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Optical spectroscopy of cooperative phenomena and their symmetries in solids

Mai, Thuc T
http://rave.ohiolink.edu/etdc/view?acc_num=osu1555629359625425

Abstract Details

2019, Doctor of Philosophy, Ohio State University, Physics.
In crystalline materials, the symmetry of the crystal lattice imposes strict conditions on the observable properties of the material. These symmetry restricted conditions can be, in turn, probed by light via the electromagnetic interaction. Studying the electromagnetic excitations in solids can reveal many fundamental properties of these systems. A quick introduction and guide to symmetry in solids will be given, with an emphasis on how it can be used to interpret spectroscopic measurements. The measurement techniques used will also be described. Time domain Terahertz spectroscopy (TDTS) is the main technique used in this dissertation. Important experimental considerations pertaining to the construction of the THz spectrometer will be given. In the multiferroic Sr_2 FeSi_2O_7, we found multiple excitations in the few meV energy scale (THz), in the material’s paramagnetic phase. Measurements with varying temperature and magnetic field revealed that these excitations are both electric and magnetic dipole active. By considering the ground state of the Fe 2+ magnetic ion in Sr 2 FeSi 2 O 7 , we concluded that our observation is coming from the spin-orbital coupled states of the ion. This realization demonstrated that spin-orbit coupling plays a crucial role in these exotic materials. Interestingly, these spin-orbital THz excitations persist into the magnetically ordered phase. The single-ion picture of the paramagnetic phase needs to be expanded theoretically to explain our observations. CaFe_2O_4 orders antiferromagnetically below ~ 200 K. Two co-existing magnetic structures (A and B phase) have been measured previously by neutron diffraction. The anti-phase boundaries between these two phases have been proposed to be the cause of the quantized magnetic excitations (magnons) measured by an inelastic neutron scattering study. We measured two antiferromagnetic resonances (magnons) with TDTS. Our observation can be explained by the orthorhombic crystal anisotropy of CaFe 2 O 4 . Mean field classical spin wave analysis confirmed that indeed the magnon modes come from the low symmetry of the crystal. These two modes were found to have the same temperature dependence of only the magnetic B phase. We did not observe the hierachy of quantized magnon modes as previously reported. However, our measurements offered a hint of another magnon mode that show temperature dependence similar to the other magnetic structure, A phase. In the last study, we demonstrated the lattice symmetry breaking effects on the Raman phonon spectrum of α-RuCl 3 at room temperature. The Raman spectrum shows multiple pairs of phonon peaks with approximately 2 cm −2 (0.248 meV or ≈ 60 GHz) apart. This observation can be explained by the symmetry lowering effect when going from a perfect honeycomb layer (D 3d point group symmetry), to the monoclinic bulk crystal (C 2h ). Furthermore, because of the honeycomb layer structure, this material is a host candidate for the long-sought-after Kitaev quantum spin liquid. Following previous experimental and theoretical studies with Raman spectroscopy on α-RuCl_3, we concluded from our data that the scattering continuum is magnetic in origin, with little to no quasi-elastic scattering contribution.
Rolando Valdes Aguilar (Advisor)
P. Chris Hammel (Committee Member)
Nandini Trivedi (Committee Member)
Douglass Schumacher (Committee Member)
76 p.
Physics
Terahertz; Time-Domain; Spectroscopy; Condensed Matter; Multiferroics; Antiferromagnet; Phonon; Raman; Quantum Spin Liquid; Symmetry Breaking; Magnon; Low energy

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Citations

  • Mai, T. T. (2019). Optical spectroscopy of cooperative phenomena and their symmetries in solids [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1555629359625425

    APA Style (7th edition)

  • Mai, Thuc. Optical spectroscopy of cooperative phenomena and their symmetries in solids. 2019. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1555629359625425.

    MLA Style (8th edition)

  • Mai, Thuc. "Optical spectroscopy of cooperative phenomena and their symmetries in solids." Doctoral dissertation, Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1555629359625425

    Chicago Manual of Style (17th edition)

osu1555629359625425
425
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This open access ETD is published by The Ohio State University and OhioLINK.