The Superconductivity And Quantum Phase Transition Of Titanium Oxide And Lanthanum Oxide

Transition metal oxides exhibit manifold physical properties comprising high temperature superconductivity,piezoelectricity,ferroelectricity,magnetism,and multiferroicity and so on.The functional properties of transition metal oxides are very sensitive to the details of electronic structure and thereby strongly influenced by the elemental compositions and defects or lattice distortions.Among these,titanium oxides represent an important class of materials due to their large variety in crystal structures and physical/chemical properties.They cover a wide range of electronic properties from metals to semiconductors and insulators and are,therefore,used in many technological applications.In recent years,the superconducting properties of titanium oxides have attracted much attention,such as in TiO,?-Ti3Os,Ti4O7,and the mechanism of superconductivity is unclear.Quantum Griffiths singularity(QGS)recently discovered is another hot point and drew tremendous attention.High quality TiO epitaxial thin films with insulating normal states offers a more universal platform for investigating QGS.Rare earth oxides also show remarkable chemical,thermal,optical,and electrical properties.Rock-salt structure LaO is recognized as an carrier transporting layer embedded in mother compounds of high-temperature superconducting layered perovskite cuprates(La,M)ZCuO4(M = Ba,Sr).On the other hand,lanthanum monoxide bulk polycrystal,synthesized several decades ago,has shown ordinary metallic conduction,and further investigations have been scarcely reported,probably due to its poor chemical stability.Superconductivity and hydrostatic pressure effect in Rock-Salt Structure LaO epitaxial thin films are worthy studying issues.In chapter l,we summarized the crystal structures and transport properties of titanium,lanthanum and its oxides,as well as the research progresses of titanium oxide superconductors;and introduced the quantum phase transition(QPT)behavior in superconductors and the research progress of quantum Griffith singularity in recent years.In chapter 2,the superconducting cubic TiO thin films were epitaxially grown on the(0001)-oriented ?-Al2O3 single crystalline substrates,and the superconducting properties of the films were systematically characterized.It was found that the superconducting transition temperature is enhanced to about 7.4 K,and the upper critical field at zero field Hc2(0)is about 13.7 T(H//(111))in the TiO thin films.The high-pressure experimental results indicated that increasing pressure weakens the superconductivity.Our work will facilitate prospects for understanding the superconducting mechanism in the titanium-based oxide superconductors as well as achieving higher temperature superconductivity.In chapter 3,the I-V characteristics and critical current density J,in TiO epitaxial thin films were systematically studied.A quasi-2D VG phase transition was revealed by analyzing the scaling behaviors of the I-V characteristics.The temperature dependence of J,at different fields indicated that ?1 pinning is the dominant pinning mechanism at the temperature range measured in the TiO thin films.In addition,a clear scaling behavior of the normalized field dependence of vortex pinning forces was observed.In chapter 4,we report the thickness and magnetic field tuned superconductor-insulator transition(SIT)with QGS in TiO thin films.The film thickness-tuned superconductivity and SIT should be related to the reduction of carrier concentration and enhancement of localization.Through investigating the low temperature transport properties of TiO thin films with different thicknesses,a magnetic field-induced SIT with diverging dynamic critical.exponent is obtained,which shows a suppressed quantum Griffiths state by the weakly insulating normal state.The QGS of SIT in the TiO thin films expands the evidence of the QGS from superconductor-metal transition(SMT)systems to a SIT system,and the obtained phase diagram of this SIT system with QGS may be applicable to different crystalline superconducting systems.These results provide considerable insights into the superconducting QPT,which is helpful for designing quantum computing devices.In chapter 5,the epitaxial La and LaO thin films were grown on commercial(110)-orented YAlO3 and LaAlO3 substrates by a pulsed laser deposition(PLD)technique.We systematically studied the superconducting properties of La and LaO epitaxial thin films under hydrostatic pressure up to-2 GPa.For La thin film,with increasing pressure,T,and normal state resistivity gradually increase.But for LaO thin film,with increasing pressure,Tc gradually increases,while the normal state resistivity near the superconducting transition first decreases and then increases.