The nature of the unconventional ordered phase occurring in CeRu2Al10 below T0 = 27 K was investigated by neutron scattering. Powder diffraction patterns show clear superstructure peaks corresponding to forbidden (h + k)-odd reflections of the Cmcm space group. Inelastic neutron scattering experiments further reveal a pronounced magnetic excitation developing in the ordered phase at an energy of 8 meV. The low-temperature behavior of intermetallic cerium compounds can be broadly typified in terms of the competition between several interaction channels (intra-atomic couplings, on-site Coulomb repulsion, hybridiza-tion between local f-electron states and itinerant conduction-band states), forming the basis of the well-known and highly successful Anderson model. 1 However, there has also been continued interest in Ce-based materials which do not seem to fit into this general framework. Among those are, for instance, the "Kondo insula-tors", as well as various compounds exhibiting multipole ordering 2 or other types of elusive "hidden order" transitions. One example of such unconventional ordering properties has been discovered very recently by Strydom 3 in the ternary compound CeRu 2 Al 10. CeRu 2 Al 10 is an YbFe 2 Al 10-type orthorhombic compound belonging to the Cmcm space group, with room-temperature lattice constants a = 9.1272Å1272Å, b = 10.282Å 282Å, and c = 9.1902Å1902Å. It has been described as a "cage" crystal structure, in which Ce atoms are separated from each other by an exceptionally large distance of 5.2 ˚ A. From the lattice constants, the Ce valence state was estimated to be close to 3+. The transport properties below room temperature are indicative of a gap in the electronic structure, 3 although the Hall effect still suggests a dominant metallic character. In this regime, the material exhibits considerable magnetic anisotropy (a: easy axis, b: hard axis). 4-6 Upon application of pressure, the system rapidly changes, first to a Kondo insulator, then to a metal above 5 GPa. 4 The striking feature of this compound is the phase transition taking place at T 0 = 27 K, which causes pronounced anomalies in various physical properties. Whereas the origin of this transition remains highly controversial, there is growing evidence that it cannot reduce to a conventional ordering of local Ce magnetic moments. The transition temperature is far too high in view of the large Ce-Ce distance and, more specifically, of the magnetic ordering temperature of 16.5 K found in GdRu 2 Al 10. 4 The drop in the magnetic susceptibility below T 0 occurring for all three magnetic field orientations H a, b, c, with an exponential behavior χ = χ 0 + A exp(−∆/T) and ∆ ∼ 100 K, is also difficult to reconcile with the behavior expected for an antiferromagnet. 4,7 Finally, 27 Al NQR/NMR experiments did not find the splitting of peaks below T 0 expected for a static order of Ce magnetic moments. 8 Alternative mechanisms such as charge-or spin-density-wave formation also have serious shortcomings. 4,8 Recently, Tanida et al. 5,7 sugge...

The nature of the unconventional ordered phase occurring in CeRu2Al10 below T0 = 27 K was investigated by neutron scattering. Powder diffraction patterns show clear superstructure peaks corresponding to forbidden (h + k)-odd reflections of the Cmcm space group. Inelastic neutron scattering experiments further reveal a pronounced magnetic excitation developing in the ordered phase at an energy of 8 meV.

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Spin dynamics in the new Kondo insulator compound CeRu2Al10 has been studied using unpolarized and polarized neutron scattering on single crystals. In the unconventional ordered phase forming below T0 = 27.3 K, two excitation branches are observed with significant intensities, the lower one of which has a gap of 4.8 ± 0.3 meV and a pronounced dispersion up to ≈ 8.5 meV. Comparison with RPA magnon calculations assuming crystal-field and anisotropic exchange couplings captures major aspects of the data, but leaves unexplained discrepancies, pointing to a key role of direction-specific hybridization between 4f and conduction band states in this compound. PACS numbers: 71.27.+a, 75.20.Hr, 75.30.Ds, 75.30.Gw, 75.30.Mb, 78.70.Nx Keywords: CeRu 2 Al 10 , inelastic neutron scattering, Kondo insulator, singlet ground state, spin-gap, magnon, anisotropyThe CeM 2 Al 10 (M = Fe, Ru, Os) compounds form a new family of Ce-based intermetallic materials with fascinating, but hitherto elusive, magnetic and transport properties. Below room temperature, they show evidence of a Kondo-insulator regime, with an increase in the electrical resistivity on cooling ascribed to the opening of a narrow "hybridization gap" in the electronic density of states [1,2]. In the standard approach [3], this mechanism should ultimately lead to a nonmagnetic, many-body singlet ground state for T → 0, as was observed experimentally for the vast majority of Kondo-insulator compounds known to date. In contrast, CeRu 2 Al 10 and CeOs 2 Al 10 order magnetically below T 0 = 27.3 K and 28.7 K, respectively [1]. Their structure is antiferromagnetic (AF) with the simple wavevector k AF = (0, 1, 0) [4][5][6]. However, there is strong experimental evidence that this ordering cannot be explained by conventional Ruderman-Kittel Kasuya-Yosida (RKKY) exchange alone: T 0 seems unrealistically high in view of the large Ce-Ce interatomic distances (5.26Å), of the weak ordered antiferromagnetic moment (µ AF = 0.32(4)-0.4µ B [5][6][7] for M = Ru) derived from neutron diffraction measurements, and of the much lower Néel temperatures found in other T Ru 2 Al 10 compounds (T N = 16.5 K in GdRu 2 Al 10 [8]). It was also reported that T 0 increases with the application of pressure [1], contrary to the general trend in Ce Kondo compounds. This unique situation has attracted considerable interest because it seems to challenge widely accepted views on Kondo insulators. Various interpretations have been proposed in terms of (i) a charge density wave associated with an energy gap opening preferentially along the b direction [9, 10], (ii) a spin-Peierls state due to the formation of spin-singlet pairs [11][12][13], or (iii) a resonatingvalence-bond state [14]. Quite remarkably, despite the large anisotropy of the paramagnetic susceptibility with χ a ≫ χ c ≫ χ b , the ordered AF moments align along the c direction [5,6]. In Refs. [15,16], this discrepancy was suggested to arise from conduction-electronf -electron (c-f ) hybridization occurring predominantly along a, and suppressing ...