Magnetic and superconducting properties of an S -type single-crystal CeCu2Si2 probed by Cu6

Abstract

We have performed 63 Cu nuclear magnetic resonance/nuclear quadrupole resonance measurements to investigate the magnetic and superconducting (SC) properties on a "superconductivity dominant" (S-type) single crystal of CeCu2Si2. Although the development of antiferromagnetic (AFM) fluctuations down to 1 K indicated that the AFM criticality was close, Korringa behavior was observed below 0.8 K, and no magnetic anomaly was observed above Tc ∼ 0.6 K. These behaviors were expected in S-type CeCu2Si2. The temperature dependence of the nuclear spin-lattice relaxation rate 1/T1 at zero field was almost identical to that in the previous polycrystalline samples down to 130 mK, but the temperature dependence deviated downward below 120 mK. In fact, 1/T1 in the SC state could be fitted with the two-gap s±-wave rather than the two-gap s++-wave model down to 90 mK. Under magnetic fields, the spin susceptibility in both directions clearly decreased below Tc, indicative of the formation of spin singlet pairing. The residual part of the spin susceptibility was understood by the field-induced residual density of states evaluated from 1/T1T , which was ascribed to the effect of the vortex cores. No magnetic anomaly was observed above the upper critical field Hc2, but the development of AFM fluctuations was observed, indicating that superconductivity was realized in strong AFM fluctuations.PACS numbers:

Observation of antiferromagnetic order as odd-parity multipoles inside the superconducting phase in CeRh$_{2}$As$_{2}$

“…The sample quality of the present single-crystal CeRh 2 As 2 is considered good compared to the singlecrystal CeCu 2 Si 2 we measured previously [23,29]. This is because the distribution of the NQR frequency [29]; the local symmetry of the As(1) site in CeRh 2 As 2 is similar to that of the Cu site in CeCu 2 Si 2 , as shown in Fig.…”

Fully gapped d -wave superconductivity in CeCu 2 Si 2

“…This gives rise to a sharp peak in χ ′′ (q, ω) below 2∆, leading to the conclusion that there must be a sign changing order parameter in CeCu 2 Si 2 [16,17]. By a similar argument, the lack of a coherence peak in NQR measurements also strongly disfavors superconductivity without a sign reversal [15,18,32]. Furthermore, given the strong Coulomb repulsion in Cebased heavy fermion superconductors, the order parameter must be anisotropic with a sign change, without run- ning into the issue of a large µ * [33].…”

“…This is also consistent with the literature results showing -wave superconductivity ( 15 , 18 ), which were not obtained at low enough temperatures to observe clear evidence for fully gapped behavior. The lack of a coherence peak below in the Cu-NQR measurements ( 15 , 18 , 32 ) can also not be accounted for by a two-gap -wave model but is readily taken into account by the anisotropic state, which changes sign along across the Fermi surface. We note that the effective gap corresponding to a band-mixing pairing is formally identical to one obtained from a pairing and the good fits to and the specific heat are also consistent with this pairing.…”

Unusual Nonmagnetic Ordered State in CeCoSi Revealed by $^{59}$Co-NMR and NQR Measurements

“…In contrast, the ν Q (T ) in CeCoSi deviates from this behavior above T 0 , and the deviation becomes remarkable as the pressure increases. Such behavior has been observed in some 4 f -electron systems [15][16][17][18][19] and discussed to originate from a CEF splitting, a valence change of the 4 f ion, or the Kondo effect. The distinct pressure dependence in CeCoSi indicates that such multiple effects influence ν Q at the Co site through the pressure-enhanced hybridization between the 4 f electron and conduction electrons.…”