| Transition metal dichalcogenides(MX2)have been the focus in recent years due to their attractive physical properties and potential application prospects.Among many MX2 materials,TaS2 is a representative material with important research value because of the rich phases and charge density wave(CDW)transitions.In this dissertation,we focus on TaS2,and modulate the ground state and physical properties of this system by various method,such as element doping,intercalation,high magnetic field and so on.The chiral CDW,superconductivity,topological properties magnetoresistance and other novel electronic states in the system under the tuning effect were investigated in detail.The main research contents of the dissertation are as follows:1.Investigate the origin of the chiral CDW induced by Ti-doping in 1T-TaS2.We obtained a complete electronic phase diagram of the Ti-doped 1T-TaS2 system through transport measurements,and found that the abnormal resistivity at the low temperature hidden under the electronic phase diagram appeared in the sample with the doping amount of 0.08.Detailed scanning tunneling microscopy(STM)measurements showed that unexpected chiral CDW phase also occurred in the sample with x=0.08.Combined with the results of first-principles calculations,we attribute the emergency of chiral CD W phase to the enhanced orbital order by Ti-doping in the center of David-star.In addition,compared with other 3d transition metal elements,only Ti doping can maintain the CDW state(the feature of flat band)of 1T-TaS2,which is also a prerequisite for the appearance of the chiral CDW.2.PbTaS2 single crystals with the same structure of 2H-TaS2 were obtained through Pb intercalation,and its superconductivity and topological properties were studied.According to the results of X-ray diffraction(XRD)measurement,the centrosymmetric structure in PbTaS2 can be obtained,and the dynamic stability of this structure is also verified by the first-principles phonon spectrum calculation.Combined the detailed measurements of magnetic and electrical transport and specific heat,PbTaS2 is determined to be a weakly coupled type-Ⅱ superconductor.The results of first principles calculations predicted that PbTaS2 with a centrosymmetric structure is also a topological Nodal-line semimetal with "drumhead-like" surface states.The results above suggest that PbTaS2 may be a potential topological superconductor.3.Investigate the origin of the large magnetoresistance(MR)in 4Hb-TaS2.Combining first-principles calculations and electrical transport measurements under the high magnetic field,the structure and electrical properties of 1T,2H and 4Hb-TaS2 are comparatively studied.We found that only the resistance of 4Hb-TaS2 exhibits a strong magnetic field dependence at the lower CDW transition,i.e.,a large MR appears.Through detailed energy band structure calculations,scanning tunneling spectroscopy(STS)and Hall effect measurements,the large MR is attributed to the combined effect of partial electron-hole compensation and high mobility holes at low temperatures.4.Investigate the effect of Cu intercalation on the ground state and electronic state of 1T-TaS2.We have grown Cu-intercalated 1T-TaS2(the intercalation amount is less than 0.05)successfully by the method of chemical vapor transport(CVT),and obtained the intercalation of Cu atoms by energy-dispersive spectroscopy(EDS)and scanning transmission eletron microscope(STEM),but the original layer spacing was not changed.The electrical transport measurement results show that even a small amount of Cu intercalation can cause a sharp change in the property of ground state and commensurate CDW(CCDW)transition.According to the results of STM/STS measurement,we also found that the obvious domain structures appear in the sample with the largest intercalation amount,and there are two kinds of electronic states(insulating state and metallic state)in different domains.We speculate that the novel electronic state may be caused by the charges transfer from Cu to TaS2 layers or the destruction of the interlayer dimerization in 1 T-TaS2 due to the intercalation of Cu. |
PDF Full Text Request