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 conductance at low field and high temperature are 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∆/kB Tc ≈ 16.6. 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 superconductivity. Such 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.

Superconductivity in LaRu3Si2 with the honeycomb structure of Ru atoms has been investigated. It is found that the normal state specific heat C/T exhibits a deviation from the Debye model down to the lowest temperature. A relation C/T = γn + βT 2 − AT lnT which concerns the electron correlations can fit the data very well. The suppression to the superconductivity by the magnetic field is not the mean-field like, which is associated well with the observation of strong superconducting fluctuations. The field dependence of the induced quasiparticle density of states measured by the low temperature specific heat shows a non-linear feature, indicating the significant contributions given by the delocalized quasiparticles.PACS numbers: 74.70. Dd, 74.25.Bt,74.10.+v Superconductivity arising from non-phonon mediated pairing, such as through exchanging the magnetic spin fluctuations, has renewed interests in condensed matter physics. The superconducting (SC) mechanism of the cuprates[1] and the iron pnictides[2], although not yet settled completely, should have a close relationship with the electron correlations.[3-5] A similar assessment may extend to many others, like heavy Fermion[6] and organic materials [7]. In this regard, the systems RT 3 Si 2 or RT 3 B 2 (R stands for the rare earth elements, like La, Ce, Y, etc., T for the transition metals, like Ru, Co and Ni, etc.) provide an interesting platform, since a variety of combinations of chemical compositions allow the system to be tuned between superconducting and magnetic, and sometimes both phases coexist. [8,9] Among these samples, the LaRu 3 Si 2 has a SC transition temperature as high as 7.8 K.[10] The material of LaRu 3 Si 2 contains layers of Ru with the honeycomb structure sandwiched by the layers of La and Si, forming a P 6 3 /m or P 6 3 space group. Preliminary experiment found that the SC transition temperature drops only 1.4 K with the substitution of 16 % La by Tm (possessing a magnetic moment of about 8µ B ), suggesting that the superconductivity is robust against the local paramagnetic moment [8]. By doping the La with Gd, a coexistence of superconductivity and the spin glass state [11] was observed. In CeRu 3 Si 2 , the SC transition temperature drops to about 1 K and a valence fluctuation model was proposed for the pairing [12]. Since the Ru atom locates just below the Fe in the periodic table, a key player in the iron pnictide superconductors, therefore it is very curious to know whether the superconductivity here is induced by the electron-phonon coupling, or by other novel mechanism, such as the electron correlations. In this paper we report the results of transport and specific heat on samples of LaRu 3 Si 2 . Our results reveal some novelties in both the SC and normal states of LaRu 2 (degree) FIG. 1: (color online) X-ray diffraction patterns of the sample LaRu3Si2. All main diffraction peaks can be indexed well by a hexagonal structure with a = 5.68Å and c = 7.13Å with Ru as the impurity phase. For some peaks the difference between the data ...

A high-quality FeSe0.5Te0.5 single crystal with Tc ∼ 14.8 K was obtained by the self-flux method. We present the temperature dependence of resistivity at various fields and the magnetization hysteresis loops at various temperatures. The upper critical field Hc2(T) with a criterion of 90% ρn follows Hc2(T) = Hc2(0)[1−T/Tc]n. A second peak (namely the fishtail effect) was observed in a large temperature region ranging from 6.5 K to 12 K. According to the theory of Dew-Hughes, we scaled the flux pinning force density for several fixed temperatures and found its maximum position around h = 0.33, here h is the reduced magnetic field. The dynamical relaxation rate Q of the vortices was measured and discussed.