Tyler O'Neal, Staff Editor ENGINEERING

MIT physicist Checkelsky demos new material for superconductivity that has potential applications in quantum supercomputing

MIT physicists and colleagues have demonstrated an exotic form of superconductivity in a new material the team synthesized only about a year ago. Although predicted in the 1960s, until now this type of superconductivity has proven difficult to stabilize. Further, the scientists found that the same material can potentially be manipulated to exhibit yet another, equally exotic form of superconductivity. Computations were performed at Harvard University.

The demonstration of finite momentum superconductivity in a layered crystal known as a natural superlattice means that the material can be tweaked to create different patterns of superconductivity within the same sample. And that, in turn, could have implications for quantum supercomputing and more.

The material is also expected to become an important tool for plumbing the secrets of unconventional superconductors. This may be useful for new quantum technologies. Designing such technologies is challenging, partly because the materials they are composed of can be difficult to study. The new material could simplify such research because, among other things, it is relatively easy to make.

“An important theme of our research is that new physics comes from new materials,” says Joseph Checkelsky, lead principal investigator of the work and the Mitsui Career Development Associate Professor of Physics. “Our initial report last year was of this new material. This new work reports the new physics.”

Checkelsky’s co-authors on the current research include lead researcher Aravind Devarakonda Ph.D. '21, who is now at Columbia University. The work was a central part of Devarakonda’s thesis. Co-authors are Takehito Suzuki, a former research scientist at MIT now at Toho University in Japan; Shiang Fang, a postdoc in the MIT Department of Physics; Junbo Zhu, an MIT graduate student in physics; David Graf of the National High Magnetic Field Laboratory; Markus Kriener of the RIKEN Center for Emergent Matter Science in Japan; Liang Fu, an MIT associate professor of physics; and Efthimios Kaxiras of Harvard University. Aravind Devarakonda PhD '21 is lead author of a paper describing an exotic form of superconductivity. Credits:Photo: Denis Paiste

New quantum material

Classical physics can be used to explain any number of phenomena that underlie our world — until things get exquisitely small. Subatomic particles like electrons and quarks behave differently, in ways that are still not fully understood. Enter quantum mechanics, the field that tries to explain their behavior and effects.

Checkelsky and colleagues discovered a new quantum material or one that manifests the exotic properties of quantum mechanics at a macroscopic scale. In this case, the material in question is a superconductor.

Checkelsky explains that fairly recently there has been a boom of realizing special superconductors that are two-dimensional, or only a few atomic layers thick. These new ultrathin superconductors are of interest in part because they are expected to give insights into superconductivity itself.

But there are challenges. For one, materials only a few atomic layers thick are themselves difficult to study because they are so delicate. Could there be another approach to plumbing their secrets?

The new material made by Checkelsky and colleagues can be thought of as the superconducting equivalent of a layer cake, where one layer is an ultrathin film of superconducting material, while the next is an ultrathin spacer layer that protects it. Stacking these layers one atop other results in a large crystal (this happens naturally when the constituent elements of sulfur, niobium, and barium are heated together). “And that macroscopic crystal, which I can hold in my hand, behaves like a 2D superconductor. It was very surprising,” Checkelsky says.

Many of the probes scientists use to study 2D superconductors are challenging to use on atomically thin materials. Because the new material is so large, “we now have many more tools [to characterize it],” Checkelsky says. The scientists used a technique that requires massive samples.

Exotic superconductors

A superconductor carries a charge in a special way. Instead of via one electron, the charge is carried by two electrons bound together in what is known as a Cooper pair. Not all superconductors are the same, however. Some unusual forms of superconductivity can only appear when the Cooper pairs can move unimpeded through the material across relatively long distances. The longer the distance, the “cleaner” the material.

The Checkelsky team’s material is extremely clean. As a result, the physicists were excited to see if it might exhibit an unusual superconducting state, which it does. In the current document, the team shows that their new material is a finite momentum superconductor upon the application of a magnetic field. This particular kind of superconductivity, which was proposed in the 1960s, has remained a fascination to scientists.

While superconductivity is usually destroyed by modest magnetic fields, a finite momentum superconductor can persist further by forming a regular pattern of regions with lots of Cooper pairs and regions that have none. It turns out this kind of superconductor can be manipulated to form a variety of unusual patterns as Cooper pairs move between quantum mechanical orbits known as Landau levels. And that means, Checkelsky says, that scientists should now be able to create different patterns of superconductivity within the same material.

“This is a striking experiment which is able to demonstrate Cooper pairs moving between Landau levels in a superconductor, something that has never been observed before. Frankly, I never anticipated seeing this in a crystal you could hold in your hand, so this is very exciting. To observe this elusive effect, the authors had to perform painstaking, high-precision measurements on a uniquely two-dimensional superconductor that they had previously discovered. It's a remarkable achievement, not only in its technical difficulty but also in its cleverness,” says Kyle Shen, professor of physics at Cornell University. Shen was not involved in the study.

Further, the physicists realized that their material also has the ingredients for yet another exotic kind of superconductivity. Topological superconductivity involves the movement of charge along edges or boundaries. In this case, that charge could travel along the edges of each internal superconducting pattern.

The Checkelsky team is currently working to see if their material is indeed capable of topological superconductivity. If so, “can we combine both new types of superconductivity? What could that bring?” Checkelsky asks.

“It’s been a lot of fun realizing this new material,” he concludes. “As we’ve dug into understanding what it can do, there have been a number of surprises. It’s really exciting when new things come out that we don’t expect.”

Japanese researchers build a nanoantenna for long-distance, ultra-secure communication

Tyler O'Neal, Staff Editor ENGINEERING

Researchers from Osaka University have improved the transfer efficiency between quantum information carriers, in a manner that's based on well-established nanoscience and is compatible with upcoming advanced communication technologies

Information storage and transfer in the manner of simple ones and zeros is insufficient for quantum technologies under development. Now, researchers from Japan have fabricated a nanoantenna that will help bring quantum information networks closer to practical use.

In a study recently, researchers from Osaka University and collaborating partners have substantially enhanced photon-to-electron conversion through a metal nanostructure, which is an important step forward in the development of advanced technologies for sharing and processing data.

 Fig.1 Conceptual illustration of efficient illumination of photons to semiconductor lateral quantum dots, by using a surface plasmon antenna and excitation of electrons in the quantum dots. copyright @ 2021 Oiwa lab.
Classical computer information is based on simple on/off readouts. It’s straightforward to use a technology known as a repeater to amplify and retransmit this information over long distances. Quantum information is based on comparatively more complex and secure readouts, such as photon polarization and electron spin. Semiconductor nano boxes known as quantum dots are materials that researchers have proposed for storing and transferring quantum information. However, quantum repeater technologies have some limitations—for example, current ways to convert photon-based information to electron-based information are highly inefficient. Overcoming this information conversion and transfer challenge is what the researchers at Osaka University aimed to address.

"The efficiency of converting single photons into single electrons in gallium arsenide quantum dots—common materials in quantum communication research—is currently too low," explains lead author Rio Fukai. "Accordingly, we designed a nanoantenna—consisting of ultra-small concentric rings of gold—to focus light onto a single quantum dot, resulting in a voltage readout from our device."

The researchers enhanced photon absorption by a factor of up to 9, compared with not using the nanoantenna. After illuminating a single quantum dot, most of the photogenerated electrons weren't trapped there, and instead accumulated in impurities or other locations in the device. Nevertheless, these excess electrons gave a minimal voltage readout that was readily distinguished from that generated by the quantum dot electrons, and thus didn't disrupt the device's intended readout.

"Theoretical simulations indicate that we can improve the photon absorption by up to a factor of 25," says senior author Akira Oiwa. "Improving the alignment of the light source and more precisely fabricating the nanoantenna are ongoing research directions in our group."

These results have important applications. Researchers now have a means of using well-established nano-photonics to advance the prospects of upcoming quantum communication and information networks. By using abstract physics properties such as entanglement and superposition, quantum technology could provide unprecedented information security and data processing in the coming decades.

German supercomputing produces new insights into element synthesis in the universe

Tyler O'Neal, Staff Editor ENGINEERING

How are chemical elements produced in our Universe? Where do heavy elements like gold and uranium come from? Using supercomputer simulations, a research team from the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany together with colleagues from Belgium and Japan, shows that the synthesis of heavy elements is typical for certain black holes with orbiting matter accumulations, so-called accretion disks. The predicted abundance of the formed elements provides insight into which heavy elements need to be studied in future laboratories — such as the Facility for Antiproton and Ion Research (FAIR), which is currently under construction — to unravel the origin of heavy elements. Sectional view through the simulation of an accretion disk from the study by Dr. Just and his colleagues. The black hole at the center is surrounded by torus-shaped matter several hundred kilometers in extent. The rotation axis of the disk is given by the z-axis, which runs at R=0 through the black hole along the vertical direction. The arrows illustrate the velocity distribution of the matter. The color shading shows the density (upper left), the proton fraction Ye (lower left), and the characteristic time scales of neutrino emission (upper right) and neutrino absorption (lower right). Values of Ye less than 0.5 indicate a high fraction of neutrons available for the r-process.

All heavy elements on Earth today were formed under extreme conditions in astrophysical environments: inside stars, in stellar explosions, and during the collision of neutron stars. Researchers are intrigued with the question in which of these astrophysical events the appropriate conditions for the formation of the heaviest elements, such as gold or uranium, exist. The spectacular first observation of gravitational waves and electromagnetic radiation originating from a neutron star merger in 2017 suggested that many heavy elements can be produced and released in these cosmic collisions. However, the question remains open as to when and why the material is ejected and whether there may be other scenarios in which heavy elements can be produced.

Promising candidates for heavy element production are black holes orbited by an accretion disk of a dense and hot matter. Such a system is formed both after the merger of two massive neutron stars and during a so-called collapsar, a rotating star's collapse, and subsequent explosion. The internal composition of such accretion disks has so far not been well understood, particularly concerning the conditions under which an excess of neutrons forms. A high number of neutrons is a basic requirement for the synthesis of heavy elements, as it enables the rapid neutron-capture process or r-process. Nearly massless neutrinos play a key role in this process, as they enable the conversion between protons and neutrons.

“In our study, we systematically investigated for the first time the conversion rates of neutrons and protons for a large number of disk configurations by means of elaborate computer simulations, and we found that the disks are very rich in neutrons as long as certain conditions are met,” explains Dr. Oliver Just from the Relativistic Astrophysics group of GSI's research division Theory. “The decisive factor is the total mass of the disk. The more massive the disk, the more often neutrons are formed from protons through the capture of electrons under emission of neutrinos and are available for the synthesis of heavy elements by means of the r-process. However, if the mass of the disk is too high, the inverse reaction plays an increased role so that more neutrinos are recaptured by neutrons before they leave the disk. These neutrons are then converted back to protons, which hinders the r-process.” As the study shows, the optimal disk mass for prolific production of heavy elements is about 0.01 to 0.1 solar masses. The result provides strong evidence that neutron star mergers producing accretion disks with these exact masses could be the point of origin for a large fraction of the heavy elements. However, whether and how frequently such accretion disks occur in collapsar systems is currently unclear.

In addition to the possible processes of the mass ejection, the research group led by Dr. Andreas Bauswein is also investigating the light signals generated by the ejected matter, which will be used to infer the mass and composition of the ejected matter in future observations of colliding neutron stars. An important building block for correctly reading these light signals is accurate knowledge of the masses and other properties of the newly formed elements. “These data are currently insufficient. But with the next generation of accelerators, such as FAIR, it will be possible to measure them with unprecedented accuracy in the future. The well-coordinated interplay of theoretical models, experiments, and astronomical observations will enable us, researchers, in the coming years to test neutron star mergers as the origin of the r-process elements”, predicts Bauswein.