Abstract. We study the ground state of the Pauli Hamiltonian with a magnetic field in R 2d . We consider the case where a scalar potential W is present and the magnetic field B is given by B = 2i ∂∂ W . The main result is that there are no zero modes if the magnetic field decays faster than quadratically at infinity. If the magnetic field decays quadratically then zero modes may appear, and we give a lower bound for the number of them. The results in this paper partly correct a mistake in a paper from 1993.
Introduction and main resultThe Pauli operator P in R n describes a charged spin-1 2 particle in a magnetic field. Along with the Dirac operator, it lies in the base of numerous models in quantum physics. The problem about zero modes, the bound states with zero energy, is one of many questions to be asked about the spectral properties of these operators.Zero modes were discovered in [AC79] in dimension n = 2. Unlike the purely electric interaction, a compactly supported magnetic field can generate zero modes, as soon as the total flux of the field is sufficiently large. Quantitatively, this is expressed by the famous Aharonov-Casher formula. The two-dimensional case is by now quite well studied; the AC formula is extended to rather singular magnetic field, moreover, if the total flux is infinite (and the field has constant sign), there are infinitely many zero modes.On the other hand, in the three-dimensional case the presence of zero modes is a rather exceptional feature, and the conditions for them to appear are not yet found, see the discussion in [MR03] and references therein.Even less clear is the situation in the higher dimensions. In [Shi91], for even n some sufficient conditions for the infiniteness of the number of zero modes were found, requiring, in particular, that the field decays rather slowly (more slowly than r −2 ) at infinity. On the other hand, in [Ogu93], again for even n, the case where a finite number of zero modes should appear was considered. Under the assumption of a rather regular behavior of the scalar potential of the magnetic field at infinity the number of zero modes was calculated. In particular, for a field with compact support or decaying faster than quadratically at infinity the formula in [Ogu93] implies the absence of zero modes, thus making a difference with the two-dimensional situation.2000 Mathematics Subject Classification. 81Q05 (Primary); 35Q40, 81Q10 (Secondary).