Valence Instability and Superconductivity in Heavy Fermion Systems

Abstract

Many cerium-based heavy fermion (HF) compounds have pressure–temperature phase diagrams in which a superconducting region extends far from a magnetic quantum critical point. In at least two compounds, CeCu 2 Si 2 and CeCu 2 Ge 2 , an enhancement of the superconducting transition temperature was found to coincide with an abrupt valence change, with strong circumstantial evidence for pairing mediated by critical valence, or charge transfer, fluctuations. This pairing mechanism, and the valence instability, is a consequence of a f – c Coulomb repulsion term U fc in the Hamiltonian. While some non-superconducting Ce compounds show a clear first order valence instability, analogous to the Ce α–γ transition, we argue that a weakly first order valence transition may be a general feature of Ce-based HF systems, and both magnetic and critical valence fluctuations may be responsible for the superconductivity in these system

Heavy fermion superconductor CeCu2Si2under high pressure: Multiprobing the valence crossover

“…4. All major characteristics are in line with previous reports, 2,7,8 the parameters ρ 0 , A, and n, obtained from simple fits to ρ(T 4.2 K,H = 0) = ρ 0 + A * T n . In particular, from the inset of Fig.…”

“…5(b)]. As mentioned earlier, such a trend had already been inferred from resistivity and a kind of "plateau" in A(p ∼ 4 GPa), 2,7,8 which has now grown into a clear maximum in the present high-quality single crystal [ Fig. 5(b)].…”

“…4 at low T , three qualitative features can already be distinguished by eye: As a function of increasing p, the extrapolated residual resistivity ρ 0 increases before a final downturn, the A coefficient drops dramatically at high p, and, maybe less obvious, at 4.3 GPa n(p) has a minimum. The VF theory 8,9 predicts for a 4f electron delocalization realized at p = p V a maximum in the associated fluctuations at slightly lower p (p p V ), coupled to n ∼ 1 and an enhancement in A and T c . Right at p = p V , the impurity scattering cross section should rise strongly, leading to a maximum in ρ 0 (p).…”

“…Instead, in CeCu 2 Si 2 the superconducting region extends far beyond the magnetic quantum critical point (located at p c ≈ 0 GPa) and exhibits an enhanced transition temperature (T c ), culminating at ∼2.4 K around 4 GPa. 2,[6][7][8] The most elaborate scenario 9,10 invokes critical charge or valence fluctuations (VF) associated with an underlying valence transition (VT), that is, delocalization of the Ce 4f electron when tuning p across the critical region (p V ). In elementary Ce (see inset of Fig.…”

“…Preceding experiments point toward distinct natures of the two superconducting (sc) phases 7,9 (related to p c and p V , respectively) and have unveiled close to p V features peculiar to the concept of critical VF, such as a T -linear resistivity and an increased residual resistivity ρ 0 . 2,8 Despite such compelling indications for the presence of critical VF in proximity to a CEP, additional signatures of the putative VT are still highly desirable in order to consolidate the decisive role of the 4f electron delocalization in the properties of CeCu 2 Si 2 and accordingly in other Ce-based compounds.…”