Crystal structure and electronic structure of quaternary semiconductors Cu2_2ZnTiSe4_4 and Cu2_2ZnTiS4_4 for solar cell absorber

We design two new I2-II-IV-VI4 quaternary semiconductors Cu$_2$ZnTiSe$_4$ and Cu$_2$ZnTiS$_4$, and systematically study the crystal and electronic structure by employing first-principles electronic structure calculations. Among the considered crystal structures, it is confirmed that the band gaps of Cu$_2$ZnTiSe$_4$ and Cu$_2$ZnTiS$_4$ originate from the full occupied Cu 3$d$ valence band and unoccupied Ti 3$d$ conducting band, and kesterite structure should be the ground state. Furthermore, our calculations indicate that Cu$_2$ZnTiSe$_4$ and Cu$_2$ZnTiS$_4$ have comparable band gaps with Cu$_2$ZnTSe$_4$ and Cu$_2$ZnTS$_4$, but almost twice larger absorption coefficient $\alpha(\omega)$. Thus, the materials are expected to be candidate materials for solar cell absorber.Comment: 4 pages, 4 figure

Moral Education in Plato\u27s Gorgias

In this thesis I argue for the theme of moral education as an important theme for Plato\u27s Gorgias. The Gorgias has been commonly understood in terms of politics and philosophy either separately or jointly. I argue that the theme of moral education is equally important and deserves attention. I first contextualize the Gorgias into the cultural background of late 5th century BCE when the decline of traditional morality stimulated the Greeks to ask whether virtues (which were traditionally conceived to be hereditary traits exclusive to members of noble family) were teach-able (and thus, could be learned). Next, I analyze the three conversations in the Gorgias as a unity that demonstrates the impact of Gorgias\u27 teaching. Then, through an analysis of the relationship between morality, education, and politics, I argue that the Gorgias as a whole presents two contrasting modes of education, namely, Gorgias way of teaching and Socrates\u27 education, between which Socrates\u27 education is preferred, but not without its problems

Anisotropies in insulating La2−x_{2-x}Srx_xCuO4_4: angular resolved photoemission and optical absorption

Due to the orthorhombic distortion of the lattice, the electronic hopping integrals along the $a$ and $b$ diagonals, the orthorhombic directions, are slightly different. We calculate their difference in the LDA and find $t_{a}^{\prime}-t_{b}^{\prime}\approx 8$meV. We argue that electron correlations in the insulating phase of La$_{2-x}$Sr$_{x}$CuO$_{4}$, i. e. at doping $x\leq 0.055,$ dramatically enhance the $(t_{a}^{\prime}-t_{b}^{\prime})$-splitting between the $a$- and $b$-hole valleys. In particular, we predict that the intensity of both angle-resolved photoemission and of optical absorption is very different for the $a$ and $b$ nodal points

Automatically learning topics and difficulty levels of problems in online judge systems

Online Judge (OJ) systems have been widely used in many areas, including programming, mathematical problems solving, and job interviews. Unlike other online learning systems, such as Massive Open Online Course, most OJ systems are designed for self-directed learning without the intervention of teachers. Also, in most OJ systems, problems are simply listed in volumes and there is no clear organization of them by topics or difficulty levels. As such, problems in the same volume are mixed in terms of topics or difficulty levels. By analyzing large-scale users’ learning traces, we observe that there are two major learning modes (or patterns). Users either practice problems in a sequential manner from the same volume regardless of their topics or they attempt problems about the same topic, which may spread across multiple volumes. Our observation is consistent with the findings in classic educational psychology. Based on our observation, we propose a novel two-mode Markov topic model to automatically detect the topics of online problems by jointly characterizing the two learning modes. For further predicting the difficulty level of online problems, we propose a competition-based expertise model using the learned topic information. Extensive experiments on three large OJ datasets have demonstrated the effectiveness of our approach in three different tasks, including skill topic extraction, expertise competition prediction and problem recommendation

Variational Structure and Two Dimensional Jet Flows for Compressible Euler System with Non-zero Vorticity

In this paper, we investigate the well-posedness theory of compressible jet flows for two dimensional steady Euler system with non-zero vorticity. One of the key observations is that the stream function formulation for two dimensional compressible steady Euler system with non-zero vorticity enjoys a variational structure, so that the jet problem can be reformulated as a domain variation problem. This allows us to adapt the framework developed by Alt, Caffarelli and Friedman for the one-phase free boundary problems to obtain the existence and uniqueness of smooth solutions to the subsonic jet problem with non-zero vorticity. We also show that there is a critical mass flux, such that as long as the incoming mass flux does not exceed the critical value, the well-posedness theory holds true

LIVING WELL WITH WORRIES, HUMILITY, AND OTHER PEOPLE: LEARNING TO REFLECT ON LIFE WITH THE ANALECTS

In this dissertation, I present three distinct papers connected through the theme of control. The Analects suggests a view of control or human efficacy that we have a great deal of control over events related to us. But our control is often partial, limited, and indeterminate. It suggests, in other words, a continuum of control that goes from having (relatively) full control over certain things to having (relatively) no control over certain things, with a middle section where we exert partial and indeterminate amount of control. Operating under this model of control, I develop and argue for three theses regarding 1) the emotion of worry when our control and concern over things should go beyond a small set; 2) humility when we are humbled by the challenge of self-cultivation and others’ contributions; and 3) relational virtue when we give up a certain control and let others partially determine not just what we do, but who we are. In the first paper, I argue that in the Analects, learning to worry well is part and parcel of the Confucian cultivation program, and it includes learning to worry broadly about non-related others, and deeply about intimate others. In the second paper, I argue that in the Analects, humility is about ritually expressed vigilance developed from a strong desire for cultivating oneself and devotion to responsibilities that honor forerunners and check against misanthropy. In the third paper, I argue that relational virtues that characterize being a good friend, spouse, sibling, or child require more than the agent’s disposition, but also acknowledgement from intimate others, and in unfortunate situations, public acknowledgement and evaluation. To be relationally virtuous, the agent does not fully determine whether she is virtuous or not