[Paper Review] First-principles investigation of effect of pressure on BaFe$_2$As$_2$

Wenhui Xie, Mingli Bao+2 more|arXiv (Cornell University)|Aug 26, 2008

Iron-based superconductors research3 citations

TL;DR

This first-principles study uses density-functional theory with the full-potential linearized augmented plane wave method to investigate pressure effects on BaFe₂As₂. It finds that the striped antiferromagnetic state remains stable up to 13 GPa, with isotropic compressibility up to 6.4 GPa, and reveals a Fermi surface topological transition at ~6 GPa linked to structural changes and loss of superconductivity.

ABSTRACT

On experimental side, BaFe$_2$As$_2$ without doping has been made superconducting by applying appropriate pressure (2-6 GPa). Here, we use a full-potential linearized augmented plane wave method within the density-functional theory to investigate the effect of pressure on its crystal structure, magnetic order, and electronic structure. Our calculations show that the striped antiferromagnetic order observed in experiment is stable against pressure up to 13 GPa. Calculated antiferromagnetic lattice parameters are in good agreements with experimental data, while calculations with nonmagnetic state underestimate Fe-As bond length and c-axis lattice constant. The effects of pressure on crystal structure and electronic structure are investigated for both the antiferromagnetic state and the nonmagnetic one. We find that the compressibility of the antiferromagnetic state is quite isotropic up to about 6.4 GPa. With increasing pressure, the FeAs$_4$ tetrahedra is hardly distorted. We observe a transition of Fermi surface topology in the striped antiferromagnetic state when the compression of volume is beyond 8% (or pressure 6 GPa), which corresponds to a large change of $c/a$ ratio. These first-principles results should be useful to understanding the antiferromagnetism and electronic states in the FeAs-based materials, and may have some useful implications to the superconductivity.

Motivation & Objective

• To understand the pressure-induced evolution of crystal structure, magnetism, and electronic structure in BaFe₂As₂.
• To clarify the role of antiferromagnetic order in determining structural and electronic properties under pressure.
• To investigate the origin of superconductivity under pressure and its connection to Fermi surface topology.
• To compare predictions from antiferromagnetic and nonmagnetic states to assess the importance of spin polarization.
• To provide a theoretical basis for interpreting experimental observations of pressure-induced superconductivity and structural transitions.

Proposed method

• Employed full-potential linearized augmented plane wave (FP-LAPW) method within density-functional theory (DFT).
• Used both local density approximation (LDA) and generalized gradient approximation (GGA) for electronic structure calculations.
• Performed self-consistent calculations with convergence criteria on charge density and atomic forces.
• Relaxed internal parameter z_As until forces per atom were below 1 mRy/a.u.
• Used high k-point sampling (500 for AFM, ≥1500 for NM) for Brillouin zone integration.
• Compared results from striped antiferromagnetic and nonmagnetic states to isolate magnetic effects.

Experimental results

Research questions

• RQ1 How does pressure affect the stability of the striped antiferromagnetic order in BaFe₂As₂?
• RQ2 What is the role of spin polarization in determining lattice parameters and Fe-As bond lengths under pressure?
• RQ3 Does a topological transition in the Fermi surface occur under pressure, and if so, at what pressure and why?
• RQ4 How does the compressibility of the crystal differ between antiferromagnetic and nonmagnetic states?
• RQ5 What is the relationship between the c/a ratio collapse and the onset of superconductivity or loss of superconducting phase?

Key findings

• The striped antiferromagnetic state is stable up to 13 GPa, with calculated lattice parameters in excellent agreement with experimental data.
• Nonmagnetic calculations underestimate the c-axis lattice constant and Fe-As bond length, and yield a smaller z_As value than experiment.
• The antiferromagnetic state exhibits nearly isotropic compressibility up to 6.4 GPa, while the nonmagnetic state shows stronger c-axis compression.
• The FeAs₄ tetrahedra remain nearly undistorted under pressure in the antiferromagnetic state.
• A Fermi surface topological transition occurs at approximately 6 GPa (or 8% volume compression), linked to a significant change in the c/a ratio.
• The transition is associated with a softening of the c-axis and may explain the loss of superconductivity at higher pressures.