Gate-tunable large second-order and third-order nonlinear Hall effect in topological insulator BiSbTeSe2

In the world of electronics, the way electricity moves through a material is often governed by strict rules of symmetry. Imagine a road where traffic can only flow in one direction unless a specific barrier is removed; this is similar to how certain electrical effects, like the classic Hall effect, require a magnetic field to break the natural balance of a system and push electrons sideways. However, physicists have discovered a more subtle family of effects that do not need a magnetic field at all. These are called nonlinear Hall effects. Instead of a simple, straight-line relationship between the current flowing through a wire and the voltage it produces, these effects create a response that changes in a more complex way when the current is pushed harder or made to oscillate. This happens when the material itself lacks a specific type of mirror symmetry, allowing electrons to scatter in a way that generates a sideways voltage even without magnets. The ability to control these effects with a simple electrical signal, rather than bulky magnets, holds great promise for creating faster, more efficient electronic devices that can process information in new ways.

Researchers at Nanjing University have now found a way to make these effects much stronger and easier to control in a specific type of material known as a topological insulator. This material, called BiSbTeSe2, is special because its interior acts as an insulator, blocking electricity, while its surface acts as a highly efficient conductor. The team built tiny devices using thin flakes of this material, sandwiching them between layers of other materials to create a dual-gate system. This setup allowed them to act like a precise tuner, using two separate electrical gates to adjust the flow of electrons on the surface without disturbing the rest of the crystal. By sending an alternating current through these devices and measuring the resulting voltages, they observed two distinct types of nonlinear Hall effects: a second-order response and a third-order response. These terms refer to how the voltage signal relates to the input current; the second-order signal appears at twice the frequency of the input, while the third-order appears at three times the frequency.

What makes this discovery particularly significant is the degree of control the researchers achieved. They found that by simply adjusting the voltage on their gates, they could turn the strength of these signals on and off, or even flip their direction. This level of tunability is a major step forward, as previous attempts to observe these effects in similar materials were often difficult to control. The team measured the efficiency of these signals and found that the second-order effect was remarkably strong, far exceeding what had been seen in other topological insulator systems. Perhaps most impressively, this strong signal remained stable even at room temperature, a condition where many delicate quantum effects usually disappear. The third-order effect was also observed with a strength comparable to the best results reported in other materials, marking the first time such a signal has been detected in a strong three-dimensional topological insulator.

To understand what was causing these signals, the researchers analyzed how the voltage changed as they varied the electrical conductivity of the material. This analysis suggested that the second-order effect arises from a combination of two mechanisms: one where electrons are deflected by impurities in a specific way, and another intrinsic property related to the shape of the electron energy bands. The third-order effect, meanwhile, appears to stem from a different intrinsic property of the electron waves and a higher-order version of the impurity deflection. The team also ruled out several alternative explanations, such as heating effects or simple electrical artifacts, confirming that the signals were genuine quantum phenomena originating from the unique surface states of the material.

The behavior of these signals changed in interesting ways as the temperature shifted. While the second-order signal persisted up to 300 Kelvin, the third-order signal faded as the temperature rose, eventually vanishing at lower temperatures than its counterpart. This difference suggests that the third-order effect is more sensitive to the dominance of the surface electrons, which become less influential as heat causes the bulk of the material to conduct electricity more freely. The researchers also examined the direction of the signals by rotating the device and found that the third-order response followed a specific two-fold pattern, which aligns with theoretical predictions for this type of effect, although the exact microscopic reason for the symmetry breaking remains an open question.

These findings establish a robust platform for future electronic components. The ability to generate strong, tunable nonlinear signals at room temperature in a material that is easy to manipulate suggests a clear path toward practical applications. Such devices could be used to build high-frequency rectifiers, which convert alternating current to direct current, or frequency multipliers that generate new signal frequencies. By proving that these effects can be engineered and controlled in a stable, room-temperature environment, the work opens the door to a new generation of nonlinear electronic logic devices that operate without the need for external magnetic fields.