Multiple quantum phase transitions and superconductivity in Ce-based heavy fermions

Investigating the A n+1B n X3n+1 Homologous Series: A New Platform for Studying Magnetic Praseodymium Based Intermetallics

The recent discovery of the A _n+1B _n X_3n+1 (A = lanthanide, B = transition metal, X = tetrel) homologous series provides a new platform to study the structure-property relationships of highly correlated electron systems. Several members of Ce _n+1Co _n Ge_3n+1 (n = 1, 4, 5, 6, and ∞) show evidence of heavy electron behavior with complex magnetic interactions. While the Ce analogues have been investigated, only n = 1, 2, and ∞ of Pr _n+1Co _n Ge_3n+1 have been synthesized, with n = 1 and 2 showing a nonsinglet magnetic ground state. The Pr analogues can provide a platform for direct comparison of highly correlated behavior. In this perspective, we discuss the impetus for synthesizing the Pr _n+1Co _n Ge_3n+1 members and present the structural characterization of the n = 3 and n = 4 members. We lay the foundation for future investigations of the Pr _n+1Co _n Ge_3n+1 family of compounds and highlight the importance of complementary methods to characterize new quantum materials.

Localized 4f-electrons in the quantum critical heavy fermion ferromagnet CeRh6Ge4

Ferromagnetic quantum critical points were predicted to be prohibited in clean itinerant ferromagnetic systems, yet such a phenomenon was recently revealed in CeRh₆Ge₄, where the Curie temperature can be continuously suppressed to zero under a moderate hydrostatic pressure. Here we report the observation of quantum oscillations in CeRh₆Ge₄ from measurements using the cantilever and tunnel-diode oscillator methods in fields up to 45 T, clearly demonstrating that the ferromagnetic quantum criticality occurs in a clean system. In order to map the Fermi surface of CeRh₆Ge₄, we performed angle-dependent measurements of quantum oscillations at ambient pressure, and compared the results to density functional theory calculations. The results are consistent with the Ce 4f electrons remaining localized and not contributing to the Fermi surface, suggesting that localized ferromagnetism is a key factor for the occurrence of a ferromagnetic quantum critical point in CeRh₆Ge₄.

Critical phenomenon in the room-temperature ferromagnet Ce0.65Mg0.35Co3 prepared by high-pressure annealing

Numerous studies have showed evidence that high-pressure annealing (HPA) can modify the crystal and electronic structure significantly, which thus probably alters the magnetic ordering with a different universality class. In this work, we investigate the effects of HPA on the critical behaviors of magnetization in a room-temperature ferromagnet Ce_0.65Mg_0.35Co₃. We observe the HPA compound after annealing at 2 GPa undergoing a second-order phase transition with a decreased Curie temperature. Using the DC magnetization data, the critical exponents β, γ and δ are calculated independently by three methods including the modified Arrott plot, the Kouvel-Fisher plot, and critical isotherm analysis. The obtained critical parameters together with the magnetization data obey the scaling equation of state, indicating that they are intrinsic and unambiguous. Furthermore, we notice that HPA not only reduces the intensity of exchange coupling, but also elongates the exchange range with J(r) ∼r^-4.467, which leads to a universality class different from that of the conventional compound and the existing theoretical models.

Evolution of Magnetic Order from the Localized to the Itinerant Limit

Quantum materials that feature magnetic long-range order often reveal complex phase diagrams when localized electrons become mobile. In many materials magnetism is rapidly suppressed as electronic charges dissolve into the conduction band. In materials where magnetism persists, it is unclear how the magnetic properties are affected. Here we study the evolution of the magnetic structure in Nd_{1-x}Ce_{x}CoIn_{5} from the localized to the highly itinerant limit. We observe two magnetic ground states inside a heavy-fermion phase that are detached from unconventional superconductivity. The presence of two different magnetic phases provides evidence that increasing charge delocalization affects the magnetic interactions via anisotropic band hybridization.

Pressure-Induced Superconductivity in Elemental Ytterbium Metal

Ytterbium (Yb) metal is divalent and nonmagnetic (4f^{14} configuration). Under pressure its valence increases significantly leading to the expectation that magnetic instabilities and other highly correlated electron effects may appear before a stable trivalent state is reached (4f^{13} configuration). We carried out electrical resistivity and ac magnetic susceptibility measurements to 179 GPa over the temperature range 1.4-295 K. No evidence for magnetic order is observed. However, Yb becomes a superconductor at 86 GPa with T_{c}≃1.4  K, increasing to 4.6 K at 179 GPa. X-ray absorption spectroscopy shows that Yb remains mixed valent to at least 125 GPa, pointing to an active role of f electrons in the emergence of superconductivity in this simple, elemental solid.