Study On The Transport Properties Of Low-Dimensional Molybdenum Purple Bronzes Under Ultra-High And Flat-Top Pulsed Magnetic Fields

Pulsed high magnetic fields have always been an important means to study phase transitions and electronic structures in strongly correlated electronic materials,and play an important role in revealing the related mechanisms behind various physical phenomena.Molybdenum purple bronze is a kind of typical low dimensional strongly correlated electronic system,and the interaction between electrons is significantly modulated by magnetic field.There are still some confusing questions about the physical properties exhibited by such materials,particularly the transport properties beyond the quantum limit and the mechanisms of phase transitions in high magnetic field.In this paper,we explore the novel electrical transport properties of molybdenum bronze based on pulsed field measurement technique with hydrostatic pressure and I-V characteristic.The main research contents of this thesis are summarized as follows:In chapter one,the charge density wave in low dimensional materials and the modulation of magnetic field on density wave are introduced.We emphasize the scientific significance of the extreme conditions on study of the Fermi surface and the phase transitions on the materials of A0.9Mo6017(A=Li,Na,K)and η-Mo4O11.The second chapter mainly introduces the high magnetic field transport measurement systems,including temperature dependent,angular dependent,pressure dependent and nonlinear I-V transport measurement systems based on 60 T single coil magnet and ultra-high field transport measurement system based on 80 T dual-coil magnet.The third chapter systematically presents the transport behavior of Na0.9Mo6O17 under the conditions of pulsed field and low temperature,including the interlayer and in-plane magnetoresistance,Hall behavior and angular dependent transport properties.Firstly,we observe the field-induced phase transition in the magnetoresistance,and obtain the high field phase diagram by temperature dependent magnetoresistance.Secondly,we focus on the analysis of Shubnikov-de Haas oscillations in high field so that we obtain several fundamental frequencies and corresponding parameters related to the properties of the Fermi pockets.We verify its two-dimensional Fermi surface properties by the angular dependent Shubnikov-de Haas quantum oscillations.Finally,according to the analysis we establish the Fermi surfaces under different phases.On the other hand,we have studied the interlayer magnetoresistance of K0.9Mo6O17 under ultra-high magnetic field up to 80 T,and found a new ultra-high frequency up to 2893 T.It is very close to the frequency in Na1.9Mo6O17 in high field,indicating that both of the compounds have the similar Fermi surface in high field.The controversy of the Fermi surfaces between two compounds is resolved.In addition,we study the transport properties of 20 layers K0.9Mo6O17 in pulsed field.The fourth chapter presents the effect on modulating the density wave ground state,superconductivity and electronic structure in η-Mo4O11 and Li0.9Mo6O17 by hydrostatic pressure.We verify that the thermal effect of the pressure cell in the pulsed field is acceptable.The interlayer and in-plane transport properties of η-Mo4011 under pressure are studied systematically.Through comparative studies between two directions,we find that the phase transition induced by magnetic field is suppressed by pressure.Based on the results above,related phase diagrams and high field Fermi surfaces under pressures are constructed.On the other hand,we introduce the superconducting transition of Li0.9Mo6O17 under pressure.The fifth chapter clarifies the nonlinear transport properties of Li0.9Mo6O17 under the steady field and pulsed field.The electron dynamics behavior in the nonlinear region is described and the modulation of the density wave gap by magnetic field is explained in combination with the properties of high field magnetoresistance.The I-V characteristic measurement based on flat top pulsed magnetic field is introduced emphatically.The influence of current thermal effect is found in the testing process,and the thermal effect is controlled in an acceptable range.It is also the first time to study the nonlinear I-V characteristics in pulsed field,which largely reveals the trend of density wave phase transition in such materials under magnetic field.The sixth chapter summarizes the research content of this paper and prospects the follow-up work.