Quantum Acoustics Reveals Vibronic Interactions in Strange Metals, Challenging Planckian Resistivity Models

The unusual electrical behaviour of strange metals has long puzzled physicists, with traditional explanations based on interactions between electrons and atomic vibrations, known as phonons, proving inadequate. Eric J. Heller from Harvard University, Alhun Aydin from Sabanci University, and Anton M. Graf, also from Harvard, alongside colleagues including Joost de Nijs, Yoel Zimmermann, and Xiaoyu Ouyang, now demonstrate that a fundamentally different approach, treating electron-lattice interactions dynamically and comprehensively, unlocks the secrets of these materials. Their work establishes a powerful connection between quantum acoustics and the behaviour of strange metals, revealing that strong interactions between vibrations and electrons drive key phenomena, including a linear relationship between resistance and temperature, the formation of polarons and charge density waves, and the resolution of long-standing anomalies in infrared light absorption. This research not only recovers established theories from a foundational model, but also uncovers a universal principle of diffusion in dynamic environments, offering a more fundamental understanding of electron behaviour than previously thought.

Collective Dynamics of Interacting Active Particles

Researchers investigate the collective behaviour of active matter, focusing on how self-propelled particles create coherent motion and patterns. The team aims to understand how interactions between these particles give rise to large-scale behaviours, such as flocking, swarming, and collective transport. This work builds upon previous studies, extending the analysis to incorporate more realistic particle shapes and the effects of fluid dynamics. The approach combines theoretical modelling, computer simulations, and experimental observations. Researchers develop a mathematical framework based on active nematic liquid crystals to describe the system’s large-scale behaviour, predicting the formation of stable, self-organised patterns like asters and spirals.

Computer simulations confirm these predictions and explore conditions difficult to study analytically, while experiments utilise colloidal particles to allow direct observation of particle movement and collective dynamics. Specific findings include the discovery of a new instability mechanism leading to temporary, highly ordered structures. The team identifies a critical interplay between particle shape, density, and interaction strength governing the system’s stability, showing that elongated particles align more readily, promoting long-range order. Furthermore, researchers quantify how interactions with the surrounding fluid affect collective motion, revealing that these interactions can significantly enhance the speed and efficiency of transport. This work provides new insights into active matter and has implications for areas including micro-robotics, bio-inspired materials, and understanding collective animal behaviour.

Quantum Acoustics and Planckian Dissipation in Metals

This research presents a comprehensive exploration of quantum acoustics and its application to understanding electron transport in metals, particularly in strongly correlated materials. The central idea is to treat lattice vibrations, or phonons, not simply as a source of scattering, but as a quantum system influencing electron dynamics, moving beyond traditional models and incorporating quantum effects. A key finding is the emergence of Planckian scattering rates in many materials exhibiting linear-in-temperature resistivity, suggesting a universal mechanism for energy dissipation governed by fundamental constants. The work proposes that phonons play a crucial role in establishing this Planckian regime, utilising path integrals to acknowledge that electron behaviour is not instantaneous, but influenced by past states.

The Stratonovich-Feynman formalism proves to be a powerful tool in this analysis. The interplay between electron localisation due to disorder and phonon interactions is a fascinating aspect of this work, suggesting that phonons can disrupt localisation, leading to improved conductivity. The use of machine learning to study this dynamic is innovative, with the predicted shift in the Drude peak, related to electrical conductivity, providing a testable prediction. The work’s comprehensive theoretical foundation, emphasis on universality, connection to experiment, and innovative techniques represent significant strengths.

Planckian Slope Reveals Electron-Lattice Coupling

Scientists have achieved a breakthrough in understanding strange metals, demonstrating that accurate treatment of electron-lattice interactions can explain several long-standing mysteries. The research reveals that a coherent treatment of these interactions, using a framework termed “quantum acoustics,” accurately predicts phenomena previously unexplained by conventional theories, with experiments demonstrating that materials can exceed a theoretical limit on resistivity and exhibit suppression of a characteristic feature of metallic conductivity. The team measured linear-in-temperature resistivity extending down to at least 50 Kelvin, and even lower under strong magnetic fields, explaining this previously unexplained linear relationship as a result of the coherent interplay between electrons and lattice vibrations. Data shows the spontaneous formation of polarons and charge density waves within computer simulations, superseding traditional approaches, with researchers establishing that the quantum acoustics framework successfully implements a model describing electron-phonon interactions, extending beyond it to accommodate more complex models.

The work accurately predicts a Planckian relaxation time and explains the persistence of linear resistivity down to extremely low temperatures, providing a mechanism for imposing symmetry on electronic properties, resolving a previous theoretical challenge. Simulations reveal that the number of lattice vibrations at room temperature is substantial, remaining significant even at very low temperatures, justifying the use of a coherent state basis for describing lattice vibrations. This work establishes a new foundation for understanding the complex behaviour of strange metals and opens avenues for exploring novel materials with tailored electronic properties.

Nonperturbative Vibronic Interactions Explain Strange Metals

Researchers have demonstrated that established understandings of electron-lattice interactions, specifically concerning phonons, require revision to fully explain the behaviour of strange metals. The team challenges previous conclusions based on static and simplified approaches, arguing these overlooked crucial time-dependent and complex physics, revealing that strong vibronic interactions play a fundamental role in several observed phenomena, including the formation of polarons and charge density waves. This work derives Planckian transport, polarons, charge density waves, and electronic properties from a model describing electron-phonon interactions, demonstrating that previously hidden physics emerges when the model is treated accurately. Furthermore, the research uncovers a generalization of electron localisation to dynamic media, identifying a universal Planckian diffusion that appears as a consequence of electron localisation and provides a more fundamental concept than the previously proposed ‘Planckian speed limit’. The findings explain the absence of a low-temperature resistivity rise in two-dimensional systems and bypass a theoretical limit on resistivity, offering a pathway to understanding linear-in-temperature resistivity extending to zero Kelvin. The authors acknowledge that further investigation is needed to fully explore the implications of Planckian diffusion and its connection to strongly correlated systems, and to determine its broader ubiquity across different materials, suggesting that this research provides a new framework for understanding the behaviour of electrons in solids and opens avenues for exploring novel quantum phenomena.