Search

The magnetic circular dichroism in threshold photoemission (TPMCD) for perpendicularly magnetized fcc Co films on Pt(111) has been revisited. A complete mapping of the spectral function $I({E}_{B},{k}_{x},{k}_{y})$ (binding energy ${E}_{B}$, momentum parallel to surface ${k}_{x}$, ${k}_{y}$) and the corresponding TPMCD asymmetry distribution ${A}_{\mathrm{MCD}}({E}_{B},{k}_{x},{k}_{y})$ has been performed for one-photon and two-photon photoemission using time-of-flight momentum microscopy. The experimental results allow distinguishing direct from indirect transitions. The measurements reveal clear band features of direct transitions from bulk bands that show a nontrivial asymmetry pattern. A significant homogeneous background with substantial asymmetry stemming from indirect transitions superposes direct transitions. Two-photon photoemission reveals enhanced emission intensity via an image potential state, acting as intermediate state. The image potential state enhances not only intensity but also asymmetry. The present results demonstrate that two-photon photoemission is a powerful method for mapping the spin-polarized unoccupied band structures and points out pathways for applying TPMCD as a contrast mechanism for various classes of magnetic materials.

Topologické izolátory (Tis) jsou zajímavé materiály, které vykazují nebývalé vlastnosti. . Zde jsme prozkoumali sloučeniny YPtBi jako příklad ze třídy polovu-Heuslerových materiálů. Topological insulators (TIs) are exciting materials, which exhibit unprecedented properties, such as helical spinmomentum locking, which leads to large torques for magnetic switching and highly efficient spin current detection. Here we explore the compound YPtBi, an example from the class of half-Heusler materials, for which the typical band inversion of topological insulators was predicted. We prepared this material as thin films by conventional cosputtering from elementary targets. By in situ time-of-flight momentum microscopy, a Dirac conelike surface state with a Dirac point 300 meV below the Fermi energy was observed, in agreement with electronic structure-photoemission calculations. Only little additional spectral weight due to other states was observed at EF , which corroborates the identification of the topologically protected surface state and is highly relevant for spintronics applications.

Angle-resolved photoemission spectroscopy (ARPES) is one of most powerful techniques to unravel the electronic properties of layered materials and, in recent decades, it has lead to significant progress in the understanding of the band structures of cuprates, pnictides, and other materials of current interest. On the other hand, its application to Mott-Hubbard insulating materials where a Fermi surface is absent has been more limited. Here we show that in these latter materials, where electron spins are localized, ARPES may provide significant information on the spin correlations which can be complementary to the one derived from neutron-scattering experiments. Sr2Cu1-xZnxO2Cl2, a prototype of a diluted spin S=1/2 antiferromagnet (AF) on a square lattice, was chosen as a test case and a direct correspondence between the amplitude of the spectral weight beyond the AF zone boundary derived from ARPES and the spin-correlation length ξ estimated from Cl35 NMR was established. It was found that even for correlation lengths of a few lattice constants, a significant spectral weight in the backbended band is present which depends markedly on ξ. Moreover, the temperature dependence of that spectral weight is found to scale with the x-dependent spin stiffness. These findings prove that the ARPES technique is very sensitive to short-range correlations and its relevance in the understanding of the electronic correlations in cuprates is discussed.