Orbital magnetization in the Nb-substituted Kagome metal CsV$_3$Sb$_5$

TL;DR

This work addresses how Nb-induced chemical pressure alters the electronic structure and orbital magnetism of the kagome metal CsV$_3$Sb$_5$ by using low-temperature soft X-ray ARPES and magnetic circular dichroism (MCDAD). The Nb doping broadens bands and opens a sizeable Dirac gap at the K point, while enabling detailed momentum-resolved mapping of three van Hove singularities near the Fermi level at the $M$ points, all within a 3D CDW framework. Strong MCDAD signals observed in the V/Nb $3d$ states, including domain-dependent sign changes, provide experimental evidence for orbital magnetism and time-reversal symmetry breaking linked to the van Hove singularities. The study thus substantiates a loop-current scenario in Nb-doped CsV$_3$Sb$_5$ and demonstrates how chemical pressure enhances orbital-moment signals, offering a detailed platform to explore CDW and superconductivity in kagome metals.

Abstract

This study uses angle-resolved photoemission spectroscopy to examine the low-temperature electronic structure of Cs(V$_{0.95}$Nb$_{0.05}$)$_3$Sb$_5$, demonstrating that partially substituting V atoms with isoelectronic Nb atoms results in \blue{an increase of the band width} and enhanced gap opening at the Dirac-like crossings due to the resulting chemical pressure. This increases the magnetic circular dichroism signal in the angular distribution (MCDAD) compared to CsV$_3$Sb$_5$, enabling detailed analysis of magnetic circular dichroism in several bands near the Fermi level. These results \blue{substantiate} the predicted coupling of orbital magnetic moments to three van Hove singularities near the Fermi level at M points. Previous studies have observed that Nb doping \blue{lowers the charge density transition temperature} and increases the critical temperature for superconductivity. This article demonstrates that Nb doping concomitantly increases the magnetic circular dichroism signal attributed to orbital moments.

Figures (8)

• Figure 1: (a) Crystal structure of the canonical kagome metal CsV$_3$Sb$_5$ at low temperatures, indicating the orthorhombic 2$\times$2$\times$4 superstructure Kautzsch2023. (b) Experimental ARPES geometry. The angle of incidence of the circularly polarized X-rays is 22.5$^\circ$ with respect to the sample a-b plane. (c) Brillouin zone of the pristine crystal structure in the high temperature phase (we use the high-temperature phase notation throughout the paper). (d) Schematic band structure of pristine CsV$_3$Sb$_5$ highlighting the electronic bands derived from the V 3$d$-orbitals forming the three van Hove singularities near the Fermi level at the M-points.
• Figure 2: (a-d) Constant energy cuts of the photoemission intensities in the $k_x - k_y$ plane at the indicated binding energies, measured at a photon energy of 170 eV for Nb-substituted CsV$_3$Sb$_5$ (probing the $\Gamma$-K-M plane - see Fig. \ref{['Fig1']}). (e-h) Band dispersions along the indicated high symmetry directions in reciprocal space. (i-p) Similar data measured at a photon energy of 155 eV (probing the A-L-H plane - see Fig. \ref{['Fig1']}).
• Figure 3: (a) Comparison of the band dispersions of pristine CsV$_3$Sb$_5$ and Nb-substituted CsV$_3$Sb$_5$ at $T=30$ K. Data for pristine CsV$_3$Sb$_5$ have been measured at a photon energy of 250 eV and the data for the Nb-substituted compound at 170 eV, both corresponding to a cut through the $\Gamma$ point along $k_z$. (b) Band dispersion along the A - L direction for Nb-substituted CsV$_3$Sb$_5$ at 30 K. The adapted intensity scale was changed at $k_x=0.3$ Å$^{-1}$ to show the backfolded band $\alpha'$. (c) Band dispersion along the M - K direction for Nb-substituted CsV$_3$Sb$_5$ at 30 K. (d) Laplacian derivative of the band dispersion along the A-L direction using the data shown in (b). (e) Laplacian derivative of the data shown in (c).
• Figure 4: (a) Sections of the photoemission intensity, $I(E_B,k_y)$, at equidistant $k_x$ values parallel to the K - M direction in the vicinity of the gap at the Dirac point from the same data set shown in Fig. \ref{['FigComp']}(c). (b) Set of energy distribution curves for equidistant momentum values along the profile indicated in the inset. The red marks indicate the maximum intensities near the Dirac point at $E_B=0.3$ eV. (c) Binding energies of the maximum intensities at lower (squares) and higher (circles) binding energy as a function of $k_y$. Parabolic fits (full lines) to the binding energies result in an energy gap of $(96\pm 8)$ meV.
• Figure 5: (a-d) Constant energy sections of the magnetic circular dichroism (MCDAD) and photoemission intensities in the $k_x - k_y$ plane at the indicated binding energies, measured at a photon energy of 170 eV for Nb-substituted CsV$_3$Sb$_5$. The photoemission intensity is overlaid with the MCDAD asymmetry in a two-dimensional color scale as indicated on the right. The maximum asymmetry of the color scale (right) is $A_{\rm max}=0.1$. (e-h) Band dispersion of the MCDAD asymmetry along the indicated lines in panel (a). (i-p) Similar data measured at a photon energy of 155 eV.