Shen Chunlei, reporter of China Science News
"My undergraduate grades are not very satisfactory, and I can't directly apply for a PhD abroad. Now that doctoral students are about to graduate, they have not yet decided where to go. From Zhejiang University to Peking University to Ohio State University in the United States, Tian Haidong describes himself as a person who takes one step at a time.
It is such a boy who thinks he is ordinary, but he is very obsessed with physics experiments. He experimentally found that when two layers of graphene are deflected to a precise angle, they can become superconductors, and proved that quantum geometry plays a key role in this. On February 15, the study was published in Nature, which is also the first Nature paper by Tian Haidong.
"I didn't make any progress in the two years I started my PhD, and I couldn't even twist graphene at a small angle." Tian Haidong did not give up, focused on how to improve the preparation method of samples, while pursuing the deflection angle of graphene, he also reaped the first results on his scientific research road.
Encounter the "Magic Horn"
Deflected bilayer graphene device Photo provided by interviewee (same below)
Graphene is a hot topic of research in the field of materials in recent years. It is a new material with a monolayer sheet structure composed of carbon atoms.
In 2018, a team of researchers at the Massachusetts Institute of Technology found that under the right conditions, if one layer of graphene is placed on another layer of graphene and the two layers of graphene are deflected to a specific angle - 1.08°, a magical superconductivity phenomenon occurs, which can transmit electrical energy without loss.
Since then, this angle has also been called "Magic Horn". The researchers began studying this deflected bilayer graphene and tried to figure out how the "magic angle" works.
In 2019, Tian Haidong's research team published their research results in Science Advance and found that superconductivity still exists in samples with a 0.93° angle. "This corner is 15% smaller than the theoretically predicted 'magic angle'." Tian Haidong introduced, "Our team was the first to find in experiments that the restrictions of the corner on superconductivity are relatively not so strict. ”
At the beginning of Nature's research, Tian Haidong was stuck while preparing the sample, and he could not control the deflection angle well. This study requires a very high degree of precise control of the corner, and on this premise, the transport properties of twisted graphene when the corner is very close to the "magic angle" can be further explored.
From not being able to control the corner to accurately controlling the corner, it is not an accident for Tian Haidong, but the result of countless perseverance and efforts.
In theory, when the rotation angle is the "magic angle", the electron movement speed is 0; When the angle of rotation is slightly deviated from the "magic angle" by 0.02°, the electron movement speed reaches 5000 m/s. "It can be seen that the speed of electron motion is extremely sensitive to the angle of rotation." Tian Haidong said.
Tian Haidong said he was lucky to prepare a sample with electrons moving at a speed of about 1,000 meters per second and experimentally observed the contribution of quantum geometry to superconductivity.
Why was quantum geometry introduced? Tian Haidong explains: "The sample we prepared is very close to the 'magic angle', according to the standards of condensed matter physics, the movement of electrons is almost blocked, but the sample still shows superconductivity. In this regard, we cannot use the speed of electrons to explain how twisted bilayer graphene works, and have to use quantum geometry. ”
Tian Haidong
Error and luck
The corner of the graphene sample is close to the "magic angle" while also having a higher quality, which is a difficult point of the experiment.
"The deviation of the corner from the 'magic angle' is random and cannot be eliminated under existing experimental conditions." Tian Haidong told China Science News, "We can only reduce the error to about 0.1° by constantly exploring and improving the sample preparation process, and the error of less than 0.02° really requires good luck." ”
Good luck doesn't fall from the sky. The slow progress of the experiment caused a lot of psychological pressure to Tian Haidong, and his supervisor, Professor Chun Ning Lau, helped him connect with a senior brother who graduated from the group to pass on his experience so that he could focus entirely on improving the sample preparation method. Tian Haidong got rid of unnecessary mental internal consumption, and the samples he slowly made were getting better and better.
In most previous studies of "magic angle" graphene, it is very challenging to observe superconductivity; It is even more difficult to observe the contribution of quantum geometry to superconductivity, because the speed of electron motion needs to be reduced.
To this end, Tian Haidong not only demonstrated slow-moving electrons in his experiments, but also gave more accurate measurements of electron motion than previous studies.
The biggest highlight of this research is that when the speed of electron motion is extremely low, there are superconductivity and transport phenomena. However, this goes against the traditional understanding of superconductivity and transport, because electrons moving so slowly should not be able to conduct electricity.
"The above highlights are also the reason why we submitted Nature, our findings are more fundamental, especially to deepen the understanding of the mechanism of high-temperature superconductivity." Tian Haidong said, "During the review process, the reviewers asked to verify the rationality of the existence of extremely low electron velocity in the flat band, which was also verified in the subsequent prepared samples. ”
In the revision of the manuscript for almost a year after submission, Tian Haidong found that how to communicate effectively with editors and reviewers is also a very particular discipline, which requires continuous learning and improvement.
Tian Haidong is doing experiments
Fledgling
Talking about his path to study, Tian Haidong told China Science News a little ashamed: "When I was an undergraduate at Zhejiang University, I didn't think about what I would do in the future. ”
Near the end of his undergraduate program, Tian Haidong chose to enter graduate school. During his master's degree at the School of Physics of Peking University, he won the national scholarship of Peking University for the academic year 2015-2016, which also strengthened his determination to study physics.
In 2017, Tian Haidong came to Ohio State University to pursue a doctorate, just when the teams of professors Liu Jinning and Marc Bockrath had just been poached from the University of California, Riverside, and the laboratory was not set up.
After enrollment, The Ohio State University's School of Physics organized a mentor for each new student, chaired by Jonathan Pelz. "After Professor Jonathan Pelz learned that I wanted to work on condensed matter physics experiments on two-dimensional materials, he recommended Professor Liu Jinning and Professor Marc Bockrath." Tian Haidong recalls that he joined Liu's team in November 2017.
In Tian Haidong's eyes, two-dimensional materials are a very active branch of condensed matter experiments, and many interesting physics studies are carried out on the platform of two-dimensional materials.
"Not only can I do the research I like, but I also meet a mentor who can help contact and promote the laboratory, be academically strict, and care about life." Tian Haidong said that through Liu Jinning's words and deeds, he gradually adapted to the work and life inside and outside the ivory tower.
"I am an ordinary student, and there are still some gaps with excellent students at home and abroad, whether it is articles or theoretical knowledge reserves, I need to accumulate." Talking about the future, Tian Haidong said that he has not planned carefully, and what he needs to do now is to work hard to make his wings full, in order to be qualified to deal with the next opportunities and challenges.
Related paper information: https://doi.org/10.1038/s41586-022-05576-2