Abstract
Different theoretical methodologies are employed to investigate the effect of hydrostatic pressure and anisotropic stress and strain on the superconducting transition temperature (Tc) of MgB2. This is done both by studying Kohn anomalies in the phonon dispersions alone and by explicit calculation of the electron–phonon coupling. It is found that increasing pressure suppresses Tc in all cases, whereas isotropic and anisotropic strain enhances the superconductivity. In contrast to trialed epitaxial growth that is limited in the amount of achievable lattice strain, we propose a different path by co-deposition with ternary diborides that thermodynamically avoid mixing with MgB2. This is suggested to promote columnar growth that can introduce strain in all directions.
Highlights
Beginning with the discovery by Nagamatsu et al in 2001 of MgB2 being a high-temperature superconductor with Tc 1⁄4 39 K,1,2 a flurry of both experimental and theoretical studies on different properties of this compound was initiated
The electronic band structure reveals two σ-bands from the valence band with px and py character and one π-band from the conduction band with pz character. These make up the Fermi surface that consists of two narrow coaxial cylindrical sheets in the Γ–A out-of-plane direction, from the px,y σ-bands, and two distinct 3D tubular networks along scitation.org/journal/jap in-plane directions, arising from bonding and antibonding pz π-bands, see, e.g., the work of Kortus et al.3 for visualization
The values are in excellent agreement, with Vienna ab initio Simulation Package (VASP)-PBE96 being slightly higher at the pressures below À10 GPa
Summary
Beginning with the discovery by Nagamatsu et al in 2001 of MgB2 being a high-temperature superconductor with Tc 1⁄4 39 K,1,2 a flurry of both experimental and theoretical studies on different properties of this compound was initiated. The record-holding binary system was Nb3Ge with Tc 1⁄4 23:2 K, making the sudden jump in Tc impressive Both the electronic and phonon band structures hold crucial information for understanding superconductivity. We benchmark different approaches that predict Tc of MgB2 under hydrostatic pressure and anisotropic stress and strain. The first studied approach relies on phonon dispersions and the pressure-induced evolution of the Kohn anomaly on the optical E2g branch. Phonons derived from both finite displacements and linear response methods are compared and differences in the identified Kohn anomalies along Γ–q (q 1⁄4 K, M, H, L) between the methodologies are quantified. We suggest that Tc can be boosted by introducing tensile lattice strain along both the c-axis, and in the ab-plane, by co-deposition of MgB2 with a combination of YB2 and either ZrB2 or HfB2 as (Zr,Y)B2 and (Hf,Y)B2
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