Visualizing spin-polarization of an altermagnet KV$_2$Se$_2$O via spin-selective tunneling

Abstract

Altermagnetism, a recently identified magnetic phase that combines vanishing net magnetization with momentum-dependent spin splitting, challenges the conventional dichotomy between ferromagnets and antiferromagnets. While several candidate materials have been proposed, direct experimental evidence linking crystal symmetry, electronic structure and d-wave spin polarization remains scarce. Here we report the visualization of a metallic d-wave altermagnet in KV2Se2O. Through spin-selective scanning tunneling microscopy powered by a topological insulator tip, we uncover symmetry-protected momentum-dependent spin splitting that follows a characteristic d-wave form factor. Our results establish KV2Se2O as a tunable platform to study the interplay between spin-valley locking, Fermi-surface instability and unconventional magnetism, and open a pathway toward symmetry-engineered spintronics without net magnetization.

Figures (4)

• Figure 1: Crystal structure and basic surface properties.(A) Unit cell of KV$_2$Se$_2$O. (B) Projection of K and V-plane with the shade purple circles represent K atoms, blue and red balls represent V atoms with local spin point down and up. Lattice constant $a$ = 3.966 Å. (C) The cleaved surface of KV$_2$Se$_2$O with have of K atoms missing (empty circles). The residual K atoms form a reconstructed $\sqrt{2}a \times \sqrt{2}a$ square lattice. The different size of the red and blue balls represent an SDW modulation. Two opposite-spin sublattices are connected by a spin group symmetry $[U_s||M_{1\bar{1}0}]$. (D) STM topography of the reconstructed $\sqrt{2}a \times \sqrt{2}a$ K-terminated surface. Impurities form arrow shape which points to four orthogonal directions. (E) d$I$/d$V$-spectrum obtained on the K-terminated surface, red and blue curves are measured at 5 K by W- and SmB$_6$-nanowire tips, respectively.
• Figure 2: Band structure of KV$_2$Se$_2$O.(A) Differential conductance (d$I$/d$V$) map measuring $35 \times 35$ nm$^2$, obtained at a bias of 40 mV ($V_{\text{mod}} = 2$ mV). (B) FFT of the map in (A), revealing the dominant scattering vector $q_x$ and $q_y$ (indicated by the white arrow). Green circles mark the Bragg peaks corresponding to the reconstruction lattice. (C) Calculated band structure of KV$_2$Se$_2$O. The inset shows CEC at 40 mV. The scattering geometries for the $q_x$ and $q_y$ components of $\mathbf{q}_1$ are indicated. Note that the Fermi surface includes the first and second Brillouin zones after folding, and for clarity only $d_{xz}$ and $d_{yz}$ orbitals of K are illustrated. (D) Theoretically calculated QPI pattern at 40 mV. (E) Evolution of the QPI signal slices within the orange rectangular region in (B) at various bias voltages. (F) Energy dispersion of $\mathbf{q}_x$. k is extracted from the scattering vector $q$ via the relation $q=2k$. Red dots represent peak positions extracted from Gaussian fitting, with error bars indicating the confidence intervals. The blue lines are guide lines.
• Figure 3: Spin polarized scattering in real space.(A) Topography of K-terminated surface with a single impurity. d$I$/d$V$ line-cut along the orang dashed arrow is presented in (B). (C, D) d$I$/d$V$ maps with strong QPI signal around an impurity measured at 5 K by a W-tip (C) and SmB$_6$-tip (D), respectively. Bias voltage is denoted in the corresponding figure. (E, F) Line cut of the d$I$/d$V$ map along three directions indicated by the dashed line in (C) and (D). All the curves are color coded. (G, H) Demonstration of the standing wave induce by QPI around an impurity that visualized by W-tip (spin insensitive) and SmB$_6$-tip (spin sensitive), respectively. The gray regions indicate the total LDOS.
• Figure 4: Visualize spin polarized band in momentum space.(A, B) FFT difference maps obtained using a W-tip at $\pm20$ mV, respectively. The green circle indicates one of the Bragg peaks. (C, D) Line profiles extracted along the $q_x$ (red arrows) and $q_y$ (black arrows) directions in (A) and (B). (E, F) FFT difference maps obtained using an SmB$_6$-tip at $\pm20$ mV. (G, H) Corresponding line profiles extracted along the $q_x$ and $q_y$ directions are presented in (E) and (F). The gray shaded regions indicate the position of the scattering vector $q$ in k space at the corresponding energy.