Search Results (39)

Since the definitions of the two-dimensional (2D) optical absorption coefficient and photoconductivity are independent of the thickness of 2D materials, they are more suitable than the dielectric function to describe the optical properties of 2D materials. Based on the many-body GW method and the Bethe–Salpeter equation, we calculated the quasiparticle electronic structure, optical absorbance, and complex photoconductivity of 2D InSe from a single layer (1L) to three layers (3L). The calculation results show that the energy difference between the direct and indirect band gaps in 1L, 2L, and 3L InSe is so small that strain can readily tune its electronic structure. The 2D optical absorbance results calculated taking into account exciton effects show that light absorption increases rapidly near the band gap. Strain modulation of 1L InSe shows that it transforms from an indirect bandgap semiconductor to a direct bandgap semiconductor in the biaxial compressive strain range of −1.66 to −3.60%. The biaxial compressive strain causes a slight blueshift in the energy positions of the first and second absorption peaks in monolayer InSe while inducing a measurable redshift in the energy positions of the third and fourth absorption peaks. Full article

The determination of energy loss of charged particles as they pass through a medium and consequently the calculation of their range within the medium are of tremendous importance in various areas of physics from both theoretical and practical perspectives. Previous works have derived approximate equations regarding the range of ions within a medium, focusing on providing simple solutions for practitioners in the radiotherapy field that do not require significant computational cost, unlike traditional Monte Carlo methods. These solutions focus on radiotherapy and are limited to specific ions’ initial speeds, which should be up to 0.65c (where c is the speed of light in vacuum). In this paper, solutions for significantly larger initial velocities are explored. A new analytical equation for determining the range of charged particles within a medium for initial velocities between 0.6c and 0.9c is presented. This equation provides excellent results when compared to the accurate numerical solution. Beyond its theoretical and mathematical interest, this solution is also reliable for radiotherapy applications. It provides excellent results for protons with initial energies between 200 MeV and 350 MeV and has the major advantage of being expressed in terms of elementary functions, making its use more straightforward compared to other approaches. Full article

This article presents the maiden investigation of the electronic structural properties of the Sr⁺ ion confined inside fullerene. The Dirac equations are solved to calculate the energy levels, probability distributions, etc. for various confinement depths of the Gaussian Annular Square Well (GASW) potential using the Multi-Configuration Dirac Hartree–Fock (MCDHF) formalism. The wavelengths, transition probabilities, and oscillator strengths are reported for the $5S_{1/2}⟷5P_{1/2}$ ($D_1$ line) and $5S_{1/2}⟷5P_{3/2}$ ($D_2$ line) transitions of the encapsulated ion. We also estimate variations in the line intensity ratio, electron density, Coulomb coupling parameter, etc. A suggested direction for the calculation of electron impact ionization cross-section using the binary-encounter Bethe (BEB) model with the present data is also given. Full article

Radiotherapy is one of the major cancer treatments that uses controlled doses of ionizing radiation to damage tumor cells. The monitoring of charged particles within a medium is of tremendous importance in radiotherapy. Monte Carlo methods can be used to estimate radiation profiles; however, despite their effectiveness, they are computationally expensive, which limits their practicality. To simplify the analysis, approximate analytical methods focused on estimating the range of charged particles and their velocity function within a medium have been previously derived. Previous solutions include non-elementary functions, such as the exponential integral function with relativistic coordinate transformations, or the use of regular perturbation methods accounting for small relativistic effects. In this paper, a much simpler approach is presented to assist practitioners in the field of radiotherapy. Using the proposed method, the particles’ range and velocities are calculated exclusively with elementary functions. The main advantage of the proposed approach, aside from its straightforward application, is its suitability for relativistic velocities. The equations derived in this paper were successfully tested at the radiotherapy level, accommodating protons with energies of up to 350 MeV. Full article

We apply extended local hidden-gauge formalism to study meson–meson interactions with the quark constituents $cc\overline{c}\overline{c}$, $cc\overline{c}\overline{b}/\overline{c}\overline{c}cb$, $cc\overline{b}\overline{b}/\overline{c}\overline{c}bb$, $bb\overline{c}\overline{b}/\overline{b}\overline{b}cb$, and $bb\overline{b}\overline{b}$, in which the exchanged mesons are the fully heavy vector mesons $J/\psi$, $B_c^{*}$ and $\mathrm{{\rm Y}}$. We solve the coupled-channel Bethe–Salpeter equation to derive two poles in the $bb\overline{c}\overline{b}$ system and two poles in the $cc\overline{c}\overline{b}$ system. There are also four charge-conjugated poles in the $\overline{b}\overline{b}cb$ and $\overline{c}\overline{c}cb$ systems. In the $bb\overline{c}\overline{b}$ system, one pole corresponds to a sub-threshold bound state when the cutoff momentum is set to $\mathrm{\Lambda }>850 \mathrm{MeV}$. The other pole in this system corresponds to a sub-threshold bound state when $\mathrm{\Lambda }>1100 \mathrm{MeV}$. In the $cc\overline{c}\overline{b}$ system, the two poles correspond to sub-threshold bound states only when $\mathrm{\Lambda }>1550 \mathrm{MeV}$ and $\mathrm{\Lambda }>2650 \mathrm{MeV}$. This makes them difficult to identify as deeply bound hadronic molecules. We propose investigating the two poles of the $bb\overline{c}\overline{b}$ system in the ${\mu }^{+}{\mu }^{-}{B}_c^{-}$ channel at the LHC. Full article

Recently, Bazhanov and Sergeev have described an Ising-type integrable model which can be identified as a sinh-Gordon-type model with an infinite number of states but with a real parameter q. This model is the subject of Sklyanin’s Functional Bethe Ansatz. We develop in this paper the whole technique of the FBA which includes: (1) Construction of eigenstates of an off-diagonal element of a monodromy matrix. The most important ingredients of these eigenstates are the Clebsh-Gordan coefficients of the corresponding representation. (2) Separately, we discuss the Clebsh-Gordan coefficients, as well as the Wigner’s 6j symbols, in details. The later are rather well known in the theory of $3D$ indices. Thus, the Sklyanin basis of the quantum separation of variables is constructed. The matrix elements of an eigenstate of the auxiliary transfer matrix in this basis are products of functions satisfying the Baxter equation. Such functions are called usually the Q-operators. We investigate the Baxter equation and Q-operators from two points of view. (3) In the model considered the most convenient Bethe-type variables are the zeros of a Wronskian of two well defined particular solutions of the Baxter equation. This approach works perfectly in the thermodynamic limit. We calculate the distribution of these roots in the thermodynamic limit, and so we reproduce in this way the partition function of the model. (4) The real parameter q, which is the standard quantum group parameter, plays the role of the absolute temperature in the model considered. Expansion with respect to q (tropical expansion) gives an alternative way to establish the structure of the eigenstates. In this way we classify the elementary excitations over the ground state. Full article

Halide perovskites are widely used as components of electronic and optoelectronic devices such as solar cells, light-emitting diodes (LEDs), optically pumped lasers, field-effect transistors, photodetectors, and $\gamma$-detectors. Despite this wide range of applications, the construction of an electrically pumped perovskite laser remains challenging. In this paper, we numerically justify that mixing two perovskite compounds with different halide elements can lead to optical properties suitable for electrical pumping. As a reference, the chosen model material was CsPbBr${}_3$, whose performance as a part of lasers has been widely recognised, with some Br atoms substituted by I at specific sites. In particular, a strong enhancement of the low-energy absorption peaks has been obtained using the ab initio Bethe–Salpeter equation. Based on these results, we propose specific architectures of ordered doping that could be realised by epitaxial growth. Efficient light emission from the bottom of the conduction band is expected. Full article

Electron impact scattering from C${}_3$F${}_6$O is studied in this work. The R-matrix method was used for the calculations of elastic, momentum transfer, and excitation cross sections. The attachment cross section was obtained through a parametric estimator based on the R-matrix outputs. The Binary-Encounter-Bethe (BEB) method was used for computing the ionization cross section. The obtained cross section set was used for the transport studies using the BOLSIG+ code, which is a two-term Boltzmann equation solver. The present calculation was performed for steady-state Townsend experimental conditions for E/N, covering a range of 100–1000 Td. The critical dielectric strength of pure C${}_3$F${}_6$O was found to be 475 Td, which is much greater than that of SF${}_6$ (355 Td). The effect of the addition of different buffer gases, such as CO${}_2$, N${}_2$, and O${}_2$, was also examined. For the C${}_3$F${}_6$O–CO${}_2$, C${}_3$F${}_6$O–N${}_2$, and C${}_3$F${}_6$O–O${}_2$ mixtures with 65%, 55%, and 60% C${}_3$F${}_6$O, respectively, the critical dielectric strength was determined to be essentially the same as that of pure SF${}_6$. The presence of synergism was confirmed for these gas mixtures. We further derived the Paschen curve using a fitting method with the transport parameters as the basic inputs. The minimum breakdown voltage of C${}_3$F${}_6$O accounted for only 55% of that of SF${}_6$. The buffer gas mixture improved the condition; however, the performance of CO${}_2$ and O${}_2$ mixtures was not satisfactory. The addition of N${}_2$ as the buffer gas significantly improved the breakdown property of the gas. The mixture of ≥99% of N${}_2$ or ≤1% of C${}_3$F${}_6$O gave a better breakdown characteristic than SF${}_6$. Any proportion ≥90% of N${}_2$ or ≤10% of C${}_3$F${}_6$O was suitable in the higher pressure ranges. The present work demonstrates the potential of C${}_3$F${}_6$O as a substitute gas for SF${}_6$ with a negligible environmental threat. Full article

The applicability of the effective models to the description of baryons and the behaviour of ratios of strange baryons to pions is discussed. In the framework of the EPNJL model, the Bethe–Salpeter equation is used to find masses of baryons, which are considered to be in a diquark-quark state. Baryon melting is discussed at a finite chemical potential, and a flavor dependence of the hadronic deconfinement temperature is pointed out. It is shown that the description of the diquark-quark state at finite chemical potential is limited due to the occurrence of Bose condensate. This effect is strongly manifested in the description of light diquarks and baryons. Both the ${\mathrm{\Lambda }}^0/{\pi }^{+}$ and ${\mathrm{\Xi }}^{-}/{\pi }^{+}$ ratios show a sharp behaviour as functions of the $T/{\mu }_B$ variable, where T and ${\mu }_B$ are calculated along the melting lines. Full article

We reconsider the semileptonic decays of $B\to D_1^{(\prime )}l{\overline{\nu }}_l$. The previous theoretical calculations predict a significantly smaller rate for the semileptonic decay of B to $D_1^{\prime }(J_l=\frac{1}{2})$ compared with that to the $D_1(J_l=\frac{3}{2})$, which is not consistent with the current experimental data. This conflict is known as the so-called ‘$\frac{1}{2}$ vs. $\frac{3}{2}$ puzzle’. In this work, we propose a simple scheme to fix this problem, where we suppose the strong eigenstates $D_1^{(\prime )}$ that do not coincide with the eigenstate of the weak interaction, since no experimental results show that the weak and the strong interactions have to share the same eigenstates. Within the framework of this tentative scheme, meson B first weakly decays to the weak eigenstates $D_{\alpha \left(\beta \right)}$ and then the latter are detected as the $D_1^{(\prime )}$ by the strong decay products $D^{*}\pi$. We predict that there exist two new particles $D_{\alpha \left(\beta \right)}$ with $J^P=1^{+}$, which were not previously identified. The good performance of the new scheme in describing the experimental data may hint at new symmetry in the weak decays of $B_q$ to $1^{+}$ heavy–light mesons. To test the scheme proposed here, we suggest an experiment to detect the difference in the invariant mass spectra of $D_1$ that is reconstructed from the B weak decay and from the strong decay products. Full article

We present an ab initio study on quasiparticle (QP) excitations and excitonic effects in two-dimensional (2D) GaN based on density-functional theory and many-body perturbation theory. We calculate the QP band structure using $GW$ approximation, which generates an indirect band gap of $4.83$ eV ($\mathrm{K}\to \Gamma$) for 2D GaN, opening up $1.24$ eV with respect to its bulk counterpart. It is shown that the success of plasmon-pole approximation in treating the 2D material benefits considerably from error cancellation. On the other hand, much better gaps, comparable to $GW$ ones, could be obtained by correcting the Kohn–Sham gap with a derivative discontinuity of the exchange–correlation functional at much lower computational cost. To evaluate excitonic effects, we solve the Bethe–Salpeter equation (BSE) starting from Kohn–Sham eigenvalues with a scissors operator to open the single-particle gap. This approach yields an exciton binding energy of $1.23$ eV in 2D GaN, which is in good agreement with the highly demanding $GW$-BSE results. The enhanced excitonic effects due to reduced dimensionality are discussed by comparing the optical spectra from BSE calculations with that by random-phase approximation (RPA) for both the monolayer and bulk GaN in wurtzite phase. Additionally, we find that the spin–orbit splitting of excitonic peaks is noticeable in 2D GaN but buried in the bulk crystal. Full article

In this work, we present a comprehensive theoretical and computational investigation of exciton fine structures of WSe${}_2$-monolayers, one of the best-known two-dimensional (2D) transition-metal dichalcogenides (TMDs), in various dielectric-layered environments by solving the first-principles-based Bethe–Salpeter equation. While the physical and electronic properties of atomically thin nanomaterials are normally sensitive to the variation of the surrounding environment, our studies reveal that the influence of the dielectric environment on the exciton fine structures of TMD-MLs is surprisingly limited. We point out that the non-locality of Coulomb screening plays a key role in suppressing the dielectric environment factor and drastically shrinking the fine structure splittings between bright exciton (BX) states and various dark-exciton (DX) states of TMD-MLs. The intriguing non-locality of screening in 2D materials can be manifested by the measurable non-linear correlation between the BX-DX splittings and exciton-binding energies by varying the surrounding dielectric environments. The revealed environment-insensitive exciton fine structures of TMD-ML suggest the robustness of prospective dark-exciton-based optoelectronics against the inevitable variation of the inhomogeneous dielectric environment. Full article

In this work, we interpret the newly observed ${\eta }_1\left(1855\right)$ resonance with exotic $J^{PC}=1^{-+}$ quantum numbers in the $I=0$ sector, reported by the BESIII Collaboration, as a dynamically generated state from the interaction between the lightest pseudoscalar mesons and axial-vector mesons. The interaction is derived from the lowest order chiral Lagrangian from which the Weinberg–Tomozawa term is obtained, describing the transition amplitudes among the relevant channels, which are then unitarized using the Bethe–Salpeter equation, according to the chiral unitary approach. We evaluate the ${\eta }_1\left(1855\right)$ decays into the $\eta {\eta }^{\prime }$ and $K{\overline{K}}^{*}\pi$ channels and find that the latter has a larger branching fraction. We also investigate its SU(3) partners, and according to our findings, the ${\pi }_1\left(1400\right)$ and ${\pi }_1\left(1600\right)$ structures may correspond to dynamically generated states, with the former one coupled mostly to the $b_1\pi$ component and the latter one coupled to the $K_1\left(1270\right)\overline{K}$ channel. In particular, our result for the ratio $\Gamma ({\pi }_1\left(1600\right)\to f_1\left(1285\right)\pi )/\Gamma ({\pi }_1\left(1600\right)\to {\eta }^{\prime }\pi )$ is consistent with the measured value, which supports our interpretation for the higher ${\pi }_1$ state. We also report two poles with a mass about 1.7 GeV in the $I=1/2$ sector, which may be responsible for the $K^{*}\left(1680\right)$. We suggest searching for two additional ${\eta }_1$ exotic mesons with masses around 1.4 and 1.7 GeV. In particular, the predicted ${\eta }_1\left(1700\right)$ is expected to have a width around 0.1 GeV and can decay easily into $K\overline{K}\pi \pi$. Full article

We conducted a systematic investigation using state-of-the-art techniques on the electronic and optical properties of two crystals of alkaline earth metal fluorides, namely rutile MgF${}_2$ and cubic SrF${}_2$. For these two crystals of different symmetry, we present density functional theory (DFT), many-body perturbation theory (MBPT), and Bethe–Salpeter equation (BSE) calculations. We calculated a variety of properties, namely ground-state energies, band-energy gaps, and optical absorption spectra with the inclusion of excitonic effects. The quantities were obtained with a high degree of convergence regarding all bulk electronic and optical properties. Bulk rutile MgF${}_2$ has distinguished ground-state and excited-state properties with respect to the other cubic fluoride SrF${}_2$ and the other members of the alkaline earth metal fluoride family. The nature of the fundamental gaps and estimates of the self-energy and excitonic effects for the two compounds are presented and discussed in detail. Our results are in good accordance with the measurements and other theoretical–computational data. A comparison is made between the excitation and optical properties of bulk rutile MgF${}_2$, cubic SrF${}_2$, and the corresponding clusters, for which calculations have recently been published, confirming strong excitonic effects in finite-sized systems. Full article

This paper review the modelling efforts regarding Generalised Parton Distributions (GPDs) using continuum techniques relying on Dyson–Schwinger and Bethe–Salpeter equations. The definition and main properties of the GPDs are first recalled. Then, we detail the strategies developed in the last decade in the meson sector, highlighting that observables connected to the pion GPDs may be measured at future colliders. We also highlight the challenges one will face when targeting baryons in the future. Full article