Discerning ground state and photoemission-induced spin textures in altermagnetic $α$-MnTe

Abstract

Recently discovered altermagnets provide a physical realization of an unconventional compensated magnetic phases with a higher partial-wave type of ordering, reminiscent of unconventional superfluid phases. Their stability under normal conditions has sparked significant research interest, spanning fields from spintronics to topological and correlated quantum materials. Spin- and angle-resolved photoemission spectroscopy (SARPES) has great promise to resolve the momentum-dependent spin textures intricately interweaved with the altermagnetic real space spin order. Using the relativistic $d$-wave-like spin polarization on one of the nodal surfaces of the altermagnetic band structure of $α$-MnTe as an example, we here identify and resolve the challenges associated with (S)ARPES studies on altermagnets and offer insights into data interpretation. We focus particularly on the role of photoemission-induced electron polarization and the coupling between light and the Néel vector of a magnetic domain. Our findings reveal an extraordinary behaviour of photoemission selection rules while using linearly-polarized light. We observe, and distinguish, polarization of photoelectrons originating from the sample's ground state spin texture, on one hand, and from the photoemission process, on the other hand. Our experimental results are supported by a combination of ab initio band-structure and 1-step photoemission calculations.

Figures (10)

• Figure 1: (a) Schematic representation of 2D $d$-wave-like out-of-plane polarization symmetry the reciprocal space. Notation of high-symmetry points of the Brillouin zone at $k_z = 0$ is shown. $d$-wave symmetry of $S_z$ is sketched on the Fermi surface cut at binding energy -0.1 eV. The Néel vector is indicated with a yellow arrow. (b) shows the spin-polarized band structure along the $\overline{\Gamma}-\overline{\text{K}}_i$, $\overline{\Gamma}-\overline{\text{M}}_i$ paths at $k_z = 0$krempasky2024altermagnetic.
• Figure 2: Calculated SARPES maps of the out-of-plane spin polarization at the $k_z=0$ nodal plane at $h\nu=82$ eV along $\overline{\text{K}}-\overline{\Gamma}-\overline{\text{K}}$ direction. The domain orientation and experimental geometry in each plot are sketched in the inset. (a)-(d) correspond to the $s$-polarized light, (e)-(h) correspond to $p$-polarized light.
• Figure 3: Constant energy slices (100 meV integration) from the photon energy scan at (a) $E_B = 150~meV$ and (b) $E_B = 500~meV$ measured with $p$-polarized light along $\overline{\text{K}}-\overline{\Gamma}-\overline{\text{K}}$ direction. Positions of high symmetry points are indicated. Constant-angle intense lines at $h\nu \approx 40$ and $48$ eV are instrumental artifacts due to high order harmonics coming from the undulator.
• Figure 4: (a)-(d) Constant energy maps 100 meV below $E_F$ for 4 different light polarizations, taken with $h\nu=78$ eV, the borders of the surface Brillouin zone are indicated; (e)-(h) corresponding bandmap cuts along the $\overline{\text{K}}-\overline{\Gamma}-\overline{\text{K}}$ directions, indicated with dashed red lines in (a) and (b); (e) and (g) show the raw data, (f) and (h) display the data with angle-integrated background subtracted for better visibility.
• Figure 5: SARPES 1-step calculations and measurements for the balanced (a) an imbalanced (b)-(d) domain configurations. The measured data were taken at $h\nu=78$ eV and the calculated ones are at $h\nu=82$ eV. The experimental geometry and domain combinations are shown in the inset.