An international research led by the Complutense University of Madrid (UCM) has managed to assemble monocrystalline layers of ceramic oxides just a few atoms thick, rotated at an arbitrary angle, which link together to form a new artificial crystal that does not exist in nature.
The work, published in “Nature,” shows that at the interface between rotated ferroelectric layers of barium titanate (BaTiO₃), emerging properties appear that could trigger a revolution in the science and technology of materials. BaTiO₃ has been known since the early 20th century and, like graphene, has been produced in the form of ultra-thin crystals.
This new generation of artificial materials arises from stacking two ultra-thin crystals of a ceramic oxide, barium titanate, rotated with respect to each other. In nature, crystals spontaneously grow with well-defined facets because they maintain the orientation of the so-called crystal axes, reaching sizes of tens of meters and weights of several tons.
Until now, modern material growth technologies have exploited this natural tendency by combining very thin layers of different materials, stacking them on top of each other while rigidly maintaining the crystalline orientation and atomic arrangement of the different layers. The result is the emergence of new and interesting properties at the interfaces or bonding surfaces between the crystalline layers, which have allowed, for example, the construction of electronic devices and their use in information and communication technologies.
In this work, oxides crystals with a new degree of freedom that does not exist in nature and was impossible to achieve until now have been fabricated: controlled rotation between atomic-thick crystalline layers, a strategy that we colloquially call “twistronics” (from the English, twistronics, the study of how the angle or torsion between layers of two-dimensional materials can change their electrical properties).
The bonding between these layers results in a characteristic structural and interaction pattern (called a moiré pattern) that, according to theoretical calculations by Hugo Aramberri and Jorge Iñiguez at the Luxembourg Institute of Science and Technology (LIST, in Luxembourg), is the origin of the emerging properties found.
The work demonstrates that the rotation between layers induces a ferroelectric state never seen before in which electric polarization vortices (whirlpools) alternate with a very small lateral size (a few atoms), and could be the information element (bits) of future memories.
The work opens new paths to increase information storage density and energy efficiency in future computer devices. Jacobo Santamaría, director of the group of Complex Materials Physics at UCM, explains that this state would allow reaching storage densities exceeding 100 terabits/in², surpassing the current limit of 1Tb/in² where computer memory information density has been stagnant for some years. This would address the technological and energy sustainability challenge of a global information storage that could surpass yotta (10²⁴) bytes in the current decade. According to the authors, this work opens new paths to increase information storage density and energy efficiency of future computer devices.
Carlos León, a researcher in the same group, adds that “beyond this, this study opens up a whole range of opportunities for observing and exploiting new effects and properties in other crystalline oxides (and not only oxides) that exhibit ferroic or multiferroic states or other collective states.”
In addition to UCM and LIST, this research involved the Institute of Materials Science in Madrid (CSIC) and the Heterostructures Laboratory with spintronic applications, also from UCM and CSIC.
Referrer: MiMub in Spanish
