Observation of orbital-angular-momentum-driven temperature modulation via the spin Peltier effect

In the solid world of materials science, controlling how heat moves is as vital as controlling how electricity flows. For decades, scientists have relied on the movement of electric charge to manage energy, but a newer frontier has opened up: the movement of angular momentum. Think of this not as a physical spinning top, but as an intrinsic property of electrons that allows them to carry a kind of rotational energy. When this rotational energy moves through a material, it can push heat around, creating cooling or heating effects at specific points. This field, known as spin caloritronics, has traditionally focused on "spin," a specific type of angular momentum. However, electrons possess another form of rotational energy called "orbital angular momentum," which arises from how they orbit the atomic nucleus. While theorists have long suspected this orbital motion could also drive heat transport, proving it in a real device has remained a significant challenge.

A team of researchers at the National Institute for Materials Science in Japan and the University of Tokyo has now provided the first direct experimental evidence that orbital angular momentum can indeed drive temperature changes. By constructing a specialized sandwich of thin films, they demonstrated that a flow of electric current can generate a stream of orbital angular momentum, which then converts into heat at a magnetic interface. This discovery opens a new pathway for managing heat in solid-state devices, suggesting that engineers might soon have a second, distinct tool—alongside spin—to control thermal energy.

The researchers built their experiment using a layered structure, starting with a magnetic insulator called yttrium iron garnet, followed by a thin sheet of platinum, and topped with a layer of copper that was allowed to naturally oxidize. The key to their success was a clever design: the copper oxide layer was not uniform. Instead, it was shaped like a wedge, meaning its thickness gradually changed from zero to sixteen nanometers across a single sample. This allowed the team to test how the effect changed as the top layer got thicker, all within one device to ensure consistent conditions. They passed an electric current through the platinum and copper layers and used a highly sensitive infrared camera to map the temperature changes on the surface of the magnetic layer below.

When the current flowed, the researchers observed a distinct pattern of heating and cooling that depended on the direction of the magnetic field. By carefully analyzing these temperature maps, they were able to separate the heat generated by the well-known "spin" effect from a new, additional contribution. They found that as the copper oxide layer grew thicker, the temperature modulation did not simply increase or stay the same. Instead, it rose to a sharp peak when the oxide layer was between six and seven nanometers thick, and then steadily declined as the layer became even thicker. This specific peak was the smoking gun. If the effect were caused by the bulk material of the copper oxide itself, the signal would have grown steadily with thickness. The fact that it peaked and then fell suggested that the effect was happening at a specific interface, likely where the metallic copper met the oxidized copper.

To confirm that this peak was indeed caused by orbital angular momentum and not some other artifact, the team performed a series of control experiments. They created samples where the copper was capped with a thin layer of aluminum to prevent it from oxidizing. In these samples, the special temperature effect vanished, leaving only the standard spin-based signal. This proved that the natural oxidation of the copper was essential for the new effect to occur. Furthermore, they inserted a thin barrier of aluminum between the platinum and the copper oxide. When this barrier was one nanometer thick, the effect persisted, but when it was two nanometers thick, the signal disappeared. This behavior confirmed that the orbital angular momentum was generated at the boundary between the copper and its oxide, traveled through the metal, and then converted into heat at the magnetic interface.

The magnitude of this new effect was striking. At the optimal thickness of the copper oxide layer, the heat modulation driven by orbital angular momentum was roughly three times stronger than the heat modulation driven by the conventional spin effect in the same device. This indicates that the process of converting electric current into heat via orbital motion is highly efficient in this specific configuration. The researchers also measured how far this effect could travel through the material, finding that the signal decayed over a distance of about eleven nanometers, a scale consistent with how orbital currents are known to move in metals.

These findings establish that orbital angular momentum is not just a theoretical concept but a real, measurable driver of heat transport in solids. By showing that the interface between metallic and oxidized copper can generate this effect, the study provides a clear design principle for future technologies. It suggests that by engineering the surfaces and interfaces of materials, scientists can harness orbital currents to create more efficient thermal management systems, potentially leading to new ways to cool electronic components or convert waste heat into useful energy. The work moves the field beyond a focus solely on spin, revealing a richer landscape of angular momentum that can be used to control the flow of energy in the solid state.