NIST researchers study twists graphene Layers has unveiled a “quantum ruler” to investigate the unique properties of materials.
Single person-atom-Thick sheets of carbon known as graphene have remarkable properties of their own. However, things can get even more interesting by layering multiple layers of two-dimensional material. When two or more overlapping graphene sheets are significantly misaligned, that is, twisted at specific angles to each other, they take on a large number of exotic identities.
These materials, known as moiré quantum materials, can suddenly generate their own magnetic field, become superconductors with zero electrical resistance, or conversely become perfect insulators, depending on the twist angle. there is.
Quantum ruler to measure the mysteries of graphene
Joseph A. Strosio and colleagues at the National Institute of Standards and Technology (NIST), along with a team of international collaborators, have developed a “quantum ruler” to measure and explore the strange properties of these twisted materials. This research could also lead to new miniature standards for electrical resistance that allow electronics to be calibrated directly on the factory floor, eliminating the need to send electronics to off-site standards laboratories.
Co-researcher Fereshteh Ghahari, a physicist at George Mason University in Fairfax, Virginia, took two layers of graphene (known as bilayer graphene) about 20 micrometers in diameter and placed them into another layer. By twisting the layers, they created a Moiré quantum material device. Gahari used the nanofabrication facility at NIST’s Center for Nanoscale Science and Technology to fabricate the device.
NIST researchers Marlou Slott and Yulia Maximenko cooled the twisted material device to a hundredth of a degree warmer. absolute temperature, which reduces the random movement of atoms and electrons and increases the ability of electrons in the material to interact. After reaching ultralow temperatures, they investigated how the energy levels of electrons within the graphene layer change when the strength of a strong external magnetic field is varied. Measuring and manipulating the energy level of electrons is important for the design and manufacturing of semiconductor devices.
Electron movement and energy levels
To measure the energy levels, the team used a versatile scanning tunneling microscope that Stroscio designed and built at NIST. When the researchers applied a voltage to the graphene bilayer in a magnetic field, the microscope recorded tiny electrical currents from electrons “tunneling” from the material to the tip of the microscope’s probe.
In a magnetic field, electrons move in circular orbits. Typically, the circular orbits of electrons in solid materials have a special relationship with the applied magnetic field. The area enclosed by each circular orbit multiplied by the applied magnetic field can only take on a set of fixed, discrete values. Quantum properties of electrons. To maintain that fixed product, when the magnetic field is halved, the area surrounded by the orbiting electrons must double.
The energy differences between successive energy levels that follow this pattern can be used like the scale on a ruler to measure the electronic and magnetic properties of a material. Small deviations from this pattern could represent new quantum rulers that can reflect the orbital magnetic properties of the particular quantum moiré materials researchers are studying.
Discoveries and suggestions
In fact, when NIST researchers varied the magnetic field applied to a Moire graphene bilayer, they found evidence of a new quantum ruler at work. The area enclosed by the electron’s circular orbit multiplied by the applied magnetic field no longer equals a fixed value. Instead, the product of these two numbers shifted by an amount that depended on the magnetization of the double layer.
This deviation was translated into a set of different scales of electron energy levels. The discovery is expected to shed new light on how electrons trapped in twisted sheets of graphene give rise to new magnetic properties.
“By using our new quantum ruler to study how circular orbits change with magnetic fields, we hope to uncover the subtle magnetic properties of these Moiré quantum materials,” Strossio said.
In Moiré quantum materials, electrons have egg-carton-shaped variations in energy, high and low, determined by the material’s electric field. The electrons concentrate in the lower energy states, or valleys, of the carton. NIST theoretical physicist Paul Haney said the large spacing between the valleys in the double layer, which is larger than the atomic spacing in a single layer of graphene or in untwisted layers, is part of the unusual magnetic properties the research team discovered. He said he was explaining.
The researchers, including colleagues at the University of Maryland at College Park and the Joint Quantum Institute, a research partnership between NIST and the University of Maryland, published their work in the journal science.
Future prospects and applications
Because the properties of Moiré quantum materials can be selected by choosing specific torsion angles and the number of thin atomic layers, this new measurement allows scientists to tailor the magnetic and electronic properties of quantum materials to their hosts. It is expected that you will gain a deeper understanding of how to optimize. Introduction to applications in microelectronics and related fields. For example, ultrathin superconductors are already known to be very sensitive detectors of single photons, and quantum Moiré superconductors rank among the thinnest superconductors.
The NIST team is also interested in other applications. Under the right conditions, Moiré quantum materials could provide a new easy-to-use standard for electrical resistance.
Current standards are based on the discrete resistance that a material takes up when a strong magnetic field is applied to electrons in a two-dimensional layer. This phenomenon, known as the quantum Hall effect, is due to the same quantized energy levels of electrons in circular orbits as described above. Individual resistance values can be used to calibrate the resistance of various electrical devices. However, because it requires a strong magnetic field, calibration can only be performed at an instrumentation facility such as NIST.
If researchers can manipulate quantum moiré material so that it has a net magnetization even in the absence of an externally applied magnetic field, they could use it to create a new portable version of the most accurate standard for resistance. Strossio said it could be created. Anomalous quantum Hall resistance standard. Calibration of electronic devices can be performed on the manufacturing floor, potentially saving millions of dollars.
Reference: “Quantum ruler of orbital magnetism in Moiré quantum materials” MR Slot, Y. Maximenko, PM Haney, S. Kim, DT Walkup, E. Strelcov, Son T. Le, EM Shih, D. Yildiz, SR Blankenship, K. Watanabe, T. Taniguchi, Y. Barlas, NB Zhitenev, F. Ghahari, JA Stroscio, October 5, 2023. science.
DOI: 10.1126/science.adf2040