arXiv subjects

V. Shahnazaryan

Publications and source records attributed to V. Shahnazaryan.

15 recordsLinked to original sources

arXiv2025

Trion polaron problem in bulk and two-dimensional materials

We develop a microscopic theoryof the trion polaron: a bound state of two electrons and one hole, dressed by longitudinal optical (LO) phonons. Starting from the Frohlich Hamiltonian, which describes the interaction of charged particles with LO phonons in three-dimensional (bulk) and two-dimensional (monolayer) polar crystals, we adopt the intermediate coupling variational approximation of Lee, Low, and Pines, and generalize it for the three-body problem. This yields an effective three-particle Hamiltonian with renormalized electron-electron and electron-hole interactions, similar to those obtained for exciton polaron and bipolaron problems. We compute the binding energies for a family of bulk perovskite materials and several atomic monolayer materials characterized by pronounced polar effects, providing quantitative benchmarks for spectroscopic measurements.

cond-mat.mtrl-sci↗

arXiv2025

Polarons in two-dimensional polar materials: All-coupling variational theory

We present a detailed and self-contained theoretical study of polarons in two-dimensional (2D) polar materials, which extends the classical macroscopic theory of Fröhlich polarons to the 2D case. The theory is fully determined by experimentally accessible parameters, the static and optical 2D polarizabilities of a monolayer, the frequency of transverse optical phonons, and the effective mass of charge carriers. We define a single dimensionless parameter, which characterizes the coupling of electrons with longitudinal optical phonons, analyze both weak- and strong-coupling regimes, and adopt the Feynman variational path-integral approach for a high-quality interpolation between these limits. Our results provide insight into the ground-state energy and effective mass of polarons in the new generation of 2D polar monolayers.

cond-mat.mes-hall↗

arXiv2025

Polarons and Exciton-Polarons in Two-Dimensional Polar Materials

We propose a macroscopic theory of optical phonons, Fr{ö}hlich polarons, and exciton-polarons in two-dimensional (2D) polar crystalline monolayers. Our theory extends the classical macroscopic formulation of the electron-phonon problem in three-dimensional (3D) polar crystals to the new generation of 2D materials. Similarly to the 3D case, in our approach, the effective electron-phonon Hamiltonian is parametrized solely in terms of macroscopic experimentally accessible quantities -- 2D polarizabilities of the monolayer at low and high frequencies. We derive the dispersion of long wave length longitudinal optical (LO) phonons, which can be viewed as a 2D form of the Lyddane-Sachs-Teller relation, and study the formation of 2D Fr{ö}hlich polarons by adopting the intermediate coupling approximation. Finally, we apply this approach to excitons in polar 2D crystals and derive an effective potential of the electron-hole interaction dressed by LO phonons. Due to a specific dispersion of LO phonons, polarons and exciton-polarons in 2D materials exhibit unique features not found in their 3D counterparts. As an illustration, the polaron and exciton-polaron binding energies are computed for a representative set of 2D polar crystals, demonstrating the interplay between dimensionality, polarizability of materials, and electron-phonon coupling.

cond-mat.mes-hall↗

arXiv2024

Theory of magnetotrion-polaritons in transition metal dichalcogenide monolayers

Magnetic field is a powerful tool for the manipulation of material's electronic and optical properties. In the domain of transition metal dichalcogenide monolayers, it allows one to unveil the spin, valley, and orbital properties of many-body excitonic complexes. Here we study theoretically the impact of normal-to-plane magnetic field on trions and trion-polaritons. We demonstrate that spin and orbital effects of a magnetic field give comparable contributions to the trion energies. Moreover, as magnetic field redistributes the free electron gas between two valleys in the conductance band, the trion-photon coupling becomes polarization and valley dependent. This results in an effective giant Zeeman splitting of trion-polaritons, in-line with the recent experimental observations.

cond-mat.mes-hall↗

arXiv2024

Exciton Spin Hall Effect In Arc-Shaped Strained WSe$_2$

Generating a pure spin current using electrons, which have degrees of freedom beyond spin, such as electric charge and valley index, presents challenges. In response, we propose a novel mechanism based on intervalley exciton dynamics in {\em arc-shaped} strained transition metal dichalcogenides (TMDs) to achieve the {\em exciton spin Hall effect} in an electrically insulating regime, without the need for an external electric field. The interplay between strain gradients and strain-induced pseudomagnetic fields results in a net Lorentz force on long-lived intervalley excitons in WSe$_2$, carrying non-zero spin angular momentum. This process generates an exciton-mediated pure spin Hall current, resulting in opposite-sign spin accumulations and local magnetization on the two sides of the single-layer arc-shaped TMD. We demonstrate that the magnetic field induced by spin accumulation, at approximately $\sim {\rm mT}$, can be detected using techniques such as superconducting quantum interference magnetometry or spatially-resolved magneto-optical Faraday and Kerr rotations.

cond-mat.mes-hall↗

arXiv2023

Exciton localization on a magnetic domain wall in MoSe$_2$-CrI$_3$ heterostructure

The existence of spontaneous magnetization that fingerprints a ground-state ferromagnetic order was recently observed in two-dimensional (2D) van der Waals materials. Despite progress in the fabrication and manipulation of the atom-thick magnets, investigation of nanoscale magnetization properties is still challenging due to the concomitant technical issues. We propose a promising approach for a direct visualization of the domain walls formed in 2D magnetic materials. By interfacing 2D magnet with a transition metal dichalcogenide (TMD) monolayer, the strong proximity effects enable pinning the TMD excitons on the domain wall. The emergent localization stems from the proximity-induced exchange mixing between spin-dark and spin-bright TMD excitons due to the local in-plane magnetization characteristic of the domain wall in the magnetic monolayer.

cond-mat.mes-hall↗

arXiv2021

Theory of nonlinear excitonic response of hybrid organic perovskites in the regime of strong light-matter coupling

We present a quantitative study of the nonlinear optical response of layered perovskites placed inside planar photonic microcavities in the regime of strong light matter coupling, when excitonic and photonic modes hybridize and give rise to cavity polaritons. Two sources of nonlinearity are specified, the saturation of the excitonic transition with increase of the optical pump and Coulomb interaction between the excitons. It is demonstrated, that peculiar form of the interaction potential, specific to multilayer structure of organic perovskites, is responsible for substantial increase of the exciton binding energy and Rabi splitting with respect to conventional semiconductor systems. This results in dominant contribution of the Rabi splitting quench effect in the nonlinear optical response. Moreover, due to the tightly bound character of excitons, the density of Mott transition is essentially higher, allowing to reach extremely large polariton blueshifts of about 50 meV, which is order of magnitude higher than in conventional semiconductors.

cond-mat.mes-hall↗

arXiv2021

Tunable strongly interacting dipolar excitons in hybrid perovskites

We study theoretically the excitonic nonlinearity in hybrid organic-inorganic Ruddlesden-Popper perovskite thin films. The composite layered structure of these materials allows for flexible modulation of their excitonic response between the limiting cases of single atomic layer and wide quasi-two-dimensional quantum well. In particular, we demonstrate that transverse electric field leads to the spatial separation of charge carriers within the inorganic layer, giving rise to strongly interacting excitons possessing built-in dipole moment. Combined with exciton binding energy of the order of hundreds of meVs, this makes hybrid perovskites an optimal platform for tailoring of nonlinear optical response at reduced dimensionality.

cond-mat.mes-hall↗

arXiv2020

Tunable optical nonlinearity for TMD polaritons dressed by a Fermi sea

We study a system of a transition metal dichalcogenide (TMD) monolayer placed in an optical resonator, where strong light-matter coupling between excitons and photons is achieved. We present quantitative theory of the nonlinear optical response for exciton-polaritons for the case of doped TMD monolayer, and analyze in detail two sources of nonlinearity. The first nonlinear response contribution stems from the Coulomb exchange interaction between excitons. The second contribution comes from the reduction of Rabi splitting that originates from phase space filling at increased exciton concentration and the composite nature of excitons. We demonstrate that both nonlinear contributions are enhanced in the presence of free electrons. As free electron concentration can be routinely controlled by an externally applied gate voltage, this opens a way of electrical tuning of the nonlinear optical response.

cond-mat.mes-hall↗

arXiv2019

Highly nonlinear trion-polaritons in a monolayer semiconductor

Highly nonlinear optical materials with strong effective photon-photon interactions (Kerr-like nonlinearity) are required in the development of novel quantum sources of light as well as for ultrafast and quantum optical signal processing circuitry. Here we report very large Kerr-like nonlinearities by employing strong optical transitions of charged excitons (trions) observed in semiconducting transition metal dichalcogenides (TMDCs). By hybridising trions in monolayer MoSe$_2$ at low electron densities with a microcavity mode, we realise trion-polaritons exhibiting significant energy shifts at very small photon fluxes due to phase space filling. Most notably, the strong trion-polariton nonlinearity is found to be 10 to 1000 larger than in other polariton systems, including neutral exciton-polaritons in TMDCs. Furthermore it exceeds by factors of $\sim 10^3-10^5$ the magnitude of Kerr nonlinearity in bare TMDCs, graphene and other widely used optical materials (e.g. Si, AlGaAs etc) in weak light-matter coupling regimes. The results are in good agreement with a theory which accounts for the composite nature of excitons and trions and deviation of their statistics from that of ideal bosons and fermions. This work opens a new highly nonlinear system for quantum optics applications enabling in principle scalability and control through nano-engineering of van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2019

Exciton routing in the heterostructure of TMD and paraelectrics

We propose a scheme for the spatial exciton energy control and exciton routing in a transition metal dichalcogenide (TMD) monolayer which lies on a quantum paraelectric substrate. It relies on the ultrasensitive response of the substrate dielectric permittivity to temperature changes, allowing for spatially inhomogeneous screening of Coulomb interaction in a monolayer. As an example, we consider the heterostructure of TMD and strontium titanate oxide SrTiO$_3$, where large dielectric screening can be attained. We study the impact of substrate temperature on the characteristic electronic features of TMD monolayers such as the particle bandgap and exciton binding energy, Bohr radius and nonlinearity (an exciton-exciton interaction). The combination of particle bandgap and exciton binding energy modulation results in the shift of the exciton resonance energy. Applying local heating, we create spatial patterns with varying exciton resonant energy and an exciton flow towards the energetically lower region of the sample.

cond-mat.mes-hall↗

arXiv2017

Exciton-exciton interaction in transition-metal dichalcogenide monolayers

We study theoretically the Coulomb interaction between excitons in transition metal dichalcogenide (TMD) monolayers. We calculate direct and exchange interaction for both ground and excited states of excitons. The screening of the Coulomb interaction, specific to monolayer structures, leads to the unique behavior of the exciton-exciton scattering for excited states, characterized by the non-monotonic dependence of the interaction as function of the transferred momentum. We find that the nontrivial screening enables the description of TMD exciton interaction strength by approximate formula which includes exciton binding parameters. The influence of screening and dielectric environment on the exciton-exciton interaction was studied, showing qualitatively different behavior for ground state and excited states of excitons. Furthermore, we consider exciton-electron interaction, which for the excited states is governed by the dominant attractive contribution of the exchange component, which increases with the excitation number. The results provide a quantitative description of the exciton-exciton and exciton-electron scattering in transition metal dichalcogenides, and are of interest for the design of perspective nonlinear optical devices based on TMD monolayers.

cond-mat.mes-hall↗

arXiv2017

Resonance fluorescence from an asymmetric quantum dot dressed by a bichromatic electromagnetic field

We present the theory of resonance fluorescence from an asymmetric quantum dot driven by a two-component electromagnetic field with two different frequencies, polarizations and amplitudes (bichromatic field) in the regime of strong light-matter coupling. It follows from the elaborated theory that the broken inversion symmetry of the driven quantum system and the bichromatic structure of the driving field result in unexpected features of the resonance fluorescence, including the infinite set of Mollow triplets, the quench of fluorescence peaks induced by the dressing field, and the oscillating behavior of the fluorescence intensity as a function of the dressing field amplitude. These quantum phenomena are of general physical nature and, therefore, can take place in various double-driven quantum systems with broken inversion symmetry.

cond-mat.mes-hall↗

arXiv2016

Attractive Coulomb interaction of 2D Rydberg excitons

We analyze theoretically the Coulomb scattering processes of highly excited excitons in the direct bandgap semiconductor quantum wells. We find that contrary to the interaction of ground state excitons the electron and hole exchange interaction between excited excitons has an attractive character both for $s$- and $p$-type 2D excitons. Moreover, we show that similarly to the three-dimensional (3D) highly excited excitons, the direct interaction of 2D Rydberg excitons exhibits van der Waals type long-range interaction. The results predict the linear growth of the absolute value of exchange interaction strength with an exciton principal quantum number, and point the way towards enhancement of optical nonlinearity in 2D excitonic systems.

cond-mat.mes-hall↗

arXiv2015

Spin currents of exciton polaritons in a microcavity with (110)-oriented quantum well

We study the polarization optical properties of microcavities with embedded (110)-oriented quantum wells. The spin dynamics of exciton polaritons in such structures is governed by the interplay of the spin-orbit splitting of exciton states, which is odd in the in-plane momentum, and the longitudinal-transverse splitting of cavity modes, which is even in the momentum. We demonstrate the generation of polariton spin currents by linearly polarized optical pump and analyze the arising polariton spin textures in the cavity plane. Tuning the excitation spot size, which controls the polariton distribution in the momentum space, one obtains symmetric or asymmetric spin textures.

cond-mat.mes-hall↗