Effects Of Pressure On Superconductivity In Titanium Oxide And Molybdenum Carbide

Superconductivity has been attracting widespread attention due to its broad application.Although many superconducting systems have been discovered,the superconducting mechanism other than conventional metals and alloys are not clear.Pressure as an important thermodynamic parameter has a direct impact on the crystal lattice,providing a clean way to study superconductivity.Once a new superconducting material is discovered,people are eager to know the pressure effect.The large pressure response prompt researchers to use atomic substitution to form chemical pressure or growth epitaxial film to form stress to achieve higher Tc under ambient pressure.Furthermore,enormous number of new superconducting materials have been discovered under pressure for decades,greatly expanding the range of materials for superconducting research.In addition,clues on the superconducting mechanism also can be obtained from the changes of superconducting properties under pressure.In the dissertation,we have studied the pressure effects on the superconducting epitaxial TiO thin films and Mo2C nanosheets which newly discovered in recent years.The variations of the superconductingand transition temperature and other basic superconducting properties of TiO,α-Mo2C β-Mo2C under pressure are revealed,which are helpful to understand the material properties and superconducting mechanism.In addition,we have researched the predicted multiferroic in the spin ladder superconductor BaFe2Se3 by the measurement of structure,dielectric and magnetic dielectric properties at different temperatures.The dissertation divided into five parts as follows:In Chapter 1,we introduced how pressure affects the superconductivity of various superconductors,including traditional superconductors with accepted theoretical foundations,cupates superconducting systems and iron-based superconducting systems with higher transition temperatures.There are also some special systems in which pressure plays an important role,such as organic superconducting systems that are very sensitive to pressure,and topological superconducting systems that have been extensively studied in recent years.Finally,we explained how pressure plays a pivotal role in improving superconducting T,and discovering new superconducting materials with some recent important research progress.In Chapter 2,we systematically studied the electrical transport properties of TiO epitaxial films under hydrostatic pressure.In TiO,we found that the superconducting transition temperature Tc decreases with increasing pressure,and the decrease of Tc is much faster than theoretically expected.By analyzing the normal transport properties,we found that the localization of carriers in the TiO film is significantly enhanced under pressure,indicating that carrier localization plays a crucial role in the superconductivity of TiO.In Chapter 3,we found that Mo2C grown by chemical vapor deposition have regular sample shapes,mainly in the form of quadrilateral and hexagonal.We studied the crystal structure by scanning transmission electron microscopy and found that the quadrilateral Mo2C is a phase with orthogonal structure,and the hexagonal Mo2C is β phase with hexagonal structure.We compared electric transport and superconductivity properties of the two phases Mo2C and found that the superconducting transition temperature and the upper critical field are quite different.Futhermore,the effects of grain boundaries were studied.In Chapter 4,we found that the two phases of Mo2C have different pressure responses by studied the electrical transport properties of a-Mo2C and β-Mo2C under high pressures.β-Mo2C is similar to conventional superconductors,a linear decrease of Tc with increasing pressure indicates an increase of the average vibration frequency of phonons under pressure.The Tc of α-Mo2C shows a dome-shape profile under pressure,which indicates that in addition to the change of the phonon properties,the pressure also has a significant effect on the electronic properties in the a-Mo2C.These results indicate that the α-Mo2C electronic properties are particular compared to the conventional BCS superconductors and β-Mo2C.In Chapter 5,we have systematically studied the temperature dependencies of structure,magnetic,electrical transport,dielectric,and magneto-dielectric properties of BaFe2Se3 single crystal.The lattice parameters show continues Debye thermal expansion throughout the temperature range from 10 K to 300 K with a structural anomaly at about 125K.The electrical transport measurements suggest that the conductivity follows thermal activated transport mechanism in two discrete temperature ranges.The inevitable thermal activation of electronic conduction affects the dielectric behaviors significantly.The magneto-dielectric coefficient is about 6%in BaFe2Se3 compound.However,narrow energy gap,smaller polarization,lower resistivity as well as the inevitable defects make the ferroelectricity of BaFe2Se3 difficult to be characterized.