First observation of superconductivity by uniaxial strain in (TMTTF)2X-salt

We studied superconductivity and spin density wave by measuring temperature dependence of resistivity of (TMTTF)2Br under uniaxial strain up to 3.7 GPa (room temperature). The superconducting phase of (TMTTF)2Br under uniaxial strain was observed below 0.8 K at 3.7 GPa for the first time, which was realized by the compression along the a-axis but not along other directions. Superconductivity was found below a slight SDW-hump in resistance with the a-axis strain; however, with the b'- and c*-axis strain both superconductivity and SDW was suppressed. The anomaly in d ln ρc*/d(1/T) was observed at T*∼0.22TSDW under uniaxial strain along the a-axis. This behavior suggests the existence of the sub-phase structure in incommensurate SDW phase under the a-axis compression. This behavior reminds us of the same tendency in (TMTSF)2PF6 in the previous reports of Kondo et al. where only the a-axis uniaxial strain favors superconductivity, even though the SDW transition is not suppressed sufficiently in both samples.