Date of Graduation

12-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Fu, Huaxiang

Committee Member

Bellaiche, Laurent

Second Committee Member

Vyas, Reeta

Keywords

Metallicity; Ferroelectric

Abstract

In this dissertation, we have thoroughly studied the effect of chemical and charge dopingon ferroelectrics (PbTiO3 and BaTiO3) and Rashba type semiconductor (BiTeI). In the first project, We investigate the polar instability and soft modes in electron-doped PbTiO3 using linear-response density functional calculations. Because, metallicity and ferroelectric-like polar distortion are mutually non-compatible, and their coexistence in the same system is an intriguing subject of fundamental interest in the field of structure phase transition. However, it is unclear what mechanism may extend the limit of metallicity that allows polar distortion. We find that ferroelectric instability can remarkably sustain up to an electron concentration of ne=0.7 per unit cell, which is beyond the limit that causes the polar catastrophe in LaAlO3/SrTiO3. Our study further reveals two unusual discoveries: (i) Electron doping can turn non-soft mode into soft mode, which leads to different microscopic mechanism for ferroelectricity when system is strongly metallic; (ii) The frequency change Δω/Δne is surprisingly flat at large ne, which is pivotal for the persistence of soft mode and polar distortion at high metallicity. We also provide an interesting physical origin—which is caused by the strong mode-mode interaction—to explain these phenomena, and the finding of this origin may further extend the limit where metallicity and polar distortion coexist. In the second project, after we understood the existence of polar-metal characteristics in PbTiO3 under electron doping, we extend our study to a more complex system of supercell PbTiO3 under chemical doping and the application of biaxial compressive strain to find room temperature polar-metal. Polar metals offer a wide range of useful properties in superconductivity, magnetoelectricity, photovoltaics, and mutli-ferroic sensors. The realization of a room temperature polar metals would be an ideal candidate for such versatile applications. Hence, using liner-response density functional calculations, we have investigated Nb-doped PbTiO3, which is under four different biaxial compressive strains (η=0%, -1%, -2%, and -3%) to alter the minimum energy of its polar mode (A2u(TO1)). We find from the total density of states of -2% biaxially strained Nb-doped PbTiO3 that the frequencies of most phonon modes are less than 300 cm−1. We also find that the extra electron acquired due to Nb-doping is localized and form small polarons around Nb site, which can be thermally actuated into a conduction state. This electron partially screens out the internal dipole moment existed in pure (without Nb-doping) PbTiO3, and preserves the ferroelectric instability. The double well potential depth of A2u(TO1) display stability under room temperature condition (KBT∼25 meV) for η=-2% and η=-3%, because the depth of the potential well for these two strains are -29.51 and -39.56 meV, respectively. However, the depth of the potential well of A2u(TO1) for unstrained (η=0%) Nb-doped PbTiO3 is -0.095 meV, which is unstable at this condition. We therefore demonstrated that metallic Nb-doped PbTiO3 can be transformed into polar metal by the application of biaxial compressive strain. As a result, strain play a prominent role to tune the physical and chemical properties of polar metals in addition to doping. Finally, in the third project, we investigate the effect of electron and hole doping on the spin-orbit interaction and electron-phonon coupling constant of BiTeI. The spin-splitting of bands by spin-orbit interaction (SOI) in systems that lack inversion symmetry have paramount importance in spin-polarized field effect transistor, magnetoelectric effect, Edelstein effect and spin Hall effect. We have performed first principle calculation to study the effect of charge doping (electron and hole) on the SOI and electron-phonon coupling constant (λ) of BiTeI. The Rashba parameter is tuned up to a maximum value of 7.46 and 6.32 eV˚A for the valance and conduction bands, respectively. The valance band Rashba parameter is so far the highest we have recognized in any work. λ of BiTeI is 0.46, and this value is almost the same as that of Al and Mo. However, the critical temperature (TC) of BiTeI in this study is 0.7 K, which is very small for practical application.

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