Browsing by Author "Chen, H. -J."

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Dirac fermion relaxation and energy loss rate near the Fermi surface in monolayer and multilayer graphene
(2014-08-07)
Luo, C. W.
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Tseng, P. S.
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Chen, H. -J.
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Wu, K. H.
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Li, L. J.
Ultrafast dynamics of Dirac fermions near the Fermi surface in monolayer and multilayer graphene are revealed using optical pump mid-infrared probe spectroscopy. The energy loss rate of Dirac fermions is also determined via energy-resolved transient transmissivity spectra, which is significantly suppressed as the number of layers in graphene increases.
Epitaxial-Strain Effects on Electronic Structure and Magnetic Properties of Hexagonal YMnO3 Thin Films Studied by Femtosecond Spectroscopy
(2013-04-01)
Wu, K. H.
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Chen, H. -J.
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Hsieh, C. C.
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Luo, C. W.
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Uen, T. M.
In this paper, (001)-oriented hexagonal YMnO3 (h-YMO) thin films with various nominal strain states were deposited on MgO(100), MgO(111), and YSZ(111) substrates by pulsed laser deposition. The in-plane orientation alignment and substrate-induced epitaxial strain between the h-YMO films and substrates are examined by X-ray diffraction (XRD) theta-2 theta, I broken vertical bar, and omega scans. The effects of epitaxial strain on the on-site Mn d-d transition energy E (dd) (T), and the N,el temperature (T (N)) in these thin films are investigated by wavelength-tunable ultrafast pump-probe techniques. We find that the changes of E (dd) (T) and T (N) depend on the type and magnitude of the epitaxial strain. The possible mechanisms for the observed strain effect on the electronic structure and associated magnetism are discussed.
Phonon dynamics in CuxBi2Se3 (x=0, 0.1, 0.125) and Bi2Se2 crystals studied using femtosecond spectroscopy
(2012-09-17)
Chen, H. -J.
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Wu, K. H.
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Luo, C. W.
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Uen, T. M.
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Juang, J. Y.
Phonon dynamics in Bi, Bi2Se2, and CuxBi2Se3 (x=0, 0.1, 0.125) single crystals was investigated using femtosecond pump-probe spectroscopy. Two damped oscillations in measured transient reflectivity changes were observed, respectively, attributed to coherent optical and acoustic phonons. The frequency shift of the phonon modes in Bi-rich Bi2Se2 crystals indicates the intercalation of the additional Bi atoms into the Bi2Se3 matrix and formation of the Bi-2 layer between the quintuple layers (QLs). The deformation of QLs in CuxBi2Se3 crystals was also found. These phonon dynamics results suggest that Cu atoms in CuxBi2Se3 crystals are predominantly intercalated between pair of QLs. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754005]
Snapshots of Dirac Fermions near the Dirac Point in Topological Insulators
(2013-12-01)
Luo, C. W.
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Wang, H. J.
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Ku, S. A.
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Chen, H. -J.
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Yeh, T. T.
The recent focus on topological insulators is due to the scientific interest in the new state of quantum matter as well as the technology potential for a new generation of THz optoelectronics, spintronics and quantum computations. It is important to elucidate the dynamics of the Dirac fermions in the topologically protected surface state. Hence we utilized a novel ultrafast optical pump mid-infrared probe to 4 explore the dynamics of Dirac fermions near the Dirac point. The femtosecond snapshots of the relaxation process were revealed by the ultrafast optics. Specifically, the Dirac fermion-phonon coupling strength in the Dirac cone was found to increase from 0.08 to 0.19 while Dirac fermions were away from the Dirac point into higher energy states. Further, the energy-resolved transient reflectivity spectra disclosed the energy loss rate of Dirac fermions at room temperature was about 1 meV/ps. These results are crucial to the design of Dirac fermion devices.
Ultrafast dynamics in topological insulators
(2013)
Luo, C. W.
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Chen, H. -J.
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Wang, H. J.
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Ku, S. A.
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Wu, K. H.
Ultrafast dynamics of carriers and phonons in topological insulators CuxBi2Se3-y (x= 0, 0.1, 0.125, y= 0, 1) was studied using femtosecond optical pump-probe spectroscopy. One damped oscillation was clearly observed in the transient reflectivity changes (Delta R/R), which is assigned to the coherent optical phonon (A(1g)(1)). According to the red shift of A(1g)(1) phonon frequency, the Cu atoms in CuxBi2Se3 crystals may predominantly intercalated between pair of the quintuple layers. Moreover, the carrier dynamics in the Dirac-cone surface state is significantly different from that in bulk state, which was investigated using optical pump mid-infrared (mid-IR) probe spectroscopy. The rising time and decay time of the negative component in Delta R/R, which is assigned to carrier relaxation in Dirac cone, is 1.62 ps and 20.5 ps, respectively.