Catastrophes, Optical Multistabilities, and Chiral Photocurrent Hysteresis in Driven Weyl Semimetals

In the vast landscape of modern materials science, some substances behave less like ordinary solids and more like exotic fluids where electrons move with almost no resistance. Among these, a class of materials known as Weyl semimetals has captured the imagination of physicists. Inside these crystals, electrons do not follow the usual rules of heavy, sluggish movement; instead, they act as if they are massless, racing through the material at a constant, maximum speed determined by the crystal structure itself. This unique behavior gives rise to strange and powerful responses when the material is hit with light, particularly light that spins as it travels, known as circularly polarized light. Scientists have long been interested in how these materials might be used to create faster electronics or new types of sensors, but a critical piece of the puzzle remained missing: how does the material itself react to the electric fields it generates when light hits it? Just as a loudspeaker cone vibrates and creates its own sound waves that can interfere with the original signal, the electrons in a Weyl semimetal create their own electric fields that can either dampen or amplify the incoming light. Understanding this self-interaction is essential for predicting how these materials will actually perform in the real world.

Researchers at the California Institute of Technology and the University of Copenhagen have now mapped out this complex interaction, revealing that the material can behave in a surprisingly unpredictable way. They focused on what happens when these crystals are driven by light at frequencies comparable to the natural rhythm of the electrons inside, a range known as the terahertz regime. In their study, they found that the electrons do not simply respond in a straight line to the strength of the light. Instead, as the light gets stronger, the material's internal response changes dramatically, leading to a phenomenon called optical multistability. This means that for a single, specific strength of incoming light, the material can settle into one of three different internal states. It is as if the material has a memory of how the light was turned on or off, leading to a situation where the internal electric field can suddenly jump to a much higher level than the outside world would suggest, or drop back down, depending on the history of the light exposure.

The key to this behavior lies in the speed limit of the electrons. Because the electrons in a Weyl semimetal have a fixed maximum speed, they cannot accelerate indefinitely no matter how strong the light becomes. When the light is weak, the electrons move in a standard, predictable way, screening out the electric field much like a conductor normally would. However, as the light intensity increases, the electrons reach their speed limit and their motion saturates. This saturation causes the material's ability to screen the electric field to weaken in a nonlinear fashion. The researchers discovered that this weakening creates a feedback loop: the internal field becomes strong enough to amplify the light further, but only up to a point where the saturation kicks in and cuts off the growth. This delicate balance allows for a weak external signal to be amplified significantly inside the material, creating a powerful internal field that can be much larger than the one applied from the outside.

This amplification is not just a theoretical curiosity; it has direct consequences for the electric current the material produces. The study shows that this internal amplification leads to a massive boost in the photocurrent, the flow of electricity generated by light. In their simulations, the researchers found that the current could reach levels of roughly 100 amperes per square millimeter, a value far higher than what would be expected if the material were simply reacting to the external light without this internal feedback. Furthermore, the current exhibits a behavior known as hysteresis. If you slowly increase the strength of the light and then slowly decrease it, the current does not follow the same path back down. Instead, it traces a different loop, meaning the material's output depends on whether the light is getting stronger or weaker. This memory effect is a hallmark of systems with multiple stable states and suggests that these materials could be used to create optical switches or memory devices that operate at extremely high speeds.

The researchers also explored how the shape of the electron's energy path, known as the Weyl cone, influences these effects. In some materials, the cone is tilted, breaking a fundamental symmetry of the crystal. When this happens, the material becomes sensitive to the "handedness" of the light. If the light spins clockwise, the material amplifies the internal field and current one way; if it spins counter-clockwise, the response is different. This helicity-selective behavior means the material can distinguish between the two directions of light spin, amplifying one while suppressing the other. This effect arises from a subtle quantum property of the electrons that causes them to move sideways when pushed, a motion that becomes amplified by the same nonlinear screening mechanism. The study confirms that these effects are not limited to idealized, perfect crystals but are generic features of Weyl semimetals, appearing regardless of whether the material has certain symmetries or not.

To reach these conclusions, the team used a combination of advanced computer simulations and mathematical models that accounted for the chaotic dance of electrons colliding with each other and the crystal lattice. They modeled the material as a thin disk, a shape that mimics real-world samples, and calculated how the light-induced currents would create surface charges that generate their own electric fields. By solving the equations that describe the interaction between the light, the electrons, and the self-generated fields, they were able to predict the exact points where the system would jump between different stable states. Their work rules out the idea that these materials would simply act as passive conductors at high light intensities, showing instead that they are active participants in the optical process, capable of reshaping the light field within them. The findings suggest that by tuning the chemical composition of these materials to adjust the electron density, scientists could control the threshold at which these dramatic jumps occur, opening the door to new ways of manipulating light and electricity in the terahertz range.

The implications of this work extend beyond just understanding a new physical phenomenon. The ability to generate such large photocurrents with relatively weak external light, combined with the hysteresis and amplification effects, points toward a future where Weyl semimetals could be used in high-speed optical computing or sensing technologies. The researchers noted that these effects are observable in samples with a thickness of about 0.1 micrometers, which is thinner than the depth light penetrates at these frequencies, making the effect robust for practical devices. While the study was conducted through simulations and theoretical analysis, the parameters used were based on realistic experimental values, giving confidence that these dramatic behaviors can be observed in a laboratory setting. The work provides a clear roadmap for how to harness the unique topology of these materials, turning their exotic quantum properties into a powerful tool for controlling light and electricity in ways that were previously thought impossible.