Accessing quasi-flat $\textit{f}$-bands to harvest large Berry curvature in NdGaSi

Abstract

In typical rare-earth lanthanide compounds, the localized 4\textit{f}-electrons have a weak effect on the electrical conduction, limiting their influence on the Berry curvature and, hence, the intrinsic anomalous Hall effect. A comprehensive study of the magnetic, thermodynamic, and transport properties of single-crystalline NdGaSi, guided by first-principles calculations, reveals a ferromagnetic ground state that induces a splitting of quasi-flat 4\textit{f} electronic bands and positions them near the Fermi energy. The observation of an extraordinarily large intrinsic anomalous Hall conductivity of 1165 $Ω^{-1}$ cm$^{-1}$ implies the direct involvement of localized states in the generation of non-trivial band crossings around the Fermi energy. The angle-resolved photoemission spectroscopy measurements provide direct evidence of non-trivial crossing of the 4\textit{f}-bands with dispersive bands. These results are remarkable when compared to ferrimagnetic NdAlSi, which differs only in a non-magnetic atom (a change in the principal quantum number \textit{n} of the outer \textit{p }orbital) with the same number of valence electrons and does not exhibit any measurable anomalous Hall conductivity.

Figures (4)

• Figure 1: (a) A tetragonal unit cell of NdGaSi. A simultaneous occupancy of Ga and Si sites (complete site mixing) imparts a centrosymmetric space group I4$_1$/amd. (b) Magnetic susceptibility ($\chi$) measured under the field-cooled (FC) condition by applying the magnetic field along the in-plane and out-of-the-plane applied magnetic field. (c) Isothermal magnetization curves of NdAlSi and NdGaSi at 2 K with the magnetic field applied along [001] and [100]. (d) Longitudinal resistivity ($\rho_{xx}$) as a function of temperature under zero magnetic field, along with a fit (red) at low temperature to obtain the coefficient of electron scattering.
• Figure 2: (a) Specific heat of NdGaSi and NdAlSi as a function of temperature. The inset shows the variation of both magnetic specific heat and entropy as a function of temperature. (b) Specific heat data below 12 K with fitting. (c) Density of states of NdGaSi and NdAlSi in the ground state. (d) Kadowaki-Woods ratio of NdGaSi and its representation among itinerant and heavy Fermion systems.
• Figure 3: (a) The band structure of NdGaSi with SOC in the FM state, with the spins aligned along [001]. (b) Magnetic field-dependent Hall resistivity ($\rho_{yx}$) at different temperatures ranging from 2 K to 30 K with $B \parallel$ [001] and $I \parallel$ [010]. (c) Temperature dependence of the anomalous Hall conductivity ($\sigma^{A}_{xy}$) and anomalous Hall angle ($\Theta_\mathrm{H}$). (d) Linear fitting of anomalous Hall conductivity $\sigma^{A}_{xy}$ vs. $\sigma^{2}_{xx}$ curve. (e) BC distribution of NdGaSi in the first Brillouin zone reveals regions of maximum positive and negative values, depicted in red and blue, respectively. (f) Energy-dependent AHC obtained from the first-principles calculations.
• Figure 4: (a) ARPES spectra of NdGaSi along $\Gamma -\mathrm{X}$ direction with incident photon energy of 120 eV recorded using linear horizontal polarized light. The spectra are presented with the overlapped band structure obtained from DFT (with SOC) represented by dotted yellow lines. Red boxes indicate the quasiflat bands observed from ARPES. (b) Experimental Fermi surface recorded using hv = 50 eV and linear polarized light. (c) Theoretical Fermi surface calculated from DFT at the $\Gamma-\mathrm{X}-\mathrm{M}$ plane ($k_z = 0$).