Spectroscopic Evidence for Electron-Boson Coupling in Half-metallic CrO2

TL;DR

This paper investigates whether many-body interactions shape the spin-polarized electronic structure of the half-metal CrO2. Using high-resolution ARPES, it maps Fermi surfaces and dispersions, extracts Re$\Sigma$ and Im$\Sigma$, and identifies a kink near 68 meV, indicating coupling to bosonic modes. A two-boson model including electron-phonon and electron-magnon interactions reproduces the self-energy and ARPES spectra, yielding $\lambda_{el-ph} \sim 0.6$ and an effective mass enhancement of about 1.6, consistent with transport and specific-heat hints. The results establish renormalized quasiparticle dynamics in CrO2 and highlight the importance of many-body interactions in half-metallic ferromagnets for spintronic applications.

Abstract

We report quasiparticle properties of the half-metal ferromagnet CrO2 by means of high-resolution angle-resolved photoemission spectroscopy (ARPES). We clearly observed the Fermi surface (FS) and band dispersion in good agreement with the previous reports. Moreover, the ARPES band dispersion reveals a distinct kink structure around 68 meV, providing the first spectroscopic evidence for the elementary excitations in CrO2. The energy scale of this feature is comparable to the Debye temperature and the $A\subm{1g}$ phonon mode, suggesting the electron-phonon interaction. From the detailed analysis, we have extracted the self-energy and found two characteristic structures in the real part of the self-energy. Assuming the existence of the electron-magnon interaction as well as the electron-phonon interaction, we could reproduce the evaluated real and imaginary parts of the self-energy as well as ARPES intensity. Our findings reveal the renormalized quasiparticle (QP) dynamics in CrO$_2$ and provide valuable insights into the fundamental many-body interactions governing half-metallic ferromagnets.

Figures (3)

• Figure 1: (color online) (a) Crystal structure of CrO$_2$ visualized by VESTA vesta. (b) Brillouin zone of CrO$_2$. (c) Symmetrized FS of CrO$_2$ at $T = 20$ K of $\Gamma^\prime$X$^\prime$R$^\prime$Z$^\prime$ plane, corresponding to the blue shaded area in (b). The data were collected at $h\nu = 114$ eV.
• Figure 2: (color online) (a) ARPES band dispersion along $\Gamma^\prime$-X$^\prime$ direction at $T$ = 20 K. (b) Photon-energy dependence of ARPES spectra in $\Gamma^\prime$X$^\prime$M$^\prime$ plane as indicated by the green shaded area in Fig. \ref{['f1']}(b). (c) Enlarged view of ARPES spectra near $E_{\mathrm {F}}$ for the rectangular region (green dashed line) in (a). The ARPES spectra of (a) and (c) were obtained at $h\nu = 114$ eV, corresponding to the green dotted line in (b).
• Figure 3: (color online) (a) ARPES band dispersion along $\Gamma^\prime$-X$^\prime$ direction in Fig. \ref{['f2']}(c). The grey solid line and the dashed line indicate the fitted results of MDCs and the bare parabolic band dispersion without the electron-boson coupling. (b) Experimental real part of self-energy Re$\Sigma$ and imaginary part of self-energy Im$\Sigma$ deduced from the ARPES band dispersion of (a). Simulation of (c) Re$\Sigma$ and (d) Im$\Sigma$ based on the two-boson model with the electron-phonon (Green dashed-and-dotted line) and electron-magnon interaction (Black dashed line). (e) Calculated self-energy using the previously reported phonon-DOS Kim2012. The dotted line indicates the real part of self-energy Re$\Sigma_{\mathrm {\textrm{el-ph}}}$ in (c) for comparison with simulation. (f) Simulated ARPES data using the self-energies in (c) and (d).