STM/STS observation of the local density of states in impurity doped Mx-Fe(SeTe) superconductors
*Akira Sugimoto1, Satoru Ishimitsu1, Hironori Ohtsubo1, Kaito Matsumoto1, Takeshi Saito1, Toshikazu Ekino1, Alexander M. Gabovich2

The STM/STS (scanning tunnel microscopy/scanning tunnel spectroscopy) measurements of the local quasiparticle density of states (LDOS) in impurity doped Mx-Fe (SeTe) (M = Zn, Co, Cu, Ni ) superconductors were carried out. Single crystal samples of Mx-FeSe0.4Te0.6 (x= 0- 0.03) were synthesized by the self-flux method using double-sealed quartz tubes. The onset superconducting critical temperatures (Tc onset) of samples with x= 0.01 and doped by Zn, Co, Cu, Ni were 15.1 K, 14.2 K, 12.7 K, 10.4 K, respectively. The STM measurements were carried out at T = 4.9 K under the pressure of P ~ 10-8 Pa. The magnitude of LDOS (given by the electrical conductance dI/dV) was obtained by numerical differentiation of the measured tunnel-current (I) – voltage (V) characteristics, I(V).

All the STM images show clear regular square arrangements of spots with the lattice spacing ~0.38 nm as well as the randomly distributed large spots, which are most possibly due to the excess Fe atoms. On the other hand, the LDOS (dI/dV) maps of all doped samples show extra energy-dependent spot structures of bright (with higher magnitudes of dI/dV) or dark (lower magnitudes) types. With increasing of the Cu doping level x, the number of such spots grew proportionally, indicating that these spots correspond to the dopant atoms. Periodical LDOS modulations were also observed. Among them, the non-energy-dispersive periodic structures (about ~4 times of a0 and b0 lattice spacing) remarkably enhanced with growing Cu dopant amount. Those patterns were similar to the checkerboard charge-order structures in cuprate superconductors. The results obtained suggest that the impurity doping induces or at least strengthens a charge order, which competes with the superconducting pairing.

Keywords: Iron 11 superconductor, STM/STS, Impurity effect