Observation of Supermodulation in LaO_0.5F_0.5BiSe₂ by Scanning Tunneling Microscopy and Spectroscopy

Abstract

We observed surface and electronic structure of LaO 0.5 F 0.5 BiSe 2 single crystal by scanning tunneling microscopy/spectroscopy (STM/STS) at 4.2 K. Square lattice composed of Bi atoms was observed at a positive sample bias voltage on the surface prepared by cleavage. At a negative sample bias voltage, a stripe structure running along Bi-Bi directions was observed as in the previous report on NdO 0.7 F 0.3 BiS 2 . Furthermore, we observed a supermodulation running along the diagonal directions with the period of about 5 times of the lattice constant. This seems to be indicative of structural instability of this system rather than electronic instability attributed to a nesting picture.Key word: Superconductivity, BiS 2 -superconductor, Scanning tunneling microscopy, Scanning tunneling spectroscopy 2 Recently, various kinds of layered BiS 2 -based superconductors, LnOBiS 2 (Ln = La, Pr, Ce, Nd, Yb, and Bi), have been discovered . These materials have been intensively investigated because their layered structure, which is composed of superconducting layers and blocking layers, is similar to those of cuprates and iron-based superconductors. Although, the superconducting transition temperature (T c ) of these BiS 2 -based materials is 3-5 K, T c strongly depends on external pressure or high-pressure annealing [2,[28][29][30][31][32][33]. For instance, LaO 1-x F x BiS 2 showing T c of around 3 K shows superconductivity at around 10 K when an external pressure is applied. This higher T c remains at ambient pressure after as-grown polycrystalline samples are annealed under high pressure. These high-pressure effects indicate that the crystal structure of these materials is rather unstable against the external perturbations.Theoretical calculations also predicted electronic and structural instability against the formation of charge density wave (CDW) [34][35][36][37][38]. A calculation of band structures predicted due to nesting at the F concentration of x=0.5, where the Fermi surface connects at around (/2, /2) in the k space [34,35]. The CDW is expected to be formed along the (, ) direction in the k space, whose direction corresponds to the diagonal directions of Bi square lattice in the real space with the period of about √2 times of the lattice constant. A calculated phonon dispersion curve found the existence of plateau at the negative energy between Γ and M point in k space, indicating a structural instability at F concentration of x=0.5 [35][36][37][38]. Although the direction of the instability agrees with that of the electronic instability, because of the plateau at the negative energy, the period of the CDW cannot be defined from the dispersion curve.These theoretical calculations suggest the superconductivity in BiS 2 based materials realizes near CDW ground state.Although the existence of the CDW was suggested by transport experiments in EuFBiS 2 , this has not been confirmed by direct observations [10]. Thus, the relation between the superconductivity and the CDW is still unclear. 3To investigate...

Material Development and Physical Properties of BiS2-Based Layered Compounds

“…Using single crystals, scanning tunneling microscopy/spectroscopy (STM/STS) has been performed to analyze the surface structure and the electronic states of BiS 2 -based compounds [58,75,158,200]. Machida et al observed checkerboardstripe electronic states for NdO 0.7 F 0.3 BiS 2 [75].…”

Structural Modulation in LaO0.9F0.1BiSe2 Single Crystals Revealed by Scanning Tunneling Microscopy/Spectroscopy

“…In addition, some Bi defects have dark streaks along one or two diagonal directions of the Bi square lattice. The density of defects was about 1% and was almost the same as that in x = 0.5 sample [4]. Thus, the Bi defect density is almost independent of the amount of F. Figure 1 (b) shows the STM image taken at a negative sample bias voltage for the same field of view as Figure 1 (a).…”

“…However, spectroscopic gap was observed at the Fermi energy EF despite of metallic bulk properties [2]. Furthermore, the unexpected supermodulation with the period of about 5 times the length of the lattice constant running along the diagonal direction of the Bi square lattice was revealed in LaO0.5F0.5BiS2 (x = 0.5) single crystals by synchrotron X-ray measurements [3] and in LaO0.5F0.5BiSe2 (x = 0.5) single crystals by STM/STS measurements [4]. Theoretically, in high F doping samples, such as LaO0.5F0.5BiS2 (x = 0.5), the phonon instability spreading from near Γ point to M point is predicted while in low F doping samples, such as LaOBiS2 (x = 0), the phonon instability around Γ point is predicted [5].…”