Surface orbital order and chemical potential inhomogeneity of the iron-based superconductor FeTe0.55Se0.45 investigated with special STM tips

The atomically clean surface of the iron-based superconductor FeTe0.55Se0.45 is investigated by low-temperature STM with different tip apex states. By manipulating a single Fe atom onto the tip apex, signatures of the orbital nature of the subsurface Fe layer of FeTe0.55Se0.45 can be identified. By preparing a charged tip state, the intrinsic spatial inhomogeneity of the chemical potential of FeTe0.55Se0.45 can be revealed. As a result, three different types of vortex bound states originating from locally varying topological properties of the FeTe0.55Se0.45 surface are observed by scanning tunneling spectroscopy.

During the last decades, a lot of efforts have been devoted to the functionalization of scanning tunneling microscope (STM) probe tips. For instance, to make the tip apex magnetically sensitive, electrochemically etched tips made of bulk magnetic materials like CrO2 (1), Cr (2), Ni (3,4) and Co (5) were used. Nonmagnetic tips covered by an ultrathin film (6,7) of magnetic materials were also developed for reducing the tip's stray field.
Carefully etched Ag (8) and Au(9) bulk tips with a sharp and smooth apex were employed in the study of tip-enhanced Raman spectroscopy and light emission (10). By decorating the tip apex with a single molecule (11), one can even get information about the chemical bonding (12,13), exchange (14) and superexchange (15) interactions of molecules on metal surfaces.
Special functionalized tips were also used for STM and scanning tunneling spectroscopy (STS) studies of superconductors. To achieve higher energy resolution, tips made of superconducting materials were employed (16). Later on, the application of superconducting probe tips was extended to the study of fundamental properties of superconductors (17,18), the transport characteristics of Josephson junctions (19)(20)(21) and recently the pair density wave state (22)(23)(24). However, most of the studies were focused on conventional superconductors and cuprates. Investigations of iron-based superconductors with special functionalized STM tips are still rare (25,26).
Here, we employ three different kinds of special STM tips to study the iron-based superconductor FeTe 0.55 Se 0. 45 . First, by making use of a very sharp tip, we have found a significant enhancement of the quasiparticle scattering from the Γ point to the X/Y point compared to a normal tip. Second, by picking up a single Fe atom from the sample surface to the tip apex, we have successfully observed a new type of order on the FeTe 0.55 Se 0.45 surface. We attribute these patterns to the sensitivity of a special tip state to the Fe d-orbital ordering. Third, we can even image the charge or chemical potential distribution of the The STM/STS measurements were performed in ultra-high vacuum (less than 10 -10 mbar) and at a temperature of 1.1 K. A lock-in technique with a bias modulation of 0.03 mV and a frequency of 893 Hz was used to record differential tunneling conductance (dI/dV) spectra.
Before recording the spectra, the STM tip has been stabilized with a sample bias voltage V stab and a tunneling current I stab . During the acquisition of dI/dV spectra, the STM feedback loop has been switched off. All the data have been obtained with a bulk Cr tip as a starting point which then has been modified by various in-situ tip treatment procedures as explained in the following.
Previous studies showed that the spatial resolution of STM images can significantly be improved by picking up a single atom from a surface onto the STM tip (27,28). Here, we repeatedly transferred individual Fe atoms onto the tip by vertical manipulation of predeposited Fe atoms on the surface of FeTe 0.55 Se 0.45 . Atomic-resolution STM images of the sample surface before and after the tip has become extremely sharp are shown in Fig. 1(a) and (e). The two bright protrusions are Fe adatoms with the left one partially buried into the surface. Fig. 1(e) clearly shows a reduction of the apparent size of the individual Fe atoms, while the Te/Se square lattice is much more clearly visible compared to Fig. 1(a). The enhanced atomic-scale contrast is also observed in the corresponding FFT maps of Fig. 1 Fig. 3(c), while with the very sharp tip, the QPI image at 1 mV also reveals the atomic lattice very clearly as shown in (g). These differences can even more clearly be seen by comparing the corresponding FFT maps in Fig. 1(d) and (h). The X/Y spot intensities in Fig. 1(h) are greatly enhanced compared to those in (d). Previously, such intensity enhancement has been observed in experiments with a high external magnetic field and was attributed to the nonconventional pairing symmetry of iron-based superconductors (29)(30)(31). However, in our case, the difference in intensity of the Fourier spots can be explained by a trivial tip sharpening effect.
By transferring single Fe atoms onto our tip apex, it is not only the spatial resolution which can be greatly improved, but the tip can additionally become sensitive to the surface Fe orbital ordering. This is supported by a newly observed stripe pattern on the surface of FeTe 0.55 Se 0.45 other than the normal Te/Se square lattice, as shown in Figure 2. Figure 2(a) and (b) display STM images of the same surface region obtained with a normal tip and a special tip having picked-up Fe atoms at its apex. One can clearly see a new periodic stripe pattern in the offdiagonal Te/Se lattice direction. These patterns can be observed everywhere on the surface: the STM image of another region imaged with the same tip is shown in Fig. 2(c,e,f). The green arrow k in Fig. 2(c) indicates the propagation direction of the periodic stripe pattern which is the same as in Fig. 2 This additional periodicity can even more clearly be identified in the corresponding FFT map of Fig. 2(c), as shown in Fig. 2(d). Besides the X/X' and Y/Y' spots which reflect the square atomic lattice symmetry, new spots K and K'(-K) appear. By carefully analyzing the FFT data we found that the angle between ΓX and ΓK is 45 o and that the length ratio of the corresponding wavevectors is 1/1.43~1/√2. These values correspond in fact to the Fe square lattice underneath the top layer Te/Se lattice. As shown in Fig. 2(g), the Fe atoms reside at the bridge sites of the top layer Te/Se lattice with a nearest neighbor distance of 1/√2 the Te/Se lattice constant. Furthermore, the atomic Fe lattice is rotated by 45 o relative to the surface Te/Se lattice. Thus, we attribute the newly observed stripe pattern to the Fe lattice below the surface ( Fig. 2(h)). However, it still needs to be investigated further which information was obtained by the special tip. It should also be noted that the brighter regions which are mainly composed of surface Te atoms always exhibit the square lattice symmetry (32).
A previous study showed that the spin-polarized Yu-Shiba-Rusinov states of individual Fe atoms on the FeTe 0.55 Se 0.45 surface can be revealed by using a bulk Cr tip with an Fe atom at its apex (26). Therefore, our tip might be spin-polarized as well. It is known that FeTe, which is quite similar to FeTe 0.55 Se 0.45 , exhibits a bi-collinear antiferromagnetic order at its surface (33,34). However, our observation here differs from the FeTe case in two ways: First, the period found for the FeTe surface measured with a spin-polarized tip is twice the Te lattice constant whereas in our case we observe a period of 1/√2 the Te lattice constant for the newly observed pattern. More importantly, the direction of the stripe pattern observed on the FeTe surface is the same as for the Te lattice, in contrast to the new pattern observed in the present study. Thus, this new pattern is not related to the bi-collinear unidirectional stripe order observed on FeTe surfaces with a spin-polarized tip. We even confirmed that the newly observed pattern is not related with magnetic order by performing magnetic-field dependent STM measurements. As shown in Figure S1, we can hardly see any contrast change when we ramp the externally applied magnetic field between 2T and -2T.
We therefore propose another type of explanation:  Fig. 2(h). Therefore, we can see the stripe period only in one direction for some special tips. It also explains why the newly observed stripe pattern only appears in the darker regions, as the d yz orbital is less screened by the chalcogenide p z orbital. We can also understand why the tip preparation success rate for this kind of contrast is so low because the tip apex Fe atom need to achieve a special configuration where only d yz -or d xz -orbitals are involved in the tunneling. This particular configuration is rather fragile and a small bias pulse can change it. We further verify our interpretation by performing current-and bias voltage-dependent measurements (Figs. 2(e,f)): We find that the stripe pattern is indeed sensitive to the sample bias voltage as can be seen in Figure 2(f). The particular stripe pattern is clearly resolved at -10 mV but disappears at -40mV. This is consistent with the fact that when imaging the surface with larger bias, more electrons from the bands including the p z orbital states of the chalcogenide atoms are involved in the tunneling process (35,36). On the other hand, the stripe pattern is not sensitive to the tunneling current as shown in Figure 2(e). Our interpretation is also consistent with our magnetic field dependent measurements because a pure orbital imaging mechanism does not involve contributions from spin-polarized tunneling.
These findings provide novel microscopic insights into the nature of orbital order in iron-based superconductors (37,38). , the tip experiences a larger negative electric field and also the negative sample bias applied may induce some additional band bending. As a result, the conduction band will be lifted again due to a higher electronic potential resulting in a reduction of tunneling events in the charged areas compared to the surroundings. Therefore, the spatial extension of the charged area decreases rapidly with decreasing tip-sample distance. In the bias-dependent experiment shown in Figs. 3(e-h), the observations can be interpreted as follows. A higher sample bias voltage will lead to two effects: on one hand, the tip-sample distance will increase in the constant-current STM mode and the electric field will decrease correspondingly. On the other hand, the electric field becomes stronger due to a larger sample bias. Therefore, the experimental results shown Our assumption of a spatially inhomogeneous charge and chemical potential distribution is consistent with the observation that not every vortex core of superconducting FeTe 0.55 Se 0. 45 in an external magnetic field shows a Majorana zero mode (46). One possible explanation is that the chemical potential in the material exhibits a spatially inhomogeneous distribution (49). In conclusion, by functionalizing the STM tip apex by a single Fe atom or a cluster of    . (b,f) FFT maps of (a,e). and denote the first-order spots of the reciprocal lattice. The second-order spots can only be seen clearly in (f). (c,g) dI/dV maps taken at 1 mV for the same regions as shown in (a,e) before (c) and after (g) tip modification. A quasiparticle interference pattern can be seen in both (c) and (g). Scale bar: 1 nm. (d,h) FFT maps of (c,g). After tip modification, the intensity of the spots at points and is greatly enhanced in (h) compared to that in (d) before the tip modification. Tunneling parameters in (a,c,e,g): V stab =-10 mV, I stab =600 pA, V osc = 0.03 mV.