Magnetoelectric Bulk Photovoltaic Effect in Centrosymmetric Solids

For decades, a fundamental rule in the physics of light and matter has held firm: to turn light directly into a steady electric current inside a solid material, the material must lack a specific kind of symmetry. Imagine a crystal structure that looks the same if you flip it inside out; scientists believed such "centrosymmetric" materials could not generate this type of electricity when hit with light. This limitation has long defined the search for new ways to harvest solar energy or detect light, forcing researchers to look only at materials with broken symmetry. The prevailing understanding was that the electric field of light pushes electrons in a way that requires this structural imbalance to create a net flow of charge.

However, light is not just an electric wave; it also carries a magnetic component that oscillates in sync with the electric field. While the electric push is strong, the magnetic push is usually so weak that it has been largely ignored in the context of generating electricity. The question remained: could this tiny magnetic force, acting together with the electric force, do something new? Specifically, could the interaction between these two fields of light generate a current in materials that were previously thought to be incapable of producing one? This question sits at the heart of a new study that challenges the old rules of how light creates electricity in solids.

A team of researchers has now demonstrated that this is indeed possible. They describe a new phenomenon they call the magnetoelectric bulk photovoltaic effect. In simple terms, they found that when light hits certain solid materials, the combined push of its electric and magnetic fields can drive a steady electric current, even if the material is perfectly symmetric and has no internal magnetic order of its own. This discovery overturns the long-held belief that breaking spatial symmetry is a strict requirement for generating light-induced currents. The researchers showed that this effect arises from a subtle quantum interaction where the magnetic part of the light couples with the electron's spin, creating a flow of electricity that was previously invisible to theory.

To understand how this works, one must look at the microscopic behavior of electrons. In these materials, the electrons do not just sit still; they possess a kind of geometric shape in their quantum state, described by properties known as the quantum metric and Berry curvature. When light strikes the material, the electric field tries to move the electrons, while the magnetic field interacts with their spin. The researchers found that the interplay between these two forces creates a "bilinear" coupling—a joint action where the electric and magnetic fields work together to push electrons in a specific direction. This process generates four distinct types of current, depending on whether the light is linearly or circularly polarized and whether the material preserves or breaks time-reversal symmetry. Crucially, because the magnetic field of light behaves differently under spatial inversion than the electric field does, this new mechanism is allowed to exist in materials that were previously ruled out.

The team did not just propose a theory; they performed detailed calculations to see if this effect could be measured in real materials. They focused on two specific examples: a non-magnetic material called monolayer Pt2HgSe3 and a magnetic material called RuF4. For the non-magnetic material, they simulated how the current would behave under linearly polarized light. Their calculations predicted a current density of approximately 467 microamperes per meter. While this number might seem small, it is comparable to currents measured in other well-known photovoltaic effects, suggesting it is large enough to be detected in a laboratory setting. The direction of this current was found to depend on the angle of the incoming light, creating a predictable pattern that could be used to identify the effect.

The results were even more striking for the magnetic material, RuF4, which possesses a specific type of magnetic order known as altermagnetism. In this material, the researchers predicted a current density on the order of 1 milliampere per meter, which is significantly larger than what they found in the non-magnetic example. What makes this finding particularly powerful is the control it offers. The study showed that by simply flipping the direction of the light's spin (its helicity) or by flipping the internal magnetic direction of the material, the electric current would reverse its direction. This means the current acts as a direct electrical readout of the magnetic state, a feature that could be invaluable for future technologies that need to sense or switch magnetic information using light.

The researchers also explored how this effect behaves in different types of light. They found that while the current in the non-magnetic material responds strongly to the angle of linearly polarized light, the current in the magnetic material can be switched on and off or reversed by changing the light from linear to circular polarization. This sensitivity to the polarization of light provides a clear signature that experimentalists can look for. The team emphasized that while the magnetic component of light is typically much weaker than the electric component, the specific quantum geometry of these materials amplifies the effect enough to make it observable. They noted that the magnitude of the current depends heavily on the frequency of the light, peaking at specific energies where the material absorbs the light most efficiently.

This work extends the known boundaries of the bulk photovoltaic effect, a phenomenon that has been studied for decades but was thought to be restricted to a narrow class of asymmetric materials. By proving that centrosymmetric solids can also generate these currents, the study opens up a vast new landscape of materials for investigation. It suggests that many common materials, which were previously ignored for this purpose, might actually be capable of converting light into electricity through this magnetoelectric mechanism. The findings also provide a new tool for probing the quantum geometry of materials, allowing scientists to map out the hidden shapes of electron states by measuring the currents they produce.

The study concludes by highlighting the potential for these effects to be used in electrical readouts of magnetic orders, particularly in the emerging field of altermagnetism. The ability to switch a current simply by changing the light's properties offers a pathway to new types of sensors and switches that operate without the need for external magnetic fields. While the current work is based on theoretical simulations and calculations, the predicted current densities are within the range of what modern experimental equipment can measure. The researchers suggest that the next step is to build these materials and shine light on them to see if the predicted currents appear in the real world, potentially confirming a new chapter in the physics of light and matter.