Studies Of Neutron Spin Resonant Mode In KCa₂Fe₄As₄F₂ And Random Singlet State In Sr₂CuTe_1-xW_xO₆

Magnetic interaction is one of the essential driving forces of various novel quantum states in strongly correlated systems.Based on large-scale scientific facilities,neutron scattering and muon spin relaxation/rotation/resonance(?SR)play irreplaceable roles in studying magnetic structures and magnetic excitations.We can use the inelastic neutron scattering technique to measure the spin excitations and their relationship with super-conductivity in unconventional superconductors.By using elastic neutron scattering and ?SR techniques,one can determine the magnetic structures and magnetic phase in materials.?SR is also used to study the static distribution and dynamic fluctuations of the internal magnetic field inside the samples.This dissertation focuses on the neutron spin resonance in KCa₂Fe₄As₄F₂iron-based superconductor and the random singlet state in a two-dimensional quantum magnet Sr₂CuTe_1-xW_xO₆and divides the research content into the following four parts:1.Single-crystal growth of KCa₂(Fe_1-xNi_x)₄As₄F₂iron-based superconductors and its phase diagram.We have successfully grown a series of KCa₂(Fe_1-xNi_x)₄As₄F₂single crystals using KAs as the flux.The phase diagram of KCa₂(Fe_1-xNi_x)₄As₄F₂ is drawn in detail by combining X-ray diffraction,electrical transport,magnetic mea-surement,and other characterization methods.These results show that Ni doping does not induce any magnetic order in this quasi-two-dimensional bilayer iron-based super-conductor but mainly plays the role as impurity scattering and gradually suppresses the superconductivity.2.Neutron spin resonance mode and its dispersion relationship of KCa₂Fe₄As₄F₂ single crystal.This chapter is one of the main results for this dissertation.Using triple-axis spectrometer,we measured the low-energy spin excitations of KCa₂Fe₄As₄F₂ in the superconducting and the normal states in detail.We observed a two-dimensional spin resonant mode with downward dispersion for the first time.This behavior is similar to that in copper oxide high-temperature superconductors with(9-wave pairing symmetry but can not explain by the spin-exciton picture under^±-wave pairing symmetry.More-over,the spin resonant energy is well above the total superconducting gap of any two of the hole and electron pockets.These results strongly challenge the common under-standing of the origin of neutron spin resonance in unconventional superconductors.It suggests that when the superconductivity becomes very quasi-two-dimensional in iron-based superconductors,the electronic behaviors are similar to those in cuprates.3.Single-crystal growth and neutron scattering studies of Rb_1-KFe₂As₂.We have successfully grown high-quality Rb_1-KFe₂As₂single crystals by using KAs-Rb As mixed self-flux method.We establish the superconducting phase diagram by combining resistance and magnetic measurements.We found that the superconducting temperature reaches its lowest value at the middle doping level,which may be related to impurity scattering.We use a time-of-flight neutron spectrometer to investigate the low-energy excitations of RbFe₂As₂.We observed excited magnetic peaks at incom-mensurate positions and the FWHM increases upon warming.Further measurements are needed to determine whether there are spin nematic fluctuations.4.Magnetic properties in the two-dimensional quantum magnet Sr₂CuTe_1-xW_xO₆studied by neutron diffraction and?SR techniques.This chapter is one of the main results for this dissertation.We found that the long-range Néel type magnetic order in the x=0 sample is suppressed by a small fraction of Wions(x=0.03).However,Néel type short-range spin correlations still survives to higher doping levels.This extreme quick suppression of the long-range order can be understood within the framework of a two-dimensional J₁-J₂Heisenberg model by considering that the super-exchanges around Te and W are very different.When x>0.03,the?SR relaxation rate exhibits quantum critical scaling with a sizeable dynamic exponent z>2 which increases with increasing doping level,consistent with the theoretical prediction for a random-singlet state.Thus,we believed that the two-dimensional random singlet state can be realized in Sr₂CuTe_1-xW_xO₆.