Exploring Novel Characteristics of Twisted Graphene Using a Quantum Metric

by Hiroshi Tanaka
Quantum Ruler

The image showcases dual bilayers of graphene employed by the team at the National Institute of Standards and Technology (NIST) to delve into the unique traits of moiré quantum materials. A closer view on the left reveals the moiré pattern that emerges when one bilayer is rotated minutely relative to the other. Source: B. Hayes/NIST

Researchers at NIST, focusing on misaligned graphene layers, have introduced a novel “quantum metric” to examine the singular attributes of this material.

Graphene, a mono-atomic layer of carbon, already exhibits impressive properties in its isolated form. However, its characteristics become even more intriguing when multiple two-dimensional sheets are stacked atop one another. When these overlying layers are slightly off-axis—rotated at specific angles—they adopt a host of unconventional identities.

Depending on the angle of misalignment, these substances, termed as moiré quantum materials, can spontaneously develop magnetic fields, transform into superconductors that offer zero electrical resistance, or, alternately, evolve into perfect insulators.

Utilizing a Quantum Metric to Unravel the Enigmas of Twisted Graphene

Joseph A. Stroscio, along with his peers at the National Institute of Standards and Technology (NIST) and a global consortium of experts, has created a “quantum metric” that enables the study of these peculiar materials’ traits. This advancement might also establish a new, downscaled standard for electrical resistance, thereby allowing for in-situ calibration of electronic devices, thus negating the need for external standards laboratories.

Fereshte Ghahari, a physicist hailing from George Mason University in Fairfax, Virginia, constructed a moiré quantum device by taking two 20-micrometer layers of graphene (known as bilayer graphene) and rotating them relative to another pair of layers. Ghahari utilized the nanofabrication facilities at NIST’s Center for Nanoscale Science and Technology for this purpose.

NIST investigators Marlou Slot and Yulia Maximenko subsequently supercooled this twisted material to temperatures barely above absolute zero, limiting random atomic and electron movements and augmenting the electrons’ interactive capabilities within the material. After achieving such low temperatures, they scrutinized how the energy levels of the electrons within the graphene layers were impacted by varying the potency of a strong external magnetic field. The assessment and alteration of these energy levels are pivotal for the development and production of semiconductor components.

A detailed representation of one of the sites in the moiré quantum material illustrates the ladder-like energy levels of electrons (red and blue dots to the right). The lattice-like background signifies that this measured energy level serves as a sort of quantum metric for determining the electrical and magnetic attributes of the material. Source: NIST/B. Hayes

Electron Behavior and Energy States

To gauge the energy states, the team employed a multifaceted scanning tunneling microscope, designed and assembled by Stroscio at NIST. Application of voltage to the graphene bilayers within the magnetic field allowed the microscope to capture the minute current emanating from tunneling electrons from the material to the probe tip of the microscope.

Within a magnetic field, electrons traverse in circular orbits. Typically, the orbits of electrons in solid substances maintain a unique correlation with an induced magnetic field: the area enveloped by each circular orbit, when multiplied by the applied field, assumes a set of fixed, discrete values, attributable to the quantum nature of electrons. To sustain this constant value, halving the magnetic field would necessitate a doubling of the area enclosed by an orbiting electron.

Variations in energy between successive, quantum-fixed energy levels can serve as benchmarks for evaluating a material’s electronic and magnetic attributes. Any subtle divergence from this pattern signals a new quantum metric reflective of the specific magnetic properties of the quantum moiré material under examination.

Insights and Implications

Remarkably, when NIST researchers altered the magnetic field applied to the moiré bilayers of graphene, they uncovered indications of a novel quantum metric. The product of the area circumscribed by the electrons’ circular orbits and the applied magnetic field no longer adhered to a set value. Instead, this product shifted in proportion to the magnetization of the bilayers.

This shift translated into a distinct set of benchmarks for the energy states of the electrons, shedding fresh light on how confined electrons in twisted graphene layers produce new magnetic attributes.

“Through the application of this new quantum metric, we aim to elucidate the intricate magnetic properties of these moiré quantum substances,” Stroscio commented.

In moiré quantum materials, electrons occupy a spectrum of energy states—peaks and troughs that resemble an egg carton—defined by the materials’ electric field. Electrons accumulate in the low-energy states or troughs of this carton. According to NIST theoretical physicist Paul Haney, the extensive separation between these troughs in the bilayers, larger than any atomic spacing in a single or multiple non-twisted layers of graphene, accounts for some of the discovered unusual magnetic properties.

The study, co-authored by researchers from the University of Maryland in College Park and the Joint Quantum Institute—a collaborative venture between NIST and the University of Maryland—has been published in the journal Science.

Future Trajectories and Applications

Given that the traits of moiré quantum materials can be manipulated by choosing specific twist angles and numbers of atom-thin layers, the new observations offer a more comprehensive comprehension of how to tailor and enhance the magnetic and electronic attributes of quantum substances for various applications in microelectronics and adjacent sectors. For example, ultra-thin superconductors are already identified as highly sensitive single-photon detectors, and quantum moiré superconductors are among the thinnest known.

Additionally, NIST has expressed interest in an alternative application: moiré quantum material could offer a new, more convenient standard for electrical resistance, potentially effecting significant cost savings.

The prevailing standard relies on discrete resistance values that manifest when a robust magnetic field is applied to electrons in a two-dimensional stratum. Known as the quantum Hall effect, this originates from the same quantized energy levels of orbiting electrons previously discussed. However, the necessity for a powerful magnetic field restricts these calibrations to metrology facilities such as NIST.

Should scientists be capable of inducing a net magnetization in quantum moiré material without an external magnetic field, it could potentially serve as a new, portable version of the most precise resistance standard known as the anomalous quantum Hall resistance standard. This could enable on-site calibrations of electronic devices, potentially leading to significant financial savings.

Reference: “A quantum metric for orbital magnetism in moiré quantum material” by M. R. Slot, Y. Maximenko, P. M. Haney, S. Kim, D. T. Walkup, E. Strelcov, Son T. Le, E. M. Shih, D. Yildiz, S. R. Blankenship, K. Watanabe, T. Taniguchi, Y. Barlas, N. B. Zhitenev, F. Ghahari, and J. A. Stroscio, published on 5 October 2023 in Science. DOI: 10.1126/science.adf2040

Frequently Asked Questions (FAQs) about Quantum Ruler

What is the main focus of this research?

The primary focus of this research is to investigate the unique properties of twisted graphene layers, referred to as moiré quantum matter, using a newly developed “quantum ruler.”

How does the “quantum ruler” work?

The “quantum ruler” measures the energy levels of electrons in the layers of graphene when exposed to varying strengths of an external magnetic field. It does this by recording the tiny current generated when electrons tunnel out from the material to a microscope probe tip. This measurement provides insights into the material’s electronic and magnetic properties.

What are the potential applications of this research?

This research has the potential to revolutionize electrical resistance standards. By manipulating moiré quantum materials, it may be possible to create a portable version of the most precise standard for resistance, known as the anomalous quantum Hall resistance standard. This could lead to on-site calibrations of electronic devices, saving significant costs.

How could this research benefit the field of microelectronics?

Understanding how to tailor and optimize the magnetic and electronic properties of quantum materials, such as moiré quantum matter, could have numerous applications in microelectronics. For example, ultra-thin superconductors, a product of this research, are highly sensitive detectors of single photons, which could be valuable in various electronic devices.

Where was this research conducted?

This research was conducted by scientists at the National Institute of Standards and Technology (NIST) in collaboration with international experts and published in the journal Science.

What is the significance of the findings related to the magnetic properties of moiré quantum materials?

The findings related to the magnetic properties provide new insights into how confined electrons in twisted graphene layers give rise to unique magnetic properties. This understanding could have implications for the development of novel materials and technologies.

What is the importance of supercooling the twisted graphene material?

Supercooling the material to temperatures just above absolute zero reduces random atomic and electron movements, enhancing the ability of electrons in the material to interact. This controlled environment is crucial for the study of energy levels in graphene layers.

What is the quantum Hall effect mentioned in the research?

The quantum Hall effect is a phenomenon where discrete resistance values occur when a strong magnetic field is applied to electrons in a two-dimensional layer. It is used as a standard for electrical resistance calibration and is related to the quantized energy levels of orbiting electrons.

More about Quantum Ruler

You may also like


Reviewer123 October 8, 2023 - 12:34 am

this text is pretty interesting they’re talkin’ ’bout quantum stuff in graphene layers & some ruler thing, idk it’s complicated but sounds cool

SciFiGeek October 8, 2023 - 12:50 am

Quantum ruler? Sounds like something outta sci-fi, but it’s real! This article explains it all, good read.

GrammarNazi October 8, 2023 - 12:55 am

They need to proofread better, saw some typos & missing commas. Otherwise, it’s informative.


Leave a Comment

* By using this form you agree with the storage and handling of your data by this website.

SciTechPost is a web resource dedicated to providing up-to-date information on the fast-paced world of science and technology. Our mission is to make science and technology accessible to everyone through our platform, by bringing together experts, innovators, and academics to share their knowledge and experience.


Subscribe my Newsletter for new blog posts, tips & new photos. Let's stay updated!

© 2023 SciTechPost