NMR Study Of Superconducting Mechanism Of Topological Superconducting Materials And Flux Annealing Effect

According to the different conductivity,materials can be divided into conductors and insulators.Topological superconductor is a kind of superconductor which is different from other superconductors according to this new standard.Topological superconductors have a metallic state with a thickness of about Inm,which is protected by topology,has no gap,and is superconducting inside.If a topological superconductor is divided into two parts,a topological protective metal state with a thickness of about 1nm will naturally form on the new surface.This unique topological property makes topological superconductors an ideal material for quantum computers,which can never make mistakes.However,the superconducting mechanism of topological superconducting materials is still controversial,so it is necessary to study its internal microscopic mechanism on the atomic scale,and nuclear magnetic resonance,NMR)is such a powerful means.NMR technology refers to the resonance transition phenomenon between the spin magnetic energy levels when the nucleus in a material is radiated by electromagnetic waves with corresponding frequency in an external magnetic field The nuclear spin absorbs the energy of electromagnetic radiation and transits to a high-energy state.Solid-state nuclear magnetic resonance(NMR)technology is widely used to detect the physical properties of materials and study the physical mechanism of material properties change because of its accuracy,rapidity and high resolution.There are three main parts in this paper.In the first part,we measured the superconducting transition temperature Tc of topological superconducting material Sr0.15Bi2Se3 in different magnetic fields by using the method of resonance frequency,and explored the superconducting mechanism of topological superconducting material Sr0.15Bi2Se3 by using nuclear magnetic resonance technology.At first,we successfully measured the superconducting transition temperature Tc and the upper critical field HC2(T)of the topological superconducting material Sr0.15Bi2Se3 by using the change of the resonant frequency of the LC circuit.We measured the superconducting transition temperature Tc of the topological superconducting material Sr0.i5Bi2Se3 to be 2.2K at zero magnetic field.It is consistent with the superconducting transition temperature reported previously.After that,we measured the superconducting phase diagrams in different magnetic fields,and found that the superconducting properties of the samples are anisotropic.When the magnetic field direction is 45 degrees from the C axis of the samples,the Tc is higher under the same magnetic field.The above experimental results show that this new resonant frequency technique is suitable for measuring the physical properties of topological superconductors.In addition,we use NMR spectroscopy and Knight shift to study the superconducting mechanism in this topological superconducting material.We measured the nuclear magnetic resonance spectrum near the superconducting transition temperature of the sample.By analyzing the nuclear magnetic resonance spectrum and Knight shift,we found that the measured Knight shift almost did not change with the decrease of temperature near the superconducting transition temperature,and no evidence of triplet state was observed.In the second part of this paper,an exploration is made on the fabrication of potential new topological superconducting materials.MOxBi2Se3 was prepared by doping Mo on the precursor of topological insulator Bi2Se3 by annealing in molten state.Through XRD characterization,it is proved that we have prepared MoxBi2Se3 samples with pure crystal structure;According to EDX characterization,the chemical ratio of the sample is proved to be Mo0.3Bi2Se3.The third part is about the nuclear magnetic resonance study of high temperature superconducting material copper oxide PCCO(Pr2-xCexCuO4-y).Here,we report an annealing phenomenon of magnetic flux vortex lattice observed by steady-state continuous radio frequency signal.In the experiment,we use the coupled antenna mutual inductance technology to input continuous wave signals into the energy storage circuit.We characterize the annealing behavior of magnetic flux lattice in superconducting samples by measuring the phase change of resonant circuit.The advantage of this phase measurement method is that the amplitude and length of continuous wave signal are flexible and adjustable,while the pulse signal parameter range is restricted by echo measurement in traditional spin echo pulse measurement.Therefore,the new method can explore the parameter space extensively,which is more conducive to the study of magnetic flux dynamics properties in high temperature superconducting materials.