Control of electronic properties ofα−(BEDT−TTF)2MHg(SCN)4(M=K,NH4)by the uniaxial strain method

Systematic uniaxial strain studies of $\alpha -{\mathrm{}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})\mathrm{}}_{2}M{\mathrm{Hg}\left(\mathrm{SCN}\right)\mathrm{}}_4$ $(M={\mathrm{K},\mathrm{N}\mathrm{H}}_4)$ have been carried out. By controlling the in-plane lattice parameters a and c independently, we find that both compounds have essentially the same phase diagram in electronic properties, although, under the hydrostatic pressures, they show quite different phase diagrams. The K salt shows the superconductivity like ${\mathrm{NH}}_4$ salt under the c-axis uniaxial strain, and the latter undergoes a transition to the density-wave state like the former under the a-axis uniaxial strain. These changes in electronic structures are verified by measuring the angle-dependent magnetoresistance. The electronic properties of these compounds can be systematically understood as a function of $c/a,$ the ratio of the in-plane lattice parameters.