First-principles study on the effective masses of zinc-blend-derived Cu_2Zn-IV-VI_4 (IV = Sn, Ge, Si and VI = S, Se)

The electron and hole effective masses of kesterite (KS) and stannite (ST) structured Cu_2Zn-IV-VI_4 (IV = Sn, Ge, Si and VI = S, Se) semiconductors are systematically studied using first-principles calculations. We find that the electron effective masses are almost isotropic, while strong anisotropy is observed for the hole effective mass. The electron effective masses are typically much smaller than the hole effective masses for all studied compounds. The ordering of the topmost three valence bands and the corresponding hole effective masses of the KS and ST structures are different due to the different sign of the crystal-field splitting. The electron and hole effective masses of Se-based compounds are significantly smaller compared to the corresponding S-based compounds. They also decrease as the atomic number of the group IV elements (Si, Ge, Sn) increases, but the decrease is less notable than that caused by the substitution of S by Se.Comment: 14 pages, 6 figures, 2 table

A Study Of Sustaining Firm Competitive Advantage Through Its Image

A mass of literature exists promoting the benefits of resource-based theory of the organization.  This literature suggests that business-level resources may be sources of competitive advantage if they are rare, valuable, nonsubstitutable, and difficult to imitate.  Unfortunately, applying this theory to research or practice requires that the study first identify specific resources that can be measured and investigated.  It also requires that this study develops and tests separate theories to explain how these resources are related to competitive advantage.  Only through these steps will this study be able to extend the resource-based view of the organization in explaining and affecting competitive advantage efforts.  The overarching theory (i.e., resource-based theory) provides only a useful guide to identifying important resources and relationships.  The current study argues that organization image is one such resource.  The ability of this resource is to enhance an organization competitive advantage.  Furthermore, the tacitness, complexity, and specificity characterized by organization image can make this advantage sustainable

Maximizing electric power through spectral-splitting photovoltaic-thermoelectric hybrid system integrated with radiative cooling

As zero-emission technologies, a daytime radiative cooling (RC) strategy developed recently, and photovoltaic (PV) and thermoelectric (TE) technologies have aroused great interest to reduce fossil fuel consumption and carbon emissions. How to integrate these state-of-the-art technologies to maximise clean electricity from the sun and space remains a huge challenge, and the limit efficiency is still unclear. In this study, a spectral-splitting PV-TE hybrid system integrated with RC is proposed to maximise clean electricity from the sun and space without any emissions. For the sun acting as a typical constant heat-flux heat source, the current thermoelectric theory overestimates the thermoelectric efficiency highly since the theory is based on constant temperature-difference conditions. A new theory based on heat-flux conditions is employed to achieve maximum thermoelectric efficiency. The PV-TE hybrid system with RC is superior to the conventional hybrid system, not only in terms of higher efficiency but also in its 24-h operation capacity. In a system with a single-junction cell, the total efficiency with 30 suns (39.4%) is higher than the theoretical PV efficiency at 500 suns (38.2%). In a hybrid system with four-junction cells, total efficiency is over 65% which is superior to most current photoelectric and thermal power systems

Scanning tunneling spectroscopy of a magnetic atom on graphene in the Kondo regime

The Kondo effect in the system consisting of a magnetic adatom on the graphene is studied. By using the non-equilibrium Green function method with the slave-boson mean field approximation, the local density of state (LDOS) and the conductance are calculated. For a doped graphene, the Kondo phase is present at all time. Surprisingly, two kinds of Kondo regimes are revealed. But for the undoped graphene, the Kondo phase only exists if the adatom's energy level is beyond a critical value. The conductance is similar to the LDOS, thus, the Kondo peak in the LDOS can be observed with the scanning tunneling spectroscopy. In addition, in the presence of a direct coupling between the STM tip and the graphene, the conductance may be dramatically enhanced, depending on the coupling site.Comment: 4 pages, 4 figures, accepted by EP

Alloy Stabilized Wurtzite Ground State Structures of Zinc-Blende Semiconducting Compounds

The ground state structures of the A$_x$B$_{1-x}$C wurtzite (WZ) alloys with $x=$0.25, 0.5, and 0.75 are revealed by a ground state search using the valence-force field model and density-functional theory total energy calculations. It is shown that the ground state WZ alloy always has a lower strain energy and formation enthalpy than the corresponding zinc-blende (ZB) alloy. Therefore, we propose that the WZ phase can be stabilized through alloying. This novel idea is supported by the fact that the WZ AlP$_{0.5}$Sb$_{0.5}$, AlP$_{0.75}$Sb$_{0.25}$, ZnS$_{0.5}$Te$_{0.5}$, and ZnS$_{0.75}$Te$_{0.25}$ alloys in the lowest energy structures are more stable than the corresponding ZB alloys. To our best knowledge, this is the first example where the alloy adopts a structure distinct from both parent phases

Thermal-hydraulic performance of mist/compressed humid air two-phase flow in an airfoil channel recuperator

The introducing water mist into the cold channel of recuperator was adopted to improve the thermal-hydraulic performance of compressed humid air in a megawatt grade humid air turbine cycle. The influences of mist content, air pressure and droplet diameter on the thermal-hydraulic performance of the humid air were numerically simulated. The results showed that when the quantity of mist increased from 0 to 16%, the pressure drop decreases by 24.35% while the Colburn factor increases by 8%. The method can effectively mediate the contradiction between heat transfer and resistance. However, the variation of air pressure had little effect on the thermal-hydraulic performance. Besides, the best heat transfer performance can be realized when the inlet droplet diameters are between 5 and 10 μm. A new heat transfer enhancement factor of mist content was proposed, and the correlations of the Nusselt number and friction factor were obtained. When enhancement factor increases from 0 to 0.16, the Nusselt number increases by 141.5%, and the friction factor decreases by 86.4%. The method of injecting mist can improve the thermal-hydraulic performance and reduce the volume of recuperator, which is further beneficial to reduce the cost and promote the thermal efficiency in the practical application of humid air turbine cycle. It has high application value in the future commercial application of humid air turbine cycle. The results and correlations can provide model and guidance for the design and application of recuperator and humid air turbine cycle