Quantum Eigenvector Continuation for Chemistry Applications

A typical task for classical and quantum computing in chemistry is finding a potential energy surface (PES) along a reaction coordinate, which involves solving the quantum chemistry problem for many points along the reaction path. Developing algorithms to accomplish this task on quantum computers has been an active area of development, yet finding all the relevant eigenstates along the reaction coordinate remains a difficult problem, and determining PESs is thus a costly proposal. In this paper, we demonstrate the use of a eigenvector continuation -- a subspace expansion that uses a few eigenstates as a basis -- as a tool for rapidly exploring potential energy surfaces. We apply this to determining the binding PES or torsion PES for several molecules of varying complexity. In all cases, we show that the PES can be captured using relatively few basis states; suggesting that a significant amount of (quantum) computational effort can be saved by making use of already calculated ground states in this manner.Comment: 13 pages, 8 figures, 3 pages of appendi

Positron annihilation lifetime spectroscopy of ZnO bulk samples

In order to gain a further insight into the knowledge of point defects of ZnO, positron annihilation lifetime spectroscopy was performed on bulk samples annealed under different atmospheres. The samples were characterized at temperatures ranging from 10 to 500 K. Due to difficulties in the conventional fitting of the lifetime spectra caused by the low intensity of the defect signals, we have used an alternative method as a solution to overcome these difficulties and resolve all the lifetime components present in the spectra. Two different vacancy-type defects are identified in the samples: Zn vacancy complexes (VZn−X) and vacancy clusters consisting of up to five missing Zn-O pairs. In addition to the vacancies, we observe negative-ion-type defects, which are tentatively attributed to intrinsic defects in the Zn sublattice. The effect of the annealing on the observed defects is discussed. The concentrations of the VZn−X complexes and negative-ion-type defects are in the 0.2–2 ppm range, while the cluster concentrations are 1–2 orders of magnitude lower.Peer reviewe

Positron annihilation lifetime spectroscopy of ZnO bulk samples

In order to gain a further insight into the knowledge of point defects of ZnO, positron annihilation lifetime spectroscopy was performed on bulk samples annealed under different atmospheres. The samples were characterized at temperatures ranging from 10 to 500 K. Due to difficulties in the conventional fitting of the lifetime spectra caused by the low intensity of the defect signals, we have used an alternative method as a solution to overcome these difficulties and resolve all the lifetime components present in the spectra. Two different vacancy-type defects are identified in the samples: Zn vacancy complexes (VZn−X) and vacancy clusters consisting of up to five missing Zn-O pairs. In addition to the vacancies, we observe negative-ion-type defects, which are tentatively attributed to intrinsic defects in the Zn sublattice. The effect of the annealing on the observed defects is discussed. The concentrations of the VZn−X complexes and negative-ion-type defects are in the 0.2–2 ppm range, while the cluster concentrations are 1–2 orders of magnitude lower.Peer reviewe

Degeneracy in excited-state quantum phase transitions of two-level bosonic models and its influence on system dynamics

Excited-state quantum phase transitions (ESQPTs) strongly influence the spectral properties of collective many-body quantum systems, changing degeneracy patterns in different quantum phases. Level degeneracies in turn affect the system’s dynamics. We analyze the degeneracy dependence on the size of two-level boson models with a u(n + 1) dynamical algebra, where n is the number of collective degrees of freedom. Below the ESQPT critical energy of these models, the energy gap between neighboring levels that belong to different symmetry sectors gets close to zero as the system size increases. We report and explain why this gap goes to zero exponentially for systems with one collective degree of freedom but algebraically in models with more than one degree of freedom. As a consequence, we show that the infinite-time average of out-of-time-order correlators is an ESQPT order parameter in finite systems with n = 1, but in systems with n > 1, this average only works as an order parameter in the mean-field limit.This project received funding through Grant No. PID2022-136228NB-C21 funded by MICIU/AEI/10.13039/501100011033 and, as appropriate, by “ERDF A way of making Europe, by ERDF/EU,” by the European Union, or by the European Union NextGenerationEU/PRTR. This work was also partially supported by the Consejería de Conocimiento, Investigación y Universidad, Junta de Andalucía and European Regional Development Fund, through Grant No. UHU-1262561 (J.K.-R. and F.P.-B.) and Grant No. US-1380840 (J.K.-R.), and Grant No. PY2000764. J.K.-R. also acknowledges support from a Spanish Ministerio de Universidades “Margarita Salas” Fellowship. Computing resources supporting this work were provided by the CEAFMC and Universidad de Huelva High Performance Computer located in the Campus Universitario “El Carmen” and funded by FEDER/MINECO Project No. UNHU-15CE-2848.Departamento de Física Aplicad

New SiS destruction and formation routes via neutral-neutral reactions and their fundamental role in interstellar clouds at low- and high-metallicity values

Context. Among the silicon-bearing species discovered in the interstellar medium, SiS and SiO stand out as key tracers due to their distinct chemistry and variable abundances in interstellar and circumstellar environments. Nevertheless, while the origins of SiO are well documented, the SiS chemistry remains relatively unexplored. Aims. Our objective is to enhance the network of Si- and S-bearing chemical reactions for a gas-grain model in molecular clouds, encompassing both low and high metallicities. To achieve this, we calculated the energies and rate coefficients for six neutral atom-diatom reactions involved in the SiCS triatomic system, with a special focus on the C+SiS and S+SiC collisions. Methods. We employed the coupled-cluster method with single and double substitutions and a perturbative treatment of triple substitutions (CCSD(T)) refined at the explicitly correlated CCSD(T)-F12 level. With these computational results in conjunction with supplementary data from the literature, we construct an extended network of neutral-neutral chemical reactions involving Si- and S-bearing molecules. To assess the impact of these chemical reactions, we performed time-dependent models employing the Nautilus gas-grain code, setting the gas temperature to 10 K and the H2 density to 2 × 10^4 cm−3 . The models considered two initial abundance scenarios, corresponding to low- and high-metallicity levels. Abundances were computed using both the default chemical network and the constrained network, enriched with newly calculated reactions. Results. The temperature dependence for the reactions involving SiS were modelled to the k(T ) = α (T/300)β exp (−γ/T ) expression, and the coefficients are provided for the first time. The high-metallicity models significantly boost the SiS production, resulting in abundances nearly four orders of magnitude higher compared to low-metallicity models. Higher initial abundances of C, S, and Si, roughly ∼2, 190, and 210 times higher, respectively, contribute to this. Around the age of 10^3 yr, destruction mechanisms become relevant, impacting the abundance of SiS. The proposed production reaction S + SiC −→ C + SiS, mitigates these effects in later stages. By expanding the gas reaction network using a high-metallicity model, we derived estimates for the abundances of observed interstellar molecules, including SiO, SO, and SO2 . Conclusions. We demonstrate the significance of both SiC+S and C+SiS channels in the SiS chemistry. Notably, the inclusion of neutral-neutral mechanisms, particularly via Si+HS and S+SiC channels, played a pivotal role in determining SiS abundance. These mechanisms carry a significance level on a par with that of the well-known and fast ion-neutral reactions

The Si + SO2 collision and an extended network of neutral–neutral reactions between silicon and sulphur bearing species

International audienceThe Si + SO2 reaction is investigated to verify its impact on the abundances of molecules with astrochemical interest, such as SiS, SiO, SO, and others. According to our results Si(3P) and SO2 react barrierlessly yielding only the monoxides SO and SiO as products. No favourable pathway has been found leading to other products, and this reaction should not contribute to SiS abundance. Furthermore, it is predicted that SiS is stable in collisions with O2, and that S(3P) + SiO2 and O(3P)+OSiS will also produce SO + SiO. Using these results and gathering further experimental and computational data from the literature, we provide an extended network of neutral-neutral reactions involving Si- and S-bearing molecules. The effects of these reactions were examined in a protostellar shock model, using the NAUTILUS gas-grain code. This consisted in simulating the physicochemical conditions of a shocked gas evolving from (i) primeval cold core, (ii) the shock region itself, (iii) and finally the gas bulk conditions after the passage of the shock. Emphasizing on the cloud ages and including systematically these chemical reactions, we found that [SiS/H2] can be of the order of ~10-8 in shocks that evolves from clouds of t = 1 × 106 yr, whose values are mostly affected by the SiS + O $\longrightarrow$SiO + S reaction. Perspectives on further models along with observations are discussed in the context of sources harbouring molecular outflows

Localized versus delocalized states: Photoluminescence from electrochemically synthesized ZnO nanowires

We analyze the near-band-edge photoluminescence of electrochemically deposited ZnO nanowires and directly correlate the photoluminescence properties with the carrier concentration in the nanowires as determined from electrochemical impedance spectroscopy. We find a donor density of 81019 cm−3 in the as-deposited nanowires and show that the near-band-edge emission results from band-to-band recombination processes delocalized states. A photoluminescence band centered at 3.328 eV scales with the diameter of the nanowires and is assigned to recombination processes involving surface states. We show that annealing at 500 °C in air reduces the donor density in the nanowires by more than one order of magnitude, leading to sharp excitonic transitions in the electrochemically deposited nanowire

Fullerene-Based Materials as Hole-Transporting/Electron Blocking Layers. Applications in Perovskite Solar Cells

Here we report for the first time an efficient fullerenebased compound, FU7, able to act as Hole-Transporting Material (HTM) and electron blocking contact. It has been applied on perovskite solar cells (PSCs), obtaining 0.81 times the efficiency of PSCs with the standard HTM, spiro-OMeTAD, with the additional advantage that this performance is reached without any additive introduced in the HTM layer. Moreover, as a proof of concept, we have described for the first time efficient PSCs where both selective contacts are fullerene derivatives, to obtain unprecedented “fullerene sandwich” PSCs