The Structural And Electrical Transport Properties Of Transition Metal Dichalcogenides With Different Stacking Orders Under High Pressure

Transition metal dichalcogenides（TMDs）are one of the most promising two-dimensional quantum material systems for applications,possessing rich physical properties in electrical,magnetic,and optical aspects due to their complicated crystal lattice,special electronic state,intrinsic electron-phonon coupling,and electronic correlation.In correlated electronic systems,understanding the relationship between superconductivity,charge density wave（CDW）,magnetic order,and other quantum states by exploring the lattice,charge,spin,and the interaction between them has become one of the hottest and most challenging research areas in condensed matter physics.Although there are many phase diagrams have been established in TMDs by combining with chemical doping,gating,reduced dimension,high pressure,etc.,the relationship between superconductivity and charge density waves in these phase diagrams is still puzzling.How crystal structure and electronic structure modulation determine the superconductivity and CDW properties of the system is still unclear.These problems are mainly due to the fact that the superconductivity and charge density waves of TMDs systems are greatly dependent on the stacking sequence,chemical composition,and coordination structures.Furthermore,existing characterization methods（such as angle-resolved photoelectron spectroscopy,scanning tunneling microscopy,neutron scattering,etc.）are difficult to combine effectively with in situ modulation methods.The combination of these factors severely limits the further exploration and development of superconductivity and charge density waves in TMDs.Based on the above problems,it is necessary to study TMDs with different interlayer coupling effects such as stacking sequences,chemical components,and coordination structures,and combine in situ regulation methods and characterization methods to reveal the influence of these factors on superconductivity and charge density waves.This thesis focuses on the high-pressure regulation of the physical properties of4Hb-and 6R-TMDs,which are alternately stacked by a trigonal prismatic（H）layer and an octahedral（T）layer.Through pressure regulation,we discover for the first time the competition between T-layer CDW and H-layer superconductivity;pressure-induced‘double layer superconductivity’of TaSe₂,and‘double-domed superconductivity’of TaS₂,and‘double-domed superconductivity’of self-intercalated 2Ha-Nb_1.08Se₂ in which no CDW is involved.Combined with the first-principles calculations we revealed the influence of the charge transfer to the‘double layer superconductivity’,and the origin of the superconducting dome of TaS₂ and 2Ha-Nb_1.08Se₂ has been revealed by Hall effect measurements,upper critical measurements,in situ high-pressure synchrotron X-ray diffraction（XRD）,and high-pressure Raman.The main results are presented below:1.Pressure-induced‘double layer superconductivity’within 4Hb-TaSe₂ and 6R-TaSe₂.Using high temperature and high pressure,we have successfully synthesized 4Hb-TaSe₂ and 6R-TaSe₂ single crystals.At ambient pressure,the superconducting behavior of the H-layer in 4Hb-TaSe₂（T_c=2.5 K）and the charge density wave of the T-layer of6R-TaSe₂(T_CDW=350 K)were observed for the first time.By combining in situ high-pressure electrical transport,high-pressure synchrotron X-ray diffraction,high-pressure Raman and the first-principles calculations,we discovered the pressure-induced‘double layer superconductivity’phenomenon in 4Hb-TaSe₂ and 6R-TaSe₂ for the first time in the TMDs system.More interestingly,we observed for the first time the competition between the superconductivity of the H-layer and the CDW of the T-layer in the superconductivity and CDW phase diagram of 4Hb-TaSe₂.This phenomenon is different from the competition between superconductivity and CDW for the same coordination layer in the past,which not only enriches the phase diagram of the existing TMDs system but also plays an important informative role for the preparation of CDW-based heterojunctions in the future.By comparing the phase diagrams of different coordination layers within 4Hb-TaSe₂ and corresponding bulk materials（1T and 2Ha-TaSe₂）,and combining theoretical calculations,we show that different coordination layers have significantly different responses to charge transfer.2.Pressure-induced‘double-domed superconductivity’within 4Hb-TaS₂ and6R-TaS₂.Based on the above research on TaSe₂,which is alternately stacked by T-layer（metal,CDW）and H-layer（superconductivity,charge density wave）,we also investigated the TaS₂ alternately stacked by T-layer（Mott insulator,CDW）and H-layer（superconductivity,charge density wave）as well.We synthesized 4Hb-TaS₂ and 6R-TaS2 single crystals by chemical vapor transport,and high-temperature quenching methods,respectively.The superconducting transition temperature of the two structures was almost the same at ambient pressure（T_c～3 K）,and anomalous signals of the electrical transport signals caused by CDW can be observed in each layer.Unlike4Hb（6R）-TaSe₂,4Hb（6R）-TaS₂ did not show‘double layer superconductivity’behavior in the highest-pressure range of the experiment,but the superconducting transition temperature of the H-layer of the two superconducting phase diagrams appeared in‘double-domed superconductivity’with pressure.Similar to 4Hb-TaSe₂,the superconducting‘dome’of the low pressure range is mainly dominated by the competition between the superconductivity of the H-layer and the CDW of the T-layer.However,unlike the superconducting re-enhancement phenomenon caused by the structural phase transition of the corresponding bulk material 2Ha-TaS₂under ultrahigh pressure,the re-enhancement of superconductivity in 4Hb-TaS₂ and 6R-TaS₂ is related to the reconstruction of Fermi surfaces.In addition,we observed abnormal signals of new electrical transport in high-pressure electrical transport experiments of 4Hb-TaS₂and 6R-TaS₂,which may be the result of pressure-induced formation of new CDW states.These results from TaS₂,combined with the above high-pressure phase diagrams of 4Hb-TaSe₂and 6R-TaSe₂,have important implications for understanding the stacking sequence and the influence of chemical components on the superconductivity and CDW states.3.Pressure-induced‘double-domed superconductivity’within 2Ha-Nb_1.08Se₂ with no CDW statesTo gain further insight into the inherent mechanism of the second superconducting dome with respect to the phase diagram of 4Hb（6R）-TaS₂,we synthesized the self-intercalated 2Ha-Nb_1.08Se₂ by high temperature and high pressure,and no superconducting and charge density wave signals were observed above 2 K at ambient pressure.The phase diagram of the superconducting transition temperature of 2Ha-Nb_1.08Se₂ with pressure also shows‘double-domed superconductivity’.Unlike the superconducting phase diagram associated with the charge density wave mentioned above,the‘double-domed superconductivity’phenomenon of 2Ha-Nb_1.08Se₂ is related to the electron interaction regulated by the carrier concentration in the recently proposed dirty limit case,i.e.,when the carrier concentration is low to high,the interaction between electrons will cross over from local to non-local,and when k_F l^*1（k_F is the Fermi wave vectors,l^*is the distance between electrons to attract each other）,the superconducting transition temperature appears maximum,forming a superconducting‘dome’shape.