The hidden state was discovered by Sergei Brazovskiy (now a leading researcher of the Theory of Locally Tunable Electronic States in Layered Materials project at NUST MISIS) together with his Slovenian colleagues back in 2014. During this experiment, which later gave rise to a boom in researching layered materials, the scientists applied ultrashort laser or voltage pulses to a tantalum disulfide sample of less than 100 nanometers.
The application of pulses resulted in the material's switching from an insulating to a conducting state (or vice versa, depending on the scientists' intentions). Moreover, the switching occurred in the course of one picosecond — much faster than in the "fastest" materials that serve as storage mediums in modern computers. Once the switch occurred, the material remained in the same state without switching back. As a result, this material became a new potential candidate for being used in the production of a new generation of storage mediums.
"Once our colleagues from Slovenia discovered the hidden state of materials, which cannot be obtained via traditional (thermodynamic) phase transition, a huge number of articles on the subject started appearing in various journals," explained Pyotr Karpov, Sergei Brazovskiy's co-author and engineer at the NUST MISIS Department of Theoretical Physics and Quantum Technologies, in an interview to RIA Novosti. "Most of these articles, however, focused on experiments and there was not much theory. Which meant that this state could be obtained in any laboratory, but why exactly this state? What were the mechanisms behind its formation, what was the nature of this state — that was still unclear. Why did the material remain altered indefinitely without switching back to its initial state? In this article, we attempted to give a theoretical explanation to all these processes."
Upon the application of voltage pulses to the layered tantalum disulfide sample, some atoms in this metal break away from the lattice, thus forming defects — charged vacancies in Wigner crystals. However, instead of distancing from each other, the charged voids coalesce into the linear chains of tantalum atoms that form the walls of the zones with different degenerate ground states — the domains. Then these chains form a global network. It is the manipulations with this exact nanonetwork that result in the material's switching between the states and storing data.
"Our goal was to explain why similar charges in these structures aggregate instead of push away from each other," Karpov said. "As it turned out, it is more energetically favorable for the positive charges to attract one another than to push each other away as far as possible, because during the formation of fractionally-charged domain walls, the charge on every atom forming these walls is brought down to a minimum, and the domain system becomes more stable, which was fully confirmed in the course of this experiment. And one can make an entire crystal switch into this mosaic state of domains and globules of domain walls."
The researchers say the theory proves that the domain state of tantalum disulfide can be used for long-term storage of data and working with this data at an ultrafast speed.