Designing Novel Quantum Materials Based On Two-dimensional Square-net Structural Motif

The materials containing two-dimensional square lattices,such as copper oxide high-temperature superconductors with tetragonal Cu O layers and iron-based superconductors with tetragonal Fe As/Se layers,usually exhibit novel physical properties.In recent years,due to the unique band structure,these materials containing two-dimensional square lattice have attracted extensive attention in exploring new quantum materials.By tuning the symmetry,vacancies,in-plane distortion and inserting magnetic layers,I have discovered four new magnetic or topological quantum material candidates based on two-dimensional square lattice.The main contents of this paper are as follows:First of all,in the parent compound of iron-based superconductor Sr Fe₂As₂,the tetragonal Fe As planes were doped with transition metal Mn.Based on the different symmetry of tetragonal Sr Fe₂As₂ and trigonal Sr Mn₂As₂,the existence of intrinsic Griffith phase has been confirmed by various techniques,including X-ray diffraction,Raman spectroscopy,scanning electron microscopy,X-ray energy spectroscopy,and scanning tunneling microscopy.By varying the doping concentration of Mn,the structural and magnetic phase diagram for Sr(Fe_1-xMn_x)₂As₂ was established.When the doping concentration is low（x<0.0973）,the spin density wave transition is gradually suppressed with the increase of Mn doping;with 0.0973<x<0.2055,Sr(Fe_1-xMn_x)₂As₂goes into the intrinsic Griffith phase,where a magnetic order emerges with the transition temperature abnormally increases with the increasing Mn doping;at higher doping concentrations,Sr(Fe_1-xMn_x)₂As₂ exhibits micro-scale phase separation（x=0.2055～0.4362）and a large misciblility gap（x=0.44～0.95）;When the concentration of Mn approaching 1,Sr(Fe_1-xMn_x)₂As₂ has a trigonal structure and acts as a narrow-bandgap antiferromagnetic semiconductor similar to Sr Mn₂As₂.Secondly,large-sized Hf Ge_0.92Te single crystals containing Ge square net with vacancies were grown for the first time by using Ge-Te flux,where multiple Dirac points and a possible two-dimensional spin orbit Dirac point were observed.Hf Ge_0.92Te crystalizes in a nonsymmorphic tetragonal space group P4/nmm（No.129）,and the vacancy of Ge square net is determined to be about 8%by single crystal X-ray diffraction.First principle calculations and angular-dependent magnetoresistance show quasi-two-dimensional features in the bulk crystals of Hf Ge_0.92Te,which can be easily exfoliated into thin flakes with a thickness of about 5 nm（6 atomic layers）.Despite the presence of Ge vacancies,multiple Dirac points including spin orbit Dirac point are still observed by angle-resolved photoemission spectroscopy.The spin orbit Dirac point mainly comes from the surface state according to the calculation,so it is posible to obtain a novel two-dimensional spin orbit Dirac point based on a Hf Ge_0.92Te monolayer.Hf Ge_0.92Te is a promising platform to explore topological phases and to study the intrinsic properties of two-dimensional spin orbital Dirac points.Thirdly,topological crystalline insulator candidate Er As S single crystal containing distorted As square nets was successfully grown by a chemical vapor transport method.Single crystal X-ray diffraction and high-resolution spherical aberration-corrected transmission electron microscopy indicate the distortion of As square net,where As atoms arrange with equal intervals along the b axis but unequally along the c axis.Due to the distortion,As atoms form zig-zag chains along the b axis,making the in-plane C₄symmetry changed into C₂ symmetry.The distorted As square nets stack along the a axis and there is a 180 degrees rotation between two neighbouring As square ntes,resulting in a new nonsymmorphic symmetry—a glide along the c axis.Paramagnetic Er As S is calculated to be a topological crystalline insulator with hourglass surface states,which are protected by the nonsymmorphic a glide along the c axis.Dirac cones with a large-energy-range linear dispersion were experimentally observed near the Fermi level,which is in good agreement with theoretical calculations.At low temperature,Er As S exhibits multiple magnetic phases,and the single crystal neutron diffraction reveals a collinear antiferromagnetic ground state（T_N=3.27 K）,which is calculated to be a trivial insulator with broken time reversal symmetry.At a little bit higher temperature(T_N<T<T_ICM=3.65 K),Er As S exhibits a wavelike-modulated incommensurate magnetic structure.Upon applying magnetic field,a magnetic pleateau phase and a field-induced ferromagnetic phase would be sequentially generated in Er As S.Due to the existence of multiple magnetic phases,Er As S is an experimentally available topological crystal insulator with hourglass surface states and magnetic-tuned topological phase transitions.Finally,a topological insulator candidate Ce Cu As₂ single crystal containing both As square net and Ce-Cu-As structural motif was successfully grown by a chemical vapor transport method.According to first principle calculations,the Dirac points near the Fermi level mainly come from the As square net and the Ce-Cu-As structral motif,showing a small global gap（～5 me V）.The in-plane semiconductor-like resistivity can be fitted with both the thermal activation model and Kondo effect at high temperature region,suggesting Ce Cu As₂ is a Kondo topological insulator candidate.A large negative magnetoresistance was observed in Ce Cu As₂ at low temperature,reaching-15%under 9 T at 2 K.By applying magnetic field with various directions,the negative magnetoresistance shows no strong dependence on the orientation of magnetic field relative to electric current,ruling out the possibility of chiral anomaly.Further investigation on the anisotropic magnetism of Ce Cu As₂ single crystal reveales a clear transition from paramagnetic to a possible spin-glass state with T_f～4.5 K.The Hall resistance exhibits obvious hole-dominant characteristics and the magnetoresistance is obviously dependent on the magnetization strength,indicating the existence of a strong spin-charge interaction.These results show 112-type materials with RE-Cu-As structural motif host various magnetic states,including possible spin-glass state or long-range magnetic order,may serve as a new building block approaching large intrinsic negative magnetoresistance and uncovering the mechanism of novel magnetoresistance in topological materials,which is essential to explore the interplay among electrical transport properties,topology,and magnetism.