Coherent magnetic excitations in a topological Kondo semimetal

Abstract

In Kondo insulators the many-body Kondo lattice effect drives the formation of bands containing heavy charge carriers with a hybridization gap, leading to insulating properties. These renormalized bands can host non-trivial topologies driven by strong electron-electron interactions, but probing narrow heavy bands at low temperatures is challenging. We use inelastic neutron scattering (INS) to probe the Kondo lattice CeNiSn, which hosts both semimetallic transport properties and a hybridization gap. The INS response exhibits momentum-dependent magnetic excitations and a spin-gap in the low-temperature Kondo coherent state, which electronic structure calculations corroborate as arising from the renormalized heavy band structure. Dynamical-mean field theory demonstrates that this renormalized band structure corresponds to a topological Kondo insulating state, and hence the INS probes bulk excitations of heavy topological bands. This identification of a Kondo insulator addresses the long-standing mystery of the electronic properties of CeNiSn, and demonstrates the manifestation of a topological many-body coherent state in spectroscopic measurements of strongly correlated narrow band materials.

Figures (4)

• Figure 1: Crystal structure and inelastic neutron scattering measurements of CeNiSn. (A) Crystal structure of CeNiSn, where red, green and blue correspond to Ce, Ni and Sn atoms respectively. Constant energy slices of inelastic neutron scattering measurements with an incident energy of $E_i=8.97$ meV at 0.3 K in the (0$KL$) scattering plane are displayed integrating over energy transfers of (B) 1.5 - 2.5 meV, and (C) 3.5 - 4.5 meV. Color plots of the excitation spectra are shown at three temperatures for momentum transfers of (D)-(F) (0, $K$ 0), and (G)-(I) (0, 0.5 $L$).
• Figure 2: Low temperature magnetic excitations of CeNiSn. Estimates of the intensity of the magnetic excitations of CeNiSn $S(\mathbf{Q},\hbar\omega)$ at 0.3 K in units of (mb/sr meV Ce), obtained by subtracting the 23 K data from that at 0.3 K, along momentum transfers (A) (0 $K$ 0), (B) (0 0.5 $L$), (C) (0 1 $L$), and (D) ($H$ 0.5 0). (E)-(H) show the corresponding imaginary part of the dynamical susceptibility calculated using linear response theory based on DFT+$U$ calculations within the random-phase approximation (Eq. \ref{['eq-chi_rpa']}).
• Figure 3: Low energy cuts of inelastic neutron scattering measurements of CeNiSn. Low energy cuts of measurements with an incident energy of $E_i=3.44$ meV as a function of energy transfer are compared for the three measured temperatures for different momentum transfers $\mathbf{Q}$, with integration widths of $\Delta H=\Delta L= \pm0.2$ r.l.u, $\Delta K=\pm0.1$ r.l.u for (A)-(C) and $\Delta H=\Delta K= \pm0.2$ r.l.u, $\Delta L=\pm0.1$ r.l.u for (D)-(F) . The reduced intensity at low energies at 0.3 K evidences the opening of an extended spin-gap. The inset of panel a highlights the quasielastic scattering present at 0.3 K.
• Figure 4: Momentum resolved spectral function of CeNiSn from dynamical mean-field theory. The calculated spectral functions are displayed for (A) 290 K, (B) 23 K, (C)12 K. (D) Eigenstates of the topological Hamiltonian $H^t$. The dashed line intersects the fully open direct gap between the $N$-th and $(N+1)$-th states. (E) The Wannier charge center (WCC) evolution calculated using $H^t$ at $k_i=0$$k_i=\pi$ ($i$=x or z) planes. The WCC evolution line intersects with the dashed line odd/even times at the $k_i=0$/$\pi$ planes respectively, indicating a strong topological insulator.