Search Results (51)

In this work, a theoretical study on the combined effects of an external electric field and a nonresonant intense laser field on the electronic properties of a quantum dot with a truncated cone shape is presented. This quantum dot was made from a type-II CdTe/CdSe heterostructure (core/shell). Using the effective mass approximation with parabolic bands and the finite element method, the Schrödinger equation was solved to analyze the confined states of electron, hole, and exciton. This study demonstrates the potential of combining nonresonant intense laser and electric fields to control confinement properties in semiconductor nanodevices, with potential applications in optoelectronics and quantum mechanics-related technologies. Full article

In this monograph, motivated by the work of Aphane, Gaba, and Xu, we explore fixed-point theory within the framework of $C^{\star }$-algebra-valued bipolar b-metric spaces, characterized by a non-solid positive cone. We define and analyze $(F_{\mathcal{H}}-G_{\mathcal{H}})$-contractions, utilizing positive monotone functions to extend classical contraction principles. Key contributions include the existence and uniqueness of fixed points for mappings satisfying generalized contraction conditions. The interplay between the non-solidness of the cone, the $C^{\star }$-algebra structure, and the completeness of the space is central to our results. We apply our results to find uniqueness of solutions to Fredholm integral equations and differential equations, and we extend the Ulam–Hyers stability problem to non-solid cones. This work advances the theory of metric spaces over Banach algebras, providing foundational insights with applications in operator theory and quantum mechanics. Full article

In the present work, we present the process of preparing CdS nanostructures based on templating synthesis using chemical deposition (CD) on a SiO₂/Si substrate. A 0.7 μm thick silicon dioxide film was thermally prepared on the surface of an n-type conduction Si wafer, followed by the creation of latent ion tracks on the film by irradiating them with swift heavy Xe ions with an energy of 231 MeV and a fluence of 10⁸ cm⁻². As a result of etching in hydrofluoric acid solution (4%), pores in the form of truncated cones with different diameters were formed. The filling of the nanopores with cadmium sulfide was carried out via templated synthesis using CD methods on a SiO_2 nanopores/Si substrate for 20–40 min. After CdS synthesis, the surfaces of nanoporous SiO_2 nanopores/Si were examined using a scanning electron microscope to determine the pore sizes and the degree of pore filling. The crystal structure of the filled silica nanopores was investigated using X-ray diffraction, which showed CdS nanocrystals with an orthorhombic structure with symmetry group 59 Pmmn observed at 2θ angles of 61. 48° and 69.25°. Photoluminescence spectra were recorded at room temperature in the spectral range of 300–800 nm at an excitation wavelength of 240 nm, where emission bands centered around 2.53 eV, 2.45 eV, and 2.37 eV were detected. The study of the CVCs showed that, with increasing forward bias voltage, there was a significant increase in the forward current in the samples with a high degree of occupancy of CdS nanoparticles, which showed the one-way electronic conductivity of CdS/SiO₂/Si nanostructures. For the first time, CdS nanostructures with orthorhombic crystal structure were obtained using track templating synthesis, and the density of electronic states was modeled using quantum–chemical calculations. Comparative analysis of experimental and calculated data of nanostructure parameters showed good agreement and are confirmed by the results of other authors. Full article

The first part of this work evaluates Al-doped ZnO (AZO) as an optically transparent top-gate material for studies on semiconductor quantum dots. In comparison with conventional Ti gates, samples with AZO gates demonstrate a more than three times higher intensity in the quantum dot emission under comparable excitation conditions. On the other hand, charges inside a process-induced oxide layer at the interface to the semiconductor cause artifacts at gate voltages above $U\approx$ 1 V. The second part describes an optical and simulation study of a vertical electric-field (F)-induced switching from a strong to an asymmetric strong–weak confinement in GaAs cone-shell quantum dots (CSQDs), where the charge carrier probability densities are localized on the surface of a cone. These experiments are performed at low U and show no indications of an influence of interface charges. For a large F, the measured radiative lifetimes are substantially shorter compared with simulation results. We attribute this discrepancy to an F-induced transformation of the shape of the hole probability density. In detail, an increasing F pushes the hole into the wing part of a CSQD, where it forms a quantum ring. Accordingly, the confinement of the hole is changed from strong, which is assumed in the simulations, to weak, where the local radius is larger than the bulk exciton Bohr radius. In contrast to the hole, an increasing F pushes the electron into the CSQD tip, where it remains in a strong confinement. This means the radiative lifetime for large F is given by an asymmetric confinement with a strongly confined electron and a hole in a weak confinement. To our knowledge, this asymmetric strong–weak confinement represents a novel kind of quantum mechanical confinement and has not been observed so far. Furthermore, the observed weak confinement for the hole represents a confirmation of the theoretically predicted transformation of the hole probability density from a quantum dot into a quantum ring. For such quantum rings, application as storage for photo-excited charge carriers is predicted, which can be interesting for future quantum photonic integrated circuits. Full article

We study a quantum cone, a novel structure composed of multiple quantum dots with gradually decreasing diameters from the base to the top. The dot distribution leads to a dispersive radiated spectrum. The blue edge of the spectrum is determined by the quantum confinement of excitons on top of the cones, while the red edge is determined by the bandgap of a semiconductor. We observe the kinetics of photoluminescence by obeying the stretch-exponential law from quantum cones formed on the surface of diamond-like carbon (DLC). They are explained by an increase in the lifetime of excitons along the height of the cone from the top to the base of the cone and an increasing concentration of excitons at the base due to their drift in the quasi-built-in electric field of the quantum cone. The possible visualization of the quantum cone tops of DLC using irradiation by a UV light source is shown. A quantum cone is an innovative nano-source of light because it substitutes for two elements in a conventional spectrometer: a source of light and a dispersive element—an ultrafast monochromator. These features enable the building of a nano-spectrometer to measure the absorbance spectra of virus and molecule particles. Full article

The time dynamics of charge accumulation at the electrochemical interface between graphene and water is important for supercapacitors, batteries, and chemical and biological sensors. By using impedance spectroscopy, we have found that measured capacitance (C_m) at this interface with the gate voltage V_gate ≈ 0.1 V follows approximate laws C_m~T^1.2 and C_m~T^0.11 (T is V_gate period) in frequency ranges (1000–50,000) Hz and (0.02–300) Hz, respectively. In the first range, this dependence demonstrates that the interfacial capacitance (C_int) is only partially charged during the charging period. The observed weaker frequency dependence of the measured capacitance (C_m) at frequencies below 300 Hz is primarily determined by the molecular relaxation of the double-layer capacitance (C_dl) and by the graphene quantum capacitance (C_q), and it also implies that C_int is mostly charged. We have also found a voltage dependence of C_m below 10 Hz, which is likely related to the voltage dependence of C_q. The observation of this effect only at low frequencies indicates that C_q relaxation time is much longer than is typical for electron processes, probably due to Dirac cone reconstruction from graphene electrons with increased effective mass as a result of their quasichemical bonding with interfacial molecular charges. Full article

Strain-free GaAs cone-shell quantum dots have a unique shape, which allows a wide tunability of the charge-carrier probability densities by external electric and magnetic fields. Here, the influence of a lateral electric field on the optical emission is studied experimentally using simulations. The simulations predict that the electron and hole form a lateral dipole when subjected to a lateral electric field. To evaluate this prediction experimentally, we integrate the dots in a lateral gate geometry and measure the Stark-shift of the exciton energy, the exciton intensity, the radiative lifetime, and the fine-structure splitting (FSS) using single-dot photoluminescence spectroscopy. The respective gate voltage dependencies show nontrivial trends with three pronounced regimes. We assume that the respective dominant processes are charge-carrier deformation at a low gate voltage U, a vertical charge-carrier shift at medium U, and a lateral charge-carrier polarization at high U. The lateral polarization forms a dipole, which can either enhance or compensate the intrinsic FSS induced by the QD shape anisotropy, dependent on the in-plane orientation of the electric field. Furthermore, the data show that the biexciton peak can be suppressed by a lateral gate voltage, and we assume the presence of an additional vertical electric field induced by surface charges. Full article

The article presents a theoretical study of Oleinik resonances in the process of scattering a gamma quantum by an ultrarelativistic electron in the field of a strong electromagnetic wave with intensities up to ${10}^{27}{\mathrm{Wcm}}^{-2}$. The resonant kinematics for three possible resonant reaction channels in a strong external field have been studied in detail. It is shown that under resonant conditions, the scattering channels of the reaction effectively split into two first-order processes according to the fine structure constant, such as the external field-stimulated Compton effect. The annihilation channel of the reaction effectively decays into direct and reverse the external field-stimulated Breit–Wheeler processes. In the absence of interference from the reaction channels, a resonant differential cross-section was obtained in a strong external electromagnetic field. The cases when the energy of the initial electrons significantly exceeds the energy of the initial gamma quanta have been studied. At the same time, all particles (initial and final) fly in a narrow cone away from the direction of wave propagation. The conditions under which the energy of ultrarelativistic initial electrons is converted into the energy of a finite gamma quantum are studied. It is shown that the resonant differential cross-section of such a process significantly (by several orders of magnitude) exceeds the corresponding nonresonant cross-section. This theoretical study predicts a number of new physical effects that may explain the high-energy fluxes of gamma quanta produced near neutron stars and magnetars. Full article

Herein, we investigate the effect of tilt mismatch on the quantum oscillations of spin transport properties in two-dimensional asymmetrically tilted Dirac cone systems. This study involves the examination of conductance oscillation in two distinct junction types: transverse- and longitudinal-tilted Dirac cones (TTDCs and LTDCs). Our findings reveal an unusual quantum oscillation of spin-polarized conductance within the TTDC system, characterized by two distinct anomaly patterns within a single period, labeled as the linear conductance phase and the oscillatory conductance phase. Interestingly, these phases emerge in association with tilt-induced orbital pseudo-magnetization and exchange interaction. Our study also demonstrates that the structure of the LTDC can modify the frequency of spin conductance oscillation, and the asymmetric effect within this structure results in a quantum beating pattern in oscillatory spin conductance. We note that an enhancement in the asymmetric longitudinal tilt velocity ratio within the structure correspondingly amplifies the beating frequency. Our research potentially contributes valuable insights for detecting the asymmetry of tilted Dirac fermions in type-I Dirac semimetal-based spintronics and quantum devices. Full article

Numerous optoelectronic devices based on low-dimensional nanostructures have been developed in recent years. Among these, pyramidal low-dimensional semiconductors (zero- and one-dimensional nanomaterials) have been favored in the field of optoelectronics. In this review, we discuss in detail the structures, preparation methods, band structures, electronic properties, and optoelectronic applications (photocatalysis, photoelectric detection, solar cells, light-emitting diodes, lasers, and optical quantum information processing) of pyramidal low-dimensional semiconductors and demonstrate their excellent photoelectric performances. More specifically, pyramidal semiconductor quantum dots (PSQDs) possess higher mobilities and longer lifetimes, which would be more suitable for photovoltaic devices requiring fast carrier transport. In addition, the linear polarization direction of exciton emission is easily controlled via the direction of magnetic field in PSQDs with C_3v symmetry, so that all-optical multi-qubit gates based on electron spin as a quantum bit could be realized. Therefore, the use of PSQDs (e.g., InAs, GaN, InGaAs, and InGaN) as effective candidates for constructing optical quantum devices is examined due to the growing interest in optical quantum information processing. Pyramidal semiconductor nanorods (PSNRs) and pyramidal semiconductor nanowires (PSNWRs) also exhibit the more efficient separation of electron-hole pairs and strong light absorption effects, which are expected to be widely utilized in light-receiving devices. Finally, this review concludes with a summary of the current problems and suggestions for potential future research directions in the context of pyramidal low-dimensional semiconductors. Full article

Integrable quantum field theories can be regularized on the lattice while preserving integrability. The resulting theories on the lattice are integrable lattice models. A prototype of such a regularization is the correspondence between a sine-Gordon model and a six-vertex model on a light-cone lattice. We propose an integrable deformation of the light-cone lattice model such that in the continuum limit we obtain the $T\overline{T}$-deformed sine-Gordon model. Under this deformation, the cut-off momentum becomes energy dependent and the underlying Yang–Baxter integrability is preserved. Therefore, this deformation is integrable but non-local: similar to the $T\overline{T}$ deformation of quantum field theory. Full article

The temperature-dependent intensities of the exciton (X) and biexciton (XX) peaks from single GaAs cone–shell quantum dots (QDs) are studied with micro photoluminescence (PL) at varied excitation power and QD size. The QDs are fabricated by filling self-assembled nanoholes, which are drilled in an AlGaAs barrier by local droplet etching (LDE) during molecular beam epitaxy (MBE). This method allows the fabrication of strain-free QDs with sizes precisely controlled by the amount of material deposited for hole filling. Starting from the base temperature T = 3.2 K of the cryostat, single-dot PL measurements demonstrate a strong enhancement of the exciton emission up to a factor of five with increasing T. Both the maximum exciton intensity and the temperature $T_{x,max}$ of the maximum intensity depend on excitation power and dot size. At an elevated excitation power, $T_{x,max}$ becomes larger than 30 K. This allows an operation using an inexpensive and compact Stirling cryocooler. Above $T_{x,max}$, the exciton intensity decreases strongly until it disappears. The experimental data are quantitatively reproduced by a model which considers the competing processes of exciton generation, annihilation, and recombination. Exciton generation in the QDs is achieved by the sum of direct excitation in the dot, plus additional bulk excitons diffusing from the barrier layers into the dot. The thermally driven bulk-exciton diffusion from the barriers causes the temperature enhancement of the exciton emission. Above $T_{x,max}$, the intensity decreases due to exciton annihilation processes. In comparison to the exciton, the biexciton intensity shows only very weak enhancement, which is attributed to more efficient annihilation processes. Full article

We analyze time-of-arrival probability distributions for relativistic particles in the context of quantum field theory (QFT). We show that QFT leads to a unique prediction, modulo post-selection that incorporates properties of the apparatus into the initial state. We also show that an experimental distinction of different probability assignments is possible especially in near-field measurements. We also analyze causality in relativistic measurements. We consider a quantum state obtained by a spacetime-localized operation on the vacuum, and we show that detection probabilities are typically characterized by small transient non-causal terms. We explain that these terms originate from Feynman propagation of the initial operation, because the Feynman propagator does not vanish outside the light cone. We discuss possible ways to restore causality, and we argue that this may not be possible in measurement models that involve switching the field–apparatus coupling on and off. Full article

It is shown that it is possible to adapt the quantum graph model of graphene to some types of nonequilateral graphynes considered in the literature; we also discuss the corresponding nanotubes. The proposed models are, in fact, effective models and are obtained through selected boundary conditions and an ad hoc prescription. We analytically recover some results from the literature, in particular, the presence of Dirac cones for $\alpha$-, $\beta$- and $(6,6,12)$-graphynes; for $\gamma$-graphyne, our model presents a band gap (according to the literature), but only for a range of parameters, with a transition at a certain point with quadratic touch and then the presence of Dirac cones. Full article