Low-Frequency Raman Modes and Electronic Excitations In Atomically Thin MoS2 Crystals

Atomically thin MoS$_{2}$ crystals have been recognized as a quasi-2D semiconductor with remarkable physics properties. This letter reports our Raman scattering measurements on multilayer and monolayer MoS$_{2}$, especially in the low-frequency range ($<$50 cm$^{-1}$). We find two low-frequency Raman modes with contrasting thickness dependence. With increasing the number of MoS$_{2}$ layers, one shows a significant increase in frequency while the other decreases following a 1/N (N denotes layer-number) trend. With the aid of first-principle calculations we assign the former as the shear mode $E_{2g}^{2}$ and the latter as the compression vibrational mode. The opposite evolution of the two modes with thickness demonstrates novel vibrational modes in atomically thin crystal as well as a new and more precise way to characterize thickness of atomically thin MoS$_{2}$ films. In addition, we observe a broad feature around 38 cm$^{-1}$ (~5 meV) which is visible only under near-resonance excitation and pinned at the fixed energy independent of thickness. We interpret the feature as an electronic Raman scattering associated with the spin-orbit coupling induced splitting in conduction band at K points in their Brillouin zone.Comment: 5 pages, 4 figure

Germinación de semillas de Pinus koraiensis Siebold & Zucc en respuesta a regímenes de luz causados por sombreo y situación de la semilla

Pinus koraiensis Siebold &amp; Zucc. (Korean pine), the dominant tree species in the mixed broadleaved Korean pine forests (regional climax), is severely restricted by its regeneration failure. To determine the effects of light regimes on P. koraiensis regeneration, the seed germination process was examined in shade houses and forest stands (before and after leaf expansion) with various light levels created by shading and seed positions. Despite the large size of P. koraiensis seeds (500-600 mg), both light intensity and quality significantly affected the germination percentage in both shade houses and forests. Substantial changes in light intensity and quality led the majority of seeds (80%) to germinate in leafless forests and shade houses, while only a minority (≤ 20%) germinated after leaf expansion in the forests. Moreover, seed germination in shade houses and leafless forests exhibited similar patterns; they consistently reached a 70% shading degree, which was optimal for the seed germination of P. koraiensis on topsoil. Seed positioning significantly affected germination for each shading degree, especially when litter and soil coverings drastically inhibited germination. In conclusion, (1) when seeds were not stressed by temperature and moisture, light irradiance played a critical role in the seed germination of P. koraiensis; (2) seed positioning, in relation to alterations in light intensity and quality, affected the germination of P. koraiensis; (3) a shade house experiment using neutral cloth provided an applicable and controllable way to monitor the P. koraiensis seed germination in early spring before leaf expansion. The light requirement for the germination of P. koraiensis played a key role in the regeneration of P. koraiensis throughout the temperate secondary forests.Pinus koraiensis Siebold &amp; Zucc. es la especie arbórea dominante en los bosques mixtos de frondosas de pinos coreanos (clímax regional) que se ve seriamente limitada por su falta de regeneración. Para determinar los efectos de los regímenes de luz sobre la regeneración de P. koraiensis se ha examinado el proceso de germinación de la semilla en cámaras de sombra y en masas forestales (antes y después de la expansión de las hojas) con diferentes niveles de luz creados por sombreado y por la localización de las semillas. A pesar del gran tamaño de las semillas de P. koraiensis (500-600 mg), tanto la intensidad de luz y la calidad han afectado significativamente el porcentaje de germinación tanto en cámaras de sombra como en los bosques. Cambios importantes en la intensidad de la luz y en su calidad ocasionaron la germinación de la mayoría de las semillas (80%) en los bosques sin hojas y cámaras de sombra, mientras que sólo una minoría (≤ 20%) germinaron después de la expansión de las hojas en el bosque. Por otra parte, la germinación de las semillas en las cámaras de sombra y en bosques sin hojas mostraron patrones similares. Constantemente han alcanzado un grado de sombreado del 70%, lo cual es óptimo para la germinación de las semillas de P. koraiensis en tierra vegetal. La posición de la semilla afecta significativamente a la germinación en cada grado de sombreo, especialmente cuando los revestimientos de desechos vegetales y de tierra inhibieron drásticamente la germinación. Como conclusiones, (1) cuando las semillas no se estresan por temperatura o humedad, la irradiación de luz juega un papel crítico en la germinación de las semillas de P. koraiensis; (2) La posicion de las semillas, en relación con alteraciones en la intensidad de la luz y la calidad, afectó la germinación de P. koraiensis, (3) un experimento en cámara de sombreo utilizando un paño neutral proporciona una manera para controlar la germinación de P. koraiensis en primavera, antes de la expansión foliar. El requisito de luz para la germinación de P. koraiensis juega un papel clave en la regeneración de los bosques secundarios templadas

Condition assessment of heritage timber buildings in operational environments

© 2017, Springer-Verlag GmbH Germany. Due to changing environments and aging, the structural resistance of the heritage buildings has been reduced significantly. It has become crucial to monitor and protect the architectural heritage buildings. The objective of this research is to monitor and assess the performance of the heritage Tibetan timber building in operational environments. A three-storey corridor part of the typical heritage building was chosen in the study. A long-term monitoring system was installed in the building to collect the structural response and temperature. Detailed finite element model was built based on site investigation and existing documents, and updated based on the temperature-based response sensitivity using the field-monitoring data. The updated model was further evaluated using the static and dynamic analysis for condition assessment of the building in operational environments. The results show that the updated model is effective and accurate to predict the structural behaviour of the building in operational environments. Based on temperature-based response sensitivity, it is capable of tracking structure performance throughout the life-cycle allowing for condition-based maintenance and structural protection

Connection stiffness identification of historic timber buildings using Temperature-based sensitivity analysis

© 2016 Elsevier Ltd The beam-column connection, called ‘Que Ti’, is the key component of historic Tibetan timber buildings to transfer shear, compression and bending loads from one structural element to another. This kind of connections can reduce the internal forces and improve the structure's ability to resist earthquakes. Its structure is very complicated and there is little information about the behaviour of this kind of semi-rigid connections. In this paper, a temperature-based response sensitivity method is proposed to identify the connection stiffness of the ‘Que-Ti’ in typical historical Tibetan buildings from temperature and strain response measurements. The semi-rigid connection is modeled as two rotational springs and one compressive spring. The temperature is treated as a measurable input and the thermal loading on the structure can be determined from the temperature variation. The numerical results show the method is effective and reliable to identify both unknown boundary conditions and the connection stiffness of the structure accurately even with 10% noise in measurements. A long-term monitoring system has also been installed in a typical historical Tibetan building and the monitoring data are used to further verify the proposed method. The experimental results show that the identified stiffnesses by the proposed method are consistent with that by finite element model updating from ambient vibration measurements

Limits from Weak Gravity Conjecture on Dark Energy Models

The weak gravity conjecture has been proposed as a criterion to distinguish the landscape from the swampland in string theory. As an application in cosmology of this conjecture, we use it to impose theoretical constraint on parameters of two types of dark energy models. Our analysis indicates that the Chaplygin-gas-type models realized in quintessence field are in the swampland, whereas the $a$ power-low decay model of the variable cosmological constant can be viable but the parameters are tightly constrained by the conjecture.Comment: Revtex4, 8 pages, 5 figures; References, minor corrections in content, and acknowledgement adde