#twistronics

What's that swirly pattern? It's a moiré, and it has potential power

Just as wave-like patterns can appear on a computer screen when pixels do not align, new research led by Flinders University is investigating atomic-scale "moiré patterns" in the promising field of ferroelectricity. The new study, with experts at Monash University and Nanyang Technological University in Singapore, seeks inroads into electrical and optical science by exploring these complex "superlattice" patterns in various ways to create new energy and material capabilities. "Similar to the pixel example, we can overlay single-atom thin layers in non-aligned ways to achieve physical properties not present in regular repetitive materials—including superconductivity, special insulating and conductive states, and ferroelectricity," says Dr. Pankaj Sharma, from the Institute for Nanoscale Science and Technology at Flinders University.

Bilayer Graphene Reveals Hidden Topology

Small rotations of a graphene layer relative to its neighboring layer expose previously unseen topological phases. [...] Stacking two sheets of graphene—and slightly twisting one relative to the other—creates a material that exhibits a rich array of electronic phenomena. Ranging from exotic insulating behavior to unconventional superconductivity, such effects arise from an interplay between strong electron–electron interactions and topologically nontrivial electronic bands. The flat, low-energy bands of twisted bilayer graphene are central to many of these phenomena, but the coupling of those bands to higher-energy ones has been poorly understood. Now Pierre Pantaleón of IMDEA Nanoscience in Spain and his colleagues have studied how changing the twist angle affects this coupling and, in turn, the material’s electron density and topological properties [1]. The results could help scientists use twisted bilayer graphene for applications in quantum computing and other quantum information technologies.

Twisted oxide crystals show how atomic patterns alone can trap or repel electrons

It has been revealed that simply twisting and stacking two layers of oxide crystals can allow the atomic arrangement itself to control the behavior of electrons. Much like the new patterns that emerge when two meshes are overlapped and rotated, a twisted oxide interface forms specific atomic configurations that act as an "invisible fence," either trapping or repelling electrons. A research team led by Prof. Si-Young Choi in the Department of Materials Science and Engineering and the Department of Semiconductor Engineering at POSTECH (Pohang University of Science and Technology), in collaboration with Prof. Chang-Beom Eom and postdoctoral researcher Kyoungjun Lee at the University of Wisconsin–Madison, and Prof. Ryo Ishikawa at the University of Tokyo, has elucidated the mechanism underlying this phenomenon in twisted oxide interfaces formed at specific rotation angles. This work was published as a cover article in the journal ACS Nano.

Twisted Bilayers Are Hard to Pin Down

Simulations have shown that the 2D sheets in twisted bilayers such as graphene can slide past one another even at small twists and under extreme compression. [...] If two egg cartons are stacked so that their respective peaks and troughs lock together, then one cannot be slid over the other. But stack them with a twist so that they no longer align, and the top carton can slide along the peaks of the bottom carton. Something similar occurs between the sheets of bilayer graphene and other 2D materials. In a regular bilayer with aligned lattices, the sheets are pinned in an energetically favorable configuration. But for certain twist angles, the two-layer superlattice lacks long-range order, no configuration is favored, and one sheet can move freely over the other. Jin Wang and Erio Tosatti at the International School for Advanced Studies (SISSA) in Italy have now found that this friction-free state is remarkably resilient [1].

Light-Induced Tuning of Twisted Quantum Materials

A laser-based approach rapidly injects charge into moiré materials and drives metal-to-insulator transitions. [...] By twisting stacks of atomically thin crystals, scientists have created structures called moiré materials, which can host useful, exotic electronic states. But switching between these states on ultrashort timescales remains a big challenge. Now Xiaoyang Zhu at Columbia University and his colleagues have demonstrated a way to use laser pulses to quickly inject charge into moiré materials and adjust their electronic behavior [1]. In a companion study, the team has shown that this process can induce certain metal-to-insulator phase transitions that had not previously been observed [2]. The new method could help scientists build ultrafast quantum devices based on moiré materials.

New Material Joins Moiré Family

Moiré systems incorporating lead iodide can host exotic quantum states and enable near-lossless electrical transport. [...] Since 2018, condensed-matter physicists have been fascinated by so-called moiré materials, stacks of atomically thin crystalline layers that are slightly twisted or otherwise misaligned relative to each other. These small misalignments change how electrons move and interact, potentially inducing superconductivity and other unusual electronic effects. Now Yan Sun at the University of Paris-Saclay and her colleagues have shown how the phenomena accessible in moiré materials can be enriched by adding lead iodide, a layered semiconductor with an intrinsically strong coupling between electron spin and orbital motion [1].

How superconductivity arises: New insights from moiré materials

How exactly unconventional superconductivity arises is one of the central questions of modern solid-state physics. A new study published in the journal Nature provides crucial insights into this question. For the first time, an international research team was able to demonstrate a direct microscopic connection between a strongly correlated normal state and superconductivity in so-called moiré materials. In the long term, these findings could contribute to the development of new quantum materials and superconductors for future quantum technologies. Professor Giorgio Sangiovanni from the Institute of Theoretical Physics and Astrophysics at Julius-Maximilians-Universität Würzburg (JMU) was involved in the study. His research is part of the Cluster of Excellence ctd.qmat—Complexity, Topology and Dynamics in Quantum Matter—at JMU and the Technical University of Dresden.

Ultrafast electron diffraction captures atomic layers twisting in response to light

A pulse of light sets the tempo in the material. Atoms in a crystalline sheet just a few atoms thick begin to move—not randomly, but in a coordinated rhythm, twisting and untwisting in sync like dancers following a beat. This atomic choreography, set in motion by precisely timed bursts of energy, happens far too fast for the human eye or even traditional scientific tools to detect. The entire sequence plays out in about a trillionth of a second. To witness it, a Cornell–Stanford University collaboration of researchers turned to ultrafast electron diffraction, a technique capable of filming matter at its fastest timescales. Using a Cornell-built instrument and Cornell-built high-speed detector, the team captured atomically thin materials responding to light with a dynamic twisting motion.