Newswise — CAMBRIDGE, Mass. --When your laptop or smartphone becomes hot, it occurs due to energy being lost in communication. The identical applies to power cables that convey electricity across urban areas. In reality, roughly 10 percent of the produced energy dissipates during the transfer of electricity. This is because the electrons, which transport electric charge, act as independent entities, colliding and brushing against other electrons as they travel together through power cables and transmission lines. All this interaction produces friction, leading to heat eventually.
However, when electrons form pairs, they can transcend the commotion and smoothly traverse a substance without any friction. This phenomenon, known as "superconductivity," can be observed in various materials, albeit under extremely cold conditions. If it becomes possible to achieve superconductivity at temperatures closer to room temperature, it could open up possibilities for the development of devices that operate without any energy loss, such as laptops and phones that remain cool, as well as highly efficient power lines. Nevertheless, before this can happen, scientists need to grasp the fundamental mechanisms underlying the pairing of electrons.
Fresh glimpses of particles forming pairs within an atomic cloud offer valuable insights into the mechanisms behind electron pairing in superconducting materials. These snapshots, captured by physicists at MIT, represent the initial images to directly observe fermions, a significant group of particles encompassing electrons, protons, neutrons, and specific atomic species, engaging in pairing behavior.
In this particular study, the MIT researchers focused on fermions, specifically potassium-40 atoms, to replicate the characteristics of electrons in specific superconducting substances. They devised a method to visualize a superchilled cluster of potassium-40 atoms, enabling them to witness the formation of particle pairs, even when positioned close together. Additionally, they identified intriguing patterns and phenomena, such as pairs arranging themselves in a checkerboard pattern that was occasionally disrupted by solitary atoms passing through.
The findings, recently published in the journal Science, offer a visual roadmap for understanding the pairing dynamics of electrons in superconducting materials. These observations provide valuable insights that can contribute to elucidating the mechanisms behind the pairing of neutrons, leading to the formation of a highly dense and dynamic superfluid within neutron stars.
Martin Zwierlein, the Thomas A. Frank Professor of Physics at MIT and one of the study's authors, emphasizes the significance of fermion pairing in superconductivity and nuclear physics. He expresses the awe and excitement experienced by the research team upon witnessing the in-situ pairing of particles for the first time, describing it as a breathtaking moment to observe these images faithfully displayed onscreen.
The co-authors of the study consist of Thomas Hartke, Botond Oreg, Carter Turnbaugh, and Ningyuan Jia, who are all affiliated with MIT's Department of Physics, the MIT-Harvard Center for Ultracold Atoms, and the Research Laboratory of Electronics. Their collective expertise and contributions have played a crucial role in conducting this research.
A decent view
Directly observing the pairing of electrons presents a formidable challenge due to their minuscule size and rapid movement, which exceeds the capabilities of current imaging techniques. Consequently, physicists, including Zwierlein, have turned to analogous systems involving atoms to gain insights into electron behavior. Despite their disparate sizes, both electrons and specific atoms share a common characteristic as fermions, particles that possess a property called "half-integer spin." When fermions with opposite spins interact, they have the ability to form pairs, akin to how electrons pair up in superconductors or certain atoms pair up within a gas cloud.
Zwierlein's research group has been dedicated to investigating the characteristics of potassium-40 atoms, a type of fermion that can exist in two distinct spin states. When a potassium atom with one spin state interacts with another atom possessing a different spin state, they have the potential to form pairs, analogous to the pairing of superconducting electrons. However, at typical room-temperature conditions, the interactions between these atoms occur rapidly, making it challenging to capture their behavior in detail.
To obtain a more detailed understanding of the particles' behavior, Zwierlein and his team conduct their studies using an extremely sparse gas consisting of approximately 1,000 atoms. They subject the gas to ultracold conditions, reducing the temperature to nanokelvin levels, which significantly slows down the atoms' motion. Additionally, the researchers confine the gas within an optical lattice, which is essentially a grid created by laser light. This lattice allows the atoms to move between lattice sites, and it provides the researchers with a precise map of the atoms' exact positions, enabling them to accurately track their movements.
In their recent study, the research team made notable improvements to their imaging technique for fermions, allowing them to temporarily immobilize the potassium-40 atoms. By freezing the atoms momentarily, they were able to capture individual snapshots of atoms with specific spin states. The researchers then superimposed the images of one atom type onto the other, enabling them to identify the locations and manner in which the two different types of atoms paired up. This approach provided valuable insights into the pairing behavior and mechanisms involved.
Zwierlein describes the challenges faced by the research team in reaching a stage where they could successfully capture these images. He emphasizes the initial difficulties encountered, such as encountering significant imaging issues and the atoms behaving unpredictably. Overcoming these obstacles required the team to solve complex problems in the laboratory over the course of several years. Zwierlein expresses his admiration for the persistence and resilience displayed by the students involved in the research. Finally achieving the ability to visualize these images brought a profound sense of elation and excitement to the team.
Pair dance
The observed phenomenon displayed atom pairing as anticipated by the Hubbard model, a well-accepted theory thought to unlock the mysteries of electron behavior in high-temperature superconductors. These materials demonstrate superconductivity at moderately elevated (yet still extremely frigid) temperatures. While this model has been utilized to test predictions regarding electron pairing in such substances, direct observation had remained elusive until this moment.
The team generated and captured multiple atom clouds, repeating the process thousands of times, and then converted each image into a digital grid format. Each grid depicted the spatial arrangement of atoms, distinguishing between the two types (represented as red and blue in their publication). These maps revealed squares within the grid where either a solitary red or blue atom was present, as well as squares where a red and blue atom formed a localized pair (represented as white). Additionally, there were empty squares devoid of both red and blue atoms (depicted as black).
Even at the level of individual images, numerous local pairs of red and blue atoms in close proximity were observed. Through the analysis of sets of hundreds of images, the team demonstrated that atoms consistently appeared in pairs. Some pairs were tightly connected within a single square, while others formed looser pairs with a separation of one or several grid spacings. This physical separation, known as "nonlocal pairing," had been predicted by the Hubbard model but had never been directly observed until now.
The scientists additionally noticed that groups of pairs exhibited a larger-scale checkerboard pattern. Interestingly, this pattern fluctuated, appearing and disappearing as one member of a pair moved beyond its designated square and temporarily disrupted the arrangement of other pairings in the checkerboard. This intriguing occurrence, referred to as a "polaron," had been predicted in theory but had never been directly observed until now.
“In this dynamic soup, the particles are constantly hopping on top of each other, moving away, but never dancing too far from each other,” Zwierlein notes.
The observed pairing behavior among these atoms is believed to be analogous to the behavior of paired electrons in superconductors. According to Zwierlein, the team's recent snapshots will contribute valuable information to enhance scientists' comprehension of high-temperature superconductors. Furthermore, these findings may offer insights into potential methods for optimizing these materials to achieve even higher and more practical temperatures for superconductivity.
Zwierlein suggests that if the density of the gas of atoms is adjusted to match the density of electrons in a metal, the observed pairing behavior could potentially occur well above room temperature. This realization instills optimism and confidence, as it indicates that such pairing phenomena could theoretically manifest at elevated temperatures. Consequently, there is no inherent limitation preventing the development of a room-temperature superconductor in the future.
This research was supported, in part, by the U.S. National Science Foundation, the U.S. Air Force Office of Scientific Research, and the Vannevar Bush Faculty Fellowship.
###
RSS