Research on Unconventional Superconductors

 
When two single-layer two-dimensional van der Waals materials with similar lattice constants are stacked vertically and slightly misaligned, they will exhibit a periodic moiré pattern, thereby changing the electronic state of the material and appearing in flat bands of electrons. In 2018, the team of Pablo Jarillo-Herrero and Cao Yuan from the Massachusetts Institute of Technology discovered a new electronic state in the double-layer graphene twisted by the magic angle of ~1.1°, which can simply realize the transition from insulator to superconductor, opening up unconventional superconductors. The door to research.
 
 
Since 2018, domestic and foreign scientific research teams represented by the team of Pablo Jarillo-Herrero and Cao Yuan have made a lot of progress in the field of magic angle graphene. Just last week, Pablo Jarillo-Herrero and Cao Yuan’s team just published their results in Nature. They performed thermodynamics and transport measurements at the same time, and studied the symmetry breaking multibody ground state and nontrivial of the magic angle twisted double-layer graphene (MATBG). Topological phenomenon.
 

 
Today, just a week later, Erez Berg, Pablo Jarillo-Herrero, Shahal Ilani Pablo and Cao Yuan team, and Andrea F. Young team independently published two papers in Nature, reporting the latest breakthrough of magic angle graphene!
 
 
Author: Asaf Rozen, Jeong Min Park, Uri Zondiner, former CAO
Corresponding author: Erez Berg, Pablo Jarillo-Herrero, Shahal Ilani
Correspondence unit: Weizmann Institute of Science, Massachusetts Institute of Technology
 
 
Author: Yu Saito, Fangyuan Yang
Corresponding author: Andrea F. Young
Correspondence unit: University of California, Santa Barbara
 
The relationship between temperature and electronic movement
Generally, electrons move more freely at higher temperatures. However, recent studies have observed that in a system composed of two stacked and slightly misplaced graphene sheets, when the temperature rises, electrons will be abnormal and "frozen".
 
The higher the temperature, the more "freeze" the electrons
The particles in the substance move randomly and vigorously at higher temperatures, causing the solid to melt into a liquid above the critical temperature. In thermodynamics, higher temperature is conducive to the formation of a state with greater entropy. Because the movement of atoms is more disorderly, the entropy of liquid matter is usually greater than that of solid matter. However, helium-3 is an exception. When the temperature rises, it freezes into a solid. This phenomenon occurs because the entropy of solid 3He is greater than that of liquid-a phenomenon related to the fluctuation of the spin (angular momentum) of 3He atoms. This behavior is called the Pomeranchuk effect. Recently, Erez Berg, Professor Shahal Ilani of the Weizmann Institute of Science in Israel, Professor Pablo Jarillo-Herrero of the Massachusetts Institute of Technology, and Professor Andrea F. Young of the University of California, Santa Barbara, etc. were in "Nature". An article published in the magazine described a similar effect in the graphene system and found that electrons "frozen" as the temperature increased.
 
Figure 1 Phase transition of magic-angle twisted double-layer graphene
 
The system consists of two layers of stacked graphene, the upper flakes and the lower flakes are twisted and misaligned, forming a periodic arrangement of atoms in a moiré pattern. When the twist angle is about 1°, the electron energy band in the twisted double-layer graphene becomes nearly flat, and the speed of the electrons is much lower than the normal speed at this time. At this time, the behavior of the electrons is controlled by the repulsive interaction between them, resulting in the appearance of phases that do not exist in 5-8 layers of graphene. At low temperatures (below 5-10 Kelvin), when the number of electrons is tuned to fill one or more 1/4 flat bands, an electrically insulating phase is usually formed due to the interaction between the electrons. On the contrary, the system either becomes a metal (low resistance) or a superconductor (zero resistance).
 
Figure 2 Pomeranchuk effect in double-layer graphene
 
Figure 3 Pomeranchuk effect in magic angle graphene
 
Metals can be widely regarded as electronic liquids, and physicists usually call them Fermi liquids. The insulator can be regarded as a solid state of electrons. The electrons are frozen in a certain position and arranged in an ordered array. In most cases, because the electrons are more ordered, the insulator state has lower entropy than the metal state. Therefore, when the temperature rises, the insulator usually becomes metal. Two recent studies published in "Nature" show that the opposite phenomenon is observed in magic-angle twisted double-layer graphene. By measuring the power transmission behavior in the system, it is found that as the temperature increases, when the number of electrons is adjusted to nearly 1/4 of the flat band, the twisted double-layer graphene will change from a metallic state to a high-resistance state, which is close to the electrical state. Insulator. This transition occurs at a temperature of about 10 K, and the near-insulating phase lasts to about 70-100 K.
Studies have shown that the Pomeranchuk effect of electrons is a phenomenon similar to that observed for 3 He atoms. In addition, in order to understand the origin of the effect, the researchers measured the entropy of the twisted double-layer graphene filled with a quarter of a flat ribbon, and found that the entropy of each electron in the high-temperature near-insulating phase is greater than that in the low-temperature metal phase. The entropy of each electron.
Where will the future research direction be?
The new discoveries also left many unanswered questions:
1. Is the low-temperature metal and high-temperature near-insulating phase separated by a first-order phase change, or is there a more stable transition?
2. Why is there no Pomeranchuk effect in the other 1/4 filling of the magic-angle twisted double-layer graphene flat ribbon structure (that is, when the flat ribbon is 1/2 and 3/4 filled)?
 
 
References
1. Rozen, A., Park, J.M., Zondiner, U. et al. Entropic evidence for a Pomeranchuk effect in magic-angle graphene. Nature 592, 214–219 (2021).
DOI: 10.1038/s41586-021-03319-3
https://doi.org/10.1038/s41586-021-03319-3
2. Saito, Y., Yang, F., Ge, J. et al. Isospin Pomeranchuk effect in twisted bilayer graphene. Nature 592, 220–224 (2021).
https://doi.org/10.1038/s41586-021-03409-2
3. Biao Lian. Nature 592, 191-193 (2021)
DOI: 10.1038/d41586-021-00843-0
https://doi.org/10.1038/d41586-021-00843-0