On a method to calculate conductance by means of the Wigner function: two critical tests

We have implemented the linear response approximation of a method proposed to compute the electron transport through correlated molecules based on the time-independent Wigner function [P. Delaney and J. C. Greer, \prl {\bf 93}, 36805 (2004)]. The results thus obtained for the zero-bias conductance through quantum dot both without and with correlations demonstrate that this method is either quantitatively nor qualitatively able to provide a correct physical escription of the electric transport through nanosystems. We present an analysis indicating that the failure is due to the manner of imposing the boundary conditions, and that it cannot be simply remedied.Comment: 22 pages, 7 figur

Antireflective nanotextures for monolithic perovskite silicon tandem solar cells

Recently, we studied the effect of hexagonal sinusoidal textures on the reflective properties of perovskite silicon tandem solar cells using the finite element method FEM . We saw that such nanotextures, applied to the perovskite top cell, can strongly increase the current density utilization from 91 for the optimized planar reference to 98 for the best nanotextured device period 500 nm and peak to valley height 500 nm , where 100 refers to the Tiedje Yablonovitch limit. [D. Chen et al., J. Photonics Energy 8, 022601, 2018 , doi 10.1117 1.JPE.8.022601] In this manuscript we elaborate on some numerical details of that work we validate an assumption based on the Tiedje Yablonovitch limit, we present a convergence study for simulations with the finite element method, and we compare different configurations for sinusoidal nanotexture

Hybrid integrators with predictive overload estimation for analog computers and continuous-time ΔΣ modulators

Continuous-time integrators are a central component in ΔΣ modulators, in analog computers, and general analog signal processing. If several integrators are interconnected, scaling plays an important role: In analog computers, scaling is performed with respect to the machine unit (MU). In ΔΣ modulators, scaling is performed in such a way that at maximum input signal the allowable dynamic range of no integrator is exceeded. In both cases the scaling is a compromise limiting the dynamic range. For analog computers, it was proposed early on to extend the dynamic range by hybrid integrators. Here, an analog range overflow is processed digitally and the analog integrator is reduced to its permissible operating range within the machine unit interval. While in earlier proposals for hybrid integrators only the subsequent integrator stage processes the overflow and works with reduced analog values, our hybrid integrator can process the overflow directly, with the analog reset process being continuous-time. In the case of highly dynamical input signals and transients, analog overload handling is further improved by a prediction of the overload that includes the currently applied input signal in the calculation. For example, with continuous-time ΔΣ modulators, overload of the analog integrator can be reliably avoided.</p

Optical simulations of advanced light management for liquid phase crystallized silicon thin film solar cells

Light management is a key issue for highly efficient liquid phase crystallized silicon LPC Si thin film solar cells and can be achieved with periodic nanotextures. They are fabricated with nanoimprint lithography and situated between the glass superstrate and the silicon absorber. To combine excellent optical performance and LPC Si material quality leading to open circuit voltages exceeding 640 mV, the nanotextures must be smooth. Optical simulations of these solar cells can be performed with the finite element method FEM . Accurately simulating the optics of such layer stacks requires not only to consider the nanotextured glass silicon interface, but also to adequately account for the air glass interface on top of this stack. When using rigorous Maxwell solvers like the finite element method FEM , the air glass interface has to be taken into account a posteriori, because the solar cells are prepared on thick glass superstrates, in which light is to be treated incoherently. In this contribution we discuss two different incoherent a posteriori corrections, which we test for nanotextures between glass and silicon. A comparison with experimental data reveals that a first order correction can predict the measured reflectivity of the samples much better than an often applied zeroth order correctio

Band dispersion in the deep 1s core level of graphene

Chemical bonding in molecules and solids arises from the overlap of valence electron wave functions, forming extended molecular orbitals and dispersing Bloch states, respectively. Core electrons with high binding energies, on the other hand, are localized to their respective atoms and their wave functions do not overlap significantly. Here we report the observation of band formation and considerable dispersion (up to 60 meV) in the $1s$ core level of the carbon atoms forming graphene, despite the high C $1s$ binding energy of $\approx$ 284 eV. Due to a Young's double slit-like interference effect, a situation arises in which only the bonding or only the anti-bonding states is observed for a given photoemission geometry.Comment: 12 pages, 3 figures, including supplementary materia

Anelastic spectroscopy study of the metal-insulator transition of Nd(1-x)EuxNiO3

Measurements are presented of the complex dynamic Young's modulus of NdNiO3 and Nd0.65Eu0.35NiO3 through the Metal-Insulator Transition (MIT). On cooling, the modulus presents a narrow dip at the MIT followed by an abrupt stiffening of ~6%. The anomaly is reproducible between cooling and heating in Nd0:65Eu0:35NiO3 but only appears as a slow stiffening during cooling in undoped NdNiO3, conformingly with the fact that the MIT in RNiO3 changes from strongly first order to second order when the mean R size is decreased. The elastic anomaly seems not to be associated with the antiferromagnetic transition, which is distinct from the MIT in Nd0.65Eu0.35NiO3. It is concluded that the steplike stiffening is due to the disappearance or freezing of dynamic Jahn- Teller (JT) distortions through the MIT, where the JT active Ni3+ is disproportionated into alternating Ni3+d and Ni3-d. The fluctuating octahedral JT distortion necessary to justify the observed jump in the elastic modulus is estimated as ~3%, but does not have a role in determining the MIT, since the otherwise expected precursor softening is not observed.Comment: 11 pages, accepted by Phys. Rev.

Density functional theory study of the multimode Jahn-Teller effect – ground state distortion of benzene cation

The multideterminental-DFT approach performed to analyze Jahn-Teller (JT) active molecules is described. Extension of this method for the analysis of the adiabatic potential energy surfaces and the multimode JT effect is presented. Conceptually a simple model, based on the analogy between the JT distortion and reaction coordinates gives further information about microscopic origin of the JT effect. Within the harmonic approximation the JT distortion can be expressed as a linear combination of all totally symmetric normal modes in the low symmetry minimum energy conformation, which allows calculating the Intrinsic Distortion Path, IDP, exactly from the high symmetry nuclear configuration to the low symmetry energy minimum. It is possible to quantify the contribution of different normal modes to the distortion, their energy contribution to the total stabilization energy and how their contribution changes along the IDP. It is noteworthy that the results obtained by both multideterminental-DFT and IDP methods for different classes of JT active molecules are consistent and in agreement with available theoretical and experimental values. As an example, detailed description of the ground state distortion of benzene cation is given