Research On Pressure Control Of Unconventional Superconductivity

Superconductivity,as a typical macroscopic quantum phenomenon,is a fundamental element and core area of condensed matter physics research.It is a common phenomenon in unconventional superconductors that superconducting state,in some ways,coexists with magnetic ordered states,such as antiferromagnetic order,spin orbit coupling state,electron nematic order,etc.Some ordered states may play a crucial role in and even directly participate in superconducting electron pairing.Therefore,understanding the relationship between these ordered states and superconducting state is helpful to understand the pairing mechanism of unconventional superconductors.As an important thermodynamic strength parameter in the equation of state,pressure is an effective means to modify crystal structure and therefore electronic structure.For most of the unconventional superconductors,the coupling/competition between electronic ordered phases is realized by applying pressure,which greatly enriches the corresponding superconducting phase diagram,and provides an important tool for better（further）understanding of unconventional superconductivity.In this thesis,we mainly introduce our research on modifying crystal structure/electronic spin-orbit coupling order or/and antiferromagnetic order by means of applying hydrostatic pressures,combining with high-pressure transport and point contact Andreev reflection spectroscopy measurements.The relationship between superconductivity and spin-orbit coupling in noncentrosymmetric superconducting Pb Ta Se₂single crystals,and between superconductivity and antiferromagnetic order in Fe-based superconductor Fe Se single crystals are studied.At the same time,a Bi Pd superconducting single crystal with a non-centrosymmetric structure was grown.The specific research contents are as follows:1.High-pressure point-contact Andreev reflection spectrum of non-centrosymmetric superconductor Pb Ta Se₂Pb Ta Se₂is a typical non-centrosymmetric superconducting system,and it provides a research platform for understanding the relationship between superconducting state and spin orbit coupling order.There is an effective antisymmetric spin-orbit coupling（ASOC）due to the absence of a spatial inversion center in the crystal structure.Theoretically,the strength of ASOC determines the mixing ratio of spin singlet and spin triplet states.At ambient pressure,we obtain the upper critical magnetic field H_c2-temperature curve by measuring the resistance-temperature curves in the magnetic field.The H_c2value at zero temperature is much smaller than that of Pauli paramagnetic field.The corresponding point contact Andreev reflection spectra show that Pb Ta Se₂is a weakly coupled s-wave BCS superconductor with two energy gaps at ambient pressure.On this basis,we modify the crystal structure of Pb Ta Se₂by applying hydrostatic pressure.The results of high-pressure transport measurements show that a first-order-like phase transition occurs in the normal state,accompanied by the change of superconducting critical temperature T_c.At the critical pressure of P_c=2.5 kbar,the T_cchanges abruptly,and the system enters the superconducting phase II.According to the first principles calculation,the microstructure of Pb Ta Se₂at high pressure is that the position of Pb atoms in the cell moves,which increases the non-centrosymmetry of the system and leads to the increase of ASOC strength.With the increase of the degree of noncentrosymmetry,the superconducting bulk state and the superconducting surface state separate from each other.The corresponding high-pressure point contact Andreev reflection spectra show that the critical temperature T_c^Aof the superconducting surface state is lower than that of the bulk superconducting phase.Theoretical analysis shows that the superconducting surface state has topological properties.The experimental results show that Pb Ta Se₂is a genuine topological superconductor only under high pressure,which provides a platform for the study of topological superconductivity.2.The growth of a non-centrosymmetric superconductor Bi Pd single crystalBased on the continuation of Pb Ta Se₂work,in order further to clarify and verify the unconventional superconductivity observed in Pb Ta Se₂,the growth and preparation of non-centrosymmetric Bi Pd superconducting single crystals was carried out.Through structure characterization and transport measurement,it is shown that the Bi Pd single crystal with a non-centrosymmetric structure is obtained,which is sufficient preparation for the subsequent pressure-dependent point contact tunneling spectroscopy experiment.3.High-pressure point-contact Andreev reflection spectrum of iron-based superconducting Fe Se single crystalsAt ambient pressure,Fe Se single crystal point contact tunneling spectra show that Fe Se is a multi band superconductor.Among them,the smaller gap has less contribution to the density of states,while the larger gap has a certain degree of anisotropy,which indicates that there is a certain degree of electron orbit selectivity in the electron pairing mechanism of Fe Se superconductors.At high pressure,when the pressure increases to P=1.07 GPa,antiferromagnetism will appear in the electron transport of Fe Se superconducting single crystals.Another set of small peaks near±7 me V will be observed in the corresponding point contact Andreev reflectance spectrum.The analysis suggests that the appearance of antiferromagnetism under certain pressure has led to the discovery of new superconducting energy gap structures in Fe Se single crystals.