Philander Smith College

Generally, formamidinium (FA)-based halide perovskite thin films are fabricated using halide precursors, such as PbI2 and FAI, but this approach often leads to stoichiometric distortions, resulting in perovskite films with structural defects and reduced crystallinities. These problems can adversely influence power conversion efficiencies and open-circuit potentials of perovskite solar cells. In this study, we propose a microcrystalline perovskite powder (MCP) synthesized by controlling the stoichiometry of the FAI precursor. We optimize the synthesis of α-FAPbI3 powder using 1.1 equiv of FAI. Interestingly, the synthesized black powder produced an excellent crystallinity and phase stability for up to six months. Remarkably, the MCP forms large colloids in solutions that are stably cohesive, promoting spontaneous nucleation and enabling the fabrication of low-defect thin films. Consequently, perovskite solar cells fabricated using the MCP display significantly improved efficiencies of 23.12% compared to those (19.64%) of the cells fabricated using the conventional PbI2 and FAI precursors. This approach highlights the potential of MCPs for use in enhancing the performances and stabilities of perovskite-based devices.

Gravity is perturbatively renormalizable for the physical states which can be conveniently defined via foliation-based quantization. In recent sequels, one-loop analysis was explicitly carried out for Einstein-scalar and Einstein-Maxwell systems. Various germane issues and all-loop renormalizability have been addressed. In the present work we make further progress by carrying out several additional tasks. Firstly, we present an alternative 4D-covariant derivation of the physical state condition by examining gauge choice-independence of a scattering amplitude. To this end, a careful dichotomy between the ordinary, and large gauge symmetries is required and appropriate gauge-fixing of the ordinary symmetry must be performed. Secondly, vacuum energy is analyzed in a finite-temperature setup. A variant optimal perturbation theory is implemented to two-loop. The renormalized mass determined by the optimal perturbation theory turns out to be on the order of the temperature, allowing one to avoid the cosmological constant problem. The third task that we take up is examination of the possibility of asymptotic freedom in finite-temperature quantum electrodynamics. In spite of the debates in the literature, the idea remains reasonable.

The cosmological constant problem is examined by taking an Einstein--scalar with a Higgs-type potential and scrutinizing the infrared structure induced by finite temperature effects. A variant optimal perturbation theory is implemented in the recently proposed quantum-gravitational framework. The optimized renormalized mass, i.e., the renormalized mass determined by the variant optimal perturbation theory, of the scalar field turns out to be on the order of the temperature. This shifts the cosmological constant problem to compatibility of the consequent perturbative analysis. The compatibility is guaranteed essentially by renormalization group invariance of physical quantities. We point out the resummation behind the invariance.

Abstract Recent studies show terrorist organizations that target only civilians almost always fail to achieve their ultimate objectives. On the contrary, groups that target combatants and civilians have better chances of success. Yet, most terrorist organizations do not directly target the state. When terrorist organizations shift their strategy from purely terrorist acts to confrontation against the target state, we may see a transition from terrorism to civil war. A terrorist organization's decision to engage in civil war largely depends on the organization's ability to alleviate the collective action problem. We argue terrorist groups with a territorial goal and groups operating in oil-rich countries are more likely to engage in civil war. The desire to gain a separate homeland is a powerful motivator to overcome the collective action problem. Terrorist organizations that operate in oil-rich countries are more likely to resort to civil war because oil dependence has the potential to increase grievances, which motivate rebellion, and resources provide a means of financing rebellion, while weakening target state institutions. Our findings confirm the existence of a territorial goal and availability of oil resources trigger transition from terrorism to civil war.

We consider the information loss paradox at different angles, from the standard semi-classical approximation of General Relativity to the recently proposed scenarios of black holes evolution caused by effects of gravity quantization. Focusing on the Firewall proposal, we study the loop effects on the geometry and boundary conditions in black hole spacetimes and analyze the energy measured by an infalling observer near their horizons. As a result we obtain a trans-Planckian energy transition for the time-dependent black hole solution on the quantum-induced AdS background, the importance of which for the black hole (in)formation is discussed.

We continue our recent endeavor in which a time-dependent black hole solution of a one-loop quantum-corrected Einstein-scalar system was obtained and its near-horizon behavior was analyzed. The energy analysis led to a trans-Planckian scaling behavior near the event horizon. In the present work, the analysis is extended to a rotating black hole solution of an Einstein–Maxwell-scalar system with a Higgs potential. Although the analysis becomes much more complex compared to that of the previous, we observe the same basic features, including the quantum-gravitational trans-Planckian energy near the horizon.

Квантовые гравитационные эффекты могут содержать в себе ключ к некоторым нерешенным проблемам теоретической физики. Проанализированы пертурбативные квантовые эффекты на границе гравитационной системы и граничные условия Дирихле, наложенные на классическом уровне. Анализ показал, что для решения типа черной дыры существует противоречие между квантовыми эффектами и граничным условием Дирихле: решение типа черной дыры, полученное из одночастично-неприводимого действия, не удовлетворяет граничным условиям Дирихле, как это можно было бы ожидать, не вдаваясь в подробности. Результатом представленного анализа также является предположение о том, что противоречие между граничным условием Дирихле и петлевыми эффектами связано с некоторым механизмом накопления информации на границе.

We have recently proposed in \cite{Park:2014tia} the quantization of pure 4D Einstein gravity through hypersurface foliation, and observed that the 4D Einstein gravity becomes renormalizable once all (or most) of the unphysical degrees of freedom are removed. In this work, we confirm this observation from a more mathematical angle. In particular, we show that the physical state condition arising from the shift vector constraint connects with the requirement that the manifold admit "totally geodesic (TG) foliation". The TG foliation, in turn, makes it possible to view the 4D manifold as abelian fibration over a 3D base. Associating the abelian fibration with the 4D diffeomorphism leads to reduction of the 4D manifold to 3D, thereby realizing and generalizing the holography of 't Hooft.