Quasiparticle interference (QPI) of the electronic states has been widely applied in scanning tunneling microscopy to analyze the electronic band structure of materials. Single-defect-induced QPI reveals defectdependent interaction between a single atomic defect and electronic states, which deserves special attention. Due to the weak signal of single-defect-induced QPI, the signal-to-noise ratio is relatively low in a standard twodimensional QPI measurement. In this paper, we introduce a projective quasiparticle interference (PQPI) method in which a one-dimensional measurement is taken along high-symmetry directions centered on a specified defect. We apply the PQPI method to the topological nodal-line semimetal ZrSiS. We focus on two special types of atomic defects that scatter the surface and bulk electronic bands. With an enhanced signal-to-noise ratio in PQPI, the energy dispersions are clearly resolved along high-symmetry directions. We discuss the defect-dependent scattering of bulk bands with the nonsymmorphic symmetry-enforced selection rules. Furthermore, an energy shift of the surface floating band is observed, and a branch of energy dispersion (q 6) is resolved. This PQPI method can be applied to other complex materials to explore defect-dependent interactions in the future.

We apply high resolution scanning tunneling microscopy to study intrinsic defect states of bulk FeSe. Four types of intrinsic defects including the type I dumbbell, type II dumbbell, top-layer Se vacancy and inner-layer Se-site defect are extensively analyzed by scanning tunneling spectroscopy.From characterized depression and enhancement of density of states measured in a large energy range, the type I dumbbell and type II dumbbell are determined to be the Fe vacancy and Se Fe defect, respectively. The top-layer Se vacancy and possible inner-layer Se-site vacancy are also determined by spectroscopy analysis. The determination of defects are compared and largely confirmed in the annular dark-field scanning transmission electron microscopy measurement of the exfoliated FeSe. The detailed mapping of defect states in our experiment lays the foundation for a comparison with complex theoretical calculations in the future. * yiyin@zju.edu.cn 1 arXiv:1908.03427v1 [cond-mat.supr-con] 9 Aug 2019 I. INTRODUCTIONAtomic defects are ubiquitous in condensed matter materials. The type, density and distribution of atomic defects can be controlled in material preparation to introduce doped carriers [1,2], tune phase transitions [3,4], pin vortices in superconductors [5,6], and provide other related applications. The microscopic effect of atomic defects has been well studied by atomic-resolved scanning tunneling microscopy (STM). The defect scattering of electronic states leads to a quasiparticle interference (QPI) pattern from which the electronic band structure of materials can be extracted [7,8]. Magnetic or nonmagnetic defects can induce a resonant in-gap state for probing pairing symmetry of high-T c superconductors [9-13]. Generally, the defect-induced change of the local density of states (DOS) includes information of the interaction between defects and the bulk material, rendering insights about the determination of defects and material properties.FeSe is the structurally simplest iron-based superconductor [14]. The critical temperature of the parent bulk FeSe is T c ∼ 9 K, which can be astonishingly enhanced to much higher values by the intercalation [15][16][17][18] or the doping with K adatoms [19][20][21][22]. A related high-T c system is the monolayer FeSe grown on SrTiO 3 substrates [23,24]. A dumbbell defect has been observed in FeSe and FeSe-related systems, with defects as scattering centers of a QPI pattern at ultra-low temperatures [25][26][27][28][29], pinning sites of nematic order [30-32] and charge order [33], and the touchstone of the paring symmetry [35][36][37].In most previous STM reports of FeSe [26,32,34,35], the voltage range of scanning tunneling spectroscopy (STS) is limited within ±20 mV, close to the superconducting gap (∆ ≈ 2.5 mV). Correspondingly such delicate spectroscopy can be used to clarify the pairing symmetry of the superconductor [26,32,34,35]. Despite of the insightful information reported from the intrinsic defects such as dumbbell defect and Se vacancy [25,[35][36][37][38], spectrosc...

Multiple ordered states have been observed in unconventional superconductors. Here, we apply scanning tunneling microscopy to probe the intrinsic ordered states in FeSe, the structurally simplest iron-based superconductor. Besides the well-known nematic order along [100] direction, we observe a checkerboard charge order in the iron lattice, which we name a [110] electronic order in FeSe. The [110] electronic order is robust at 77 K, accompanied with the rather weak [100] nematic order. At 4.5 K, The [100] nematic order is enhanced, while the [110] electronic order forms domains with reduced correlation length. In addition, the collective [110] order is gaped around [−40, 40] meV at 4.5 K. The observation of this exotic electronic order may shed new light on the origin of the ordered states in FeSe.