1+1>2, win-win cooperation, seems to have become a more universal law in the field of quantum materials. In quantum materials, those phenomena that reach a delicate balance on multiple degrees of freedom or energy scales are the basic interest of condensed matter physics research.

When two single-layer graphenes with similar lattice constants are stacked vertically and slightly misaligned, they will exhibit a periodic moiré pattern, thereby changing the electronic state and phase transition of the material, resulting in novel physical properties. In March 2018, the Pablo Jarillo-Herrero research group of the Massachusetts Institute of Technology (Cao Yuan’s doctoral supervisor at MIT) discovered a new electronic state in the double-layer graphene twisted by the magic angle of ~1.1°, which can easily realize the insulator to superconductor The transformation has opened the door to unconventional superconductor research.

This new discovery has pushed graphene to a new level of development, and also pushed the two-dimensional moiré superlattice to the commanding heights of cutting-edge research.

Today, Pablo Jarillo-Herrero of the Massachusetts Institute of Technology and AF Young of the University of California, Santa Barbara, etc., published two papers on the Moiré superlattice based on magic angle graphene at the same time, reporting the unconventional ferroelectricity and Magnetic control has brought a new direction to the innovation of a new generation of quantum materials and electronic devices.

1. Nature: Discover super-normal ferroelectricity

Ferroelectric materials are usually formed by the space separation between the average centers of positive and negative charges in the unit cell, and have electrically switchable electric dipoles. Generally speaking, graphene (a material composed only of carbon atoms) does not exhibit ferroelectricity.

However, Pablo Jarillo-Herrero and Qiong Ma et al. found unconventional ferroelectric properties in graphene-based moiré heterostructures. They found that a switchable ferroelectricity was realized in the Bernal stacked double-layer graphene between two hexagonal boron nitride layers.

The researchers found that by aligning the double-layer graphene with the top or bottom boron nitride crystals and introducing the Moiré superlattice potential, the graphene resistance has obvious hysteresis behavior. The test results found that the response function of the displacement field and the electron filling is extremely amazing, beyond the range of conventional ferroelectrics. Furthermore, the researchers used non-local single-layer graphene sensors to directly detect iron polarization and found that there is an unconventional parity electronic ordering in the double-layer graphene/boron nitride moiré system.

This emerging moiré ferroelectric characteristic is expected to realize ultra-fast, programmable and atomic-level ultra-thin carbon-based storage devices.

2. Nature: Discover the super conventional magnetic switch

Magnetism is usually derived from the combined effect of Fermi statistics and repulsive Coulomb interaction, which is conducive to the realization of the ground state of non-zero electron spin. For a long time, spin magnetism can only be controlled indirectly by electric field

In view of this, the University of California, Santa Barbara, A. F. Young, et al. achieved direct electric field control of the magnetic state in the orbital Chern insulator for the first time through experiments. In this magnetic system, the topological structure of the non-flat zone is conducive to the long-range order of orbital angular momentum, but the spin is still disordered.

The researchers used the van der Waals heterojunction composed of double-layer rotating graphene as the research object to realize the narrow and topologically unimportant valley projection Moiré microstrip. In these bands, when each molar unit cell is filled with 1 to 3 electrons, the quantum anomalous Hall effect can be observed, and its lateral resistance is approximately equal to h/2e2 (where h is Planck’s constant and e is the electron The charge of ), indicating that the spontaneous polarization of the system is a single valley projection zone with Chern number equal to 2. When each mole unit cell is filled with 3 electrons, the sign of the quantum anomalous Hall effect can be reversed through the field effect control of the chemical potential.

This transition has a certain hysteresis and can be used to prove the reversal of the magnetic state caused by the non-volatile electric field. Theoretical analysis shows that this effect is caused by the topological edge state, which drives the change of the magnetization characteristics, thereby promoting the reversal of the more favorable magnetic state.

In short, the direct control of the magnetic state by voltage can be used to electrically pattern the nonvolatile magnetic domain structure of the chiral edge state. It has important applications in the field of reconfigurable microwave circuit elements and even ultra-low power magnetic memory. prospect.

1.Zhiren Zheng et al. Unconventional ferroelectricity in moiré heterostructures.Nature 2020.

https://www.nature.com/articles/s41586-020-2970-9

2.H. Polshyn et al. Electrical switching of magnetic order in an orbital Cherninsulator. Nature 2020.

https://www.nature.com/articles/s41586-020-2963-8

Information source: Nanoman

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