Spectroscopic evidence for the direct involvement of local moments in the pairing process of the heavy-fermion superconductor CeCoIn5

Abstract

The microscopic mechanism for electron pairing in heavy-fermion superconductors remains a major challenge in quantum materials. Some form of magnetic mediation is widely accepted with spin fluctuations as a prime candidate. A novel mechanism, "composite pairing" based on the cooperative two-channel Kondo effect directly involving the f-electron moments has also been proposed for some heavy fermion compounds including CeCoIn 5 . The origin of the spin resonance peak observed in neutron scattering measurements on CeCoIn 5 is still controversial and the corresponding hump-dip structure in the tunneling conductance is missing. This is in contrast to the cuprate and Fe-based high-temperature superconductors, where both characteristic signatures are observed, indicating spin fluctuations are likely involved in the pairing process. Here, we report results from planar tunneling spectroscopy along three major crystallographic orientations of CeCoIn 5 over wide ranges of temperature and magnetic field. The pairing gap opens at T p ~ 5 K, well above the bulk T c = 2.3 K, and its directional dependence is consistent with d x2-y2 symmetry. With increasing magnetic field, this pairing gap is suppressed as expected but, intriguingly, a gaplike structure emerges smoothly, increasing linearly up to the highest field applied. This field-induced gaplike feature is only observed below T p . The concomitant appearance of the pairing gap and the fieldinduced gaplike feature, along with its linear increase with field, indicates that the f-electron local moments are directly involved in the pairing process in CeCoIn 5 .

Peculiar Physics of Heavy-Fermion Metals: Theory versus Experiment

“…As a result, ∆σ d (V) remains approximately the same below the superconducting T c [15,31]. It is seen from Equation ( 25) and Figure 10 that with rising temperatures, the asymmetry diminishes and finally vanishes at T ≥ 40 K. Such a behavior has been observed in measurements on the HF metal CeCoIn 5 [84,85], displayed in Figure 10. Under the application of a magnetic field B at sufficiently low temperatures k B T µ B B, where k B and µ B are the Boltzmann constant and the Bohr magneton, the strongly correlated Fermi system transits from the NFL to the LFL regime [15,86].…”

“…As we have seen above, the asymmetry of the tunneling conductivity vanishes in the LFL state [15,31,70,82]. It is seen from Figure 11, that ∆σ d (V), displayed in Figure 10 and extracted from experimental data [85], vanishes in the normal state at sufficiently high magnetic fields applied along the easy axis and low temperatures k B T << µ B (B − B c ) with the critical field B c 5 T in agreement with the prediction, see, e.g., [15,31,87]. Under this condition, the system transits from the NFL to the LFL behavior, with the resistance ρ becoming a quadratic function of temperature, ρ(T) ∝ T 2 [15].…”

Unconventional Superconductivity from Fermi Surface Fluctuations in Strongly Correlated Metals

“…9). In the magnitude of T c as well as in the spin-singlet nature of the pairing and other properties of its superconducting state, CeRhIn 5 is similar to its high-chemical-pressure counterpart CeCoIn 5 under ambient conditions 3 . In both cases, the spin anisotropy is relatively small 9 , i.e., the SU(2) limit should apply.…”

Direct evidence for Cooper pairing without a spectral gap in a disordered superconductor above T c

“…For conventional, elemental superconductors, pairing and condensation take place concurrently when cooling below the critical temperature ( T c ). By contrast, disordered superconductors exhibit unusual normal state properties above T c ( 1 – 9 ), which, analogous to high-temperature ( 10 – 14 ), interface ( 15 – 17 ), and heavy fermion superconductors ( 18 , 19 ), were thought to be a consequence of preformed or fluctuating Cooper pairs that could exist above T c . However, there is no direct experimental evidence of this.…”

“…Enhanced noise has even been observed at bias voltages greater than the superconducting gap. Fourth, several spectroscopic techniques show (partially filled) gaps in the spectral weight at the Fermi level, frequently called pseudogaps, that persist above T c , in disordered, cuprate, interfacial, and heavy fermion superconductors ( 1 , 7 , 10 , 16 , 18 ). These observations, which differ in many respects from expectations for the conventional metallic state in ordinary metals, have been interpreted as being due to a finite population of paired electrons.…”