Effect of disorder and impurities on the superconducting properties of materials

Project coordinator: Croitoru M.D.

Employees of the center: Vagov A.V., Krasavin A.V., Lukyanov A.E., Neverov V.A., Shanenko A.A.,

External cooperation: Chen Ya.

Subject and purpose of research

The relationship between disorder and superconductivity is a very interesting and intriguing phenomenon in condensed matter physics. It is well known that conventional superconductors with a uniform order parameter are insensitive to low concentrations of nonmagnetic impurities. This property is known as Anderson's theorem. In the strong disorder regime, superconductivity is destroyed and even the superconductor-insulator transition occurs. Between these extremes, the most interesting regime can occur, where disorder even enhances superconductivity, as seen in some alloys or granular materials. The mechanisms of this enhancement are still not entirely clear and are being actively investigated.

Many of the properties of disordered materials can only be understood if one takes into account the fact that disorder in nature is almost never completely random. In real materials, the position of impurities and defects correlates with each other, and often quite strongly. This correlation can have a critical effect on the properties of materials. In many materials, from ferroelectric and thermoelectrics to photovoltaic perovskites and ionic conductors, disorder correlation or even just correlated deviation from ideal crystal periodicity plays a key role in their functionality.

The focus of modern research on materials is increasingly focused on obtaining precisely such properties of materials that are a consequence of correlated disorder. It is expected that the correlations will provide functional properties that are unattainable in conventional crystalline structures and in systems, where disorder is completely random. However, until now, such studies have practically not touched superconductivity. This is partly due to the lack of theoretical methods capable of adequately describing superconductivity in the presence of strong and, at the same time, correlated disorder. The research carried out at our Center fills this significant gap in the physics of superconductors.

Latest results and publications

Disorder with long-range correlations to enhance superconducting properties

As is known, the positions of impurities and defects in real materials correlate with each other. These correlations have been shown to have a strong effect on superconductivity, making it more robust and less sensitive to disorder potential. This confirms that the superconducting properties can be controlled not only by the total density of impurities and defects, but also by their spatial correlations.

Long-Range Correlated Disorder a Means for Enhancement of Superconductivity

V.D. Neverov, A.E. Lukyanov, A. V. Krasavin, A. Vagov, and M. D. Croitoru

Communications Physics (2022), in press

Microscopic description of surface superconductivity

Using the method of the Bogolyubov-de Gennes (BdG) equations for the strong coupling model, it was demonstrated that the critical temperature of superconductivity nucleation near the sample surface can be significantly increased compared to its bulk value. To demonstrate this effect,  numerical solution of the BdG equations were used, including the continuum and Anderson approximations. Calculations were performed for a wide range of parameters of the system under investigation. It was demonstrated that

• an increase in the critical temperature near the surface occurs due to quantum interference of various states of the BdG, contributing to the order parameter ("surface ordering" - a concept proposed by L. N. Bulaevsky and V. L. Ginzburg in 1964).
• the surface induced increase in the critical temperature is largest when the conduction band is symmetrical with respect to the Fermi level. Both the continuous and Anderson approximations cannot capture the main features of the surface effect.
• Analysis within the framework of the strongly coupled BdG surface model with surface disorder revealed that the surface effect of superconductivity is not robust with respect to surface roughness.