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Regulating the electronic structures and carrier dynamics in 2D materials via elastic strain is a fascinating avenue for tailoring their optoelectronic properties at atomic scale. Here, the abnormal response of indirect excitons to non‐uniform elastic strain in MoS2 flakes is reported. The non‐uniform elastic strain in the MoS2 flakes is introduced by transferring them to pre‐prepared convex cross structures on a SiO2/Si substrate, where the topography and strain distribution are determined by atomic force microscopy and Raman spectroscopy, respectively. It is observed that the emission energy of the indirect excitons shows an unexpected blue‐shift followed by a red‐shift with increasing the local tensile strain, which is in sharp contrast to the linear red‐shift of the direct excitons. Density functional theory calculations reveal that the abnormal energy shift of the indirect excitons in the MoS2 flakes arises from the strain‐induced competition between two indirect bandgap transitions and the spatial exciton funnel effect in the non‐uniform strain field. This work provides new insights into the elastic strain effect on the indirect excitons in 2D semiconductors, which possesses potential applications in flexible optoelectronic devices. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides layered nano‐crystals are emerging as promising candidates for next‐generation optoelectronic and quantum devices. In such systems, the interaction between excitonic states and atomic vibrations is crucial for many fundamental properties, such as carrier mobilities, quantum coherence loss, and heat dissipation. In particular, to fully exploit their valley‐selective excitations, one has to understand the many‐body exciton physics of zone‐edge states. So far, theoretical and experimental studies have mainly focused on the exciton–phonon dynamics in high‐energy direct excitons involving zone‐center phonons. Here, ultrafast electron diffraction and ab initio calculations are used to investigate the many‐body structural dynamics following nearly‐ resonant excitation of low‐energy indirect excitons in MoS2. By exploiting the large momentum carried by scattered electrons, the excitation of in‐plane K‐ and Q‐ phonon modes are identified with 푬′ symmetry as key for the stabilization of indirect excitons generated via near‐infrared light at 1.55 eV, and light is shed on the role of phonon anharmonicity and the ensuing structural evolution of the MoS2 crystal lattice. The results highlight the strong selectivity of phononic excitations directly associated with the specific indirect‐ exciton nature of the wavelength‐dependent electronic transitions triggered in the system. [ABSTRACT FROM AUTHOR]

An indirect exciton is a bound state of an electron and a hole in spatially separated layers. Two-dimensional indirect excitons can be created optically in heterostructures containing double quantum wells or atomically thin semiconductors. We study theoretically the transmission of such bosonic quasiparticles through nanoconstrictions. We show that the quantum transport phenomena, for example, conductance quantization, single-slit diffraction, two-slit interference, and the Talbot effect, are experimentally realizable in systems of indirect excitons. We discuss similarities and differences between these phenomena and their counterparts in electronic devices.

The article "Abnormal Response of Indirect Excitons to Non‐Uniform Elastic Strain in MoS2 Flakes" published in Advanced Optical Materials discusses the unique behavior of indirect excitons in response to non‐uniform elastic strain in MoS2 flakes. Researchers found that indirect excitons exhibit a different response compared to direct excitons due to strain-induced competition between bandgap transitions and the spatial exciton funnel effect in the non-uniform strain field. This study sheds light on the complex interactions between excitons and strain in two-dimensional materials. [Extracted from the article]

Layered bosonic dipolar fluids have been suggested to host a condensate of interlayer molecular bound states. However, experimental observation has remained elusive. Motivated by two recent experimental works [C. Hubert et al., Phys. Rev. X9, 021026 (2019) and D. J. Choksy et al., Phys. Rev. B 103, 045126 (2021)], we theoretically study, using numerically exact quantum Monte Carlo calculations, the experimental signatures of collective interlayer pairing in vertically stacked indirect exciton (IX) layers. We find that IX energy shifts associated with each layer evolve nontrivially as a function of density imbalance following a nonmonotonic trend with a jump discontinuity at density balance, identified with the interlayer IX molecule gap. This behavior discriminates between the superfluidity of interlayer bound pairs and independent dipole condensation in distinct layers. Considering finite temperature and finite density imbalance conditions, we find a cascade of Berezinskii–Kosterlitz–Thouless (BKT) transitions, initially into a pair superfluid and only then, at lower temperatures, into complete superfluidity of both layers. Our results may provide a theoretical interpretation of existing experimental observations in GaAs double quantum well (DQW) bilayer structures. Furthermore, to optimize the visibility of pairing dynamics in future studies, we present an analysis suggesting realistic experimental settings in GaAs and transition metal dichalcogenide (TMD) bilayer DQW heterostructures where collective interlayer pairing and pair superfluidity can be clearly observed. [ABSTRACT FROM AUTHOR]

Motivated by the experiment of electrostatic conveyor belt for indirect excitons (Winbow et al 2011 Phys. Rev. Lett. 106 196806), we studied the exciton patterns for understanding the exciton dynamics. By analyzing the exciton diffusion, we found that the patterns mainly came from the photoluminescence of two kinds of excitons. The patterns near the laser spot came from the hot excitons which can be regarded as the classical particles. However, the patterns far from the laser spot come from the cooled or coherent excitons. Considering the finite lifetime of Bosonic excitons and of the interactions between them, we built a time-dependent nonlinear Schrödinger equation including the non-Hermitian dissipation to describe the coherent exciton dynamics. The real-time and imaginary-time evolutions were used alternately to solve the Schrödinger equation to simulate the exciton diffusion accompanied by the exciton cooling in the moving lattices. By calculating the escape probability, we obtained the transport distances of the coherent excitons in the conveyor, consistent with the experimental data. The cooling speed of excitons was found to be important in coherent exciton transport. Moreover, the plateau in the average transport distance cannot be explained by the dynamical localization-delocalization transition induced by the disorders., (© 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

As shown, taking into account the centrifugal energy in the Hamiltonian of an exciton with spatially separated electron and hole (the hole moves in the germanium quantum dot, and the electron is localized over the spherical interface of the silicon matrix–quantum dot) leads to the appearance of quasistationary states in the zone of surface exciton states, which transform into stationary states with increasing quantum dot radius. As determined, the absorption spectra of the nanosystem interband consist of energy bands, which are formed by electron transitions between quasi-stationary and stationary states, and the intraband absorption spectra consist of zones formed by electron transitions between stationary states. [ABSTRACT FROM AUTHOR]

The optical properties of dielectric nanostructures strongly depend on dielectric nanoparticles (NP) polarizability which can take the high values even interacting with low-intensity radiation. In this article, we are trying to find out if this is possible for electron transits to indirect exciton (IX) states at the NP surface. Because the exciton energy spectrum depends on NP shape and size, we research these factors influence on NP polarizability within the dipole approximation framework. For a quantitative assessment, we use a nanosystem containing a spherical dielectric alumina nanoparticle in a dielectric medium with a lower permittivity. We estimated the impact of NP size effect on the NP energy spectrum and the NP polarizability in case of exciton transitions to spatially separated electron and hole condition at the interface of dielectric nanoparticle and dielectric medium. Thus, using the variational method, we found that NP polarizability can achieve many orders of magnitude higher values than for atomic and molecular electron transitions case. It's take place for infrared, visible and ultraviolet radiation ranges. Also was found that size-dependence of IE energy spectrum weakly affects on NP polarizability. The obtained results can be used for creating the nano- and heterostructures for advanced nanophotonic applications that operate in conditions of weak optical fields. [ABSTRACT FROM AUTHOR]

Strongly interacting bosons display a rich variety of quantum phases, the study of which has so far been focused in the dilute regime, at a fixed number of particles. Here we demonstrate the formation of a dense Bose-Einstein condensate in a long-lived dark spin state of 2D dipolar excitons. A dark condensate of weakly interacting excitons is very fragile, being unstable against a coherent coupling of dark and bright spin states. Remarkably, we find that strong dipole-dipole interactions stabilize the dark condensate. As a result, the dark phase persists up to densities high enough for a dark quantum liquid to form. The striking experimental observation of a step-like dependence of the exciton density on the pump power is reproduced quantitatively by a model describing the nonequilibrium dynamics of driven coupled dark and bright condensates. This unique behavior marks a dynamical condensation to dark states with lifetimes as long as a millisecond, followed by a brightening transition at high densities. [ABSTRACT FROM AUTHOR]

Correlated phases in Hall systems have topological character. Multilayer configurations of planar electron systems create the opportunity to change topological phases on demand using macroscopic factors, such as vertical voltage. We present an analysis of such phenomena in close relation to recent experiments with multilayer Hall setups including GaAs and graphene multi-layers. The consequences of the blocking or not of the inter-layer electron tunneling in stacked Hall configurations are analyzed and presented in detail. Multilayer Hall systems are thus tunable topological composite nanomaterials, in the case of graphene-stacked systems by both intra- and inter-layer voltage.

We report the observation of a doublet structure in the low-temperature photoluminescence of interlayer excitons in heterostructures consisting of monolayer MoSe2 and WSe2. Both peaks exhibit long photoluminescence lifetimes of several tens of nanoseconds up to 100 ns verifying the interlayer nature of the excitons. The energy and line width of both peaks show unusual temperature and power dependences. While the low-energy peak dominates the spectra at low power and low temperatures, the high-energy peak dominates for high power and temperature. We explain the findings by two kinds of interlayer excitons being either indirect or quasi-direct in reciprocal space. Our results provide fundamental insights into long-lived interlayer states in van der Waals heterostructures with possible bosonic many-body interactions. [ABSTRACT FROM AUTHOR]

We study the time-dependent quantum scattering of a spatially indirect exciton by an external potential, taking fully into account the relative quantum dynamics of the electron-hole ( e - h ) pair. Exact calculations for an e - h wave packet show that transfer of energy between centre-of-mass (c.m.) and relative degrees of freedom may result in a genuine correction to the evolution during the scattering and eventually at asymptotic times. We show in experimentally relevant regimes and device configurations, that transmission resonances, tunnelling probabilities, diffraction patterns and wave packet fragmentation of indirect excitons are largely determined by the internal dynamics, and could not be reproduced by point-like dipole models or mean-field calculations. We show that a properly-designed local self-energy potential to be added to the c.m. Hamiltonian embeds the effects of the c.m.-internal motion correlation at a small fraction of the computation load needed for full-propagation calculations. The explicit form of this self-energy emphasises the dominant role of internal virtual transitions in determining scattering coefficients of indirect excitons. [ABSTRACT FROM AUTHOR]

Recently, the concept of the terahertz bosonic cascade laser (BCL) based on a parabolic quantum well (PQW) embedded in a microcavity was proposed. We refine this proposal by suggesting transitions between indirect exciton (IX) states as a source of terahertz emission. We explicitly propose a structure containing a narrow-square QW and a wide-parabolic QW for the realisation of a bosonic cascade. Advantages of this type of structures are in large dipole matrix elements for terahertz transitions and in long exciton radiative lifetimes which are crucial for realisation of threshold and quantum efficiency BCLs. [ABSTRACT FROM AUTHOR]

In this paper, we theoretically study the spin dynamics of a spatially trapped Bose-Einstein condensate of indirect excitons (IXs). A spontaneous parity-symmetry breaking spin bifurcation for bright excitons is reported under non-resonant linearly polarized pumping. Above a certain pumping amplitude, condensation for bright excitons spontaneously and stochastically adopts one of two spin-polarized configurations S z depending on initial occupations. This phenomenon of spontaneous spin bifurcations is attributed to the small energy splitting and difference in dissipation rates between bright and dark excitons in IX systems. The considered systems thus have potential applications in spin memories, spin switches and to simulate spin interactions in condensed lattices. [ABSTRACT FROM AUTHOR]

The increasing role of two-dimensional (2D) devices requires the development of new techniques for ultrafast control of physical properties in 2D van der Waals (vdW) nanolayers. A special feature of heterobilayers assembled from vdW monolayers is femtosecond separation of photoexcited electrons and holes between the neighboring layers, resulting in the formation of Coulomb force. Using laser pulses, we generate a 0.8 THz coherent breathing mode in MoSe 2 /WSe 2 heterobilayers, which modulates the thickness of the heterobilayer and should modulate the photogenerated electric field in the vdW gap. While the phonon frequency and decay time are independent of the stacking angle between the MoSe 2 and WSe 2 monolayers, the amplitude decreases at intermediate angles, which is explained by a decrease in the photogenerated electric field between the layers. The modulation of the vdW gap by coherent phonons enables a new technology for the generation of THz radiation in 2D nanodevices with vdW heterobilayers.

The investigation on the formation of indirect exciton states in atomic layer doped structures is presented. A GaAs-based system with n -type and p -type delta-doping layers is proposed and the separately confined electron and hole states are calculated in the effective mass approximation, with the exciton states being determined trough a diagonalization of the Hamiltonian for the electrostatically coupled electron–hole system. The binding energy of the s -type electron-heavy-hole-like exciton state is calculated as a function of the inter-doping-layer distance and the optical absorption coefficient related with inter-exciton-state transitions is reported. [ABSTRACT FROM AUTHOR]

Abstract: The peculiarities and the possibility of a control of exciton condensed pulse movement in semiconductor double quantum wells under the slot in the metal electrode are studied. The condensed phase has been considered phenomenologically with the free energy in Landau–Ginzburg form taking into account the finite value of the exciton lifetime. It was shown that the exciton condensed phase pulse moves along the slot driven by an external linear potential. If the exciton density is high enough for the formation of the condensed phase then the pulse moves maintaining a constant value of a maximum density during exciton lifetime, while the exciton gas phase pulse diffuses. The penetration of the exciton condensed phase pulse through a barrier and its stopping by the barrier have been studied. Additionally, it was shown that the exciton pulse in the condensed phase can be amplified and recovered after damping by an additional laser pulse. Solutions for the system of excitons in double quantum wells under the slot in the electrode under steady irradiation in the form of bright and dark autosolitons were found. [Copyright &y& Elsevier]

We propose a semiconductor heterostructure that allows an effective control of the shape of the carriers wavefunctions by varying just one main structural parameter. The structure is formed by a type II quantum dot and a type I quantum well. We present the results of calculations for a particular system consisting of an InP/GaAs quantum dot and an InGaAs/GaAs quantum well using a simple effective mass model that provides a good insight on our structure. We show that the wavefunction of the carrier that remains outside the dot changes from a spheroidal to a ring-like shape depending mainly on the separation between the well and the dot layers. This change has a significant impact on relevant excitonic properties such as its lifetime and electrical dipole, and it also determines the possibility of observing the optical Aharonov-Bohm effect. [ABSTRACT FROM AUTHOR]

Abstract: Formation and movement of an exciton pulse in an inhomogeneous potential are studied. It is shown that the pulse does not blur and the maximum of the exciton density in the pulse remains constant during the exciton lifetime if the pulse is formed from the condensed phase. The path, traversed by the excitons, can be increased by imposing an additional laser pulse on the system. Thereby, such a system can be used for information transmission over the exciton condensed phase. [Copyright &y& Elsevier]

Abstract: The lifetime of electrostatically trapped indirect excitons in a field-effect structure based on coupled AlGaN/GaN quantum wells has been theoretically studied. Within the plane of a double quantum well, indirect excitons are trapped between the surfaces of the AlGaN/GaN heterostructures and a semitransparent metallic top gate. The trapping mechanism has been assumed to be a combination of the quantum confined Stark effect and local field enhancement. In order to study the trapped exciton lifetime, the binding energy of indirect excitons in coupled quantum wells is calculated by finite difference method in the presence of an electric field. Thus, the lifetime of trapped excitons is computed as a function of well width, AlGaN barrier width, the position of double quantum well in the device and applied voltage. [Copyright &y& Elsevier]

Abstract: The description is presented for the dependence of the indirect exciton condensate density at the ring as a function of the polar angle at zero temperature with the involvement of the processes of formation and recombination of the excitons. In particular, starting from the quasi one-dimensional Gross–Pitaevskii equation with a spatially uniform generating term, we derive an exact analytical solution yielding the fragmentation of an exciton ring which is probably observed in the experiments. [Copyright &y& Elsevier]

Abstract: In this work we study, both experimentally and theoretically, in-plane localized ring patterns of photoluminescence (PL) from indirect excitons in GaAs/AlGaAs coupled quantum wells (QWs). These spatially pinned PL centers (PLCs) are attributed to transverse current filaments which cross the structure and inject hot electrons in the QWs. We model the experimental data with a set of drift–diffusion and Poisson equations, a thermalization equation and a quantum mass action law. The quantum statistical corrections are explicitly included in the description. [Copyright &y& Elsevier]

Abstract: The theory of what happens to a superfluid in a random field, known as the “dirty boson” problem, directly relates to a real experimental system presently under study by several groups, namely excitons in coupled semiconductor quantum wells. We consider the case of bosons in two dimensions in a random field, when the random field can be large compared to the repulsive exciton–exciton interaction energy, but is small compared to the exciton binding energy. The interaction between excitons is taken into account in the ladder approximation. The coherent potential approximation (CPA) allows us to derive the exciton Green''s function for a wide range of the random field strength, and in the weak-scattering limit CPA results in the second-order Born approximation. For quasi-two-dimensional excitonic systems, the density of the superfluid component and the Kosterlitz–Thouless temperature of the superfluid phase transition are obtained, and are found to decrease as the random field increases. [Copyright &y& Elsevier]

We report on the optical properties of CdTe/ZnTe quantum dot superlattices with different thickness of the spacer between neighboring layers of quantum dots. It is found that for a spacer width larger than 20 monolayers (ML) of ZnTe, only one emission band is observed which is ascribable to excitonic recombination in isolated CdTe islands. On the other hand, if the ZnTe spacer width is smaller than 20 ML, a second luminescence band evolves on the low energy side of the main PL peak, which we attribute to recombination of excitons in spatially correlated CdTe quantum dots. This additional emission is characterized by a small oscillator strength and by a very long (∼20 ns) decay time, as evidenced by the photoreflectance and the time resolved luminescence spectroscopy. Thus, it is interpreted as being due to a recombination of electrons and holes localized in two spatially separated but neighboring CdTe quantum dots. [Copyright &y& Elsevier]

Strain-engineering in 2D transition metal dichalcogenide (TMD) semiconductors has garnered intense research interest in tailoring the optical properties via strain-induced modifications of the electronic bands in TMDs, while its impact on the exciton dynamics remains less understood. To address this, an extensive study of transient optical absorption (TA) of both W- and Mo-based single-crystalline monolayer TMDs grown by a recently developed laser-assisted evaporation method is performed. All spectral features of the monolayers as grown on fused silica substrates exhibit appreciable redshifts relating to the existence of strain due to growth conditions. Moreover, these systems exhibit a dramatic slowing down of exciton dynamics (100s of picoseconds to few nanoseconds) with an increase in carrier densities, which strongly contrasts with the monolayers in their freestanding form as well as in comparison with more traditionally grown TMDs. The observations are related to the modifications of the electronic bands as expected from the strain and associated population of the intervalley dark excitons that can now interplay with intravalley excitations. These findings are consistent across both the Mo- and W-based TMD families, providing key information about the influence of the growth conditions on the nature of optical excitations and fostering emerging optoelectronic applications of monolayer TMDs., (© 2022 Wiley-VCH GmbH.)

Correlated phases in Hall systems have topological character. Multilayer configurations of planar electron systems create the opportunity to change topological phases on demand using macroscopic factors, such as vertical voltage. We present an analysis of such phenomena in close relation to recent experiments with multilayer Hall setups including GaAs and graphene multi-layers. The consequences of the blocking or not of the inter-layer electron tunneling in stacked Hall configurations are analyzed and presented in detail. Multilayer Hall systems are thus tunable topological composite nanomaterials, in the case of graphene-stacked systems by both intra- and inter-layer voltage. [ABSTRACT FROM AUTHOR]

Indirect excitons (IX) in semiconductor heterostructures are bosons, which can cool below the temperature of quantum degeneracy and can be effectively controlled by voltage and light. IX quantum Bose gases and IX devices were explored in GaAs heterostructures where an IX range of existence is limited to low temperatures due to low IX binding energies. IXs in van der Waals transition-metal dichalcogenide (TMD) heterostructures are characterized by large binding energies giving the opportunity for exploring excitonic quantum gases and for creating excitonic devices at high temperatures. TMD heterostructures also offer a new platform for studying single-exciton phenomena and few-particle complexes. In this work, we present studies of IXs in MoSe 2 /WSe 2 heterostructures and report on two IX luminescence lines whose energy splitting and temperature dependence identify them as neutral and charged IXs. The experimentally found binding energy of the indirect charged excitons, that is, indirect trions, is close to the calculated binding energy of 28 meV for negative indirect trions in TMD heterostructures [Deilmann, T.; Thygesen, K. S. Nano Lett. 2018, 18, 1460]. We also report on the realization of IXs with a luminescence line width reaching 4 meV at low temperatures. An enhancement of IX luminescence intensity and the narrow line width are observed in localized spots.

The possible phases and the nanoscale particle correlations of two-dimensional interacting dipolar particles is a long-sought problem in many-body physics. Here we observe a spontaneous condensation of trapped two-dimensional dipolar excitons with internal spin degrees of freedom from an interacting gas into a high density, closely packed liquid state made mostly of dark dipoles. Another phase transition, into a bright, highly repulsive plasma, is observed at even higher excitation powers. The dark liquid state is formed below a critical temperature Tc ≈ 4.8 K, and it is manifested by a clear spontaneous spatial condensation to a smaller and denser cloud, suggesting an attractive part to the interaction which goes beyond the purely repulsive dipole-dipole forces. Contributions from quantum mechanical fluctuations are expected to be significant in this strongly correlated, long living dark liquid. This is a new example of a two-dimensional atomic-like interacting dipolar liquid, but where the coupling of light to its internal spin degrees of freedom plays a crucial role in the dynamical formation and the nature of resulting condensed dark ground state.

Temperature-dependent single-particle spectroscopy is used to study interfacial energy transfer in model light-harvesting CdSe/CdS core-shell tetrapod nanocrystals. Using alternating excitation energies, we identify two thermalized exciton states in single nanoparticles that are attributed to a strain-induced interfacial barrier. At cryogenic temperatures, emission from both states exemplifies the effects of intraparticle disorder and enables their simultaneous characterization, revealing that the two states are distinct in regards to emission polarization, spectral diffusion, and blinking.