Scattering Alters Conductivity in Materials, Reversing Signals after One Driving Cycle

Azaz Ahmad and colleagues at the BITS Pilani-Hydrabad Campu in collaboration with Indian Institute of Technology Mandi, Indian Institute of Technology Bombay, SRM University and National Institute of Technology present a semiclassical theory detailing frequency-dependent magneto-optical transport, incorporating the effects of orbital magnetic moment, Weyl cone tilt, and intervalley scattering. Their analysis reveals how these elements influence the longitudinal magneto-optical conductivity, potentially suppressing the chiral anomaly under specific conditions. Frequency-dependent conductivity serves as a key set of tools for probing chiral relaxation in magneto-optical experiments operating across the MHz-THz range, offering new insights into the fundamental properties of these materials. Intervalley scattering explains sign reversal in Weyl semimetal conductivity The longitudinal magneto-optical conductivity (LMOC) in untilted Weyl semimetals experienced a sign reversal, transitioning from positive to negative values, when subjected to strong intervalley scattering in the weak ac regime. Previously, modelling this frequency-dependent behaviour required approximations unable to consistently account for key factors like orbital magnetic moment and band tilt. This reversal, indicative of suppressed chiral anomaly, a fundamental symmetry in physics, was impossible to predict accurately without a thorough theoretical framework. The chiral anomaly, a quantum mechanical effect, manifests as an imbalance in the number of left- and right-handed Weyl fermions, leading to an anomalous current; its suppression has significant implications for understanding charge transport in these materials. Researchers at Austin developed a semiclassical Boltzmann theory to model electron behaviour, incorporating orbital magnetic moment, Weyl cone tilt, and intervalley scattering to determine the full conductivity tensor under a static magnetic field. A sign reversal of the longitudinal magneto-optical conductivity (LMOC) occurs in untilted Weyl semimetals when intervalley scattering is strong and the alternating current (ac) driving force is weak. This suppression of the chiral anomaly, a fundamental property linked to asymmetry in particle physics, was demonstrated through a new semiclassical Boltzmann theory incorporating the orbital magnetic moment of electrons. The Boltzmann theory, a cornerstone of non-equilibrium statistical mechanics, allows for the calculation of transport coefficients like conductivity by averaging over the distribution of electron momenta and energies. The inclusion of a scattering matrix within this framework accurately describes the rate of electron scattering between different energy bands and momenta, crucial for modelling intervalley scattering. The moment induces linear responses to magnetic fields, differing from the quadratic responses caused by the chiral anomaly itself. Furthermore, the direction of band tilt, the distortion of the energy bands within the material, sharply alters the LMOC, with transverse tilt creating symmetric behaviour and parallel tilt resulting in asymmetric responses. In particular, a negative LMOC appears naturally with parallel tilt, but requires the presence of orbital magnetic moment for transverse tilt. This highlights the interaction of these factors, and these findings are applicable across a frequency range of MHz-THz. The Weyl cone tilt arises from the breaking of inversion symmetry in the material, leading to an asymmetry in the energy dispersion of the Weyl fermions. This tilt significantly influences the electron dynamics and, consequently, the magneto-optical response. The observed asymmetry in LMOC with parallel tilt suggests a preferential direction for electron transport, potentially exploitable in device applications. Terahertz conductivity modelling clarifies chiral anomaly suppression in Weyl semimetals Weyl semimetals, materials exhibiting unusual electronic behaviour, are receiving increasing attention from scientists due to their potential applications in next-generation devices. Accurately modelling their response to electromagnetic forces at very high frequencies, specifically in the MHz-THz range, has proven challenging, however. Existing theories often simplify the complex interaction of factors governing electron movement, limiting their predictive power. These materials are characterised by linearly dispersing electronic bands known as Weyl cones, which give rise to unique topological properties and allow for the existence of chiral fermions. The topological protection of these surface states makes Weyl semimetals promising candidates for spintronic and quantum computing applications. This modelling reveals that while strong scattering between electron energy levels can suppress a fundamental symmetry known as the chiral anomaly, the precise conditions required for this suppression remain unclear. Understanding how Weyl semimetals respond to electromagnetic radiation at terahertz frequencies is important for developing faster, more efficient electronic components. This modelling provides a framework for interpreting experimental results and refining material designs. The terahertz frequency range is particularly relevant as it corresponds to the energy scale of many collective electronic excitations in these materials, such as plasmons and interband transitions. Accurately capturing the dynamics at these frequencies is essential for predicting the material’s optical and electronic properties. The University of Arkansas at Austin scientists detailed how electron scattering can suppress a key symmetry, the chiral anomaly, within Weyl semimetals, which exhibit unique electronic properties. Linking frequency to conductivity, this modelling provides a diagnostic tool for material analysis and offers insights that will begin to accelerate the development of advanced electronic devices. A semiclassical Boltzmann theory capable of modelling frequency-dependent magneto-optical transport within Weyl semimetals, a class of materials exhibiting unique electronic characteristics, has been established. Collisions between electrons in different energy levels, known as intervalley scattering, alongside factors like orbital magnetic moment, are incorporated into the theory, providing a more complete picture of electron behaviour than previous approximations. The findings demonstrate that strong intervalley scattering can reverse electrical conductivity under specific conditions, suppressing the chiral anomaly, a fundamental symmetry in particle physics, and offering a new method for probing chiral relaxation. The ability to control and suppress the chiral anomaly through material design and external stimuli opens up possibilities for novel electronic devices with tailored functionalities. Further research will focus on extending this theory to incorporate more complex scattering mechanisms and exploring the effects of disorder and impurities on the magneto-optical response of Weyl semimetals. The research established a semiclassical Boltzmann theory modelling frequency-dependent magneto-optical transport in Weyl semimetals. This modelling incorporates electron collisions, known as intervalley scattering, and demonstrates how these collisions can reverse electrical conductivity and suppress the chiral anomaly. Understanding this interplay between scattering and fundamental symmetries provides a new method for probing chiral relaxation within these materials. The findings offer insights that may begin to accelerate the development of advanced electronic devices, and future work will explore more complex scattering mechanisms within Weyl semimetals. 👉 More information🗞 Semiclassical theory of frequency dependent linear magneto-optical transport in Weyl semimetals🧠 ArXiv: https://arxiv.org/abs/2604.11527 Tags: