For the last forty years, the semiconductor industry has lead the technology for manufacturing integrated circuits. The most common one, the silicon complementary metal-oxide semiconductor (CMOS), has the advantage of predictably scale the dimension of each transistor smaller with each generation. This is known as “Moore’s law”, originally stating that the number of components per integrated circuits is expected to double yearly.
In recent years the semiconductor industry has lagged behind the Moore’s law trend. At the same time, information technology currently consumes around 8% of global electrical energy, and is growing rapidly. The semiconductor industry has identified a need for a new technology to supercede silicon CMOS, with lower energy per switching operation. There is now a worldwide basic-research effort to find ways to overcome this issue.
This action aimed to develop and characterize new materials for future electronics. Our project investigated the possibility to use a family of semiconducting 2 dimensional materials, the so called transition metal dichalcogenides (TMDs), to search for electronic phase transitions which (1) cause large conductivity changes (e.g. from metal to insulator) and (2) are extremely sensitive to external electric fields. Our approach explores twisted van der Waals heterostructures, in which two atomically thin layers are stacked one upon another but with a twist, as a promising new platform for electronically tuned phase transitions.
The project tried to address the following scientific questions:
● How do insulating phases arise in twisted moiré heterostructures?
● How does unconventional superconductivity arise in twisted moiré heterostructures?
● What new electronic phases can be realized in twisted moiré heterostructures? The use of semiconductor heterostructures adds spin-orbit coupling and spin non-degenerate bands, not available in graphene. New phases, such as spin liquids and time-reversal symmetry-broken topological phases (quantum anomalous Hall effect) have been predicted.
● Can correlated phases be realised at much higher temperatures in semiconductor moirés? The interaction strength is anticipated to be significantly larger in TMD moirés. This is promising for pushing interacting phases to room temperature and above.
● How can the charge-density-dependent phase diagram be engineered? Can it be used for ultra-low-energy switching? The understanding of the phase diagram of insulating, conducting, and superconducting phases, and their dependence on twist angle and charge density, will allow the engineering of highly sensitive switches, where tuning the charge density in the moiré will drive the system across a quantum critical point, with large conductivity changes. The project will understand this switching behaviour and evaluate its usefulness in low energy electronics.
