Episode 3: Quantum ruler measures orbital magnetism in moiré quantum matter

In this podcast episode, MRS Bulletin’s Rahul Rao interviews Fereshte Ghahari of George Mason University about the use of a scanning tunneling microscope (STM) to measure the electronic and magnetic properties of moiré quantum materials. Ghahari and collaborators twisted two layers of graphene at a specific angle, then chilled the material to suppress as much motion as possible. They ran an STM across the material while varying the magnetic field. They could precisely observe how those field changes affected the energy levels of the electrons, realizing that they could use those discrete energy levels as a “quantum ruler.” “We hope these new measurements help researchers to optimize these magnetic and electronic properties of quantum materials for specific applications,” says Prof. Ghahari. By manipulating the electrons in moiré quantum matter and shifting its twist angles, materials researchers may be able to improve on materials that are useful for microelectronics or superconductors, for example. This work was published in a recent issue of Science. 

Transcript

Rahul Rao: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Rahul Rao. Take a material that is one atom thick, like graphene. Now, stack multiple layers such that their atoms are subtly misaligned with each other. What you have is called a moiré quantum material. A moiré quantum material has the potential to become anything from a perfect insulator to a superconductor. It all depends on how, exactly, you have misaligned its layers.

Fereshte Ghahari: I took two layers of graphene and twisted them at a specific angle.

Rahul Rao: That’s Fereshte Ghahari, a physicist at George Mason University. Thanks to her group and her colleagues at the nearby National Institute for Standards and Technology, or NIST, researchers can now measure moiré quantum materials’ electronic and magnetic properties. The key to their method is a scanning tunneling microscope, or STM. 

Fereshte Ghahari: Local measurements such as STM can provide specific information about the electronic structure, which is not accessible by area-average measurements such as transport.  

Rahul Rao: Ghahari crafted a pair of twisted graphene bilayers and sent the resultant moiré quantum material to NIST. There, researchers chilled the material to just a few millikelvins, suppressing as much motion as possible. Then, the researchers ran an STM across the material while varying the magnetic field. They could precisely observe how those field changes affected the electrons’ energy levels. The researchers realized that they could use those discrete energy levels as a sort of ruler. 

Fereshte Ghahari: In moiré quantum materials, you have this twist angle inhomogeneity, which often complicates the interpretation of transport measurements. In STM, however, we can measure the twist angle exactly at each location and its related physics.

Rahul Rao: Testing their so-called “quantum ruler” proved rather challenging. For instance, they needed properly configured moiré quantum materials, and it is difficult to fabricate those without bumping their delicate twist angles out of place. For another, any residue on the material can throw off the measurement rather drastically.

 Fereshte Ghahari: In STM measurement, you bring a conducting tip near the surface and record the tiny current of electrons that tunnel out of the material into the tip. In the presence of residues, the tip can become contaminated. As a result, you won’t be able to measure this tiny current.

Rahul Rao: But their test was a success. Ghahari is very excited about its potential for advancing research into moiré quantum materials.
 Fereshte Ghahari: With the new quantum ruler, we hope to probe the subtle magnetic and topological properties of a large variety of moiré quantum materials. Also, because the properties of these materials can be chosen by selecting a specific twist angle or a number of layers. We hope these new measurements help researchers to optimize these magnetic and electronic properties of quantum materials for specific applications.

Rahul Rao: By subtly manipulating the electrons in moiré quantum matter and shifting its twist angles, materials scientists may be able to craft better materials that are useful for microelectronics or superconductors. This work was published in a recent issue of Science. My name is Rahul Rao from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.