Muon-spin-relaxation and inelastic neutron scattering investigations of the caged-type Kondo semimetals: CeT₂Al₁₀ (T = Fe, Ru and Os)

Recently, Ce-based caged-type compounds with the general formula CeT ₂Al₁₀ (T = Fe, Ru and Os) have generated considerable interest due to the Kondo semiconducting paramagnetic ground state (down to 40 mK) observed in CeFe₂Al₁₀ and anomalously high magnetic ordering temperature with spin gap formation at low temperatures in Kondo semimetals CeRu₂Al₁₀ and CeOs₂Al₁₀. The formation of long-range magnetic ordering out of the Kondo semiconducting/semimetallic state itself is extraordinary and these are the first examples of this enigmatic coexistence of electronic ground states. These compounds also exhibit strong anisotropy in magnetic and transport properties, which has been explained on the basis of single-ion crystal electric field anisotropy in the presence of strongly anisotropic hybridization between localized 4f-electron and conduction electrons. Furthermore, they also exhibit a remarkable modification of magnetic and transport properties with doping on Ce, or T or Al sites. In this article, we briefly discuss the bulk properties of these compounds, giving a detailed discussion on our muon-spin-relaxation (μSR) investigations and inelastic neutron scattering (INS) results. We present the μSR and the INS results of Ce(Ru_1-xFe _x)₂Al₁₀ and CeOs₂Al₁₀ as well as the μSR results of NdFe₂Al₁₀, NdOs ₂Al₁₀ and YFe₂Al₁₀ for comparison. The zero-field μSR spectra clearly reveal coherent two-frequency oscillations at low temperatures in CeT₂Al₁₀ (T = Ru and Os) and Ce(Ru_1-xFe_x)₂Al₁₀ (x = 0.3-0.5), which confirms the long-range magnetic ordering with a reduced moment of the Ce. On the other hand, the μSR spectra of Ce(Ru_1-xFe _x)₂Al₁₀ (x = 0.8 and 1) down to 1.2 and 0.04 K, respectively, exhibit a temperature independent Kubo-Toyabe (KT) term confirming a paramagnetic ground state. INS measurements on CeT ₂Al₁₀ (T = Ru and Os) exhibit sharp inelastic excitations at 8 and 11 meV at 5 K due to an opening of a gap in the spin excitation spectrum. A spin gap of 8-12 meV at 7 K, with a strong Q-dependent intensity, is observed in the magnetic ordered state of Ce(Ru_1-xFe _x)₂Al₁₀ with x = 0.3 and 0.5 which remarkably extends into the paramagnetic state of x = 0.8 and 1. The observation of a spin gap in the paramagnetic samples (x = 0.8 and 1) is an interesting finding in this study and it challenges our understanding of the origin of the semiconducting energy gap in CeT₂Al₁₀ (T = Ru and Os) in terms of a hybridization gap opening only a small part of the Fermi surface, gapped spin waves or a spin-dimer gap. Furthermore, the μSR study of NdFe₂Al₁₀ below T_N exhibits a clear sign of two frequency oscillations, which are absent in NdOs₂Al₁₀. Moreover, the μSR study of YFe₂Al₁₀, which has been proposed as a compound exhibiting ferromagnetic critical fluctuations did not reveal any clear sign of critical magnetic fluctuations down to 60 mK, within the ISIS μSR time window, which is unexpected for a T → 0 quantum phase transition (QPT).