| The competition among charge-spin-lattice interactions in the correlated electronic material system leads to many novel condensed matter phenomena.The competition among these interactions also makes it possible to use a variety of methods to regulate various condensed matter properties within the material.The study of superconductivity,magnetism and electrical transport properties is the most lasting topic in condensed matter physics.The regulation of these properties can further increase our understanding of these competition between different interactions in condensed matter systems.By changing the carrier concentration(charge)in the material,adjusting the interlayer distance(lattice)between the superconducting layer or the magnetic layer(spin),we can tune the coupling strength of these interactions to control different phase transitions.In this dissertation,we used the electrochemical intercalation method to insert organic ions into the interlayer of FeSe single crystal to obtain(CTA)0.3FeSe and(TBA)0.3FeSe,which increased the superconducting transition temperature to 45 K and 50 K,respectively.The CTA+ ion with long chain structure is inserted into the interlayer of FeSe in the form of double layer,while the TBA+ion with spherical structure is inserted into the interlayer in the form of single layer.The magnetic topological material MnBi2Te4(Bi2Te3)n(n=1,2)single crystal was synthesized by the vertical Bridgeman method,and its magnetic and transport properties were systematically characterized.With the insertion of the nonmagnetic layer Bi2Te3,the coupling strength between the magnetic layers gradually decreases,the antiferromagnetic transition temperature gradually decreases,too.Moreover,there is a ferromagnetic hysteresis occurs at 2 K,which is due to the magnetocrystalline anisotropy and the competition of different magnetic interactions.The above work achieved the regulation of superconductivity and magnetism by changing the carrier concentration and interlayer coupling strength in layered materials.The dissertation is divided into four chapters as follows:1.IntroductionThis chapter is divided into five parts to summarize the research background.It includes the history of superconductivity,the research progress of FeSe-based superconductors,the research progress of organic-inorganic hybrid two-dimensional superlattices and the introduction of typical material synthesis methods.2.Organic-ion-intercalated FeSe-based superconductorsIn this chapter,we used the electrochemical intercalation method to insert the chain-like organic ion CTA+ into the interlayer of FeSe single crystal,and obtained(CTA)0.3FeSe with superconducting transition temperature up to 45 K.After inserting the organic ion CTA+,the interlayer distance of the adjacent FeSe increased from 5.5 ?to 14.5 ?.According to the results of the X-ray diffraction pattern and high-resolution transmission electron microscopy(HRTEM),we proposed the lateral bilayer structure model which consists of double-layer of CTA+chain and one layer of FeSe.In the pressure range of 0-1 GPa,the superconducting transition temperature obtained from the magnetic susceptibility curve is gradually suppressed from 45 K at atmospheric pressure to 40 K at 1 GPa.Superconducting phase II of intercalated FeSe-based superconductors was also observed at higher pressure.At the same time,we observed the tightly packed CTA+ions in the scanning tunneling microscope(STM),which confirmed the accuracy of the crystal structure model proposed previously.3.FeSe-based superconductors with superconducting transition temperature up to 50 KIn this chapter,we still used the electrochemical intercalation method to insert spherical organic ion TBA+into the FeSe interlayer to obtain the(TBA)0.3FeSe crystal with regular shape,which allows us to study its transport properties.After insertion of TBA+ions,the interlayer distance of the adjacent FeSe increased from 5.5 ? to 15.5 A.According to the results of X-ray diffraction pattern and transmission electron microscopy,and the size and dimension of the TBA+ion,we proposed the crystal structure model of(TBA)0.3FeSe.(TBA)0.3FeSe is composed of single-layer FeSe and singlelayer TBA+organic ions alternately stacking along the c-axis direction.The electrical transport measurement showed that the initial superconducting transition temperature of(TBA)0.3FeSe was as high as 50 K,and the resistance reached zero at 42 K.With the increase of the applied magnetic field,the initial superconducting transition temperature did not change significantly,and the superconducting transition gradually widened,showing the typical characteristics of two-dimensional superconductivity.Under external pressure,the superconducting transition temperature was gradually suppressed from 50 K at atmospheric pressure to 30 K at 2.5 GPa in the range of 0-2.5 GPa.The results of further NMR resonance,high-field magnetic susceptibility and thermal transport show that there is a pseudo-gap behavior up to 60 K in(TBA)0.3FeSe,indicating that the electron pairing has begun to occur at this temperature.4.Magnetic and transport properties in magnetic topological insulator MnBi2Te4(Bi2Te3)n(n=1,2)In this chapter,the magnetic topological insulator MnBi2Te4(Bi2Te3),(n=1,2)single crystals were obtained by inserting the nonmagnetic layer Bi2Te3 into the magnetic layer MnBi2Te4,and their magnetic and transport properties were systematically characterized.By inserting a nonmagnetic Bi2Te3 layer,the antiferromagnetic coupling strength between the magnetic layers decreases gradually.The antiferromagnetic transition temperature gradually decreased from 25 K in MnBi2Te4 to 12.3 K in MnBi4Te7 and 10.5 K in MnBi6Te10.At low temperature of 2 K,due to the competition of magnetic interaction and magnetocrystalline anisotropy,the magnetic susceptibility curve shows an abnormal ferromagnetic hysteresis curve.This ferromagnetic hysteresis at low temperature is beneficial to the quantum anomalous Hall effect at zero field. |
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