Research

Schematic single-molecule device (image: J.W. Ciszek
Schematic single-molecule device (image: J.W. Ciszek)

Contents

Overview

The Natelson Group is interested in nanoscale condensed matter physics, in particular the electronic, magnetic, and optical properties of systems with characteristic dimensions approaching the atomic scale. A variety of unusual phenomena arise in this size regime, and an understanding of the properties of matter on these scales will likely be essential for future technologies.

A molecular
junction out of equilibrium
Figure 1. A molecular junction out of equilibrium.

Imagine taking some nanoscale structure and somehow hooking it up to a battery via a couple of wires. As the voltage across the nanosystem is increased, many things can happen. An electric field develops across the structure, and charge rearranges itself in response to that field. When there is conduction, electrons are driven from one side of the nanosystem to the other, pushing the distribution of electrons (as a function of both position and energy) out of equilbrium. Those electrons interact with each other, and energy can pass between them; the electrons can also deposit energy into the vibrational motion of the atoms, driving those degrees of freedom out of equilibrium. The electrons can interfere quantum mechanically, though their coherence (and therefore the importance of quantum interference) degrades as they interact with their environment. The spins of the electrons can become entangled with each other. All of this takes place on very short time and distance scales, and somehow leads to the macroscopic world we see all around us. Sometimes electron-electron interactions play only a minor role in determining material properties (as in aluminum or silicon); in other material systems, however, the electron-electron repulsion can be critically important. Our group tries to understand the physics in both driven and "correlated" systems, through experiments and their analysis. We are particularly interested in trying to get information at the nanoscale in nanoscale structures driven out of equilibrium, and in materials where strong electron-electron interactions determine material properties.

Techniques now exist for controllably fabricating structures with characteristic lengths below 10 nm, through both "top-down" methods (combining electron beam lithography, directional etching, and metal deposition) and "bottom-up" approaches (chemical fabrication, self-assembly). We can now examine "traditional" solid state systems at previously unexplored scales by making samples in this newly accessible regime. Further, we can develop tools for studying novel materials such as organic semiconductors and other molecular and molecular-scale objects.

In addition to learning a lot of neat physics, people in this research group develop skills in micro- and nanofabrication, sensitive electrical measurements, various microscopy techniques, semiconductor physics, low temperature physics, and vacuum systems. Graduate students or interested undergraduates are encouraged to send email (natelson@rice.edu).

Atomic- and molecular-scale junctions

It is now possible to examine the electronic, magnetic, and optical properties of materials down to the atomic scale in certain circumstances. These systems are of fundamental scientific interest and are likely to be relevant to next-generation technologies.

This research has been supported in part by the Robert A. Welch Foundation, The Research Corporation, the Packard Foundation, the National Science Foundation, the W. M. Keck Program in Quantum Materials at Rice.

One overarching topic of interest in truly nanoscale devices is dissipation. If you connect a wire from one terminal of a battery to the other, current flows and the wire gets warm. The organized chemical energy of the battery ends up dissipated in the disorganized vibrations of the atoms that constitute the wire. We understand this process well when we consider macroscopic wires, but if your conductor of interest consists of only a small number of atoms, the flow of energy and the nature of dissipation are much more challenging to assess experimentally.

Atomic-scale metal junctions

In some ways the simplest kind of nanoscale device is an atomic-scale junction between two pieces of metal. Imagine breaking a metal wire into two pieces. At the last instant before the wire breaks, the two sides are linked by an atomic-scale connection. The electronic conduction of such a junction are dominated by quantum effects even at room temperature. For metals with mostly s-type conduction electrons (e.g., Au) the conductance of an atomic point contact is given by 2e2/h, where e is the electronic charge and h is Planck's constant.

Figure 2. Shot noise suppression in atomic-scale
junctions
Figure 2. Shot noise suppression in atomic-scale junctions.

We have developed methods to measure high frequency, broadband (200-500 MHz) noise in atomic-scale structures. Current noise (mean square fluctuations about the average current) can reveal much about correlations between the motions of electrons. If electrons transit a system (without inelastic scattering) with some characteristic rate but are otherwise uncorrelated (Poisson statistics), the current noise is white (frequency independent out to some cutoff) and given by 2eI, where I is the average current. Adding in correlations changes this result. For example, perfectly coordinated electron equally spaced in time would suppress the current noise all the way to zero. Conversely, electrons travelling in bunches rather than independently would enhance the noise. Noise in quantum nanostructures is known to show suppression under particular circumstances (see here for example). We can see this inherently quantum mechanical suppression of shot noise even at room temperature, as reported here. We have been looking at the evolution of this noise as a function of the bias across the junctions, both in ensembles of junctions (as reported here) and in individual junctions (as reported here). Of particular interest is an apparent increase in noise at relatively large currents and voltages above simple expectations, and whether this is due to electron-electron and electron-vibrational inelastic processes.

In collaboration with the Untiedt group at Alicante, we have found evidence for Kondo physics in atomic-scale junctions between ferromagnetic metals. This is rather surprising, since Kondo physics involves antiferromagnetic process that couple a local electronic spin to the spins of conduction electrons. It appears that the undercoordination of the atom(s) at the contact region modifies the local electronic structure, making this physics possible. This system is an attractive target for noise measurements, as predictions exist regarding noise in the Kondo regime.

Single-molecule transistors

Schematic single-molecule transistor
Figure 3. Schematic single-molecule transistor

We are one of a relatively small number of research groups in the world who have successfully made single-molecule transistors (SMTs). A readable article for nonexperts is here (pdf). SMTs are three-terminal devices, with source and drain electrodes to pass current into and out of a molecule, and also a gate electrode to shift molecular level energies up or down relative to the source and drain. These structures generally act like single-electron devices, shown schematically at right. In the limit shown, electronic conduction through SMTs is strongly affected by the single-particle level spacing (the energy required to promote an electron from the highest occupied level to the lowest unoccupied level) and the Coulomb charging energy (the energy needed to add an additional electron to the molecule).

Level structure of SMT
Figure 4.Level structure of SMT

In SMTs, the small molecular size implies that both of these energy scales can be hundreds of meV, vastly higher than in conventional metal or semiconductor single-electron devices. As a result, physical chemistry issues such as molecular vibrational modes and conformational changes can become relevant. Depending on the strength of the molecule/electrode coupling, higher order tunneling processes can strongly affect conduction as well. In molecules with unpaired spins, magnetic effects can result in the development of strongly correlated electronic states (e.g.the Kondo resonance) that span the device. The details of the contact also determine the relative alignment of the electrode and molecular levels, with major implications for transport. The interplay between all of these issues means that conduction through SMTs can exhibit a rich variety of phenomena.

Prof. Natelson has written an extensive review article about single-molecule transistors. Please contact Prof. Natelson if you would like a copy. Similar matters are discussed in these two papers (1, 2).

We have used these devices to study several interesting physics and physical chemistry problems: