Search

Since the discovery of high-temperature superconductivity in cuprates, finding more unconventional superconductors and understanding their superconducting pairing mechanism has been an important theme in condensed matter physics. Recently, ternary Cr-based superconductors A2Cr3As3 (A=K, Rb, Cs) and ACr3As3 (A=K, Rb) were reported, which own quasi-one-dimensional crystal structure, containing[(Cr3As3)-] linear chains. A2Cr3As3 belongs to P6m2 space group, and ACr3As3 crystallizes in a centrosymmetric structure with the space group P63/m. Many experiments, such as nuclear magnetic resonance, London penetration depth, show that A2Cr3As3 is an unconventional superconductor. However, these A2Cr3As3 compounds are extremely unstable in air. Here, we study the superconducting gap of the air-stable RbCr3As3 single crystal, using ultralow-temperature thermal conductivity measurement. The resistivity of RbCr3As3 single crystal shows a superconducting transition temperature Tczero at 6.6 K. The normal-state resistivity data from 20 K to 8 K are fitted to (T)=0 + AT2, which gives a residual resistivity of 0=781 cm. Then, the thermal conductivity of RbCr3As3 single crystal is measured at temperature down to 80 mK and in magnetic fields up to 9 T. In zero field, residual linear term 0/T=7.5 WK-2cm-1 is observed, which is about 24% of its normal-state value, suggesting nodes in the superconducting gap. At low field, the 0/T of RbCr3As3 shows a relatively faster field dependence than single-gap s-wave superconductors. These results reveal that RbCr3As3 is likely an unconventional superconductor with superconducting gap nodes, although the exact superconducting gap symmetry and structure for this quasi-one-dimensional superconductor needs further investigation.

The temperature dependence of thermal conductivity and electronic conductivity in superconductor MgB 2 , MgCNi 3 and Bi 2 Sr 2 Ca 0.9 Ce 0.1 Cu 2 O 8+y are presented. The thermal conductivity of Bi 2 Sr 2 Ca 0.9 Ce 0.1 Cu 2 O 8+y shows a peak under T c observ ed in cuprates, but those of MgB 2 and MgCNi 3 decrease monotonously. We calculate the elec tron thermal condu ctivity and the phonon thermal conductivity by Wiedemann-Franz law. The electron s contribute a large fraction to the thermal conductivity in normal state of MgB 2 and MgCNi 3 because of the non-localigation of mutual eff ect of electrons. The analysis on electronic thermal conductivity indicates that the scattering by impurities prevails in electronic thermal resistance of both samples.

The thermal conductivity of MgB2 polycrystalline sample in the mixed state has been investigated as a function of magnetic field (0-7 T) and temperature (5-45 K). The thermal conductivity increased rapidly in low magnetic fields, and became saturation in higher fields. This behaviour correlates with the two-gap electronic structure of MgB2. Analysis indicates that the remarkable en hancement of thermal conductivity of MgB2 polycrystalline sample in the mixed state cannot be completely attributed to the electronic contribution.

The temperature-dependent thermopower and resistivity of MgB2 were measured.In the range of 100—300K, the thermopower presents positive nearly linear temperature dependence,indicating that the charge carrier is hole and it is consistent with the energy band pattern.In contrast,the resistivity follows T2 relationship in the same temperature range.Below 100K,thermopower and resistivity change their temperature dependence.The thermopower presents a hump between the superconducting transition temperature Tc (zero resistance,36.6K) and 100K.It has the characteristic of the phonon drag,indicating the strong electron-phonon interaction.Some important parameters are estimated according to experimental results,such as Fermi energy EF and energy band width.

The temperature dependences of resistivity and Hall coefficient of MgB2 and Mg0.93Li0.07B2 were measured, and the temperature dependence of cot-ΘH was calculated. The result of resistivity indicates that MgB2 and Mg0.93Li0.07B2 nearly have the same transition temperature. The-possible effect on transition temperature has been discussed. The measurement of Hall coefficient suggests the type of charge carriers is hole, and the Hall coefficient of Mg0.93Li0.07B2 becomes a little smaller compared with that of MgB2 which is attributed to Li doping . Both the decrease with increasing temperature-and-the temperature dependence of cotangent of Hall angle are proportional to the square of temperature.

Temperature dependence of magnetoresistance in the range of 2—300K and the Δρ /ρ０-B relation at 2K were measured in quasi-two dimensional charge- density wave compound (PO2)4(WO3)2m( m=6). The enhancement of the magnetoresistance at low temperatures was analyzed by use of the magnetic breakdown model. The first Peierls energy gap in the char ge-density wave states was estimated to be about 3.0meV.

The in-plane resistivity ρab(T) and the out-of-plane ρc(T) have been extensively measured for pure single crystal Bi2Sr2CaCu2O8+δ(Bi2212) annealed at different oxygen pressures.The ρc(T) and anisotropy (ρc(T)/ρab(T)) decrease rapidly with increasing carrier concentration.It is found that the out-of-plane resistivity decreases linearly with decreasing temperature down to about 120K for the overdoped sample,its resistivity anisotropy is of weak temperature dependence.The data of ρc(T) and ρc(T)/ρab(T) are well fitted by the bipolaron theory proposed by Alexandrov and Mott.

The electrical transport of polycrystalline samples SmxC60 has been investigated by resistivity and magnetoresistance (MR) measurements. The transport mechanism is different for the samples with different x in SmxC60 system. The temperature-dependent resistivity can be explained by the fluctuation-induced-tunneling model for the sample SmC60. The transport properties for the sample Sm2.75C60 appear to be dominated by weak localization and electron-electron interactions. The MR data of the sample Sm2.75C60 can be fitted by a weak-localized model with strong electron-electron interactions, but that of the sample SmC60 cannot.