Insights from Systematic DFT Calculations on Superconductors

Abstract

We present three systematic approaches to use of Density Functional Theory (DFT) for interpretation and prediction of superconductivity in new or existing materials. These approaches do not require estimates of free parameters but utilize standard input values that significantly influence computational resolution of reciprocal space Fermi surfaces and that reduce the meV-scale energy variability of calculated values. Systematic calculations on conventional superconductors show that to attain a level of resolution comparable to the energy gap, two key parameters, Δk and the cut-off energy, must be optimized for a specific compound. The optimal level of resolution is achieved with k-grids smaller than the minimum reciprocal space separation between key parallel Fermi surfaces. These approaches enable estimates of superconducting properties including the transition temperature (Tc) via (i) measurement of the equivalent thermal energy of a phonon anomaly (if present), (ii) the distribution of electrons and effect on Fermi energy (EF) when subjected to a deformation potential and (iii) use of parabolic, or higher order quartic, approximations for key electronic bands implicated in electron–phonon interactions. We demonstrate these approaches for the conventional superconductors MgB2, metal substituted MgB2 and boron-doped diamond.

An Inquiry into Two Intriguing Values of the Critical Current Density of Bi-2212

“…Parameters characterizing j c1 : It is gratifying to note that our suggestion that regardless of the physical attributes of an SC, its Fermi energy (i.e., μ at T = 0) plays an important role in determining its properties is being taken note of, as evidenced by [12].…”

DFT techniques for resolution of significant practical materials problems

“…Other parameters are chosen to guarantee that the total energy convergence is within 1.0x10 -6 eV atom -1 , maximum force at 0.01 eV Å -1 , maximum stress at 0.02 GPa and maximum atom displacement at 5x10 -4 Å. For calculations on superconductors, we require resolution of very fine meV features; hence, we use a very fine k-grid (k<0.01 Å -1 ) to guarantee high accuracy [17,21]. For battery materials, the relevant energies for comparison are in the eV range for which bands are relatively flat.…”

“…Extensive calculations on superconductors have resulted in excellent correlations with experimental results; in particular, after understanding the importance of calculation setup parameters such as k-grid resolution and extent of Brillouin zone calculations [12][13][14][15][16][17][18][19][20][21]. An update on these learnings was presented to the XVI Meeting of Physics [19].…”

“…Figure 5 shows the experimental set-up and temperature dependent spectra for carbon doped MgB2. Evaluation of these results provides new insights on the atomistic mechanisms that determine superconductivity [23] and highlights the importance of superlattice concepts to our understanding of quasi-particle behaviour [12,21,22].…”

“…In the future, computational modelling will be ubiquitous for R&D on all materials to guide syntheses and to assist with interpretation of experimental results. We present two case studies of our DFT investigations on practical materials in the fields of battery materials [3][4][5][6][7][8][9][10][11] and superconductivity [12][13][14][15][16][17][18][19][20][21][22][23], validated by comparing predicted property values with experimentally measured values. To achieve this outcome, we use an extensive combination of advanced characterization techniques including X-ray Photoemission Spectroscopy (XPS), Electron Energy Loss Spectroscopy (EELS) and Terahertz/Far Infra-Red Spectroscopy…”