Understanding the properties of metals and semiconductors underpins modern electronics; new techniques to adjust nanometer-scale properties can mean unexpected changes in how the materials interact with heat and electricity. In a recent article in Science, an international team of researchers, led by Junqiao Wu at the University of California, Berkeley and Olivier Delaire at Duke University, reported new behavior in vanadium dioxide nanobeams, where electrons moving in the beams conduct electricity, but not heat.

“My group has been studying VO₂ for many years,” Wu says. The compound was already known for showing strange behavior. As the surroundings of materials change, their microscopic structures vary too. That change in phase can mean materials shift from being metals that conduct electricity to insulators that do not. “As vanadium dioxide has a metal-insulator phase transition in which electrical conductivity rises by many orders of magnitude, it is natural to ask whether the thermal conductivity also rises drastically.”  When the scientists tested the thermal conductivity of VO₂, they made an exciting discovery: “We found the opposite, …. the discovery of the violation of the Wiedemann–Franz law.”

The Wiedemann–Franz law, proposed in 1853, describes the relationship between the electrical and thermal conductivity of metals: increasing electrical conductivity normally means an increase in the heat transmitted. Decades before the discovery of electrons, Wiedemann and Franz had described how the particles carry energy. In vanadium dioxide, this relationship breaks down.

The electrons in a solid metal are termed the Fermi liquid, in a simple description. In this theory, metals have a sea of free-flowing electrons that move through space but repel each other. The large, positively charged nuclei of the atoms are locked in place, forming the crystal structure of the material. Because the nuclei cannot move, any energy that moves through the material must be due to the electrons, leading to the Wiedemann–Franz law. When violations of this law were found in the past, they were typically in experiments happening at very cold temperatures. “Fermi liquid theory is an intellectual triumph, but it clearly does not apply to all electrical conductors,” says Rafael Jaramillo of the Massachusetts Institute of Technology, an expert on conductivity changes at insulator–metal transitions.

Working at Lawrence Berkeley National Laboratory’s Molecular Foundry, the researchers assembled and tested microscopic devices to explore vanadium dioxide’s properties. Large samples can be complicated, with structural defects and variations that can introduce errors. Hence one-dimensional (1D) nanobeams, made from a single crystal, were used to remove this uncertainty. The nanobeams were assembled into a configuration spanning the gap between pads that could produce and measure heat and electric current. Careful assembly was critical, according to Wu. “To measure thermal conductivity of [a] single nanobeam reliably was very challenging. One has to eliminate the contact thermal resistance, to avoid damage to the material [and] to remove any strain/doping effect.”

More challenges still had to be overcome. Heat can also be transmitted by phonons—quantum-mechanical vibrations in the material. The phonon components need to be accounted for to confirm that the heat is transmitted only by the motion of electrons. The Delaire group first used density functional theory to define the model of the metal and insulator structures, then performed high-level quantum mechanical calculations with those structures to describe how the electrons would carry heat. When the thermal conductivity of the electrons was compared with the electrical conductivity, the researchers found that vanadium dioxide’s electrons conduct electricity without conducting heat. Unlike earlier experiments, this effect persists even at higher temperatures, up to 152°F (67°C), opening an array of potential applications for the material.

This behavior is the result of electrons moving in a way that is different from what simple models predict. Rather than the Fermi liquid model’s random motions, the electrons in vanadium dioxide work together as a team across relatively large distances in the material to transmit electricity.

The scientists are planning to search for this phenomenon, called strong electron correlation, in other materials. Single-crystal 1D materials may share similarity in chemical bonding with vanadium dioxide. “Systems such as VO₂ are often more like molecular solids than traditional metals, in their chemical bonding and electronic properties,” Jaramillo says. The team is hoping to discover how widely this can be generalized; Wu plans to determine whether “such [a] non-quasiparticle conduction mode exists universally in this wide class of materials, and whether the violation of the Wiedermann–Franz law could be used as a fingerprint of that.” This could lead to a more general approach that does not require the Delaire team’s sophisticated simulation approach to remove other sources of conduction. “We would all benefit from a consensus on how to determine this important parameter,” Jaramillo says.

In fully understanding this effect, a new class of materials with applications in efficient electronic devices could be created. “We keep being amazed at the richness of physical phenomena displayed in this seemingly simple compound,” Delaire says.

Read the abstract in Science.