Physicists have managed to watch individual hydrogen atoms move on metal surfaces at very low temperatures — in defiance of classical physics.
Quantum theory, which has just celebrated its 100th birthday, allows particles to break the rules of classical physics. Although, in a classical sense, a particle may not have sufficient energy to cross |a given barrier, quantum theory says that there is still a definite probability that it can penetrate the barrier in a process known as quantum tunnelling. This process is at the heart of many phenomena — from the formation of chemical bonds to what distinguishes a metal from an insulator. Writing in Physical Review Letters, Lauhon and Ho1 report that they have tracked and visualized the quantum tunnelling of individual atoms for the first time.
By using a scanning tunnelling microscope (STM), the researchers were able to monitor the motion of individual hydrogen atoms on a metal surface. They found that the atoms remain mobile down to temperatures as low as 9 K. Classically, thermal diffusion or motion is expected to fade away as the temperature is lowered. But the constant movement of hydrogen implies that there is a quantum effect that allows the atoms to tunnel along the metal's surface. Quantum tunnelling of atoms at low temperatures has been inferred from experiments on relatively large groups of atoms, but never before has quantum motion been observed so directly — one atom at a time.
In condensed-matter physics, quantum tunnelling of atoms is believed to play a key role in phenomena such as the diffusion of impurities in solids and the properties of glasses at low temperatures. An atom typically 'rests' in an energy well (Fig. 1a). It can tunnel to another well if it is light enough and if the energy barrier between the wells is sufficiently small. Because hydrogen is so light, it is particularly open to the possibility of quantum tunnelling2. On the surfaces of metals, the constant movement of hydrogen has been reported down to low temperatures3,4. But whether this diffusion arises from classical thermal motion or from quantum tunnelling is unclear. Some of the uncertainty can be attributed to the fact that previous experiments measured the average behaviour of a group of atoms, and so could not resolve the role of surface defects in the tunnelling process. A localized probe, such as the tip of an STM, sidesteps these complications.
The STM uses the principle of electron tunnelling to give us a close-up view of atoms at surfaces5. A stream of electrons tunnels between a sharp metallic tip and the surface under investigation as the tip skims across the surface. A feedback circuit keeps the flow of electrons constant by adjusting the distance between the tip and the surface. The recorded trajectory of the tip is then used to form an image.
The presence of a hydrogen atom on the surface is seen as a dip or 'hole' on an STM image (Fig. 1b). This is because an STM image of an isolated atom on a surface depends on how much the atom modifies the surface's electronic structure. Isolated hydrogen atoms provide very little attraction for the conduction electrons of a metal surface and, as Lauhon and Ho1 show, they appear only as small dips in the STM images of copper at low temperatures (9 K). An identical image is obtained for deuterium, hydrogen's heavier isotope, because the two are electronically equivalent. But when hydrogen and deuterium are chemically bound to the surface, the STM can distinguish between them because their different masses cause them to vibrate at different frequencies6.
The rate at which atoms move across a surface can be measured by making an STM 'movie' of the sample. Alternatively, the tip can lock on to an atom and keep constant track of its location. Using both techniques, Lauhon and Ho measured the rate of diffusion of hydrogen and deuterium across a copper surface at different temperatures. From their observations, they ruled out the effect of the STM tip on their measurements. At higher temperatures, they saw both atoms obeying a diffusion law that is consistent with classical motion caused by thermal excitation. But as the surface temperature was lowered, the behaviour of hydrogen and deuterium began to differ dramatically. Deuterium follows classical physics and slows down to a halt. But hydrogen shows a very weak temperature-independent motion, which starts at about 65 K and is still present at 9 K, the lowest temperature used in the experiment. Because hydrogen is much lighter than deuterium, this movement provide strong evidence that quantum tunnelling is at work.
A particle's ability to tunnel is also affected by its immediate environment. For example, lattice vibrations (phonons) and conduction electrons scattering from the atom as it tunnels can hinder or promote the tunnelling action. The interaction of a tunnelling atom with its environment dissipates energy and can disrupt the tunnelling process, reduce it to a mere hop, or stop it altogether7. Detailed analysis of the STM-measured diffusion rate for hydrogen by Lauhon and Ho identifies different temperature regimes in which phonon- or electron-scattering predominates. Most intriguing is their observation that at the lower temperatures in the experiment, the tunnelling rate increases as the surface is cooled. This classically impossible behaviour shows that hydrogen tunnelling improves over longer periods of time as the surface gets colder. Perhaps reducing the temperature by a further factor of 10 or 100 will reveal a new regime in which hydrogen atoms can eventually tunnel over greater distances.
Nowadays, the focus of much tunnelling research is to determine the degree to which this quantum concept can be extended to the macroscopic world8. As the size of the tunnelling object is increased, the effect of the environment becomes much more important. The situation in Lauhon and Ho's experiments is still microscopic, yet it provides a remarkably clean setting to get at the heart of environmental effects on the tunnelling process. The STM can also be used to position atoms carefully so as to design different tunnelling hurdles and to examine how the tunnelling process can be controlled. Perhaps learning how to manipulate the outcome of the tunnelling events at the nanometre scale would have some useful applications in making devices on this scale.
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Journal of Superconductivity and Novel Magnetism (2012)