archives@tulane.edu / The electronic, spin, phonon and magnetic behavior govern the electrodynamics of solid materials. The different compositions and symmetries mix all the degrees of freedom leading to varieties of interesting phenomena such as metal-to-insulator transition, nonreciprocal directional dichroism and topological states. The study of the behavior of electrons, spins and phonons is crucial to reveal the physics behind the mysterious phenomena. The nature of terahertz time domain spectroscopy (THz TDS) which has low photon energy and contains phase information makes this technique very powerful to probe the physics of spins, electrons, phonons and magnons where the resonance energy is in the THz range. The multiferroic materials are studied by using THz-TDS with strong dc magnetic field. Multiferroic material is one of complex materials that simultaneously contain ferroelectricity and magnetism. Many fascinating physical phenomena are discovered in multiferroics, including magneto-dielectric effect and nonreciprocal directional dichroism. The magneto-dielectric effect, change in dielectric function in applied magnetic field, is studied in multiferroic CaBaCo4O7. We analyze the dynamics of phonons to clarify the individual phonon contribution to the magneto-dielectric effect. We observe giant nonreciprocal directional dichroism in the multiferroic material FeZnMo3O8, which is defined as the difference in absorption coefficient for linearly polarized light waves travelling in the opposite direction. A spin excitation is determined as the origin of nonreciprocal effect in the multiferroic FeZnMo3O8 by using THz-TDS. The nonreciprocal effect from magneto-chiral dichroism is also observed in BaCoSiO4 crystal where the material simultaneously possesses the chiral structure and magnetization. The polarimetry of transmitted THz light through BaCoSiO4 is carefully analyzed. We attribute the change in polarization in the zero magnetic field to the chirality of the structure. Nonlinearity of semiconductor InSb due to intense THz electric field is investigated quantitatively by using THz-TDS. The effective mass approximation breaks down when the intense THz pulse is applied to the semiconductor. We develop a predictive model that replaces the effective mass with a realistic band structure and retains the Drude parameters, the electron density and scattering rate, to accurately calculate the experimental observations (saturable absorption and amplitude-dependent refractive index) in InSb. / 1 / Shukai Yu

We studied the pressure effect of polycrystal BiMnO3 (type-I multiferroic) and single crystal Co3TeO6 (type-II multiferroic) with different magnetic fields and pressures. With the primary objective of understanding the pressure effect on BiMnO3, complex multiple magnetic transitions (kink I, II and III) are observed under the maximum applied pressure of 15.94 kbar (~1.6 GPa). Kink I, a long-range soft ferromagnetic transition at TcI ~ 100 K under ambient pressure, is suppressed completely at 11.74 kbar. Kink II emerges at 8.66 kbar along with TcII ~ 93 K. Kink II is a long-range soft ferromagnetic the same as kink I but canted in nature. Kink III, a canted antiferromagnetic transition at TcIII ~ 72.5 K appears along with kink II also at 8.66 kbar. These results indicate the complicated correlation between the lattice distortion and the spin configuration under pressures and magnetic fields in multiferroic system. Whereas, two distinct anomalies (T1 ~ 26 K and T2 ~ 18 K) are observed on single crystal Co3TeO6 in magnetic susceptibility, specific heat, and neutron diffraction measurements. Interestingly, the strong anisotropic magnetic variations are also noticed in high-magnetic-field hysteresis measurements with applied magnetic field parallel to a- and c- axes. Dielectric studies were also carried out in different magnetic fields at the temperature range 5 ¡V 300 K. Concomitantly, frequency-independent step-like dielectric anomaly is observed around 18 K, coinciding with the transition of magnetic susceptibility, specific heat, and neutron diffraction. The dielectric constant is also modified by external magnetic fields. These experimental results strongly suggest the multiferroicity of Co3TeO6. From temperature-dependent X-ray diffraction studies, it is evident that a structural distortion appears around 18 K, responsible of dielectric and/or magnetic ordering. The transition at 18 K is disappeared under pressure above 9.82 kbar, indicative of suppressing structural distortion. Similarly, the lattice distortion and the spin configuration under pressures are important factors for multiferroic property. Through the specific heat and pressure-dependent susceptibility, the structural distortion probably results from the magnetic ordering, indication the dielectric anomaly at 18 K.

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