Optical signatures of states bound to vacancy defects in monolayer MoS2_2

We show that pristine MoS$_2$ single layer (SL) exhibits two bandgaps $E_{g\parallel}=1.9$ eV and $E_{g\perp}=3.2$ eV for the optical in-plane and out-of-plane susceptibilities $\chi_\parallel$ and $\chi_\perp$, respectively. In particular, we show that odd states bound to vacancy defects (VDs) lead to resonances in $\chi_\perp$ inside $E_{g\perp}$ in MoS$_2$ SL with VDs. We use density functional theory, the tight-binding model, and the Dirac equation to study MoS$_2$ SL with three types of VDs: (i) Mo-vacancy, (ii) S$_2$-vacancy, and (iii) 3$\times$MoS$_2$ quantum antidot. The resulting optical spectra identify and characterize the VDs.Comment: 5 pages, 5 figure

Electronic and Optical Properties of Vacancy Defects in Transition Metal Dichalcogenides

A detailed first-principle study has been performed to evaluate the electronic and optical properties of single-layer (SL) transition metal dichalcogenides (TMDCs) (MX${}_2$; M= transition metal such as Mo, W and X= S, Se, Te), in the presence of vacancy defects (VDs). Defects usually play an important role in tailoring electronic, optical, and magnetic properties of semiconductors. We consider three types of VDs in SL TMDCs i) $X$-vacancy, $X_{2}$-vacancy, and iii) $M$-vacancy. We show that VDs lead to localized defect states (LDS) in the band structure, which in turn give rise to sharp transitions in in-plane and out-of-plane optical susceptibilities, $\chi_{\parallel}$ and $\chi_{\perp}$. The effects of spin orbit coupling (SOC) are also considered. We find that SOC splitting in LDS is directly related to the atomic number of the transition metal atoms. Apart from electronic and optical properties we also find magnetic signatures (local magnetic moment of $\sim\mu_{B}$) in MoSe$_{2}$ in the presence of Mo vacancy, which breaks the time reversal symmetry and therefore lifts the Kramers degeneracy. We show that a simple qualitative tight binding model (TBM), involving only the hopping between atoms surrounding the vacancy with an on-site SOC term, is sufficient to capture the essential features of LDS. In addition, the existence of the LDS can be understood from the solution of the 2D Dirac Hamiltonian by employing infinite mass boundary conditions. In order to provide a clear description of the optical absorption spectra, we use group theory to derive the optical selection rules between LDS for both $\chi_{\parallel}$ and $\chi_{\perp}$.Comment: 14 pages, 11 figure

Dirac electrons in the presence of matrix potential barrier: application to graphene and topological insulators

Scattering of a 2D Dirac electrons on a rectangular matrix potential barrier is considered using the formalism of spinor transfer matrices. It is shown, in particular, that in the absence of the mass term, the Klein tunneling is not necessarily suppressed but occurs at oblique incidence. The formalism is applied to studying waveguiding modes of the barrier, which are supported by the edge and bulk states. The condition of existence of the uni-directionality property is found. We show that the band of edge states is always finite with massless excitations, while the spectrum of the bulk states, depending on parameters of the barrier, may consist of the infinite or finite band with both, massive and massless, low-energy excitations. The effect of the Zeeman term is considered and the condition of appearance of two distinct energy dependent directions corresponding to the Klein tunneling is found.Comment: published versio

Horizontal Scroll Compressor for Refrigeration Applications

New efficiency regulations, as well as trends in applications of new low GWP refrigerants, force compressors to be re-designed, especially for A3 refrigerants, because of the regulatory limitations of the refrigerant charge. The paper addresses compressor design allowing for more efficient use of the refrigerant charge. An oil sump retains significant amount of refrigerant diluted in the oil. Elimination of the oil sump allows to use the limited refrigerant charge more effectively. Furthermore, it reduces the size of the compressor and simplifies the compressor structure. Another benefit of this type of compressor is its orientation flexibility-the same compressor can be installed vertically or horizontally. These design features are beneficial in close-coupled systems, as well as transport (truck/trailer) refrigeration systems. The sump-less design requires a revision of friction pairs in the compressor: radial bearings, thrust bearings, tip-tobase contact and others. Existing refrigeration compressors rely on the continuous oil flow for journal bearing lubrication, while the lubrication mechanism in the sump-less compressor relies on the oil mist. This design leverages the approach utilized in the automotive scroll compressors, but the requirements and conditions for the compressor operation and reliability are different. The paper discusses the major design steps and results of preliminary reliability and system tests with an experimental determination of the optimal oil charge

Probing La(0.7)Sr(0.3)MnO3 multilayers via spin wave resonances

La(0.7)Sr(0.3)MnO3/BiFeO3 and La(0.7)Sr(0.3)MnO3/PbZr20Ti80O3 epitaxial heterostructures have been grown on SrTiO3 substrates. Spin wave resonances are used to study interface properties of the ferromagnetic La(0.7)Sr(0.3)MnO3. We find that the addition of the BiFeO3 or PbZr20Ti80O3 causes out-of-plane surface pinning of the La(0.7)Sr(0.3)MnO3. We are able to place new limits on the exchange constant D of La(0.7)Sr(0.3)MnO3 grown on these substrates and confirm the presence of uniaxial and biaxial anisotropies caused by the SrTiO3 substrate.Comment: 7 pages, 5 figures, 3 table