Using resonant x-ray spectroscopies combined with density functional calculations, we find an asymmetric biaxial strain-induced d-orbital response in ultrathin films of the correlated metal LaNiO3 which are not accessible in the bulk. The sign of the misfit strain governs the stability of an octahedral "breathing" distortion, which, in turn, produces an emergent charge-ordered ground state with an altered ligand-hole density and bond covalency. Control of this new mechanism opens a pathway to rational orbital engineering, providing a platform for artificially designed Mott materials.

We have succeeded in establishing the crystal-field ground state of CeRu2Al10, an orthorhombic intermetallic compound recently identified as a Kondo insulator. Using polarization dependent soft x-ray absorption spectroscopy at the Ce M4,5 edges, together with input from inelastic neutron and magnetic susceptibility experiments, we were able to determine unambiguously the orbital occupation of the 4f shell and to explain quantitatively both the measured magnetic moment along the easy a axis and the small ordered moment along the c-axis. The results provide not only a platform for a realistic modeling of the spin and charge gap of CeRu2Al10, but demonstrate also the potential of soft x-ray absorption spectroscopy to obtain information not easily accessible by neutron techniques for the study of Kondo insulators in general.Comment: submitted to Physical Review B Rapid Communication

The interplay of structural, orbital, charge, and spin degrees of freedom is at the heart of many emergent phenomena, including superconductivity. Unraveling the underlying forces of such novel phases is a great challenge because it not only requires understanding each of these degrees of freedom, it also involves accounting for the interplay between them. Cerium-based heavy fermion compounds are an ideal playground for investigating these interdependencies, and we present evidence for a correlation between orbital anisotropy and the ground states in a representative family of materials. We have measured the 4f crystal-electric field ground-state wave functions of the strongly correlated materials CeRh 1−x Ir x In 5 with great accuracy using linear polarization-dependent soft X-ray absorption spectroscopy. These measurements show that these wave functions correlate with the ground-state properties of the substitution series, which covers long-range antiferromagnetic order, unconventional superconductivity, and coexistence of these two states.heavy fermions | crystal fields | X-ray absorption | rare earth W hy do many chemically and structurally highly similar compounds develop different ground states? This seemingly simple question still eludes a straightforward description despite intense research. However, it is specifically pressing in view of the quest for a deeper insight into unconventional superconductivity.We here investigate heavy fermion metals, i.e., rare earth or actinide materials, in which a plethora of phenomena including antiferromagnetism and unconventional superconductivity can be observed. In these compounds, the f electrons hybridize with the conduction electrons (cf -hybridization), and, in analogy to the Kondo effect in diluted systems, the local magnetic moments can be screened in these so-called "Kondo lattices" at sufficiently low temperatures. However, the Kondo effect competes with the Ruderman-Kittel-Kasuya-Yosida interaction, which typically favors long-range magnetic order. As a result of this competition, a quantum phase transition from magnetically ordered to paramagnetic, more itinerant f electron behavior can take place. The balance of both interactions can be tuned by external parameters such as pressure, magnetic field, or doping (1). Non-Fermi liquid behavior and, of interest here, unconventional superconductivity often occur in the vicinity of such quantum critical points. However, despite research over the last 30 y, the ability to predict conditions favorable for superconductivity has remained elusive. Here, the cerium-115 family CeMIn 5 with M = Co, Ir, and Rh is ideally suited for a systematic study because ground states and changes in Fermi volumes in this family can be tuned easily by substitutions of one M element for another (2-25).These heavy fermion compounds CeMIn 5 crystallize in the tetragonal HoCoGa 5 -type structure as depicted in Fig. 1, with In1 being the in-plane and In2 the out-of-plane indium. Fig. 2 shows the substitution phase diagram of CeRhIn 5 , wher...

We have successfully determined the hitherto unknown sign of the B(4)(4) Stevens crystal-field parameter of the tetragonal heavy-fermion compound CeCu(2)Si(2) using vector q-dependent nonresonant inelastic x-ray scattering experiments at the cerium N(4,5) edge. The observed difference between the two different directions, q∥[100] and q∥[110], is due to the anisotropy of the crystal-field ground state in the (001) plane and is observable only because of the utilization of higher than dipole transitions possible in nonresonant inelastic x-ray scattering. This approach allows us to go beyond the specific limitations of dc magnetic susceptibility, inelastic neutron scattering, and soft x-ray spectroscopy, and provides us with a reliable information about the orbital state of the 4f electrons relevant for the quantitative modeling of the quasiparticles and their interactions in heavy-fermion systems.

Topological insulators form a novel state of matter that provides new opportunities to create unique quantum phenomena. While the materials used so far are based on semiconductors, recent theoretical studies predict that also strongly correlated systems can show non-trivial topological properties, thereby allowing even the emergence of surface phenomena that are not possible with topological band insulators. From a practical point of view, it is also expected that strong correlations will reduce the disturbing impact of defects or impurities, and at the same increase the Fermi velocities of the topological surface states. The challenge is now to discover such correlated materials. Here, using advanced x-ray spectroscopies in combination with band structure calculations, we infer that CeRu4Sn6 is a strongly correlated material with non-trivial topology.