arXiv subjects

Meera M. Parish

Publications and source records attributed to Meera M. Parish.

At least 19 recordsLinked to original sources

arXiv2026

Multi-dimensional spectroscopy of mobile excitons in two-dimensional semiconductors

Multi-dimensional coherent spectroscopy (MDCS) goes beyond standard linear-response probes and provides a powerful tool for investigating correlations between quasiparticles such as excitons (bound electron-hole pairs). Here we present a microscopic theory of MDCS that accounts for the delocalized nature of excitons in two-dimensional semiconductors. In contrast to the more phenomenological few-level approaches typically employed for modelling MDCS, our theory features mobile excitons with continuous momentum degrees of freedom. We find that the energy continuum associated with exciton momenta is crucial for producing interaction-induced decoherence, as well as capturing the interference between different exciton-polaron quasiparticles in the case of charge-doped semiconductors. Crucially, our calculated MDCS spectra agree well with recent experiments on doped monolayer MoSe$_2$ [Hao et al., Nature Communications 8, 15552 (2017)], and they suggest that the interactions between exciton polarons depend strongly on phase-space filling effects, where exciton polarons compete for electrons. Our results demonstrate that microscopic approaches allow one to gain new insights from the fine structure of MDCS on two-dimensional semiconductors, and they illustrate the utility of microscopic approaches to modelling MDCS experiments more generally.

cond-mat.mes-hall↗

arXiv2026

Coupled-channel approach to scattering of hybrid excitons

We consider the interactions of hybrid excitons in a two-dimensional semiconductor bilayer, where spatially direct and indirect excitons are hybridized by interlayer charge-carrier tunneling. Starting from a microscopic electron-hole description, we construct realistic pseudopotentials for exciton-exciton interactions and use them as inputs to a coupled-channel scattering integral equation. This enables non-perturbative calculations of hybrid-exciton scattering beyond standard perturbative theories, and highlights the importance of energy-dependent scattering and channel mixing. In particular, we show that the interaction strength of hybrid excitons exhibits a rapid growth with increasing energy, which we find is inherited from their indirect-exciton component. We further demonstrate that dielectric screening affects the direct and indirect channels in distinct ways, leading to markedly different interaction strengths across experimentally relevant dielectric environments. Finally, we show that the hybrid-exciton scattering strength can be electrically tuned via the Stark shift, which controls the direct-indirect detuning and hence the hybridization of the two exciton modes.

cond-mat.mes-hall↗

arXiv2026

Very strong light-matter coupling in patterned GaAs heterostructures

The very strong light-matter coupling regime enables the non-perturbative modification of matter properties via light. Using a patterned GaAs/AlGaAs waveguide with twelve wide quantum wells, we demonstrate hybridization of heavy- and light-hole excitons within a single polariton state and show that, at finite magnetic field, the presence of the light-hole exciton suppresses coupling to the heavy-hole Rydberg excitons and unbound scattering states. We develop a fully microscopic theory that accounts for the combined effects of the magnetic field and light-matter coupling on the excitons, providing an accurate description of the experimental results beyond a perturbative coupled-oscillator framework. This identifies quantum well width as a key control parameter for engineering the light-induced hybridization of matter wave functions in polaritons, which, in turn, can play a crucial role in the optical non-linearities.

cond-mat.mes-hall↗

arXiv2026

Magnetic-field driven hybridization of heavy- and light-hole Rydberg excitons in GaAs quantum wells

We present a combined theoretical and experimental study of ground and excited Rydberg exciton states in wide GaAs quantum wells exposed to a magnetic field in the Faraday geometry. We employ a multiband exciton model based on the Luttinger Hamiltonian, which captures valence-band mixing between heavy- and light-hole states induced by both the quantum well confinement and the magnetic field, and we develop an efficient numerical approach to solve for both ground- and excited-state excitons. The method treats Coulomb interactions, magnetic confinement, and band mixing on an equal footing, enabling a systematic characterization of exciton energies, oscillator strengths, and orbital composition. We show that band hybridization increases with magnetic field and is significantly more pronounced for higher excited states, where it sets in at lower fields and strongly modifies their properties. The theoretical predictions are validated by polarization-resolved magneto-reflectance measurements up to 9 T on GaAs/Al$_{0.4}$Ga$_{0.6}$As quantum wells of 20 nm width. We find excellent agreement for both the diamagnetic shift and Zeeman splitting of the ground state and the first four Rydberg excitons. Our results demonstrate that valence-band mixing plays a crucial role in determining the magnetic-field dependence of excited exciton states and must be properly included for a quantitative description of magneto-excitons in wide GaAs quantum wells.

cond-mat.mes-hall↗

arXiv2026

Optical properties of Fermi polarons in a GaInP/MoSe2 monolayer heterostructure

Engineering optical properties, such as luminescence purity and charge transfer, is crucial for harnessing the application potential of atomically thin transition metal dichalcogenides (TMDCs). While electrostatic gating is widely applied to gain charge control in TMDC monolayers, charge transfer can also be engineered via coupling of TMDC monolayers at semiconductor III/V, organic, or van der Waals interfaces. This confers great advantages, such as ease in implementation and compatibility in device integration. Here, we shed light on the optical properties of many-particle complexes emerging at the GaInP/MoSe2 interface as a highly relevant material combination to manipulate the optical properties of TMDCs in integrated photonic devices. Our study verifies its nature as a type II hetero-interface, which bears the feasibility to display disorder-free photoluminescence. Through optical absorption measurements, we verify that the charged complexes acquire substantial oscillator strength. Furthermore, temperature-dependent photoluminescence, supported by a microscopic theory framework, evidences the suppression of the characteristic carrier recoil effect that was previously observed in the photoluminescence of trions in TMDCs. These phenomena allow us to identify the optical signatures at the TMDC-GaInP interface as Fermi polaron quasiparticle resonances, which are of high importance in researching Bose-Fermi mixtures in condensed matter systems.

cond-mat.mtrl-sci↗

arXiv2026

Quantum droplets in a resonant Bose-Fermi mixture

We study the canonical problem of a Fermi gas interacting with a weakly repulsive Bose-Einstein condensate at zero temperature. To explore the quantum phases across the full range of boson-fermion interactions, we construct a versatile variational ansatz that incorporates pair correlations and correctly captures the different polaron limits. Remarkably, we find that self-bound quantum droplets can exist in the strongly interacting regime, preempting the formation of boson-fermion dimers, when the Fermi pressure is balanced by the resonant boson-fermion attraction. This scenario can be achieved in experimentally available Bose-Fermi mixtures for a range of boson-fermion mass ratios in the vicinity of equal masses. We furthermore show that a larger fermion density instead yields phase separation between a Bose-Fermi mixture and excess fermions, as well as behavior reminiscent of a liquid-gas critical point. Our results suggest that first-order quantum phase transitions play a crucial role in the phase diagram of Bose-Fermi mixtures.

cond-mat.quant-gas↗

arXiv2025

Role of impurity statistics and medium constraints in polaron-polaron interactions

We consider the behavior of a small density of mobile impurities (polarons) immersed in a quantum gas, a generic scenario that can be realized in cold atomic gases, liquid helium mixtures, and doped semiconductors. We present a unified theoretical framework for understanding polaron quasiparticles beyond the single-impurity limit, and we identify two key factors that control the polaron-polaron interactions: (i) the statistics of the impurities, including whether or not they are degenerate, and (ii) the constraints on the medium response, i.e., whether the medium density or chemical potential is held fixed. By constructing wave functions for two bosonic, fermionic, or distinguishable impurities immersed in a Bose or Fermi gas, we derive rigorous results for the polaron interactions in the limit of weak impurity-medium coupling. We furthermore obtain an exact relationship between the polaron interactions at fixed medium density and at fixed chemical potential, a result which is valid for arbitrary interaction strength. Our work provides an important guide for understanding experiments, and it acts as a starting point for future strong-coupling theories of polaron interactions that capture all of the effects identified in this work.

cond-mat.quant-gas↗

arXiv2025

Realization of repulsive polarons in the strongly correlated regime

Mobile impurities interacting with a quantum medium form quasiparticles known as polarons, a central concept in many-body physics. While the quantum impurity problem has been extensively studied with ultracold atomic gases, repulsive polarons in the strongly correlated regime have remained elusive. Typically, the impurity atoms bind into molecules or rapidly decay into deeper lying states before they can acquire an appreciable dressing cloud. Here, we report on the realization of polarons in a strongly repulsive quasi-two-dimensional quantum gas. Using a superfluid of $^6$Li dimers, we introduce impurities by promoting a small fraction of the dimers into higher levels of the transverse confining potential. These novel synthetic-spin polarons give access to the strongly repulsive regime where common decay channels are suppressed. We extract key polaron properties - the energy, quasiparticle residue, and effective mass - using trap modulation and Bragg spectroscopy. Our measurements are well captured by a microscopic T-matrix approach and quantum Monte Carlo simulations, revealing deviations from mean-field predictions. In particular, we measure a significant enhancement of the polaron mass, with values exceeding twice the free dimer mass. Our demonstration of a stable repulsive Bose polaron establishes a platform for studying impurity physics in low-dimensional and strongly correlated systems.

cond-mat.quant-gas↗

arXiv2025

Scattering resonances and pairing in a Rabi-coupled Fermi gas

We investigate the possibility of using a Rabi drive to tune the interactions in an atomic Fermi gas. Specifically, we consider the scenario where two fermion species (spins) are Rabi coupled and interacting with a third uncoupled species. Using an exact calculation within a minimal low-energy model, we derive analytical expressions for the effective scattering length and effective range that characterize the collisions between a Rabi-dressed atom and an atom from the third species. In particular, we find that new scattering resonances emerge in the Rabi-coupled system, which we demonstrate are linked to the existence of hybrid two-body bound states. Furthermore, we show via a generalized Thouless criterion that the scattering properties have a direct impact on the superfluid transitions in the Rabi-coupled Fermi gas. The presence of Rabi-induced resonances thus has implications for the investigation of many-body physics with driven atomic gases.

cond-mat.quant-gas↗

arXiv2025

Hybrid-pair superfluidity in a strongly driven Fermi gas

We explore the paired superfluid phases of a Fermi gas in the presence of a continuous Rabi drive. We focus on the case where two components are strongly coupled by the drive, forming hybrid superpositions, and interacting with an uncoupled third component. Using a generalized Bardeen- Cooper-Schrieffer (BCS) ansatz, we show that there are two coupled superfluid order parameters, and we obtain the associated free energy and quasiparticle excitation spectrum. We find that we can drive BCS-BCS, BCS-Bose-Einstein condensate (BEC) and BEC-BEC crossovers purely by varying the detuning of the Rabi drive from the bare transition, with the precise crossover depending on the sign of the underlying interactions between the coupled and uncoupled components. We furthermore identify an exotic excited branch which features both normal to BCS superfluid transitions, as well as a BCS-BEC-BCS crossover. Introducing a generalized Thouless criterion, we show that this behavior is reflected in the critical temperature for superfluidity. Our Rabi-coupled scenario also gives possesses additional thermodynamic properties related to the pseudospin of the coupled components, which provide novel signatures of the state of the many-body system. The Rabi-driven Fermi gas thus emerges as a unique platform for engineering and probing a rich array of multi-band superfluid phases.

cond-mat.quant-gas↗

arXiv2025

Medium-enhanced polaron repulsion in a dilute Bose mixture

We investigate the fundamental problem of a small density of bosonic impurities immersed in a dilute Bose gas at zero temperature. Using a rigorous perturbative expansion, we show that the presence of the surrounding medium enhances the repulsion between dressed bosonic impurities (polarons) in the regime of weak interactions. Crucially, this differs from prevailing theories based on Landau quasiparticles, which neglect the possibility of quantum degenerate impurities and predict an exchange-induced attraction. We furthermore show that the polaron-polaron interactions are strongly modified if the medium chemical potential rather than the density is held fixed, such that the medium-induced attraction between thermal impurities becomes twice the expected Landau effective interaction. Our work provides a possible explanation for the differing signs of the polaron-polaron interactions observed in experiments across cold atomic gases and two-dimensional semiconductors, and it has important implications for theories of quasiparticles and quantum mixtures in general.

cond-mat.quant-gas↗

arXiv2025

Resonantly enhanced polariton-mediated superconductivity in a doped transition metal dichalcogenide monolayer

We present a proposal for achieving light-induced superconductivity using exciton polaritons - hybrid light-matter particles of excitons (bound electron-hole pairs) and microcavity photons. In contrast to previous theories of polariton-mediated superconductivity, which typically require multiple semiconductor layers, we show that superconductivity can be induced within a single semiconductor monolayer with inverted conduction bands, such as in the tungsten-based transition metal dichalcogenides. The key ingredient is that we can resonantly excite exciton polaritons into bands that are different from those occupied by the doped electrons, thus avoiding any Pauli blocking effects. Crucially, we can exploit the trion fine structure (i.e., multiple exciton-electron bound states) and tune the electron-polariton interactions via Feshbach resonances. Our theory of polariton-mediated superconductivity includes the energy dependence of the polariton-mediated interactions between electrons, as well as the polariton-induced changes to the electron quasiparticles. We find that superconductivity at elevated temperatures is within reach of current experiments.

cond-mat.mes-hall↗

arXiv2025

Universal Efimov Scaling in the Rabi-Coupled Few-Body Spectrum

We investigate the behavior of the Efimov effect -- a universal quantum few-body phenomenon -- in the presence of an external driving field. Specifically, we consider up to three bosonic atoms, such as $^{133}$Cs, interacting with a light atom, such as $^{6}$Li, where the latter has two internal spin states $\{\uparrow, \downarrow\}$ that are Rabi coupled. Assuming that only the spin-$\uparrow$ light atom interacts with the bosons, we find that the Rabi drive transposes the entire Efimov spectrum such that the Efimov trimers and tetramers are centered around the Rabi-shifted two-body scattering resonance. Crucially, we show that the Rabi drive preserves the trimers' discrete scaling symmetry, while universally shifting the Efimov three-body parameter, leading to a log-periodic modulation in the spectrum as the Rabi drive is varied. Our results suggest that Efimov physics can be conveniently explored using an applied driving field, opening up the prospect of an externally tunable three-body parameter.

cond-mat.quant-gas↗

arXiv2025

Quantum Droplets of Light in Semiconductor Microcavities

Quantum droplets are dilute self-bound configurations of bosons that result from the balance between a mean-field attraction and a repulsion induced by quantum fluctuations. Such droplets have been successfully realized in cold atomic gases and represent a signature of their quantum nature. Here, we predict the existence of a similar droplet phase in a solid-state system, involving polaritons formed from the strong coupling between excitons (bound electron-hole pairs) and photons in a semiconductor microcavity. We consider a spin mixture of exciton-polaritons near a biexciton Feshbach resonance, which allows one to tune the interspecies interactions to be attractive and comparable in magnitude to the intraspecies repulsion. We find that self-bound quantum droplets are achievable for realistic parameters in atomically thin semiconductors, and that they can be detected via their excitation spectrum and spatial profile. This exotic phase could potentially lead to polariton condensation at lower thresholds and it opens an alternative avenue to achieve the long-sought quantum polaritonic regime.

cond-mat.mes-hall↗

arXiv2025

Valley-mediated singlet- and triplet-polaron interactions and quantum dynamics in a doped WSe$_2$ monolayer

In doped transition metal dichalcogenides, optically created excitons (bound electron-hole pairs) can strongly interact with a Fermi sea of electrons to form Fermi polaron quasiparticles. When there are two distinct Fermi seas, as is the case in WSe$_2$, there are two flavors of lowest-energy (attractive) polarons -- singlet and triplet -- where the exciton is coupled to the Fermi sea in the same or opposite valley, respectively. Using two-dimensional coherent electronic spectroscopy, we analyze how their quantum decoherence evolves with doping density and determine the condition under which stable Fermi polarons form. Because of the large oscillator strength associated with these resonances, intrinsic quantum dynamics of polarons as well as valley coherence between coupled singlet- and triplet polarons occur on sub-picosecond time scales. Surprisingly, we find that a dark-to-bright state conversion process leads to a particularly long-lived singlet polaron valley polarization, persisting up to 200-800 ps. Valley coherence between the singlet- and triplet polaron is correlated with their energy fluctuations. Our finding provides valuable guidance for the electrical and optical control of spin and valley indexes in atomically thin semiconductors.

cond-mat.mes-hall↗

arXiv2024

Light-enhanced dipolar interactions between exciton polaritons

We consider the scenario of excitons in a semiconductor bilayer that are strongly coupled to cavity photons, leading to the formation of dipolar exciton polaritons (dipolaritons). Using a realistic pseudopotential for the dipolar interactions, we exactly determine the scattering between dipolaritons, accounting for the hybridization between interlayer and intralayer excitons. Similar to conventional non-dipolar polaritons, we find that the light-matter coupling enhances the interactions between dipolaritons by forcing excitons to scatter at energies that would otherwise be forbidden in ordinary exciton-exciton collisions. However, we show that this light enhancement is larger for long-range dipolar interactions than for short-range intralayer interactions, and is sensitive to the (non-uniform) dielectric environment of the bilayer. Crucially, we find that the largest dipolariton interactions are achieved for transition metal dichalcogenide bilayers in vacuum. Our results thus reveal the optimal dipolariton setup for realizing strong photon correlations.

cond-mat.mes-hall↗

arXiv2024

Rydberg Exciton-Polaritons in a Magnetic Field

We theoretically investigate exciton-polaritons in a two-dimensional (2D) semiconductor heterostructure, where a static magnetic field is applied perpendicular to the plane. To explore the interplay between magnetic field and a strong light-matter coupling, we employ a fully microscopic theory that explicitly incorporates electrons, holes and photons in a semiconductor microcavity. Furthermore, we exploit a mapping between the 2D harmonic oscillator and the 2D hydrogen atom that allows us to efficiently solve the problem numerically for the entire Rydberg series as well as for the ground-state exciton. In contrast to previous approaches, we can readily obtain the real-space exciton wave functions and we show how they shrink in size with increasing magnetic field, which mirrors their increasing interaction energy and oscillator strength. We compare our theory with recent experiments on exciton-polaritons in GaAs heterostructures in an external magnetic field and we find excellent agreement with the measured polariton energies. Crucially, we are able to capture the observed light-induced changes to the exciton in the regime of very strong light-matter coupling where a perturbative coupled oscillator description breaks down. Our work can guide future experimental efforts to engineer and control Rydberg excitons and exciton-polaritons in a range of 2D materials.

cond-mat.mes-hall↗

arXiv2024

Efficient calculation of trion energies in monolayer transition metal dichalcogenides

The reduced dielectric screening in atomically thin semiconductors leads to remarkably strong electron interactions. As a result, bound electron-hole pairs (excitons) and charged excitons (trions), which have binding energies in the hundreds and tens of meV, respectively, typically dominate the optical properties of these materials. However, the long-range nature of the interactions between charges represents a significant challenge to the exact calculation of binding energies of complexes larger than the exciton. Here, we demonstrate that the trion binding energy can be efficiently calculated directly from the three-body Schr\"odinger equation in momentum space. Key to this result is a highly accurate way of treating the pole of the electronic interactions at small momentum exchange (i.e., large separation between charges) via the Land\'e subtraction method. Our results are in excellent agreement with quantum Monte Carlo calculations, while yielding a substantially larger ratio of the trion to exciton binding energies than obtained in recent variational calculations. Our numerical approach may be extended to a host of different few-body problems in 2D semiconductors, and even potentially to the description of exciton polarons.

cond-mat.mes-hall↗