Exciton Mass Research Articles

Polaron-induced changes in moiré exciton propagation in twisted van der Waals heterostructures.

Twisted transition metal dichalcogenides (TMDs) present an intriguing platform for exploring excitons and their transport properties. By introducing a twist angle, a moiré superlattice forms, providing a spatially dependent exciton energy landscape. Based on a microscopic many-particle theory, we investigate in this work polaron-induced changes in exciton transport properties in the exemplary MoSe2/WSe2 heterostructure. We demonstrate that polaron formation and the associated enhancement of the moiré exciton mass lead to a significant band flattening. As a result, the moiré inter-cell tunneling and the propagation velocity undergo noticeable temperature and twist-angle dependent changes. We predict a reduction of the hopping strength ranging from 80% at a twist angle of 1° to 30% at 3° at room temperature. The provided microscopic insights into the spatio-temporal exciton dynamics in presence of a moiré potential further expand the possibilities to tune charge and energy transport in 2D materials.

Exciton–Polaritons in CsPbBr3 Crystals Revealed by Optical Reflectivity in High Magnetic Fields and Two‐Photon Spectroscopy

Cesium lead bromide (CsPbBr3) is a representative material of the emerging class of lead halide perovskite semiconductors that possess remarkable optoelectronic properties. Its optical properties in the vicinity of the bandgap energy are greatly contributed by excitons, which form exciton polaritons due to strong light–matter interactions. Exciton–polaritons in solution‐grown CsPbBr3 crystals are examined by means of circularly polarized reflection spectroscopy measured in high magnetic fields up to 60 T. The excited 2P exciton state is measured by two‐photon absorption. Comprehensive modeling and analysis provides detailed quantitative information about the exciton–polariton parameters: exciton binding energy of 32.5 meV, oscillator strength characterized by longitudinal–transverse splitting of 5.3 meV, damping of 6.7 meV, reduced exciton mass of 0.18m0, exciton diamagnetic shift of 1.6 μeV T−2, and exciton Landé factor . It is shown that the exciton states can be described within a hydrogen‐like model with an effective dielectric constant of 8.7. From the measured exciton longitudinal–transverse splitting, Kane energy of Ep = 15 eV is evaluated, which is in reasonable agreement with values of 11.8–12.5 eV derived from the carrier effective masses.

Novel strategy for in situ construction of Cercis chinensis cluster-like photocatalytic heterojunctions

Developing high-performance catalysts is a crucial area of research for efficiently utilizing light energy. In this study, a lamellar spherical structure of Cu2(OH)2CO3 was utilized to synthesize Cercis chinensis clustered CuO/g-C3N4 heterojunctions. These heterojunctions were then further improved through in situ orogenesis initiated by the steam and CO2 produced during the decomposition of Cu2(OH)2CO3. This strategy avoided the dilemma of multi-step synthesis of traditional heterojunctions, and further overcame the trouble of removing templates. The heterojunction results in a significant increase in CH4 production in the photocatalytic CO2 reduction reaction. The rate increases to 18.61 μmol g−1 h−1, which is 4.68 times higher than that of pure g-C3N4 and 16.32 times higher than that of CuO. Moreover, the heterojunction effective exciton separation, mass transfer, and distinct morphology result in a CH4 selectivity of over 75%. In comparison, pure g-C3N4 exhibits a CH4 selectivity of 52%, while CuO demonstrates a selectivity of 36.3%. This work provides a potential and simple strategy for the in situ construction of novel photocatalysts with heterojunctions and special morphologies.

Two-dimensional Wannier-Mott exciton diamagnetic shift

We present a model of diamagnetic shifts for two-dimensional Wannier-Mott excitons in out-of-plane magnetic fields, including environmental dielectric effects. We indicate the reasons for the magnetic-field-induced increase of exciton mass and size. In the weak-field limit, we deduce the diamagnetic coefficient and explain why its value is significantly different between excitonic states. Our results agree well with experimental data.

Retrieval of material properties of monolayer transition metal dichalcogenides from magnetoexciton energy spectra

Reduced exciton mass, polarizability, and dielectric constant of the surrounding medium are essential properties for semiconducting materials, and they have been extracted recently from the magnetoexciton energies. However, the acceptable accuracy of the suggested method requires very high magnetic intensity. Therefore, in the present paper, we propose an alternative method of extracting these material properties from recently available experimental magnetoexciton s-state energies in monolayer transition-metal dichalcogenides (TMDCs). The method is based on the high sensitivity of exciton energies to the material parameters in the Rytova-Keldysh model. It allows us to vary the considered material parameters to get the best fit of the theoretical calculation to the experimental exciton energies for the $1s$, $2s$, and $3s$ states. This procedure gives values of the exciton reduced mass and $2D$ polarizability. Then, the experimental magnetoexciton spectra compared to the theoretical calculation also determine the average dielectric constant. Concrete applications are presented only for monolayers WSe$_2$ and WS$_2$ from the recently available experimental data; however, the presented approach is universal and can be applied to other monolayer TMDCs. The mentioned fitting procedure requires a fast and effective method of solving the Schr\"{o}dinger equation of an exciton in monolayer TMDCs with a magnetic field. Therefore, we also develop such a method in this paper for highly accurate magnetoexciton energies.

Spin-mediated Mott excitons

Motivated by recent experiments on Mott insulators, in both iridates and ultracold atoms, we theoretically study the effects of magnetic order on the Mott-Hubbard excitons. In particular, we focus on spin-mediated doublon-holon pairing in Hubbard materials. We use several complementary theoretical techniques: Mean-field theory to describe the spin degrees of freedom, the self-consistent Born approximation to characterize individual charge excitations across the Hubbard gap, and the Bethe-Salpeter equation to identify bound states of doublons and holons. The binding energy of the Mott exciton is found to increase with increasing the N\'eel order parameter, whereas the exciton mass decreases. We observe that these trends rely significantly on the retardation of the effective interaction, and require consideration of multiple effects from changing the magnetic order. Our results are consistent with the key qualitative trends observed in recent experiments on iridates. Moreover, the findings could have direct implications on ultracold atom Mott insulators where the Hubbard model is the exact description of the system and the microscopic degrees of freedom can be directly accessed.

An analytical model for organic bulk heterojunction solar cells based on Saha equation for exciton dissociation.

The power conversion efficiencies of organic solar cells (OSCs) have been greatly improved in recent years. However, latest experimental data of high efficiency OSCs, the sublinear relationship between the short circuit current density (Jsc) and light intensity (Pin), and the effects of energetic disorder in bulk heterojunction organic solar cells have not been understood. An analytical model for high-efficiency OSCs is proposed, which takes most physical factors into account that have been ignored in most previous models, including practical solar spectra and absorption spectra, degeneracy effect, exciton effect, space charge limited current, and unified mobility expression dependent on temperature, electric field and density, etc. Three analytical iterative methods are proposed to solve the strong non-linear Poisson equation and the drift-diffusion equations. The method for the drift-diffusion equations involves introducing two constant coefficients and determining their values self-consistently by demanding the space averages of approximate drift and diffusion currents equal to the averages of accurate ones. The theoretical results for five high-efficiency OSCs are in good agreement with experimental data, including current-voltage curves, light intensity-dependent Jsc and open-circuit voltage (Voc) curves. The effects of energetic disorder in bulk heterojunction organic solar cells, and the sublinear relationship Jsc ∝ Pαin (α < 1) can be well explained. The Saha equation for exciton dissociation and the space-charge-limited-current (SCLC) effect are important for modelling high-efficiency OSCs. The Voc ∼ Pin relationship can be influenced by many factors. But, the Jsc ∼ Pin relationship can be mainly and slightly influenced by the exciton effect and energetic disorder, respectively. When aiming to realize higher performance OSCs, one should decrease six material parameters, including the energetic disorder, exciton mass, deep level impurity concentration, the ratios of electron and hole mobilities, densities of states for electrons and holes, and potential barriers at the anode and cathode. The performance parameters of 15 triad compounds are predicted by using ab initio Eg and absorption spectra from the literature along with other input parameters taken from previous optimized values, and the efficiency of two compounds was found to exceed 35%.

Renormalization of excitonic properties by polar phonons.

We employ quasiparticle path integral molecular dynamics to study how the excitonic properties of model semiconductors are altered by electron-phonon coupling. We describe ways within a path integral representation of the system to evaluate the renormalized mass, binding energy, and radiative recombination rate of excitons in the presence of a fluctuating lattice. To illustrate this approach, we consider Fröhlich-type electron-phonon interactions and employ an imaginary time influence functional to incorporate phonon-induced effects nonperturbatively. The effective mass and binding energies are compared with perturbative and variational approaches, which provide qualitatively consistent trends. We evaluate electron-hole recombination rates as mediated through both trap-assisted and bimolecular processes, developing a consistent statistical mechanical approach valid in the reaction limited regime. These calculations demonstrate how phonons screen electron-hole interactions, generically reducing exciton binding energies and increasing their radiative lifetimes.

Symmetry-dependent exciton-exciton interaction and intervalley biexciton in monolayer transition metal dichalcogenides

The multivalley band structure of monolayer transition metal dichalcogenides (TMDs) gives rise to intravalley and intervalley excitons. Much knowledge of these excitons has been gained, but fundamental questions remain, such as how to describe them all in a unified picture with their correlations, how are those from different valleys coupled to form the intervalley biexciton? To address the issues, we derive an exciton Hamiltonian from interpair correlations between the constituent carriers-fermions of two excitons. Identifying excitons by irreducible representations of their point symmetry group, we find their pairwise interaction depending on interacting excitons’ symmetry. It is generally repulsive, except for the case excitons from different valleys, which attract each other to form the intervalley biexciton. We establish a semianalytical relationship between the biexciton binding energy with exciton mass and dielectric characteristics of the material and surroundings. Overall, by providing insight into the nature of diverse excitons and their correlations, our theoretical model captures the exciton interaction properties permitting an inclusive description of the structure and energy features of the intervalley biexciton in monolayer TMDs.

Engineering the radiative dynamics of thermalized excitons with metal interfaces

As a platform for optoelectronic devices based on exciton dynamics, monolayer transition metal dichalcogenides (TMDCs) are often placed near metal interfaces or inside planar cavities. While the radiative properties of point dipoles at metal interfaces has been studied extensively, those of excitons, which are delocalized and exhibit a temperature-dependent momentum distribution, lack a thorough treatment. Here, we analyze the emission properties of excitons in TMDCs near planar metal interfaces and explore their dependence on exciton center-of-mass momentum, transition dipole orientation, and temperature. Defining a characteristic energy scale k B T c = (ℏk)2/2m (k being the radiative wavevector and m the exciton mass), we find that at temperatures T ≫ T c and low densities where the momentum distribution can be characterized by Maxwell–Boltzmann statistics, the modified emission rates (normalized to free space) behave similarly to point dipoles. This similarity in behavior arises due to the broad nature of wavevector components making up the exciton and point dipole emission. On the other hand, the narrow momentum distribution of excitons for T < T c can result in significantly different emission behavior as compared to point dipoles. These differences can be further amplified by considering excitons with a Bose Einstein distribution at high phase space densities, such as in a condensate phase. We find suppression or enhancement of emission relative to the point dipole case by several orders of magnitude. These insights can help optimize the performance of optoelectronic devices that incorporate 2D semiconductors near metal electrodes and can inform future studies of exciton radiative dynamics at low temperatures. Additionally, these studies show that nanoscale optical cavities are a viable pathway to generating long-lifetime exciton states in TMDCs.

Transition from metal to semiconductor by semi-hydrogenation of borophene

Borophene has triggered a surge of interest due to its outstanding properties including mechanical flexibility, polymorphism, and optoelectrical anisotropy. Very recently, a novel semi-hydrogenated borophene, called ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H, was synthesized in large-scale freestanding samples, exhibiting excellent air-stability and semiconducting nature. Herein, using the density functional theory (DFT) and many-body perturbation theory (MBPT), we investigate the electronic and excitonic optical properties of ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H borophene. The DFT results reveal that hydrogenation breaks the mirror symmetry and increases the buckling height of pure ${\ensuremath{\alpha}}^{\ensuremath{'}}$ borophene, which results in an orbital hybridization and indirect band gap of 1.49 eV in ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H borophene. Moreover, the optical spectrum achieved from solving the Bethe-Salpeter equation (i.e., $\mathrm{GW}+\mathrm{BSE}$) shows an optical band gap of 2.40 eV, which corresponds to a strongly bound and stable bright exciton with a binding energy of 1.18 eV. The mean value of absorption within the visible area is 1.68 (1.13)$\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{4pt}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ for $E\ensuremath{\parallel}x$ ($E\ensuremath{\parallel}y$) polarization, which shows a linear dichroism for visible light. The effective mass and Bohr radius of the ground-state exciton are 0.78 ${m}_{0}$ and 2.03 \AA{}, respectively, which demonstrates the characteristics of Frenkel exciton. The excitonic states are robust against tension up to 10%, under which the monolayer is dynamically stable. We also study the bilayer ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H borophene with different stackings. For the weak van der Waals interactions between the layers, the bilayer preserves most of the structural and electronic properties of the monolayer. Our study exposes the underlying physics behind the structural, electronic, and optical properties of ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H borophene and suggests it as a very promising candidate for flexible optoelectronic applications.

Reviving modulational instability with third-order dispersion

It is well-known that third-order dispersion (TOD) plays no role in the onset of modulation instability in the nonlinear Schrödinger (NLS) family of systems. Here we demonstrate that the characteristics of modulational instability can be dramatically altered by the TOD in two different nonlinear systems, namely Wannier exciton mass in a ZnCdSe/ZnSe semiconductor superlattice and nonlinear pulse/beam propagation in a Kerr-like waveguide under non-slowly varying envelope approximation (non-SVEA), with spatial dispersion. Further we reveal the results of numerical simulations suggesting the existence of recently much studied Akhmediev-type breathers and Peregrine-like rogue waves due to the nonlinear development of MI stemming from the TOD.

Second-harmonic generation of blue series excitons and magnetoexcitons in Cu2O

Nonlinear optical studies of the yellow and green exciton series in ${\mathrm{Cu}}_{2}\mathrm{O}$ have been reported for more than 40 years. Because of the band structure (the two highest even-parity valence and lowest conduction bands), the $S$ excitons of the two lowest exciton series are dipole-forbidden for one-photon absorption and thus dipole-allowed for two-photon absorption. There is an odd-parity higher conduction band that leads with the two even-parity valence bands to the blue and violet exciton series. We report on second-harmonic generation (SHG) of the blue exciton series. The odd-parity $S$-exciton SHG is due to a dipole-quadrupole excitation and a dipole emission process. Because of their high oscillator strength density, polariton effects have to be taken into account, since resonances might be shifted to higher energies by up to $10\phantom{\rule{4pt}{0ex}}\mathrm{me}\mathrm{V}$ compared to the transverse exciton energies. The polariton dispersion for the blue excitons up to $n=4$ is calculated and compared to the experimental results. In magnetic fields up to $10\phantom{\rule{4pt}{0ex}}\mathrm{T}$ applied in a Voigt configuration ($\mathbf{B}\ensuremath{\perp}\mathbf{k}$), SHG of $S$ excitons by a dipole-dipole excitation is observed, which is due to the admixture of dipole-dipole excited $P$ excitons by the effective electric field from the magneto-Stark effect (MSE). From the analysis of the diamagnetic shift and the MSE interaction of the three-level system of $1S, 2S$, and $2P$ excitons, we derive experimental results for the ratio $\ensuremath{\langle}{r}_{n,l}^{2}\ensuremath{\rangle}/{\ensuremath{\mu}}_{X}$ between the average of radius squared for the three states and the reduced exciton mass. For higher principal quantum number states, we observe magnetoexcitons up to $n=8$. We analyze their magnetic field dependence and derive the electron effective-mass values for the crystalline orientations [111], $[1\overline{1}0]$, and [001].

High-Temperature Excitonic Bose–Einstein Condensate in Centrosymmetric Two-Dimensional Semiconductors

The realization of high-temperature excitonic Bose-Einstein condensation (BEC) in practical materials poses great challenges, because of strict constraints in symmetry, exciton binding, lifetime, and interaction. Here, using first-principles methods and symmetry analysis, we propose a new route to realize high-temperature excitonic BEC in centrosymmetric 2D materials, exploiting the parity symmetry of band edges and reduced Coulomb screening. We demonstrate it by taking monolayer TiS3 as an example, whose lowest-energy exciton shows small exciton mass, small Bohr radius, large binding, and long lifetime simultaneously. The phase diagram of electron-hole systems is further constructed, showing that both BEC and superfluidity can be realized at high temperature and in a broad range of exciton density. Importantly, we reveal that the high-temperature character of excitonic BEC is robust against thickness, beneficial for its experimental observation. By application of this general strategy to 2D materials in the database, monolayer AuBr and BiS2 are identified as promising candidates for high-temperature excitonic BEC.

Temperature dependency of excitonic effective mass and charge carrier conduction mechanism in CH3NH3PbI3\u2212xClx thin films

In this paper we explain the temperature dependence of excitonic effective mass and charge carrier conduction mechanism occurs in CH3NH3PbI3−xClx thin films prepared by chemical dip coating (CDC), spray pyrolysis (Spray) and repeated dipping-withdrawing (Dipping). Hall Effect study confirmed that prepared CH3NH3PbI3−xClx samples are p-type semiconductor having carrier concentration of the order of ~ 1016 cm−3. The charge carrier mobility, mean free path and mean free life time were found to decrease with increasing temperature due to polaronic effect. The excitonic effective mass is estimated to (0.090–0.196)me and excitonic binding energy (15–33) meV, well consistent with Wannier-Mott hydrogenic model and the nature of exciton is likely to be Mott-Wannier type. From electrical measurement, it was observed that charge carrier conduction in CH3NH3PbI3−xClx is governed by migration of {mathrm{I}}^{-} and CH3N {mathrm{H}}_{3}^{+} vacancies and vacancy-assisted diffusion processes depending on temperature.

Complexity of the dipolar exciton Mott transition in GaN/(AlGa)N nanostructures

The Mott transition from a dipolar excitonic liquid to an electron-hole plasma is demonstrated in a wide GaN/(Al,Ga)N quantum well at $T=7$K by means of spatially-resolved magneto-photoluminescence spectroscopy. Increasing optical excitation density we drive the system from the excitonic state, characterized by a diamagnetic behavior and thus a quadratic energy dependence on the magnetic field, to the unbound electron-hole state, characterized by a linear shift of the emission energy with the magnetic field. The complexity of the system requires to take into account both the density-dependence of the exciton binding energy and the exciton-exciton interaction and correlation energy that are of the same order of magnitude. We estimate the carrier density at Mott transition as $n_\mathrm{Mott}\approx 2\times 10^{11}$cm$^{-2}$ and address the role played by excitonic correlations in this process. Our results strongly rely on the spatial resolution of the photoluminescence and the assessment of the carrier transport. We show, that in contrast to GaAs/(Al,Ga)As systems, where transport of dipolar magnetoexcitons is strongly quenched by the magnetic field due to exciton mass enhancement, in GaN/(Al,Ga)N the band parameters are such that the transport is preserved up to $9$T.

Molecular Design Strategy for Simultaneously Strong Luminescence and High Mobility: Multichannel CH-π Interaction.

It is a big challenge to achieve high-performance organic semiconductor materials integrating both high luminescence efficiency and carrier mobility, because they are commonly regarded as a pair of contradiction. Here, combining a tight-binding model and density functional theory/time-dependent density functional theory, we propose a theoretical protocol to characterize the luminescence efficiency via an excitonic effective mass and charge transport ability via charge effective mass at the same level. Applying this protocol to a series of organic semiconductor materials, we find that the multichannel CH-π interaction can induce a heavy excitonic effective mass and light charge effective mass, which effectively balance the light-emitting efficiency and carrier mobility. Thus, a practical molecular design strategy is figured out to exploit novel organic semiconductor materials with strong luminescence and fast carrier transport simultaneously.

Emergent chiral symmetry in a three-dimensional interacting Dirac liquid

We compute the effects of strong Hubbardlike local electronic interactions on three-dimensional four-component massless Dirac fermions, which in a noninteracting system possess a microscopic global U(1) ⊗ SU(2) chiral symmetry. A concrete lattice realization of such chiral Dirac excitations is presented, and the role of electron-electron interactions is studied by performing a field theoretic renormalization group (RG) analysis, controlled by a small parameter ϵ with ϵ = d−1, about the lower-critical one spatial dimension. Besides the noninteracting Gaussian fixed point, the system supports four quantum critical and four bicritical points at nonvanishing interaction couplings ∼ ϵ. Even though the chiral symmetry is absent in the interacting model, it gets restored (either partially or fully) at various RG fixed points as emergent phenomena. A representative cut of the global phase diagram displays a confluence of scalar and pseudoscalar excitonic and superconducting (such as the s-wave and p-wave) mass ordered phases, manifesting restoration of (a) chiral U(1) symmetry between two excitonic masses for repulsive interactions and (b) pseudospin SU(2) symmetry between scalar or pseudoscalar excitonic and superconducting masses for attractive interactions. Finally, we perturbatively study the effects of weak rotational symmetry breaking on the stability of various RG fixed points.

Twist-angle dependence of moir\xe9 excitons in WS2/MoSe2 heterobilayers

Moiré lattices formed in twisted van der Waals bilayers provide a unique, tunable platform to realize coupled electron or exciton lattices unavailable before. While twist angle between the bilayer has been shown to be a critical parameter in engineering the moiré potential and enabling novel phenomena in electronic moiré systems, a systematic experimental study as a function of twist angle is still missing. Here we show that not only are moiré excitons robust in bilayers of even large twist angles, but also properties of the moiré excitons are dependant on, and controllable by, the moiré reciprocal lattice period via twist-angle tuning. From the twist-angle dependence, we furthermore obtain the effective mass of the interlayer excitons and the electron inter-layer tunneling strength, which are difficult to measure experimentally otherwise. These findings pave the way for understanding and engineering rich moiré-lattice induced phenomena in angle-twisted semiconductor van der Waals heterostructures.

Hole and Electron Effective Masses in Single InP Nanowires with a Wurtzite-Zincblende Homojunction.

The formation of wurtzite (WZ) phase in III-V nanowires (NWs) such as GaAs and InP is a complication hindering the growth of pure-phase NWs, but it can also be exploited to form NW homostructures consisting of alternate zincblende (ZB) and WZ segments. This leads to different forms of nanostructures, such as crystal-phase superlattices and quantum dots. Here, we investigate the electronic properties of the simplest, yet challenging, of such homostructures: InP NWs with a single homojunction between pure ZB and WZ segments. Polarization-resolved microphotoluminescence (μ-PL) measurements on single NWs provide a tool to gain insights into the interplay between NW geometry and crystal phase. We also exploit this homostructure to simultaneously measure effective masses of charge carriers and excitons in ZB and WZ InP NWs, reliably. Magneto-μ-PL measurements carried out on individual NWs up to 29 T at 77 K allow us to determine the free exciton reduced masses of the ZB and WZ crystal phases, showing the heavier character of the WZ phase, and to deduce the effective mass of electrons in ZB InP NWs (me= 0.080 m0). Finally, we obtain the reduced mass of light-hole excitons in WZ InP by probing the second optically permitted transition Γ7C ↔ Γ7uV with magneto-μ-PL measurements carried out at room temperature. This information is used to extract the experimental light-hole effective mass in WZ InP, which is found to be mlh = 0.26 m0, a value much smaller than the one of the heavy hole mass. Besides being a valuable test for band structure calculations, the knowledge of carrier masses in WZ and ZB InP is important in view of the optimization of the efficiency of solar cells, which is one of the main applications of InP NWs.