Light exposure switches heat flow in barium titanate by altering its structure, offering a reversible method for dynamic thermal control in ferroelectric materials.
(Nanowerk Spotlight) We can switch off a current with a flick of a transistor, route electrons with atomic precision, and shape complex digital systems from streams of moving charge. But heat, the stray byproduct of all this activity, remains untamed. In laptops, phones, power grids, and data centers, heat builds up and lingers. It resists redirection or control. The result is waste, wear, and inefficiency that grows with the complexity of the devices we build.
This imbalance reflects a deeper asymmetry in modern materials science. Electricity flows through mobile charges that respond easily to external forces. Heat moves through vibrating atoms whose behavior is locked into the structure of the crystal. While we have learned to manipulate electrons with fields and switches, phonons, the quantum vibrations that carry heat, have proven far less cooperative.
To change how heat moves through a solid, researchers have usually relied on permanent strategies. They add disorder, introduce embedded structures, or chemically alter the material. These methods can lower thermal conductivity, but once applied, they cannot be reversed. The thermal behavior of the material is fixed.
The real challenge is to make thermal conductivity something that can be controlled dynamically. Materials that respond to changing conditions could help cool electronic components automatically, improve energy harvesting, or reduce thermal waste in miniature devices. Achieving that level of control would require a way to influence phonons directly.
One route involves phase transitions, in which a material shifts from one atomic arrangement to another. These transitions can sometimes be triggered by electric fields or strain, and they often change how phonons behave. If the shift can be controlled in real time, then so can the material’s thermal conductivity.
In a new study published in Advanced Functional Materials (“Optical Control of the Thermal Conductivity in BaTiO3“), a group of physicists in Spain reports that light can be used to do exactly this. Working with the well-known material barium titanate, they show that light alone can trigger a structural transformation that alters how heat moves through the crystal.
By simulating how injected charge modifies atomic interactions, the team reveals a reversible mechanism for switching thermal conductivity with precision. This approach requires no heating, no structural damage, and no permanent modification of the material.
The authors used quantum mechanical simulations to study how barium titanate responds to photoexcitation. This material is a ferroelectric, meaning it naturally exhibits a built-in electric polarization in its low-temperature phase. At room temperature, barium titanate forms a tetragonal structure with an asymmetric arrangement of atoms. The direction of polarization aligns with the elongated axis of this structure. However, above a certain temperature known as the Curie point, the crystal becomes cubic and symmetric, and the polarization disappears.
In their study, the researchers focused on a different way to reach this nonpolar cubic phase. Instead of increasing temperature, they simulated what happens when light injects electrons into the material. These electrons populate higher-energy states and weaken the long-range interactions that stabilize the polar structure. When the photoexcited charge density exceeds a threshold, the crystal relaxes into a nonpolar cubic phase even at room temperature.
This transformation has a direct impact on how heat flows through the material. In its polar state, barium titanate supports a low-frequency vibrational mode called a soft phonon. This mode contributes to thermal transport and strongly interacts with other phonons. When the structure becomes cubic, the soft mode disappears, and the resulting changes in phonon scattering lead to a reduction in thermal conductivity.
Panels a and b show how the atomic vibrations in barium titanate change under light exposure. In panel a, under dark conditions, certain low-frequency vibrations are unstable, supporting the material’s polar structure. In panel b, after light injects charge into the crystal, these unstable vibrations are suppressed, stabilizing a nonpolar structure that alters how heat moves through the material. (Image: Reprinted from DOI:10.1002/adfm.202425424, CC BY)
The researchers quantified this effect by calculating the lattice thermal conductivity of the material for different levels of photoexcited charge. They found that at charge densities just above the threshold needed to induce the phase transition, the conductivity dropped sharply, especially in the directions perpendicular to the original polarization axis. In these directions, the conductivity decreased by about thirty percent. As the charge density increased further, the effect became less pronounced. At very high levels, the conductivity began to recover or even increase in the direction parallel to the former polarization axis.
This non-monotonic behavior reflects a balance between two competing effects. On one hand, the removal of the soft mode increases phonon scattering, which lowers thermal conductivity. On the other hand, the structural change also reduces the volume of the material, which tends to stiffen the lattice and increase the speed of sound. Faster phonons usually carry heat more efficiently. The net result depends on which effect dominates. At low excitation levels, scattering wins out, and heat flow decreases. At higher levels, the stiffening begins to counteract the scattering losses.
To explore this balance further, the researchers computed the phonon lifetimes, which indicate how long each vibration persists before being scattered. They compared lifetimes in the polar tetragonal phase and the light-stabilized cubic phase. At low excitation, lifetimes in the cubic phase were consistently shorter, confirming the dominance of anharmonic scattering. At higher excitation, this difference diminished, and in some cases reversed. The phonons in the cubic phase became more stable, explaining the observed recovery in thermal conductivity.
They also compared the light-induced cubic phase to the conventional high-temperature cubic phase that forms when barium titanate is simply heated above its Curie point. Although both phases share the same crystal symmetry, they are stabilized by different mechanisms. The high-temperature phase arises from thermal motion, while the light-induced phase results from electronic excitation.
This difference shows up in the behavior of phonons. The high-temperature phase exhibits stronger scattering at low frequencies, leading to lower thermal conductivity. In contrast, the light-induced phase retains longer phonon lifetimes and carries heat more effectively, despite having similar structure.
One important consideration is the possible contribution of electrons to thermal transport. In semiconductors and insulators, this contribution is usually negligible, but photoexcitation introduces mobile charges. To test this, the researchers computed the electronic thermal conductivity separately. They found it was orders of magnitude smaller than the phonon contribution, even under illumination. This confirms that the changes in thermal behavior are driven by phonons, not by electrical conduction.
These findings suggest that light offers a powerful tool for controlling heat transport in ferroelectrics. The approach is reversible, tunable, and fast, and it does not rely on altering the material’s composition or waiting for thermal equilibration. It also avoids the energy costs associated with heating or mechanical deformation.
The significance of this work extends beyond barium titanate. Similar mechanisms may operate in other materials where polarization depends on long-range electrostatic interactions. In such systems, photoexcited charges could screen these interactions and trigger structural changes that affect phonon dynamics. Potential candidates include sodium niobate and solid solutions involving potassium niobate or barium titanate itself. However, not all ferroelectrics are suitable. In some, such as lead titanate, the polarization is too robust to be suppressed by light alone.
The study adds to a small but growing body of research on photoinduced phase transitions and their effects on heat transport. While these effects have been studied in low-dimensional materials and charge density wave systems, their exploration in ferroelectrics is still in early stages. This work provides a clear demonstration that structural changes induced by light can lead to substantial and reversible modulation of thermal conductivity.
By treating phonons as tunable elements rather than passive carriers of heat, the authors open the door to new forms of thermal control. In principle, materials designed with this capability could be used in adaptive thermal switches, energy conversion devices, or electronics that respond intelligently to their own temperature profiles. The findings point to a future in which light, already used to control charge and spin, can also be used to reshape how solids conduct heat.
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Riccardo Rurali (Institut de Ciència de Materials de Barcelona)
, 0000-0002-4086-4191 corresponding author
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