Robust spin splitting and fermiology in a layered altermagnet

Abstract

Altermagnetism defies conventional classifications of collinear magnetic phases, standing apart from ferromagnetism and antiferromagnetism with its unique combination of spin-dependent symmetries, net-zero magnetization, and anomalous Hall transport. Although altermagnetic states have been realized experimentally, their integration into functional devices has been hindered by the structural rigidity and poor tunability of existing materials. First, through cobalt intercalation of the superconducting 2H-NbSe$_2$ polymorph, we induce and stabilize a robust altermagnetic phase and using both theory and experiment, we directly observe the lifting of Kramers degeneracy. Additionally, we present spectroscopic insight into a previously hinted low-temperature phase, and provide evidence of its electronic origin. While shedding light on overlooked aspects of altermagnetism, these findings open pathways to spin-based technologies and lay a foundation for advancing the emerging field of altertronics.

Figures (3)

• Figure 1: Surface-dependent crystalline and electronic structure.a. Schematic of Co$_{1/4}$NbSe$_{2}$ formed by 2H-NbSe$_2$ planes linked by Co atoms in a $2\times2$ reconstruction. b. LEED data (25, $\sim120\K$) showing the $2\times2$ reconstruction: the orange circles indicate the $2\times2$ features, while the blue circles are the primitive ones. c--d. Schematics showing the possible surface terminations after cleaving: Se and Co. These two terminations are clearly visible in both STM and micro-ARPES data. e. Se-termination (characterized by the triangular tiling) and f. Co-termination, measured by STM, and their corresponding Fourier transforms. g. micro-ARPES spatial map acquired by measuring the Se 3d and Co 3p core levels with yellow corresponding to Se-terminated regions. h. Core levels acquired from the two different surface terminations (curve colors matching the circle colors in g); the highlighted spectral features are attributed to surface replicas for the Se-termination. i--j. Corresponding Fermi surfaces and (E, k) dispersions, collected at 25 with p-polarized photons at 47.5 (black indicates high intensity in the ARPES scale).
• Figure 2: Fermiology, DFT, and spin splitting. Fermi surfaces at bulk $\Gamma$ (51, a.), A (75, b.), and an intermediate point near $k_z=\pi/2c$ (55, c. - this energy is favorable in terms of matrix elements to detect the splitting in the Fermi surface). d. Zoom-in on the splitting, with and without DFT calculations. The arrows indicate sections were the bands are prominently separated. e. (E, k)-spectra at an intermediate point between $\Gamma$ and $A$ (44 shows more favorable matrix elements in this direction - See Supporting Information for additional energies) using p- (LH, left) and s-polarized (LV, right) light, with DFT calculations on the lower panel. f. DFT spin-polarized 2D Fermi surfaces of Co$_{1/4}$NbSe$_2$ at $k_z=0$ , $k_z=\pi/c$, and $k_z=\pi/2c$. g. MDCs at the Fermi level from e., confirming multiple split bands. h. DFT spin-polarized bands of Co$_{1/4}$NbSe$_2$ unfolded in the large Brillouin zone of ($1\times1$) NbSe$_2$ along $\mathrm{M}-\Gamma-\mathrm{M}$ for $k_z=0$, $k_z=\pi/c$, and $k_z=\pi/2c$, with spectral weight $P_{\vec{K}m}$ defined in Eq. \ref{['eq:eq1']}. i. spin-ARPES at opposite k-points (55p-polarized, as in c.), showing EDCs at $k_{s1}$ and MDCs at the Fermi level (insets).
• Figure 3: Low-temperature phase transition. ARPES spectra (black indicates high intensity in the ARPES scale) collected at two distinct photon energies: 37.5 (top row) and 47.5 (bottom row), and for temperatures above and below $T_0$. These are shown for both a.p-polarization (LH) and b.s-polarization (LV). c. The difference between spectra collected above and below $T_0$; orange (purple) corresponds to a negative (positive) difference. The strong electronic renormalization, indicating a transition, manifests as a decrease in the overall bandwidth, while $k_F$ remains constant. d. EDCs collected at different photon energies (indicated along the vertical axis) for spectra below $T_0$ (orange) and above it (purple). These are shown for two k values (k1 and k2) as indicated in a.e.$\Delta R/R$ signal as measured by ultrafast reflectivity at $t-t_0\sim100f\s$ for 150µ□ fluence. The two zero-crossings of the $\Delta R/R$ signal at $T_0\approx50\K$ and $T_1\approx130\K$ are highlighted.