Researchers from the Russian Quantum Center, ITMO University, and Moscow Institute of Physics and Technology have demonstrated that the collective behavior of electrons in ultrathin materials can be observed using light. The corresponding study was published in Physical Review B and implemented as part of the quantum computing roadmap. The new approach will shed light on the complex states of substances essential for the development of next-gen materials, quantum electronics, and solid-state quantum simulators.

^{Credit: sakkmesterke / photogenica.ru}

The finding benefits one of the key branches of modern physics and quantum technology that studies complex materials in which particles behave as a single system. Since collective effects underlie many promising phenomena, including superconductivity, unconventional magnetism, and novel electronic states, their study is vital for developing future materials, quantum devices, and more advanced electronics.

Normally, electrons can move freely in solids; however, if the material is cooled and its particle density decreases, electrons start to repel one another and instead of moving chaotically, form a regular lattice at some point. This state is known as a Wigner crystal. 

For their study, the researchers chose to observe the electronic crystal in monolayer tungsten diselenide – a semiconductor up to one nanometer in size, which is characterized as a 2D material and is about a hundred thousand times thinner than a human hair.

The key difference was in the mode of observation. While such states are commonly studied via electrical measurements or strong magnetic fields, in their project the scientists successfully observed signs of the electronic crystal optically – i.e. through the material’s response to light and without applying an external magnetic field.

In their experiment, the researchers formed the electronic crystal by cooling the material past 26K (roughly -247°C) and fine-tuning the electronic density. For that, ITMO scientists developed a theoretical model and an experimental protocol that allowed them to determine the crystal’s period via the reflectance spectra of 2D materials.

“2D heterostructures are one of the most promising platforms for studying quantum substance states. At ITMO, we developed a theoretical model and experimental protocol to look into the structure of the electronic crystal through optical changes. This is a major step towards having such studies in solid-state systems; previously, such experiments were performed exclusively using ultracold atoms, which are significantly more complex and expensive,” notes Ivan Iorsh, an author of the paper and a chief research associate at ITMO’s Faculty of Physics.

Unlike other methods, the approach allows researchers to observe electron states with almost zero contact. With no more need for multiple electronic contacts or strong magnetic fields, they can now conveniently examine strongly interactive electronic systems.

In the long term, the 2D structures may be used to build solid-state quantum simulators – quantum platforms that help model the behavior of complex materials. Such challenges often remain beyond the capabilities of conventional supercomputers as they have to observe the collective behavior of a great number of particles.

“We were the first to demonstrate that such complex electron states in monolayer tungsten diselenide can be observed via light. Our experiment shows that electrons in an atomically thin semiconductor come together in a regular lattice and thus affect the material’s optical response. This finding provides a new tool for the scientific community to study quantum states in which multiple particles behave as a single system,” elaborates Alexander Chernov, an author of the paper and the leader of the research team at the Russian Quantum Center and Moscow Institute of Physics and Technology.

The study is a part of a research initiative related to the development of 2D material-based quantum simulators within the quantum computing roadmap – the national quantum project pursued in Russia since 2020.