Abstract-Light-induced electron localization in a quantum Hall system

• T. Arikawa,
• K. Hyodo,
• Y. Kadoya
• & K. Tanaka

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys4078.html

An insulating bulk state is a prerequisite for the protection of topological edge states¹. In quantum Hall systems, the thermal excitation of delocalized electrons is the main route to breaking bulk insulation². In equilibrium, the only way to achieve a clear bulk gap is to use a high-quality crystal under high magnetic field at low temperature. However, bulk conduction could also be suppressed in a system driven out of equilibrium such that localized states in the Landau levels are selectively occupied. Here we report a transient suppression of bulk conduction induced by terahertz wave excitation between the Landau levels in a GaAs quantum Hall system. Strikingly, the Hall resistivity almost reaches the quantized value at a temperature where the exact quantization is normally disrupted by thermal fluctuations. The electron localization is realized by the long-range potential fluctuations, which are a unique and inherent feature of quantum Hall systems. Our results demonstrate a new means of effecting dynamical control of topology by manipulating bulk conduction using light.

Abstract-Collective antenna effects in the terahertz and infrared response of highly aligned carbon nanotube arrays

L. Ren, Q. Zhang, C. L. Pint, A. K. Wójcik, M. Bunney Jr., T. Arikawa, I. Kawayama, M. Tonouchi, R. H. Hauge, A. A. Belyanin, and J. Kono

http://prb.aps.org/accepted/82072O75N94Eb81ee2493a953b908e0024fb04a9d

We study macroscopically-aligned single-wall carbon nanotube arrays with uniform lengths via polarization-dependent terahertz and infrared transmission spectroscopy. Polarization anisotropy is extreme at frequencies less than ~ 100 cm-1 with no sign of attenuation when the polarization is perpendicular to the alignment direction. The attenuation for both parallel and perpendicular polarizations increases with increasing frequency, exhibiting a pronounced and broad peak around 450 cm-1 in the parallel case. We model the electromagnetic response of the sample by taking into account both radiative scattering and absorption losses. We show that our sample acts as an effective antenna due to the high degree of alignment, exhibiting much larger radiative scattering than absorption in the mid/far-infrared range. Our calculated attenuation spectrum clearly shows a non-Drude peak at ~ 450 cm-1 in agreement with the experiment.

Abstract-Terahertz Time Domain Spectroscopy of Gold Nanorod/Polymer Films

K. S. S. Christie1, 2, J. K. Young3, Y. Mukai4, W. S. Rockward1, K. Tanaka4, T. A. Searles1, T. Arikawa4

1Department of Physics, Morehouse College, Atlanta, Georgia 30314, USA

2 NanoJapan Program, Rice University, Houston, Texas 77005, USA

3 Department of Electrical & Computer Engineering, Rice University, Houston, Texas 77005, USA

4 Department of Physics, Kyoto University, Kyoto, 606-8502, Japan

http://nanojapan.rice.edu/Downloads/2012%20RQI/Christie,%20Kofi_RQI%20Abstract%20and%20Poster.pdf

 Nanoparticles have distinct electrical and vibrational properties from bulk materials originating from the quantum confinement and surface effect. Bioengineers are currently able to exploit these properties for applications in biosensing, using the surface plasmon resonance wavelength of gold nanorods to monitor changes in their local environment. THz-TDS provides scientists with new opportunities to study low frequency phonons, and low frequency phonons in gold nanoparticles are explicatory of their morphology. Here, terahertz time-domain spectroscopy (THz-TDS) was used to study the vibrational behavior of gold nanorods embedded in a poly(vinyl alcohol) matrix. The nanorods’ aspect ratios (diameter x length) of 30.7 x 81.6 nm, 30.7 x 84.0 nm, 16.2 x 39.5 nm, 18.7 x 52.2 nm, and 18.5 x 56.5 nm are confirmed by visible/near-infrared absorption spectroscopy and transmission electron microscopy. The frequencies of the phonon modes are expected to be proportional to the longitudinal and transverse sound velocity in the material and inversely proportional to the size of the Au nanorods. We discuss how THz-TDS offers a solid method to determine nanoparticle morphology