Flat band driven itinerant magnetism in the Co-pnictides CaCo$_2$As$_2$ and LaCo$_2$P$_2$

Abstract

Flat bands can induce strong electron correlation effects that help stabilize both magnetic and superconducting states. Here, we carry out angle-resolved photoemission spectroscopy and density functional theory calculations to study the electronic structure of the Co-pnictides CaCo$_2$As$_2$ and LaCo$_2$P$_2$. We find that, while the $k_z$ Fermi topology of ferromagnetic LaCo$_2$P$_2$ is markedly 2-dimensional, antiferromagnetic CaCo$_2$As$_2$ develops a 3D Fermi surface described by a $zig-zag$-like band dispersion perpendicular to the Co-As plane. Furthermore, the magnetism is driven by the electronic correlations of the flat bands with $d_{xy}$ and $d_{z^2}$ orbital character at the Fermi level. Our results link the electronic dimensionality and the magnetic order, and emphasize the critical role of the As-As and P-P bond strength along the $c$-direction to understand the electronic band structure and the rich phase diagram of transition metal pnictides.

Figures (4)

• Figure 1: (a) Conventional unit cell of the Co-pnictides with $\mathrm{ThCr_2Si_2}$-type structure, where A = Ca, La, and X = As, P. (b) Brillouin zone (BZ) and the projected BZ on the (001) surface, along the high symmetry directions. (c-d) Isotropic ($\sigma^+$+$\sigma^-$) x-ray absorption spectroscopy (XAS) of $\mathrm{CaCo_2As_2}$ and $\mathrm{LaCo_2P_2}$, respectively, labeling the different transitions in the inset.
• Figure 2: (a-b) Atomically resolved density of states (DOS) of $\mathrm{CaCo_2As_2}$ for the PM (a) and AFM (b) phases. (c) k$_x$-k$_z$ Fermi surface of $\mathrm{CaCo_2As_2}$ at T=20 K exhibiting a zig-zag dispersion along the c-direction, superimposed with the Fermi momentum, (k$_f$), extracted from the momentum distribution curves (MDCs). (d,e) Fermi surface of $\mathrm{CaCo_2As_2}$, corresponding to the $k_z=0$ and $k_z=\frac{\pi}{c}$ planes, taken with $h\nu=78\,\mathrm{eV}$ and $h\nu=105\,\mathrm{eV}$, respectively. (f-g) Total and atomically resolved DOS for the PM (f) and FM (g) order of $\mathrm{LaCo_2P_2}$. (h) k$_x$-k$_z$ Fermi surface of $\mathrm{LaCo_2P_2}$, T=20 K. (i,j) k$_x$-k$_y$ Fermi surface at the $k_z=0$ and $k_z=\frac{\pi}{c}$ planes of $\mathrm{LaCo_2P_2}$, taken with $h\nu=85\,\mathrm{eV}$ and $h\nu=100\,\mathrm{eV}$, respectively, and T=20 K. The yellow dashed lines that highlight the electron pockets are a guide to the eye.
• Figure 3: (a-b) Valence band dispersion of CaCo$_2$As$_2$ along the high symmetry $\mathrm{X}-\Gamma-\mathrm{M}$ line at $k_z=0$ (E$_\mathrm{i}$=78 eV), T=150 K. (c) Orbital resolved DFT calculation of the d orbitals of Co at $k_z=0$ in the paramagnetic state. (d,e) VB spectrum along the high symmetry $\mathrm{R}-\mathrm{Z}-\mathrm{A}$ line. (f) Atomic-orbital contribution DFT calculation of the d orbitals of Co at $k_z=\frac{\pi}{c}$. (g-h) VB dispersion at low temperature and $k_z=0$ (i) DFT calculation for $k_z=0$, and (j-i) for $k_z=\frac{\pi}{c}$. See supp. inf. Fig. 6 for high-resolution VBs that identify the bands crossing the Fermi level in (h) and (k).
• Figure 4: (a-b) VB dispersion along the high symmetry $\mathrm{X}-\Gamma-\mathrm{M}$ line for LaCo$_2$P$_2$, $k_z=0$ (E$_\mathrm{i}$=85 eV), T=150 K. (c) Orbital resolved VB at k$_z$=0. (d,e) Band dispersion following the $\mathrm{R}-\mathrm{Z}-\mathrm{A}$ path, E$_\mathrm{i}$=100 eV. (f) Orbital contribution to the band structure at $k_z=\frac{\pi}{c}$. (g-h) Band dispersion at low temperature, T=20 K and k$_z$=0 plane. (i) VB DFT calculation at k$_z$=0. (j-k) Band dispersion and (l) DFT calculation at the $k_z=\frac{\pi}{c}$ plane.