| There are many novel phenomena in condensed matter physics, among them are strong correlation and non fermi liquid materials. In this thesis superconductors con-taining kagome lattice and SmB6 are highly focused because of either their fantastic properties and potential applications. Materials with kagome lattice behave strong spin fluctuation because of spin frustration and this may be the origin of high temperature superconductor. SmB6 was recently been demonstrated to be topological kondo insu-lator. We will extensively study these two kinds of materials which possess the two unique properties.In chapter 1, I will briefly introduce the development of superconductivity. Then I will basically introduce condensed matter physics that is related to superconductivity, such as Fermi liquid and magnetic properties etc. Finally we give an brief introduction to the topological insulator.The principle and apparatus for low-temperature specific heat are described in chapter 2.In chapter 3, we investigated the layered superconductor LaRu3Si2. In this com-pound, transition metal Ru atoms form the kagome lattice, and materials containing kagome lattice usually show some exotic properties. Through transport measurement, especially from the data of low temperature specific heat data, we determined an s-wave superconductivity in this compound. We also doped this sample with Fe and Co atoms to the Ru site respectively, and the doped samples showed a distinct suppression of superconducting transition temperature. The distinct behavior indicates Fe atoms act as magnetic impurities while Co atoms are not, which is consistent with the theoretical study. These results further demonstrated an s-wave parity in the parent compound. Ba2/3Pt3B2, which is similar to the structure of LaRu3Si2 shows almost the same phys-ical properties. The Wilson ratio we calculated for this sample is as large as 34, which is beyond intuition indicating strong correlation in this system.In chapter 4, Electric transport and scanning tunneling spectrum (STS) have been investigated on polycrystalline samples of the new superconductor Bi4O4S3. A weak insulating behavior in the resistive curve has been induced in the normal state when the superconductivity is suppressed by applying a magnetic field. Interestingly, a kink appears on the temperature dependence of resistivity near 4 K at all high magnetic fields above 1 T when the bulk superconductivity is completely suppressed. This kink associated with the upper critical field as well as the wide range of excess conduc-tance at low fields and high temperatures is explained as the possible evidence of strong superconducting fluctuation. From the tunneling spectra, a superconducting gap of about 3 meV is frequently observed yielding a ratio of 2△/kBTC～17. This value is much larger than the one predicted by the BCS theory in the weak coupling regime 2△/kBTC～3.53, which suggests the strong coupling superconductivity in the present system. Furthermore, the gapped feature persists on the spectra until 14 K in the STS measurement, which suggests a prominent fluctuation region of supercon-ductivity. Such a superconducting fluctuation can survive at very high magnetic fields, which are far beyond the critical fields for bulk superconductivity as inferred both from electric transport and tunneling measurements.In chapter 5, we studied the Kondo topological insulator SmB6. By using the arc melting method we can get high quality single crystals. Resistivity saturated at low temperature indicate the existence of surface states, and the ARPES data show obvious surface state, which demonstrated that the SmB6 is a topological Kondo insulator. So it offers a good opportunity to learn this new kind of topological insulator.Finally, a detailed summary is presented in chapter 6. |
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