The peak effect and the quasiorder–disorder phase transition for type-II conventional and high-Tc superconductors have been investigated by taking into account both the quenched disorder and the thermal fluctuations of the vortex lattice. The peak value of the critical current density, the exact peak position and its corresponding half-width for a constant temperature as well as for a constant applied magnetic field have been calculated for type-II superconducting films and bulk materials in the non-dispersive regime of the vortex bundle, and all of the results from the experiment are in agreement.

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The charge transport of electron doped Mott insulators on a triangular lattice is investigated within the t-J model. The conductivity spectrum shows a low-energy peak and an unusual midinfrared band, while the resistivity is characterized by a crossover from the high temperature metallic-like to low temperature insulating-like behavior, in qualitative agreement with experiments. Our results also show that the mechanism that causes this unusual charge transport is closely related to a competition between the kinetic energy and magnetic energy in the system. PACS: 74.25.Fy, 74.25.Gz, The recent discovery of superconductivity in doped cobaltates, Na x CoO 2 · yH 2 O with the superconducting transition temperature T c ∼ 5K, has generated great interests due to the role of strong electron correlations 1 . This compound has a lamellar structure consisting of the two-dimensional (2D) CoO 2 layers separated by a thick insulating layer of Na + ions and H 2 O molecules, where the one-half spin Co 4+ ions are arranged on a triangular lattice. This structure is similar to cuprates superconductors in the sense that they also have a layered structure of the square lattice of the CuO 2 plane separated by insulating layers 2 . It has been well established that Cu 2+ ions exhibit an antiferromagnetic (AF) long-range order (AFLRO) in the parent compounds of cuprate superconductors, where superconductivity occurs when the AFLRO state is suppressed by hole or electron doping 2,3 . However, Na x CoO 2 ·yH 2 O is viewed as an electron doped Mott insulator, where superconductivity appears with electron doping 1 . A fundamental similarity between cuprate and cobaltate superconductors has been seen in the decreases in the superconducting transition temperature for both underdoped and overdoped materials 4 . The optimal doping for superconductivity occurs at 0.3 electrons per Co above the ground-state Na 0 CoO 2 · 1.3H 2 O, which is a half-filled two-electron t 2g derived band, while for cuprates, the optimal doping occurs at 0.15 holes (electrons) added to the half-filled band of the parent compound 4 . On the other hand, doped Mott insulators on a triangular lattice, where the geometric spin frustration exists, are also of interests in their own right, with many unanswered fascinating questions. Historically the undoped Mott insulator on a triangular lattice was firstly proposed to be a model for microscopic realization of the spin liquid due to the existence of the strong spin frustration 5 . It has been argued that this spin liquid state may play a crucial role in the mechanism for superconductivity in doped cuprates 6 . Thus the unexpected finding of superconductivity in doped cobaltates has raised the hope that it may help solve the unusual physics in doped cuprates.Some experimental studies have revealed a non-Fermi liquid behavior in doped cobaltates 1,7,8 . Among the striking features in the normal-state, a hallmark is the charge transport, where the resistivity shows a temperature linear variation in a wide range of temperature...

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