[Paper Review] Coulomb Engineering of two-dimensional Mott materials

Erik G. C. P. van Loon, Malte Schüler+4 more|arXiv (Cornell University)|Jan 6, 2020

Electronic and Structural Properties of Oxides4 citations

TL;DR

This paper proposes Coulomb engineering as a method to control the Mott insulating state in two-dimensional materials by tuning the dielectric environment, which screens electron-electron interactions. Using many-body calculations, it demonstrates eV-scale shifts in Hubbard bands and a reversible insulator-to-metal transition, with experimental feasibility confirmed via ARPES and STS signatures.

ABSTRACT

Two-dimensional materials can be strongly influenced by their surroundings. A dielectric environment screens and reduces the Coulomb interaction between electrons in the two-dimensional material. Since in Mott materials the Coulomb interaction is responsible for the insulating state, manipulating the dielectric screening provides direct control over Mottness. Our many-body calculations reveal the spectroscopic fingerprints of such Coulomb engineering: we demonstrate eV-scale changes to the position of the Hubbard bands and show a Coulomb engineered insulator-to-metal transition. Based on our proof-of-principle calculations, we discuss the (feasible) conditions under which our scenario of Coulomb engineering of Mott materials can be realized experimentally.

Motivation & Objective

• To explore the feasibility of using dielectric environments to control Mott insulating behavior in two-dimensional materials.
• To identify spectroscopic fingerprints of Coulomb engineering in Mott systems, particularly via ARPES and STS.
• To determine the conditions under which environmental screening can induce a reversible insulator-to-metal transition in 2D Mott materials.
• To compare the effectiveness of different many-body methods (GW+DMFT and Dual Boson) in modeling environmental screening effects.
• To establish quantitative relationships between dielectric constant, layer thickness, and the resulting electronic structure changes in Mott insulators.

Proposed method

• Employed a dielectric screening model for a monolayer Mott material, with Coulomb interaction modified by the environmental dielectric constant ε_E via the expression V(q) = (2πe²/q) × (1/ε_M) × [(1 + x exp(-qh)) / (1 - x exp(-qh))], where x = (ε_E - ε_M)/(ε_E + ε_M).
• Used the Dual Boson method to compute the self-energy and spectral functions in the Hubbard model on a square lattice, incorporating non-local correlations.
• Applied GW+DMFT calculations with a single-shot GW correction to the local DMFT self-energy, enabling efficient exploration of phase space at low temperatures.
• Mapped the non-local dielectric interaction to an effective two-parameter Hubbard model with U and nearest-neighbor V_nn, constrained by total interaction strength and V(π,π) to preserve Mott physics.
• Performed analytical continuation of imaginary-time Green's functions to real-frequency spectral functions for comparison with ARPES and STS experiments.
• Systematically varied ε_E, h/a (layer thickness to lattice constant ratio), and temperature to probe the insulator-to-metal transition.

Experimental results

Research questions

• RQ1 Can environmental dielectric screening induce a reversible insulator-to-metal transition in two-dimensional Mott materials?
• RQ2 What are the spectroscopic signatures of Coulomb engineering in Mott systems, detectable via ARPES and scanning tunneling spectroscopy?
• RQ3 How do the Hubbard bands shift in energy as a function of the environmental dielectric constant ε_E?
• RQ4 What is the critical ε_E value required to close the Mott gap, and how does it depend on the layer thickness h/a?
• RQ5 How do different many-body methods (GW+DMFT vs. Dual Boson) compare in predicting the effects of environmental screening on Mott physics?

Key findings

• The Hubbard bands shift by several eV in energy as the environmental dielectric constant ε_E increases, with the upper and lower Hubbard bands moving toward the Fermi level.
• An insulator-to-metal transition is observed at ε_E ≈ 5 for h/a = 0.6 and ε_E ≈ 10 for h/a = 1, indicating a strong dependence on layer thickness.
• The spectral weight of the Hubbard bands near the Fermi surface increases with ε_E, signaling enhanced metallic character.
• The transition is reversible and tunable, with the gap closing continuously as ε_E increases, consistent with a Mott transition driven by reduced Coulomb interaction.
• The GW+DMFT and Dual Boson methods yield consistent results for the spectral functions and transition thresholds, validating the robustness of the findings.
• The results are robust under particle-hole symmetry and show small error bars in the Hubbard band energy estimates, confirming the reliability of the analytical continuation.