Researchers show that capacitance can play a key role in probing correlated states of semiconductor moiré materials.
In recent years, researchers have increasingly focused on experimenting with new materials that could be used to study electronic correlation phenomena, which refers to the interaction between electrons in the electronic structure of a quantum system. Van der Waals (vdW) moiré materials are particularly promising for this phenomenon. These materials consist of strongly bonded two-dimensional (2D) layers that are bound in the third dimension through weaker dispersion forces.
The term, moiré refers to the specific pattern produced when an opaque ruled pattern with gaps is placed onto a similar pattern. Recent studies have indicated robust and correlated insulating states at both integer and fractional filling factors of semiconducting materials with a moiré pattern.
Researchers at Cornell University and the National Institute for Materials Science in Japan have explored the thermodynamic properties of these robust correlated states. Their studies, published in Nature Nanotechnology, shows that capacitance can play a key role in probing correlated states of semiconductor moiré materials.
Generally, the capacitance of a parallel plate capacitor depends on its geometry, or in particular, the distance between the two plates. However, the calculations by Veit Elser, who is a co-author in the paper, suggests that the capacitance when the sample plate is in a phase mixture of electron crystals could in fact be infinite.
The team tested this theory experimentally, and measured the capacitance of a parallel capacitor that consists of the moiré sample as one plate and a thin sheet of metal as the second plate.
“The two plates were separated by an experimentally variable distance,” Kin Fai Mak, one of the researchers who carried out the study, said. “The capacitance is intimately connected to the electronic compressibility (a thermodynamic quantity) of the sample, which is a measure of how compressible the electrons are when they are subjected to an external electric field.”
“One of the most important achievements of our study was a significant enhancement of the measured capacitance compared to the geometrical value,” Mak said. “To the best of our knowledge, this is probably the largest enhancement reported to date. Because of sample disorders, however, the observed enhancement is far from infinity as predicted by Veit’s original calculations. One could imagine further capacitance enhancement with better samples in the future.”
The findings can help in the development of electronic devices. And their work demonstrates that the capacitance of semiconductor moiré superlattices can be significantly enhanced, which means that the charge storage of devices made of these materials could be improved.