Synthesis And Physical Property Of Hole-type Binary Fe_1-xSe Superconductor Single Crystals

Among iron-based superconductors,binary FeSe superconductor is structurally the simplest,which only consists of FeSe₄-tetrahedron conducting-layer（T>T_c）or superconducting-layer（T<T_c）.Simultaneously,the binary FeSe display unique and rich physics.At ambient pressure,the superconducting transition temperature（T_c）of FeSe superconductors could be greatly increased from bulk materials T_c～8.5 K to T_c～40 K by high-pressure,intercalation and ionic-liquid regulation.What’s more,a remarkably high gap opening temperature of～65 K has been observed in FeSe monolayer on Sr Ti O₃ substrate.Therefore,the simplest binary FeSe is regarded as the most important prototype for investigating the superconductivity mechanism of iron based superconductors and has drawn continuous research attention.Bulk FeSe does not order magnetically,and has been shown to display significant anisotropic antiferromagnetic（AFM）fluctuations.It undergoes a tetragonal（P4/nmm）to orthorhombic（Cmma）structural transition upon cooling to T_s～80-90 K.The orthorhombic distortion（（a-b）/（a+b）～0.3%）is weak,and has been proposed as driven by the development of a long-range nematic order with unequal occupations of iron 3d_xz and 3d_yz orbitals.This is dissimilar to iron arsenide superconductor systems with a tetragonal（P4/nmm）to orthorhombic（Cmma）structural transition closely followed by a long-range AFM ordering.These different leading instabilities in iron selenide and arsenide compounds complicate a unified understanding of iron-based superconductivity.Nematic order in FeSe system has been intensively studied experimentally and theoretically in connection with superconductivity.However,considerable controversy still remains.Previous investigations of binary FeSe superconductors were mainly focused on nearly stoichiometric（Fe/Se?1）FeSe sample.However,these single crystal samples with a constant T_c（～9 K at ambient pressure）,electrical transport properties in the normal state present electron-dominated behavior.So far,systematic study of the key ingredient for superconducting pairing is still lacking,because of it is very difficult to fabricate Fe-deficient Fe_1-xSe single crystal samples by conventional high-temperature growth methods due to structural transformations or decomposition reactions at high temperature.So,it is necessary to find an effective control parameter in experimental in order to follow the envolution of normal state and superconducting state properties of FeSe system,However,sample fabrication of FeSe superconductors by conventional growth methods always tend to show characteristics of nearly stoichiometric（Fe/Se?1）.The samples fabrication obstacle of FeSe-based superconducting single crystals has recently been overcome by our team Prof.X L Dong.Novel hydrothermal ion-exchange,Matrix-assisted Hydrothermal Epitaxial（MHE）and ion-deintercalation techniques（HID）were developed,provide a favorable platform for investigating superconductivity mechanism of FeSe-based superconductors.A series of purify phase with different T_c’s Fe_1-xSe single crystals have been prepared via soft chemical hydrothermal ion-deintercalation（HID）method,which provide a favorable platform for investigating superconductivity mechanism of FeSe-based superconductors.Some important experiment results have been obtained by systematic physical properties measutements and analysis.We establish a more complete phase diagram for FeSe system,across the iron-deficient Fe_1-xSe（Fe/Se<1）and nearly stoichiometric FeSe（Fe/Se?1）,the main research is as follows:（1）A series of purify phase with different T_c’s Fe_1-xSe single crystals have been prepared via improvement soft chemical hydrothermal ion-deintercalation（HID）method.These Fe_1-xSe single crystals were characterized as iron-deficient in the chemical stoichiometry,with just a few per cent amount of the Fe-deficiency x.The T_c can be tuning from zero up to 9 K with decreasing Fe-deficiency x.The Fedeficiency x 5.3%for superconductivity to emerge in the prototypical Fe_1-xSe,and then T_c reaches the optimal value of 9 K smoothly.（2）The temperature dependence of Hall coefficient R_H indicated that HID Fe_1-xSe samples exhibits almost compensated semimetal in the normal state and share almost the same temperature dependence of R_H with CVT FeSe sample for temperature far higher T_c.However,at lower temperatures above T_c,distinctly different hole-and electron-dominant regions develop in the HID（R_H>0）and CVT（R_H<0）samples,respectively.This is because of introduce holes into this binary system by creating vacant Fe-sites in HID Fe_1-xSe samples,here p=2x/（1-x）is the doped hole concentrations relative to that（p=0）of stoichiometric FeSe.（3）The nematic/orthorhombic phase transition in Fe_1-xSe single crystals is unexpectedly absent by specific-heat and crystallographic characterizations but its no impact on auperconductivity at ambient pressure,which indicated that the orbital（below T_s）long-range orders have no concern with the superconductivity.T_c of FeSe system reach the optimal value of 9 K in the close vicinity of Fe-deficient concentrations x^h-e～1.4%,no matter hole-type Fe_1-xSe single crystals or electron-type FeSe single crystals.Upon entering the electron-dominated regime,the tetragonal-to-orthorhombic phase transition is observed only at higher electron-doping level（x 1.4%）of the e-dominated regime.Simultaneously,the T_c of FeSe sysyem tends to decreasing with decreasing Fe-deficient concentrations x（increasing electron doping）.These results suggests that the electronic states developing with the orbital nematicity do not favor the superconductivity.（4）The electronic correlation is found to be strongly enhanced in hole-dominated Fe_1-xSe,as compared with electron-dominated FeSe,from the magnetic susceptibility and electrical transport measurements in the normal state.Similar feature was also observed in heavily hole-doped iron arsenide AFe₂As₂（A=K,Rb,Cs）superconductors.Importantly,the T_c of hole-dominated Fe_1-xSe rises gradually from zero to 9 K with weakening strength of electronic correlation,which is associated with increasing electron doping（i.e.decreasing concentrations of introduced holes p and Fe-vacancies x）.This is analogous to that observed in cuprate superconductors derived from strongly correlated region.（5）The temperature dependence of upper critical field H_c2（T）data extracted from the magnetotransport data.For the samples with higher T_c（?7.2 K）,the temperature dependence can also be fitted by a single-band Werthamer-Helfand-Hohenberg（WHH）formula,besides the two-band model,for the samples with lower superconducting transition temperature T_c（<7.2 K）,the temperature dependence of H_c2 is appropriately described by an effective two-band model.Such a T_c-dependent change in H_c2（T）behavior showing an inherent link between the changes of intrinsic superconducting and normal state properties in FeSe system.In conclusion,the experimental findings presented here have important implication for the underlying physics of iron-based superconductivity in connection with the cuprate superconductors.