Browsing by Author "Sankar, R."

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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.
;
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]
Probing the bulk electronic states of Bi2Se3 using nuclear magnetic resonance
(2012-08-21)
Young, Ben-Li
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Lai, Zong-Yo
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Xu, Zhijun
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Yang, Alina
;
Gu, G. D.
We report a nuclear magnetic resonance (NMR) study of Bi2Se3 single crystals grown by three different methods. All the crystals show nine well-resolved peaks in their Bi-209 NMR spectra of the nuclear quadrupolar splitting, albeit with an intensity anomaly. Spectra at different crystal orientations confirm that all the peaks are purely from the nuclear quadrupolar effect, with no other hidden peaks. We identify the short nuclear transverse relaxation time (T-2) effect as the main cause of the intensity anomaly. We also show that the Bi-209 signal originates exclusively from bulk, while the contribution from the topological surface states is too weak to be detected by NMR. However, the bulk electronic structure in these single crystals is not the same, as identified by the NMR frequency shift and nuclear spin-lattice relaxation rate (1/T-1). The difference is caused by the different structural defect levels. We find that the frequency shift and 1/T-1 are smaller in samples with fewer defects and a lower carrier concentration. Also, the low-temperature power law of the temperature-dependent 1/T-1 (proportional to T-alpha) changes from the Korringa behavior alpha = 1 in a highly degenerate semiconductor (where the electrons obey Fermi statistics) to alpha < 1 in a less degenerate semiconductor (where the electrons obey Boltzmann statistics).
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.