A Current-Driven Single-Atom Memory

Top row: (left to right): Christian Schirm, Manuel Matt, Fabian Pauly. Bottom row (left to right): Elke Scheer, Juan Carlos Cuevas, Peter Nielaba.

Authors: Christian Schirm¹, Manuel Matt¹, Fabian Pauly¹, Juan Carlos Cuevas², Peter Nielaba¹, Elke Scheer¹

Affiliation:
¹Department of Physics, University of Konstanz, Germany.
²Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, Spain.

Building functional devices at the atomic scale is a central vision of nanotechnology. Recently we demonstrated a two-state electrical switch with basically a single atom acting as moving part [1]. The switch consists of an aluminum atomic contact that is connected to electrodes at each side. Applying positive or negative currents above a certain threshold will change the state. Lower currents can be used to read out the switching status by simply measuring the resistance. This qualifies the device for future potential applications in high-density solid-state memories.

The single-atom switch was analyzed in close collaboration between experimental and theoretical physicists. Quantum mechanical transport channels of the electrical conductance served as an important link between experiment and theory. (The conductance is the inverse of the electrical resistance.) On the one hand these channels could be determined experimentally by measuring non-linear current-voltage characteristics in the superconducting state at very low temperatures [2]. On the other hand the channels could be calculated through a combination of molecular dynamics simulations, electronic structure theory, and transport theory expressed in terms of non-equilibrium Green's functions [4]. The theory enabled us to identify atomic configurations for both states of the experimentally realized switches. The comparison showed that in some cases the observed switching effect can be attributed to the geometrical rearrangement of a single atom.

Fig 1: Aluminum bridge with a 100 nm constriction, fabricated by electron beam lithography (see left panel). Due to the use of an elastic substrate, the aluminum structure can be stretched by carefully bending the sample. Since we measure the conductance continuously, we can stop the breaking procedure at a constriction narrowed to a single atom (see sketch in the right panel).

In the experiment we created an atomic contact of aluminum by the mechanically controllable break-junction technique using a lithographically fabricated bridge with a 100 nm wide constriction [3]. Fig. 1 shows a scanning electron micrograph (in false colors) of such an aluminum sample on an elastic substrate and illustrates the breaking process to reach an atomic-sized contact. Next, as shown in the graph in Fig. 2, we applied a slowly increasing electrical current and measured the conductance simultaneously. At some threshold current an electromigration process takes place, which can be observed as a jump in the conductance. The current is now decreased until the next jump is detected, then increased and so on. After some rearrangements the system may reversibly switch between two conductance states. A comparison of the conduction channel signatures verifies that these states remain exactly the same.

Fig 2: Applying a careful electromigration protocol when an atom-size contact has formed, a two-state atomic switch may develop. For this the current through the contact is increased slowly with time. When a jump in the conductance is detected, the current is reversed. After some time the system may switch reversibly between two conductance states. The inset shows the rectangular conductance-current hysteresis of a successfully generated two-level switch. The unit of conductance is G₀=2e²/h, also called the “quantum of conductance”.

In molecular dynamics simulations we performed a stretching procedure similar to the preparation of the contact in the experiment. The simulations generate realistic contact geometries and transport data as we verified by comparing experimental and theoretical conductance histograms [4,5]. Fig. 3 shows two atomic geometries as obtained in a stretching process. The calculated total conductance and individual conductance channels were in good agreement with several experimentally realized switches.

Fig 3: Molecular dynamics simulation of geometries corresponding to conductance values and channel signatures found in the experiment. The primary difference between both geometries is a relocation of the central atom and the breaking of two bonds to move from state “1” to state “0” (symbolized by the red scissors). The conductance and the individual channels are given in units of G₀.

The project demonstrates how theory and experiment can work hand in hand to advance nanotechnology. The atomic switch fulfills several technological requirements. For instance, it is relatively easy to fabricate due to the two-terminal configuration. Our work can be considered as a feasibility study of a one-bit atomic-size memory [6]. Potentially, an array of such switches can be implemented in a cross-bar architecture of solid-state memories when further technological issues, such as the stable operation at room temperature, can be solved.

References:
[1] C. Schirm, M. Matt, F. Pauly, J. C. Cuevas, P. Nielaba, E. Scheer, "A current-driven single-atom memory", Nature Nanotechnology 8, 645–648 (2013). Abstract. 
[2] E. Scheer, P. Joyez, D. Esteve, C. Urbina, M. H. Devoret, “Conduction channel transmissions of atomic-size aluminum contacts”, Physical Review Letters, 78, 3535–3538 (1997). Abstract. 
[3] J. M. van Ruitenbeek, A. Alvarez, I. Piñeyro, C. Grahmann, P. Joyez, M. H. Devoret, D. Esteve, C. Urbina, "Adjustable nanofabricated atomic size contacts", Review of Scientific Instruments, 67, 108 (1996). Abstract. 
[4] F. Pauly, M. Dreher, J. K. Viljas, M. Häfner, J. C. Cuevas, P. Nielaba, "Theoretical analysis of the conductance histograms and structural properties of Ag, Pt, and Ni nanocontacts", Physical Review B, 74, 235106 (2006). Abstract. 
[5] See supplementary information in reference [1].
[6] Sense Jan van der Molen, "Single-atom switches: Toggled with electrical current", Nature Nanotechnology, 8, 622 (2013). Abstract.