25th Anniversary Plenary Lecture I

Chair: Hideo Hosono, Tokyo Institute of Technology

Monday 11:00–11:45

Electrical Properties of Nanocrystalline Media: Optical Conductivity and Non-Drude Behavior

Koichi Shimakawa (1,2)

1. Department of Electrical and Electronic Engineering, Gifu University, Gifu 501-1193, Japan

2. Department of General and Inorganic Chemistry, University of Pardubice, Pardubice 530 02, Czech Republic

It is well known that terahertz (THz) spectroscopy, through complex conductivity, can be helpful in understanding electron dynamics over nanometer length scales, in particular, in nanomaterials [1]. Deviations from the Drude behavior of free carriers within or near the THz range is quite dramatic in nanostructured materials, such as metals, semiconductors, and oxides [1,2].

The anomalous behavior of the complex optical conductivity has been usually interpreted by a generalization of the Drude model (Drude-Smith model) [3], while its physical basis is not clear. It is shown that a series sequence of transport involving grains and grain boundaries produces non-Drude THz conductivity in nanomaterials [2]. The present model represents a different point of view than the well accepted Drude-Smith model. In the present review, it is shown how the complex conductivity is calculated and the real and imaginary parts of THz conductivity are obtained. Of a particular interest is a Lorentz-type conductivity which is also induced by a series sequence of free carrier and tunneling carrier transports.

Fitting of the present model to the experimental data on different material systems (nanostructured metals, semiconductors, and oxides) produces reasonable physical parameters such as the number of carrier, scattering time, and mobility. We now know the importance of the grain-boundary effects which dominate overall features of nanostructured materials. It is of interest to note that the present model can be also applied to the optical conductivity in conducting polymers [4].

[1] J. Lloyd and T.-In Jeon, J. Infrared Milli TeraHz Waves 33, 871 (2012)

[2] K. Shimakawa, T. Itoh, H. Naito, and S. O. Kasap, Appl. Phys. Lett. 100, 132102 (2012); ibid 100, 239901 (2012); J. Non-Cryst. Solids 358, 2378 (2012)

[3] N. V. Smith, Phys. Rev. B 64, 155106 (2001)

[4] K. Shimakawa, H. Naito, and S. O. Kasap, Philos. Mag. Lett. 89, 673 (2009)

Keywords: nanomaterial, dielectric function, THz conductivity, optical conductivity, non-Drude relaxation