Condensed-Matter Physics & Materials Science Seminar

In strongly correlated systems, interactions between various microscopic degrees of freedom with similar energy scales can induce strong competition and frustration, leading to exotic phenomena. Here we use neutron scattering technique to study several strongly correlated systems to show how the competition and interplay between these degrees of freedom can induce different phases and properties. 1) In the pressure-induced superconductor CrAs, the competition between various magnetic interactions lead to a noncollinear helimagnetic order. In addition, CrAs exhibits a spin reorientation at a critical pressure (Pc ~ 0.6 GPa), which is accompanied by a lattice anomaly and coincides with the emergence of bulk superconductivity, indicating the strong interplay between magnetic, structural and electronic degrees of freedom. 2) FeSe, the structurally simplest iron-based superconductor, shows nematic order at 90 K, but no magnetic order in the parent phase. Our neutron scattering experiments reveal both stripe and Neel spin fluctuations that are coupled to the nematicity. The competition between these two phases suppress the magnetic order and drive the system into a nematic quantum disordered paramagnet. Similar phenomenon is observed in YFe2Ge2, in which the magnetic order is suppressed by the competition between stripe type AFM phase and in-plane FM phase. 3) In the heavily electron-doped FeSe based superconductor Li0.8Fe0.2ODFeSe (Tc=41 K), a twisted dispersion of spin excitations is observed which may be caused by the competition between itinerant and local electrons, analogous to the hole-doped cuprates which host remarkably high Tc as well. 4) In the two-dimensional triangular lattice antiferromagnet YbMgGaO4, due to the strong spin-orbit coupling and crystalline electric field (CEF), the low-lying crystal field ground state is a Kramers doublet. The geometric frustration is enhanced by the anisotropic interactions and a quantum sp