Dai, Pengcheng2019-05-162019-05-162019-052019-02-28May 2019Wang, Weiyi. "Neutron scattering studies of doped iron pnictides." (2019) Diss., Rice University. <a href="https://hdl.handle.net/1911/105379">https://hdl.handle.net/1911/105379</a>.https://hdl.handle.net/1911/105379Unraveling the mechanism of unconventional superconductivity is one of the long-sought crown jewels in condensed matter physics. Outstanding among many of the challenges is the critical importance of electron-electron interactions, which gives rise to new emergent collective electronic orders, including magnetism, electronic nematic order, and superconductivity. These degrees of freedom are closely intertwined in iron-based superconductors. The interplay between these orders, can be unraveled by studying their co-evolution across phase diagrams, which nature conveniently grants us access through chemical substitution. Such an understanding provides the foundation for a description that unifies these different orders as different flavors of the same electrons, and sets the stage for future applications of these fascination materials. Within this thesis I present several neutron scattering works on prototypical iron-based superconductors, derived from BaFe$_{2}$As$_{2}$ and NaFeAs. Neutron scattering directly probes the magnetic correlations of atoms, and it also allows for the measurement of the structural properties that directly couple to the electronic nematic order. Complemented by physical property measurements that directly probe superconductivity, this enables us to carry out a comprehensive survey of how these entangled orders behave across the temperature and doping phase diagram. In Ni doped NaFeAs, we found both nematic and magnetic orders compete with superconductivity, demonstrating an intricate balance between the different orders that results from the fact they are different manifestations of the same electrons. At high temperature or high doping-levels, where these orders were assumed to be absent, using high-resolution neutron diffraction and neutron Larmor diffraction techniques we uncovered local orthorhombic distortions, indicating electronic nematic order persists on a local length scale, over a much large region of the phase diagram than previous thought. Our observation of these unexpected local orthorhombic distortions demonstrate fluctuations of the electronic nematic order that are maximized near a putative quantum critical point, when coupled with random strain fields, result in locally pinned electronic nematic states, from which global superconductivity precipitates. In a similar vein, locally-broken fourfold rotational symmetry is also observed in heavily Cu-doped Ba(Fe$_{1-x}$Cu$_{x}$)$_2$As$_2$. Combining transport, magnetic susceptibility, elastic and inelastic neutron scattering measurements, we identified for the first time the existence of short-range magnetic order over a large doping range beyond the putative quantum critical point. This discovery reveals while the system on average is tetragonal and exhibit four-fold rotational symmetry, small patches can exhibit locally broken fourfold symmetry, giving rise to magnetic order. Our findings point to an inherent tendency towards the lowering of crystallographic symmetry, on a local scale if not globally, providing the backdrop from which superconductivity emerges. Our neutron scattering results on partially detwinned NaFe$_{0.985}$Co$_0.015$As show that neutron spin resonance in this system only appears at the antiferromagnetic wave vector. Combined with electronic band analysis, our results indicate that the neutron spin resonances in NaFe$_{0.985}$Co$_0.015$As arise mostly from quasiparticle excitations between the hole and electron Fermi surfaces with the $d_{\rm yz}$ orbital character. Our analysis suggests that intraorbital quasiparticle scattering of the $d_{\rm yz}$-$d_{\rm yz}$ orbitals are important for superconductivity. The combination of these works provides a significant advance in the understanding of the interplay between electronic nematic order, magnetism and unconventional superconductivity in iron-based superconductors, underscores the physics at the local scale which is critical for potential applications, and motivates future studies to examine the intricate interplay of complex orders at the local scale.application/pdfengCopyright is held by the author, unless otherwise indicated. Permission to reuse, publish, or reproduce the work beyond the bounds of fair use or other exemptions to copyright law must be obtained from the copyright holder.Unconventional superconductorsNeutron scatteringStrong correlated systemsMagnetismThesis2019-05-16