Structure, magnetism and transport in atomic size contacts

Resumen

From a macroscopic point of view, matter can be considered as a continuum, but when materials are reduced to the nanoscale the discreteness of atoms cannot be neglected anymore. Moreover, atoms are constituted by a nucleus and a surrounding electronic cloud which determines many of the interesting properties of materials (electrical conduction, heat transport, magnetism...). Reaching the size of the characteristic lenghts of those electrons gives rise to new surprising effects where their quantum nature is highlighted. The electronic distribution and interactions are strongly altered, leading to the emergence of striking and often counterintuitive properties. In the above context, Nanoscience is the search for and study of new properties which appear as we reduce the size of materials to the nanometric scale. An important aim is the application of those new phenomena to the development of nanodevices. For this purpose, we need not only to deeply understand the rules that govern matter at the nanoscale but also to control the fabrication of nanostructures with atomic precision. A valuable instrument in the pursuit of those goals is the Scanning Tunneling Microscope (STM), which, since its invention by Bining and Rohrer in 1986, has turned into an essential tool for characterization and manipulation at the nanoscale. Its precision allows from imaging with atomic resolution to manipulate a single atom or even to alter the composition of a molecule. In adition, it is a perfect technique for the study of electronic transport characteristics, in the conduction tunneling regime but also in contact. This thesis is dedicated to the fabrication and study of different properties in metallic atomic size contacts. These can be defined as a bridge between two pieces of the same metal when they are connected by only a few atoms. The monoatomic contact is the limiting case when such a connection is consituted by only one atom, that is, the smallest possible contact between two pieces of the same material. This work is devoted to the study of the interrelation of transport, structure and magnetism: transport is influenced by structure and therefore structure can be determined by means of the electronic transport; magnetism is expected to influence the transport and at the same time the magnetic properties will strongly depend on structure, etc. This manuscript focuses on experimental methods and results, but their corresponding interpretation is in many cases based on theoretical simulations and/or calculations made by collaborators. Metallic atomic contacts do not seem to have a direct application, but they are a perfect workbench for the study of electronic transport in nanostructures. There also exists a considerable amount of proposals suggesting nanostructures as the next revolution in the design of electronic devices. For instance, molecular electronics searches molecules which could play the role of transistors, switches, ... In any case, those devices should be connected by electrodes in order to be part of a circuit, a fact which adds another reason for the study of metallic nanocontacts. In a bulk metal, electrons fill continuous bands of energy, where, under certain conditions, they can be considered to form a nearly free gas. In contrast, in an isolated atom, electrons are confined to discrete energy orbitals. Atomic contacts present an intermediate situation: electrons are confined in two of the spatial dimensions to just the size of a few atoms, the order of magnitude of the electronic wavelenght in metals. Electronic transport is then driven by a number of transversal modes or channels and the conductance strongly depends on the size, the chemical valence of the material and the specific geometry of the contact. All of this is exposed in detail in Chapter 1 of this thesis. The aim of this first chapter is to provide the fundamentals and experimental determination of conductance in atomic size contacts. Chapter 2 focuses on the fabrication of atomic size contacts. The most common techniques are described and selected experimental results are shown as application examples. A higher level of detail is used in the case of fabrication by electrochemical methods, since this is a novel technique that we have implemented and optimized in our laboratory in Alicante. We show our achievements in terms of control and feasibility of the technique in order to reproduce results already obtained by other more spread out techniques. Specifically, we show indications of the existence of exceptionally stable diameters during the contact dissolution process (shell structure). This work was done in close collaboration with Dr. Ancuta Mares and Prof. Jan van Ruitenbeek and Prof. Victor Climent. The last result mentioned above is already a good example of how structure determines the conductance and therefore of the amount of information concerning structure that might be accessible through conductance measurements. Precisely in Chapter 3 we study the process of formation of a monoatomic contact at 4.2K by means of the electronic transport. This process may happen with a sudden jump from the tunneling to the contact regime for certain materials (such as Au, Ag, Cu,...), while for others this transition is smooth (W). The statistical analysis of data in the cases where jump to contact formation occurs, provides information about the different structural configurations for the monoatomic contact. In addition, the occurrence (or non-occurrence) of jump to contact is expected to be related with elastic properties so we discuss the possible relation between mechanical macroscopic properties and the different phenomenologies at the formation of the one-atom contact. This work has been done in collaboration with Prof. María José Caturla, Prof. Juan José Palacios and Prof. Jan van Ruitenbeek. On the other hand, size is not the only scale that may alter the electronic configuration of a material. The exploration of new (higher or lower) energy scales returns frequently a rich unexpected phenomenology. Low temperatures techniques have in this way revealed the existence of surprising electronic ground states, unpredictable at room temperature (superconductivity, Kondo effect, ...). The results shown in Chapter 4 are a good example of how scale matters when looking at materials properties5. We found that the behavior of ferromagnetic materials can surprisingly change when we scale down its size and temperature. Electronic transport and magnetism are not independent phenomena either, and even less at nanostructures. This influence of magnetism in electronic transport at the nanometric scale is a hot topic in condensed matter research due to its perspectives of application, for instance, in devices for information storage. Controlling a single magnetic moment orientation by an electric current can be used as a writing procedure, while one can afterwards read the information by looking at the influence of the magnetic moment orientation in the electronic transport. For a few years, an intriguing, controversial question has been which should be the signatures of magnetism in the electronic transport in atomic size con- tacts 6, specially when we fabricate atomic contacts of pure ferromagnetic materials such as Fe, Co or Ni. Our results are conclusive and unexpected, the spin hosted at the central atom of the contact is screened by means of the Kondo effect. An extensive introduction to this effect as well as experimental results are shown in Chapter 4. Part of those experiments were performed in the laboratory of Prof. Douglas Natelson and the theory which completes this work was developed by Dr. David Jacob, Dr. Joaquín Fernández-Rossier and Dr. Juan José Palacios. The last chapter probably contains the most important result of this thesis. It emphasizes the strong influence of coordination in the electronic structure and, therefore, in the electronic transport and magnetic properties. The central atom of the contact stops being a bulk ferromagnetic atom and its magnetic moment is screened. This result should be taken into account at the time of studying magnetism in other low coordinated systems as surfaces or nanoparticles. In addition, our results point out the importance of electronic correlations between electrons in nanostructures, a fact that is often neglected by many approximations typically used for transport calculations in those systems.