Magnets and superconductors are often perceived as incompatible — or at least, that has been the belief by scientists for ages. However, a recent discovery by MIT physicists is reshaping this long-held notion.
In a publication released today in the journal Nature, the researchers claim to have unearthed a “chiral superconductor”—a substance that transmits electricity without resistance, which intriguingly is also fundamentally magnetic. Moreover, this extraordinary superconductivity was found in an unexpectedly common material: graphite, the primary component of pencil lead.
Graphite comprises countless layers of graphene—atomically thin, lattice-like carbon sheets—stacked together that can readily flake off under pressure, such as when writing on paper. A solitary flake of graphite can include millions of graphene layers, typically arranged so that alternating layers align. Yet occasionally, some graphite samples contain minuscule regions where graphene layers are arranged in a unique pattern, resembling a staircase of shifted layers.
The MIT team found that when four or five graphene layers are assembled in this “rhombohedral” pattern, the resultant structure demonstrates exceptional electronic qualities not observed in bulk graphite.
In their latest investigation, the physicists isolated microscopic flakes of rhombohedral graphene from graphite and subjected them to an array of electrical evaluations. They discovered that when the flakes were cooled to 300 millikelvins (approximately -273 degrees Celsius), the material transitioned into a superconductor, allowing any electrical current traversing through it to flow without resistance.
Additionally, they observed that by varying an external magnetic field, the flakes could toggle between two distinct superconducting states, much like a magnet. This indicates that the superconductor possesses intrinsic magnetism. Such toggling behavior is not observed in conventional superconductors.
“Common wisdom dictates that superconductors dislike magnetic fields,” says Long Ju, assistant professor of physics at MIT. “However, we believe this is the first time a superconductor has been observed to behave as a magnet with such clear and straightforward evidence. This is quite peculiar because it contradicts conventional beliefs regarding superconductivity and magnetism.”
Ju is the senior author of the study, which features co-authors from MIT including Tonghang Han, Zhengguang Lu, Zach Hadjri, Lihan Shi, Zhenghan Wu, Wei Xu, Yuxuan Yao, Jixiang Yang, Junseok Seo, Shenyong Ye, Muyang Zhou, and Liang Fu, alongside collaborators from Florida State University, the University of Basel in Switzerland, and the National Institute for Materials Science in Japan.
Graphene Twist
In typical conductive materials, electrons move in a chaotic manner, racing past one another and bouncing off the atomic lattice. Each time an electron collides with an atom, it essentially encounters resistance, dissipating energy mainly in the form of heat. Conversely, when specific materials are cooled to extremely low temperatures, they can become superconducting; this enables electrons to pair up, known in physics as “Cooper pairs.” Rather than scattering, these paired electrons glide through the material resistance-free. Thus, in a superconductor, no energy gets lost during transit.
Since the initial observation of superconductivity in 1911, physicists have repeatedly shown that zero electrical resistance defines a superconductor. A defining characteristic was noted in 1933 when physicist Walther Meissner discovered that a superconductor can expel an external magnetic field. This “Meissner effect” is partly due to the electron pairs in the superconductor, which collectively repel any magnetic field.
Physicists have generally assumed that all superconducting materials must exhibit both zero electrical resistance and a natural magnetic repulsion. Indeed, these two traits could facilitate Maglev, or “magnetic levitation,” trains, where a superconducting track repels and levitates a magnetized vehicle.
Ju and his team had no reason to doubt this assumption during their experiments at MIT. Over recent years, they have investigated the electrical properties of pentalayer rhombohedral graphene. The researchers have noted unexpected characteristics in the five-layer, staircase-like graphene structure, the latest of which is its ability to allow electrons to subdivide into fractions of themselves. This phenomenon occurs when the five-layer structure is placed on top of a sheet of hexagonal boron nitride (similar to graphene) and slightly misaligned by a specific angle, or twist.
Curious about how electron fractions might alter under varying conditions, the researchers followed up their initial findings with similar tests, this time by misaligning the graphene and hexagonal boron nitride structures. To their astonishment, they found that when they misaligned the two materials and passed an electrical current through it at temperatures below 300 millikelvins, they recorded zero resistance. The phenomenon of electron fractions seemed to vanish, revealing instead superconductivity.
The researchers further investigated this new superconducting state’s response to an external magnetic field. They applied a magnet to the material, alongside a voltage, and measured the resulting electrical current. As they toggled the magnetic field from negative to positive (analogous to north and south polarity) and back, they noted that the material sustained its superconducting, zero-resistance condition, except in two cases, at each magnetic polarity. During these instances, resistance momentarily surged before reverting to zero, resuming the superconducting state.
“If this were a standard superconductor, it would remain at zero resistance until the magnetic field reached a critical threshold that would terminate superconductivity,” remarks Zach Hadjri, a first-year student in the group. “In contrast, this material appears to alternate between two superconducting states, similar to a magnet that initially points upward and can flip downward when a magnetic field is applied. Thus, it seems this is a superconductor that also functions as a magnet. Which is quite perplexing!”
“One of a Kind”
Despite how counterintuitive this discovery might seem, the team observed the same behavior in six additional samples. They suspect that the unique structure of rhombohedral graphene is essential. The material has a very straightforward arrangement of carbon atoms. When cooled to near absolute zero, thermal fluctuations are minimized, allowing electrons within the material to decelerate, become aware of one another, and interact.
Such quantum interactions can lead to electron pairing and superconductivity. These interactions may also motivate electrons to synchronize. Specifically, electrons can collectively occupy one of two opposing momentum states, or “valleys.” When all electrons reside in a single valley, they spin in one direction, as opposed to the other direction. In traditional superconductors, electrons can inhabit either valley, and pairs of electrons typically consist of electrons of opposite valleys that cancel each other’s effects. Overall, the pair thus has zero momentum and does not exhibit a spin.
However, within the team’s material structure, they suspect that all electrons interact so that they share the same valley or momentum state. Consequently, when electrons pair, their overall superconducting pairs possess “non-zero” momentum and a spin, which, along with many other pairs, can give rise to internal superconducting magnetism.
“You can envision the two electrons in a pair spinning clockwise or counterclockwise, which correlates with a magnet directed up or down,” explains Tonghang Han, a fifth-year group member. “Therefore, we believe this is the first instance of a superconductor behaving as a magnet due to the electrons’ orbital motion, which is recognized as a chiral superconductor. It’s unparalleled. It also represents a candidate for a topological superconductor that could facilitate robust quantum computation.”
“Every aspect we’ve uncovered in this material has been entirely unexpected,” notes Zhengguang Lu, a former postdoc in the group and currently an assistant professor at Florida State University. “However, because this is such a simple system, we think we have an excellent opportunity to grasp what is occurring and could demonstrate some profound and fundamental principles of physics.”
“It is truly astonishing that such an exotic chiral superconductor arises from such straightforward components,” adds Liang Fu, professor of physics at MIT. “Superconductivity in rhombohedral graphene is sure to yield significant insights.”
The portion of the research conducted at MIT was supported by the U.S. Department of Energy and a MathWorks Fellowship.