Physics has two very important directions, topological quantum physics and high temperature superconductivity.When I was at Tsinghua University in 2013, I led a research team and discovered the quantum anomalous Hall effect in topological matter.
In the last hundred years, superconductivity and high temperature superconductivity have been the most important research directions in physics.The phenomenon of superconducting zero resistance was first discovered in 1911 by Kamalin Ones, who won the Nobel Prize in Physics in 1913.In the past century or so, seven or eight scientists have won the Nobel Prize in physics five times in the field of superconductivity.
The study of superconductivity and superconductivity has promoted a deep understanding of nature, especially the law of resistance transmission, which is why people have been paying attention for more than 100 years.
In the field of superconductivity, a very important research direction is to raise the temperature of superconductivity.When Ones first discovered superconductivity, the temperature was very low at liquid helium 4K—about minus 270 degrees Celsius.Not only that, liquid helium is scarce and expensive, now about 200 yuan per liter.If the superconducting temperature is raised to 77K, that is, liquid nitrogen temperature, minus 200 degrees Celsius, it is very considerable.Because liquid nitrogen is cheap and about the same price as mineral water, the cost of getting zero resistance is much lower.
Of course, the greatest human dream is to achieve superconductivity at room temperature.If it can be realized, transmission lines will no longer be greatly consumed by current and heat, and many appliances will be greatly reduced by the application of room temperature superconductor.The mountain floating in Avatar is actually a room temperature superconductivity phenomenon.So room-temperature superconductivity brings a lot of imagination, but it still exists only in science fiction movies.
In 1986, two Swiss scientists discovered high-temperature superconductivity in copper oxides—that is, superconductivity in liquid nitrogen.They won the Nobel Prize in Physics the following year (1987), and it is the first time that they have won the Nobel Prize in physics.
Nearly 40 years later, the question of why high-temperature superconductivity occurs in copper oxides, which are not metals, continues, making it an unsolved century problem in physics.At least 14 Nobel laureates and thousands of researchers have come up with their own theories and proposals – most of which claim to be unconventional or bizarre, but often contradictory.This led to the "Battle of the Physical Giants".
In the process of solving this problem, there are two important problems, the symmetry of superconducting pairs: anisotropy and anisotropy.Traditionally, copper oxide HTS is paired with anisotropic (d-wave)—the fundamental amount of its wave function in different directions is different, which is a mainstream understanding in HTS research.
But after a lot of reading, we find that unconventional copper oxide HTS may be explained by conventional superconductivity theory: HTS is not so "unconventional", but a normal superconductivity.
Based on this idea, in 2012, we discovered the interface high-temperature superconductivity resulting from the combination of single-layer FeSe and SrTiO3 substrates.Although the temperature is only close to that of liquid nitrogen, it is a new high-temperature superconducting system based on the speculation of several physical parameters.The key is that this is a very simple system of high temperature superconductivity, not anisotropic symmetry, but a simple isotropic (s-wave) symmetry.
This finding challenges the mainstream view, which is very difficult.In 2021, we published a paper entitled "S-wave pairing in corner ultra-thin bismuth-strontium-calcium-copper-oxygen Josephson junction", in which the symmetry of copper oxide high-temperature superconductors is isotropic – a spherical state, not a mallet state.Our work took a long time from submission to publication, especially in the process of preparing thousands of high-quality Josephson junctions with atomic-level flat interfaces.That is to say, we are at the highest level, the highest quality of structural devices, different from the mainstream view of the discovery.
In the face of such world challenges, it is difficult to put forward ideas that contradict mainstream consensus, but we have been doing so for more than a decade.SUST and Tsinghua have six young professors with more than 20 graduate students, using the world's most advanced instruments and equipment, through the highest quality control of materials and devices, with the highest level of experimental technology.Perhaps it doesn't matter if we prove ourselves wrong in the end, because if we use the most reliable experiments to figure out a scientific puzzle, the effect will be enormous.
Therefore, in order to challenge the world's scientific difficulties, I think it is necessary not only to have excellent experimental skills, solid physical and theoretical foundations, but also to question consensus or mainstream views.This is a very important point in the critical spirit.
Of course, this kind of persistence is very difficult for students who are under pressure to graduate and to prove their "green pepper.I think we older scientists should first guide them to master the basics, develop innovative skills, and make them dare to challenge difficult problems.At the same time, we should build a healthy academic environment so that everyone can do scientific research with confidence and dare to do difficult and truly innovative work.