Single Crystal Growth And Physical Properties Of Quantum Functional Materials

Quantum functional materials present a variety of novel quantum effects and rich physical phenomena due to the coupling and competition of charge,spin,orbit,lattice and some other quantum orders.In this dissertation,we mainly introduce the crystal growth and research of physical properties of several kinds of quantum functional materials,including the crystal growth and related physical properties research of"111"type iron-based superconductors,the crystal growth and pressure effect research of the newly found topological insulator KHgAs,which possess"hourglass"surface state,the crystal growth and pressure effect research of Fe₂As compound,which is isostructural to"111"type iron-based superconductors.The main results are as follows:1.The crystal growth and systematic rsearch using scanning tunnelling microscope of Li Fe_1-xCo_xAs.We have grown a series of high-quality single crystals of Li Fe_1-xCo_xAs and used scanning tunneling microscopy to systematic study the global effects of a finite density of Co substitution on the superconducting ground state.With the increase of Co concentration,we observe that the global superconducting gap is progressively suppressed and the spectral bottom evolves from a U shape to a V shape.By quantitative analysis,we find that the coupling strength 2?_L/K_BT_C remains a constant as the Co concentration x increases,suggest that the superconductivity is destroyed via a mechanism which decreases the pair susceptibility strength,but not the coupling strength.Our systematic experimental-theoretical analysis of the Co impurity effect uncovers that the Born-limit nonmagnetic scattering is the driving force of the superconducting quantum phase transition in Li Fe_1-xCo_xAs.2.The crystal growth and V-doping effect study of NaFe_1-xV_xAs.We have grown a series of high-quality single crystals of NaFe_1-xV_xAs,which were characterized via magnetic susceptibility,resistance,and specific heat,to study the hole-doping effect of NaFe As.The phase diagram of NaFe_1-xV_xAs was obtained.Both the structural and magnetic transitions are gradually suppressed with increasing V concentration.The superconducting transition temperature T_c increases from ～8K of the parent NaFe As to the maximum value of ～15K at the optimal doping level x=0.007,and then decreases rapidly to zero when x=0.012.It is speculated that the strong scattering effect of V impurities dominates the suppression of T_c and V doping has a small effect on the structural and SDW transitions.3.Crystal growth and investigation of pressure effect of hourglass fermion compound KHgAs.We have successfully growth high quality single crystals of newly found hourglass fermion compound KHgAs,which were characterized via transport properties under high pressure,synchrotron radiation X-ray diffraction combined with crystal structure predictions.The transport measurement under high pressure indicated that,with the increase of pressure,KHgAs transforms from semiconducting behavior into metallic state and finally induce superconductivity at about 21GPa.The transition temperature of onset increases rapidly at first and then been suppressed slowly as the pressure increases.High pressure X-ray diffraction indicates that there exist three high-pressure phases up to 55GPa.Interestingly,the superconductivity appears at around 21GPa is coincide with the emergence of the second high-pressure phase（phaseⅢ）,this result indicates that there may be some intrinsic connection between superconductivity and phase III.High pressure X-ray diffraction combined the crystal structure prediction indicates the crystal structures of three high-pressure phases are Pnma structure,P6₃mc structure,and P6₃/mmc-Ⅱ structure,respectively.The current result will enrich the research on the topological superconductors.The results of structure transition under high pressure will also have certain guiding significance to materials of similar structure.4.Crystal growth and investigation of high-pressure physical properties of Fe₂As.We have growth Fe₂As single crystals and the study of its physical properties via transport properties under pressure and high-pressure synchrotron radiation X-ray diffraction.According to the transport properties under pressure,within 100GPa,Fe₂As keeps its metallic state and no superconductivity is observed above 2K.The high-pressure X-ray experiments indicate a high-pressure phase transition happens at about 27GPa,which rules out the possibility of pressure-induced superconductivity in the tetragonal P4/nmm phase of Fe₂As.After the refinement of the high-pressure X-ray data,the pressure dependence of lattice constants and unit cell volumes for P4/nmm phase were plotted and the bulk modulus B₀ was obtained to be 168.6GPa.