Capacitance of Atomic Junctions

Abstract

We report the behavior of the electrochemical capacitance for a variety of atomic junctions using ab initio methods. The capacitance can be classified according to the nature of conductance and shows a remarkable crossover from a quantum dominated regime to that of a classical-like geometric behavior. Clear anomalies arise due to a finite density of states of the atomic junction as well as the role played by the atomic valence orbitals. The results suggest several experiments to study contributions due to quantum effects and the atomic degree of freedom. [S0031-9007(98)06031-1] PACS numbers: 73.40.Cg, 61.16.Ch, 61.43.BnFor a very small conductor in which quantum effects play a role, it is not difficult to imagine that capacitance of the conductor may behave differently from the familiar classical case. For a small conductor its discrete nature of electron energy levels can be important, and the quantum correction due to the finite density of states (DOS) of the plates is known as the quantum capacitance [1,2]. For a microscopic sized conductor in which quantum coherence is maintained, one must consider the role played by the leads which connect the conductor to the outside world. One also needs to consider the finite screening length of the interacting electrons when it is not small compared with the system size. At these very small length scales, the relevant capacitance is the electrochemical capacitance C ϵ edQ͞dm, which is a quantity depending on the electronic properties of the conductor [2].Capacitance plays a central role in many phenomena such as the Coulomb blockade, and it is very important for several experimental techniques. However, there have been no quantitative predictions of capacitance for microscopic systems. Thus, we do not yet know its dependence on the atomic valence orbitals, the environment (e.g., the leads), and the shape, size, and other properties of a nanosystem. The purpose of this work is to make this connection by theoretically investigating capacitance of atomic junctions. In particular we shall study junctions formed by a small cluster of atoms, shown in the right panel of Fig. 1, where the clusters are sandwiched in between two metallic leads. We emphasize small system size where quantum effects are dominant and we answer a number of very relevant questions: (i) What is the value of the capacitance of these atomic junctions? (ii) What is the effect of atomic orbitals? (iii) How do we characterize the behavior of the capacitance? (iv) For the tip-substrate system familiar to scanning tunneling microscopy (STM), what is the C C͑d͒? Although atomic nanosystems are of great current interest [3], these fundamental questions, which are important to ac transport, have not been addressed. It is clear that capacitance and many other mechanical and electrical properties of atomic junctions can be obtained only from detailed first principle analysis [4]. This will be our approach.The theoretical analysis consists of four steps. First, we determine the atomic cluster shape by extensive quant...

Nonclassical Behavior in the Capacitance of a Nanojunction

“…In addition to the geometry and dielectric constant, which are sufficient to determine the capacitance at the macroscales and microscales, the capacitance of a nanostructure can be affected by quantum effects [1]. Theoretical studies have shown the capacitance quantum corrections to come mainly from the finite density of states (DOS) of the nanosized electrodes, the finite screening length to the electron-electron interaction, and quantum tunneling [1][2][3][4][5][6]. So far, no experiment has been made to demonstrate these quantum effects, in spite of its importance in many phenomena such as Coulomb blockade (CB) and device applications.…”

Electronic structure of cylindrical simple-metal nanowires in the stabilized jellium model

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Nonlinear Spin Polarized Transport through a Ferromagnetic-Nonmagnetic-Ferromagnetic Junction

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