Electronic and crystal structures of organic superconductors under the uniaxial strain

The interrelation between the electronic properties and the crystal structure was studied in the two-dimensional organic superconductor α-(BEDT-TTF)₂NH₄Hg(SCN)₄ under the uniaxial strain. For this purpose, a novel type of four-circle X-ray diffractometer and a pressure cell were developed. The lattice parameter was found to change as expected from the uniaxial strain created in the sample. The structure analysis based on 936 Bragg reflections suggests that the electronic system becomes more isotropic in the conducting plane and the density-of-states at the Fermi level increases by about 7% under the uniaxial strain created by the piston pressure of 0.1 GPa applied parallel to the b^*-axis. This change in the density-of-states is consistent with the observed increase in the superconducting critical temperature from 1.5 to 2 K in the framework of the BSC expression.