Physical Properties Of Several New Pnictides Quantum Materials At Cryogenic Temperature

Quantum material is a broader description of various materials with emergent phenomena in condensed matter physics in recent years,which including superconductors,spin liquids,heavy fermions,topological insulators,and topological semimetals.The pnictides can adopt a variety of structures,and their properties can be tuned by temperature,magnetic field,and pressure in a wide range.Our group has rich experience in the research of pnictides quantum materials,with emphasize in searching for novel superconductors and studying new topologically non-trivial materials.In this thesis,we studied several pnictides quantum materials and the main results are as follows:1.We have successfully synthesized La₃Cr_10-xN₁₁ polycrystalline samples by solid state method,which is isostructural with cubic nitride Pr₃Cr_10-xN₁₁.The superconducting transition temperature T_c from resistivity measurement is 4.6 K,and magnetization and specific heat measurements confirm it is bulk property.The upper critical field of H_c2 is about 12.2 T,exceeding the Pauli paramagnetic limit.Electronic structure calculations show that most of density of states at the Fermi energy is contributed by Cr 3d electrons,thus it is a new Cr based superconductor.However,compared to La₃Cr_10-xN₁₁ and Pr₃Cr_10-xN₁₁,Ce₃Cr_10-xN₁₁ by rare earth element substitution does not show any signature of superconductivity above 1.8 K.2.We have successfully synthesized high quality CaMn₂P₂ single crystals by flux method.The compound adopts the trigonal CaAl₂Si₂-type structure,different from that of 122 series iron based superconductors.It has an insulating ground state.Magnetization measurements show no apparent magnetic phase transition under 400K,nor a large ordered magnetic moment.Different from other AMn₂Pn₂(A=Ca,Sr,and Ba,and Pn=P,As,and Sb)compounds with the same structure,heat capacity C_p(T)and?(T)reveal that CaMn₂P₂ has a first-order transition at T=69.5 K,which is not affected by magnetic field.The transition shifts to higher temperature upon increasing pressure and the compound can not be metalized at least below 10 GPa.The emergence of new peaks in Raman spectra below the transition,clearly suggests a change in crystal symmetry through the transition.The combination of the structural,transport,thermal and magnetic measurements suggests that CaMn₂P₂ is a insulator showing correlation effect both above and below the transition.3.We have successfully synthesized high quality SrAs₃ single crystals by flux method.SrAs₃ has been predicted to be a topological nodal ring semimetal with a low crystalline symmetry monoclinic structure.We utilized Focused ion beam(FIB)technique and fabricated a series of devices,in which the excitation current flows strictly along different crystalline principle axis,then carried out low temperature,high magnetic field electric transport measurements.The data from devices is in good accordance with data from SrAs₃ single crystals.We clearly observed the anisotropy of resistivity,the bulk Fermi surfaces are determined by quantum oscillation in magnetoresistivity.By combining quantum oscillation and Hall data,it is shown that SrAs₃ has multiple charge carriers with low density and high mobility,similar to other topological semimetal.However,a robust negative longitudinal magnetoresistance is observed when the external magnetic field is aligned along both a-and c-axis,after excluding other possible explanations,we suggest it should be caused by the existence of topologically protected nodal ring in SrAs₃.