Neutron-Scattering Studies of Spin Dynamics in Pure and Doped CeB6

Magnetism in heavy-fermion metals is governed by the competition between the Kondo screening, which tends to quench the localized magnetic moments and to form a nonmagnetic ground state, and the Ruderman–Kittel–Kasuya–Yosida (RKKY) coupling mechanism via the conduction electrons, which supports long-range magnetic order. This competition results in an amazingly rich variety of possible ground states. An important ingredient in the f-electron system is the strong spin-orbit coupling, which leads to the presence of new eigenstates that may be described in terms of the multipolar moments. Some heavy-fermion materials show long-range ordered multipolar phases, which are invisible to conventional diffraction techniques [1,2]. This so-called “hidden order” has been observed in a variety of compounds containing 4f- and 5f-elements, like URu₂Si₂, NpO₂, YbRu₂Ge₂ , and CeB₆ . Unraveling the underlying structure and a wide range of associated phenomena, ranging from quantum criticality to orbital ordering and unconventional superconductivity, requires a deep understanding of the interplay among these degrees of freedom.

The most well-studied member of this family of compounds is cerium hexaboride, CeB₆ [3], which is considered a textbook example of a system with magnetically hidden order. As a simple-cubic system with only one f electron per cerium ion, CeB₆ is of model character to investigate the interplay of orbital phenomena with magnetism. It is difficult to identify the symmetry of hidden-order states in common x-ray or neutron scattering experiments, as there is no signal in the zero field; however, alternative techniques such as neutron diffraction in the external field [4], resonant x-ray scattering [5–8], and ultrasonic investigations [9,10] can be applied.

Another possible method for characterizing hidden order is to look at the magnetic excitation spectrum, which carries the imprint of the multipolar interactions and the hidden order parameter in its dispersion relations [11, 12]. Using a specific candidate model, the dispersion is calculated and then compared to that measured with inelastic neutron scattering. Until recently, only a limited amount of data which show the presence of dispersing excitations measured along a few high-symmetry directions in an applied magnetic field were available [13]. Early attempts to compare such calculations [14–17] with experiments showed that only the strongest modes at high-symmetry points could be identified. The review of recent neutron-scattering results presented in this chapter is intended to satisfy the need for more accurate inelastic neutron-scattering experiments as a function of field and temperature, explicitly mentioned by theoreticians [16], giving us the opportunity to identify existing excitation branches and conclusively compare them with the theoretically predicted multipolar excitations.