Superconductivity in quantum-dot superlattices composed of quantum wire networks

Based on calculations using the local density approximation, we propose quantum wire networks with square and plaquette type lattice structures that form quantum dot superlattices. These artificial structures are well described by the Hubbard model. Numerical analysis reveals a superconducting ground state with transition temperatures $T_c$ of up to 90 mK for the plaquette, which is more than double the value of 40 mK for the square lattice type and is sufficiently high to allow for the experimental observation of superconductivity.Comment: 10 pages, 4 figure

Miniband-related 1.4–1.8 μm luminescence of Ge/Si quantum dot superlattices

The luminescence properties of highly strained, Sb-doped Ge/Si multi-layer heterostructures with incorporated Ge quantum dots (QDs) are studied. Calculations of the electronic band structure and luminescence measurements prove the existence of an electron miniband within the columns of the QDs. Miniband formation results in a conversion of the indirect to a quasi-direct excitons takes place. The optical transitions between electron states within the miniband and hole states within QDs are responsible for an intense luminescence in the 1.4–1.8 µm range, which is maintained up to room temperature. At 300 K, a light emitting diode based on such Ge/Si QD superlattices demonstrates an external quantum efficiency of 0.04% at a wavelength of 1.55 µm

The boundary integral method for magnetic billiards

We introduce a boundary integral method for two-dimensional quantum billiards subjected to a constant magnetic field. It allows to calculate spectra and wave functions, in particular at strong fields and semiclassical values of the magnetic length. The method is presented for interior and exterior problems with general boundary conditions. We explain why the magnetic analogues of the field-free single and double layer equations exhibit an infinity of spurious solutions and how these can be eliminated at the expense of dealing with (hyper-)singular operators. The high efficiency of the method is demonstrated by numerical calculations in the extreme semiclassical regime.Comment: 28 pages, 12 figure

A composite electrodynamic mechanism to reconcile spatiotemporally resolved exciton transport in quantum dot superlattices

Quantum dot (QD) solids are promising optoelectronic materials; further advancing their device functionality depends on understanding their energy transport mechanisms. The commonly invoked near-field F\"orster resonance energy transfer (FRET) theory often underestimates the exciton hopping rate in QD solids, yet no consensus exists on the underlying cause. In response, we use time-resolved ultrafast stimulated emission depletion (TRUSTED) microscopy, an ultrafast transformation of stimulated emission depletion (STED) microscopy to spatiotemporally resolve exciton diffusion in tellurium-doped CdSe-core/CdS-shell QD superlattices. We measure the concomitant time-resolved exciton energy decay due to excitons sampling a heterogeneous energetic landscape within the superlattice. The heterogeneity is quantified by single-particle emission spectroscopy. This powerful multimodal set of observables provides sufficient constraints on a kinetic Monte Carlo simulation of exciton transport to elucidate a composite transport mechanism that includes both near-field FRET and previously-neglected far-field emission/reabsorption contributions. Uncovering this mechanism offers a much-needed unified framework in which to characterize transport in QD solids and additional principles for device design.Comment: 47 pages, including supplemen

Examination Of Callaway-Holland-Based Thermal Conductivity Calculation For Nano-Phononic Crystals

Phononic crystals are periodic structured materials whose frequency spectrum is characterized by band gaps, which are regions in frequency space where acoustic or elastic waves cannot propagate. Nano scale phononic crystals have shown promise for reducing thermal conductivity and improving the thermoelectric figure of merit. Correctly calculating the thermal conductivity of nano phononic crystals has become increasingly important due to the growing research interest in the thermal properties of these materials. A widely used expression to calculate thermal conductivity, presented by Klemens and expressed in terms of the relaxation time by Callaway and Holland, originates from the Boltzmann transport equation. In its most general form, this expression involves a direct summation of the heat current contributions from individual phonons of all wavevectors and polarizations in the first Brillouin zone. In common practice, the expression is simplified by three assumptions commonly applied in bulk materials: first, the isotropic assumption that converts the summation over wavevector to an integral over wavevector magnitude; second, the assumption that phonon-phonon scattering rates for nano-phononic crystals can be described by the same empirical expressions commonly used for bulk materials and fitted to experimental data in bulk materials; third, the effective material assumption that the thermal transport can be modeled by treating the nano-phononic crystal as a single bulk effective medium with properties dictated by the nano-phononic crystal dispersion relation. The accuracy of nano-phononic crystal thermal conductivity predictions using these three assumptions need to be validated. In this dissertation, we propose to verify these assumptions one by one. First, to investigate the isotropic assumption, the thermal conductivities of bulk Si, Si/Ge superlattices, and Si/Ge quantum dot superlattices have been calculated using both the isotropic and direct summation methods, and the results show that the differences between the two methods increase substantially with supercell size. These differences arise because the vibrational modes neglected in the isotropic assumption provide an increasingly important contribution to the thermal conductivity for larger supercells. To avoid the significant errors that can result from the isotropic assumption, direct summation is recommended for thermal conductivity calculations in superstructures. Second, to investigate the assumption of the empirical phonon-phonon scattering rates from bulk material, work to calculate the phonon-phonon scattering rates from the empirical equations has been done and compared against the results from an established normal mode analysis method, which provides more accurate results. The fundamental reasons behind the difference between the empirical method and the NMA method will be discussed. Finally, the effective material assumption will be briefly examined by using Green Kubo Modal Analysis method. Overall, this dissertation will provide direction in the correct thermal conductivity calculation for nano-phononic crystals

INTERTEMPORAL PRICE ADJUSTMENTS IN THE BEEF MARKET: A REDUCED FORM ANALYSIS OF WEEKLY DATA

An intertemporal reduced form model is estimated for boxed beef, carcass, and slaughter prices on a weekly basis. The results indicate that prices respond jointly to changes in economic information within weeks t and t – 1, supporting time-series studies showing farm and wholesale prices to be nearly instantaneously related. However, the existence of market uncertainty entails significant intertemporal lags, revealed by prices stabilizing 9-14 weeks subsequent to a market shock. The model results imply that postponing marketings of fed cattle to capitalize on expected price advantages would be risky and that selling cattle carcass grade and weight is more favorable when prices respond to increases in beef production.Demand and Price Analysis, Livestock Production/Industries,

Simulation of nanostructure-based high-efficiency solar cells: challenges, existing approaches and future directions

Many advanced concepts for high-efficiency photovoltaic devices exploit the peculiar optoelectronic properties of semiconductor nanostructures such as quantum wells, wires and dots. While the optics of such devices is only modestly affected due to the small size of the structures, the optical transitions and electronic transport can strongly deviate from the simple bulk picture known from conventional solar cell devices. This review article discusses the challenges for an adequate theoretical description of the photovoltaic device operation arising from the introduction of nanostructure absorber and/or conductor components and gives an overview of existing device simulation approaches.Comment: Invited paper, accepted for publication in IEEE Journal of Selected Topics in Quantum Electronic