[Paper Review] Semiconductor to Metal Transition, Dynamical Stability and Superconductivity of Strained Phosphorene

G. Q. Huang, Zhongwen Xing|arXiv (Cornell University)|Sep 25, 2014

2D Materials and Applications4 citations

TL;DR

This study uses first-principles calculations to demonstrate that vertical strain and interlayer distance tuning in bilayer phosphorene (BLP) can induce a semiconductor-to-metal transition and stabilize a dynamical phase with enhanced electron-phonon coupling. The key finding is that BLP can become a promising BCS superconductor with a predicted critical temperature (Tc) of ~10 K when the interlayer distance is reduced to 0.7d₀, driven by strong interlayer van der Waals interactions that enhance superconducting pairing.

ABSTRACT

Very recently, field-effect transistors based on few-layer phosphorene crystals with thickness down to a few nanometres have been successfully fabricated, triggering interest in this new functional two-dimensional material. In this work, we apply the first-principles calculations to studying the evolution of electronic structures and lattice dynamics with vertical strain for monolayer and bilayer phosphorenes. It is found that, by changing the thickness of phosphorene or the strain applied on it, its band gap width can be well tuned, and there will appear a transition from semiconductor to metal. In particular, the bilayer phosphorene may become a good BCS superconductor by adjusting the interlayer distance, in which the interlayer van der Waals coupling is favorable to the dynamical stability against strain.

Motivation & Objective

• To investigate the electronic and vibrational response of monolayer and bilayer phosphorene under vertical strain.
• To determine the dynamical stability limits of strained phosphorene and compare monolayer and bilayer systems.
• To explore the potential for superconductivity in bilayer phosphorene by tuning interlayer distance.
• To understand the role of van der Waals interactions in stabilizing metallic and superconducting phases in bilayer phosphorene.

Proposed method

• First-principles density functional theory (DFT) calculations using the PWSCF program in the Quantum-ESPRESSO package.
• Use of ultrasoft pseudopotentials and GGA-PBE exchange-correlation functional with a 30 Ry plane-wave cutoff.
• Inclusion of van der Waals corrections via the DFT-D2 method to accurately model interlayer interactions.
• Linear response theory applied to compute dynamical matrices and phonon dispersion relations.
• Calculation of electron-phonon coupling via the Eliashberg spectral function α²F(ω) and self-consistent EP matrix elements.
• Estimation of superconducting transition temperature Tc using the Allen-Dynes modified McMillan equation with μ* = 0.1.

Experimental results

Research questions

• RQ1 How does vertical strain affect the band gap and electronic structure of monolayer and bilayer phosphorene?
• RQ2 What is the range of dynamical stability for strained monolayer and bilayer phosphorene under vertical strain?
• RQ3 How does interlayer distance tuning influence the electron-phonon coupling and potential for superconductivity in bilayer phosphorene?
• RQ4 What is the role of interlayer van der Waals interactions in stabilizing metallic and superconducting phases in bilayer phosphorene?

Key findings

• The band gap of bilayer phosphorene closes at an interlayer distance of 0.85d₀, indicating a semiconductor-to-metal transition.
• Bilayer phosphorene becomes dynamically unstable when the interlayer distance is reduced to 0.65d₀ or less, defining a stable metallic phase in the range 0.65d₀ < d < 0.85d₀.
• The Eliashberg spectral function α²F(ω) shows a significant enhancement of low-frequency phonon modes with decreasing interlayer distance, indicating stronger electron-phonon coupling.
• The electron-phonon coupling constant λ increases dramatically to 1.45 at d = 0.7d₀, enhancing superconducting pairing.
• The estimated superconducting transition temperature Tc reaches approximately 10 K at d = 0.7d₀, indicating bilayer phosphorene can be a good BCS superconductor.
• The dynamical stability range for bilayer phosphorene under vertical strain is wider than for monolayer phosphorene due to stabilizing interlayer van der Waals interactions.