Abstract-Electromagnon with Sensitive Terahertz Magnetochromism in a Room-Temperature Magnetoelectric Hexaferrite

Sae Hwan Chun, Kwang Woo Shin, Hyung Joon Kim, Seonghoon Jung, Jaehun Park, Young-Mi Bahk, Hyeong-Ryeol Park, Jisoo Kyoung, Da-Hye Choi, Dai-Sik Kim, Gun-Sik Park, J. F. Mitchell, and Kee Hoon Kim

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.027202

An electromagnon in the magnetoelectric (ME) hexaferrite ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{2.5}{\mathrm{Co}}_2{\mathrm{Fe}}_{24}{O}_{41}$ (${\mathrm{Co}}_{2}Z$-type) single crystal is identified by time-domain terahertz (THz) spectroscopy. The associated THz resonance is active on the electric field ($E^{\omega }$) of the THz light parallel to the $c$ axis ($\parallel \left[001\right]$), whose spectral weight develops at a markedly high temperature, coinciding with a transverse conical magnetic order below 410 K. The resonance frequency of 1.03 THz at 20 K changes $-8.7\%$ and $+5.8\%$ under external magnetic field ($H$) of 2 kOe along [001] and [120], respectively. A model Hamiltonian describing the conical magnetic order elucidates that the dynamical ME effect arises from antiphase motion of spins which are coupled with modulating electric dipoles through the exchange striction mechanism. Moreover, the calculated frequency shift points to the key role of the Dzyaloshinskii-Moriya interaction that is altered by static electric polarization change under different $H$.

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Abstract-Anisotropic high-field terahertz response of free-standing carbon nanotubes

Byounghwak Lee1, Ali Mousavian1, Michael J. Paul1, Zachary J. Thompson1, Andrew D. Stickel1, Dalton R. McCuen1, Eui Yun Jang2, Yong Hyup Kim2, Jisoo Kyoung3, Dai-Sik Kim3 and Yun-Shik Lee1,a)

1 Department of Physics, Oregon State University, Corvallis, Oregon 97331-6507, USA

2 School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-744, South Korea

3 Department of Physics and Astronomy, Seoul National University, Seoul 151-747, South Korea

a) Electronic mail: leeys@physics.oregonstate.edu

Appl. Phys. Lett. 108, 241111 (2016); http://dx.doi.org/10.1063/1.4954222
http://scitation.aip.org/content/aip/journal/apl/108/24/10.1063/1.4954222

We demonstrate that unidirectionally aligned, free-standing multi-walled carbon nanotubes (CNTs) exhibit highly anisotropic linear and nonlinear terahertz (THz) responses. For the polarization parallel to the CNTaxis, strong THz pulses induce nonlinear absorption in the quasi-one-dimensional conducting media, while no nonlinear effect is observed in the perpendicular polarization configuration. Time-resolved measurements of transmitted THz pulses and a theoretical analysis of the data reveal that intense THz fields enhance permittivity in carbon nanotubes by generating charge carriers.