Phonon-Localization-Driven Decoupling of Dual-Channel Transport for Record-Low Intrinsic Lattice Thermal Conductivity

Heat moves through solid materials in two distinct ways, a duality that has long puzzled scientists trying to create the world's most effective thermal insulators. In materials that do not conduct electricity, heat travels primarily through vibrations of the atoms that make up the crystal structure. For decades, researchers believed that to stop this heat, they had to scatter these vibrations as much as possible, much like throwing obstacles in the path of a runner. However, a deeper understanding of physics has revealed a more complex reality: these atomic vibrations behave both like tiny particles bouncing through a lattice and like waves that can tunnel through barriers. The challenge has been that the strategies used to stop the particle-like motion often inadvertently help the wave-like motion, creating a fundamental limit to how cold a material can get. This competition has kept the best insulators from reaching their theoretical minimum, leaving a gap between what is possible and what has been achieved.

A team of researchers has now found a way to break this deadlock by designing a material that stops both types of heat movement at the same time. By focusing on a specific family of crystals that form long, twisting chains, they discovered that the atoms in these structures become so tightly confined that their vibrations essentially freeze in place. This phenomenon, known as phonon localization, creates a state where the vibrations are so sluggish that they cannot move as particles, nor can they tunnel as waves. The result is a material with a record-breaking ability to block heat, offering a new blueprint for engineering the ultimate thermal insulators.

The researchers focused their study on a group of compounds made from indium, gallium, or aluminum combined with selenium and iodine. These materials form a unique architecture: long, one-dimensional tubes made of atoms that twist into a helix, similar to a spiral staircase. These tubes are packed together, but the forces holding them to one another are incredibly weak, like the gentle attraction between two sheets of paper, while the bonds inside the tubes are strong. This structural arrangement, combined with the presence of heavy atoms, creates a perfect storm for trapping heat. The heavy atoms move slowly, and the weak connections between the tubes prevent the vibrations from spreading easily from one chain to the next.

When the team simulated the behavior of these crystals, they found that the vibrations within the material became highly localized. Instead of traveling freely through the crystal, the vibrations were stuck in small regions, unable to gain momentum. This confinement had a dual effect. First, it stopped the particle-like flow of heat because the vibrations simply could not move fast enough to carry energy. Second, and more surprisingly, it also stopped the wave-like tunneling. Usually, when vibrations are crowded together in a complex crystal, they can overlap and tunnel through each other, allowing heat to sneak through even when particle flow is blocked. But in these twisted chains, the vibrations were so localized and the energy levels so distinct that the wave-like tunneling was also crushed. The two channels of heat transport, which usually compete or compensate for each other, were effectively decoupled and suppressed simultaneously.

The numbers from their simulations are striking. For a compound called indium selenide iodide, the heat flowing between the chains at room temperature was measured at just 0.198 watts per meter-kelvin. This is an exceptionally low value, but even more remarkable were the results for the related compounds gallium selenide iodide and aluminum selenide iodide. These materials achieved even lower values, dropping to 0.086 and 0.089 watts per meter-kelvin respectively at room temperature. To put this in perspective, these values are so low that they approach the theoretical limit for how little heat a solid crystal can conduct. Even at high temperatures of 900 kelvin, these materials maintained their insulating power, with values dropping further to 0.058 and 0.059 watts per meter-kelvin. This behavior is unusual because many materials that block heat well at low temperatures start to conduct more heat as they get hotter, but these crystals continued to get better at insulating as the temperature rose.

The key to this success lies in the specific design of the material. The researchers showed that the heavy atoms and the weak connections between the chains forced the vibrations into a state where they had almost no speed. In the language of physics, the group velocity, which describes how fast a vibration travels, was reduced to a near standstill. This happened because the vibrations were compressed into flat energy bands, meaning they had no room to move or spread out. The team confirmed this by analyzing the electron distribution, which showed that the electrons were tightly bound within the chains but completely loose and spread out between them. This confirmed that the chains were held together by very weak forces, allowing the vibrations to become trapped.

This work suggests that the path to the lowest possible thermal conductivity does not lie in making crystals more complex or adding more impurities to scatter heat, but rather in creating a specific type of confinement. By designing materials where vibrations are forced to stay put, scientists can stop heat from moving in any form. The study provides a clear example of how dimensional confinement—restricting the material to a one-dimensional shape—can be used to engineer properties that were previously thought to be impossible to achieve in a single crystal. These findings offer a new direction for creating materials that can protect sensitive electronics from heat or improve the efficiency of devices that convert waste heat into electricity, by finally overcoming the fundamental trade-off that has limited thermal insulation for so long.