A superconductor is a material that conducts direct current with no electrical resistance. This lack of resistance supports very high currents without loss, and makes superconductors very attractive for use in power transmission. Also, the persistence of current in a closed loop can be used in various applications to make large permanent magnets. For example, today’s Magnetic Resonance Imaging (MRI) machines use superconductor magnets to achieve a magnetic field strength 30 000 times stronger than the Earth’s field. Superconductors also make for highly efficient electronics with extremely high levels of performance.
An important feature of superconductors, which confirms the existence of the state, is that they expel the magnetic field from within the material and also do not allow the magnetic field to penetrate. This phenomenon in superconductors is called the Meissner effect. This makes them powerful electromagnets, some with the ability to levitate trains. Superconducting circuits are also a promising technology for quantum computing because they can be used as qubits, the basic units of quantum processors.
However, until recently, the ultra-low temperatures (close to absolute zero, 0 Kelvin) and ultra-high pressures (>104 bar) necessary to achieve the superconducting state makes it inconvenient to implement. An ideal superconductor would be at room temperature and pressure. If such a superconductor could be economically mass-produced, it could revolutionise electronics. Despite decades of intense research efforts, such a state is yet to be realised.
In recent decades, researchers have developed a class of so-called high-temperature superconductors, defined as materials with a critical temperature, Tc (the temperature below which the material behaves as a superconductor) above 77 K (−196.2 °C), which is the boiling point of liquid nitrogen. They are only ‘high-temperature’ relative to previously known superconductors, which function at even colder temperatures.
New room-temperature superconductors promise to change that. As the name suggests, room-temperature superconductors don’t need special equipment to cool them. Some do need to be pressurised, but only to a level that can be achieved by using strong metallic casings.
In March this year, eleven researchers at the University of Rochester, New York, USA announced in Nature a new material that is a superconductor at room temperature. It is a synthesised compound of nitrogen-doped lutetium hydride exhibiting super conductivity at a critical temperature, Tc, of 21°C (294 K). While at room temperature, the pressure required is still high at 104 bar, and would restrict the use of this superconductor (see Dasenbrock-Gammon et al., 2023. Nature, 615, 244–250).
However, subsequent published criticism of this research, which claims that it does not present evidence for true superconductivity, has resulted in this paper being retracted from Nature in November, 2023.
In June this year, two research teams in China, one led by Jianjun Ying at the University of Science and Technology of China and the other led by Changqing Jin at the Chinese Academy of Sciences, published their work together on the production of a single element superconductor with the highest critical temperature to-date of -237°C (36 K) This was using Scandium (Sc) compressed between two diamonds. While much below room temperature, the Tc is comparable to that of classic multielement superconductors. However, it is still at a pressure of 104 bar. (See Physical Review Letters, 130, 256002).
A more recent development in July this year, by Hyun-Tak Kim and colleagues at William and Mary College in Virginia, USA, is the formulation of a multi-element superconductor at room temperature and pressure. The synthetised material, called LK-99, is a modified lead apatite crystal and its resistance is near zero at 30° C (See New Scientist, 5 August 2023). The conductor exhibits the Meissner effect when a millimetre-sized sample of LK-99 is placed on a magnet. This is illustrated in a still from a video in the above New Scientist paper, page 10. Only one edge of LK-99 levitates, due to only that part being superconductive. Two papers, not peer reviewed, reporting this development, are doi.org/kk42 and doi.org/kk43. The latter is by Kim’s colleagues at Korea University in South Korea.
While the above development is considered by many to be an important breakthrough in the quest to have easy-touse superconductors, some are sceptical of the results from LK-99 and claim it is too early to say that the evidence of superconductivity is certain.
Clearly this is a space to be watched!