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Study Reveals Mysterious Electronic Signature of Iron-Based Superconductors

Physicists at Rice University have used an ingenious method to draw a detailed map revealing the “rules of the road” for electrons, both in the critical moments and in normal conditions, just before the material changes into a superconductor. This ingenious method causes unruly iron selenide crystals to snap into alignment.

Using a technique to snap unruly crystals of iron selenide into alignment, Rice University physicists (from left) Ming Yi, Qimiao Si, Tong Chen, Han Wu, and colleagues have drawn an electronic road map that reveals the quantum mechanical “rules of the road” that electrons must follow in the enigmatic superconductor. Image Credit: Tommy LaVergne/Rice University.

Physicist Ming Yi and collaborators have drawn a band structure map specifically for iron selenide—a material that has puzzled physicists for a long time due to its behavioral complexity and structural simplicity. The study has been published online in the Physical Review X (PRX), the American Physical Society journal.

This map describes the material’s electronic states. It is a visual summary of data collected by measuring a single iron selenide crystal while it was being cooled to the point of superconductivity.

For the study, Yi started the angle-resolved photoemission spectroscopy experiments when she was doing her PhD at the University of California, Berkeley. The experiments are technically complex and coaxed the crystal to discharge electrons by using strong synchrotron light from the Stanford Synchrotron Radiation Lightsource (SSRL).

In a sense, these measurements are like taking photographs of electrons that are flying out of the material. Each photograph tells the lives the electrons were living right before being kicked out of the material by photons. By analyzing all the photos, we can piece together the underlying physics that explains all of their stories.

Ming Yi, Assistant Professor, Department of Physics and Astronomy, Rice University

Red-Light Cameras for Electrons

The electron detector monitored the speed as well as the direction traveled by electrons when discharged from the crystal. That data contained significant clues regarding the quantum mechanical laws governing the traffic patterns at a greater, microscopic scale, where major aspects of superconductivity are expected to emerge.

According to Yi, such rules are encoded in the electronic structure of a material.

They’re like an electronic fingerprint of a material. Each material has its own unique fingerprint, which describes the allowed energy states electrons can occupy based on quantum mechanics. The electronic structure helps us decide, for example, whether something will be a good conductor or a good insulator or a superconductor.

Ming Yi, Assistant Professor, Department of Physics and Astronomy, Rice University

When Things Go Sideways

Computers, smartphones, and wires heat up during use because of electrical resistance, costing billions of dollars every year in terms of cooling bills for data center or lost power on electric grids.

That waste could be eliminated by superconductivity—the zero-resistance flow of electricity. However, physicists have found it difficult to interpret and describe the behavior of unusual superconductors such as iron selenide.

The first iron-based superconductors were identified in 2008 when Yi was in graduate school. She has spent her career investigating these superconductors. In each of these superconductors, a layer of iron measuring one-atom-thick is closely packed between other elements.

The atoms in this layer of iron are organized in checkerboard squares at room temperature. However, upon cooling the materials close to the point of superconductivity, a shift occurs in the iron atoms, causing the squares to become rectangular. Such a change results in nematicity, or direction-reliant behavior, which is considered to play a crucial but unidentified role in superconductivity.

Iron selenide is special because in all of the other iron-based materials, nematicity appears together with magnetic order,” Yi added. “If you have two orders forming together, it is very difficult to tell which is more important, and how each one affects superconductivity. In iron selenide, you only have nematicity, so it gives us a unique chance to study how nematicity contributes to superconductivity by itself.”

Performing Under Pressure

The electrons’ traffic patterns are the upshot of nematicity, and these patterns are caused by the quantum rules. The traffic patterns of electrons may be rather different for electrons that flow right to left, along the rectangles’ long axis when compared to the electrons that flow up and down along the short axis.

However, it has been very difficult to get a clear look at those traffic patterns present in iron selenide. This difficulty can be attributed to twinning—a property of the crystals that makes the rectangles to haphazardly change direction by 90°. Twinning implies that long-axis rectangles will run left to right approximately 50% of the time and will run up and down the other half.

In the case of iron selenide, twinning made it unfeasible to get clear, whole-sample measurements of the material’s nematic order until Pengcheng Dai and Tong Chen, both physicists at Rice University, published an ingenious solution to the issue in May.

Based on a detwinning method devised by Dai and collaborators in 2014, Chen discovered that fragile crystals of iron selenide could be detwinned by pasting them atop a stronger layer of barium iron arsenide and then applying a slight pressure by turning a screw. Through this method, all the nematic layers in the iron selenide are snapped into alignment.

The co-authors of the PRX paper are Dai and Chen. According to Yi, the detwinning method was instrumental in obtaining clear information about the effect of nematicity on the electronic behavior of iron selenide.

This study would not have been possible without the detwinning technique that Pengcheng and Tong developed. It allowed us to take a peek at the arrangements of electronic states as the material system gets ready for superconductivity. We were able to make precise statements about the availability of electrons belonging to different orbitals that could participate in superconductivity when nematic rules have to be obeyed.

Ming Yi, Assistant Professor, Department of Physics and Astronomy, Rice University

A Path Forward

According to Yi, the data show that the extent of nematic shifts in iron selenide is similar to the changes quantified in more challenging iron-based superconductors that also include magnetic order.

This indicates that the nematicity that has been noted in iron selenide might be a universal aspect of all iron-based superconductors, irrespective of the existence of long-range magnetism, added Yi. She hopes that her information would enable theorists to investigate that possibility and more.

This set of measurements will provide precise guidance for theoretical models that aim to describe the nematic superconducting state in iron-based superconductors,” she added. “That’s important because nematicity plays a role in bringing about superconductivity in all of these materials.”

Other co-authors include Han Wu and Qimiao Si, both from Rice University; Heike Pfau of Lawrence Berkeley National Laboratory; Yan Zhang and Zirong Ye, both from Peking University; Yu He, Dung-Hai Lee, and Robert Birgeneau, all from UC Berkeley; Makoto Hashimoto and Donghui Lu, both from the SSRL; Zhixun Shen from Stanford University; and Rong Yu from Renmin University.

RCQM makes use of international collaborations and the strengths of over 20 research groups at Rice University to deal with queries associated with quantum materials.

RCQM is supported by Rice University’s offices of the provost and the vice provost for research, the Brown School of Engineering, the Wiess School of Natural Sciences, the Smalley-Curl Institute, and the departments of Materials Science and NanoEngineering, Electrical and Computer Engineering, and Physics and Astronomy.

The study was funded by the Department of Energy (DE-AC02-05-CH11231, KC2202, DE-SC0012311, DE-SC0018197), the Alfred P. Sloan Foundation, the Robert A. Welch Foundation (C-2024, C-1839, C-1411), the National Science Foundation of China (11374361, 11674392), and the Miller Institute for Basic Research in Science, and the Ministry of Science and Technology of China (2016YFA0300504), the German Science Foundation (DFG-PF 947/1-1).


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