Abstract. We establish an asymptotic estimate of the lowest eigenvalue µ(bF) of the Schrödinger operator −∇ 2 bF with a magnetic field in a bounded 2-dimensional domain, where curl F vanishes non-degenerately, and b is a large parameter. Our study is based on an analysis on an eigenvalue variation problem for the Sturm-Liouville problem. Using the estimate, we determine the value of the upper critical field for superconductors subject to non-homogeneous applied magnetic fields, and localize the nucleation of superconductivity. §1. IntroductionIn this paper we study the lowest eigenvalue µ(bF) for the Schrödinger operator −∇ 2 bF with a magnetic field F, where curl F vanishes non-degenerately in a bounded 2-dimensional domain, and derive asymptotic estimates for the large parameter b. This problem arises in the mathematical theory of describing the nucleation phenomenon for superconductors subject to non-homogeneous applied magnetic fields. Using the estimates of µ(bF), we determine the value of the upper critical field and the location of nucleation of superconductivity. This problem is interesting to us also because of its connection to the problem of estimating the lowest eigenvalue and describing the (bounded) eigenfunctions 1 of the Schrödinger operator with a non-degenerately vanishing magnetic field in the entire plane R 2 and in the half-plane R 2 + . A type 2 superconductor subject to an applied magnetic field will exhibit many interesting phenomena. It is well-known that, if the applied magnetic field is homogeneous and decreases from the upper critical value H C3 , superconductivity nucleates at the surface of the sample. The estimate of the upper critical field and the localization of the nucleation of superconductivity have been studied by many physicists; see Saint-James and De Gennes Key words and phrases. Schrödinger operator with a magnetic field, eigenvalue, GinzburgLandau system, superconductivity, nucleation, upper critical field, Sturm-Liouville operator, Riccati type equation.1 In this paper, for convenience, when considering eigenvalue problems in the entire plane or in the half-plane, we call a non-trivial bounded solution an eigenfunction. Therefore, the eigenfunctions associated with the lowest eigenvalue need not lie in L 2 .