Visualizing Uniaxial-strain Manipulation of Antiferromagnetic Domains in Fe1+YTe Using a Spin-polarized Scanning Tunneling Microscope

Summary

Using uniaxial strain combined with spin-polarized scanning tunneling microscopy, we visualize and manipulate the antiferromagnetic domain structure of Fe_1+yTe, the parent compound of iron-based superconductors.

Abstract

The quest to understand correlated electronic systems has pushed the frontiers of experimental measurements toward the development of new experimental techniques and methodologies. Here we use a novel home-built uniaxial-strain device integrated into our variable temperature scanning tunneling microscope that enables us to controllably manipulate in-plane uniaxial strain in samples and probe their electronic response at the atomic scale. Using scanning tunneling microscopy (STM) with spin-polarization techniques, we visualize antiferromagnetic (AFM) domains and their atomic structure in Fe_1+yTe samples, the parent compound of iron-based superconductors, and demonstrate how these domains respond to applied uniaxial strain. We observe the bidirectional AFM domains in the unstrained sample, with an average domain size of ~50-150 nm, to transition into a single unidirectional domain under applied uniaxial strain. The findings presented here open a new direction to utilize a valuable tuning parameter in STM, as well as other spectroscopic techniques, both for tuning the electronic properties as for inducing symmetry breaking in quantum material systems.

Introduction

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.

Protocol

NOTE: The U-shaped body is made of 416-grade stainless steel, which is stiff and has a low coefficient of thermal expansion (CTE), ~9.9 μm/(m∙°C), as compared to ~17.3 μm/(m∙°C) for 304-grade stainless steel.

1. Mechanical uniaxial-strain device

1. Clean the U-shaped device, the micrometer screws (1–72 corresponding to 72 rotations per inch), the Belleville spring disks, and the base plate by sonicating them separately in acetone first and then in isopropanol, for 20 min each, in an ultrasonic bath sonicator. This removes any impurities/particles. This process should be carried out in the hood.
2. Bake them in an oven for 15–20 min to get rid of any water residue and to degas.
3. Using a sharp razor blade, while observing under an optical microscope, cut the Fe_1+YTe sample to size, namely 1 mm x 2 mm x 0.1 mm.
4. Assemble the parts together as shown in Figure 1C, first panel. The opening inside the U is 1 mm and can be tuned smaller or large by a pair of micrometer screws located on the sides of the device.

2. Application of the strain

1. In two separate dishes, mix silver epoxy (H20E) and nonconductive epoxy (H74F) according to the instructions on the epoxy data sheet.
2. On the U-shaped device, apply a thin layer of silver epoxy (H20E) to create electrical contact, and mount the sample (of a size of 1 mm x 2 mm x ~0.1 mm) with its long axis oriented along the b-axis of the Fe_1+yTe sample, on top of the device, across the 1 mm gap, as shown in Figure 1C. In a convection oven, bake the device for 15 min at 120 °C.
3. Cover the two sides of the sample with nonconductive epoxy so that the sample is firmly supported on the device. Bake for 20 min at 100 °C.
   1. Using an optical microscope, examine the position of the sample from all angles to check for a parallel alignment of the sides of the sample with the gap.
   2. Optionally, place samples within the gap and enforced by H20E and H74F epoxy (Figure 1C).
4. Under an optical microscope, apply compressive strain by rotating the micrometer screw while observing the surface of the sample.
   NOTE: Here we applied a 50° strain, but this can be modified depending on the amount of strain to be applied to the sample. The pressure is transferred to the sample by a series of Belleville spring disks. There should be no cracks or bending of the sample after the pressure is applied.
5. Screw the device onto the base plate as shown in Figure 1B.
   1. Apply a thin layer of silver epoxy (H20E) from the base plate onto the U-shaped device to create electrical contact between the sample and the plate. Bake for 15 min at 120 °C. Measure the electrical contact using a multimeter.
   2. Using a thin layer of H74F nonconducting epoxy, glue an aluminum post (the same size as the sample) onto the strained sample, perpendicular to the a-b cleaving plane. Bake the assembled device for 20 min until the epoxy is cured.

3. Transfer of the device to the scanning tunneling microscope head

1. Transfer the staining device with the sample and the post through the loading dock of the variable-temperature, ultrahigh vacuum scanning tunneling microscope, to the analysis chamber (see Figure 2A).
2. Using an arm manipulator, knock off the aluminum post in ultrahigh vacuum at room temperature, to expose a freshly cleaved surface.
3. Immediately transfer the device (with the strained sample) in situ with another set of manipulators to the scanning tunneling microscope chamber and to the microscope head (see Figure 2B), which has been cooled down to 9 K. Carry out all experiments at 9 K.
4. Allow the sample to cool down overnight before carrying out the next steps.

4. Carrying out the STM experiments

1. Prepare the Pt-Ir tips prior to each experiment by field emission on a Cu (111) surface that has been treated with several rounds of sputtering and annealing.
2. Using the voltage applied to the piezoelectric materials in the microscope by an external controller, move the sample stage to align with the tip, then follow by approaching the sample.
3. Once the tip is a few Å away from the sample and the tunneling current is registered on the oscilloscope, take topographs at different setpoint biases and setpoint currents.
   NOTE: The scanning tunneling microscope is controlled by manufacturer-provided controller and software. For the operation of the microscope, please refer to the user manual/tutorials (http://www.rhk-tech.com/support/tutorials/).

Results

STM topographs were measured in constant current mode with a setpoint bias of -12 meV applied to the sample and a setpoint current of -1.5 nA collected on the tip. Pt-Ir tips were used in all experiments. To achieve spin-polarized STM, the scanning tunneling microscope tip has to be coated with magnetic atoms, which can be quite challenging. In this case of studying Fe_1+yTe, the sample itself provides a simple means of achieving this. The excess irons (y...

Discussion

All operations required to move the samples into and inside the STM are carried out using sets of arm manipulators. The STM is maintained at low temperatures by liquid nitrogen and liquid helium, and the sample cools down for at least 12 h before being approached. This allows the sample and microscope temperature to reach thermal equilibrium. To isolate electric and acoustic noise, the STM is placed in an acoustic and radio frequency shielded room. The microscope head is further suspended from springs for optimized instr...

Materials

Name	Company	Catalog Number	Comments
Belleville spring disks	McMaster Carr
Fe(1.1)Te			Single Crystal
H20E	Epoxy Technology
H74F	Epoxy Technology
Micrometer screws	McMaster Carr
Stainless Steel sheets (416)	McMaster Carr