ReCALL: Electrical Resistivity of Concentrated Alloys

Electrical resistivity is one of the oldest and well-known characteristics of solid materials. For instance, it separates metals from insulators: For metals, the electrical resistivity is low and it increases with temperature, while insulators have the large resistivity which goes down with temperature. However, many technologically important metallic alloys with relatively high resistivity seem to violate this simple distinction. Specifically, their electrical resistivity does not behave like the one in normal metals, and can even decrease with temperature, as if the material was insulating. The origin of this behavior is not yet fully understood. In this project, we propose to use a unique combination of methods based on the ab initio approach to study peculiar electrical properties of highly resistive alloys, with the goal of unraveling the relation between the local atomic structure and the temperature evolution of the resistivity. Numerous theories have been put forward to explain the origin of the negative temperature dependence of the resistivity in metallic alloys. However, the main difficulty of these theories is that they all rely on simplified models, providing conflicting qualitative explanations of the phenomenon. Therefore, to distinguish between plausible alternative mechanisms, a quantitative comparison to experimental data is needed. In our approach, we will study the temperature-dependent resistivity using ab initio simulations of metallic alloys in all their complexity. To achieve this, we will combine a method that has successfully been applied to accurately describe complex multicomponent alloys with a formalism for calculating electrical resistivity that has recently been extended to correctly take into account the temperature dependence. Two major outcomes from the project are expected. On the one hand, we will use our novel methodology to identify the mechanism of the unusual resistivity behavior in disordered alloys. On the other hand, the sensitivity of the electrical resistivity to the atomic structure of materials makes it an important tool for monitoring the structure evolution of alloys during their production. The application of this characterization technique to many technologically important alloys, such as, e.g., austenitic steels, high-entropy alloys, or bulk metallic glasses, is not possible due to the poorly understood peculiar behavior of their resistivity. Our novel methodology will enable one to interpret experimentally obtained resistivity data, giving access to important local atomic properties, such as atomic short-range order or random displacements, which are very difficult to measure in complex multicomponent alloys using conventional spectroscopic methods.

Imagine trying to walk down a crowded hallway. If everyone is standing still, it's relatively easy. But if everyone starts jostling around, it becomes much harder to get through. This is a lot like how electrons feel, when they make their way through densely packed atoms, as electricity flows through a metal wire. Scattering of electrons on atoms leads to electrical resistance. When the metal is cool, the atoms are mostly still, but as it heats up, they vibrate more and more vigorously. This atomic jostling scatters the electrons, making it harder for them to flow. That is why in common metals, such silver, copper, or bronze, the resistance increases as they are heated up. However, there is a strange and fascinating class of metallic alloys that breaks this rule entirely. When you heat them up, their resistance actually decreases. It's as if the hallway becomes easier to navigate the more people start to move around. This counter-intuitive behavior has puzzled scientists for decades. While many theories have been proposed, they were often based on oversimplified models that couldn't capture the true complexity of these unique materials. Our project introduces a computational method that may finally resolve this mystery. The main challenge in studying these alloys is their inherent messiness. The random mix of different atoms, combined with the chaotic vibrations from heat, breaks the neat, orderly structure that makes calculations manageable. To overcome this, we combined a special computational methodology designed for random alloys and a rather general theoretical formalism to calculate resistance with the goal to investigate electrical properties ab initio, that is, based on the fundamental laws of quantum mechanics. For the first time with such accuracy, our computational approach successfully reproduced this strange temperature effect in alloys known for this behavior. With this ab initio tool at our disposal, we could isolate and analyze all the factors that contribute to resistance. This allowed us to definitively rule out several older explanations and relate the phenomenon to effects that trace back to the structure of atoms themselves. These effects become apparent only when the material is modeled in full details, which is why earlier simplified models failed to capture them. Our methodology does more than just resolves a long-standing scientific issue. It equips us with a predictive tool to design entirely new materials with custom-tailored electrical properties. Such materials could be invaluable for creating next-generation precision electronics, sensors, and other technologies where a specific response to temperature is not just desired, but essential.