By measuring how easily electron pairs flow through "magic-angle" graphene, scientists have made significant progress in understanding its extraordinary properties.

Superconducting materials function similarly to a carpool lane on a congested highway—just as commuters who travel together can bypass regular traffic, paired electrons move through the material without resistance.

However, the ease with which these electron pairs flow depends on various factors, including their density within the material. This property, known as "superfluid stiffness," is a crucial indicator of a material's superconducting capabilities.

Figure 1. Physicists Measure Superconductivity in Magic-Angle Graphene

Physicists at MIT and Harvard University have directly measured superfluid stiffness in “magic-angle” graphene for the first time. This material consists of two or more atomically thin graphene sheets twisted at a precise angle, unlocking a range of extraordinary properties, including unconventional superconductivity. Figure 1 shows Physicists Measure Superconductivity in Magic-Angle Graphene.

Magic-angle graphene holds promise as a key component for future quantum-computing devices, but the exact mechanism behind its superconductivity remains unclear. Measuring its superfluid stiffness provides crucial insights into this phenomenon.

The team's findings indicate that superconductivity in magic-angle graphene is primarily influenced by quantum geometry—the conceptual "shape" of quantum states that can exist within the material.

The findings, published today in Nature, mark the first direct measurement of superfluid stiffness in a two-dimensional material. To achieve this, the research team developed a novel experimental technique that can now be applied to study other 2D superconducting materials.

“There’s a whole family of 2D superconductors waiting to be explored, and we are just scratching the surface,” says study co-lead author Joel Wang, a research scientist at MIT’s Research Laboratory of Electronics (RLE).

The study’s co-authors include researchers from MIT’s main campus and MIT Lincoln Laboratory: co-lead author and former RLE postdoc Miuko Tanaka, along with Thao Dinh, Daniel Rodan-Legrain, Sameia Zaman, Max Hays, Bharath Kannan, Aziza Almanakly, David Kim, Bethany Niedzielski, Kyle Serniak, Mollie Schwartz, Jeffrey Grover, Terry Orlando, Simon Gustavsson, Pablo Jarillo-Herrero, and William D. Oliver. Kenji Watanabe and Takashi Taniguchi of Japan’s National Institute for Materials Science also contributed to the research.

Resonance in Magic-Angle Graphene

Since its isolation and characterization in 2004, graphene has emerged as a remarkable material. Composed of a single, atom-thin sheet of carbon arranged in a precise chicken-wire lattice, graphene boasts exceptional strength, durability, and conductivity for both electricity and heat.

In 2018, Pablo Jarillo-Herrero and his team discovered that stacking two graphene sheets at a specific "magic" angle creates a new material—magic-angle twisted bilayer graphene (MATBG)—that exhibits entirely novel properties, including superconductivity. In this state, electrons form Cooper pairs instead of repelling each other, allowing them to move as a superfluid with zero resistance, enabling friction-free electrical currents.

"Even though Cooper pairs experience no resistance, an electric field is still needed to push them into motion," explains Joel Wang. "Superfluid stiffness measures how easily these particles move, which is key to driving superconductivity."

Scientists typically measure superfluid stiffness in superconducting materials using microwave resonators. These devices oscillate at characteristic microwave frequencies, much like a vibrating violin string. When a superconducting material is placed inside, it alters the resonator’s frequency, particularly affecting its "kinetic inductance"—a shift that directly correlates to the material’s superfluid stiffness.

Until now, these measurement methods have only been suitable for large, thick material samples. The MIT team recognized that to measure superfluid stiffness in atomically thin materials like MATBG, a new approach would be necessary.

“Compared to MATBG, the typical superconductor studied using resonators is 10 to 100 times thicker and larger in area,” says Wang. “We weren’t sure if such a tiny material would produce any measurable inductance at all.”

A recorded signal

The challenge of measuring superfluid stiffness in MATBG lies in attaching the fragile material to a microwave resonator without degradation. To solve this, the MIT team, led by Will Oliver’s group, developed a method to seamlessly connect a tiny MATBG sample to an aluminum resonator. They used conventional methods to assemble MATBG and sandwiched it between insulating hexagonal boron nitride layers to preserve its properties. After making a sharp contact between MATBG and aluminum, they sent a microwave signal through the resonator and measured the shift in its frequency to infer the material’s superfluid stiffness.

The results showed a tenfold increase in superfluid stiffness compared to conventional expectations, suggesting that MATBG’s quantum geometry plays a key role. This discovery highlights the potential of using quantum technology to study strongly interacting particle systems. The research was funded by several U.S. defense and science organizations. A related study on magic-angle twisted trilayer graphene also appears in the same Nature issue.

Source:MIT NEWS

Cite this article:

Janani R (2025), Physicists Investigate a Crucial Property of Superconductivity In "Magic-Angle" Graphene, AnaTechMaz, pp. 136