Browsing by Author "Loganathan, Vaideesh"

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    First principles approach to strongly correlated materials
    (2019-04-19) Loganathan, Vaideesh; Nevidomskyy, Andriy H
    Theoretical studies stemming from first-principles calculations have played a crucial role in understanding the plethora of interesting properties exhibited by condensed matter systems. Density functional theory (DFT) based calculations have been widely used to study the electronic structure in various classes of materials. Strongly correlated materials possess unusual properties that challenge the assumptions of band-structure theory. Examples of such materials include Mott insulators, unconventional superconductors, heavy fermion materials, etc. Simplified models such as the Hubbard model have been developed to account for the correlated behavior of electrons. The DFT+U method as well as the many-body based Dynamical mean field theory (DMFT) methods provide a better description of strongly correlated materials. In this work, we study the aspects of the Mott transition in a transition metal compound, \ce{Sr_2Mn_2O_3As_2}. The study was experimentally motivated by measurements showing an interplay between the magnetic ordering and transport properties. We first obtain the electronic structure using DFT+U, showing signs of a Mott transtion. We identify the orbitals involved in magnetic ordering and the Mott transition. In order to model the system, we construct an effective tight-binding Hamiltonian involving two orbitals with the help of Wannier functions. We then solve the model using the approaches of DMFT as well as Variational cluster approximation (VCA). The results show the opening of the Mott gap in an orbital-selective fashion, i.e. the orbitals develop a gap at different critical values of the Hubbard interaction. In this dissertation, I first introduce the theoretical aspects of DFT, Wannier functions methodology, and DMFT. In the next chapter, I discuss the study on the material, \ce{Sr_2Mn_2O_3As_2}. I summarise the results obtained from DFT and DMFT to describe an orbital-selective Mott transition. In the last chapter, I include a few applications of DFT calculations to complement recent experimental findings in Yb-based heavy fermion compounds, and magnetic materials with competing interactions.
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    Nonsymmorphic symmetry-protected band crossings in a square-net metal PtPb4
    (Springer Nature, 2022) Wu, Han; Hallas, Alannah M.; Cai, Xiaochan; Huang, Jianwei; Oh, Ji Seop; Loganathan, Vaideesh; Weiland, Ashley; McCandless, Gregory T.; Chan, Julia Y.; Mo, Sung-Kwan; Lu, Donghui; Hashimoto, Makoto; Denlinger, Jonathan; Birgeneau, Robert J.; Nevidomskyy, Andriy H.; Li, Gang; Morosan, Emilia; Yi, Ming; Rice Center for Quantum Materials
    Topological semimetals with symmetry-protected band crossings have emerged as a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic symmetries in particular have been shown to play an important role in protecting the crossings along a line (rather than a point) in momentum space. Here we report experimental and theoretical evidence for Dirac nodal line crossings along the Brillouin zone boundaries in PtPb4, arising from the nonsymmorphic symmetry of its crystal structure. Interestingly, while the nodal lines would remain gapless in the absence of spin–orbit coupling (SOC), the SOC, in this case, plays a detrimental role to topology by lifting the band degeneracy everywhere except at a set of isolated points. Nevertheless, the nodal line is observed to have a bandwidth much smaller than that found in density functional theory (DFT). Our findings reveal PtPb4 to be a material system with narrow crossings approximately protected by nonsymmorphic crystalline symmetries.
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    Possible Mott transition in layered Sr2Mn3As2O2ﾠsingle crystals
    (American Physical Society, 2019) Chen, Chih-Wei; Wang, Weiyi; Loganathan, Vaideesh; Carr, Scott V.; Harriger, Leland W.; Georgen, C.; Nevidomskyy, Andriy H.; Dai, Pengcheng; Huang, C.-L.; Morosan, E.
    Single crystals of Sr2Mn3As2O2 have been grown for the first time, for which we show a possible layer-selective Mott insulator behavior. This compound stands out as a hybrid structure of MnO2 and MnAs layers, analogously to the active CuO2 and FeAs layers, respectively, in the cuprate and iron-based high-temperature superconductors. Electrical transport, neutron diffraction measurements, together with density functional theory calculations on Sr2Mn3As2O2 single crystals converge toward a picture of independent magnetic order at T1∼79 K and T2∼360 K for the two Mn sublattices, with insulating behavior at odds with the metallic behavior predicted by calculations. Furthermore, our inelastic neutron-scattering studies of spin-wave dispersions for the Mn(1) sublattice reveal an effective magnetic exchange coupling of SJ∼3.7 meV. This is much smaller than those for the Mn(2) sublattice.