Exciton Parameters Research Articles

Bright Excitonic Fine Structure in Metal-Halide Perovskites: From Two-Dimensional to Bulk.

The optical response of two-dimensional (2D) perovskites, often referred to as natural quantum wells, is primarily governed by excitons, whose properties can be readily tuned by adjusting the perovskite layer thickness. We have investigated the exciton fine structure splitting in the archetypal 2D perovskite (PEA)2(MA)n-1PbnI3n+1 with varying numbers of inorganic octahedral layers n = 1, 2, 3, and 4. We demonstrate that the in-plane excitonic states exhibit splitting and orthogonally oriented dipoles for all confinement regimes. The evolution of the exciton states in an external magnetic field provides further insights into the g-factors and diamagnetic coefficients. With increasing n, we observe a gradual evolution of the excitonic parameters characteristic of a 2D to three-dimensional transition. Our results provide valuable information concerning the evolution of the optoelectronic properties of 2D perovskites with the changing confinement strength.

Excitonic condensation and metal-semiconductor transition in AA bilayer graphene in an external magnetic field

In this paper, the effects of the external transverse magnetic field $B$ (perpendicular to the surface of the layers) on the electronic and excitonic properties are studied in the AA-stacked bilayer graphene (BLG). The effects of the Coulomb interactions and excitonic pairing have been taken into account and analyzed in detail within the bilayer Hubbard model. Both half-filling and partial filling regimes have been taken into account and the magnetic field dependence of a series of physical parameters was found. It is shown that the difference between the average electron concentrations in the layers vanishes at some critical value of magnetic field $B_{c}$ and the chemical potential is calculated numerically above and below that value. The role of the Coulomb interactions on the average carrier concentrations in the layers has been analyzed, and the excitonic order parameters have been calculated for different spin orientations. We found a possibility for the particle population inversion between the layers when varying the external magnetic field. The calculated electronic band structure in the AA-BLG shows the presence of metal-semiconductor transition, governed by the strength of the applied magnetic field or the interlayer interaction potential. We show that for high magnetic fields the band-gap is approaching the typical values of the gaps in the usual semiconductors. It is demonstrated that, at some parameter regimes, the AA-BLG behaves like a spin-valve device, by permitting the electron transport with only one spin direction. All calculations have been performed at the zero-temperature limit.

Zeeman and Davydov splitting of Frenkel excitons in the antiferromagnet CuB2O4

The optical spectra of antiferromagnetic copper metaborate ${\mathrm{CuB}}_{2}{\mathrm{O}}_{4}$ are characterized by an exceptionally rich structure of narrow absorption lines due to electronic transitions within the magnetic ${\mathrm{Cu}}^{2+}$ ions, but their unambiguous identification and behavior in magnetic fields have remained far from being fully understood. We study the polarized magnetoabsorption spectra of this tetragonal antiferromagnet with high spectral resolution across the energy range of $1.4055--1.4065$ eV in magnetic fields up to 9.5 T for temperatures from 1.6 up to the N\'eel temperature ${T}_{N}=20$ K. We observe a set of eight absorption lines at $T=1.6$ K in magnetic fields exceeding 1.4 T, which we identify as arising from Frenkel excitons related to the ground and first excited states of the ${\mathrm{Cu}}^{2+}$ ions. The number of these excitons is defined by the presence of the four ${\mathrm{Cu}}^{2+}$ ions with doubly degenerate spin state $S=1/2$ at the $4b$ positions in the crystallographic unit cell. The energies of these excitons are determined by the exchange interaction of 0.5 meV of the ${\mathrm{Cu}}^{2+}$ ions in the excited state with the surrounding ions and by the Davydov splitting of 0.12 meV. In large magnetic field the observed Zeeman splitting is controlled by the anisotropic $g$-factors of both the ground and excited states. We develop a theoretical model of Frenkel excitons in the magnetic field that accounts for specific features of the spin structure and exchange interactions in ${\mathrm{CuB}}_{2}{\mathrm{O}}_{4}$. The model is used for fitting the experimental data and evaluation of the Frenkel exciton parameters, such as the Davydov splitting, the molecular exchange energy, and the $g$-factors of the ground and excited states of the ${\mathrm{Cu}}^{2+}$ ions.

Экситонные спектры объемных кристаллов BiI-=SUB=-3-=/SUB=-

The luminescence, reflection and absorption spectra of layered BiI3 crystals on samples of different quality are studied in a wide temperature range. Based on the data obtained, the parameters of the direct exciton, which has a high oscillator strength, are estimated

Exciton Spectra of BiI3 bulk crystals

The luminescence, reflection and absorption spectra of layered BiI3 crystals on samples of different quality are studied in a wide temperature range. Based on the data obtained, the parameters of the direct exciton, which has a high oscillator strength, are estimated. Keywords: Bismuth(III) iodide, exciton, localized states

Non-trivial dependence of spectral characteristics of excitons in quantum wells on the resonant optical excitation power

Basic exciton parameters, such as the energy of exciton transition and the radiative and nonradiative broadenings, are experimentally studied by means of reflectance spectroscopy for a heterostructure with a 14-nm GaAs/AlGaAs quantum well. Particular attention is paid to the nonradiative broadening which is sensitive to densities of free carriers and long-lived nonradiative excitons. A sublinear increase of the broadening of the heavy-hole and light-hole exciton resonances is observed when the light-hole exciton resonance is excited with increasing power. A simple model is developed, which allows one to well reproduce the observed dependence. Keywords: exciton, quantum well, nonradiative broadening.

Interference of exciton-polariton waves in GaSe nanocrystals

Absorption, reflection, photoluminescence and wavelength modulation transmission spectra of GaSe single crystals were investigated in temperature rage 300 – 10 K. From fitting of contours of excitonic reflection spectra parameters of excitons as energy of transversal exciton ω0 = 2.1212 eV, longitudinal transversal splitting ωLT = 2.3 meV, effective mass M = 2.5m0, background permittivity εb= 5.2 and damping constant γ = 1.4 were determined. Effective masses of electrons (mC1 = 0.37m0) and holes (mV1 = 2.13m0) in the Brillouin zone center were estimated. An interference of waves of upper and lower branches of exciton-polaritons was found out in wavelength modulation transmission spectra of GaSe nanocrystals.

Нетривиальная зависимость спектральных характеристик экситонов в квантовых ямах от мощности резонансного оптического возбуждения

Basic exciton parameters, the energy of exciton transition and the radiative and nonradiative broadenings, are experimentally studied by means of reflectance spectroscopy for a heterostructure with the 14-nm GaAs/AlGaAs quantum well. Particular attention is paid to the nonradiative broadening which is sensitive to optical creation of free carriers and long-lived nonradiative excitons. A sublinear increase of the broadening of the heavy-hole and light-hole exciton resonances is observed when the light-hole exciton resonance is excited with increasing power. A simple model is developed, which allows one to well reproduce the observed dependence.

Reflectivity and Photoreflectivity Spectra of Structures with Quantum Wells Based on ZnO

Reflectivity and modulated photoreflectivity spectra of ZnO and Zn1 – xMgxO epitaxial layers as well as ZnO/Zn1 – xMgxO quantum wells have been studied at normal and oblique light incidence. Comparison of experimental and calculated spectra allowed refinement of certain parameters of excitons in these structures and the order of energy bands in ZnO.

Excitonic and electronic transitions in Me-Sb2Se3 structures.

The optical anisotropy of the Sb2Se3 crystals was investigated at 300 and 11 K. Excitonic features of four excitons (A, B, C, and D) were observed in the optical spectra of the Sb2Se3 single crystals and in the photoelectric spectra of the Me–Sb2Se3 structures. The exciton parameters, such as the ground (n = 1) and excited (n = 2) state positions and the binding energy (Ry), were determined. The effective mass of the electrons at the bottom of the conduction band (mc* = 0.67m0) as well as the holes at the four top valence bands (mv1* = 3.32m0, mv2* = 3.83m0, mv3* = 3.23m0 and mv4* = 3.23m0) were calculated in the Г-point of the Brillouin zone. The magnitude of the valence band splitting V1–V2 due to the spin–orbit interaction (Δso = 35 meV) and the crystal field (Δcf = 13 meV) were estimated in the Brillouin zone center. The energy splitting between the bands V3–V4 was 191 meV. The identified features were discussed based on both the theoretically calculated energy band structure and the excitonic band symmetry in the Brillouin zone (k = 0) for crystals with an orthorhombic symmetry (Рnma). The photoelectric properties of the Me–Sb2S3 structures were investigated in the spectral range 1–1.8 eV under E||c and E⟂c polarization conditions and at different applied voltages.

Strain-engineered room temperature cavity polariton in ZnO whispering gallery microcavity

Cavity polaritons, originating from the strong coupling effect between cavity modes and excitons, have been tremendously investigated because of their high potential in new or quantum optoelectronic devices. Among them, realization of their dynamic controllability is highly desirable for real applications. In this work, we present a dynamic modulation of cavity polaritons via strain-engineering in the ZnO whispering gallery microcavity. By using a home-made strain apparatus, a nearly uniaxial stress is precisely applied to ZnO microwires, thus inducing an internal strain along its c-axis. Dynamic tuning of cavity polariton modes is clearly characterized by angle-resolved microphotoluminescence spectroscopy. The coupled oscillator's model with fitting parameters of strain-dependent excitons' energies can well describe the modes' redshift. Moreover, we show that the modes' redshift can also be understood by the strain-induced refractive index increase, described by the Lorentz model, of which the underlying mechanism is also related to light-exciton strong coupling. Finally, the strain-engineered dynamic tuning of polariton lasing modes is demonstrated as well, and this actually provides convincing evidence for the polariton states surviving above the threshold. This work indicates that strain-engineered cavity polaritons would pave an alternative way for developing polariton-based integrated or flexible optoelectronic devices.

Exciton properties and optical spectra of light harvesting complex II from a fully atomistic description.

We present a fully atomistic simulation of linear optical spectra (absorption, fluorescence and circular dichroism) of the Light Harvesting Complex II (LHCII) trimer using a hybrid approach, which couples a quantum chemical description of the chlorophylls with a classical model for the protein and the external environment (membrane and water). The classical model uses a polarizable Molecular Mechanics force field, thus allowing mutual polarization effects in the calculations of the excitonic properties. The investigation is performed both on the crystal structure and on structures generated by a μs long classical molecular dynamics simulation of the complex within a solvated membrane. The results show that this integrated approach not only provides a good description of the excitonic properties and optical spectra without the need for additional refinements of the excitonic parameters, but it also allows an atomistic investigation of the relative importance of electronic, structural and environment effects in determining the optical spectra.

Chemometrics-spectroscopic study of the effect of temperature and pre-micellar to post-micellar forms of various surfactants on the dimerization of nickel and copper phthalocyanines

The dye-surfactant aggregations have various applications in, industry, biochemistry, pharmacology, chemical sensor, wastewater treatment and for removal modeling of dyes. Interactions of surfactant-dye can change the intensity, the position, and the shape of the bands in the UV–Vis spectra. The effect of the nonionic and ionic surfactants such as Triton X-100 (TX-100), Triton X-114 (TX-114), sodium dodecyl sulfate (SDS), cetyl trimethyl ammonium bromide (CTAB) and cetyl pyridinium chloride (CPC) was studied on the spectra of two metallophthalocyanines. The interaction and aggregation behavior was investigated and discussed by chemometrics-spectroscopy results. The equilibrium constant (KD) of under studied systems at various temperatures were calculated by multivariate curve resolution (MCR) method. Besides the thermodynamic parameters were calculated by using the van't Hoff equation (the inverse temperature dependence of the KD). The excitonic parameters were determined for some of the studied systems at various conditions. The sudden blueshift of dimer-band was observed by increasing the concentration of surfactants from 0.25×CMC to 8 × CMC (628–613 nm for CPC/CTAB-CuPcTS system, 628–613 nm for CTAB-NiPcTS system and 621–607 nm for CPC-NiPcTS system). The H-dimer was notable for cationic surfactant-dye systems. The relative position of the dye molecules was studied against the surfactant's micelle. As it is expected, the strong interaction was not observed between the dyes and the anionic and nonionic surfactants. The KD values were increased by increasing the concentration of surfactant from pre-micellar to micellar concentration, and then it was decreased in the post-micellar concentration at the constant concentrations of dye.

Spectrophotometric-chemometrics study of the effect of solvent composition and temperature on the spectral shape and shift of copper and nickel phthalocyanines in different aqueous-nonaqueous mixed solvents.

Phthalocyanines (Pcs) are green-blue colored aromatic macrocyclic compounds used extensively in the dyeing industry. The assembly phenomenon for dye molecules is directly traceable by most of the spectroscopic methods. In this investigation, the monomer-dimer equilibria of copper and nickel phthalocyanines tetra sulfonic acid tetrasodium salts (CuPcTS, NiPcTS) have been investigated by spectrophotometric and chemometrics methods in binary mixtures of H2O-DMSO and H2O-CH3CN. The dimerization constants, (KD), enthalpies and entropies of CuPcTS and NiPcTS have been calculated by studying the UV-Vis spectra at different concentrations of dyes (10-6 to 10-4molL) and in the temperature range 298-343 K and in some samples up to 353 K by multivariate curve resolution (mcr) methods. By increasing the temperature, the value ofKD decreases. The inverse temperature dependence of KD(van't Hoff equation) was used for determination of ΔH0 and ΔS0and following that ΔG0of the dimerization reactions. As a result, upon aggregation, an increase in the intensity of the new shoulder at (~600 nm) and the Q-band at (662-670 nm) and concomitant decrease of the dimer band at (630-624 nm) are observed for all of the samples in different solvents composition. Therefore, the H-dimer type of these pigments was notable, in studied binary solvents. The effect of the solvent composition, concentration dye, and temperature on the spectral responses, the exciton parameters and concentration distribution diagrams of the two pigments were studied and discussed. The density functional theory (DFT) calculations were done in the aqueous and gas phase that they were included the HOMO-LUMO energies and the simulated UV-Vis spectrum. These calculations were beneficial for studying the UV-Vis spectrum of it in the aqueous phase for checking experimental data.

Оptical properties and electronic band structure of SnS single crystals

Reflection and absorption spectra of SnS single crystals (Pnma (D2h16) space group) were investigated at low and room temperatures. Features attributed to three excitonic states were found out. Parameters of excitons and bands were determined. The first excitonic states are formed at Γ-Y direction of Brillouin zone and have hole effective mass mV1* = 2.6m0 and electron effective mass mC1* = 1.3m0. The second excitonic states near X point in direction Γ-X and the third excitonic states in U point of Brillouin zone are formed. Excitonic transitions in U point are allowed in both polarizations and effective mass of holes in U point (mV1* = 3.46m0) is more than one at Γ-Y direction (mV1* = 2.6m0). Polarized transitions in reflection spectra of E||b and E⊥b polarizations in wide energy range of 1–6 eV were revealed. Optical functions (n, k, ε1 and ε2) for polarizations E||b and E⊥b in the wide energy range by Kramers-Kronig relations were calculated.

Many-body effects in twisted bilayer graphene at low twist angles

We study the zero-temperature many-body properties of twisted bilayer graphene with a twist angle equal to the so-called `first magic angle'. The system low-energy single-electron spectrum consists of four (eight, if spin label is accounted) weakly-dispersing partially degenerate bands, each band accommodating one electron per Moir{\'{e}} cell per spin projection. This weak dispersion makes electrons particularly susceptible to the effects of interactions. Introducing several excitonic order parameters with spin-density-wave-like structure, we demonstrate that (i)~the band degeneracy is partially lifted by the interaction, and (ii)~the details of the low-energy spectrum becomes doping-dependent. For example, at or near the undoped state, interactions separate the eight bands into two quartets (one quartet is almost filled, the other is almost empty), while for two electrons per Moir\'{e} cell, the quartets are pulled apart, and doublets emerge. When the doping is equal to one or three electrons per cell, the doublets split into singlets. Hole doping produces similar effects. As a result, electronic properties (e.g., the density of states at the Fermi energy) demonstrate oscillating dependence on the doping concentration. This allows us to reproduce qualitatively the behavior of the conductance observed recently in experiments [Cao et al., Nature {\bf 556}, 80 (2018)]. Near half-filling, the electronic spectrum loses hexagonal symmetry indicating the appearance of a many-body nematic state.

Inhomogeneous Broadening of the Exciton Band in Optical Absorption Spectra of InP/ZnS Nanocrystals.

In this work, we have simulated the processes of broadening the first exciton band in optical absorption spectra (OA) for InP/ZnS ensembles of colloidal quantum dots (QDs). A phenomenological model has been proposed that takes into account the effects of the exciton–phonon interaction, and allows one to analyze the influence of the static and dynamic types of atomic disorder on the temperature changes in the spectral characteristics in question. To vary the degree of static disorder in the model system, we have used a parameter δ, which characterizes the QD dispersion in size over the ensemble. We have also calculated the temperature shifts of the maxima and changes in the half-width for the exciton peaks in single nanocrystals (δ = 0), as well as for the integrated OA bands in the QD ensembles with different values of δ = 0.6–17%. The simulation results and the OA spectra data measured for InP/ZnS nanocrystals of 2.1 nm (δ = 11.1%) and 2.3 nm (δ = 17.3%), are in good mutual agreement in the temperature range of 6.5 K–RT. It has been shown that the contribution of static disorder to the observed inhomogeneous broadening of the OA bands for the QDs at room temperature exceeds 90%. The computational experiments performed indicate that the temperature shift of the maximum for the integrated OA band coincides with that for the exciton peak in a single nanocrystal. In this case, a reliable estimate of the parameters of the fundamental exciton–phonon interaction can be made. Simultaneously, the values of the specified parameters, calculated from the temperature broadening of the OA spectra, can be significantly different from the true ones due to the effects of static atomic disorder in real QD ensembles.

Master equation study of excitonic processes limiting the luminous efficacy in phosphorescent organic light-emitting diodes

We use a master equation model for numerical simulations of triplet loss mechanisms in phosphorescent organic light-emitting diodes (OLEDs). In addition to the IV characteristics, which were successfully simulated in our previous work, the measurements of luminous efficacy (LE) for the OLEDs with various doping profiles of the emitter fac-tris(2-phenylpyridine)iridium (III) (Ir(ppy)3) can be reproduced based on a single set of excitonic parameters, which is a strong indication of the validity of the model. In order to minimize the strong Dexter diffusion of triplets out of the emission layer (EML) and the nonradiative decays, an additional exciton blocking layer and stronger exciton confinement on guest molecules are investigated in the simulations to improve the device efficiency. With the modifications, the LE of the lowest-doped OLED with 4% Ir(ppy)3 can be more than twofold increased at a luminance up to 20000cd/m2. However, due to the enhanced triplet-triplet annihilation, the roll-off in electrophosphorescence efficiencies under high current injections of the upgraded devices becomes stronger, which can nevertheless be suppressed by a higher emitter concentration in the EML.

Exciton spectroscopy of optical reflection from wide quantum wells

Optical spectroscopy of resonant reflection has been used for both experimental and theoretical studies of the exciton-light interaction in wide, high-quality quantum-well structures with the widths exceeding the exciton Bohr radius by an order of magnitude or more. The light mixes the low-lying confined exciton states which are captured by the generalized model developed by M. M. Voronov et al. [Phys. Solid State 49, 1792 (2007)]. We demonstrate that the high-energy confined states in the wide QWs can still be described by the standard model in which the amplitude reflection coefficient from the QW is a sum of individual size-quantized exciton resonances. The excitonic parameters extracted from fitting the experimental spectrum to the standard model agree with those obtained by the numerical solution of the two-particle Schr\odinger equation in a rectangular quantum well. The measured and microscopically calculated spectra are compared with those found with the widely used model of the center-of-mass exciton quantization and the polaritonic model. The comparison shows that the two approximate models considerably underestimate the interaction of confined excitons with light because they ignore the strong modification of the exciton wave function near the QW interfaces.

Spin texture and spin current in excitonic phases of the two-band Hubbard model

Using the mean-field approximation, we study the $k$-space spin textures and local spin currents emerged in the spin-triplet excitonic insulator states of the two-band Hubbard model defined on the square and triangular lattices. We assume a noninteracting band structure with a direct band gap and introduce $s$-, $p$-, $d$-, and $f$-type cross-hopping integrals, i.e., the hopping of electrons between different orbitals on adjacent sites with four different symmetries. First, we calculate the ground-state phase diagrams in the parameter space of the band filling and interaction strengths, whereby we present the filling dependence of the amplitude and phase of the excitonic order parameters. Then, we demonstrate that the spin textures (or asymmetric band structures) are emerged in the Fermi surfaces by the excitonic symmetry breaking when particular phases of the order parameter are stabilized. Moreover, in case of the $p$-type cross-hopping integrals, we find that the local spin current can be induced spontaneously in the system, which does not contradict the Bloch theorem for the absence of the global spin current. The proofs of the absence of the global spin current and the possible presence of the local spin currents are given on the basis of the Bloch theorem and symmetry arguments.