Energy Level Shifts Research Articles

Ab-initio calculations of temperature dependent electronic structures of inorganic halide perovskite materials.

Despite wide interest in halide perovskite materials, it is still challenging to accurately calculate their electronic structure and its temperature dependence. In this work, we present ab-initio calculations of the temperature dependence of the electronic structure of CsPbX3 materials (X = Cl, Br or I) in the cubic form and of the zero temperature electronic structure of the orthorhombic phase of these materials. Phonon-induced temperature dependent band energy renormalization was calculated within the framework of Allen-Heine-Cardona theory, where we exploited the self-consistent procedure to determine both the energy level shifts and their broadenings. The phonon spectrum of the materials was obtained using the self-consistent phonon method since standard density functional perturbation theory calculations in harmonic approximation yield phonon modes with imaginary frequencies due to the fact that the cubic structure is not stable at zero temperature. Our results suggest that low energy phonon modes mostly contribute to phonon-induced band energy renormalization. The calculated values of the band gaps at lowest temperature where the material exhibits a cubic structure are in good agreement with experimental results from the literature. The same is the case for the slope of the temperature dependence of the band gap for the CsPbI3 material where reliable experimental data are available in the literature. We also found that phonon-induced temperature dependence of the band gap is most pronounced for the conduction band minimum and valence band maximum, while other bands exhibit a weaker dependence.

Oxynitride K2Ba6.72Si16O40-1.5yNy:0.28Eu2+ phosphor with high thermal stability realized by crystal field engineering.

Extensive research has gone into modifying the chemical composition of phosphors to achieve desirable optical properties. Here, oxynitride phosphors K2Ba6.72Si16O40-1.5yNy:0.28Eu2+ were synthesized by introducing N3- (y) into a K2Ba6.72Si16O40:0.28Eu2+ lattice. An uneven shrinking of the cell parameters a, b, and c was observed through a combination of X-ray diffraction studies and Rietveld refinements. This shrinking caused a large centroid shift (εc) and splitting of the 5d energy level (εcfs), thus inducing the broadening of the excitation spectra (104 → 127 nm, y = 0 → y = 12) and the red shift of the emission spectra (501 → 543 nm, y = 0 → y = 12). The modified series of samples have a broad excitation spectrum, suitable of use in UV, near-UV, and blue light-emitting LEDs. In addition, the optimal sample, K2Ba6.72Si16O31N6:0.28Eu2+, benefits from an increased activation energy and thermal stability.

Polaronic suppression of shuttle vibrations

A re-entrant behavior of electron shuttling is shown to occur in a nanoelectromechanical transistor made of magnetic material where spin-polarized electrons are injected into a quantum dot with a single electron level split into two by an external magnetic field. A suppression of shuttle vibrations occurs at a certain value of a bias voltage that starts to allow for transport also through the upper energy level of the dot, while for a further increase of the voltage shuttling recovers. The effect is due to a time-dependent polaronic shift of the dot energy level, which results in a reduction of the supply of electric power to the mechanical motion.

Control and regulation of the performance of fullerene-based dye-sensitized solar cells with a D-D-A structure by external electric fields.

We investigated the modulating effect of an electric field (Fext) on the photovoltaic properties of triphenylamine-based sensitizers with a D-D-A structure and compared the photovoltaic parameters at different electric field intensities. The results show that Fext can effectively adjust the photoelectric properties of the molecule. From the change of the parameters that measures the degree of electron delocalization, it can be seen that the Fext can effectively strengthen the electronic communication and promote the charge transfer process within the molecule. And the dye molecule under a strong Fext has a narrower energy gap, more favorable injection, regeneration driving force and a larger conduction band energy level shift, which ensures that the dye molecule can exhibit larger Voc and Jsc under a strong Fext. The results of calculations on the photovoltaic parameters of dye molecules show that dye molecules can exhibit better photovoltaic performance under the action of Fext, which provides beneficial predictions and prospects for the development of highly efficient DSSCs.

N-Type doping of BiVO4 with different F-doped concentrations for improving the electronic character of BiVO4 as a photoanode nanomaterial for solar water splitting: a first-principles study.

Atom doping has been realized as an effective way to improve the photocatalytic performance of the most promising photoanode material, BiVO4, but the effects of doping mass concentration still need to be explored. In this work, the effects of F-doping with different doping mass concentrations (1%, 2%, 5%, 10%, 15%, and 20%) on the electronic character of BiVO4 were examined theoretically using density functional theory (DFT) calculations. The thermal stability of BiVO4 with different F-doped mass concentrations was confirmed using formation energy (Ef) calculations though F-doped BiVO4 becomes harder as the mass concentration of induced dopants increases. n-Type doping effects on the electronic character of BiVO4 were observed upon F-doping, leading to the energy level of CBM shifting far away from the Fermi level and giving F-doped BiVO4 metallic character. Moreover, a linear relationship between the frontier energy level shifts and the total charge transfer amounts from doped F atoms to other atoms involved in F-doped BiVO4 was observed, which means the oxidizing capacity of the VBM is increased and the reducing capacity of the CBM is decreased upon increased F-doped mass concentration. Moreover, the recombination of photogenerated electron-hole pairs is suppressed by F-doping strategies, which will not change a lot with the increased F-doped mass concentration. This means atom doping is an effective strategy to improve the photocatalytic efficiency of the BiVO4, but the number of atoms introduced into BiVO4 should be appropriate.

Crystal growth and scintillation characterizations of mixed halide scintillators K2LaBr5-xIx: Ce (x = 0, 1, 2)

The purpose of this research is to investigate the luminescence and scintillation properties of mixed halide scintillators K2LaBr5-xIx. The crystals of K2LaBr5-xIx:2% Ce (x = 0, 1, 2) have been synthesized by high-temperature solid-phase method and grown by vertical Bridgman method. The crystals are measured by X-ray induced luminescence spectra, optical transmittance photoluminescence spectrum, photoluminescence decay time, and pulse height spectrum. Under the UV and X-ray excitation, all of the crystal samples have typical Ce3+ double peaks corresponding to the energy level shifts of 5d1→2F5/2 and 5d1→2F7/2, with emission centers ranging from 350 to 400 nm. The photoluminescence decay time of the three crystals of K2LaBr5-xIx: Ce is about 20 ns. Under 662 keV γ-rays excitation from a 137Cs source, the energy resolution is 8.2%, 15% and 10% (FWHM), respectively.

Isotope shifts in cadmium as a sensitive probe for physics beyond the standard model

Isotope shifts (ISs) of atomic energy levels are sensitive probes of nuclear structure and new physics beyond the standard model. We present an analysis of the ISs of the cadmium atom (Cd I) and singly charged cadmium ion (Cd II). ISs of the 229 nm, 326 nm, 361 nm and 480 nm lines of Cd I are measured with a variety of techniques; buffer–gas-cooled beam spectroscopy, capturing atoms in a magneto-optic-trap, and optical pumping. IS constants for the D1 and D2 lines of Cd II are calculated with high accuracy by employing analytical response relativistic coupled-cluster theory in the singles, doubles and triples approximations. Combining the calculations for Cd II with experiments, we infer IS constants for all low-lying transitions in Cd I. We benchmark existing calculations via different many-body methods against these constants. Our calculations for Cd II enable nuclear charge radii of Cd isotopes to be extracted with unprecedented accuracy. The combination of our precise calculations and measurements shows that King plots for Cd I can improve the state-of-the-art sensitivity to a new heavy boson by up to two orders of magnitude.

Tunable Energy-Level Alignment in Multilayers of Carboxylic Acids on Silver

Precise energy-level alignment between a metal electrode and an organic semiconductor is required to reduce contact resistance and enhance the efficiency of organic-semiconductor-based devices. Here, we introduce monolayer-thick charge-injection layers (CILs) based on aromatic carboxylic acids that can induce an energy-level shift in subsequent layers by up to 0.8 eV. Through a gradual chemical transformation of the as-deposited molecules, we achieve a highly tunable energy-level shift in the range of 0.5 eV. We reveal that the work function and energy-level positions in the CIL increase linearly with the density of induced dipoles. The energy-level position of subsequent layers follows the changes in the CIL. Our results thus connect the energy-alignment quantities, and the high tunability would allow precise tuning of the active layers deposited on the CIL, which marks a path towards efficient charge-injection layers on metal electrodes.

The Key Role of Subtle Substitution for a High‐Performance Ester‐Modified Oligothiophene‐Based Polymer Used in Photovoltaic Cells

Comprehensive SummaryIn organic solar cells (OSCs), developing high‐performing easily synthesized photoactive materials is essential for pursuing cost‐ effective balance. Herein, we have designed and synthesized a pair of wide‐band‐gap polymers (PBDE4T‐0F and PBDE4T‐2F), using the low synthesis cost dicarboxylic ester‐substituted quaterthiophene as the building block. Despite the minor change of molecular structure for polymer PBDE4T‐xF, the fluorine substituent in polymer PBDE4T‐2F greatly enhances its interchain aggregation. The higher aggregation tendency of ester‐modified polymer in solution is beneficial for reducing both the aggregate size and π‐π stacking distance of blend film, which contribute to the highly efficient exciton dissociation and symmetric charge transport. An impressive power‐conversion efficiency (PCE) of 16.1% is achieved for the PBDE4T‐2F:BTP‐eC9‐based device, while its counterpart only delivers a PCE of 5.8% with distinctly lower short‐circuit current density (Jsc) and fill factor. Notably, the aggregation effect of donor polymer has also been found to be associated with the energy level shifts, and thus the variation of charge transfer energy and voltage losses for blend system. The results suggest that simultaneously reduced voltage loss and increased Jsc can be expected by further finely tuning the aggregation behavior of the ester‐modified oligothiophene‐based donor polymer.

Hadronic vacuum polarization correction to atomic energy levels

The shift of atomic energy levels due to hadronic vacuum polarization is evaluated in a semiempirical way for hydrogenlike ions and for muonic hydrogen. A parametric hadronic polarization function obtained from experimental cross sections of $e^-e^+$ annihilation into hadrons is applied to derive an effective relativistic Uehling potential. The energy corrections originating from hadronic vacuum polarization are calculated for low-lying levels using analytical Dirac-Coulomb wave functions, as well as bound wave functions accounting for the finite nuclear size. Closed formulas for the hadronic Uehling potential of an extended nucleus as well as for the relativistic energy shift in case of a point-like nucleus are derived. These results are compared to existing analytic formulas from non-relativistic theory.

Advanced characterization of organic–metal and organic–organic interfaces: from photoelectron spectroscopy data to energy-level diagrams

Organic–metal and organic–organic interfaces account for the functionality of virtually all organic optoelectronic applications and the energy-level alignment is of particular importance for device performance. Often the energy-level alignment is simply estimated by metal work functions and ionization energies and electron affinities of the organic materials. However, various interfacial effects such as push back, mirror forces (also known as screening), electronic polarization or charge transfer affect the energy-level alignment. We perform x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) measurements on copper-hexadecafluorophthalocyanine (F16CuPc) and titanyl-phthalocyanine (TiOPc) thin films on Ag(111) and use TiOPc bilayers to decouple F16CuPc layers from the metal substrate. Even for our structurally well-characterized model interfaces and by stepwise preparation of vacuum-sublimed samples, a precise assignment of vacuum-level and energy-level shifts remains challenging. Nevertheless, our results provide guidelines for the interpretation of XPS and UPS data of organic–metal and organic–organic interfaces.

Approximate eigenvalue solutions with diatomic molecular potential under topological defects and Aharonov-Bohm flux field: application for some known potentials

In this paper, we determine the approximate eigenvalue bound state solution of the radial Schrodinger equation in three-dimension in the presence of the Aharonov-Bohm flux field with two-term diatomic molecular potential in point-like global monopole (PGM) defect. We analyse the effects of factors, such as topological defects, background curvature associated with topological defects as well as the potential. We also show that the energy levels shift due to the presence of the magnetic flux which gives us an analogue of the Aharonov-Bohm effect for the bound state. Finally, we apply the quantum system for some known potential models, such as q-deformed Hulthen-type potential and Manning-Rosen potential, and discussed the effects of various factors on the eigenvalue solutions.

RELATIVISTIC ENERGY APPROACH TO ATOMIC SYSTEMS IN A STRONG ELECTROMAGNETIC FIELD IN PLASMAS

The fundamentals of a consistent relativistic approach to determination of radiation and spectral characteristics of the atomic systems (atoms, multicharged ions in plasmas conditions) in a strong external electromagnetic field (EMF), which is based on a relativistic energy formalism (REA) (adiabatic Gell-Mann and Low formalism) and relativistic many-body perturbation theory (RMBPT) formalism for atomic systems in a plasmas are presented. Within an energy approach in relativistic approximation the Gell-Mann and Low formula expresses the imaginary part of an atomic level energy shift E through the QED scattering matrix, which includes an interaction as with an EMF as with the photon vacuum field (spontaneous radiative decay). It results in possibility of an uniform simultaneous consideration of spontaneous and (or) induced, radiative processes and their interference. The radiation atomic lines position and shape fully determine a spectroscopy of the plasmas atomic systems in an EMF. The effective modified technique, based on the Ivanova-Ivanov method of differential equations, for computing the infinite sums in expressions for a spectral line shifts and broadening is shortly described. The fundamentals of a formalism of the relativistic gauge-invariant RMBPT with the optimized Dirac-Kohn-Sham (DKS) and Debye-Hückel zeroth approximations are presented.

Sample-shape dependent energy levels in organic semiconductors

Energy levels in an organic semiconductor are mainly determined by the molecular orbital energies of the constituent molecules. Recent studies, however, have revealed that the energy levels can be altered as much as 1 eV by the molecular orientation in the film or the molecular mixing ratio in the binary film, owing to the intermolecular electrostatic interaction. Because of the long-range nature of Coulomb interaction, theory predicts that the electrostatic energy should depend on the sample shape. In this article, we examine the coverage-dependent energy levels of zinc phthalocyanine and per-fluorinated zinc phthalocyanine in the submonolayer region with ultraviolet photoelectron spectroscopy (UPS) and low-energy inverse photoelectron spectroscopy (LEIPS). Using the procedure we reported previously, we separately evaluated the electronic polarization energy and electrostatic energy as a function of coverage. Unlike the electronic polarization, which contributes only as much as 10 meV, the electrostatic energy contributes as much as 120 meV to the coverage-dependent energy shift. We conclude that the shift in energy levels by changing the coverage is attributed to the sample shape-dependent energy level, owing to the long-range nature of the charge--permanent quadrupole interaction.

Perturbative Diagonalization for Time-Dependent Strong Interactions

We present a systematic method to implement a perturbative Hamiltonian diagonalization based on the time-dependent Schrieffer-Wolff transformation. Applying our method to strong parametric interactions we show how, even in the dispersive regime, full Rabi model physics is essential to describe the dressed spectrum. Our results unveil several qualitatively different results, including realization of large energy-level shifts, tunable in magnitude and sign with the frequency and amplitude of the pump mediating the parametric interaction. Crucially, Bloch-Siegert shifts, typically thought to be important only in the ultrastrong or deep-strong coupling regimes, can be rendered large even for weak dispersive interactions to realize points of exact cancelation of dressed shifts (``blind spots'') at specific pump frequencies. The framework developed here highlights the rich physics accessible with time-dependent interactions and serves to significantly expand the functionalities for control and readout of strongly interacting quantum systems.

Interface dipole at nickel oxide surface to enhance the photovoltage of perovskite solar cells

Inorganic nickel oxide (NiOx)-based inverted perovskite solar cells (PSCs) are reported to be more stable compared to those using organic hole transport layer materials. However, the Fermi level of NiOx is relatively away from the valence band of perovskite (PVK) layer, causing a large energy level shift at the NiOx/PVK interface and a weak built-in electric field strength. These increase the possibility of recombination, reduce the effective charge collection, seriously affect the efficiency of the device. In this work, an interface dipole with -CF3 end group is anchored to the NiOx/PVK interface, leading to a more matched energy level structure and greatly reducing the energy barrier at the NiOx/PVK interface. Moreover, the addition of Si-CF3 significantly improves the conductivity of NiOx and blunts the defects to large extent. The dipole interlayer significantly improves the built-in electric field and thus increases the photovoltage. Ultimately, the modified device achieves a score of 19.58% efficiency, much better than 17.98% of the control device.

Highly Sensitive Measurement of a Megahertz rf Electric Field with a Rydberg-Atom Sensor

Rydberg atoms have great potential in electric field measurement and have an advantage with a large frequency bandwidth from the kHz to the THz scale. However, the sensitivity for measuring a weak MHz electric field signal is limited by the spectroscopic resolution, because the weak electric field induces only a small perturbation of the population and energy level shift of the Rydberg atoms. Here, we report highly sensitive measurement of a weak MHz electric field using electromagnetically induced transparency with Rydberg atoms in a thermal atomic system. Using the heterodyne method on a 30-MHz electric field, we successfully measure the minimum electric field strength to be \textcolor{black}{37.3 $\mathrm{\mu V/cm}$} with a sensitivity up to $-65$ dBm/Hz and a linear dynamic range over 65 dB. Additionally, we measure an amplitude-modulated signal and demodulate the signal with a fidelity over 98\%. This work extends the sensitivity of atomic sensors for measuring MHz electric fields, which advances atomic electric field-sensing technology.

Bespoke magnetic field design for a magnetically shielded cold atom interferometer

Quantum sensors based on cold atoms are being developed which produce measurements of unprecedented accuracy. Due to shifts in atomic energy levels, quantum sensors often have stringent requirements on their internal magnetic field environment. Typically, background magnetic fields are attenuated using high permeability magnetic shielding, with the cancelling of residual and introduction of quantisation fields implemented with coils inside the shield. The high permeability shield, however, distorts all magnetic fields, including those generated inside the sensor. Here, we demonstrate a solution by designing multiple coils overlaid on a 3D-printed former to generate three uniform and three constant linear gradient magnetic fields inside the capped cylindrical magnetic shield of a cold atom interferometer. The fields are characterised in-situ and match their desired forms to high accuracy. For example, the uniform transverse field, Bx, deviates by less than 0.2% over more than 40% of the length of the shield. We also map the field directly using the cold atoms and investigate the potential of the coil system to reduce bias from the quadratic Zeeman effect. This coil design technology enables targeted field compensation over large spatial volumes and has the potential to reduce systematic shifts and noise in numerous cold atom systems.

The shift of the constant energy level after the interaction of particles with the polarizing phonons

The shift of the electron energy level due to its interaction with the polarizing phonons is investigated here for the particles whose energy does not depend on the wave vector. We also analyze the influence of the value of the constant of electron - phonon interaction and other parameters of the problem on the electron energy change. The methods of Green's functions, presented by the the Feynman diagrams and using the double permutations, are present here. The paper contains the analysis of the columns of the corresponding double permutations, the analytical expressions of which form the mass operator presented by the chain fractions. We received the expressions for admissibles and inadmissibles columns for phonon components of double permutations. The numerical calculations allow to conclude about the energy shift and how it depends on the parameters.

Interaction of blackbody radiation with rubidium and caesium atoms in small-angular-momentum Rydberg states

We examine the possibilities of refining an asymptotic description and quantitative calculations of the effects induced by thermal blackbody radiation (BBR) of the environment on the Rydberg states of atoms. Numerical values are calculated and asymptotic expressions are proposed for simplified estimates of natural lifetimes and threshold photoionisation cross sections for Rydberg states of rubidium and caesium atoms with large values of the principal quantum number, n ⩾ 20, and small orbital momenta, l = 0, 1, 2, 3. Based on analytical expressions, we present numerical estimates for the contributions of photoionisation probabilities to the BBR-induced broadening of the Rydberg energy level, as well as the contributions of continuum integrals to thermally induced shifts in the Rydberg-state energy levels.