Metal violates law of physics

Scientists have developed a material capable of conducting electricity without conducting heat.

Marilyn Chung/Berkeley LabBerkeley Lab scientists Junqiao Wu, Fan Yang, and Changhyun Ko (l-r) are working at the nano-Auger electron spectroscopy instrument at the Molecular Foundry, a DOE Office of Science User Facility. They used the instrument to determine the amount of tungsten in the tungsten-vanadium dioxide (WVO2) nanobeams.

A metal has stunned scientists by defying one of the oldest empirical laws of physics.

Scientists discovered that vanadium dioxide, which can switch from an insulator to a metal at 67 degrees C (152 F), is not restricted by the Wiedemann-Franz Law.

This law, stemming from the work of two German physicists in 1853, states good conductors of electricity are also good thermal conductors.

The team found that the thermal conductivity attributed to the electrons in vanadium dioxide was just a tenth of what may have been predicted, meaning it conducts electricity without conducting heat.

“This was a totally unexpected finding,” said principal investigator Junqiao Wu, a physicist at Berkeley Lab’s Materials Sciences Division and a UC Berkeley professor of materials science and engineering.

Junqiao Wu/Berkeley LabVanadium dioxide (VO2) nanobeams synthesized by Berkeley researchers show exotic electrical and thermal properties. In this false-color scanning electron microscopy image, thermal conductivity was measured by transporting heat from the suspended heat source pad (red) to the sensing pad (blue). The pads are bridged by a VO2 nanobeam.

“It shows a drastic breakdown of a textbook law that has been known to be robust for conventional conductors. This discovery is of fundamental importance for understanding the basic electronic behaviour of novel conductors.”

Using results from simulations and X-ray scattering experiments, the researchers were able to assess the proportion of thermal conductivity attributable to the vibration of the material’s crystal lattice, or phonons, and to the movement of electrons.

“The electrons were moving in unison with each other, much like a fluid, instead of as individual particles like in normal metals,” said Wu.

“For electrons, heat is a random motion. Normal metals transport heat efficiently because there are so many different possible microscopic configurations that the individual electrons can jump between.

“In contrast, the coordinated, marching-band-like motion of electrons in vanadium dioxide is detrimental to heat transfer as there are fewer configurations available for the electrons to hop randomly between.”

The amount of electricity and heat that vanadium dioxide conducts can be altered when it is mixed with other materials.

Postdoctoral researcher Fan Yang said this gave the materials differing thermal conductivity at high and low temperatures presenting potential uses in engines and windows.

“By tuning its thermal conductivity, the material can efficiently and automatically dissipate heat in the hot summer because it will have high thermal conductivity, but prevent heat loss in the cold winter because of its low thermal conductivity at lower temperatures,” he said.