Transition-metal dichalcogenide heterostructure solar cells: A numerical study

We evaluate the tunneling short-circuit current density $J_{TU}$ in a $p$-$i$-$n$ solar cell in which the transition metal dichalcogenide heterostructure (MoS$_2$/WS$_2$ superlattice) is embedded in the intrinsic $i$ region. The effects of varying well and barrier widths, Fermi energy levels and number of quantum wells in the $i$ region on $J_{TU}$ are examined. A similar analysis is performed for the thermionic current $J_{TH}$ that arises due to the escape and recapture of charge carriers between adjacent potential wells in the $i$-region. The interplay between $J_{TU}$ and $J_{TH}$ in the temperature range (300 K - 330 K) is examined. The thermionic current is seen to exceed the tunneling current considerably at temperatures beyond 310 K, a desirable attribute in heterostructure solar cells. This work demonstrates the versatility of monolayer transition metal dichalcogenides when utilized as fabrication materials for van der Waals heterostructure solar cells.Comment: 9 pages, 4 figs, Journal of Mathematical Chemistry (2016

Quantum Zeno effect on Quantum Discord

We examine the quantum Zeno effect on the dynamics of quantum discord in two initially entangled qubits which are subjected to frequent measurements via decoherent coupling with independent reservoirs. The links between characteristic parameters such as system bias, measurement time duration, strength of initial entanglement between the two qubit systems and the dynamics of quantum discord are examined in the case of two initial state configurations. For weak or unsharp measurements, the quantum discord, which is an intrinsically distinct entity from concurrence, serves as a reliable indicator of the crossover point in Zeno to anti-Zeno transitions. However at highly precise quantum measurements, the monitoring device interferes significantly with the evolution dynamics of the monitored system, and the quantum discord yields indeterminate values in a reference frame where the observer is not an active constituent of the subsystems.Comment: Section on highly precise quantum measurements and exceptional points, typo

Non-hermitian exciton dynamics in a photosynthetic unit system

The non-hermitian quantum dynamics of excitonic energy transfer in photosynthetic systems is investigated using a dissipative two-level dimer model. The approach is based on the Green's function formalism which permits consideration of decoherence and intersite transfer processes on comparable terms. The results indicate a combination of coherent and incoherent behavior at higher temperatures with the possibility of exceptional points occurring at the coherent-incoherent crossover regime at critical temperatures. When each dimer site is coupled equally to the environmental sources of dissipation, the excitonic wavepacket evolves with time with a coherent component, which can be attributed to the indistinguishability of the sources of dissipation. The time evolution characteristics of the B850 Bchls dimer system is analysed using typical parameter estimates in photosynthetic systems, and the quantum brachistochrone passage times are obtained for a range of parameters.Comment: misprints correcte

Binding energies of composite boson clusters using the Szilard engine

We evaluate the binding energies of systems of bosonic and fermionic particles on the basis of the quantum Szilard engine, which confers an energetic value to information and entropy changes. We extend treatment of the quantum information thermodynamic operation of the Szilard engine to its non-trivial role in Bose-Einstein condensation of the light mass polariton quasiparticle, and binding of large multi-excitonic complexes, and note the same order of magnitudes of exchange and extraction energies in these disparate systems. We examine the gradual decline of a defined information capacitive energy with size of the boson cluster as well as the influence of confinement effects in composite boson systems. Can quantum informational entropy changes partly explain the observations of polariton condensates? We provide energy estimates using the system of polariton condensates placed in a hypothetical quantum Szilard engine, and briefly discuss the importance of incorporating entropy changes introduced during quantum measurements, and in the interpretation of experimental results.Comment: 12 pages, 6 figs, one numerical typ

Exciton formation assisted by longitudinal optical phonons in monolayer transition metal dichalcogenides

We examine a mechanism by which excitons are generated via the LO (longitudinal optical) phonon-assisted scattering process after optical excitation of monolayer transition metal dichalcogenides. The exciton formation time is computed as a function of the exciton center-of-mass wavevector, electron and hole temperatures, and carrier densities for known values of the Fr\"ohlich coupling constant, LO phonon energy, lattice temperature, and the exciton binding energy in layered structures. For the monolayer MoS$_2$, we obtain ultrafast exciton formation times on the sub-picosecond time scale at charge densities of 5 $\times$ 10$^{11}$ cm$^{-2}$ and carrier temperatures less than 300 K, in good agreement with recent experimental findings ($\approx$ 0.3 ps). While excitons are dominantly created at zero center-of-mass wavevectors at low charge carrier temperatures ($\approx$ 30 K), the exciton formation time is most rapid at non-zero wavevectors at higher temperatures ($\ge$ 120 K) of charge carriers. The results show the inverse square-law dependence of the exciton formation times on the carrier density, consistent with a square-law dependence of photoluminescence on the excitation density. Our results show that excitons are formed more rapidly in exemplary monolayer selenide-based dichalcogenides (MoSe$_2$ and WSe$_2$) than sulphide-based dichalcogenides (MoS$_2$ and WS$_2$).Comment: 9 pages, 4 figs, J. Appl. Phys. 120 (2016

Dielectric constant of monolayer transition metal dichalcogenides across excitonic resonances

We analyze the dielectric-function spectra of low dimensional transition metal dichalcogenides (TMDCs) using a fully analytical model of the complex dielectric function that is applicable in fractional dimensional space. We extract the dimensionalities of the $A$ and $B$ excitons as well as their Lorentzian broadening widths by fitting the model to experimental data in the spectral range of photon energies (1.5 - 3 eV). Our results show the significant contribution of the lowest ground exciton state to the dielectric properties of exemplary monolayer materials (MoS$_2$, MoSe$_2$ and WSe$_2$). The exciton dimensionality parametrizes the processes that underlie confinement and many-body Coulomb effects as well as substrate screening effects, which simplifies the analysis of electro-optical properties in low dimensional systems. This study highlights the potential of theoretical models as valuable tools for interpreting the optical spectrum and in seeking an understanding of the correlated dynamics between the $A$ and $B$ excitons on the dielectric function of TMDCs.Comment: 10 pages, typos fixe

Influence of the Pauli exclusion principle on scattering properties of cobosons

We examine the influence of the Pauli exclusion principle on the scattering properties of composite bosons (cobosons) made of two fermions, such as the exciton quasiparticle. The scattering process incorporates boson-phonon interactions that arise due to lattice vibrations. Composite boson scattering rates increase with the entanglement between the two fermionic constituents, which comes with a larger number of available single-fermion states. An important role is played by probabilities associated with accommodating an incoming boson among the remaining unoccupied Schmidt modes in the initial composite system. While due attention is given to bi-fermion bosons, the methodology is applicable to any composite boson made up of smaller boson fragments. Due to super-bunching in a system of multiple boson condensates such as bi-bosons, there is enhanced scattering associated with bosons occupying macroscopically occupied Schmidt modes, in contrast to the system of bi-fermion pairs.Comment: 11 pages, 1 figur

Exceptional points and quantum correlations in precise measurements

We examine the physical manifestations of exceptional points and passage times in a two-level system which is subjected to quantum measurements and which admits a non-Hermitian description. Using an effective Hamiltonian acting in the two-dimensional space spanned by the evolving initial and final states, the effects of highly precise quantum measurements in which the monitoring device interferes significantly with the evolution dynamics of the monitored two-level system is analysed. The dynamics of a multipartite system consisting of the two-level system, a source of external potential and the measurement device is examined using correlation measures such as entanglement and non-classical quantum correlations. Results show that the quantum correlations between the monitored (monitoring) systems is considerably decreased (increased) as the measurement precision nears the exceptional point, at which the passage time is half of the measurement duration. The results indicate that the underlying mechanism by which the non-classical correlations of quantum systems are transferred from one subsystem to another may be better revealed via use of geometric approaches.Comment: 13 pages, misprints fixe

Quantum information processing attributes of J-aggregates

We examine the unique spectroscopic features which give rise to quantum information processing attributes of one-dimensional J-aggregate systems, and as revealed by entanglement measures such as the von Neumann entropy, Wootters concurrence and Wei-Goldbart geometric measure of entanglement. The effect of dispersion and resonance terms in the exciton-phonon interaction are analyzed using Green function formalism and present J-aggregate systems as robust channels for large scale energy propagation for a select range of parameters. We show that scaling of the third order optical response $\chi^{(3)}$ with exciton delocalization size provides an experimentally demonstrable measure of quantifying multipartite entanglement in J-aggregates.Comment: 19 pages, 3 new figures adde

Effect of the Pauli exclusion principle on the singlet exciton yield in conjugated polymers

Optical devices fabricated using conjugated polymer systems give rise to singlet exciton yields which are high compared to the statistically predicted estimate of 25% obtained using simple recombination schemes. In this study we evaluate the singlet exciton yield in conjugated polymers systems by fitting to a model that incorporates the Pauli exclusion principle. The rate equations which describe the exciton dynamics include quantum dynamical components (both density and spin-dependent) which arise during the spin-allowed conversion of composite intra-molecular excitons into loosely bound charge-transfer (CT) electron-hole pairs. Accordingly, a crucial mechanism by which singlet excitons are increased at the expense of triplet excitons is incorporated in this work. Non-ideal triplet excitons which form at high densities, are rerouted via the Pauli exclusion mechanism to form loosely bound CT states which subsequently convert to singlet excitons. Our derived expression for the yield in singlet exciton incorporates the purity measure, and provides a realistic description of the carrier dynamics at high exciton densities.Comment: 8 pages, 4 figures, Accepted for Applied Physics A (2015