Newswise — In specific circumstances, typically in extremely low temperatures, certain substances undergo a structural transformation that enables the emergence of novel superconducting properties. This transformation, referred to as a "nematic transition," holds the potential to induce a state in materials wherein electrons can flow without any resistance, thereby advancing superconductivity. Physicists are actively exploring this phenomenon as a promising avenue for achieving frictionless electron conduction in materials.
But what exactly drives this transition in the first place? The answer could help scientists improve existing superconductors and discover new ones.
MIT physicists have made a significant discovery regarding the nematic transition in a specific class of superconductors, which challenges the assumptions held by many scientists. They have identified the crucial factor that triggers this transition, and it turns out to be surprisingly different from what was previously believed.
In their research, the physicists focused on investigating iron selenide (FeSe), a two-dimensional material recognized as the highest-temperature iron-based superconductor. This material exhibits a transition to a superconducting state at relatively elevated temperatures, reaching as high as 70 kelvins (approximately -300 degrees Fahrenheit). While still requiring ultracold conditions, this transition temperature surpasses that of the majority of known superconducting materials.
The greater the temperature at which a material can achieve superconductivity, the more potential it holds for practical applications. This includes the development of advanced electromagnets for enhanced precision and lighter MRI machines, as well as the creation of high-speed trains that utilize magnetic levitation.
To explore various applications and advancements, scientists must first gain an understanding of the underlying mechanism behind the nematic switch observed in high-temperature superconductors such as iron selenide. In other iron-based superconducting materials, researchers have observed that this switch occurs when individual atoms undergo a sudden change in their magnetic spin, aligning themselves with a preferred magnetic direction in a coordinated manner. This phenomenon is crucial to comprehend in order to unravel the mysteries of these materials and unlock their full potential.
The MIT team made a notable discovery regarding iron selenide, revealing a distinct mechanism for its nematic switch. Unlike other iron-based superconductors, iron selenide experiences a collective shift in the orbital energy of its atoms, rather than a coordinated change in magnetic spins. This subtle yet significant distinction opens up new avenues for exploring unconventional superconductors, providing fresh insights and opportunities for scientific exploration in this field.
Riccardo Comin, the Class of 1947 Career Development Associate Professor of Physics at MIT, highlights the significance of their study by stating, "Our study reshuffles things a bit when it comes to the consensus that was created about what drives nematicity. There are many pathways to achieve unconventional superconductivity, and this discovery offers an additional avenue to realize superconducting states." This acknowledgment emphasizes the importance of their findings, which challenge the existing understanding and expand the possibilities for exploring and harnessing unconventional superconductors.
Comin and his colleagues are preparing to publish their findings in a study that will be featured in Nature Materials. The study's co-authors from MIT include Connor Occhialini, Shua Sanchez, and Qian Song. Collaborating with them are Gilberto Fabbris, Yongseong Choi, Jong-Woo Kim, and Philip Ryan from Argonne National Laboratory. This collaborative effort brings together experts from both institutions to contribute their expertise and insights to the research.
Following the thread
The term "nematicity" derives its origins from the Greek word "nema," which translates to "thread." This terminology draws a parallel to thread-like structures, such as the body of a nematode worm. However, in a broader context, "nematicity" is also employed to describe conceptual threads or connections, particularly in relation to coordinated physical phenomena. For example, in the field of liquid crystals, nematic behavior is observed when molecules align in coordinated lines, resembling threads, showcasing the interconnected nature of this phenomenon.
In the past few years, physicists have employed the concept of nematicity to describe a coordinated shift that triggers a material's transition into a superconducting state. This shift is driven by strong interactions among electrons, leading the material as a whole to undergo infinitesimal stretching, akin to microscopic taffy, in a specific direction that enables unhindered electron flow along that path. The key question has been to identify the type of interaction responsible for this stretching phenomenon. In certain iron-based materials, this stretching appears to be induced by atoms that spontaneously align their magnetic spins in the same direction. As a result, scientists have generally assumed that most iron-based superconductors undergo a similar spin-driven transition.
Iron selenide, however, deviates from this prevailing trend. Unlike other iron-based materials, iron selenide exhibits a unique characteristic: despite achieving the highest temperature at which it undergoes superconductivity among iron-based materials, it does not display any discernible coordinated magnetic behavior. This peculiarity sets iron selenide apart from the rest, challenging the notion that a coordinated spin-driven transition is a universal feature among iron-based superconductors. The absence of such magnetic behavior in iron selenide highlights its distinct nature and suggests the existence of alternative mechanisms driving its superconducting properties.
According to Sanchez, an MIT postdoctoral researcher and NSF MPS-Ascend Fellow, iron selenide presents the most elusive narrative among these materials. In this particular case, the absence of magnetic order complicates the understanding of nematicity. Therefore, unraveling the underlying causes of nematic behavior in iron selenide necessitates a meticulous examination of how electrons organize themselves in the vicinity of iron atoms and the consequent effects that occur as these atoms undergo stretching or separation. By delving into these intricate details, researchers hope to shed light on the enigmatic origins of nematicity in iron selenide.
A super continuum
In their recent study, the researchers utilized ultrathin samples of iron selenide, measuring millimeters in length, which were affixed to a slender titanium strip. To replicate the structural stretching observed during a nematic transition, they physically stretched the titanium strip, thereby inducing a corresponding stretching effect on the iron selenide samples. By incrementally stretching the samples by minute increments, on the order of a fraction of a micron, they carefully observed for any properties that exhibited coordinated shifts. This experimental approach allowed them to investigate and analyze the response of iron selenide to controlled stretching, providing insights into the behavior and characteristics of the material during the nematic transition.
To investigate the movement of atoms and electron behavior in each sample, the team employed ultrabright X-rays. This allowed them to track and monitor the atomic dynamics and electron activity within the material. As the stretching progressed, the researchers observed a distinct and coordinated change in the orbital configurations of the atoms. Atomic orbitals represent energy levels that electrons can occupy around an atom. In the case of iron selenide, electrons can occupy one of two orbital states around each iron atom, typically chosen randomly. However, during the stretching process, the team discovered that the electrons exhibited a significant preference for one orbital state over the other. This pronounced preference indicated a clear and coordinated shift, unveiling a novel mechanism of nematicity, as well as its association with the emergence of superconductivity in iron selenide.
Using ultrabright X-rays, the team monitored the motion of atoms and the behavior of electrons in each sample. After a specific point, they observed a distinct and synchronized alteration in the orbital patterns of the atoms. Atomic orbitals represent energy levels available for electrons within an atom. In the case of iron selenide, electrons can occupy one of two orbital states around an iron atom. Ordinarily, the selection of which state to occupy is random. However, the team discovered that as they elongated the iron selenide, its electrons increasingly favored one orbital state over the other. This discovery indicated a noticeable and coordinated shift, accompanied by a novel mechanism of nematicity and superconductivity.
This research was supported by the Department of Energy, the Air Force Office of Scientific Research, and the National Science Foundation.
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