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

Systematic uniaxial strain studies of α-(BEDT-TTF)₂MHg(SCN)₄ (M = K, NH₄) 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 NH₄ 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.