Browsing by Author "Day-Roberts, Ezra"

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    Data for Reexamination of the electronic phase diagram of doped NiS₂: Electronic, magnetic, and structural inhomogeneity across the Mott insulator-metal transition
    (2024-11-04) Tao, Yu; Das, Bhaskar; Calder, Stuart; Day-Roberts, Ezra; Maiti, Moumita; Lee, Yeon; Komar, Caitlyn; Birol, Turan; Leighton, Chris; leighton@umn.edu; Leighton, Chris; Leighton Electronic and Magnetic Materials Lab; Chemical Engineering and Materials Science, University of Minnesota
    Pyrite-structure NiS₂ is, in principle, a model antiferromagnetic Mott insulator that can be electron doped, hole doped, and bandwidth controlled. Despite decades of study, however, the electronic and magnetic behavior of NiS₂ have proven challenging to understand. Here, we build on recent advances establishing surface conduction in NiS₂ to completely reexamine the electronic phase behavior of electron- and hole-doped single-crystal Ni₁₋ₓCuₓS₂ and Ni₁₋ₓCoₓS₂. Magnetometry, heat capacity, neutron diffraction, and electronic transport measurements suggest that prior work missed vital details of the magnetic ordering in this system. While electron and hole doping rapidly increase the antiferromagnetic ordering temperature (by as much as 4-fold by x ≈ 0.1), signatures remain of antiferromagnetic and weak ferromagnetic ordering at the same temperatures as in undoped NiS₂. As these undoped ordering temperatures remain constant, the associated magnetic moments are diminished by doping, strongly implicating electronic/ magnetic phase coexistence across the Mott insulator-metal transition. Substantial structural changes and inhomogeneity accompany these evolutions, highlighting the importance of structural-chemical-electronic-magnetic coupling in NiS₂. The insulator-metal transition is also strongly electron/hole asymmetric, which we interpret with the aid of complementary dynamical mean-field theory results. These findings significantly revise and advance our understanding of the electronic phase behavior of this prototypical Mott insulator, highlighting the essential role of electronic, magnetic, structural, and chemical inhomogeneity across the Mott transition. This dataset contains all digital data in the published paper of the same name.
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    Electronic Spectrum and Ordered States of Quasicrystals and Pyrites
    (2022-01) Day-Roberts, Ezra
    The relative strength of the electronic kinetic energy with respect to the electron-electron interaction controls what type of ground state the electronic system has. For small interaction strengths one has Fermi liquids that can be modeled in terms of quasi-electrons. In this weak interaction regime there are instabilities of the Fermi liquid that drive transitions to states like ferromagnets, antiferromagnets, and superconductors. When electron-electron interactions dominate one finds novel ground states such as Mott insulators. In this thesis I study three systems characterized by different interaction strengths. First, I solve the free-particle problem of a tight-binding model on the Penrose quasicrystal. Despite its simplicity, system has a macroscopic fraction of exactly zero-energy states (ZES). I show that all of these states derive from a staggered sublattice mismatch that forms self-similar domains over long distances. This explains the protection of ZES from magnetic fields and any perturbation that preserves the nearest-neighbor structure of the lattice. To do this I construct a Real Space Renormalization Group type approach that shows that all zero energy states come from this staggered mismatch. Second, I study the ferromagnetic transition in Fe$_{x}$Co$_{1-x}$S$_2$ where interactions are weak. I use Density Functional Theory (DFT) to compare the magnetic transition induced by chemical doping and electrostatic gating and find that the latter requires larger numbers of added carriers to induce ferromagnetism due to the absence of Co states near the Fermi level. I derive a tight-binding model from Maximally Localized Wannier Functions and use this to argue that this is a Stoner type transition. Finally, I investigate the Mott insulating state in NiS$_2$ where interactions are strong. I use Dynamical Mean Field Theory combined with DFT to treat the Mott state, which is not captured by DFT alone. I obtain a temperature/carrier density phase diagram for the insulator-metal transition and compare it with isovalent selenium substitution. A metal is obtained in both cases for sufficient doping, but under electrostatic gating there is significant incoherent weight at the Fermi level while selenium substitution yields well defined bands.