arXiv subjects

Hanan Dery

Publications and source records attributed to Hanan Dery.

At least 19 recordsLinked to original sources

arXiv2026

Energy renormalizations of resident carriers and excitons in transition metal dichalcogenide monolayers

Energy renormalizations of resident carriers and excitons are studied theoretically, and compared with recent experiments of electrostatically-doped WSe$_2$ monolayers. The calculated energy renormalization of resident carriers, subjected to strong out-of-plane magnetic field, reveals the importance of dynamical screening in transition metal dichalcogenides. The energy renormalization of tightly bound excitons is analyzed through the exchange interaction between the electron (or hole) component of the exciton and resident carriers that share the same spin and valley quantum numbers. Our theory explains the weak energy shift of excitonic resonances despite the strong energy renormalization of resident carriers. We identify the dependence of the energy renormalization on the envelope function of a tightly-bound exciton, showing that unlike free electron-hole pairs, this energy renormalization is not the added renormalizations of a resident electron and resident hole.

cond-mat.mes-hall↗

arXiv2025

High luminescence efficiency of multi-valley excitonic complexes in heavily doped WSe2 monolayer

Monolayers of group-VI transition-metal dichalcogenides (TMDs) are two-dimensional semiconductors that exhibit exceptionally strong light-matter coupling yet typically suffer from low emission quantum yields. In this letter, we investigate the heavily n-doped regime of a WSe$_2$ monolayer and show that multi-particle excitonic complexes produce photoluminescence signals up to two orders of magnitude stronger than in the neutral state. Time-resolved photoluminescence and differential reflectivity measurements reveal that the quantum yield rises with carrier density and exceeds 50% for electron concentrations above 10$^{13}$ cm$^{-2}$. These findings establish TMD monolayers as a platform for exploring excitonic complexes in high-density electron gases and point toward new opportunities for efficient, atomically thin light emitters.

cond-mat.mes-hall↗

arXiv2025

Excitons in moir\'{e} superlattices with disordered electrons

Moir\'{e} superlattices in transition metal dichalcogenides (TMDs) heterobilayers exhibit various correlated insulating states driven by long-range Coulomb interactions, and these states crucially alter exciton resonances, particularly at fractional fillings. We revisit a theoretical framework to investigate the doping dependence of exciton spectra by extending hydrogenic exciton wavefunctions, systematically analyzing how the 1$s$, 2$s$, and 3$s$ Rydberg states respond to moir\'e-induced mixing of $s$- and $p$-type orbitals. Notably, while the 1$s$ state remains relatively robust against doping, higher Rydberg excitons show strong redshifts and oscillator-strength quenching near specific fractional fillings. We incorporate both defect-induced quasi-ordering and thermal fluctuations to capture realistic device conditions, employing a large supercell approach. By selectively randomizing a subset of electrons or utilizing classical Monte Carlo simulations, we present direct calculations of exciton spectra under varying defect densities and temperatures. Our results emphasize how even moderate disorder or finite temperature can partially or completely suppress characteristic moir\'{e} exciton physics. Especially, we show how the 2$s$ exciton states respond to the phase transition in correlated electron states. This comprehensive picture not only clarifies recent experimental observations but also provides a framework to guide the design of moir\'{e}-based optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Ten-valley excitonic complexes in charge-tunable monolayer WSe$_2$

Excitons dominate the optical response of two-dimensional (2D) semiconductors. Strong interactions produce peculiar excitonic complexes, which provide a testing ground for exciton and quantum many-body theories. Here, we report a hitherto unobserved many-body exciton that emerges upon filling both the K and Q valleys of WSe$_2$. We optically probe the exciton landscape using charge-tunable devices with unusually thin dielectrics that facilitate doping up to several $10^{13}$ cm$^{-2}$. We observe the emergence of the thermodynamically stable complex when 10 valleys are electrostatically filled. We gain insight into its physics using magneto-optical measurements. Our results are well-described by a model where the number of distinguishable Fermi seas interacting with the photoexcited electron-hole pair defines the complex's behavior. In addition to expanding the repertoire of excitons in 2D semiconductors, this complex could probe the limit of exciton models and answer open questions about screened Coulomb interactions in 2D semiconductors.

cond-mat.mes-hall↗

arXiv2025

Excitons in fractionally-filled moir\'{e} superlattices

Long-range Coulomb forces give rise to correlated insulating states when charge particles populate a moir\'{e} superlattice at certain fractional filling factors. Such behavior is characterized by a broken translation symmetry wherein particles spontaneously form a Wigner crystal. Focusing on the experimental findings of Xu et al. [Nature \textbf{587}, 214 (2020)], we present a theory that captures the correlated insulating state of a fractionally-filled moir\'{e} superlattice through the energy shift and change in oscillator strength of the exciton absorption resonance. The theory shows that the experimental findings can only be supported if the electrons reside in a charge-ordered state (i.e., electrons are not randomly distributed among the sites of the moir\'{e} superlattice). Furthermore, we explain why the energy shifts of exciton resonances are qualitatively different in cases that the superlattice is nearly empty compared with a superlattice whose sites are doubly occupied.

cond-mat.mtrl-sci↗

arXiv2025

Landau-level composition of bound exciton states in magnetic field

We present a theory that studies the state composition of a bound exciton in magnetic field. Using a basis set made of products of free electron and hole wavefunctions in Landau gauge, we derive a secular equation which shows the relation between Landau levels (LLs) of the electron and hole when a bound exciton is formed. Focusing on excitons in the light cone, we establish a scattering selection rule for the interaction of an electron in LL $n_\text{e}$ with a hole in LL $n_\text{h}$. We solve the resulting secular equation and identify a simple pairing law, $n_\text{e} = n_\text{h} + l$, which informs us on the construction of a bound exciton state with magnetic quantum number $l$, and on the interaction of the exciton magnetic moment with magnetic field. We obtain good agreement between theory results and recent measurements of the diamagnetic shifts of exciton states in WSe$_2$ monolayers.

cond-mat.mtrl-sci↗

arXiv2024

Energy shifts and broadening of excitonic resonances in electrostatically-doped semiconductors

Tuning the density of resident electrons or holes in semiconductors provides crucial insight into the composition of excitonic complexes that are observed as absorption or photoluminescence resonances in optical studies. Moreover, we can change the way these resonances shift and broaden in energy by controlling the quantum numbers of the resident carriers with magnetic fields and doping levels, and by selecting the quantum numbers of the photoexcited or recombining electron-hole (e-h) pair through optical polarization. We discuss the roles of distinguishability and optimality of excitonic complexes, showing them to be key ingredients that determine the energy shifts and broadening of optical resonances in charge-tunable semiconductors. A distinguishable e-h pair means that the electron and hole undergoing photoexcitation or recombination have quantum numbers that are not shared by any of the resident carriers. An optimal excitonic complex refers to a complex whose particles come with all available quantum numbers of the resident carriers. All optical resonances may be classified as either distinct or indistinct depending on the distinguishability of the e-h pair, and the underlying excitonic complex can be classified as either optimal or suboptimal. The universality of these classifications, inherited from the fundamental Pauli exclusion principle, allows us to understand how optical resonances shift in energy and whether they should broaden as doping is increased. This understanding is supported by conclusive evidence that the decay of optical resonances cannot be simply attributed to enhanced screening when resident carriers are added to a semiconductor. Finally, applying the classification scheme in either monolayer or moire heterobilayer systems, we relate the energy shift and amplitude of the neutral exciton resonance to the compressibility of the resident carrier gas.

cond-mat.mtrl-sci↗

arXiv2024

Component exchange theory of trions

Treating the trion problem as an effective two-body system with exciton and electron components, we identify component exchange as the reason leading to trion formation. This mechanism can be visualized as a hole that toggles back and forth between two electrons. The coined term component exchange is meant to inform that the repeated change of the electron component of the exciton originates from the exchange interaction. We develop a Bethe-Salpeter Equation for trions, showing that a bound trion state emerges if the electron and electron of the exciton are distinguishable particles (e.g., having opposite spins or residing in different valleys of the Brillouin zone). Similar to numerical techniques that treat the trion as a three-body problem, the trion Bethe-Salpeter Equation yields similar binding energy without using parameters beyond effective masses and dielectric constants.

cond-mat.mtrl-sci↗

arXiv2024

Effects of dynamical dielectric screening on the excitonic spectrum of monolayer semiconductors

We present a new method to solve the dynamical Bethe-Salpeter Equation numerically. The method allows one to investigate the effects of dynamical dielectric screening on the spectral position of excitons in transition-metal dichalcogenide monolayers. The dynamics accounts for the response of optical phonons in the materials below and on top the monolayer to the electric field lines between the electron and hole of the exciton. The inclusion of this effect unravels the origin of a counterintuitive energy blueshift of the exciton resonance, observed recently in monolayer semiconductors that are supported on ionic crystals with large dielectric constants. A surprising result is that while energy renormalization of a free electron in the conduction band or a free hole in the valence band is controlled by the low-frequency dielectric constant, the bandgap energy introduces a phase between the photoexcited electron and hole, rendering contributions from the high-frequency dielectric constant also important when evaluating self-energies of the exciton components. As a result, bandgap renormalization of the exciton is not the sum of independent contributions from energy shifts of the conduction and valence bands. The theory correctly predicts the energy shifts of exciton resonances in various dielectric environments that embed two-dimensional semiconductors.

cond-mat.mtrl-sci↗

arXiv2024

Breakdown of the static dielectric screening approximation of Coulomb interactions in atomically thin semiconductors

Coulomb interactions in atomically thin materials are uniquely sensitive to variations in the dielectric screening of the environment, which can be used to control quasiparticles and exotic quantum many-body phases. A static approximation of the dielectric response, where increased dielectric screening is predicted to cause an energy redshift of the exciton resonance, has been until now sufficient. Here, we use charge-tunable exciton resonances to study screening effects in transition metal dichalcogenide monolayers embedded in materials with dielectric constants ranging from 4 to more than 1000. In contrast to expectations, we observe a blueshift of the exciton resonance exceeding 30 meV for larger dielectric constant environments. By employing a dynamical screening model, we find that while the exciton binding energy remains mostly controlled by the static dielectric response, the exciton self-energy is dominated by the high-frequency response. Dielectrics with markedly different static and high-frequency screening enable the selective addressing of distinct many-body effects in layered materials and their heterostructures, expanding the tunability range and offering new routes to detect and control correlated quantum many-body states and to design optoelectronic and quantum devices.

cond-mat.mes-hall↗

arXiv2023

Excitons and trions in monolayer semiconductors with correlated electrons

We revisit low-temperature optical spectra of transition-metal dichalcogenide monolayers and point to a possible crystallization of electrons (or holes) at low to moderate charge densities. To calculate the excitonic spectra under such conditions, we introduce the recursion method and compute how the charge density affects the energies, linewidths, and oscillator strengths of exciton and trion complexes. Equally important, we study how excitons and trions in the monolayer evolve when the charge particles gradually transition to a periodic Wigner lattice. The results provide valuable information on the ability to detect whether the particles are ordered through the exciton spectrum. Finally, we calculate the change in exciton energy in cases that the added charge particles have similar and dissimilar quantum numbers (spin and valley) to those of the electron or hole in the exciton. The results of this work shed new light on important optical properties of monolayer semiconductors.

cond-mat.mes-hall↗

arXiv2023

Excitons in periodic potentials

The energy band structure of excitons is studied in periodic potentials produced by the short-range interaction between the exciton and electrons of Wigner or Moir\'{e} lattices. Treating the exciton as a point-like dipole that interacts with the periodic potential, we can solve a simple one-body problem that provides valuable information on excitons in many-body problem settings. By employing group theory, we identify the excitonic energy bands that can couple to light and then quantify their energy shifts in response to a change in the period of the potential. This approach allows us to emulate the response of optically active exciton and trion states to a change in electron density. We gain important insights on the relation between the electron order in a Wigner crystal and the energy blueshift of the bright exciton. We discuss the consequences of this relation in the context of optical absorption experiments in monolayer semiconductors.

cond-mat.mtrl-sci↗

arXiv2022

Intervalley electron-hole exchange interaction and impurity-assisted recombination of indirect excitons in WS$_2$ and WSe$_2$ monolayers

The variety of excitonic states in tungsten-based dichalcogenide monolayers stems from unique interplay between the spin and valley degrees of freedom. One of the exciton species is the indirect exciton (momentum or valley dark), which is responsible to a series of resonances when the monolayer is charge neutral. We investigate the short-range electron-hole exchange interaction of the indirect exciton, as well as its recombination mechanism mediated by impurities. The analysis provides thorough understanding of the energy and polarization of the zero-phonon indirect exciton resonance in the emission spectrum.

cond-mat.mes-hall↗

arXiv2022

Turning many-body problems to few-body ones in photoexcited semiconductors using the stochastic variational method in momentum space, SVM-k

We develop an efficient computational technique to calculate composite excitonic states in photoexcited semiconductors through the stochastic variational method (SVM). Many-body interactions between an electron gas and the excitonic state are embodied in the problem through Fermi holes in the conduction band, introduced when electrons are pulled out of the Fermi sea to bind the photoexcited electron-hole pair. We consider the direct Coulomb interaction between distinguishable particles in the complex, the exchange-induced band-gap renormalization effect, and electron-hole exchange interaction between an electron and its conduction-band hole. We provide analytical expressions for potential matrix elements, using a technique that allows us to circumvent the difficulty imposed by the occupation of low-energy electron states in the conduction band. We discuss the computational steps one should implement in order to perform the calculation, and how to extract kinetic energies of individual particles in the complex, average inter-particle distances, and density distributions.

cond-mat.mtrl-sci↗

arXiv2022

Tetrons, pexcitons, and hexcitons in monolayer transition-metal dichalcogenides

We present a comprehensive theoretical analysis of composite excitonic states in doped transition-metal dichalcogenide monolayers. Making use of the pair distribution function, we introduce a method to include the effect of screening in the interaction between the excitonic state and Fermi-sea electrons. Employing the screened potential, we study tetrons and pexcitons in which a trion is bound to one and two holes in the conduction band, respectively. A conduction-band hole denotes the lack of electrons with certain quantum numbers at the vicinity of the trion. We then analyze the hexciton complex wherein conduction-band holes facilitate binding of three electrons with one valence-band hole. We introduce a simple model from which one can readily calculate the binding energy of the third (satellite) electron in the hexciton complex. Finally, we compare the simulated results with experiment and point out the shortcomings and successes of the analysis.

cond-mat.mes-hall↗

arXiv2022

Composite excitonic states in doped semiconductors

We present a theoretical model of composite excitonic states in doped semiconductors. Many-body interactions between a photoexcited electron-hole pair and the electron gas are integrated into a computationally tractable few-body problem, solved by the variational method. We focus on electron-doped ML-MoSe$_2$ and ML-WSe$_2$ due to the contrasting character of their conduction bands. In both cases, the core of the composite is a tightly-bound trion (two electrons and valence-band hole), surrounded by a region depleted of electrons. The composite in ML-WSe$_2$ further includes a satellite electron with different quantum numbers. The theory is general and can be applied to semiconductors with various energy-band properties, allowing one to calculate their excitonic states and to quantify the interaction with the Fermi sea.

cond-mat.mtrl-sci↗

arXiv2022

Hexcitons and oxcitons in monolayer WSe$_2$

In the archetypal monolayer semiconductor WSe$_2$, the distinct ordering of spin-polarized valleys (low-energy pockets) in the conduction band allows for studies of not only simple neutral excitons and charged excitons (i.e., trions), but also more complex many-body states that are predicted at higher electron densities. We discuss magneto-optical measurements of electron-rich WSe$_2$ monolayers, and interpret the spectral lines that emerge at high electron doping as optical transitions of 6-body exciton states ("hexcitons") and 8-body exciton states ("oxcitons"). These many-body states emerge when a photoexcited electron-hole pair interacts simultaneously with multiple Fermi seas, each having distinguishable spin and valley quantum numbers. In addition, we identify the energies of primary and satellite optical transitions of hexcitons in the photoluminescence spectrum.

cond-mat.mtrl-sci↗

arXiv2021

Relaxation and darkening of excitonic complexes in electrostatically-doped monolayer semiconductors: Roles of exciton-electron and trion-electron interactions

We present photoluminescence measurements in monolayer WSe$_2$, which point to the importance of the interaction between charged particles and excitonic complexes. The theoretical analysis highlights the key role played by exchange scattering, referring to cases wherein the particle composition of the complex changes after the interaction. For example, exchange scattering renders bright excitonic complexes dark in monolayer WSe$_2$ on accounts of the unique valley-spin configuration in this material. In addition to the ultrafast energy relaxation of hot excitonic complexes following their interaction with electrons or holes, our analysis sheds light on several key features that are commonly seen in the photoluminescence of this monolayer semiconductor. In particular, we can understand why the photoluminescence intensity of the neutral bright exciton is strongest when the monolayer is hole-doped rather than charge neutral or electron-doped. Or the reason for the dramatic increase of the photoluminescence intensity of negatively-charged excitons (trions) as soon as electrons are added to the monolayer. To self-consistently explain the findings, we further study the photoluminescence spectra at different excitation energies and analyze the behavior of the elusive indirect exciton.

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