Russian scientists demonstrate how disorder contributes to emergence of unusual superconductivity

Researchers at HSE University and MIPT have investigated how the composition of electrons in a superconductor influences the emergence of intertype superconductivity—a unique state in which superconductors display unusual properties. It was previously believed that intertype superconductivity occurs only in materials with minimal impurities. However, the scientists discovered that the region of intertype superconductivity not only persists but can also expand in materials with a high concentration of impurities and defects. In the future, these superconductors could contribute to the development of highly sensitive sensors and detectors. The study has been published in Frontiers of Physics.

In ordinary materials, there is always at least some resistance, a property that hinders the flow of electric current and results in energy loss. However, certain materials, when cooled to extremely low temperatures, transition into a state where resistance is entirely eliminated. This state is known as superconductivity, and the materials exhibiting it are called superconductors. 

When a material enters a superconducting state, it completely expels external magnetic fields, such as those generated by electromagnets or current-carrying conductors. However, if the external magnetic field becomes too strong, the superconductor loses its properties and reverts to its normal state. 

Superconductors are traditionally classified into two types. Their classification into type I or type II depends on the material's behaviour in a magnetic field and the value of the Ginzburg–Landau parameter, which, in turn, depends on the material's characteristics as well as the presence of impurities and defects. If it is below a certain value, the material is classified as a type I superconductor; if it is above that value, it is classified as a type II superconductor. In type I superconductors, the magnetic field is expelled from the material until its intensity reaches a critical value. After that, the field penetrates the material, and superconductivity is lost. In type II superconductors, the situation is different: the magnetic field begins to penetrate once the field strength exceeds a minimum threshold, but superconductivity is maintained. The field penetrates in the form of vortices—narrow current-carrying tubes, within which a magnetic field is present. These vortices form an ordered lattice structure.

However, there is a narrow region around the critical value of the Ginzburg–Landau parameter where superconductivity exhibits intermediate properties between type I and type II. This state is known as intertype superconductivity. Unusual magnetic field configurations differing from lattices arise in this state, including vortex clusters, chains, and giant vortices, which give rise to new magnetic properties distinct from the classical ones. 

Initially, intertype superconductivity was observed only in pure superconductors with minimal impurities. However, a recent study by scientists at the HSE MIEM Centre for Quantum Metamaterials and the MIPT Centre for Advanced Methods of Mesophysics and Nanotechnology revealed that the region of intertype superconductivity is maintained in superconductors with a high concentration of impurities and defects. This is possible in multiband superconductors, where multiple 'types' of electrons with different properties coexist. Electrons from different energy bands respond differently to impurities: some are more affected, while others are less so. Moreover, the extent of interaction with impurities can be controlled, for instance, by irradiating the material with ions, which allows for the expansion of the intertype superconductivity region. 

The scientists' findings contribute to the current understanding of superconductivity types and how their properties change under different conditions. This is crucial for the effective use of superconductors in cables and high-power magnets, as the current and magnetic properties of a superconductor depend on its type. It is also valuable for the development of new, highly sensitive devices.

Pavel Marychev

'The study broadens our understanding of superconductivity and the classical classification of superconductors, which has been around for about 70 years. We have shown that the combination of disorder and multiband effects fundamentally alters the properties of superconductors and opens up the possibility of exploring rare and exotic superconducting states. Since the magnetic field configurations in intertype superconductivity are sensitive to temperature and current parameters, such superconductors could be used in highly sensitive sensors and detectors in the future,' according to Pavel Marychev, Research Fellow at the HSE Centre for Quantum Metamaterials.