombohedral structural distortion related to the CMR, conventional superconductor MgB₂, 3D Fermi states of quasicrystalline AlNiCo, etc. Buried heterostructures. The paradigm LaAlO₃/SrTiO₃ interface embeds mobile 2D electron gas whose signal can be accentuated using resonant photoexcitation of the interface Ti³⁺ ions at hv ~ 460 eV. The recent push in energy resolving power towards 20'000 and variable X-ray polarization have enabled resolution of separate d_xy-, d_yz- and d_xz-derived subbands in the interface quantum well, with their Fourier composition modulating their ARPES response through k-space. The peak-dip-hump spectral lineshape manifests polaronic coupling of the interface electrons, immediately reducing their mobility. The polaronic hump scales up with temperature, explaining the mobility drop above 200K. Oxygen vacancies increase the electron concentration, as expressed by the Luttinger count of the FS, and reduce the polaronic coupling. These findings extend to other oxide heterostructure systems. Buried impurities. Resonant SX-ARPES applied to the paradigm diluted magnetic semiconductor GaMnAs has identified the ferromagnetic Mn impurity band, and established its energy alignment and mechanism of hybridization with the host GaAs bands. Combining the previous p-d exchange and double-exchange models, these results suggest a microscopic picture of the GaMnAs ferromagnetism based on the Anderson impurity model. Another example in this field is InFeAs showing the ferromagnetism induced by doped highly mobile electron carriers. [1] V.N. Strocov et al, Synchr. Rad. News 27, N2 (2014) 31