Date of Graduation

5-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Huaxiang Fu

Committee Member

Laurent Bellaiche

Second Committee Member

Hugh Churchill

Keywords

Density Functional Calculations; Local Magnetic Moment; Monolayer FeSe; Phase competition; Spin exchange interaction; Spin frustration

Abstract

In this dissertation, we present two projects to study superconducting FeSe monolayer using first-principles density functional calculations. Monolayer FeSe/SrTiO3 system has very different properties as compared to its bulk counterpart in terms of critical superconducting temperature, Fermi surface topology and antiferromagnetic (AFM) stability. For FeSe monolayer, local magnetic moment (LMM) and AFM fluctuation are closely linked to superconductivity. However, LMM is not studied enough for FeSe monolayer. For FeSe/SrTiO3 system, the substrate SrTiO3 constrains the in-plane lattice constant for FeSe and also plays a crucial role in the onset and enhancement of superconductivity by providing charge transfer. The height of Selenium atom (hSe) above Fe-plane is also linked to superconductivity. Furthermore, spin exchange interaction plays an important role to study the magnetic properties for FeSe superconductor. In our first project, we constrain the local magnetic moment on Fe atoms using density functional theory and investigate how LMM in FeSe monolayer alters the total energy, heights of Se atoms, band structure, and the electronic properties, for three different AFM spin arrangements which consist of the checkerboard (CB), collinear (CL), and pair-checkerboard (PC) spin phases. We find that (i) the total energy decreases drastically in all three spin structures when LMM develops, showing that the existence of LMM significantly stabilizes the system. The optimal LMM is found to be 2.23, 2.54, and 2.47 μB , respectively, in the CB, CL, and PC spin phases. (ii) The heights of Se atoms increase markedly with LMM, demonstrating a strong magnetostriction effect. Also intriguingly, we find that the Se heights are insensitive to spin ordering, displaying a rather universal dependence on LMM in three different AFM spin phases. (iii) We find that LMM can significantly alter the electron band structures and Fermi surfaces. In our second project, we combine density functional theory and semi-classical approach and determine the first, second and third nearest exchange interaction (J1,J2 and J3) as well as first and second biquadratic exchange (K1 and K2). Also, we find the total energies for three commensurate antiferromagnetic (AFM) spin arrangements [checkerboard (CB), collinear (CL) and pair-checkerboard (PC)] as well as two incommensurate phases (q, π) and (q, q). We find that (i) J1,J2 and J3 all decrease linearly with the increase in hSe whereas interestingly, K1 values increase. Though J1 and J2 both decrease, J2/J1 ratio in fact increases with increase in hSe. (ii) For smaller charge transfer, the incommensurate phase (q, π) is found to be the magnetic ground state for monolayer FeSe which we believe is a new knowledge. Also, the energy difference between between (q, π) and CL phase (second most stable) is always less than 6.8 meV for all hSe showing strong spin fluctuation between them. (iii) The larger charge transfer is found to significantly affect the exchange interaction as well as magnetic ground state. While J1, J2 and J3 values for larger charge transfer are significantly higher than smaller charge transfer, J1 and J2 are affected at different rate. Moreover, for larger charge transfer, the CL phase becomes the most stable phase unlike the incommensurate phase (q, π) for the smaller charge.

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