| Superconductivity remains to be one of the most fascinating and intriguing states in condensed matter physics since its discovery in mercury at4.2K in1911. Scientists have spent half a century to develop a theory, owing to the breakthrough in1957by Bardeen, Cooper, and Schrieffer, we know that superconductivity is a condensate of electron pairs, so-called Cooper pairs, which form due to an attractive interaction between electrons. Because a pair of identical fermions is antisymmetric with respect to the exchange of one fermion with another, the spin and spatial components of the Copper pair wave function must have opposite exchange symmetries. Even though electrons repel each other because of Coulomb force, at low energies there can be an effective attraction mediated by electron-phonon coupling. The spin singlet pair state with an isotropic spatial components (s-wave) turns out to satisfy, and is recognized as the conventional superconductors. As a result of electron-phonon pairing mechanism, it has predicted the upper limit of superconducting transition temperature in conventional superconductors is40K, which disillusioned large scale application of high temperature superconductivity.BCS theory has succeeded in explaining element and alloy superconductors. Superconductivity, which was thought as one of the best understood many-body problems in physics, faced the challenge when a heavy-fermion superconductor CeCu2Si2was discovered in1979. In1986the discovery of high-temperature superconductivity in cuprate compounds with layered perovskite structure announced the era of unconventional superconductivity to a wider community, what’s intriguing is that its parent compounds is antiferromagnetic Mott insulators. The magnetism previous thought incompatible with superconductivity could play an essential role for unconventional superconductivity, and became one of the guiding strategies in the nineties in the search for new materials. Trailing behind the cuprate, a novel Tc=26K iron-based superconducting compound LaO1-xFxFeAs was discovered in2008. Compared with cuprate,the undoped material in iron-based superconductors is also antiferromagnetic,while its parent compound is metallic. At one time, magnetism was believed to compete with superconductivity, but recent results show that it may play an important role in unconventional superconductivity. The thesis presents the study of heavy-fermion superconductor Ce2PdIng and superconductor Ca2Ir4Sn13through low temperature transport measurement. These include:1. The in-plane resistivity p and thermal conductivity κ of the heavy-fermion superconductor Ce2PdIn8single crystal were measured down to50mK. A field-induced quantum critical point, occurring at the upper critical field Hc2, is demonstrated from the p(T)～T near Hc2and p(T)～T2when further increasing the field. The large residual linear term κ/T at zero field and the rapid increase of κ (H)/T at low field give evidence for nodal superconductivity in Ce2PdIns.the jump of κ (H)/T near Hc2suggests a first-order-like phase transition at low temperature. These results mimic the features of the famous CeCoIns superconductor, implying that Ce2PdIn8may be another interesting compound to investigate for the interplay between magnetism and superconductivity.2. We report thermal conductivity measurement down to50mK on Ca3Ir4Sn13single crystals, in which superconductivity with Tc=7K was claimed to coexist with ferromagnetic spin-fluctuations. In zero magnetic field, no residual linear term κo/T is found. In low magnetic fields, κ (H)/T shows a slow field dependence. These results demonstrate that the superconducting gap of Ca3Ir4Sn13is nodeless, thus rule out an exotic nodal gap caused by ferromagnetic spin-fluctuations. The relatively fast increase of κ (H)/T with increasing the field may result from gap anisotropy, or multiple isotropic gaps with different magnitudes. |
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