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Angular momentum blockade in nanoscale high-T_c superconducting grains

We discuss the angular momentum blockade in small d-wave superconducting grains in an external field. We find that abrupt changes in angular momentum state of the condensate, angular momentum blockade, occur as a result of changes in the angular momentum of the condensate in an external magnetic field. The effect rep- resents a direct analogy with the Coulomb blockade. We use the Ginzburg-Landau formalism to illustrate how a magnetic field induces a deviation from the d-wave symmetry which is described by a (d + id')-order parameter. We derive the behavior of the volume magnetic susceptibility as a function of the magnetic field, and corresponding magnetization jumps at critical values of the field that should be experimentally observable in superconducting grains.