Single Crystal Growth And Superconducting Anisotropy Of Low-dimensional Superconducting Materials FeSe And Li_0.9Mo₆O₁₇

Low-dimensional materials refer to materials whose conductive electrons are confined to be along a one-dimensional line or in a two-dimensional plane,thus their physical properties such as electrical conductivity and magnetism are different in each lattice direction.Since their electrons have a lower dimension,they have novel important properties,which can improve people's understanding of the materials and promote people to develop new technological applications.Superconducting materials are one of the most important materials for solving energy problems.Different from traditional superconducting materials,low-dimensional superconducting materials exhibit abundant physical phenomena,such as multiphase transition and structural diversity and have important applications in the development of materials science,device physics,instrument technology and other disciplines.At the same time,the novel quantum phenomena brought by the low-dimensional and anisotropic characters of the electrons have attracted great attention.Fe-selenide,FeSe,belongs to the quasi-two-dimensional superconducting materials in the family of low-dimensional superconducting materials,and lithium molybdenum purple bronze Li0.9Mo6O17 is a quasi-one-dimensional superconducting material.Both of which are non-traditional superconducting materials.The former has a relatively simple crystal structure and similar Fermi surface topology and representative properties as those of other iron-based superconductors.Therefore,FeSe has been used as a model system to study the superconducting mechanism and properties of iron-based superconductors.And the latter,Li0.9Mo6O17,is highly conductive,and has layered zigzag crystal structure,its macroscopic properties including electrical,magnetic and thermal properties are novel and unique.However,the microscopic mechanisms,such as whether they are related to its layered zigzag crystal structure or its internal MoO6 polyhedron arrangement,etc.,are not clear.Further research is necessary.The work of this paper has two main aspects.First,high purity FeSe single crystal samples were successfully grown by AlCl3-KCl flux method.Then,a novel resonance frequency technique is used.The technique uses the change of the sample magnetic susceptibility and resistivity in the sample coil in a LC circuit during a superconducting phase transition,by the measurement of the circuit resonance frequency.From which,the superconducting anisotropy of FeSe,the critical temperature Tc,the upper critical field HO2(T)and other physical quantities are successfully obtained.We found that when the Maki parameter a was taken into account,the temperature change of the upper critical field conforms to a Werthamer-Helfand-Hohenberg(WHH)model,thus demonstrating an important influence of spin paramagnetic effect in superconductivity.Based on the WHH model,as temperature T tends to be 0,the values of Hc2,//c(0)and Hc2,//ab(0)are close to be the Pauli limit values at H0//c and H0//ab,respectively.In comparison to those of other typical iron-based superconductors,a lower upper critical field,a lower critical temperature Tc and higher superconducting anisotropy were found in FeSe.These results also demonstrate that the novel resonance frequency technique is a sensitive probe in detecting rich properties of unconventional superconductors.Second,high quality single crystals of Li0.9Mo6O17 were grown using a temperature-gradient flux method.In addition,the direction of crystal axis was determined by single crystal x-ray diffraction.Its quasi-one-dimensional physical properties were measured by our Nuclear Magnetic Resonance(NMR)experiments.The results show that the NMR spectrum of 7Li has three peaks,which includes one central peak and two side peaks.This is expected by the NMR theory,because the nuclear spin of 7Li I=3/2,with the spin quantum number m to be m=-3/2,-1/2,+1/2,and+3/2.We also measured the nuclear spin lattice relaxation time Ti of the 7Li nucleus.The results exhibit a metal-insulator phase transition at temperature(TMI),a Korringa relation above TMI.The Korringa relation indicates that there is a weak electron correlation in the material in the high-temperature regime.And then as the temperature decreases,the nuclear spin lattice relaxation rate 1/T1 of the 7Li nucleus increases,indicating that there is a development of anti-ferromagnetic spin fluctuations.As the temperature decreases further,metal-nonmetallic phase transition and superconductivity appear.