Correlation between ground state and orbital anisotropy in heavy fermion materials

Abstract

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...

Electronic Structures of Ferromagnetic CeAgSb2: Soft X-ray Absorption, Magnetic Circular Dichroism, and Angle-Resolved Photoemission Spectroscopies

“…It has been shown that the analysis of the LD effects using the ligand field multiplet (LFM) model gives a good quantitative description of the CEF ground state 26,[28][29][30] . In CeAgSb 2 , the CEF splitting energies from the ground state to the first and second excited states (∆ 1 and ∆ 2 ) have been determined to be 5.2 meV (≈60 K) and 12.5 meV (≈145 K) from the INS measurements, in which the CEF ground state is |±1/2 and the first and second exited levels are dominated by |±3/2 and |±5/2 , respectively 8 .…”

Emergent magnetic anisotropy in the cubic heavy-fermion metal CeIn3

“…In tetragonal materials, such as the related material CeRhIn 5 , the orbital character of the 4f wavefunctions can also be manipulated as a function of doping or pressure. 24 As seen in this high field study, the exchange interactions can vary significantly, and even change sign, as the orbital character changes (see Supplementary Information). The ability to produce a simple model capturing the consequences of strong spin-orbit coupling and varying crystal electric fields in the prototypical heavy fermion CeIn 3 is an important step toward a microscopic understanding that couples the spin degrees of freedom to the conduction electrons leading to unconventional quantum critical and superconducting states in f-electron systems.…”

Exploring small energy scales with x-ray absorption and dichroism

“…These simulations were obtained from full atomic multiplet calculations as implemented in the Quanty Package [26,27]. To obtain the best match between calculated and experimental isotropic spectra, the f f (df ) Slater Integrals were reduced by 42.5% (17.5%) from their respective Hartree-Fock values, well in accordance with previous work [7,9,11,14,28]. In addition, the Ce 3d core hole spin orbit coupling constant ζ 3d was slightly readjusted to reproduce the experimental separation between the M 4 and M 5 edges.…”

“…In a solid environment, the degeneracy of the rare earth 4f ground configuration is lifted by the crystal field in general, causing both an anisotropic 4f charge distribution and, in conjunction with spin orbit coupling, (single ion) magnetic anisotropy. Unraveling the crystal field induced level structure thus constitutes an essential part of understanding the low temperature physics and of establishing correlations between local 4f symmetry at low temperature on the one hand and macroscopic ground state properties on the other.In this respect, the usefulness of probing the 4f configuration with linear polarized soft x-rays [6] has been demonstrated for a variety of Ce compounds in recent years [7][8][9][10][11][12][13][14], allowing to settle several open issues, where other experiments left room for diverging interpretations. Its magnetic variants, x-ray magnetic linear and circular dichroism (XMLD, XMCD) constitute sensitive element and orbital specific probes of magnetic polarization and anisotropy [15][16][17][18][19][20].…”