The paramagnetic response of mixed-valence samarium hexaboride has been studied by inelastic neutron scattering using a low-absorption double-isotope single crystal of 154Sm11 B6. Measurements were performed for energy transfers 0

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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We report magnetic neutron-diffraction and electrical resistivity studies on single crystals of the heavy-fermion antiferromagnet CeRhIn5 at pressures up to 2.3 GPa. These experiments show that the staggered moment of Ce and the incommensurate magnetic structure change weakly with applied pressure up to 1.63 GPa, where resistivity, specific heat and NQR measurements confirm the presence of bulk superconductivity. This work places new constraints on an interpretation of the relationship between antiferromagnetism and unconventional superconductivity in CeRhIn5.

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Samarium hexaboride (SmB6) is a Kondo insulator, with a narrow gap due to hybridization between localized and conduction electrons. Despite being an insulator, many samples show metal-like properties. Rare-earth purification is exceedingly difficult, and nominally pure samples may contain 2% or more of impurities. Here to determine the effects of rare-earth doping on SmB6, we synthesized and probed a series of gadolinium-doped samples. We found a relationship between specific heat and impurity moment screening which scales systematically. Consistent with this finding, our neutron scattering experiments of a high purity sample of doubly isotopic 154Sm11B6 show no intrinsic excitations below the well-established 13 meV spin-exciton. The result of introducing impurities into a Kondo insulator is incompletely understood, but it is clear from our measurements that there is a systematic relationship between rare-earth impurities and metal-like properties in SmB6.

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 ...

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