Theory of transient spectroscopy of multiple quantum well structures

A theory of the transient spectroscopy of quantum well (QW) structures under a large applied bias is presented. An analytical model of the initial part of the transient current is proposed. The time constant of the transient current depends not only on the emission rate from the QWs, as is usually assumed, but also on the subsequent carrier transport across QWs. Numerical simulation was used to confirm the validity of the proposed model, and to study the transient current on a larger time scale. It is shown that the transient current is influenced by the nonuniform distribution of the electric field and related effects, which results in a step-like behavior of the current. A procedure of extraction of the QW emission time from the transient spectroscopy experiments is suggested.Comment: 5 pages, 4 figures, to be published in J. Appl. Phy

MEASURING RESEARCH BENEFITS WITH IMPORT BAN RESTRICTIONS, QUALITY CHANGES, NON-MARKET INFLUENCES ON ADOPTION AND FOOD SECURITY INCENTIVES

Hundreds of studies have been conducted since Griliches (1958) suggested a method for measuring the benefits to agricultural research. Alston et al. (2000) reviewed 292 studies that reported 1,886 observations of returns to agricultural R&D. While some studies consider complicated cases, most studies have focused solely on R&D policy. However, it is common that agricultural R&D policy is combined with other policies, such as a price support, subsidy policy, non-market inducement, and import restrictions. Naturally, when the benefits of agricultural research are measured without taking the related policies into account, biased estimates are likely. The South Korean rice industry provides a good example where several factors affected the adoption of new technology, causing the complications in the measurement of the research benefits. The agricultural R&D policy in Korea for rice was combined with factors such as an import ban, a price support and government purchase program, yield-quality tradeoffs, and arguments that there are external benefits, like a public good, associated with self-sufficiency. It is therefore important to evaluate the benefits of agricultural research appropriately and carefully given these conditions. The revolution in rice production in Asia was triggered in 1966 with the development of IR8 by the International Rice Research Institute (IRRI). As soon as the potential of IR8 was recognized, IR8 was released to Asian rice-producing countries (IRRI). Korea also received small sets of breeding materials from the IRRI (Chandler, 1992). In 1972, Korea introduced the first high-yielding variety developed through agricultural research in Korea. However, the high yielding âTong-ilâ rice was inferior in taste to conventional rice. The argument that prevailed in the 1970s (and continues to the present day) is that self-sufficiency in rice is a public good in Korea. This attitude affected the adoption of Tong-il. The Korean government strongly encouraged farmers to adopt the new varieties through a government purchase policy as well as non-market government inducements. The government purchase policy that had been in place since 1950s was expanded in the 1970s. Most of the Tong-il rice produced was purchased and used by the government or released back on the market at discounted prices. Local officials and extension workers were provides incentives to achieve high local adoption rates. In some extreme cases, officials and extension workers physically removed other rice from fields that had already been planted. An import ban was also imposed to maintain high domestic prices. The economic evaluation of high yielding rice involves innovations to deal with several elements: ⢠The basic economics of agricultural research and development (productivity-enhancing technology): measuring agricultural research benefits without considering the complicating factors. A new method to measure a supply shift caused by agricultural R&D will be presented, considering spillovers. ⢠The trade policy that affected returns to higher yields: world price is considered as an opportunity cost. The research benefit in open economy will be smaller than the benefit in closed economy. ⢠The price support and government purchase program for a fixed amount of rice from each farm: considering that the remaining amounts were sold in the market, this policy acts like a decoupled income transfer that does not affect the payoff of agricultural research. ⢠The yield-quality tradeoffs of the technology: this tradeoff should be evaluated as a loss because the gross benefits of research would increase if there had not been a quality change. ⢠Non-market incentives for the adoption of Tong-il rice: the effect of non-market adoption is similar to that of regulation in that it generally prevents profit-maximizers from making adjustments. ⢠The public good argument related to âfood securityâ and self-sufficiency. This paper measures the net research benefits derived from the development of Tong-il performed in Korea given this complex setting. Despite the importance of this case, no studies have measure the payoff to investments to development and adoption of high yielding varieties under the multitude of policies and other complications outlined above. Conceptual studies and a few examples from other countries have dealt with some of these issues, but no study has evaluated the issues fully in a real and important setting. For example, Lokollo (2002) explored the effects of modern technology on its adoption and productivity in Indonesia. The Philippines have also implemented a government purchase policy like Korea during the green revolution period (Martinez, Shively and Masters, 1998). Both of the cases are very similar to the Korean case, in that the green revolution was attained through several government policies. However, neither of them sought to analyze the agricultural R&D effects in the complicating setting, like the Korean case. One difficulty of the R&D-related studies lies in measuring a supply shift caused by agricultural research. To measure research-induced supply shifts, many R&D-related studies introduced complicating methods, such as production functions, supply-response functions, and productivity functions. One innovation of this paper is to present a new formula for measuring a research-induced supply shift. A horizontal supply shift (J) evaluated at the current price can be measured as follows, J=h/(1-δ+δh), where h indicate yield gains of new varieties over old varieties and adoption rate of high-yielding varieties, respectively. In the above formula, only two parameters are used and they are usually available in most cases. No specific forms of supply-and demand functions or elasticities are assumed. The net returns to research on Tong-il is quite large when all the complicating factors listed above were ignored, but substantially smaller once those factors are accounted for. A tentative conclusion is that the effect of the innovation is likely to be much smaller if the development and dissemination of new technology evaluated along with the applicable market-distorting policies. This research is important in order to properly account for this episode in the economic history of agricultural development of Korea. This paper also provides lessons for other developing countries, which take a similar path with the Korean case to their agricultural development. We expect the presentation to generate considerable interest. It deals with a major event that involves important current issues in the development of agriculture and food security in developing countries. The paper applies innovations in the economics of agricultural R&D, which has long been a major topic for agricultural economists.Research and Development/Tech Change/Emerging Technologies,

Plasma mechanisms of resonant terahertz detection in two-dimensional electron channel with split gates

We analyze the operation of a resonant detector of terahertz (THz) radiation based on a two-dimensional electron gas (2DEG) channel with split gates. The side gates are used for the excitation of plasma oscillations by incoming THz radiation and control of the resonant plasma frequencies. The central gate provides the potential barrier separating the source and drain portions of the 2DEG channel. Two possible mechanisms of the detection are considered: (1) modulation of the ac potential drop across the barrier and (2) heating of the 2DEG due to the resonant plasma-assisted absorption of THz radiation followed by an increase in thermionic dc current through the barrier. Using the device model we calculate the frequency and temperature dependences of the detector responsivity associated with both dynamic and heating (bolometric) mechanisms. It is shown that the dynamic mechanisms dominates at elevated temperatures, whereas the heating mechanism provides larger contribution at low temperatures, T=35-40 K.Comment: 7 pages, 4 figure

Electronic states in heterostructures formed by ultranarrow layers

Low-energy electronic states in heterosrtuctures formed by ultranarrow layer (single or several monolayers thickness) are studied theoretically. The host material is described within the effective mass approximation and effect of ultranarrow layers is taken into account within the framework of the transfer matrix approach. Using the current conservation requirement and the inversion symmetry of ultranarrow layer, the transfer matrix is written through two phenomenological parameters. The binding energy of localized state, the reflection (transmission) coefficient for the single ultranarrow layer case, and the energy spectrum of superlattice are determined by these parameters. Spectral dependency of absorption in superlattice due to photoexcitation of electrons from localized states into minibands is strongly dependent on the ultranarrow layers characteristics. Such a dependency can be used for verification of the transfer matrix parameters.Comment: 7 pages, 7 figure

Activating Magnesium Electrolytes through Chemical Generation of Free Chloride and Removal of Trace Water

Mg batteries are attractive next-generation energy storage systems due to their high natural abundance, inexpensive cost, and high theoretical capacity compared to conventional Li-ion based systems. The high energy density is achieved by electrodeposition and stripping of a Mg metal anode and requires the development of effective electrolytes enabled by a mechanistic understanding of the charge-transfer mechanism. The magnesium aluminum chloride complex (MACC) electrolyte is a good model system to study the mechanism as the solution phase speciation is known. Previously, we reported that minor addition of Mg(HMDS)₂ to the MACC electrolyte causes significant improvement in the Mg deposition and stripping voltammetry resulting in good Coulombic efficiency on cycle one and, therefore, negating the need for electrochemical conditioning. To determine the cause of the improved electrochemistry, here we probe the speciation of the electrolyte after Mg(HMDS)₂ addition using Raman spectroscopy, ²⁷Al nuclear magnetic resonance spectroscopy, and ¹H–²⁹Si heteronuclear multiple bond correlation spectroscopy on MACC + Mg(HMDS)₂ at various Mg(HMDS)₂ concentrations. Mg(HMDS)₂ scavenges trace H₂O, but it also reacts with MACC complexes, namely, AlCl₄⁻, to form free Cl⁻. We suggest that although both the removal of H₂O and the formation of free Cl⁻ improve electrochemistry by altering the speciation at the interface, the latter has a profound effect on electrodeposition and stripping of Mg

A Synthetic DNA Walker for Molecular Transport

Inspired by kinesin movement along a microtubule, we demonstrate a processive bipedal DNA walker. Powered by externally controlled DNA fuel strands, the walker locomotes with a 5 nm stride by advancing the trailing foot to the lead at each step. Real-time monitoring of specific bidirectional walker movement is achieved via multiplexed fluorescence quenching

Does Earnings Management Amplify The Association Between Corporate Governance And Firm Performance?: Evidence From Korea

This paper aims to examine whether earnings management strengthens the causal links between corporate governance and firm performance. It examines the association between corporate governance and real activity-based earnings management and extends it to firm performance. This study involves 1,104 listings on the Korean Stock Exchange and finds that real activity-based earnings management decreases if firms have a well-established governance system, and such earnings management could strengthen the causal link between corporate governance and firm performance as measured by Tobin’s Q. Our study results are the first empirical evidences that real activity-based earnings management is effectively controlled by a corporate governance system and that it has links between corporate governance and performance. This provides the importance of corporate governance which could effectively constrain real activity-based earnings management, such that eventually influences the firm’s performance. In particular, it provides useful insights into corporate structures in which ownership is highly concentrated. Our findings are of great importance for Korea, in which the predominant business structure for large enterprises is that of the chaebol (equivalent to the Japanese keiretsu), which consists of conglomerates of many smaller companies and in which the structure of corporate governance is that of owner control