Browsing by Author "Chang, C. P."

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Coulomb scattering rates of excited carriers in moderate-gap carbon nanotubes
(2006-06-01)
Chiu, C. W.
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Shyu, F. L.
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Chang, C. P.
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Chuu, D. S.
;
Lin, M. F.
Temperature induces some free carriers in moderate-gap carbon nanotubes, which, thus, creates the low-frequency intraband e-h excitations. Such excitations are the effective deexcitation channels for the excited electrons (holes) in conduction (valence) bands, while they are very weak. The Coulomb decay rates of the lowest two conduction bands are sensitive to the changes in wave vector, radius, and temperature. However, those of the band-edge states depend on chirality weakly. These results directly reflect the strong energy dispersions and the temperature-dependent electron distributions. The calculated results are roughly consistent with the experimental measurements from the ultrafast optical spectroscopies.
Picosecond Creation of Switchable Optomagnets from a Polar Antiferromagnet with Giant Photoinduced Kerr Rotations
(2019-08-30)
Sheu, Y. M.
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Chang, Y. M.
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Chang, C. P.
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Li, Y. H.
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Babu, K. R.
On-demand spin orientation with a long polarized lifetime and an easily detectable signal is the ultimate goal for spintronics. However, there still exists a trade-off between controllability and stability of spin polarization, awaiting a significant breakthrough. Here, we demonstrate switchable optomagnet effects in (Fe1-xZnx)(2)Mo3O8, from which we can obtain tunable magnetization (spanning from -40% to 40% of a saturated magnetization) that is created from zero magnetization in the antiferromagnetic state without magnetic fields. It is accomplishable by utilizing circularly polarized laser pulses to excite spin-flip transitions in polar antiferromagnets that have no spin canting, traditionally hard to control without very strong magnetic fields. The spin controllability in (Fe1-xZnx)(2)Mo3O8 originates from its polar structure that breaks the crystal inversion symmetry, allowing distinct on-site d-d transitions for selective spin flip. By chemical doping, we exploit the phase competition between antiferromagnetic and ferrimagnetic states to enhance and stabilize the optomagnet effects, which result in long-lived photoinduced Kerr rotations. The present study creating switchable giant optomagnet effects in polar antiferromagnets sketches a new blueprint for the function of antiferromagnetic spintronics.