Fundamental exciton linewidth broadening in monolayer transition metal dichalcogenides

Monolayer Transition Metal Dichalcogenides (TMDS) are highly luminescent materials despite being sub-nanometer thick due to the ultra-short ($<1$ ps) radiative lifetime of the strongly bound bright excitons hosted by these materials. The intrinsically short radiative lifetime results in a large broadening in the exciton band with a magnitude that is about two orders greater than the spread of the light cone itself. The situation calls for a need to revisit the conventional light cone picture. We present a modified light cone concept which places the light line $(\hbar cQ)$ as the generalized lower bound for allowed radiative recombination. A self-consistent methodology, which becomes crucial upon inclusion of large radiative broadening in the exciton band, is proposed to segregate the radiative and the non-radiative components of the homogeneous exciton linewidth. We estimate a fundamental radiative linewidth of $1.54\pm0.17\$meV, owing purely to finite radiative lifetime in the absence of non-radiative dephasing processes. As a direct consequence of the large radiative limit, we find a surprisingly large ($\sim 0.27$ meV) linewidth broadening due to zero-point energy of acoustic phonons. This obscures the precise experimental determination of the intrinsic radiative linewidth and sets a fundamental limit on the non-radiative linewidth broadening at $T = 0$ K

Foreign Capital Inflow, Skilled-Unskilled Wage Gap, and Welfare in the Presence of the Informal Sector

This paper attempts to analyse the impact of trade liberalisation on the skilled- unskilled wage gap and the level of welfare of developing countries, which are generally characterised by large “informal” labour markets. A neo-classical full-employment four-sector model has been developed, where the informal sector produces either a final product or an intermediate product on subcontracting basis. Evidence shows that in either case, trade liberalisation, in the form of an increase in foreign capital inflow, widens the skilled-unskilled wage gap of the economy under some reasonable conditions. It also shows that as a result of an increase in the foreign capital inflow, the level of welfare of the economy increases, when the informal sector producesCapital Inflow, Informal Sector

Direct observation of giant binding energy modulation of exciton complexes in monolayer MoSe2_2

Screening due to surrounding dielectric medium reshapes the electron-hole interaction potential and plays a pivotal role in deciding the binding energies of strongly bound exciton complexes in quantum confined monolayers of transition metal dichalcogenides (TMDs). However, owing to strong quasi-particle bandgap renormalization in such systems, a direct quantification of estimated shifts in binding energy in different dielectric media remains elusive using optical studies. In this work, by changing the dielectric environment, we show a conspicuous photoluminescence (PL) peak shift at low temperature for higher energy excitons (2s, 3s, 4s, 5s) in monolayer MoSe$_2$, while the 1s exciton peak position remains unaltered - a direct evidence of varying compensation between screening induced exciton binding energy modulation and quasi-particle bandgap renormalization. The estimated modulation of binding energy for the 1s exciton is found to be 58.6% (70.5% for 2s, 78.9% for 3s, 85% for 4s) by coating an Al$_2$O$_3$ layer on top, while the corresponding reduction in quasi-particle bandgap is estimated to be 248 meV. Such a direct evidence of large tunability of the binding energy of exciton complexes as well as the bandgap in monolayer TMDs holds promise of novel device applications.Comment: 19 pages including supplemental informatio

Trade Liberalisation, Health Care and International Fragmentation: The Role of Health Capital Mobility

This paper delves into the complex relationship between health trade through international fragmentation and health trade through commercial presence. A neo-classical full employment four sector static general equilibrium model has been developed, where the three sectors produce final products except the health intermediate goods producing sector. The paper shows that expansion of health trade through commercial presence implies, under some reasonable conditions, enhancement of the volume of health trade through international fragmentation. It also shows that the composite volume of trade in health services through international fragmentation and commercial presence increases the size of the health care in our stylised small open economy. JEL Classification: I10, F11, F21, D58 Keywords: Health Sector, Health Intermediate Sector, International Fragmentation and International Health Capital Mobilit

Polarized and narrow excitonic emission from graphene-capped monolayer WS2_2 through resonant phonon relaxation

The broadening and polarization of excitonic luminescence in monolayer TMDs largely suffer from inhomogeneity and temperature - an unresolved problem to date. In this work, through few-layer-graphene encapsulation of monolayer WS$_2$, we reduce the inter-excitonic energy separation, which then can have a narrow resonance with a specific phonon mode of our choice. The resulting single-step exciton relaxation with the resonating phonon mode significantly suppresses the inhomogeneous broadening, allowing us to achieve the narrowest exciton linewidth of 1.06 meV (which translates to 0.19 meV after deconvolution with the excitation laser linewidth). The single-phonon resonance helps to achieve a high quantum efficiency despite graphene encapsulation. The technique is powerful in tuning the exciton polarization during relaxation by choosing a specific resonating phonon mode. For example, the valley coherence (polarization) improves from $\sim$68% ($\sim$40%) to $\sim$90% ($\sim$75%) on resonance with 2$A_1$' and $A_1$' modes respectively. We further demonstrate a strong polarization reversal on resonance with a chiral phonon mode. Strikingly, the above features remain robust against temperature (up to 200 K) and sample age (few months in ambient condition). The findings will lead to clean excitonic measurements without requiring cryogenic cooling

Foreign Capital Inflow, Skilled-Unskilled Wage Gap, and Welfare in the Presence of the Informal Sector

This paper attempts to analyse the impact of trade liberalisation on the skilledunskilled wage gap and the level of welfare of developing countries, which are generally characterised by large “informal” labour markets. A neo-classical full-employment foursector model has been developed, where the informal sector produces either a final product or an intermediate product on subcontracting basis. Evidence shows that in either case, trade liberalisation, in the form of an increase in foreign capital inflow, widens the skilled-unskilled wage gap of the economy under some reasonable conditions. It also shows that as a result of an increase in the foreign capital inflow, the level of welfare of the economy increases, when the informal sector produces a final product. However, when the informal sector produces an intermediate product on subcontracting basis, the level of welfare of the economy falls

Observation of perfect valley coherence in monolayer MoS2_2 through giant enhancement of exciton coherence time

In monolayer transition metal dichalcogenide semiconductors, valley coherence degrades rapidly due to a combination of fast scattering and inter-valley exchange interaction. This leads to a sub-picosecond valley coherence time, making coherent manipulation of exciton a highly formidable task. Using monolayer MoS$_2$ sandwiched between top and bottom graphene, here we demonstrate perfect valley coherence by observing 100% degree of linear polarization of excitons in steady state photoluminescence. This is achieved in this unique design through a combined effect of (a) suppression in exchange interaction due to enhanced dielectric screening, (b) reduction in exciton lifetime due to a fast inter-layer transfer to graphene, and (c) operating in the motional narrowing regime. We disentangle the role of the key parameters affecting valley coherence by using a combination of calculation (solutions of Bethe-Salpeter and steady-state Maialle-Silva-Sham equations) and choice of systematic design of experiments using four different stacks with varying screening and exciton lifetime. To the best of our knowledge, this is the first report in which the valley coherence timescale has been significantly enhanced beyond the exciton radiative lifetime in monolayer semiconductors

Health Care Quality vs Health Care Quantity: A General Equilibrium Analysis

This paper attempts to relate the issues of health care quality with international trade. For this purpose we have mixed both flavours of Heckscher-Ohlin-Samuelson and Neo- Heckscher-Ohlin frameworks and developed a hybrid type of trade theoretic general equilibrium model. In such a set up we have shown that a movement from a regime of international health capital immobility to a regime of international health capital mobility may lead to an expansion of the health quality exporting sector. Apart from quality aspect of health services, the quantity aspect of health care has been also considered in this study. Moreover, from that hybrid model we have illustrated that the sizes of health care and composite export sector expand, where as import sector of our small open economy contracts

Liberalization and Welfare Conditions of a Developing Economy: A General Equilibrium Analysis

The paper attempts to analyze the impact of trade liberalization policy, in terms of FDI, on the level of informal competitive wage rate as well as on the size of the informal sectors of a developing economy with dualistic economic structure in a general equilibrium framework. The wage rate earned by the informal workers has been considered here as a proxy for their living standard. In this paper it is found that FDI raises the level of wage rate of the informal workers and consequently raises their standard of living. It is also found in this paper that FDI expands both formal and informal manufacturing sectors in the urban areas whereas it contracts the rural agricultural informal sector. In this structure an attempt has also been made to analyze the effects on the welfare level of the economy for a drive towards liberalization through FDI by assuming Sen (1974) type social welfare function which considers inequality in income distribution