Study Of Electronic Phase Diagram In Unconventional Superconductors By Tunneling Spectroscopy

Since the development of the tunneling spectroscopy in the 1960s,it has be-come an essential method to study the unconventional superconductivity.Due to the high energy resolution,tunneling spectroscopy can be used to detect su-perconducting energy gap,pseudogap,electron-boson coupling etc.In this the-sis,we studied tunneling spectra on several unconventional superconductors by point-contact technique and introduced the design and setting up of a new Com-bined Laser Molecular Beam Epitaxy and Scanning Tunneling Microscope system(Combi-LMBE-STM system).The main contents are as follows:(1)We studied the origin of several anomalies in the point-contact tunneling spectra of(Li1-xFex)OHFeSe,LiTi2O4 and La1.9Ce0.1CuO4-?.Although these features are quite similar to some intrinsic superconducting properties,such as Andreev reflection,multigap superconductivity,p/d-wave pairing symmetry,they cannot be accounted for by modified BTK theory,but stem from critical current effects.It is important to make a distinction between these anomalies and intrinsic superconducting properties.(2)We studied the electrical transport properties and tunneling spectra of the[001]-oriented LiTi2O4 thin films.An anomalous magnetoresistance(MR)was observed which changes from nearly isotropic negative to prominently positive with the decrease of temperature.We suggested that the positive MR can be ascribed as the orbital-related state which is also supported by the fact that the energy gap decreases as a quadratic function of magnetic field.The negative MR stems from local spin fluctuations or spin-orbit scattering.(3)We studied the tunneling spectra of the[111]-,[110]-,and[001]-oriented LiTi2O4 thin films.Several prominent bosonic modes are observed in the[111]-and[110]-oriented films but not in the[001]-oriented ones,and these modes exist at T = 2Tc and beyond the Hc2.The energy scale of these modes is close to phonon modes which can be confirmed as electron-phonon coupling.We suggested that the anisotropic electron-phonon coupling origins from oxygen vacancies enhanced Jahn-Teller distortion.The robust phonon modes in tunneling spectra may be enhanced by orbital-related state.(4)We investigated the origination of normal state gap(NSG)in Pr2CuO4±?thin films.The NSG can be detected in several films with various Tc.We found that the NSG is insensitive to magnetic field but can be suppressed by temper-ature.More importantly,the magnitude of the NSG increases with increasing point-contact junction resistance on the superconducting films,which is further enhanced in the nonsuperconducting samples with more oxygen disorder.We ar-gued that the NSG stems from disorder enhanced electron-electron interactions.(5)We introduced the design,setting up and debugging of a new Combi-LMBE-STM system.The system is consist of Combi-LMBE chamber,STM cham-ber,sample transfer chamber,preparation chamber,load-lock chamber etc.It can be used to grow combi-films and measure STM/STS in-situ.So far,the vacuum can be better than 5×10-10Torr,and temperature is down to 6K,and the holding time for liquid Helium is around 48h.We have successfully obtain atomic resolu-tion in room temperature and atomic steps in low temperature.The key technical indexes have already met the requirements.The first run is expected in recently.