Optical Studies of Novel Electronic Materials
Abstract
We have made a Kerr rotation (KR) measurement instrument with a 635~nm laser that is sensitive enough to measure KR angle of few micro-radians. The KR instrument was used to study the current induced polarization rotation of light as it is reflected off the surface of topological insulator Bi2Te2Se (BTS221). In our experiment, we observe a linear response of Kerr rotation with current and the Kerr rotation for both s and p-polarized light is negative. From the dependence of this polarization rotation on the current in the TI sample, the angle of incidence of light and the light polarization, we interpret that the rotation is induced by electric field induced Pockels effect associated with the surface containing broken inversion symmetry. For both s and p-polarized light, we did not observe any dependence of reflectivity on the current. The KR instrument was also used to study control magnetic samples (CoFeB and nickel films). We also present a theory for KR including both the magneto-optical constant and the Pockel's constant. The theory describes our experimental results very well. We have performed optical spin voltaic (voltage) (OSV) effect experiments from an interface between a paramagnetic metal (platinum or chromium) and a ferromagnetic insulator Y3Fe5O12 (YIG). We confirm photo spin voltaic (PSV) effect from the Pt/YIG samples, using a halogen light source. In the OSV experiment, using a pulsed laser, the measured voltage depends on the absorption coefficient of the metal and YIG leading to a light induced temperature gradient perpendicular to the metal/YIG interface. The experiment results lead us to conclude that the OSV originates from interface-spin Seebeck effect (ISSE) in the experiment using the pulsed laser and not from the PSV or the bulk spin Seebeck effect. Raman spectroscopy was used to study SmNiO3 (SNO) and hydrogenated electron-doped SmNiO3 (HSNO), in the temperature range 80-300~K. We have studied SmNiO3 samples doped using two methods: (1) hydrogen gas annealing and, (2) salt (NaCl) water electrochemical technique. In the temperature dependence measurements of SNO, we observed peak shift and interpret the shift due to the anharmonic process and also from spin-phonon coupling below the Neel temperature. We observe a mode around 620-625~cm-1 in HSNO, which we assign to an infrared (IR) active Ni-O breathing phonon mode of SmNiO3. This Raman mode can be fitted with a Fano line shape, which we interpret as due to the electron-phonon coupling in the doped SmNiO3 despite being insulating.
Degree
Ph.D.
Advisors
Chen, Purdue University.
Subject Area
Condensed matter physics|Optics|Nanotechnology
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