Preparation And Physical Properties Study Of Iron-Selenium-based Superconducting Single Crystals

Unconventional superconductivity including the iron-based superconductivity is the core of contemporary condensed matter physics,the origin of unconventional superconductivity has not been resolved.Studying of the correlation between normal physical properties and superconductivity of iron-based superconductors is very important for understanding this problem.The iron chalcogenide compounds FeSe_1-xS_x-FeSe_1-yTe_yare isoelectronically substituted superconducting systems with simple crystal structure but rich physical properties(including related physics of superconducting order,electron nematic order,and antiferromagnetic order).Therefore,this isovalent-substituted system,particularly without extra intercalates or carrier-doping effects,provides a good platform for studying the intrinsic properties of unconventional superconductors in the normal and superconducting state.This thesis mainly focuses on the study of the correlation between the superconducting and normal state physical properties in isoelectronic FeSe_1-xS_x-FeSe_1-yTe_y systems.The following are the important progress made in this article:(1)Establishment of the combined electronic phase diagram of FeSe_1-xS_x-FeSe_1-yTe_y superconducting systems.In this work,firstly,we synthesize the low-Te substituted FeSe_1-yTe_y(0.04?y?0.30)single crystals by hydrothermal ion-deintercalation(HID)method,we not only solve the problem of single crystal synthesis in this composition range,but also eliminate the chemical-phase-separation phenomenon commonly seen in polycrystalline samples.Based on this and comprehensive physical property measurements,we can establish the joint phase diagram of FeSe_1-xS_x-FeSe_1-yTe_y systems.Single crystal samples of other components of the system can be synthesized by conventional methods.For example,we successfully synthesize FeSe_1-xS_x(0.19?x?0.0)and high-Te substituted FeSe_1-yTe_y(0.61?y?1)single crystals through chemical vapor transport(CVT)and self-flux(with post annealing)methods,respectively.According to past reports,the CVT-FeSe_1-xS_x undergoes a tetragonal to orthogonal phase transition(nematic phase)at temperature T_s,manifesting itself as the specific heat jump,resistivity kink,nonlinear Hall resistivity,sudden drop in Hall coefficient(even to negative value)and d_xz/d_yz band splitting.Based on combined experiments of the specific heat,electrical transport,and angle-resolved photoemission spectroscopy(ARPES),no signature of the tetragonal-symmetry-broken transition to orthorhombic(nematic)phase is observed in the HID-FeSe_1-yTe_y samples.Furthermore,we find that the highly mobile electronic band closely related to the electronic nematicity does not contribute to superconductivity.In addition,we find a characteristic temperature T^*in the tetragonal phase region much higher than the superconducting transition temperature,at which the temperature dependence of the Hall coefficient exhibits a local minimum.More importantly,we find that the superconducting transition temperature T_c is positively correlated with the Hall-dip temperature T^*across the FeSe_1-xS_x and FeSe_1-yTe_y systems.This suggests that the physics related to the origin of unconventional superconductivity occurs in the high-temperature tetragonal phase region,rather than in the lower-temperature nematic phase region.(2)Identification and implication of the field-aligned spin nematicity in FeSe_1-xS_xand FeSe_1-yTe_y systems.The in-plane angular magnetoresistance(AMR)experiments show that there is a common field-induced spin nematicity in isovalent-substituted FeSe_1-xS_x(x=0,0.07,0.13,1)and FeSe_1-yTe_y(y=0.06,0.61,1)single crystals.The maximum applied magnetic field of the AMR experiments is 9 T,and the temperature range measured is from 15 K to 150 K.We find that,regardless of the presence or absence of electronic nematic(T_s)or antiferromagnetic order(T_N)in the sample,AMR exhibits an obvious twofold anisotropy under a characteristic temperature of T_sn((?)T_s/T_N).The characteristic temperature T_sn is located in the tetragonal phase region,only in the unsubstituted FeSe,T_sncoincides with the nematic phase transition temperature T_sn.The emergence of this anisotropic AMR is a manifestation of the temperature-dependent anisotropic carrier-scattering effect due to the field-induced spin nematicity that are enhanced below T_sn.This field-induced spin nematicity is ubiquitous in the entire FeSe_1-xS_x and FeSe_1-yTe_y systems.Importantly,we find that in the nematic FeSe_1-xS_x samples,the characteristic temperature T_sn of the field-induced spin mematic order changes little with the substitution x,but the electronic nematic transition T_s is reduced rapidly.This indicates that for the nematic FeSe_1-xS_x,the weakening of the electronic correlation effect caused by isoelectronic substitution directly affects the electronic nematic order,rather than the spin correlation.This indicates that the isoelectronic substitution affects the electronic channel more directly than the spin channel of the nematically ordered FeSe_1-xS_x.In addition,we find that as the characteristic temperature T_snof the field-induced spin nematicity increases,the superconducting transition temperature T_c of the FeSe_1-xS_x and FeSe_1-yTe_y samples is significantly suppressed simultaneously.(3)Unusual normal and superconducting state properties of the high-quality Fe_1-dSe single-crystal flakes synthesized by hydrothermal method.Previous reports have shown that the electronic nematic order in FeSe single crystal exhibits many abnormal characteristics in electrical transport measurement.Among them,its normal state resistivity appears an obvious kink near the structural phase transition temperature T_s,with metallic behavior in a large temperature range.The dependence of the Hall resistivity on the magnetic field shows an obvious concave nonlinear characteristic when T<T_s.And the Hall coefficient drops sharply below T_s,with its sign changing from positive to negative.This electronic carrier-dominated transport feature is due to the existence of a highly mobile electron band closely related to the nematic order.Therefore,preparation of FeSe sample without electronic nematic order and comparasion with nematic ones will be extremely helpful to clarify the interaction between superconductivity and electronic nematic order.In this work,we synthesize high-quality Fe_1-dSe single crystal flakes through a novel hydrothermal method,and study its normal and superconducting state properties(including upper critical magnetic field)through electrical transport measurements under a zero magnetic field and a strong magnetic field as high as 38.5 T.We find the normal-state resistivity of hydrothermal Fe_1-dSe exhibits a metal-nonmetal transition,and all the above electronic nematic characteristics(including highly mobile electron bands)are absent.For example,there is no resistivity kink related to structural phase transition,nonlinear Hall resistivity disappears,and Hall coefficient remains positive,that is,electrical transport is always hole-dominated at low temperature.Interestingly,corresponding to the disappearance of the electronic nematic order,the superconducting T_c(=13.2 K)of hydrothermal Fe_1-dSe is significantly higher than that of nematic FeSe(?9 K).Moreover,the hydrothermal Fe_1-dSe exhibits an isotropic and higher zero-temperature upper critical magnetic field(H_c2(0)?42 T),and its value reaches 1.8 times of the Pauli limit.This experimental study provides new evidence that the high-mobility electron band closely related to the electronic nematic order in FeSe does not favor the superconductivity.Therefore,it is necessary to explore the origin of unconventional superconductivity in the tetragonal phase region with a broader perspective beyond the nematic phase region,which is consistent with the results of the first part of this article.In addition,the experimental and fitting results of the upper critical magnetic field show that the hydrothermal Fe_1-dSe has an intrinsic strong orbital limit field and spin-orbit interaction.The newly synthesized high-quality hydrothermal Fe_1-dSe flakes also provide a superior platform for further experimental research.(4)We have grown high-quality(Li_1-xFe_xOH)FeSe single-crystal films with different T_c on a metallic single-crystal substrate by a novel hydrothermal method.Benefiting from the well conductive substrate,we systematically study the evolution of c-direction resistivity of the(Li_1-xFe_xOH)FeSe system with different T_c for the first time.We find that the c-direction resistivity of the non-superconducting samples exhibit an insulating behavior,the c-direction resistivity of the films with T_c<30 K exhibit semiconducting behavior,the c-direction resistivity of the films with T_c>30 K exhibit a nonmetal-metal crossover as the temperature decreases,accompanied by the linear resistivity behavior above T_c.