The dominant role of critical valence fluctuations on high Tc superconductivity in heavy fermions

Abstract

Despite almost 40 years of research, the origin of heavy-fermion superconductivity is still strongly debated. Especially, the pressure-induced enhancement of superconductivity in CeCu 2 Si 2 away from the magnetic breakdown is not sufficiently taken into consideration. As recently reported in CeCu 2 Si 2 and several related compounds, optimal superconductivity occurs at the pressure of a valence crossover, which arises from a virtual critical end point at negative temperature T cr .In this context, we did a meticulous analysis of a vast set of top-quality high-pressure electrical resistivity data of several Ce-based heavy fermion compounds. The key novelty is the salient correlation between the superconducting transition temperature T c and the valence instability parameter T cr , which is in line with theory of enhanced valence fluctuations. Moreover, it is found that, in the pressure region of superconductivity, electrical resistivity is governed by the valence crossover, which most often manifests in scaling behavior. We develop the new idea that the optimum superconducting T c of a given sample is mainly controlled by the compound's T cr and limited by non-magnetic disorder. In this regard, the present study provides compelling evidence for the crucial role of critical valence fluctuations in the formation of Cooper pairs in Ce-based heavy fermion superconductors besides the contribution of spin fluctuations near magnetic quantum critical points, and corroborates a plausible superconducting mechanism in strongly correlated electron systems in general. * gernot.scheerer@unige.ch 2

Nodal superconductivity and superconducting domes in the topological Kagome metal CsV3Sb5

“…While the critical valence fluctuations may play a dominant role on the superconductivity of the second dome in CeCu2(Si1-xGex)2 (ref. 34 ), the high-pressure superconducting phase of the iron chalcogenides remains enigmatic 33 . Since the second dome in CsV3Sb5 is away from the CDW order, another exotic pairing state may exist in this V-based superconductor.…”

Novel attractive pairing interaction in strongly correlated superconductors

“…This result is consistent with the fact that the Kondo critical point prompts optimal superconductivity as obtained in CeCu 2 Si 2 , [26] as well as in many other HF superconductors. [8,9,11,[52][53][54] However, T c is terminated below a critical T K which can be obtained from Eq. (9) as…”

“…The first heavy-fermion (HF) superconductor CeCu 2 Si 2 [6] was widely believed to be an unconventional superconductor. [7][8][9][10] Subsequently, more HF superconductors, [11] followed by cuprate, and pnictide superconductors are discovered to feature unconventional pairings with either nodal d-wave, or nodeless but sign-reversal s ± -pairing symmetry, or their various irreducible combinations. [12] However, the pairing symmetry, and the pairing mechanism in the primitive CeCu 2 Si 2 compound are recently called into questions.…”

“…[24,27,28] The valence fluctuation, which is ubiquitous in HF compounds, [29] can promote superconductivity with unconventional pairing mechanism. [8,9,26,30] Kondo coupling can induce various unconventional pairings. [10,[31][32][33][34][35][36] Following the overwhelming evidence of conventional pairing symmetry, the electron-phonon coupling problem with strong Coulomb interaction is revisited recently.…”

Clamp cells for high pressure neutron scattering at low temperatures and high magnetic fields at Heinz Maier-Leibnitz Zentrum (MLZ)

“…External pressure (even moderate) is a convenient tuning parameter, which can create novel spin states (e.g. superconductivity in heavy fermion compounds [1]). Neutron scattering is a standard method to study the magnetic ordering at the microscopic level.…”