THz radiation induced circular Hall effect in graphene

We report on the observation of the circular transversal terahertz photoconductivity in monolayer graphene supplied by a back gate. The photoconductivity response is caused by the free carrier absorption and reverses its sign upon switching the radiation helicity. The observed dc Hall effect manifests the time inversion symmetry breaking induced by circularly polarized terahertz radiation in the absence of a magnetic field. For low gate voltages, the photosignal is found to be proportional to the radiation intensity and can be ascribed to the alignment of electron momenta by the combined action of THz and static electric fields as well as by the dynamic heating and cooling of the electron gas. Strikingly, at high gate voltages, we observe that the linear-in-intensity Hall photoconductivity vanishes; the photoresponse at low intensities becomes superlinear and varies with the square of the radiation intensity. We attribute this behavior to the interplay of the second- and fourth-order effects in the radiation electric field which has not been addressed theoretically so far and requires additional studies

THz radiation induced circular Hall effect in graphene

We report on the observation of the circular transversal terahertz photoconductivity in monolayer graphene supplied by a back gate. The photoconductivity response is caused by the free carrier absorption and reverses its sign upon switching the radiation helicity. The observed dc Hall effect manifests the time inversion symmetry breaking induced by circularly polarized terahertz radiation in the absence of a magnetic field. For low gate voltages, the photosignal is found to be proportional to the radiation intensity and can be ascribed to the alignment of electron momenta by the combined action of THz and static electric fields as well as by the dynamic heating and cooling of the electron gas. Strikingly, at high gate voltages, we observe that the linear-in-intensity Hall photoconductivity vanishes; the photoresponse at low intensities becomes superlinear and varies with the square of the radiation intensity. We attribute this behavior to the interplay of the second- and fourth-order effects in the radiation electric field which has not been addressed theoretically so far and requires additional studies.Comment: 8 figure

Transition, the Evolution of Stock Market Efficiency and Entry into EU: The Case of Romania

In this paper we demonstrate that the measurement of stock market efficiency is an important activity in establishing whether eastern European countries satisfy the Copenhagen Criteria for EU membership. Specifically, we argue that developing an efficient stock market should be an important policy focus for countries with aspirations to join the EU as it helps to demonstrate the existence of a functioning market economy. We illustrate this issue by examining the evolution of stock market efficiency in the Bucharest Stock Exchange from mid-1997 to September 2002. We use a GARCH model on daily price data and model the disturbances using the Student-t distribution to allow for ‘fat-tails’. We find strong evidence of inefficiency in the Bucharest Stock Exchange in that the lagged stock price index is a significant predictor of the current price index. This result is robust to the inclusion of variables controlling for calendar effects of the sort that have been observed in more developed stock markets. The level of inefficiency appears to diminish over time and we find evidence consistent with stock market efficiency in Romania after January 2000. Copyright Springer 2005stock market efficiency, transition, G14, P34,