Spin-polarized scanning tunneling microscopy (SP-STM) has been used extensively to study magnetic properties of nanostructures. Using SP-STM to visualize magnetic order in strongly correlated materials on an atomic scale is highly desirable, but challenging. We achieved this goal in iron tellurium (Fe(1+ y)Te), the nonsuperconducting parent compound of the iron chalcogenides, by using a STM tip with a magnetic cluster at its apex. Our images of the magnetic structure reveal that the magnetic order in the monoclinic phase is a unidirectional stripe order; in the orthorhombic phase at higher excess iron concentration (y > 0.12), a transition to a phase with coexisting magnetic orders in both directions is observed. It may be possible to generalize the technique to other high-temperature superconductor families, such as the cuprates.

The interplay of electronic nematic modulations, magnetic order, superconductivity and structural distortions in strongly correlated electron materials calls for methods which allow characterizing them simultaneously -to allow establishing directly the relationship between these different phenomena. Spin-polarized STM enables studying both, electronic excitations as well as magnetic structure in the same measurement at the atomic scale. Here we demonstrate preparation of magnetic tips, both ferromagnetic and antiferromagnetic, on single crystals of FeTe. This opens up preparation of spin-polarized tips without the need for sophisticated ultra-high vacuum preparation.PACS numbers: 74.55.+v, 74.70.Xa In many unconventional superconductors, the superconducting phase is reached from a magnetically ordered state by some external tuning parameter, such as doping, pressure or chemical substitution. Superconductivity emerges in close vicinity to a magnetically ordered phase [1]. This suggests an intimate relation between magnetism and superconductivity in these materials. Often, the phase diagrams exhibit even regimes of coexistence between the two, however the important question about whether the two coexist or compete at the microscopic level remains unresolved. One difficulty in probing their relation at the atomic scale is that most methods employed to characterize magnetic order, such as neutron scattering, probe a macroscopic sample volume, rendering statements about local phase separation difficult. A method which has been very successful to characterize both superconductivity and magnetism locally on an atomic scale is Scanning Tunneling Microscopy (STM). It has provided important information both about local variations in the superconducting properties and charge ordering in strongly correlated electron materials [2][3][4] and, using magnetic tips in spin-polarized STM, it has also been shown to allow for characterization of magnetism at the atomic scale in nanostructures [5,6]. Application of spin-polarized STM to strongly correlated materials has recently been demonstrated in the nonsuperconducting parent compound of the iron chalcogenide superconductors [7], providing real space images of the magnetic structure Fe 1+δ Te. Preparing and calibrating a magnetic tip for spin-polarized STM measurements has been an important obstacle towards its application to strongly correlated electron materials.In this work, we demonstrate preparation of spinpolarized tips and the characterization of their magnetic properties on Fe 1+y Te. Presence of small amounts of excess iron proves instrumental in the preparation of spin-polarized tips on this material. Specifically we show preparation of both ferromagnetic and antiferromagnetic clusters at the apex of the tip and the characterization of the magnetization of the tip-cluster as a function of field.

In many high temperature superconductors, small orthorhombic distortions of the lattice structure result in surprisingly large symmetry breaking of the electronic states and macroscopic properties, an effect often referred to as nematicity. To directly study the impact of symmetry-breaking lattice distortions on the electronic states, using low-temperature scanning tunnelling microscopy we image at the atomic scale the influence of strain-tuned lattice distortions on the correlated electronic states in the iron-based superconductor LiFeAs, a material which in its ground state is tetragonal with four-fold (C4) symmetry. Our experiments uncover a new strain-stabilised modulated phase which exhibits a smectic order in LiFeAs, an electronic state which not only breaks rotational symmetry but also reduces translational symmetry. We follow the evolution of the superconducting gap from the unstrained material with C4 symmetry through the new smectic phase with two-fold (C2) symmetry and charge-density wave order to a state where superconductivity is completely suppressed.

Since the discovery of iron-based superconductors, a number of theories have been put forward to explain the qualitative origin of pairing, but there have been few attempts to make quantitative, material-specific comparisons to experimental results. The spin-fluctuation theory of electronic pairing, based on first-principles electronic structure calculations, makes predictions for the superconducting gap. Within the same framework, the surface wave functions may also be calculated, allowing, e.g., for detailed comparisons between theoretical results and measured scanning tunneling topographs and spectra. Here we present such a comparison between theory and experiment on the Fe-based superconductor LiFeAs. Results for the homogeneous surface as well as impurity states are presented as a benchmark test of the theory. For the homogeneous system, we argue that the maxima of topographic image intensity may be located at positions above either the As or Li atoms, depending on tip height and the setpoint current of the measurement. We further report the experimental observation of transitions between As and Li-registered lattices as functions of both tip height and setpoint bias, in agreement with this prediction. Next, we give a detailed comparison between the simulated scanning tunneling microscopy images of transition-metal defects with experiment. Finally, we discuss possible extensions of the current framework to obtain a theory with true predictive power for scanning tunneling microscopy in Fe-based systems.