Metallic hydrogen, which is under pressure of about four and a half million atmospheres, can have the highest critical transition temperature in the series of high-temperature conductors. According to preliminary calculations of the Italo-German group of scientists of theoretical physicists, the critical temperature of the element is 242 K (minus thirty-one degrees Celsius).
Hydrogen gas turns into a liquid at a temperature of 20 K. If you lower the temperature by another 6 K, then you can translate the element into a solid state. Hanington and Wigner in 1935 suggested the production of hydrogen in the laboratory. In their opinion, it was necessary to use high pressure - about 25 GPa (one GPa is approximately equal to ten thousand atmospheres). So, under the influence of high pressure, the element will turn into a hydrogen isotope - from a dielectric element to a conductive one. It should be noted that the gas in the initial state has conductive properties. Just like metals, an element conducts electricity, while it may not be in a solid state. In other words, hydrogen may also be a liquid having metallic properties.
In 1971, the work of Soviet theoretical scientists led by Kagan was published. A group of physicists argued that metallic hydrogen can be metastable. This means that after the cessation of exposure to high pressure, the element will not go back to its original state - a gas with dielectric properties. However, it is still unclear whether this stage will be long enough to have time to use metallic hydrogen.
The first success in the experimental plan was obtained in 1975, in February. A group of scientists led by Vereshchagin created metallic hydrogen. Under the influence of a temperature of 4.2 K in a thin layer of an element using diamond anvils, also subjected to a pressure of the order of 300 GPa, a decrease in the electrical resistance of the gas by a factor of millions was observed. This indicated the transition of hydrogen to the metallic state.
To obtain high pressure, a diamond anvil is used. It is presented in the form of two artificial diamonds that are pressed against each other by means of a press. As a result, on the cut, the diameter of which is on the order of several tenths of a millimeter, the necessary pressure is formed. At this site in the cell is a cooled sample. Equipment is supplied to the sample in the same place: miniature thermocouples, electrodes and other measuring instruments.
The next stage in the work of scientists was to clarify the possibility of a subsequent transition of the metallic state to the superconducting one. The first to ask this problem was Neil Ashcroft. The theorist predicted that metallic hydrogen would have “exotic” properties when exposed to high temperatures in excess of 200 K.
More recently, the work of German and Italian physicists was published. The authors argue that due to the electron-phonon mechanism for the formation of Cooper pairs, a record critical temperature of 242 K is achieved. At the same time, however, high pressure is also needed - about 450 GPa, which in turn is four and a half million times higher than atmospheric pressure.
In the electron-phonon formation of Cooper pairs when moving in a periodic lattice in a crystal, an electron attracts the nearest positively charged ions. In this case, a slight deformation of the lattice occurs, and the concentration of the positive charge increases for a short time. Due to the increased concentration, another electron is attracted. So, both electrons are attracted. At a nonzero temperature, ions oscillate around their equilibrium states. Phonons are quanta of these oscillations.