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June 5, 2002

Electronic newsroom

02-44

 

Conducting-Insulating Materials Reveal Their Secrets: Explanatory Graphics


Figure 1: Energy of excited electrons as a function of their direction of motion in an insulating-conducting material made of planes of atoms. The colors show the various levels of intensity of the light emitted by the electrons (the lowest intensity is blue and the highest intensity is yellow). The yellow regions in (a) represent excited electrons acting together in a collective motion, but a close-up of the top of the figure reveals that some of these electrons move individually instead of collectively below a certain temperature, as shown in (b) for a temperature of 30 Kelvin (-405 degrees Fahrenheit). Such "individualistic" behavior is absent at higher temperatures, as shown in (c) for a temperature of 180 Kelvin (-135 degrees Fahrenheit).
 


Figure 2 (a) (left): Intensity of electrons excited by the ultraviolet synchrotron light as a function of their energy for four different values of the temperature in the same material as in Figure 1. A high intensity of electrons acting individually appears only at temperatures below 180 Kelvin (-135 degrees Fahrenheit), as revealed by the intensity signals for temperatures of 95 and 30 Kelvin (-288 and -405 degrees Fahrenheit, respectively).

Figure 2 (b) (right): Resistance as a function of temperature in the same material as in Figure 1. Resistance along the planes (black curve) is compared with the resistance perpendicular to the planes (red curve). For temperatures above the critical temperature (denoted TM), the material is both conducting and insulating. As the temperature is decreased, the material becomes more conductive (or less resistive) within the planes, which is the characteristic of a conductor, while it is more resistive in the direction perpendicular to the planes, a characteristic of an insulator. For temperatures below TM, the material behaves like a conductor both within and perpendicular to the planes with decreasing temperatures. In this case, the conducting behavior in the direction perpendicular to the planes is due to single electrons moving between the planes. (The arrows make the correspondence between the curves and the vertical scales.)


Figure 3: For the second insulating-conducting material, again made of atomic planes: (a) Energy of excited electrons as a function of their direction of motion. The high intensity that is characteristic of the individual particles is visible only at very low temperatures and is less apparent than in Figure 1. (b) Intensity of the electrons emitted by the light as a function of their energy for four different values of the temperature. The signal starts to be visible below 180 Kelvin (-135 degrees Fahrenheit), but its amplitude is not as high as that for the previous material, indicating fewer electrons are available to conduct electricity between the planes. (c) Resistance as a function of temperature within (black curve) and between (red curve) the planes. Though the changes in the resistance behavior are similar to those in Figure 2, the resistance values are lower than in the previous material.

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The U.S. Department of Energy's Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies. Brookhaven also builds and operates major facilities available to university, industrial, and government scientists. The Laboratory is managed by Brookhaven Science Associates, a limited liability company founded by Stony Brook University and Battelle, a nonprofit applied science and technology organization.