Electrical transport studies of low-dimensional organic conductors and chalcogenide superconductors under high pressures

Abstract

Many novel phenomena such as superconductivity, metal-insulator transition, and Fermi-surface reconstruction under high-pressure have been reported. High-pressure generally reduces lattice constant in turn modifies band structure. Due to these general effects, many interesting physical phenomena are expected. Recently, the discovery of 200 K superconductor under 155 GPa shows the possibility of the room-temperature superconductor under high-pressure. On top of that, adjusting U/W ratio in the Mott insulators shows many exotic material properties such as an unconventional universality classes or superconductivity. We measured electrical transports of various materials under high-pressure. High-pressure was achieved by three different kinds of pressure cells

hybrid piston cylinder cell, diamond anvil cell, and cubic anvil cell. These pressure cells have different pressure range and pressure quality. Hybrid piston cylinder cell can reach up to 3.0 GPa and diamond anvil cell can reach up to higher than 60 GPa. Although both cells use liquid medium, unexpected uniaxial pressure can be applied higher than medium solidification pressure. On the other hands, cubic anvil cell can apply highly hydrostatic pressure up to 10.0 GPa after liquid medium turns into solid. In this thesis, we focused on low-dimensional materials such as 2H-PdxTaSe2, La-doped CaFeAs2, and P-doped BaFe2As2 with the quasi two-dimensional crystal structures and Nb2Pd1Se5, and [(S,S)-DM-MeDH-TTP]2AsF6 with the quasi one-dimensional crystal structures. In two dimensional materials, superconductivity was enhanced in measured superconductor samples and insulating behavior such as charge density wave, spin density wave, and insulating gap were suppressed by pressure. In addition, some superconductors show quantum critical behaviors under high-pressure. In 2H-PdxTaSe2, incommensurate and commensurate charge density wave were suppressed and superconductivity was enhanced under pressure. We also show that the superconductivity and charge density wave are competing in these materials. This enhancement of superconductivity was also observed in CaFeAs2, BaFeAs2, and Nb2PdxSe5 single crystals. In the [(S,S)-DM-MeDH-TTP]2AsF6 one-dimensional organic conductor, we observed critical exponents δ, β, and γ of an electrical conductivity which are considered as an order parameter of metal-insulator transition. The quantum criticality in two-dimensional metal-insulator transition was suggested in theoretical calculation in 2011 and discovered experimentally in 2015. We have found the quantum criticality in [(S,S)-DM-MeDH-TTP]2AsF6 with the critical exponent zν = 1. This obtained value of zν = 1 is the same as theoretical calculation for Tomonaga-Luttinger liquid model. This implies that the universality class is determined only by global factor such as dimensionality or symmetry, not microscopic factor such as a microscopic interaction.