Pomeranchuk-like electronic localization above 100 K in twisted MoS$_{2}$

In the microscopic world of modern electronics, scientists are learning to build materials atom by atom, stacking thin sheets like a deck of cards to create entirely new rules for how electricity flows. When two such sheets are slightly rotated against each other, they form a giant, repeating pattern called a moiré superlattice. This pattern acts like a new kind of landscape for electrons, trapping them in narrow valleys where they interact strongly with one another. Usually, when electrons get too crowded or too cold, they freeze into a rigid, insulating state. However, in a few special materials, heating them up can actually make them freeze even harder. This counterintuitive behavior, where warmth strengthens order rather than breaking it, is known as the Pomeranchuk effect, named after a physicist who first predicted it in liquid helium decades ago. Understanding how and why this happens in solid materials could unlock new ways to control electricity and information, but finding a material that shows this effect clearly at temperatures we can easily reach has been a major challenge.

A team of researchers has now found a way to observe this strange phenomenon in a material called twisted molybdenum disulfide, or MoS2. By carefully stacking two layers of this semiconductor and twisting them at very specific angles, they created a platform where electrons behave in surprising ways. The scientists focused on the "conduction band," which is the path electrons take when they are free to move and carry a current, rather than the "valence band" where they usually sit still. While previous studies on similar materials mostly looked at the stationary electrons, this team managed to study the moving ones, overcoming a significant technical hurdle: making stable electrical connections to the material without damaging it at extremely low temperatures. They succeeded by using a special design that exposes the material through a tiny window, allowing them to measure the flow of electricity from near absolute zero up to room temperature.

The researchers tested several samples, each twisted at a slightly different angle, ranging from about 2.3 degrees to 3.5 degrees. They discovered that the behavior of the electrons changed dramatically depending on this angle. In the samples with the smallest twist, the electrons formed a stable, ordered state that could survive temperatures as high as 160 Kelvin, which is well above the boiling point of liquid nitrogen. As they increased the twist angle to an intermediate level, this stability collapsed, and the ordered state could only survive at temperatures around 20 to 30 Kelvin. But the most surprising discovery came from the sample with the largest twist angle of roughly 3.5 degrees. In this specific configuration, when the researchers heated the material, the electrons did not start moving more freely as expected. Instead, the heat made them get stuck in place even more firmly. This is the hallmark of the Pomeranchuk-like effect: thermal energy, which usually melts ice or melts metal, was instead helping to lock the electrons into a localized, insulating state.

This heat-induced locking was particularly strong at a specific density where there was exactly one electron for every repeating unit of the pattern. As the temperature rose from near absolute zero to about 50 Kelvin, the electrical resistance of the material increased, signaling that the electrons were becoming more localized. Even more remarkably, at a density of two electrons per unit, this strange behavior persisted at temperatures above 100 Kelvin, despite the signal being weaker. The researchers also tested how these electrons reacted to magnetic fields to understand what was holding them in place. They found that the effect was highly sensitive to the direction of the magnetic field. A magnetic field applied perpendicular to the material made the electrons stick even tighter, but a field applied parallel to the surface had almost no effect. This is different from similar effects seen in twisted graphene, where the electrons respond more equally to magnetic fields from any direction. The strong difference suggests that the electrons in this twisted MoS2 are not just spinning in a simple way; they are likely influenced by a complex mix of their location in the layers and their specific quantum properties related to the material's atomic structure.

The study suggests that these electrons are behaving like a heavy fluid, moving much slower than normal electrons, which hints at a state of matter known as a heavy Fermi liquid. The researchers propose that the electrons are forming local magnetic moments that fluctuate, and the heat helps to stabilize these fluctuations, effectively trapping the electrons. This discovery is significant because it shows that the twist angle is not just a dial to turn up or down the strength of interactions, but a tool that can fundamentally change the nature of the material itself. By tuning the angle, scientists can switch between robust high-temperature order, a fragile intermediate state, and this unusual regime where heat reinforces order. The fact that this effect can be observed at temperatures above 100 Kelvin makes it a promising candidate for future studies, offering a new window into how complex quantum states can emerge and be controlled in semiconducting materials.