Hydrothermal Synthesis,Electronic Transport,and Magnetic Properties Of Superconducting FeSe_1-xS_x Single Crystals

As the simplest iron-based superconductor,FeSe has a unique ground state in which superconductivity coexists with a nematic order without long-range magnetic ordering,providing a significant opportunity to investigate the correlation between the nematic phase and the unconventional superconductivity in iron-based superconductors（IBSs）.Previous studies have shown that the nematic phase was gradually suppressed by isovalent S substitution in FeSe_1－xS_x,meanwhile,the superconducting gap structure abruptly changed once the nematic phase vanished at the quantum critical point x～0.17,indicating that the nematic phase plays a key role on the pairing mechanism.Unfortunately,the crystals with high S doping level have not yet synthesized via the conventional crystal growth method until now,which limited the further physical properties of the system.This thesis has reported the successful synthesis of FeSe_1-x-x S_x（0≤x≤1）single crystals through improved hydrothermal method,and investigated the superconductivity related electrical transport and magnetic properties.The specific research results are as follows:（1）Firstly,we have successfully prepared tetragonal FeSe_0.36S_0.64 single crystal via optimized hydrothermal process and by taking K_0.8Fe_1.6Se_0.7S_1.3 single crystals as matrix,which has the same layered feature as well as FeSe/FeS,with lattice constant c=5.1884（?）.The resistivity measurement results show that the superconducting transition temperature T_c～5 K,and the zero resistance temperature is 3.8 K,which is consistent with the temperature where the magnetic susceptibility diamagnetic signal appears,and the residual resistivity ratio RRR=ρ（300 K）/ρ（5 K）≈5.5.The upper critical fields calculated according to the WHH model are H _c2^ab（0）=11 T,H^c_c2（0）=2.3 T,respectively.And the upper critical fields anisotropy factor Γ≈4.8.The Hall effect measurement shows the dominance of the electronlike charge carriers and nonlinear behavior of Hall coefficient R_H for FeSe_0.36S_0.64,and the magnetoresistance shows an obvious violation of the Kohler’s scaling rule which can be attributed to the multiband effect.（2）By optimizing the hydrothermal reaction method,FeSe_1-xS_x（0≤x≤1）single crystals with different S doping ratios have been successfully synthesized through a series of different S-doped K_0.8Fe_1.6Se_2-xS_x（0≤x≤2）single crystals as matrix and controlling the relative ratios of selenourea and thiourea during the hydrothermal reaction.The controllable hydrothermal method provides a useful reference for the synthesis of other high quality layered single crystal samples,especially those that cannot be grown by the conventional crystal growth method.Structural characterization show that all the peaks show obvious right shift with the increasing concentration of sulfur,and the lattice parameters c show decreasing monotonically from 5.53（?）for FeSe to 5.03（?）for FeS in the whole range.Through detailed electrical transport characterizations of FeSe_1-xS_x single crystals,all of the samples show superconductivity,and the superconducting transition temperature appear as V-shape with the doping concentration,and the results revealed that the resistance of the moderate sulfur doped content FeSe_1-xS_x（0.32≤x≤0.86）single crystals possessed the clear upturn behavior,which is the first time observed in FeSe_1-xS_x bulk samples at ambient pressure.Based on the above results,we construct the complete doping-temperature（x-T）phase diagram of FeSe_1-xS_x single crystals via hydrothermal method by the data of T_c,T_s,and T^*versus the sulfur content x.The typical competition between the V-shaped x-T_c and the dome-shaped x-T^* and x-T_p indicates the upturn behavior of the resistivity of FeSe_1－xS_x single crystals,which is similar with the case of the high-pressure SDW phase of FeSe and FeSe_1-xS_x bulk samples with low S content and thin films of FeSe_1－xS_x,may be attributed to the antiferromagnetic order from the positive pressure introduced by sulfur doping.