Heavy fermion superconductor CeCu2Si2under high pressure: Multiprobing the valence crossover

Abstract

The first heavy fermion superconductor CeCu 2 Si 2 has not revealed all its striking mysteries yet. At high pressures, superconductivity is supposed to be mediated by valence fluctuations, in contrast to ambient pressure, where spin fluctuations most likely act as pairing glue. We have carried out a multiprobe (electric transport, thermopower, ac specific heat, Hall and Nernst effects) experiment up to 7 GPa on a high-quality CeCu 2 Si 2 single crystal. For the first time, the resistivity data make it possible quantitatively to draw the valence crossover line within the p-T plane and to locate the critical end point at 4.5 ± 0.2 GPa and a slightly negative temperature. In the same pressure region, remarkable features have also been detected in the other physical properties, presumably acting as further signatures of the Ce valence crossover and the associated critical fluctuations: We observe maxima in the Hall and Nernst effects and a sign change and a strong sensitivity on magnetic field in the thermopower signal.

Coincidence of magnetic and valence quantum critical points in CeRhIn5 under pressure

“…Hence our results are in a broad agreement with previous theoretical and experimental studies. Furthermore, according to Watanabe and Miyake [5], the coincidence of p c and p v consistently [4]. Note that pv is set as the zero on the horizontal axis for both cases.…”

“…[4]. Following the procedure, we define p 50% as the pressure corresponding to 50% of the resistivity drop compared to the value at 1.78(2.32) GPa for the a(c)-axis data, and ρ norm as ρ norm = [ρ * −ρ * (p 50% )]/ρ * (p 50% ).…”

“…For most of such materials, SC appears in the vicinity of a magnetic quantum critical point (QCP) at p c , leading to the belief that spin fluctuations are responsible for the Cooper pairing [2]. On the other hand, Ce-valence fluctuations may also act as the pairing glue [3] and the corresponding critical endpoint (CEP) at p v can be deduced by resistivity scaling analysis [4]. It is noteworthy that the relative position between p c and p v may vary for different systems, probably depending on the hybridization strength (V ) between Ce-4f and conduction electrons [5,6].…”

Theoretical Prediction and Spectroscopic Fingerprints of an Orbital Transition inCeCu2Si2

“…Superconductivity in this compound is observed in a wide range of pressures from 0 to 7 GPa with the SC critical temperature T c featuring two maxima: T c ≈ 0.6 K at P c ¼ 0.45 GPa, and T c ≈ 2 K at P Ã c ≈ 4.5 GPa [2][3][4][5]. This double-dome shape of the SC region has also been observed in isoelectronic CeCu 2 Ge 2 [6] and differs from the SC phases in other Ce 122-type compounds (CePd 2 Si 2 [7], CeRh 2 Si 2 [8]), which exhibit a single-dome SC phase in a much narrower range of pressures around an antiferromagnetic (AFM) quantum critical point.…”

“…The normalstate resistivity around P Ã c is described by ρ ¼ ρ 0 þ AT n , with a large enhancement of ρ 0 and a non-Fermi liquid exponent n ≈ 1 [14]. Recent multiprobe transport measurements clearly revealed the proximity of a critical point close to P Ã c [4,5]. It has been proposed [15] that P Ã c is associated with the critical end point of a first-order valence transition (VT), and that the associated critical fluctuations may provide the pairing mechanism in the high-pressure SC phase [13].…”