Scanning Tunneling Microscopy/Spectroscopy Of Transition Metal Dichalcogenides

Although high temperature superconductors?HTSCs?have been discovered for more than thirty years,understanding of unconventional superconductor phase diagrams remains an unsolved mystery.The coexistence of superconduc-tivity and other ordered states is a common phenomenon in most unconven-tional superconductors,such as antiferromagnetic orders?AFM?,charge density wave?CDW?,spin density wave?SDW?,nematicity,checkerboard,stripe,and so on.Some of them may play an important role in the superconducting pair-ing state or even directly participate as the pairing glue.Therefore,it's help-ful for exploring the mechanism of superconducting pairing by understanding the microcosmic coexistence mechanism of multiple ordered states.However,the complexity of the phase diagrams brings much difficulties.Therefore,take transition metal dichalcogenides?TMDs?as model systems which contain both superconductivity and CDW will provide a appropriate reference to HTSCs.In this thesis,we mainly introduce our research on the relation of superconductiv-ity and CDW in the typical TMDs material 2H-NbSe₂ by Scanning Tunneling Microscopy/Spectra?STM/STS?.We focus on the competition between superconductivity and CDW in Ta doped 2H-NbSe₂.CDW transition temperatures associated with the resistance anomaly decrease in pace with the doping of Ta,and the superconducting tem-peratures increase gradually,which shows a possible competitive relationship.Superconducting temperature drops rapidly once CDW feature disappeared,sen-sitive to the magnitude of residual resistance ratio?RRR?.Topographic images of samples without CDW feature on the Resistance-Temperature curves show a patch of local short range CDW domains.The suddenly dropping of Su-perconducting temperatures occur after the correlation length been compara-ble with superconducting coherence length ?.2H-TaxNb1-xSe2 show large lin-ear magneto-resistance?LMR?in the CDW state and obey Kohler's rule,while deviate these regularities after disappearance of the CDW resistance anomaly.LMR and Kohler's rule breakdown when CDW is suppressed by high pressure.Superconducting gaps observed on STS also increase in pace with the CDW decrease.In conclude,we think superconductivity compete with CDW in 2H-NbSe₂.We identify the fine structures on the low energy STS whose energies are identical with phonon energies are caused by the inelastic electron tunneling spectroscopy?IETS?,including the kinks in tunneling spectra at ±35mV,which historically were identified as CDW gap edges.The intensity of these fine struc-tures in d2I/dV2 is modulated by the charge density,this may reflect the role of electron-phonon coupling playing in the mechanism of CDW.A new CDW emerges in a distorted area on the surface of 2H-NbSe₂,coexisting with the famil-iar 3×3 CDW on the microscopic scale,which is different from report on the same system about another unidirectional CDW domains neighboring 3x3 CDW.It have an apparent impact in the relatively high energy electronic states without altering the electronic states near the Fermi level at all.The new phenomena re-fresh the understanding of CDW in 2H-NbSe₂ under strain,and may be helpful to reveal the mechanism of CDW.We investigated the local 1T phase on the cleaved surface of a 2H-NbSe₂ single crystal after introduction of tip-sample interactions through bias voltage pulses during tunneling at low temperatures.A CDW with a???×???reconstruction was observed and the well-known 3x3 CDW was observed far away.The enhanced electron correlation provides excellent oppor-tunity to clarify the relation between correlation and superconductivity,helping to explore the mechanism of unconventional superconductors.We observed t-wo types of strain-induced superstructures on the cleaved surface of 2H-NbSe₂ single crystal.Distinct change occurs in the density of states?DOS?from the triangular phase to the stripe one.However,the detected superconducting gaps are nearly homogeneous for all domains in the same layer,while very different between the layers below and above a step.Furthermore,the neighboring layers have an identical critical temperature much smaller than that of the bulk.We could thus conclude that the superconductivity observed on this reconstructed surface is induced by proximity effect due to its adjacency to the underlying bulk 2H-NbSe₂.Modulation of superconductivity caused by the charge ordering of the superstructure need more theoretical consideration and may shed light on the correlation between superconductivity and CDW.WTe₂ has attracted tremendous attentions due to its various novel proper-ties.We have studied the local density of states?LDOS?on the freshly-cleaved surface of WTe₂.The measured LDOS is in good agreement with our first-principle calculations,demonstrating WTe₂ to be a low-carrier-density semimetal with high mobility of charge carriers,which is critical to explain the extremely large magnetoresistance?XMR?in WTe₂ like in some other semimetals.Mean-while,the magnetic field driven metal-insulator transition is excluded by the measurements of field dependent LDOS.A very sensitive response of the LDOS to the inherent surface deformation has been observed,presenting a unique way to understand the electronic properties of the semimetals like WTe₂.In the next two chapters I mainly introduce my participating in work of instrument building.The first one is the project of helium-3 unltra-low temper-ature STM,and the other are point-contact spectroscopies based on PPMS.