[Paper Review] Excitonic Instability in Ta2Pd3Te5 Monolayer

Jingyu Yao, Haohao Sheng+8 more|arXiv (Cornell University)|Jan 2, 2024

2D Materials and ApplicationsMaterials Science65 references3 citations

TL;DR

This study predicts that monolayer Ta₂Pd₃Te₅ is an intrinsic excitonic insulator due to a large excitonic binding energy (633 meV) exceeding its small single-particle band gap (130 meV), as revealed by first-principles GW-BSE calculations. The instability arises from nearly degenerate band-edge states with identical C₂z symmetry, enabling finite 2D polarization and strong electron-hole pairing without structural distortion.

ABSTRACT

By systematic theoretical calculations, we have revealed an excitonic insulator (EI) in the Ta2Pd3Te5 monolayer. The bulk Ta2Pd3Te5 is a van der Waals (vdW) layered compound, whereas the vdW layer can be obtained through exfoliation or molecular-beam epitaxy. First-principles calculations show that the monolayer is a nearly zero-gap semiconductor with the modified Becke-Johnson functional. Due to the same symmetry of the band-edge states, the two-dimensional polarization $α_{2D}$ would be finite as the band gap goes to zero, allowing for an EI state in the compound. Using the first-principles many-body perturbation theory, the GW plus Bethe-Salpeter equation calculation reveals that the exciton binding energy is larger than the single-particle band gap, indicating the excitonic instability. The computed phonon spectrum suggests that the monolayer is dynamically stable without lattice distortion. Our findings suggest that the Ta2Pd3Te5 monolayer is an excitonic insulator without structural distortion.

Motivation & Objective

• To identify intrinsic excitonic insulator behavior in two-dimensional van der Waals materials.
• To investigate whether Ta₂Pd₃Te₅ monolayer exhibits excitonic instability due to strong electron-hole correlation.
• To determine if the excitonic state persists without structural distortion, distinguishing it from known candidates like 1T-TiSe₂ and Ta₂NiSe₅.
• To evaluate the role of symmetry-protected band-edge states in enabling large excitonic binding energy.
• To explore the material's potential for experimental realization via tunability and exfoliation.

Proposed method

• First-principles density functional theory (DFT) with the modified Becke-Johnson (MBJ) functional to compute electronic structure and band gap.
• Application of the GW-BSE method in the G₀W₀ approximation to obtain accurate single-particle band gap and excitonic binding energy.
• Calculation of 2D static dielectric polarization α₂D to assess the response to band gap closure.
• Systematic strain engineering to probe stability of E_g and E_b across compressive and tensile strains.
• Phonon spectrum analysis to rule out structural instabilities or charge density wave transitions.
• Analysis of excitonic wavefunction symmetry and phase structure using Yambo code to confirm 1s-like character at the Γ point.

Experimental results

Research questions

• RQ1 Does Ta₂Pd₃Te₅ monolayer exhibit an excitonic insulator phase with E_b > E_g?
• RQ2 What is the origin of the large excitonic binding energy in this 2D van der Waals material?
• RQ3 Is the excitonic instability robust against strain and structural distortions?
• RQ4 How do the symmetry properties of the band-edge states influence the excitonic response?
• RQ5 Can this material host a topologically protected excitonic insulator state without charge density wave order?

Key findings

• The GW-BSE calculation yields an excitonic binding energy of 633 meV, significantly exceeding the single-particle band gap of 130 meV, confirming excitonic instability.
• The band-edge states at the Γ point have identical C₂z symmetry eigenvalues, leading to dipole-forbidden transitions and finite 2D polarization as the band gap approaches zero.
• The excitonic binding energy remains nearly constant across a wide range of uniaxial strains (from -2% to +2%), indicating intrinsic stability.
• No structural instability is found in the phonon spectrum, ruling out charge density wave or Peierls distortion as the origin of the insulating state.
• The material's nearly zero-gap semiconductor nature, combined with weak screening and tunable chemical potential, creates ideal conditions for excitonic condensation.
• The predicted excitonic insulator state is robust in few-layer and bulk forms due to preserved band-edge symmetry and weak interlayer coupling.