Don’t worry about room temperature for now. Have you really figured out superconductivity?

Produced by: Science Popularization China

Author: Lu Jianlong

Producer: China Science Expo

Recently, Ranga Dias of the University of Rochester announced that his team had discovered a material that can achieve superconductivity at 21 degrees Celsius and 10,000 standard atmospheres. As soon as the news came out, it caused heated discussions in the physics community and even the scientific community.

The reason why it has attracted heated discussion is, firstly, that room-temperature superconductivity has always been the supreme holy grail in the minds of physicists, and secondly, because this paper was previously withdrawn because it could not be reproduced by other laboratories. Now, only half a year later, the paper has been published in Nature again.

What is the significance of this research? Why is room temperature superconductivity so important? This has to start with the discovery of the superconductivity phenomenon.

The first person to discover superconductivity

As we all know, superconductivity is the phenomenon that the electrical resistance of a conductor is zero under certain conditions (such as temperature, pressure, magnetic field, etc.). On April 8, 1911, Dutch physicist Heike Kamerlingh Onnes discovered superconductivity through experiments.

Heike Kamerlingh Onnes

(Image source: Wikipedia)

1911 is a year that is easily overlooked in the history of physics. At that time, the special theory of relativity had been born for 6 years, the general theory of relativity had to wait for another 4 years, and the completion of quantum mechanics was far away 15 years later. Physicists at that time did not have a clear understanding of physical phenomena at the microscopic level. They could only rely on various complicated and fragmentary theoretical tools, and contradictions between theories were endless.

Before Onnes observed superconductivity, physicists had no consensus on the conductivity of conductors near absolute zero. Some physicists even speculated that the current would almost completely stop in a conductor near absolute zero. In other words, the resistance of the conductor would tend to infinity at this time. Among them was William Thomson, 1st Baron Kelvin, one of the most famous physicists in the 19th century, who was used to name the absolute temperature scale.

On April 8, 1911, Onnes cooled mercury in liquid helium and found that mercury cooled to 4.2K (about -269℃) suddenly lost its resistance. Onnes immediately realized the significance of this discovery and subsequently published a series of research papers on this phenomenon.

Schematic diagram of superconductivity in mercury

(Image source: Wikipedia)

As the first person in history to observe the superconductivity phenomenon, Onnes' discovery settled the previous debate among physicists about the properties of conductors at low temperatures, and started the magnificent journey of mankind to climb to the peak of superconductivity. It also won him the Nobel Prize in Physics just two years later.

However, the superconductivity observed by Onnes requires a temperature of 4.2K. Such a low temperature means that superconductors have basically no possibility of any daily practical use.

If we want to apply superconductors in daily life, such as transmitting electricity from power plants to thousands of households over long distances with almost no loss, then operating temperatures and pressures close to those in daily life are essential. Therefore, after confirming the existence of superconductivity, how to push the temperature and pressure that produce superconductivity to a state close to that of daily life has become a long-cherished wish of physicists.

The pursuit of "high-temperature" superconductivity

Subsequently, physicists discovered the superconductivity phenomenon in other materials, accompanied by an increasingly higher superconducting critical temperature.

Among them, Lanthanum barium copper oxide, jointly discovered by German physicist Johannes Georg Bednorz and Swiss physicist Karl Alexander Müller in 1986, is the first high-temperature (high temperature here is relative) superconductor in human history. Its superconducting critical temperature (35K) is significantly higher than that of previous superconducting materials based on niobium such as Nb3Sn and Nb3Ge.

This discovery was soon awarded the 1987 Nobel Prize in Physics, followed by the birth of a series of copper oxide high-temperature superconductors, including the first high-temperature superconductor in human history whose superconducting critical temperature (93K) exceeded the boiling point of liquid nitrogen (77K) - the famous yttrium barium copper oxide (YBCO).

Why do we emphasize the boiling point of liquid nitrogen? This is because if the superconducting critical temperature of a conductor exceeds the boiling point of liquid nitrogen, it means that we can easily cool it into a superconductor with cheap liquid nitrogen, and the application cost is greatly reduced compared with previous superconductors.

Critical temperatures of different superconducting materials

(Image source: Wikipedia)

Physicists have also discovered that higher superconducting critical temperatures can be obtained by applying pressures much greater than the standard atmospheric pressure to the relevant experimental materials.

For example, in 2015, people discovered that applying a pressure of about 150GPa (about 1.5 million standard atmospheres) to hydrogen sulfide (H2S) can cause it to undergo a superconducting phase transition at a "high temperature" of 203K (about -70°C).

One of the materials with the highest superconducting critical temperature under ultra-high pressure conditions observed in experiments is LaH10, whose corresponding pressure and critical temperature are approximately 170GPa and 250K (approximately -23°C), respectively. This temperature is very close to zero degrees Celsius. Readers living in the Northeast must be familiar with this temperature.

However, ultra-high pressure conditions are the gap between laboratory discoveries and daily practical applications. After all, large-scale ultra-high pressure equipment means astronomical costs. In practical applications, in addition to material properties, cost control must also be considered.

A closer look at the phenomenon behind superconductivity

Along with the progress in experiments, physicists also want to understand the physical principles behind the superconductivity phenomenon, so theoretical research on the superconductivity phenomenon has been moving forward.

As early as 1950, Russian physicists Vitaly Ginzburg and Lev Landau proposed the Ginzburg-Landau theory named after them. It is a phenomenological model used to describe macroscopic superconductivity and does not involve the microscopic mechanism behind the superconductivity.

Vitaly Ginzburg (left) and Lev Landau (right)

Image source: Wikipedia

It's like, when studying thermal phenomena, physicists have a set of theories called thermodynamics, which studies some macroscopic properties of objects (such as temperature, pressure, etc.), and does not involve deeper microscopic concepts (such as the atoms that make up the object, etc.).

In the Ginzburg-Landau theory, there is a Ginzburg-Landau equation. From the Ginzburg-Landau equation, we can also obtain two important physical quantities, namely coherence length and penetration depth. The ratio of these two characteristic lengths is the basis for physicists to divide type I superconductors and type II superconductors.

What are Type I and Type II superconductors? Simply put, Type I superconductors have a critical magnetic field value. Once the external magnetic field strength exceeds this critical value, the entire conductor is no longer a superconductor; Type II superconductors have two critical magnetic field values. If the external magnetic field strength is between these two critical values, there are still some areas inside the conductor with zero resistance. When the external magnetic field strength exceeds the two critical values, the conductor loses its superconductivity.

The first theory on the microscopic mechanism of superconductivity in history was born in 1957. Its creators were American physicists John Bardeen, Leon Cooper and John Robert Schrieffer, so it was named BCS theory.

According to the BCS theory, the interaction between electrons and phonons (phonons are not real particles like electrons, but quasi-particles, a concept created by scientists to understand the atomic vibrations inside conductors from the perspective of particles) in conductors will lead to attractive forces between electrons, which in turn form electron pairs called Cooper pairs. Cooper pairs in a condensed state can flow freely inside a conductor without obstacles like a superfluid, which is the source of the superconducting properties of conductors at ultra-low temperatures.

However, the BCS theory is not the ultimate theory that can explain all superconducting phenomena. For example, the copper oxide high-temperature superconductor mentioned above cannot be explained by the BCS theory. These superconductors that cannot be described by the BCS theory are called unconventional superconductors. The earliest unconventional superconductor was CeCu2Si2 discovered in 1979. Its superconducting critical temperature is only 0.6K, which is much lower than the copper oxide high-temperature superconductor, which is also an unconventional superconductor.

The study of the microscopic mechanisms behind unconventional superconductors is an active area of ​​research. A famous example is the resonating valence bond theory proposed in 1987 by American physicist Philip Warren Anderson and Indian physicist Ganapathy Baskaran.

Covalent bonds between electrons

(Image source: Wikipedia)

In this theory, the electrons in the copper oxide lattice form covalent bonds between adjacent copper ions and are fixed. After doping, these electrons can form mobile Cooper pairs, thus producing superconductivity. The twisted double-layer graphene that became popular on the Internet in 2018 is also an unconventional superconductor.

So far, we are still a long way from fully understanding unconventional superconductors.

Conclusion

The material discovered by Dias' team can achieve superconductivity at 21 degrees Celsius and 10,000 standard atmospheres. Although 10,000 standard atmospheres may seem high, compared to other high-temperature superconductors mentioned above that require millions of standard atmospheres, this can be called "near normal pressure". If this discovery is subsequently confirmed by other research groups, it will undoubtedly be a huge experimental leap. As for whether it can be confirmed, we still need to wait and see.

Of course, even if it is confirmed, the new material discovered by Dias' team is still a long way from daily application, because the superconducting critical temperature and pressure are not the only factors that need to be considered. The critical current density and critical magnetic field are also important. In addition, the engineering problems from small-scale preparation in the laboratory to large-scale industrial production are also unavoidable challenges.

What do you think about this matter?