Competing4f-electron dynamics in Ce(Ru1−xFex)<

Abstract

We have carried out muon spin relaxation (SR), neutron diffraction and inelastic neutron scattering (INS) investigations on polycrystalline samples of Ce(Ru 1-x Fe x ) 2 Al 10 (x=0, 0.3, 0.5, 0.8 and 1) to investigate the nature of the ground state (magnetic ordered versus paramagnetic) and the origin of the spin gap formation as evident from the bulk measurements in the end members. Our zero-field SR spectra clearly reveal coherent two-frequency oscillations at low temperature in x=0, 0.3 and 0.5 samples, which confirms the long-range magnetic ordering of the Ce-moment with T N =27, 26 and 21 K respectively. On the other hand the SR spectra of x=0.8 and x=1 down to 1.4 K and 0.045 K, respectively exhibit a temperature independent KuboToyabe term confirming a paramagnetic ground state. The long-range magnetic ordering in x=0.5 below 21 K has been confirmed through the neutron diffraction study. INS measurements of x=0 2 clearly reveal the presence of a sharp inelastic excitation near 8 meV between 5 K and 26 K, due to an opening of a gap in the spin excitation spectrum, which transforms into a broad response at and above 30 K. Interestingly, at 4.5 K the spin gap excitation broadens in x=0.3 and exhibits two clear peaks at 8.4(3) and 12.0(5) meV in x=0.5. In the x=0.8 sample, which remains paramagnetic down to 1.2 K, there is a clear signature of a spin gap of 10-12 meV at 7 K, with a strong Q-dependent intensity. Evidence of a spin gap of 12.5(5) meV has also been found in x=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 gap in CeT 2 Al 10 (T=Ru and Os) compounds in terms of hybridization gap opening only a small part of the Fermi surface, gapped spin waves, or a spin-dimer gap.

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“…The broad maximum in the susceptibility indicates the opening of a spin or hybridization gap (Fig.4). On the other hand, the susceptibility of CeFe 2 Al 10 along the a-axis exhibits a broad maximum near 75 K without any sign of magnetic ordering [28,33], which indicates that the Ce ions are in the valence fluctuating or mixed valence state, as confirmed by L 3 -edge x-ray absorption spectroscopy [46]. Further the strong anisotropic behavior in the paramagnetic state of all three compounds reveals the presence of strong hybridization between 4f-and conduction electrons.…”

“…3), have generated considerable interest in both theoretical and experimental condensed matter physics [28,[29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48]. This interest arose due to the various interesting ground states observed in this family of Ce-compounds as well as their caged type crystal structure that is important for the enhanced thermoelectric properties.…”

“…Rather, an anisotropic exchange interaction, due to hybridization between 4f-and conduction-electrons, is needed to fit the data. The latter two contributions are also investigated in Ce(Ru 1-x Fe x ) 2 Al 10 with x=0.8 and CeFe 2 Al 10 , which have nonmagnetic ground states down to 1.2 K and 40 mK, respectively [46].…”

“…The inelastic neutron scattering study (INS) on CeT 2 Al 10 (T=Fe, Ru and Os) reveals a clear sign of the spin-gap formation of 8 meV in T=Ru [44,46], 11 meV in T=Os [28,38] and 12 meV in T=Fe [46,54]. The temperature dependent study of T=Ru and Os reveals that the spin gap decreases with increasing temperature in T=Ru and Os [28,44,46], but a finite value of the gap still exists above T N (Fig.5).…”

“…The temperature dependent study of T=Ru and Os reveals that the spin gap decreases with increasing temperature in T=Ru and Os [28,44,46], but a finite value of the gap still exists above T N (Fig.5). By raising the temperature still further (above 40 K), the INS response becomes very broad, of quasi-elastic-type for T=Ru and Os [28,44,46].…”