| Superconducting materials correlated with the charge order and spin order are not only the re-search hotspot in physics,but also closely relate to future life and modern science and technology with the huge application prospects.Low-dimensional molybdenum purple bronzes have always been at-tracting the attention of scientists because of the low-dimensional characteristics and easy manipulation of the electrons,especially a series of charge density wave and superconducting transitions at low tem-peratures.In addition,the development of pulsed field technology provides a powerful experimental platform for further research in the strongly correlated systems.Combining high magnetic fields with low temperatures,the interaction between the spin and charge orders can induce many novel quantum transport phenomena,and then help to reveal the relevant mechanisms.Therefore,this paper system-atically presents the transport properties of three typical low-dimensional molybdenum purple bronzesη-Mo4O11,γ-Mo4O11and Li0.9Mo6O17under high magnetic fields.The main research contents of this paper include the following parts:The first chapter introduces some basic physical concepts related to the charge density wave,as well as the research progress onη-Mo4O11,γ-Mo4O11and Li0.9Mo6O17.In addition,we provide an overview on the long-standing research issues of the charge density wave state in purple bronzes and highlight the importance of the high field studies on the related systems.The second chapter introduces two single crystal growth methods of the chemical vapor transport and the temperature gradient flux.We also introduce the experimental platform under the DC field and pulsed field.Finally,we describe the electrical transport system and its measurement principle in pulsed fields.In the third chapter,the phase transition and quantum oscillations induced by high magnetic fields are studied forη-Mo4O11.In the high-field magnetoresistance measurements,we observe the field-induced phase transition and high-frequency quantum oscillations ofη-Mo4O11.Through Fourier analysis,it is found that the Fermi surfaces corresponding to these high-frequency quantum oscilla-tions are consistent with the Fermi surfaces of the normal state.On the other hand,the Hall resistance ofη-Mo4O11gradually decreases and finally tends to zero in high magnetic fields,indicating that the carriers increase with the magnetic field.Therefore,the quantum oscillations in the high-field state and the low-field state should have different origins.We find that the Fermi surface ofη-Mo4O11is increasing with the magnetic field.Finally,the Fermi surfaces of the normal state are induced while the energy gap is completely suppressed above 45 T.We propose a model of the Fermi surface recon-struction,which well explains the abundant properties induced by high magnetic fields ofη-Mo4O11.Chapter 4 presents the magnetization,specific heat and transport studies onγ-Mo4O11under high fields.We first observe nonlinear transport characteristics inγ-Mo4O11.We find that the inter-layer magnetoresistance ofγ-Mo4O11shows obvious quantum oscillations in high-field state.Through systematic analysis,we propose that the energy gap ofγ-Mo4O11is nearly fully opened at low temper-atures.Therefore,nonlinear I-V characteristics similar to those in the quasi-one-dimensional molybde-num blue bronzes can be observed.A small amount of residual carriers undergo a magnetic breakdown in high magnetic fields up to 60 T,resulting in the quantum oscillations.Finally,we systematically compare the similarities and differences betweenη-Mo4O11andγ-Mo4O11in the related properties of charge density wave.We propose that there is a larger energy gap inγ-Mo4O11thanη-Mo4O11.Thus,higher magnetic fields are required to achieve the higher magnetic breakdown frequency inγ-Mo4O11.In chapter 5,we systematically study the unconventional superconductivity and rich phase tran-sitions induced by high magnetic fields in the quasi-one-dimensional molybdenum purple bronze Li0.9Mo6O17.The study on the upper critical fields supports Li0.9Mo6O17as an ideal candidate with triplet superconducting pairing symmetry.We also find that the I-V curves along the three crystallo-graphic axes of Li0.9Mo6O17show significant nonlinear transport behaviors.This nonlinear feature is significantly enhanced under the external magnetic fields.The nonlinear transport behavior may be related to the coexistence of space charge effect,local collisions,and ionization avalanche break-down effect.Several distinct phase transitions are observed in the multi-directional magnetoresistance measurements of Li0.9Mo6O17in high magnetic fields.Based on the results,we construct the phase diagram,and propose that the complex competition between superconducting order and density wave order is responsible for the multiple phase transitions of Li0.9Mo6O17.In the sixth chapter,we summarize the research content of the full text,and provide the perspec-tives on the related studies. |
