Two charges on plane in a magnetic field I. "Quasi-equal" charges and neutral quantum system at rest cases

Low-lying bound states for the problem of two Coulomb charges of finite masses on a plane subject to a constant magnetic field $B$ perpendicular to the plane are considered. Major emphasis is given to two systems: two charges with the equal charge-to-mass ratio (quasi-equal charges) and neutral systems with concrete results for the Hydrogen atom and two electrons (quantum dot). It is shown that for these two cases, but when a neutral system is at rest (the center-of-mass momentum is zero), some outstanding properties occur: in double polar coordinates in CMS $(R, \phi)$ and relative $(\rho, \varphi)$ coordinate systems (i) the eigenfunctions are factorizable, all factors except for $\rho$-dependent are found analytically, they have definite relative angular momentum, (ii) dynamics in $\rho$-direction is the same for both systems being described by a funnel-type potential; (iii) at some discrete values of dimensionless magnetic fields $b \leq 1$ the system becomes {\it quasi-exactly-solvable} and a finite number of eigenfunctions in $\rho$ are polynomials. The variational method is employed. Trial functions are based on combining for the phase of a wavefunction (a) the WKB expansion at large distances, (b) the perturbation theory at small distances (c) with a form of the known analytically (quasi-exactly-solvable) eigenfunctions. For the lowest states with relative magnetic quantum numbers $s=0,1,2$ this approximation gives not less than 7 s.d., 8 s.d., 9 s.d., respectively, for the total energy $E(B)$ for magnetic fields $0.049\, \text{a.u.} < B < 2000\, \text{a.u.}$ (Hydrogen atom) and $0.025\, \text{a.u.}\eqslantless B \eqslantless 1000\, \text{a.u.}$ (two electrons).