Disorder-driven non-Anderson transition in a Weyl semimetal

What is it about?

In most materials, how electrons move determines whether they behave like metals, semiconductors, or insulators. When a material becomes disordered—meaning its atoms are no longer perfectly arranged—electrons can get trapped, and the material can switch from conducting electricity (metal) to blocking it (insulator). This phenomenon, known as the Anderson transition, has been the cornerstone of our understanding of disorder in materials for decades. However, theorists have long predicted that another kind of transition—called a non-Anderson disorder-driven transition—could also exist. Unlike the Anderson transition, this new type does not trap electrons but still changes how they move and how the material behaves. Until now, no one had directly observed this in real materials. In our study, we investigated a special quantum material called a Weyl semimetal, specifically the compound NdAlSi. Weyl semimetals are unique because they host “Fermi arcs,” special electronic states that exist only on their surfaces and are protected by the material’s topology—a kind of built-in geometric property of the electronic structure. Using advanced experimental techniques such as angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM), we observed what happens when disorder is introduced to the surface of NdAlSi. We found that as the disorder increased, these surface states gradually disappeared, not because electrons were localized, but because the material underwent a fundamental change—from a Weyl semimetal to a diffusive metal. This discovery provides the first experimental proof of a non-Anderson disorder-driven transition. It opens new pathways for understanding how disorder can reshape quantum materials, revealing that randomness itself can create entirely new states of matter.

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Photo by Zheng Yang on Unsplash

Photo by Zheng Yang on Unsplash

Why is it important?

In the world of quantum materials, disorder—tiny imperfections or randomness in the atomic arrangement—is often considered a nuisance that disrupts the smooth flow of electrons. Traditionally, scientists believed that the only major effect of disorder was to make materials insulating by trapping electrons, a process known as Anderson localization. This idea has shaped our understanding of disordered materials for over half a century. However, modern theoretical physics has suggested that disorder might sometimes do something far more surprising: cause a non-Anderson disorder-driven transition. In this type of transition, electrons do not get trapped, yet the material still undergoes a profound transformation in its quantum state. Until now, this phenomenon had never been experimentally verified, leading many to think it might be a purely theoretical curiosity. Our research provides the first direct experimental evidence that such a non-Anderson transition can indeed occur in real materials. We studied a special quantum material called a Weyl semimetal (specifically NdAlSi), which hosts unusual surface electronic states known as Fermi arcs. These Fermi arcs are one of the hallmarks of topological materials—systems whose electronic properties are protected by deep geometric principles rather than simple chemistry. By using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM), we could watch in real time what happens as the surface of the material becomes increasingly disordered. We observed that the Fermi arcs—normally very stable—gradually disappeared, signaling a shift from a topological Weyl semimetal to a diffusive metal. Crucially, this occurred without any signs of Anderson localization, confirming a long-standing theoretical prediction. What makes our work unique and timely is that it bridges a decades-long gap between theory and experiment, validating a new class of quantum phase transitions. This discovery not only deepens our understanding of how disorder reshapes electronic behavior but also suggests new ways to engineer or control quantum states in future materials. These insights could ultimately inform the design of next-generation electronic, spintronic, and quantum devices where disorder is not just tolerated—but harnessed.

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