Molecular Interactions Induced by a Static Electric Field in Quantum Mechanics and Quantum Electrodynamics.

Semiconductor quantum dots are usually compared to artificial atoms, because their electronic structure consists of discrete energy levels as for natural atoms. These artificial systems are integrated in solid materials and can be localized with a spatial precision of the order of nanometers. Besides, they conserve their quantum properties even at quite high temperatures (∼ 10 K). These properties make quantum dots one of the most suitable systems for the realization of quantum devices and computers. However, the energy states and the optical properties of quantum dots are much more complicated than for atoms, because a quantum dot is never an isolated potential well. Instead, its electronic structure depends on the crystallin structure of the semiconductor material and on the Coulomb ion-electron and electron-electron correlations. In particular, the presence of several valence bands and their mixing, induced by quantum confinement, gives rise to novel properties which are still not completely understood and exploited in applications. To get a major advance in this field, a full deterministic control of the spatial shape of the quantum confinement is needed, combined with a deeper understanding of the connections between electronic and optical properties. This thesis work has these two main objectives. We realized and experimentally studied different quantum dot systems, in pyramidal hetero-structures grown with MOCVD techniques. These systems allowed the realization of several different geometries for the carrier confining potential, with a precision in the order of nanometers. The optical characterization has been obtained in particular by means of polarization-resolved microphotoluminescence, magneto-photoluminescence, excitation photoluminescence (PLE), and interferometry techniques. For single quantum dots, we have observed and characterized for the first time new excitonic complexes, arising from excited hole states. This allowed a full caracterizatioon of the valence band hole states in our peculiar system. By means of photon correlation measurements, we have also experimentally demonstrated that, even in presence of a large family of exciton states, these quantum dot systems can emit single photons. We have then realized much more complex quantum dot structures, double dot systems (quantum dot molecules) and a completely new system called Dot-in-Dot (DiD). This latter is composed by a small inner dot surrounded by an electrostatic potential well (which can be considered as an outer elongated dot). Such a composite system is characterized by a strong valence band mixing. This state superposition is however very sensitive to small variations of the confining potential. Therefore the degree of valence band mixing can be easily switched by the introduction of a week external field. Since the valence band mixing determines the polarization properties of the emitted light, the DiD changes the polarization properties of its emission spectrum under the action of an external field. In particular, we have experimentally demonstrated this effect for an external static magnetic field, while we have numerically predicted a very similar effect for a static electric field. In the latter case, the polarization switching is a direct consequence of the quantum confined Stark effect induced in the DiD. Hence the DiD appears to be an ideal candidate for realizing emitters of single photons with tunable and controllable polarization.

The effects of a static external electric field on the ground state electronic structure of a porphine-quinone (PQ) complex have been studied by using density functional theory (DFT). The energies of the excited states have been calculated with time-dependent density functional theory (TDDFT) and with the approximate coupled cluster singles and doubles (CC2) method. The geometries of porphine and quinone have been optimized with B3LYP. The influence of the external electric field on the PQ complex has been studied at six different intermolecular distances between 2.5 and 5.0 Å with the BH&HLYP functional. An external electric field clearly affects the orbitals localized mostly on quinone but not the orbitals localized on porphine. Additionally, the effect of the external field increases with the increasing intermolecular distance. The optical absorption spectrum of porphine obtained by using the BH&HLYP functional is consistent with the Gouterman model and with the spectrum previously calculated with CAM-B3LYP. The potential energy curves of the Q and B states and the lowest charge transfer (CT) states of the PQ complex calculated by using the BH&HLYP with TDDFT functional have also been compared with those obtained with the CC2 method. Both methods show that the lowest CT state is clearly above the Q states when no external field is applied. Therefore, when the Q states of a porphine-quinone system are excited, the conical intersection is not possible and cannot thus provide a path for electron transfer (ET). The calculations show that the Q and B states are affected by the field much less than the lowest CT state. Consequently, the calculations show that the CT state crosses the Q and B states at certain field strengths. Thus, it is possible that the external electric field triggers ET in porphine-quinone systems via conical intersection.

The external static electric field (SEF) of man-made origin brings to the substantially increased SEF background in a human environment the biological activity of which is a moot question. The paper reports on rats blood plasma/serum proteome modifications by means of 1D polyacrilamide gel electrophoresis and clotting process alterations after the short- and long-term SEF exposures of 200 kV/m. The results indicate decrease of fast α1 and α2 globular proteins in plasma coinciding with clotting acceleration after the short-term SEF, and attenuation of clotting-dependent proteome modifications reflected with incomplete coagulation after the long-term SEF exposure. Increased lysozyme activity in serum unlike plasma was observed after both SEF exposures. Applied model of the high-voltage SEF environment indicates dependence of biological systems functioning on the external SEF.

The theory of the photodetachment of negative ions in an external static electric field developed earlier is applied to photodetachment of the sulphur and chlorine negative ions. The photodetachment cross section in the presence of the electric field is modified by a modulation factor which is calculated including the rescattering effect, i.e. the scattering of the electron wave reflected by the electric field's potential barrier. The modulation factor is expressed in terms of the scattering lengths for electron-neutral-atom scattering. The scattering lengths are calculated by the method of extrapolation of potential parameters along isoelectronic series of positive ions and corresponding neutral atoms. The low-energy electron scattering cross sections for the S and Cl atoms are obtained, and the Ramsauer effect for these atoms is studied. The rescattering effect in the photodetachment cross sections is small, and cannot explain the observed reduction of the oscillations in the modulation factor.

A general comparison is presented between random electrodynamics and quantum electrodynamics for the two systems which can be solved exactly in both theories, free electromagnetic fields and point dipole oscillators. The $N$-point correlation functions of the fields are computed in both theories and are found to differ in general because of the dependence upon the order of the quantum operators within products of operators. However, if all products of quantum operators are symmetrized by taking all permutations of the operator order, then the two theories give identical results for the correlation functions. Analogous results hold to all orders in the fine-structure constant for dipole oscllators in quantum and random electrodynamics. The theories agree only if the quantum operator products are symmetrized. In the limit that the oscillator couplings to the radiation fields vanish, th oscllators can be regarded as mechanical oscillators in quantum mechanics and in random mechanics. The theory of random mechanics is defined in terms of this limit which uncouples a mechanical oscillator from the radiation field. The average values of oscillator variables in random mechanics agree with those of symmetrized products in quantum mechanics. The question is then raised as to the physical significance of the many quantum operators which differ only in the order of their factors. It is pointed out that some operator products which are regarded as physically important, such as the square of the angular momentum, indeed involve unsymmetrized products of operators. On this account the average values of the angular momentum squared in the ground state of an isotropic three-dimensional harmonic oscillator differ between the random-mechanical and quantum-mechanical descriptions. However, there seems to be no case in which experiments have shown that the (unsymmetrized) quantum operator value is to be preferred to that provided by random mechanics. The presence of thermal radiation is next treated for free electromagnetic fields and for dipole-oscillator systems. Despite extraordinary differences in the points of view toward thermal radiation taken by the two theories, the conclusion is the same as that found for zero temperature; the two theories agree in their average values if all products of quantum operators are symmetrized. Finally, as a further example of the power of random electrodynamics to give an account of phenomena where Lorentz's classical electron theory failed, we investigate the diamagnetism of a charged three-dimensional isotropic oscillator. The mathematical descriptions at finite temperature are developed in full random electrodynamics and quantum electrodynamics and in second-order perturbation theory in quantum mechanics.

The quantum states of an electron-hole pair in one-dimensional semiconductors under a static electric field are theoretically analyzed using a two-band model with on-site Coulomb interaction. In the absence of static field, the electron and hole are always bound, forming an exciton regardless of the Coulomb interaction strength, in contrast to what occurs in higher-dimensional semiconductors. The static field modifies the wave function of the electron-hole pair, turning bound states into continuum states. However, at low static fields, the linear optical spectra resemble those of the unbiased semiconductor, exhibiting a quadratic redshift of the main exciton absorption line as the field increases. When the static field exceeds a critical threshold, the exciton dissociates and the linear optical spectra exhibit signatures of the Wannier-Stark ladder with squally spaced peaks, making them a valuable tool for experimentally probing exciton dissociation.

Magnetic nanoparticles in a non-polar ferrofluid can reassemble in external electric fields. The resulting electric field-driven structural changes in ferrofluids are expected to influence the magnetic response of ferrofluids to an alternating magnetic field. Hence, it should be possible to control the magnetic susceptibility of ferrofluids by electric forces. To address the problem of a possible electro-magnetic coupling, a low-frequency alternating current magnetic susceptibility of a ferrofluid based on insulating oil and iron oxide nanoparticles is measured under a static electric field. The electric field is generated by applying a voltage on a pair of electrodes embracing the ferrofluid and acts parallel and perpendicular to the probing alternating magnetic field. The susceptibility is measured simultaneously with the ferrofluid's temperature and a leakage current. A noticeable susceptibility decrease with increasing voltage is found. The susceptibility decrease is partially caused by the temperature increase. Based on theoretical calculations, it is found that the detected temperature increase cannot be the only reason for the measured susceptibility decrease. Other mechanisms resulting from magnetic nanoparticle interactions with the electric field (nanoparticle trapping due to dielectrophoresis, electric field-induced nanoparticle aggregation) must contribute to the susceptibility drop in a static electric field.

Effect of static external electric field on bulk and interfaces in organic solar cell systems: A density-functional-theory-based study

A ‘modulated potential model’ is presented to describe the potential of diatomic molecules in static electric fields, and investigate the static electric effect on the multiphoton process. The multiphoton transition of HF in static electric and intense laser fields has been investigated using the Lie algebra approach. Multiphoton transition information is obtained by solving the time-evolution operator. This method makes the solving of the Schrödinger equation easier and makes the computation time shorter.

The diradical and ionic natures of open-shell singlet systems have been investigated using new definitions of the diradical and ionic characters as well as of their densities within the valence configuration interaction (VCI) model with two electrons in two active orbitals. The two-site symmetric and asymmetric diradical models are examined by using these diradical/ionic characters. For realistic compounds, we investigate a diradicaloid diphenalenyl and a rectangular graphene nanoflake in the presence of an external static electric field, as well as π-stacked phenalenyl-derivative dimers with varying the intermonomer distance, where the central carbon atoms in the phenalenyl rings are substituted by boron (B) and nitrogen (N) atoms, respectively. It is found that the increase of charge asymmetricity induced by the static electric field decreases the diradical character and finally induces an ionic character in the ground state, while the first excited state is changed from pure ionic to diradical-dominant as the field amplitude increases. On the other hand, when increasing the intermonomer distance, the B/N substitution in the phenalenyl dimer changes the electronic state from open-shell singlet with small diradical character to closed-shell with large ionic character. These results indicate that the application of a static electric field to diradicaloids and the asymmetric substitution of a pancake bonded π-dimer combined with the variation of intermonomer distance could tune the diradical/ionic characters and therefore control the nonlinear optical responses.

Quantum electrodynamics in an external constant field

Interaction with an external field in quantum electrodynamics

We consider the minimal coupling of two photons to neutral scalar and pseudoscalar fields, as, for instance, in the case of the Higgs boson and axion, respectively. In this framework, we analyze the photon dispersion relations in the presence of static and homogeneous external magnetic and electric fields, by taking into account the contribution of the imaginary part of the scalar/pseudoscalar self-energy. We show that this contribution cannot be neglected when it is of the same order as the photon-scalar/pseudoscalar mixing term. In addition to the usual lightlike photon propagation mode, with a refraction index $n>1$, a massive mode with mass of the order of the coupled boson mass can be induced, provided that the external field is above a particular critical value. Depending on the values of the external field, photon energy, and mass of the scalar/pseudoscalar particle, the scalar/pseudoscalar width could induce a sizable rate of photon splitting in two photons due to a strong resonant phenomenon. This effect has no practical laboratory applications for the Higgs physics due to the very large critical external magnetic or electric fields involved, for a photon energy of the order of a TeV. However, it can have relevant consequences in the axion physics or in any other scenario where light neutral scalar/pseudoscalar fields have minimal coupling with two photons.

The model of the generalized effective potential of the quadrupole field is considered. The model is based on the stability limit of the charged particles' oscillations in the composition of the high-frequency and static quadrupole electric fields. The representation of the polyharmonic oscillations in the variable fields by harmonic oscillations in the field of the generalized effective potential is approximate. Its accuracy increases at the stability limits. With the use of the model the problem of ion oscillations' excitation under the superposition on the quadrupole RF field of the static quadrupole and homogeneous electric fields is solved.

Nonlinear corrections on electromagnetic fields in vacuum have been expected. In this study, we have theoretically considered nonlinear Maxwell’s equations in a one-dimensional cavity for a classical light and external static electromagnetic fields. A general solution for the electromagnetic corrective components including that of a longitudinal standing wave was derived after a linearization. The main purpose is to give a detailed feature of the previously reported resonant behavior [Shibata, Euro. Phys. J. D 74:215 (2020)], such as the effect of external static fields and the polarization fluctuation. These results favor the development of new and effective method for experiment.Graphic abstract