Study On Thermodynamic Phase Diagram And Electromagnetic Properties Of Two-Band Superconducting Thin-Films

In 2001,Japanese scientists Akimitsu et al.discovered the superconductivity in magnesium diboride with the critical temperature about 40 K.This binary compound with the advantages of simple structure,easy preparation and high critical supercurrent density is considered as a highly promising superconductor.Theoretical calculations have suggested that MgB2 is a two-gap superconducting material.Subsequently,angle-resolved photoemission spectroscopy,scanning tunneling microscopy,specific heat measurements and other related experiments have confirmed the picture of two superconducting energy gaps in MgB2.This discovery led to an increasing focus on the research of multi-gap superconductivity and relevant theoretical problems.Since 2008,a variety of iron-based superconductors with multi-band characteristics and high critical magnetic field have raised a boom on the superconductivity research in the scientific community.The early exploration of multi-band superconductors primarily focused on bulk systems.With the continuous progress of experimental methods,low-dimensional multi-gap superconductivity has gradually become a frontier topic in the field of material science.Due to the strong correlation between superconducting properties and sample dimensions,layered superconductors with high transition temperatures such as copper oxides,magnesium diboride and iron-based systems are of great significance in discovering new high-temperature superconducting materials and exploring the physical mechanisms in these compounds.Up to now,there is still no consensus on the phase transition and the form of order parameters in the high-quality two-dimensional superconductors,and explorations to elucidate these issues are still necessary.In this paper,based on the theory of two-band superconductivity we provide detailed theoretical investigations and numerical calculations of the thermodynamic phase diagram and electromagnetic properties in the two-gap superconducting films.In addition,we also explore the potential applications of these superconductors in nuclear science.The main research contents are as follows:(1)We propose a fractional vortex statistical model to study the multiple topological phase transitions in the two-gap superconducting thin-films.Based on the two-band Ginzburg-Landau(GL)theory,we can obtain the expressions of vortex-vortex interactions in the system.The real-space renormalization-group(RG)analysis on these interactions is carried out and the recursion relations are derived thereafter.Taking the typical two-band superconducting film MgB2 as an example,we find that this two-dimensional system undergoes the Berezinskii-Kosterlitz-Thouless(BKT)phase transitions at 20 K and 30 K respectively.Meanwhile,the thin-film superconductor hosts an exotic plasma phase featuring fractional flux vortices and antivortices between these two BKT transition temperatures.We also discuss the effect of interlayer coupling on the fractional vortex statistical model and the modified RG equations also show multiple BKT topological transitions in this two-band system.In addition,based on the two-coil mutual-inductance method we propose an experimental scheme for measuring the superfluid density of MgB2 thin films to verify our theoretical predictions.(2)We investigate the influence of boundary effects on the critical temperature in the two-gap superconducting films.Based on the two-band Bogoliubov-de Gennes theory,we study the boundary effect between the two-gap superconductor and the insulator(or vacuum).New boundary terms are introduced into two-band GL formalism,which modifies the boundary conditions for the corresponding order parameters of superconductor.The theory allows for a mean-field calculation of the critical temperature suppression with the decrease in FeSe film thickness.For this system,we can obtain the characteristic length scales of the boundary effect as 50 and 70 nm respectively.The theoretical results can perfectly explain the dramatic suppression of superconducting transition temperature when the film thickness is reduced to the same order of these length scales.Our investigation thus suggests that the sample thickness has great significance for the superconducting performance in low-dimensional superconducting materials.(3)We conduct a detailed study on the electromagnetic properties of the multi-band superconducting films based on the two-band GL theory and the Neumann boundary condition.We first derive the two-band GL equations for the two-dimensional superconductor,and compute the temperature dependence of upper critical field in arbitrary direction and critical supercurrent density through the films.Our theoretical calculations are agreement with the experimental data of quasi-two-dimensional NbSe2 superconductors,Co-doped BaFe2As2 superconducting films,two-dimensional FeSe systems and LaAlO3/SrTiO3 superconducting interface.All of these results strongly indicate that the aforementioned thin-film materials are all two-gap.s-wave superconductors.In addition,we also analyze the abnormal behavior of the upper critical field and superfluid stiffness at different gate voltages for the LaAlO3/SrTiO3 interface.And our GL model can qualitatively understand the transition from single-band to two-band superconductivity in the applied gate voltages.Besides,a trijunction experiment is proposed to probe the s++or s± pairing symmetry in the superconducting films.(4)We explore the potential applications of two-band superconducting thin films in nuclear science.First,we provide a detailed explanation of nuclear fusion technology and its significance in future clean energy.Then this paper elaborates on the structure and principle of the tokamak device for controlled nuclear fusion.The superconducting magnets have great significance for the improvement and upgrading of the facility.Up to now,some superconductors like niobium titanium alloy have been utilized in tokamak tests.However,the main superconducting materials in use are low-temperature superconductors,which have high cooling costs and low working efficiency.Our research shows that the two-band superconducting thin-films with excellent electromagnetic properties may have a good application prospect in new tokamak equipments,and possess certain referential value for achieving controllable nuclear fusion in the future research.As mentioned above,we systematically study the superconducting properties of various thin-film materials based on the theory of two-band superconductivity.With the theoretical analysis and numerical calculations,we have enriched the thermodynamic phase diagrams of two-band superconducting thin films and investigated the electromagnetic properties of corresponding systems,which also provides the theoretical support for the future applications of these superconductors in nuclear science.