We study the fluctuations of an energy level as a function of the number of electrons $m$ added to a Coulomb-blockade quantum dot. A microscopic calculation in the limit of Koopmans' theorem predicts that the standard deviation of these fluctuations behaves as $\sqrt{ m}$ in the absence of surface charge but is linear in $m$ when the effect of a surface charge in a finite geometry is included. The microscopic results are compared to a parametric random-matrix approach. We estimate the number of electrons it takes to scramble the spectrum completely in terms of the interaction strength, the dimensionless Thouless conductance, and the symmetry class.
Categories: cond-mat.mes-hall cond-mat.str-el