Visualizing Uniaxial-strain Manipulation of Antiferromagnetic Domains in Fe₁₊<em>_Y</em>Te Using a Spin-polarized Scanning Tunneling Microscope

High-temperature superconductivity in cuprates and iron-based superconductors is an intriguing state of quantum matter^1,2. A major challenge in understanding superconductivity is the locally intertwined nature of various broken symmetry states, such as electronic nematic and smectic phases (that break rotational and translational symmetries of the electronic states), with superconductivity^3,4,5,6,7. Manipulation and deliberate tuning of these broken symmetry states is a key objective toward understanding and controlling superconductivity.

Controlled strain, both uniaxial and biaxial, is a well-established technique to tune the collective electronic states in condensed matter systems^{8,9,10,11,12,13,14,15,16,17,18,19,20,21,22}. This clean tuning, without the introduction of disorder through chemical doping, is commonly used in various kinds of experiments to tune bulk electronic properties^23,24,25,26. For example, uniaxial pressure has proved to have an immense effect on superconductivity in Sr₂RuO₄¹³ and cuprates²⁷ and on the structural, magnetic, and nematic phase transitions of iron-based superconductors^10,14,28,29 and was recently demonstrated in tuning the topological states of SmB₆²⁴. However, the use of strain in surface-sensitive techniques, such as STM and angle-resolved photoemission spectroscopy (ARPES), has been limited to in situ-grown thin films on mismatched substrates^26,30. The major challenge with applying strain to single crystals in surface-sensitive experiments is the need to cleave the strained samples in ultrahigh vacuum (UHV). In the last few years, an alternative direction has been to epoxy a thin sample on piezo stacks^9,10,18,31 or on plates with different coefficients of thermal expansion^19,32. Yet in both cases, the magnitude of the applied strain is quite limited.

Here we demonstrate the use of a novel mechanical uniaxial-strain device that allows researchers to strain a sample (compressive strain) without constraints and simultaneously visualize its surface structure using STM (see Figure 1). As an example, we use single crystals of Fe_1+yTe, where y = 0.10, the parent compound of the iron chalcogenide superconductors (y is the excess iron concentration). Below T_N = ~60 K, Fe_1+yTe transitions from a high-temperature paramagnetic state into a low-temperature antiferromagnetic state with a bicollinear stripe magnetic order^26,33,34(see Figure 3A,B). The magnetic transition is further accompanied by a structural transition from tetragonal to monoclinic^26,35. The in-plane AFM order forms detwinned domains with the spin structure pointing along the long b-direction of the orthorhombic structure³⁴. By visualizing the AFM order with spin-polarized STM, we probe the bidirectional domain structure in unstrained Fe_1+yTe samples and observe their transition into a single large domain under applied strain (see the schematic in Figure 3C-E). These experiments show the successful surface tuning of the single crystals using the uniaxial-strain device presented here, the cleaving of the sample, and the simultaneous imaging of its surface structure with the scanning tunneling microscope. Figure 1 shows the schematic drawings and pictures of the mechanical strain device.