[Paper Review] Breakdown of the static dielectric screening approximation of Coulomb interactions in atomically thin semiconductors

Amine Ben Mhenni, Dinh Van Tuan+11 more|arXiv (Cornell University)|Feb 28, 2024

Electronic and Structural Properties of OxidesMaterials Science3 citations

TL;DR

This study demonstrates that the static dielectric screening approximation fails in atomically thin semiconductors, as increasing static dielectric constant leads to a blueshift of exciton resonances—up to 30 meV—due to dominant high-frequency dielectric response in exciton self-energy. The authors show that while binding energy depends on static screening, self-energy is governed by dynamic screening, enabling selective control of many-body states via engineered dielectrics.

ABSTRACT

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.

Motivation & Objective

• To investigate the validity of the static dielectric screening approximation in atomically thin transition metal dichalcogenide (TMD) monolayers.
• To determine how varying the dielectric environment affects exciton resonances and many-body interactions in 2D materials.
• To explore whether dynamic dielectric response, particularly high-frequency screening, plays a critical role in excitonic behavior beyond static approximation.
• To enable selective tuning of exciton binding energy and self-energy through materials with mismatched static and high-frequency dielectric constants.
• To open new pathways for designing dielectric-engineered quantum devices and probing exotic correlated phases in 2D heterostructures.

Proposed method

• Gate-tunable optical spectroscopy was used to track exciton resonances in monolayer WSe2 embedded in dielectrics with static dielectric constants ranging from 4 to over 1000.
• Experiments were performed across hBN, TiO2, and SrTiO3 to vary the static dielectric constant while maintaining a narrow range of high-frequency dielectric constants.
• Photoluminescence (PL) and reflection spectroscopy were employed to measure exciton energy shifts and trion binding energies under electron doping.
• A dynamical screening model was developed to separate contributions of static and high-frequency dielectric responses to exciton binding energy and self-energy.
• Charge doping was carefully controlled near neutrality to isolate dielectric effects from carrier density shifts.
• Dielectric constants were characterized via ellipsometry and theoretical modeling to ensure accurate assignment of screening parameters.

Experimental results

Research questions

• RQ1 Does increasing the static dielectric constant of the environment cause a redshift or blueshift in exciton resonance energy in monolayer TMDs?
• RQ2 To what extent does the high-frequency dielectric response influence exciton self-energy compared to static screening?
• RQ3 Can materials with large disparities between static and high-frequency dielectric constants be used to selectively tune different many-body effects in 2D semiconductors?
• RQ4 How do trion binding energies respond to changes in static dielectric screening, given their short-range nature?
• RQ5 Can dynamical screening effects be leveraged to engineer and control correlated quantum phases in van der Waals heterostructures?

Key findings

• An unexpected blueshift of up to 30 meV in the exciton resonance energy was observed in WSe2 when the static dielectric constant increased from 4 (hBN) to over 1000 (SrTiO3).
• The exciton binding energy remains primarily governed by the static dielectric constant, consistent with conventional understanding.
• In contrast, the exciton self-energy is dominated by the high-frequency dielectric response, which is not captured by the static approximation.
• The trion binding energy in both WSe2 and MoSe2 shows only weak dependence on the static dielectric constant, with changes of a few meV, indicating insensitivity to long-range screening.
• The non-monotonic behavior of trion binding energy in WSe2 across different dielectrics is attributed to residual charge doping inconsistencies rather than intrinsic screening effects.
• Materials with strong frequency-dependent dielectric functions enable selective tuning of distinct many-body effects, offering a new knob for controlling quantum states in 2D heterostructures.