Aug 13, 2020 (Nanowerk Spotlight) Graphene, a two-dimensional material that draws the attention of scientists from around the globe due to its exotic and never-before-seen properties, keeps on surprising. It became world-famous as the first ever discovered two-dimensional material which is stronger than steel and a better conductor than silver, yet flexible and stretchable. In 2018, a group of scientists from MIT showed that when two graphene layers are stacked on top of each other and twisted precisely by a specific tiny “magic angle”, one can induce superconducting and insulating phases depending on the electric density (read more: “Insulator or superconductor? Physicists find graphene is both“). In other words, by changing the density of electrons with a pair of gates, one can switch from a phase in which electrons move without resistance (superconductivity) to a phase where the flow of electrons is completely blocked (Mott insulating phase). This behavior is extremely desirable because it significantly reduces energy losses in electronic devices making them more efficient and durable. However, precisely rotating two sheets of graphene is nothing but a challenging task. To illustrate this, imagine that someone gives you two pieces of material that is 100.000 thinner than a strand of hair, smaller than a dust and on top of that transparent and tells you to align them with a precision better than 0.1°. Not a trivial task! The work recently published in Nature (“Evidence of flat bands and correlated states in buckled graphene superlattices”), proposes an alternative way to obtain similar phases. The experimental research led by Rutgers University, USA (X. Lai, E. Andrei) and the University of Chinese Academy of Sciences, China (J. Mao, Y. Jiang), complemented with theoretical support from the University of Antwerp, Belgium (S. Milovanovic, M. Andelkovic, L. Covaci, F.M. Peeters), showed that periodically strained graphene harbors correlated electronic phases similar to those previously observed in magic angle twisted bilayer graphene. Scientists at the University of Manchester (Y. Cao, A.K. Geim) and the Institute of material Science in Tsukuba Japan (K. Watanabe and T. Taniguchi) contributed to the study. Figure 1. Spontaneous generation of periodically buckled graphene by thermal expansion mismatch. (Image courtesy of the researchers) Their approach relies on periodically straining a graphene sheet rather than stacking layers on top of each other. Graphene has a negative thermal expansion coefficient. This means that unlike most materials, when heated graphene contracts instead of expanding. Thus, when placed on a substrate with positive thermal expansion coefficients, graphene will contract while the substrate expands. This generates additional stress that graphene tries to compensate by stretching or compressing its surface. However, if the generated stress is larger than some critical value, in-plane deformation is no longer favorable and the material buckles, as sketched in Figure 1. In their research published in Nature, researchers observed the spontaneous appearance of various buckling patterns in graphene placed on different substrates, as shown in Figure 2. Figure 2. Topography of experimentally observed periodic patterns. (Image courtesy of the researchers) Buckling arises due to aforementioned mismatch between the thermal expansion coefficients of graphene and the substrate but the final pattern configuration is a result of interplay between different factors including substrate type, temperature, defects, morphology, etc. Interestingly, these periodic patterns have a profound effect on the motion of the electrons in the material. As a peculiar phenomenon in graphene, strain bends the trajectories of electrons, similar to real magnetic fields. Consequently if these fields are strong enough, a periodic buckling superlattice will confine the electrons in periodic arrays, as shown in Figure 3. The buckling slows down the electron flow in graphene to the point where electrons become aware of each other and start interacting. These interactions manifest themselves through the appearance of insulating and superconducting phases, as in the case of twisted bilayer graphene. Upon changing the gate voltage, the authors detected the appearance of an insulating phase in their sample confirming existence of electron-electron correlations. Figure 3. Confinement of the electron wavefunction by the periodic strain superlattice. (Image courtesy of the researchers) This study paves the way for a new direction in search of room-temperature superconductivity. Although initial research did not confirm existence of superconducting phase, buckled two-dimensional materials present promising platform that offers plethora of possibilities for modifying and controlling electronic behavior in these materials. Further analysis is needed to establish the relation between different buckling configurations and conditions needed for their appearance. This is a crucial step, according to the authors, since by controlling the buckling one can switch between metal, insulating, and superconducting phase simply by changing electron density, propelling this field into the on-demand material design technology of tomorrow. Provided by the University of Antwerp as a Nanowerk exklusive

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