Conductance modulation in spin field-efect transistors under finite bias voltages

The conductance modulations in spin field-effect transistors under finite bias voltages were studied. It was shown that when a finite bias voltage is applied between two terminals of a spin field-effect transistor, the spin precession states of injected spin-polarized electrons in the semiconductor channel of the device will depend not only the gate-voltage controlled Rashba spin-orbit coupling but also depend on the bias voltage and, hence, the conductance modulation in the device due to Rashba spin-orbit coupling may also depend sensitively on the bias voltage.Comment: 7 pages, 3 figures, to appear in Physical Review B, (April, 2004

Essays on Reputations and Dynamic Games

This dissertation consists of three essays on reputations and dynamic games. I investigate how incomplete information, Bayesian Learning and strategic behavior interplay in different dynamic settings. In Chapter 1, I study reputation effects between a long-lived seller and different short-lived buyers where buyers enter the market at random times and only observe a coarse public signal about past transactions. The signal measures the difference between the number of good and bad outcomes in a biased way: a good outcome is more likely to increase the signal than a bad outcome to decrease it. The seller has a short-run incentive to shirk, but makes high profits if it were possible to commit to high effort. I show if there is a small but positive chance that the seller is a commitment type who always exerts high effort and if information bias is large, equilibrium behavior of the seller exhibits cyclic reputation building and milking. The seller exerts high effort at some values of the signals in order to increase the chance of reaching a higher signal and build reputation. Once the seller builds up his reputation through reaching a high enough signal, he exploits it by shirking. In chapter 2, I study the reputation effect in which a long-lived player faces a sequence of uninformed short-lived players and the uninformed players receive informative but noisy exogenous signals about the type of the long-lived player. I provide an explicit lower bound on all Nash equilibrium payoffs of the long-lived player. The lower bound shows when the exogenous signals are sufficiently noisy and the long-lived player is patient, he can be assured of a payoff strictly higher than his minmax payoff. In Chapter 3 I study optimal dynamic monopoly pricing when a monopolist sells a product with unknown quality to a sequence of short-lived buyers who have private information about the quality. Because past prices and buyers’ purchase behavior convey information about private signals, they jointly determine the public belief about the quality of the monopolist’s product. The monopolist is essentially doing experimentation in the market because every price charged generates not only current period profit but also additional information about the quality. I focus on information structures with a continuum of signals. Under a mild regularity condition on information structures, I show that in equilibrium, the optimal price is an increasing function of the public beliefs. In addition, I fully characterize information cascade sets in terms of information structure. I find that the standard characterization in terms of boundedness of information structure in the social learning literature no longer holds in the presence of a monopoly. In fact, whether herding occurs or not depends more on the values of the conditional densities of the signals at the lowest signal

Effects of spin imbalance on the electric-field driven quantum dissipationless spin current in pp-doped Semiconductors

It was proposed recently by Murakami et al. [Science \textbf{301}, 1348(2003)] that in a large class of $p$-doped semiconductors, an applied electric field can drive a quantum dissipationless spin current in the direction perpendicular to the electric field. In this paper we investigate the effects of spin imbalance on this intrinsic $spin$ Hall effect. We show that in a real sample with boundaries, due to the presence of spin imbalance near the edges of the sample, the spin Hall conductivity is not a constant but a sensitively $position$-$dependent$ quantity, and due to this fact, in order to take the effects of spin imbalance properly into account, a microscopic calculation of both the quantum dissipationless spin Hall current and the spin accumulation on an equal footing is thus required. Based on such a microscopic calculation, a detailed discussion of the effects of spin imbalance on the intrinsic spin Hall effect in thin slabs of $p$-doped semiconductors are presented.Comment: 8 pages, 2 figures, An extended version with detailed calculations To appear in Phys. Rev.

Mixture of Bilateral-Projection Two-dimensional Probabilistic Principal Component Analysis

The probabilistic principal component analysis (PPCA) is built upon a global linear mapping, with which it is insufficient to model complex data variation. This paper proposes a mixture of bilateral-projection probabilistic principal component analysis model (mixB2DPPCA) on 2D data. With multi-components in the mixture, this model can be seen as a soft cluster algorithm and has capability of modeling data with complex structures. A Bayesian inference scheme has been proposed based on the variational EM (Expectation-Maximization) approach for learning model parameters. Experiments on some publicly available databases show that the performance of mixB2DPPCA has been largely improved, resulting in more accurate reconstruction errors and recognition rates than the existing PCA-based algorithms

Finite element analysis of porously punched prosthetic short stem virtually designed for simulative uncemented hip arthroplasty

Background: There is no universal hip implant suitably fills all femoral types, whether prostheses of porous short-stem suitable for Hip Arthroplasty is to be measured scientifically. Methods: Ten specimens of femurs scanned by CT were input onto Mimics to rebuild 3D models; their *stl format dataset were imported into Geomagic-Studio for simulative osteotomy; the generated *.igs dataset were interacted by UG to fit solid models; the prosthesis were obtained by the same way from patients, and bored by punching bears designed by Pro-E virtually; cements between femora and prosthesis were extracted by deleting prosthesis; in HyperMesh, all compartments were assembled onto four artificial joint style as: (a) cemented long-stem prosthesis; (b) porous long-stem prosthesis; (c) cemented short-stem prosthesis; (d) porous short-stem prosthesis. Then, these numerical models of Finite Element Analysis were exported to AnSys for numerical solution. Results: Observed whatever from femur or prosthesis or combinational femora-prostheses, “Kruskal-Wallis” value p > 0.05 demonstrates that displacement of (d) ≈ (a) ≈ (b) ≈ (c) shows nothing different significantly by comparison with 600 N load. If stresses are tested upon prosthesis, (d) ≈ (a) ≈ (b) ≈ (c) is also displayed; if upon femora, (d) ≈ (a) ≈ (b) < (c) is suggested; if upon integral joint, (d) ≈ (a) < (b) < (c) is presented. Conclusions: Mechanically, these four sorts of artificial joint replacement are stabilized in quantity. Cemented short-stem prostheses present the biggest stress, while porous short-stem & cemented long-stem designs are equivalently better than porous long-stem prostheses and alternatives for femoral-head replacement. The preferred design of those two depends on clinical conditions. The cemented long-stem is favorable for inactive elders with osteoporosis, and porously punched cementless short-stem design is suitable for patients with osteoporosis, while the porously punched cementless short-stem is favorable for those with a cement allergy. Clinically, the strength of this study is to enable preoperative strategy to provide acute correction and decrease procedure time

Force-free higher derivative Einstein-Maxwell theory and multi-centered black holes

We investigate which 4-derivative extensions of Einstein-Maxwell theory admit multi-extremal black hole solutions with gravitational force balanced by Coulomb force. We obtain a set of constraints on the 4-derivative couplings by exploring various probe limits in multi-black hole systems. It turns out that these constraints are tighter than those needed to protect the mass-charge ratio of extremal black holes from higher derivative corrections. In fact, they are so strong that the Majumdar-Papapetrou multi-black solutions are unmodified by the force-free combinations of the 4-derivative couplings. Explicit examples of such 4-derivative couplings are given in 4- and 5-spacetime dimensions.Comment: 30 page