Using some 2-D carbides or nitrides of transition metals, a team of scientists have computationally designed a new composite quantum material that exhibits an exotic quantum property called Rashba splitting, in colossal scale, in a metallic environment. This material can help interfacing with other substrates (2D substrates like graphene) in spintronic devices like spin transistors, spin diodes, and spin filters that take advantage of electron spin, a quantum property of electrons, to achieve higher performance.
Quantum materials are compounds with exotic physical properties that arise due to quantum effects like quantum fluctuations, quantum coherence, and quantum entanglement, having no counterpart in the classical world. Thus, realisation of these properties demands unconventional, out-of-the-box ideas. But they hold the promise of revolutionizing quantum technology, such as quantum computing, communication, sensors, and memory devices.
Physicists as well as chemists are engaged in intense research to design, characterize and synthesize novel quantum material and tailor their properties to address the world’s most pressing technological needs. Computer simulation comes handy in this path. Quantum materials and their properties are computationally predicted before these are tried out in the laboratory.
A research team under the leadership of Prof. Tanusri Saha Dasgupta at the S.N. Bose National Centre for Basic Sciences, an autonomous institution of the Department of Science and Technology (DST) focused their computational research on 2-D quantum materials, which are materials with confined geometry in one of the directions. 2-D materials are important as they are easier to assimilate in devices.
The team focused on creating composite 2-D quantum materials, which are quantum materials exhibiting two apparently different quantum properties, but connected by the basic requirement of symmetries. In their study, by proper choice of materials ingredients, the workers managed to demonstrate the existence of two distinct quantum phenomena, ---Rashba effect, (a momentum-dependent splitting of spin bands) and nonlinear anomalous Hall effect, arising from anomalous velocity of the electrons, in the same 2-D material.
The team chose two dimensional carbides or nitrides of transition metals technically called MXenes and zeroed down on Mo2CO2. Starting with this parent material, they computationally designed a Janus structure Mo2COX, named after the two-faced Roman God. For this, they replaced one oxygen atom from among the two on two terminating surfaces of the 2-D structure, with halogen atoms.
In such a Janus structure, inversion symmetry is broken and the two surfaces formed become structurally, and compositionally different, thus exhibiting distinctly different properties. “The choice of terminating atoms is guided by the large potential difference created between the two terminating layers, as required for generating large Rashba splitting”, argues the team in their paper published in Physical Review in February 2023.
“While most of the Rashba materials are semiconducting, scarcity of semiconductors having large Rashba splitting hinders the growth of spintronic devices in actual practice,” the authors wrote in their paper. The team has attempted to fulfill this lacuna by designing 2-D material exhibiting colossal Rashba splitting in a metallic environment, which is expected to be useful as they can be integrated by interfacing with other substrates in spintronic devices.
Even as Rashba splitting in the designed 2-D material was being computed for different X atoms in the Janus structure, the material under study was found to also have the ability to display another quantum property called the anomalous Hall Effect under the effect of longitudinal DC electric field along a particular direction.
Janus MXenes are predicted to display multiple quantum properties
This study led to the first-ever discovery of a 2-D composite quantum material. The authors have also calculated the thermal and dynamical stability of these materials and shown that when fabricated in the laboratory, these Janus MXenes will be stable.
With the 2D composite material designed computationally, the scientists hope that the challenges of manufacturing Janus MXenes in the lab and at a large scale will be gradually overcome to bring benefits for devices, energy security, and the economy.