Statistical properties of electrochemical capacitance in disordered mesoscopic capacitors

We numerically investigate the statistical properties of electrochemical capacitance in disordered two-dimensional mesoscopic capacitors. Based on the tight-binding Hamiltonian, the Green's function formalism is adopted to study the average electrochemical capacitance, its fluctuation as well as the distribution of capacitance of the disordered mesoscopic capacitors for three different ensembles: orthogonal (symmetry index beta = 1), unitary (beta = 2), and symplectic (beta = 4). It is found that the electrochemical capacitance in the disordered systems exhibits universal behavior. In the case of a single conducting channel, the electrochemical capacitance follows a symmetric Gaussian distribution at weak disorders as expected from the random matrix theory. In the strongly disordered regime, the distribution is found to be a sharply one-sided form with a nearly constant tail in the large capacitance region. This behavior is due to the existence of the necklace states in disordered systems, which is characterized by the multiresonance that gives rise to a large density of states. In addition, it is found that the necklace state also enhances the fluctuation of electrochemical capacitance in the case of a single conducting channel. When the number of conducting channels increases, the influence of necklace states becomes less important. For a large number of conducting channels, the electrochemical capacitance fluctuation develops a plateau region in the strongly disordered regime. The plateau value is identified as universal electrochemical capacitance fluctuation, which is independent of system parameters such as disorder strength, Fermi energy, geometric capacitance, and system size. Importantly, the universal electrochemical capacitance fluctuation is the same for all three ensembles, suggesting a superuniversal behavior.