CCB Research: New Nonlinear Hall Effect Discovered
Antiferromagnet unveils novel Nonlinear Hall Effect enabling wireless energy
In a groundbreaking scientific achievement, a team of Harvard scientists have harnessed the unique properties of antiferromagnetic materials and the quantum metric to unveil a novel nonlinear Hall effect. This breakthrough not only expands our understanding of fundamental physics but also paves the way for exciting advancements in electronics and quantum computing.
This discovery began by examining the duality between the fundamental laws of motion for the largest (stars, galaxies) and the smallest (an electron) objects in our universe. The theory of general relativity teaches us that the motion of stars and galaxies are governed by the geometry of the space-time. In empty space, the space-time geometry is completely flat, but in the vicinity of giant mass, the space-time geometry warps, leading to striking astronomical phenomena such as the gravitational waves and the gravitational lens. In this work, published in Science, the team of Harvard scientists discovered that the motion of the smallest particle (electron) is governed by a similar geometry, known as the quantum metric. This leads to fundamentally new motions of electrons known as the nonlinear Hall effect.
“The nonlinear Hall effect helps us to understand fundamental questions about electron motion,” Suyang Xu, Assistant Professor of Chemistry. “It has a lot of potential applications including a new way to conduct wireless energy harvesting.”
The researchers detailed how they discovered the nonlinear Hall effect by inducing quantum metric by interfacing an even-layered antiferromagnetic material with black phosphorous. The quantum metric is a mathematical tool that helps us measure how far apart or close different quantum states are from each other. In their investigation, the team was able to successfully harvest wireless electromagnetic energy via the non-linear Hall effect, which holds promise for the transfer of power wirelessly.
The work was undertaken by several members of Xu’s lab, including Anuyan Gao, Yu-Fei Liu, Damien Berube, Jian-Xiang Qiu, Houchen Li, Christian Tzschaschel, Thao Dinh, Zhe Sun, and Sheng-Chin Ho, in collaboration with physicists and scientists from several other universities.
The Hall effect, a well-known phenomenon, describes the perpendicular voltage generated when from an electric current flow. It causes charges to accumulate on one side of the conductor, creating a voltage known as the Hall voltage. This effect is used in Hall sensors to measure the strength of magnetic fields and determining charge carriers in materials. These sensors are found in many electronic devices, including phones, compasses, and steering wheels.
Typically, the Hall voltage increases in a linear manner as the applied electric field strength rises. New research by the team has uncovered an intriguing departure from this conventional behavior by utilizing antiferromagnets.
Antiferromagnets, unlike traditional magnets, possess unique properties that make them ideal candidates for this innovative application. While ferromagnets are commonly associated with magnets that attract or repel, antiferromagnets have an intriguing characteristic: their magnetic moments align in a way that cancels out their overall magnetism. This property makes AFMs a promising material in quantum computing.
In their experiment, the researchers designed and fabricated a device consisting of a thin film of antiferromagnetic material, MnBi2Te4, sandwiched between two layers of black phosphorous. When exposed to a current at cold temperatures, the researchers observed a nonlinear relationship between the Hall voltage and the magnetic field strength, defying the linear pattern observed in traditional Hall effects. This novel nonlinear Hall effect switches direction upon reversing the antiferromagnet spins and exhibits distinct scaling that suggests a non-dissipative nature. The implications of this finding are far-reaching with the potential to discover quantum metric responses.
The team also utilized the antiferromagnetic nonlinear Hall effect to convert electromagnetic radiation into DC electricity without a magnetic field. They injected microwave radiation into the platform, which absorbed the energy and converted it into an electrical current.
“Without any optimization, our device was able to turn ~40% of the energy absorbed into DC electricity,” Xu said. “The frequency dependence is very flat, extending to tens of gigahertz and is only limited by the frequency available from our source.”
The ability to harvest electromagnetic energy using antiferromagnets opens a plethora of possibilities for enabling next-generation wireless communication and powering electronic devices, including phones, laptop computers, and wearable technology. This breakthrough could also enable the development of devices that operate autonomously without the need for battery replacements or external power sources.
Going forward, Xu is looking at ways to grow these materials that can be larger in lateral dimensions and designing an external antenna that can collect these microwaves more efficiently.
“We are going to be conducting similar experiments with different materials” Anuyan Gao said. “We want to demonstrate that the quantum metric is general like the Berry Curvature and not just specific to the material.”
The abstract is available below and the article is available at Science.
ABSTRACT
Quantum geometry in condensed matter physics has two components: the real part quantum metric and the imaginary part Berry curvature. Whereas the effects of Berry curvature have been observed through phenomena such as the quantum Hall effect in 2D electron gases and the anomalous Hall effect (AHE) in ferromagnets, quantum metric has rarely been explored. Here, we report a nonlinear Hall effect induced by quantum metric dipole by interfacing even-layered MnBi2Te4 with black phosphorus. The quantum metric nonlinear Hall effect switches direction upon reversing the AFM spins and exhibits distinct scaling that is independent of the scattering time. Our results open the door to discovering quantum metric responses predicted theoretically and pave the way for applications that bridge nonlinear electronics with AFM spintronics.