Possible Long-Range Order with Singlet Ground State in CeRu₂Al₁₀

Abstract

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...

Contrasting effect of La substitution on the magnetic moment direction in the Kondo semiconductorsCeT2Al10(T=Ru,Os)

“…The inset in the Fig. 2(a) shows the La composition dependence of T N of Ce 1−x La x Ru 2 Al 10 , which reveals that T N decreases almost linearly and becomes zero near x≥0.7 [12,29]. In this critical region of compositions the heat capacity exhibits a rise at low temperature suggesting the presence of non-Fermi-liquid (NFL) behavior close to a quantum critical point (QCP) [29].…”

“…This interest arose due to the various ground states observed in this family of Cecompounds. An unusually sharp phase transition near 27 K in the magnetic susceptibility of CeRu 2 Al 10 has been attributed to a spin-dimer formation [12,13]. The resistivity of CeRu 2 Al 10 exhibits a sharp drop near 27 K resembling an insulator-metal transition [2].…”

Incommensurate-commensurate magnetic phase transition inSmRu2Al10

“…[2][3][4][5] The most prominent feature is its high transition temperature, T 0 =27.3 K for CeRu 2 Al 10 and 28.7 K for CeOs 2 Al 10 , respectively, which are much higher than T N =16 K of GdRu 2 Al 10 . 6 There are several noteworthy properties associated with this phase transition.…”

Amorphization Effect for Kondo Semiconductor CeRu2Al10

“…Tanida et al proposed that the longrange magnetic order was suppressed randomly in Ce dilute region for La substitution of CeRu 2 Al 10 . The authors pointed out that the resistivity minimum can be explained by a typical impurity Kondo effect, where Ce exists as a magnetic impurity [3,4]. Moreover, the semiconducting band gap of CeRu 2 Al 10 is broken by La substitution.…”