Measurements of electronic band structure in CeCoGe$_3$ by angle-resolved photoemission spectroscopy

TL;DR

The paper reports a comprehensive ARPES study of CeCoGe3 in the non-magnetic regime, mapping its electronic structure across the 3D Brillouin zone. The authors find substantial agreement with LDA+G calculations that assume localized Ce 4f electrons, after a rigid energy shift of about 180 meV, and identify two new features: a surface state and band folding indicative of unit-cell doubling. They provide evidence for topological features, including nodal lines and possible Weyl crossings near the Fermi energy, inferred from polarization-dependent dispersions. These results illuminate the interplay of topology, strong correlations, and surface effects in a non-centrosymmetric heavy-fermion compound and bear relevance for superconductivity under pressure.

Abstract

We report a comprehensive study of the electronic structure of CeCoGe$_3$ throughout the entire Brillouin zone in the non-magnetic regime using angle-resolved photoemission spectroscopy (ARPES). The electronic structure agrees in large part with first principles calculations, including predicted topological nodal lines. Two new features in the band structure are also observed: a surface state and folded bands, the latter which is argued to originate from a unit cell reconstruction.

Figures (6)

• Figure 1: CeCoGe$_3$ (a) crystal structure (conventional unit cell). Arrows indicate primitive lattice vectors. (b) Brillouin Zone with high symmetry points labeled.
• Figure 2: Core level spectroscopy. (a) Core level survey. (b) Momentum-integrated valence band and localized contributions from the Ce $4f$ derived states. Inset shows original energy vs momentum cut, together with integration window. Panels (a)-(b) taken at 125 eV. (c) Ge $3d$ level taken at 140 eV, together with fitting to two doublets. Dashed line is residual of fit. (d) photon energy dependence of intensity ratio of two fitted doublets (e) calculated IMFP as a function of photon energy for Ge $3d_{1/2}$ and $3d_{3/2}$ (left axis) as well as valence electrons (right axis).
• Figure 3: Cuts parallel to $\Gamma-\Sigma$ at different $k_z$. (a)-(e) Energy vs momentum cuts at photon energies indicated in each panel. Spectra collected with RCP+LCP light. Pink vertical arrows point to 'x-shaped' feature and horizontal pink arrow points to dispersing feature not directly captured in calculation. (f)-(j) LDA+G calculations from Ref. ivanov2021renormalized corresponding to each data panel above. Calculations have been shifted to lower binding energy (towards $E_F$) by 180 meV for better agreement with data. Vertical dashed lines denote high symmetry points in (f),(h) and (j), and the edge of the BZ at that value of $k_z$ otherwise. Right half of panels (f),(g),(i),(j) shows overlay of calculation from half a BZ away along $k_z$, with color of these dashed lines corresponding to legend labels below. Color bar on right applies to all ARPES image plots in manuscript.
• Figure 4: Constant energy maps at $E_F$. (a) Fermi surface map in $\Gamma-X-Z$ plane taken with LH light in photon energy range 26-118 eV. Yellow dots mark high symmetry points. Solid and dashed lines correspond to locations of maps in panels (c)-(g). (b) Fermi surface map in same momentum space region as (a) but taken with LCP+RCP light. (c)-(g) Constant energy maps in $k_x$-$k_y$ plane, taken with LH light. Data taken at select photon energies to closely match to the following planes along $\overline{\Gamma Z}$: $\Gamma$ plane, $k_z = 0.25$$\overline{\Gamma Z}, 0.5$$\overline{\Gamma Z}, 0.75$$\overline{\Gamma Z},$ and the $Z$ plane. Column 1 is taken at lower photon energies (solid lines in (a)). Second column is taken at higher photon energy (dashed lines in (a)), but closely matching locations in BZ. Third column is calculated Fermi surfaces for the $k_z$ values probed by data.
• Figure 5: Surface-like features in CeCoGe$_3$. All cuts are taken parallel to $\Gamma-\Sigma$ at different photon energies with LH polarization. Calculations are shown in measured plane (solid) and half a BZ away in $k_z$ (dashed). Pink arrow marks one instance of this surface band in each panel. (a) 42 eV (b) 66 eV (near Z plane) (c) 86 eV.