Characterization of the critical magnetic field in the Dirac-Coulomb equation

TL;DR

This paper analyzes the critical magnetic field in the Dirac-Coulomb equation for relativistic hydrogenic atoms, revealing limitations of the Landau level ansatz and providing a new eigenvalue problem-based characterization.

Contribution

It introduces a scaling-based analytical characterization of the critical magnetic field in the Dirac-Coulomb equation, highlighting the inaccuracies of the Landau level ansatz for large Z.

Findings

Critical magnetic fields are huge in Tesla but moderate in dimensionless units.

Landau level ansatz overestimates the critical field for large Z.

Scaling property enables a new eigenvalue problem formulation.

Abstract

We consider a relativistic hydrogenic atom in a strong magnetic field. The ground state level depends on the strength of the magnetic field and reaches the lower end of the spectral gap of the Dirac-Coulomb operator for a certain critical value, the critical magnetic field. We also define a critical magnetic field in a Landau level ansatz. In both cases, when the charge Z of the nucleus is not too small, these critical magnetic fields are huge when measured in Tesla, but not so big when the equation is written in dimensionless form. When computed in the Landau level ansatz, orders of magnitude of the critical field are correct, as well as the dependence in Z. The computed value is however significantly too big for a large Z, and the wave function is not well approximated. Hence, accurate numerical computations involving the Dirac equation cannot systematically rely on the Landau level ansatz. Our approach is based on a scaling property. The critical magnetic field is characterized in terms of an equivalent eigenvalue problem. This is our main analytical result, and also the starting point of our numerical scheme.