Researchers Discover Second-Order Nonlinear Valley Nernst Effect Originating from Quantum Metric

The flow of heat can, under specific conditions, generate electrical current, a phenomenon known as the Nernst effect, and researchers now demonstrate a unique variation of this effect in a specific material. Ying-Li Wu, alongside Jia-Liang Wan from Hunan University and Xiao-Qin Yu, investigate the nonlinear valley Nernst effect in strained bilayer graphene, revealing an intrinsic electrical current generated perpendicularly to an applied temperature gradient. This current arises from fundamental quantum properties of the material, independent of imperfections that usually hinder such effects, and offers a potentially robust pathway for novel thermoelectric devices. The team’s theoretical work shows that manipulating strain within the graphene layers allows control over the direction of this current, opening possibilities for designing materials with tailored thermoelectric responses.

This current originates from fundamental quantum mechanical properties, notably the quantum metric, and crucially, remains unaffected by electron relaxation time, indicating a robust and inherent origin. The effect requires both inversion and time-reversal symmetries, with a single mirror symmetry proving essential for its emergence in two-dimensional materials. Understanding and controlling this effect is vital for developing next-generation electronic technologies that exploit the valley degree of freedom, potentially leading to lower power consumption and enhanced functionality.

The research demonstrates that this effect emerges in uniaxially strained gapless bilayer graphene when a temperature gradient is applied. Altering the strain from compressive to tensile reverses the direction of the generated current, offering a potential method for controlling the effect. This investigation explores the fundamental relationship between symmetry, strain, and the emergence of this nonlinear effect, providing insights into the material’s quantum mechanical properties.

Nonlinear Transport and Quantum Effects in 2D Materials

This research focuses on nonlinear thermoelectric and transport phenomena in two-dimensional materials, particularly graphene. It explores the origins of these effects, the role of material properties like band structure, strain, and topology, and their potential use in novel devices. A central theme is the importance of the quantum metric and Berry curvature in generating these nonlinear responses.

Key concepts explored include nonlinear thermoelectric effects, where materials generate voltages or currents in response to temperature gradients in a nonlinear fashion, and the nonlinear Hall effect, which generates a transverse voltage in response to an applied electric field in a nonlinear way. The quantum metric describes how the wavefunction of an electron changes with momentum and plays a crucial role in determining nonlinear transport properties. Related to this is Berry curvature, which arises from the phase of the electron wavefunction and influences its motion. Researchers also investigate topological materials, materials with unique electronic band structures, and strain engineering, which modifies material properties by applying mechanical stress.

The research highlights that the quantum metric and Berry curvature are fundamental to understanding and controlling nonlinear transport phenomena in two-dimensional materials. They provide a mechanism for generating nonlinear responses absent in conventional materials. Strain engineering proves to be a viable method for tuning electronic band structure and enhancing nonlinear transport effects. Topological properties also play a significant role in determining these nonlinear responses. These nonlinear effects offer possibilities for novel devices with enhanced performance and new functionalities.

Understanding nonlinear responses is crucial for developing advanced materials and devices that overcome the limitations of linear response theory. A deep theoretical understanding of the underlying physics, including the quantum metric, Berry curvature, and topology, is essential for designing and optimising materials for these applications.

Nonlinear Valley Current From Quantum Metric Properties

This research presents a theoretical analysis of the nonlinear valley Nernst effect, demonstrating the generation of a valley current perpendicular to an applied temperature gradient in materials possessing specific symmetries. The study reveals that this current originates from quantum mechanical properties, specifically the quantum metric, and is independent of electron relaxation time, suggesting a robust and fundamental origin. Importantly, the effect requires both inversion and time-reversal symmetries to be present, and a single mirror symmetry is identified as crucial for its emergence in two-dimensional systems.

Researchers investigated this effect in strained bilayer graphene, finding that the nonlinear valley Nernst effect can be induced by applying a temperature gradient perpendicular to the direction of strain. They observed that changing the strain from compressive to tensile reverses the sign of the generated current, offering a potential avenue for controlling this effect. Future work could focus on exploring the effect in different materials and investigating the influence of external factors, potentially leading to novel thermoelectric devices.