Antiferromagnetism,Nematicity And Their Quantum Criticality In Iron Pnictides

The mechanism of the unconventional superconductivity and the related exot-ic electronic states in correlated materials is a long-standing question in condensed matter physics.The iron-based superconductors discovered in 2008 show a rich variety in materials which can be tuned to superconductivity by many kinds of tuning parameters,such as chemical substitution,external pressure,layer interca-lation and film interface effect,providing a new platform to study the underlying physics of unconventional superconductivity.These tuning parameters also result in complex phase diagrams with exotic orders and states like magnetism,nematic-ity and non-Fermi-liquid state adjacent to superconductivity.Despite numerous studies up to now,no consensus has been reached on the origin of the iron-based superconductivity,but the adjacent magnetic or nematic fluctuations are sug-gested to be very important to superconductivity.In addition,superconductivity emerges around the end points of nematic or magnetic orders,indicating the pos-sible presence of a quantum critical point?QCP?under the superconducting dome.Clearly,a solid understanding of these exotic orders and fluctuations is a neces-sary route to the physics of the unconventional superconductivity in iron-based superconductors.In this thesis,I will introduce our studies on the magnetism,nematicity and the related quantum criticality in BaFe₂As₂system,which is a typical iron-based superconducting system.The main works are listed below:?1?The spin nematicity in BaFe_2-xNi_xAs₂is studied by inelastic neutron scat-tering,revealing a complete picture of the low-energy spin dynamics under the in-fluence of nematic phase.The detailed temperature dependence of low-energy spin fluctuations at??,0?and?0,??in a uniaxial-stress detwinned BaFe_1.935Ni_0.065As₂sample is established.Both intensity and spin-spin correlation start to show anisotropic behavior at high temperature due to the external stress,while the re-duction of the spin-spin correlation length at?0,??happens just below T_s,which is first observed experimentally.Our results favor the idea that treats the spin degree of freedom as the driving force of the electronic nematic order.?2?By substituting Cr for 3%of Fe,the superconductivity in BaFe₂(As_1-xP_x)₂is fully suppressed and an anti-ferromagnetic quantum critical point is revealed with non-Fermi-liquid behaviors observed.BaFe₂(As_1-xP_x)₂is the most promising system which may host a QCP among different families of iron-based supercon-ductors.However,the QCP is always hidden by the superconductivity and may be avoided by the transition to superconductivity as suggested in some works.In addition,both magnetic and nematic QCPs are suggested to exist,while which quantum critical fluctuation is responsible for the non-Fermi-liquid behaviors is not clear.By slightly Cr doping,the superconductivity is fully suppressed and the underlying magnetic QCP emerges with the position shifted to higher P-doping than expected in BaFe₂(As_1-xP_x)₂.The non-Fermi-liquid behaviors like linear temperature dependence of resistivity,enhancement of effective mass of electrons and the unconventional?/T scaling further confirm the presence of QCP.Howev-er,Cr doping seems to destroy the nematic correlations,making it not a nematic QCP.Our results reveal the QCP in BaFe₂(As_1-xP_x)₂system and suggest that the non-Fermi-liquid behaviors in iron-based superconductors can be solely resulted from the anti-ferromagnetic quantum critical fluctuations.