Graphene can host multiple superconducting states, MIT study finds

Graphene can host multiple superconducting states, MIT study finds

Researchers at the Massachusetts Institute of Technology have discovered that a specific microscopic structure found in natural graphite can host multiple different superconducting states. The findings, published in the journal Nature, reveal a new family of unconventional superconducting states within rhombohedral graphene.

Superconductivity is an electronic state of matter where electrons pair up and move through a material with zero resistance. While thousands of superconducting materials exist, it is rare for a single material to exhibit multiple forms of this state.

The Nature of Rhombohedral Graphene

The team focused on rhombohedral graphene, a natural structure consisting of a stack of four or five graphene layers. Unlike "magic-angle" graphene, which requires artificial stacking and twisting at precise orientations, rhombohedral graphene occurs naturally in graphite. The layers are stacked with each one slightly offset from the previous one, creating a pattern similar to the steps in a staircase.

To isolate these samples, researchers used an exfoliation process — typically involving Scotch tape — on blocks of graphite before searching for the specific staircase-like pattern. Long Ju, the Lawrence C. And Sarah W. Biedenharn Associate Professor of Physics at MIT, noted that while some may view carbon as a simple, boring carbon material, it can be controlled by tuning experimental knobs, such as electrical voltages.

Previous research by Ju's group found that this structure could host fractional electron charge and a rare chiral form of superconductivity. For the latest study, the team shifted their approach. While they had previously observed superconductivity by adding electrons to the samples via electrical doping, they now investigated what happens when electrons are removed.

Unconventional Responses to Magnetic Fields

By progressively lowering the electron density and applying external electric currents to measure resistance, the researchers identified four different superconducting states. These experiments were conducted in collaboration with Dominik Zumbühl’s group at the University of Basel in Switzerland, utilizing a laboratory capable of maintaining ultracold temperatures and high magnetic fields.

The results contradicted the behavior of classical superconductors. Normally, magnetic fields destroy superconductivity by severing the bonds between paired electrons. However, the MIT and Basel teams observed three superconducting states that persisted in a parallel magnetic field of up to around 9 tesla, approximately 180,000 times stronger than the Earth's magnetic field.

Further surprises emerged when the magnetic field was applied in a perpendicular orientation. At a specific electron density, the superconductivity did not just persist but was actually enhanced.

"The superconductivity actually is enhanced, as in, the transition temperature goes from 55 millikelvin to probably 90 millikelvin,"

Long Ju, Associate Professor of Physics at MIT, via MIT News

Ju added that under these conditions, the material could withstand an additional 50 or 60 percent of extra current before the superconducting state was destroyed.

The Theory of Aligned Spins

The mechanism behind these states remains unclear. In conventional superconductivity, "Cooper pairs" consist of electrons with opposite spins, which are easily pulled apart by magnetic fields. To explain the rhombohedral graphene findings, the team proposes that at certain electron densities, electrons may pair up with aligned spins. In this scenario, a magnetic field would pull the spins in the same direction, preserving their alignment and the resulting superconductivity.

Broader Implications

The study was supported in part by the U.S. Office of Naval Research, and device fabrication was performed at MIT.nano. The researchers acknowledge that their theory regarding aligned spins requires further experimental and theoretical investigation.