Se-NMR study of superconductor FeSe under pressure

Binary FeSe with $T_{c}$=8 K shows the simplest crystal structure in iron-based superconductors recently reported. Moreover, FeSe is reported that $T_{c}$ increases with applying pressure and the effect of pressure for $T_{c}$ is attracted. Thus the study on FeSe is quite important to investigate the role of iron-network in Fe-based layer. We have carried out Se-NMR measurements under pressure in order to clarify the symmetry of superconducting gap, the magnetic properties of normal state and the origin of pressure dependence of $T_{c}$ in FeSe. In the normal state, the nuclear spin-lattice relaxation rate 1/$T_{1}$ is in proportion to temperature (Korringa relation) at absent pressure. The Korringa relation was also observed under pressure, 0.7 GPa, However the observed value of 1/$T_{1}$ at 0.7 GPa is about twice as large as that at absent pressure. Since 1/($T_{1}T)$ is proportional to the square root of the density of state at Fermi energy $D(E_{F})$, this increase of 1/$T_{1}$ suggests the increase of $D(E_{F})$ by the applying pressure. In the superconductor, 1$/T_{1}$ abruptly decreases by superconducting transition and obeys the cube of $T$ from 7 K to 1.5 K at 2 T. the reduction of 1/$T_{1}$ was also observed below 10K under pressure. The increase of $T_{c}$ by applying pressure is conformed from microscopic viewpoint.