# Presentation: 512862

## Electrical Transport in Thin Film NanostructuresHanno H. Weitering, The University of Tennessee, DMR 0244570

*Competing periodicities in a single atom wire*

*Fractional quantum numbers in a phase slip*

- The smallest wire width in mass produced electronic devices is about 50 nm, or about 500 atoms across. The ultimate limit of thinness would be wires of only one atom wide. Such wires can be made now, although not for any working electronic device, and it is important to know their properties for future technology.
- Atom wires can be created via self-assembly on vicinal silicon surfaces. We addressed their formation, stability, metallicity, and the role of disorder. Gallium atom wires show entropic disorder that limits their structural perfection. On the other hand, Au-induced atom wires reveal three competing charge density wave instabilities (or a multiband Peierls instability). Phase slips in these density waves represent fractionally charged quasi-particles that can be imaged and spatially manipulated with the tip of a scanning tunneling microscope.

- Phys. Rev. Lett.* ***96***, *076801 (2006) and AIP Physics News Update # 767

**Notes:**
Necklace of atoms
Atom wires generate a lot of attention because of their extremely small dimensions: an atom is typically on the order of 0.5 nanometer (a nanometer is one-billionth of a meter). Depending on their properties they could serve as interconnects in future atomic or molecular scale computer chips. Instead of neatly placing the atoms in a row with a Scanning Tunneling Microscope, there is a smarter way: make nature do the job for you. This principle is called self-organization.

The team, consisting of Dr. Weitering from the University of Tennessee, and Paul Snijders and Sven Rogge from Delft, produced atom wires using this approach. Gold atoms were evaporated in an ultra-high vacuum and deposited on a special kind of silicon surface. This surface is sawed from a silicon crystal in a deliberately slanted way in order to generate a periodic “flight of stairs” made of the planes of atoms arriving under an angle at the surface. It appears that in this environment the gold atoms organize themselves in perfect rows parallel to the steps of the stairway – atom wires have been fabricated, resembling the chain of beads in a necklace, but instead made of atoms.

Electrons in an atom wire
Electrons always have a negative charge and therefore avoid each other. But in the atom wires studied here they cannot dodge one another because the diameter of the wire, being only one atom wide, is too small for evasion. A phenomenon that can occur in such wires is the formation of a so-called Charge Density Wave (CDW) – a periodic modulation of the density of charge. Electrons cluster in a spatial undulating pattern, with a wavelength exactly fitting an integer amount the interatomic distance in the wires. Generally, by transforming to this smart arrangement, the system attains a more favorable state with a lower energy; the so-called “ground state” has been reached. But the measurements show that this is not always the case.

Two transitions
The experiments were performed with a Scanning Tunneling Microscope capable of imaging surfaces with atomic resolution. These measurements showed, surprisingly, that in these self-organized wires two transitions with different wavelengths take place, one after the other. A doubling of the wavelength was observed when the system was cooled down from room temperature. The measurements indicate that this is a conventional Charge Density Wave. But fully unexpected at lower temperatures was the observation of another transition in the same atomic wire, this time having a tripled wavelength. At intermediate temperatures both states can even be observed to coexist in the wires. It is the first time that two subsequent Charge Density Wave transitions are observed in one chain.

Fractional electrons
This study of atom wires with an STM generates yet another unique result: the observation of phase slips. Phase slips are locations where the periodicity of the CDW makes a “mistake;” in between wavelengths of three atomic distances suddenly one period of four atomic distances is located. Nobel laureate Schrieffer already predicted in 1981 that these “mistakes” possess a charge equal to 1/3 or 2/3 of an electron charge – despite the fact that electrons cannot be split at all! Also the magnetic moment of the electron (spin) exhibits this peculiarity; these phase slips either have no spin at all or half a spin. The exceptionality of these experiments is that this is the very first time that such fractional quantum numbers are accessible for further research in a spatially resolved experiment.

## Electrical Transport in Thin Film NanostructuresHanno H. Weitering, The University of Tennessee, DMR 0244570

**Education:**
- Two graduate students (Murat Özer and Eun Ju Moon) and one postdoc (Jiandong Guo) were supported by this award.
- Murat Özer was first author on a paper in the March 2006 issue of *Nature Physics* showing that films only a few atom layers thick can carry enormous supercurrents—defying theories that superconductivity is typically weak at the nanoscale. For this work, he won the prestigious Nottingham Prize for best student presentation at the 2006 Physical Electronics Conference at Princeton University. He also received the University Chancellor’s Citation for Professional Promise and a Paul H. Stelson Research Fellowship from the Physics Department. Murat Özer received his Ph.D. degree at UT in the summer of 2006. He is currently postdoc at the University of Texas in Austin.
- Dr. Jiandong Guo pioneered optical studies of thin film nanostructures. He left the group in October 2005 to become a junior physics professor at the Chinese Academy of Sciences in Beijing. Eun Ju Moon began her thesis research in January 2005. She has build dedicated measurement equipment for in-situ studies of quantum transport in thin film nanostructures and is continuing her Ph.D. dissertation research.

**Societal Impact:**
- Nanoscience represents a very promising avenue for future innovations in *e.g.* the physical sciences, medicine, and information technology. A key requirement for making functional nanodevices is the ability to acquire perfect control of their structure and morphology. A viable way to accomplish this is to exploit quantum mechanical laws while tuning and assembling nano structures. This work represents an important case-study, showing: (1) how quantum mechanics can be used to control the structure and morphology of thin film nanostructures at the atomic level, and (2) how the “quantum engineered morphology” of thin films relates to one of their most appealing functionalities, namely dissipation-free electrical conductivity or “superconductivity.”