The ordinary graphite in pencil lead is proving to be surprisingly multifaceted at the microscale.
In a study appearing today in the journal Nature , MIT researchers report that a certain microscopic structure found in natural graphite can host multiple superconducting states. Superconductivity is an electronic state of matter in which electrons pair up and glide through a material with zero resistance.
While there are thousands of materials that are known to be superconductors, it is rare for one material to host multiple forms of superconductivity.
The researchers discovered the multiple superconducting states in atomically thin exfoliations of graphite, known as graphene. Specifically, graphene is a single-atom-thin sheet of carbon atoms arranged precisely in a microscopic lattice. The team made its discoveries in samples of rhombohedral graphene, which is a natural structure within graphite consisting of a stack of four or five graphene layers.
Interestingly, the researchers found that several of the new superconducting states in rhombohedral graphene are able to persist in the presence of a magnetic field, which normally kills superconductivity.
And in a further surprise, these superconducting states even get stronger when exposed to a magnetic field.
Overall, the findings reveal a new family of unconventional superconducting states in one seemingly simple material.
"People might assume that this is a simple, boring carbon material," says Long Ju, the Lawrence C. and Sarah W. Biedenharn Associate Professor of Physics at MIT. "But we can control this material by tuning certain experimental 'knobs,' such as electrical voltages. This is how a simple physical material can exhibit so many different superconducting properties."
It's still unclear exactly how each of the multiple superconducting states arise, or how they are able to persist under a magnetic field, when normally superconductivity should fade.
"From a fundamental physics point of view, it's very exotic that a magnetic field doesn't kill superconductivity, and instead it boosts it," Ju says. "We have provided a lot of experimental results and provided the nutrition that people can absorb to try to think about what's going on here."
The study's MIT co-authors include co-first authors Junseok Seo and Shenyong Ye, together with Tonghang Han, Zhenghan Wu, Wei Xu, Jixiang Yang, Emily Aitken, Prayoga Liong, Phatthanon Pattanakanvijit, Zach Hadjri, and Mingda Li. External collaborators are co-first author Armel Cotten and members of Dominik Zumbuhl's group at the University of Basel in Switzerland, plus others at Florida State University, the University of Florida, Gainesville, and the National Institute for Materials Science in Japan.
Natural steps
Graphene and other atomically thin, two-dimensional materials can exhibit unexpected electronic, magnetic, thermal, and physical properties. And when two or more sheets of graphene are stacked and twisted at precise orientations, the "magic-angle" structure can suddenly host weird and exotic phenomena.
Ju's group has been probing the exceptional properties of graphene. But rather than artificially stacking and twisting layers, they have looked for interesting behavior in naturally occurring graphene structures. In recent years, they have unearthed surprising electronic properties in rhombohedral graphene. This particular configuration consists of graphene layers stacked on top of each other, each one slightly offset from the last, similar to the steps in a staircase.
Rhombohedral graphene can be found naturally in ordinary graphite. But to find it first requires exfoliating a block of graphite (usually with Scotch tape), then searching the exfoliated sample for the telltale staircase-like pattern, which researchers can then isolate for further experimentation.
Using this approach, Ju and his colleagues have been able to isolate and probe samples of four- and five-layer rhombohedral graphene. They have so far discovered that the structure can host a rare, "chiral" form of superconductivity , as well as fractional electron charge , among other behavior.
In the flow
For their new study, the team took a slightly different approach in studying rhombohedral graphene. Previously, they electrically "doped" their samples, progressively adding electrons as they passed a separate electric current into the material. They then measured the voltage, or essentially the force that pushes the current through the material, and looked for instances when the voltage dropped to zero, indicating that the current was passing through without resistance.
In this way, the team has observed superconductivity when adding electrons to rhombohedral graphene. So they wondered: What might happen if they did the opposite, and took electrons away?
In their new study, the team looked for signs of superconductivity as they carefully removed electrons from rhombohedral graphene, progressively lowering the material's electron density, as they applied a separate, external electric current to measure the electrical resistance. In these experiments, they also applied external magnetic field along directions parallel and perpendicular to the graphene plane. These experiments were carried out in collaboration with Zumbuhl's group in Switzerland, who provided access to a laboratory setup in which graphene samples could be exposed to high magnetic fields and ultracold temperatures.
In these experiments, the researchers found that at certain electron densities, four different superconducting states emerged. What's more, three of the states persisted in the presence of a relatively high magnetic field.
Normally, magnets destroy superconductivity by severing the bond between the paired electrons gliding through the material.
But in Ju's experiments, the team observed three superconducting states that survived in a magnetic field up to around 9 tesla, which is about 180,000 times stronger than the Earth's magnetic field. In these instances, the magnetic field they applied was in a parallel orientation with respect to the plane of the material. When they switched the magnetic field to a perpendicular orientation, they discovered another surprise: At a certain electron density, superconductivity not only persisted, but increased. The material was able to continue superconducting, at higher temperatures than predicted.
Every superconducting material has a critical temperature below which electrons can conduct without resistance, and above which superconductivity cannot persist. But the team found that, at a certain electron density, and in the presence of a perpendicular magnetic field, superconductivity in rhombohedral graphene was able to survive beyond the material's critical temperature that corresponds to zero magnetic field.
"The superconductivity actually is enhanced, as in, the transition temperature goes from 55 millikelvin to probably 90 millikelvin," Ju explains. "At the same time, the material can take another 50 or 60 percent extra current before superconductivity gets destroyed. And that is very unusual."
The researchers are unsure of what microscopic behavior is enabling multiple and unconventional superconducting states, though they propose one idea. Conventional superconductivity emerges when electrons pair up. These "Cooper pairs" consist of electrons with opposite spin, and it's thought that a magnetic field can pull the spins out of their opposite configurations, and as a result, break up superconductivity.
Instead, the team proposes that perhaps in rhombohedral graphene, and at certain electron densities, electrons can pair up with aligned spins. Any magnetic field would still pull on the spins, but in the same direction, preserving their alignment, and their superconductivity.
The researchers acknowledge that the idea needs much more investigation, both experimentally and theoretically. For now, they see the results as a demonstration of what new and exotic phenomena can emerge in a seemingly simple material, with the right measurements and controls.
"We can control the simplest of chemicals - carbon - and structurally alter the material, which is part of our fun," says lead author Junseok Seo, who is a graduate student in Ju's group. "We're not only dealing with what nature gives us, but we're applying additional controls to change it to something that nature does not give us, but that can exist in the same material."
This work was supported, in part, by the U.S. Office of Naval Research. Device fabrication was carried out, in part, at MIT.nano.
