Unconventional heavy-fermion superconductorCeCoIn5:dc magnetization study at temperatures down to 50 mK

Abstract

dc magnetization measurements on CeCoIn 5 reveal a first-order phase transition at H c2 for both Hʈa and c axes in the isothermal magnetization M (H) below 0.7 K, indicating a strong Pauli paramagnetic suppression in the even-parity pairing. M (T) in the normal state above H c2 exhibits non-Fermi-liquid behavior down to 150 mK, implying the existence of antiferromagnetic fluctuations behind the unconventional superconductivity. We observed an unusual peak effect for Hʈc in fields 5-30 kOe below 150 mK(ϭ0.06T c ), whose anomalous temperature dependence cannot be simply explained by ordinary mechanisms.Since in 1979, 1 HF superconductivity has been attracting interest in the field of strongly correlated electron systems. Recent experimental and theoretical progress indicates that most of the HF superconductors are likely to be of an unconventional type. Until recently, to our best knowledge, CeCu 2 Si 2 was the only Ce-based HF superconductor at ambient pressure. The superconductivity in CeCu 2 Si 2 , however, is rather difficult to understand because of the complicated magnetic phase diagram. 2 Quite recently, two tetragonal Ce-based HF compounds have been discovered by Petrovic et al. to become superconducting at ambient pressure: CeXIn 5 ͓XϭIr ͑Ref. 3͒ and Co ͑Ref. 4͔͒. To our best knowledge, CeCoIn 5 has the highest T c (ϭ2.3 K) among the HF superconductors known at present. The specific heat, 4 thermal conductivity, 6 and NMR relaxation rate 8 of CeCoIn 5 show power-law temperature dependencies below T c , suggesting an unconventional superconductivity with anisotropic energy gap. Very recently, the NMR Knight-shift measurement 7,8 has revealed even-parity pairing in the superconducting state, and the angle-dependent thermal-conductivity measurement 9 has identified that the gap symmetry is k x 2 Ϫk y 2 , pointing to the fact that the pairing interaction is mediated by magnetic fluctuations. An interesting observation with respect to this point is the non-Fermiliquid ͑NFL͒ behavior in the specific heat divided by temperature C/T, showing a remarkable upturn on cooling when the superconductivity is suppressed by magnetic field. 4,5 To our best knowledge the origin of the NFL behavior has not been clarified yet.One of the features of the HF superconductors is that the orbital limiting field is relatively high despite low T c , because of the small Fermi velocity of the carriers. In addition, the HF superconductors possess quite large normal-state paramagnetic susceptibility at sufficiently low temperature, reflecting the high density of states. These facts lead to an interesting situation in which the paramagnetic energy near H c2 becomes a significant fraction of the superconducting condensation energy. 10,11 It was theoretically pointed out that a second-order transition at H c2 changes into a first-order one below ϳ0.56 T c for the singlet pairing, provided that the normal-state spin susceptibility is large enough. 12,13 Subsequent theoretical studies predicted that in the case of a clean limit (lӷ 0 ) a firs...

Superconductivity and Non-Fermi-Liquid Behavior in the Heavy-Fermion Compound CeCo1−xNixIn5

“…6) The magnetic origin of the SC pairing is inferred from the d-wave (d x 2 −y 2 ) symmetry of the SC gap determined by thermal conductivity, specific heat, and conductance measurements. [7][8][9] In magnetic fields, the spin degrees of freedom are significantly coupled with the stability of the SC order; a strong Pauli paramagnetic effect gives rise to a first-order transition at the SC upper critical field H c2 below 0.7 K, 7,[10][11][12] and the SC phase coexistent with AFM spin modulation evolves just below H c2 at very low temperatures. [13][14][15][16][17][18] Furthermore, the existence of the AFM-QCP at ∼ H c2 is strongly suggested from the observations of the NFL behavior in the paramagnetic phase above H c2 , including the − ln T divergence in specific heat divided by temperature, the T -linear dependence in magnetization, and electrical resistivity.…”

“…[13][14][15][16][17][18] Furthermore, the existence of the AFM-QCP at ∼ H c2 is strongly suggested from the observations of the NFL behavior in the paramagnetic phase above H c2 , including the − ln T divergence in specific heat divided by temperature, the T -linear dependence in magnetization, and electrical resistivity. 10,19,20) In fact, the long-range AFM orders are generated by substituting the ions for the elements in CeCoIn 5 , such as Nd for Ce, 21,22) Rh for Co, [23][24][25][26] and Cd, Hg, and Zn for In. [27][28][29][30] In contrast to the doping effect by those ions, the substitution of Sn for In simply suppresses the SC phase without generating the AFM order.…”

Field-Induced Quantum Critical Point and Nodal Superconductivity in the Heavy-Fermion SuperconductorCe2PdIn8

“…4, ðHÞ=T of CeCoIn 5 shows an abrupt jump near H c2 . This is evidence for the first-order transition [5,[9][10][11][12], which apparently corresponds to the disappearance of the Q phase at H c2 [17,18] Fig. 2.…”

“…While there is no static magnetism in CeCoIn 5 at zero field, a field-induced antiferromagnetic (AF) quantum critical point (QCP) has been clearly demonstrated by resistivity and specific heat measurements [7,8]. Initially, it was very puzzling why the AF QCP is located right at the upper critical field H c2 .Meanwhile, the observations of first-order phase transition at low temperature and H c2 and a second magnetization and specific heat anomaly well inside the superconducting state have been interpreted as the signature of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state [5,[9][10][11][12]. The novel FFLO state with broken spatial symmetry was predicted in the 1960s [13,14], but it has never been experimentally verified before.…”

“…Meanwhile, the observations of first-order phase transition at low temperature and H c2 and a second magnetization and specific heat anomaly well inside the superconducting state have been interpreted as the signature of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state [5,[9][10][11][12]. The novel FFLO state with broken spatial symmetry was predicted in the 1960s [13,14], but it has never been experimentally verified before.…”