Browsing by Author "Li, Shutong"

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    Chemical bonding and Born charge in 1T-HfS2
    (2021-04-09) Neal, Sabine N; Li, Shutong; Birol, Turan; Musfeldt, Janice L; li000269@umn.edu; Li, Shutong
    We combine infrared absorption and Raman scattering spectroscopies to explore the properties of 1T-HfS2- a heavy transition metal chalcogenide with strong spin-orbit coupling due to incorporation of the 5d center. We employ the LO-TO splitting of the Eu mode along with a reevaluation of mode mass, unit cell volume, and dielectric constant to reveal the Born effective charge. We find ZB*= 5.33e, in excellent agreement with complementary first principles calculations. In addition to resolving controversy over the nature of chemical bonding in this system, we decompose the Born charge into polarizability and local (ionic) charge. We find α= 5.07 Å3 and Z*= 5.19e, respectively. In order to understand how ZB* relates to the nominal 4+ charge of the Hf center, we decompose the theoretical Born effective charge into band-by-band contributions, and find that polar displacement-induced charge transfer from sulfur p to hafnium d orbitals is responsible for the enhancement of Born charge. 1T-HfS2 is thus an ionic crystal with strong and dynamic covalent effects.
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    First-principles Study of Lattice Dynamics in Crystals
    (2022-06) Li, Shutong
    Lattice dynamics is a key component in solid state physics. It helps the understanding of many physical properties like structural phase transitions and ferroelectricity. Density functional theory, as a first-principles method, is used to investigate the lattice dynamics in this thesis. Followed by an introduction of density functional theory and lattice dynamics, I first study the strain-suppresed polarization switching barriers in layered perovskites. It is shown that the epitaxial strain is strongly coupled with the free energy of different crystal structures, which enables us to tune the energy difference between stable and transition states. The concept of distortion symmetry group is also utilized here to model the switching process accurately. Second, the idea of free-carriers-induced ferroelectricity is introduced. Free charge carriers is typically detrimental to proper ferroelectricity, but it is not the case for hybrid improper ferroelectrics. This unexpected phenomenon will be explained by the electron-enhancement of oxygen octahedral rotation. Group theory analysis and Landau free energy are also carefully looked into in this system. Third, the nature of chemical bonding in transition metal dichalcogenides (TMD) is investigated using Wannier functions. My DFPT results indicate anomalous ionic charges of HfS2 in the in-plane direction, which is also confirmed by infrared and Raman spectrum from our collaborators. The study of Wannier functions attributes this robust ionicity to the hybridization of Hf and S orbitals. Finally, this dissertation is concluded by a brief comment of future opportunities and challenges in this research field.
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    Simulation data from: Free carrier induced ferroelectricity in layered perovskites
    (2021-06-08) Li, Shutong; Birol, Turan; li000269@umn.edu; Li, Shutong
    Doping ferroelectrics with carriers is often detrimental to polarization. This makes the design and discovery of metals that undergo a ferroelectric-like transition challenging. In this letter, we show from first principles that the oxygen octahedral rotations in perovskites are often enhanced by electron doping, and this can be used as a means to strengthen the structural polarization in certain hybrid-improper ferroelectrics -- compounds in which the polarization is not stabilized by the long range Coulomb interactions but is instead induced by a trilinear coupling to octahedral rotations. We use this design strategy to predict a cation ordered Ruddlesden-Popper compound that can be driven into a metallic ferroelectric-like phase via electrolyte gating.
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    Suppressing The Ferroelectric Switching Barrier in Hybrid Improper Ferroelectrics
    (2020-08-26) Birol, Turan; Li, Shutong; tbirol@umn.edu; Birol, Turan
    Integration of ferroelectric materials in novel technological applications requires low coercive field materials, and consequently, design strategies to reduce the ferroelectric switching barriers. In this first principles study, we show that biaxial strain, which has a strong e ect on the ferroelectric ground states, can also be used to tune the switching barrier of hybrid improper ferroelectric Ruddlesden-Popper oxides. We identify the region of the strain-tolerance factor phase diagram where this intrinsic barrier is suppressed, and show that it can be explained in relation to strain induced phase transitions to nonpolar phases.