The in-plane resistivity rho and thermal conductivity kappa of the FeAs-based superconductor KFe2As2 single crystal were measured down to 50 mK. We observe non-Fermi-liquid behavior rho(T) approximately T{1.5} at H{c{2}}=5 T, and the development of a Fermi liquid state with rho(T) approximately T{2} when further increasing the field. This suggests a field-induced quantum critical point, occurring at the superconducting upper critical field H{c{2}}. In zero field, there is a large residual linear term kappa{0}/T, and the field dependence of kappa_{0}/T mimics that in d-wave cuprate superconductors. This indicates that the superconducting gaps in KFe2As2 have nodes, likely d-wave symmetry. Such a nodal superconductivity is attributed to the antiferromagnetic spin fluctuations near the quantum critical point.

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The in-plane thermal conductivity of the iron selenide superconductor FeSe x ͑T c = 8.8 K͒ was measured down to 120 mK and up to 14.5 T ͑Ӎ3 / 4H c 2 ͒. In zero field, the residual linear term 0 / T at T → 0 is only about 16 W K −2 cm −1 , less than 4% of its normal-state value. Such a small 0 / T does not support the existence of nodes in the superconducting gap. More importantly, the field dependence of 0 / T in FeSe x is very similar to that in NbSe 2 , a typical multigap s-wave superconductor. We consider our data as strong evidence for multigap nodeless ͑at least in ab plane͒ superconductivity in FeSe x . This kind of superconducting gap structure may be generic for all Fe-based superconductors.

The in-plane thermal conductivity κ of electron-doped iron-arsenide superconductor BaFe1.9Ni0.1As2 (Tc = 20.3 K) single crystal was measured down to 70 mK. In zero field, the absence of a residual linear term κ0/T at T → 0 is strong evidence for nodeless superconducting gap. In magnetic field, κ0/T shows a slow field dependence up to H = 14.5 T (≈ 30% Hc 2 ). This is consistent with the superconducting gap structure demonstrated by angle-resolved photoemission spectroscopy experiments in BaFe1.85Co0.15As2 (Tc = 25.5 K), where isotropic superconducting gaps with similar size on hole and electron pockets were observed.

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The in-plane resistivity and thermal conductivity of the heavy-fermion superconductor Ce 2 PdIn 8 single crystals were measured down to 50 mK. A field-induced quantum critical point, occurring at the upper critical field H c2 , is demonstrated from the ðTÞ $ T near H c2 and ðTÞ $ T 2 when further increasing the field. The large residual linear term 0 =T at zero field and the rapid increase of ðHÞ=T at low field give evidence for nodal superconductivity in Ce 2 PdIn 8 . The jump of ðHÞ=T near H c2 suggests a first-order-like phase transition at low temperature. These results mimic the features of the famous CeCoIn 5 superconductor, implying that Ce 2 PdIn 8 may be another interesting compound to investigate for the interplay between magnetism and superconductivity. The interplay between magnetism and superconductivity has been a central issue for heavy-fermion superconductors [1], high-T c cuprates [2], and iron pnictides [3]. Among them, one particularly interesting case is the heavy-fermion superconductor CeCoIn 5 , with T c ¼ 2:3 K at ambient pressure [4]. Its superconducting gap has d-wave symmetry [5,6]. 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. The possible FFLO state at the low-temperaturehigh-field (LTHF) corner of the H À T phase diagram of CeCoIn 5 has stimulated extensive studies [15].More recently, NMR, neutron scattering, and muon spin rotation ( SR) experiments have provided clear evidence for a field-induced magnetism in this LTHF part of the phase diagram [16][17][18][19][20][21]. It was identified as a spin-density wave (SDW) order with an incommensurate modulation Q ¼ ð0:44; 0:44; 0:5Þ. Interestingly, this SDW order disappears in the normal state above H c2 , showing that magnetic order and superconductivity in CeCoIn 5 are directly coupled [16,17]. While this has nicely explained the field-induced AF QCP at H c2 [7,8], the physical origin of this LTHF superconducting Q phase is still under debate. For example, Yanase and Sigrist have suggested that the incommensurate SDW order is stabilized in the FFLO state by the appearance of the Andreev bound state localized around the zeros of the FFLO order parameter [22]. Aperis, Varelogiannis, and Littlewood have argued that the Q phase is a pattern of coexisting condensates: a d-wave singlet superconducting state, a staggered -triplet superconducting s...