The work, published in Frontiers in Thermal Engineering by Huimin Huang and Qiang Cheng of the State Key Laboratory of Coal Combustion at Huazhong University of Science and Technology, combines first-principles density functional theory calculations with fluctuation-dissipation theory to model two suspended χ3-phase borophene sheets separated by a vacuum gap of 10 nanometers. The researchers applied uniaxial strain ranging from −4 percent compression to +4 percent tension along either the x-axis, which corresponds to the borophene lattice direction a, or the y-axis, perpendicular to it. Their central finding is stark: the direction of strain matters as much as its magnitude. Straining along the x-axis produced a nonlinear response, with the normalized heat transfer coefficient first dipping slightly below unity and then climbing to 1.72 at +4 percent tension. Straining along the y-axis, by contrast, drove a monotonic, switch-like surge, with the coefficient reaching 8.96 at the same tension—roughly a ninefold amplification of near-field heat flow.

The physical mechanism behind this anisotropy lies in borophene’s dielectric function, the quantity that describes how the material’s electrons respond to electromagnetic fields. Borophene is intrinsically anisotropic: its optical response along the lattice direction differs sharply from the perpendicular direction, a property the team confirmed by calculating the complex dielectric function over the frequency range of 0 to 4 electronvolts. Under x-axis compression, the real part of the dielectric function εxx maintains a pronounced resonance peak around 0.8 to 1.0 electronvolts whose intensity grows with increasing compression, while the imaginary part exhibits a stable absorption peak near 1.1 to 1.3 electronvolts that intensifies without shifting position. Compression along the loading direction therefore strengthens the electronic polarization response in that direction, whereas the perpendicular component εyy remains nearly untouched.

Y-axis strain operates through an entirely different channel. Compression along the y-axis leaves the main resonance and absorption structures essentially intact, producing only slight local intensity fluctuations. But large tensile strain along the y-direction—particularly at +3 and +4 percent—triggers a distinct uplift in the low-frequency region of the real dielectric function near zero energy, in both εxx and εyy. This enhanced low-frequency polarization background does not destroy the original resonance modes; instead, it raises the material’s ability to support low-energy evanescent-wave coupling. Because near-field heat transfer between two closely spaced sheets is dominated by the tunneling of evanescent waves at frequencies set by the surface electromagnetic modes, this strain-induced dielectric uplift modifies the resonance condition of those modes and dramatically increases their coupling probability within the dominant heat-transfer frequency window.

The electronic band structure calculations reinforce this picture. The team found that χ3-borophene remains metallic under all investigated strain conditions, with several bands continuously crossing the Fermi level. X-axis strain produces only moderate changes in the electronic structure near the Fermi level, indicating limited reconstruction of low-energy states. Y-axis tensile strain, however, induces pronounced band flattening and denser electronic states near the Fermi level, especially at +4 percent tension. This densification of available states enhances low-energy interband transitions, strengthens the low-frequency dielectric response, and ultimately feeds through to the near-field heat-transfer spectrum via the Fresnel reflection coefficients and the energy transmission coefficient of evanescent waves.

The spectral heat transfer coefficient h(ω) reveals these effects quantitatively. Under single-layer x-axis compression, the main spectral peak—located around ω ≈ 2 × 10^14 radians per second—rises from 4.6 × 10^−16 to 5.5 × 10^−16 joules per radian per kelvin per square meter at −4 percent strain, with the peak position essentially stable. Under single-layer y-axis tension, the response is far more dramatic: the peak value climbs to 9.56 × 10^−15 at +4 percent, roughly an order of magnitude above the corresponding x-direction condition. The team attributes this staged, nonlinear surge to the strain-induced enhancement of the low-frequency dielectric response, which improves the surface electromagnetic response and strengthens evanescent-wave coupling across the dominant frequency range.

When both borophene sheets are strained simultaneously in the same direction—what the authors call bilayer co-directional strain—the modulation effects amplify considerably. Bilayer x-axis tension at +4 percent produces a normalized coefficient of 3.64, with the spectral peak reaching 2.10 × 10^−15, approximately 4.6 times the unstrained value, accompanied by a redshift suggesting strengthened interfacial mode coupling. Bilayer y-axis tension is the standout configuration: the normalized heat transfer coefficient rises from 0.65 under −4 percent compression to 1.31 at +2 percent, then leaps to 28.11 at +3 percent and 125.92 at +4 percent. The spectral peak under this configuration reaches 1.72 × 10^−13, roughly 220 times the unstrained state. The reason both sheets undergo similar y-axis tensile-strain-induced dielectric modulation, so the simultaneous enhancement of the low-frequency dielectric response at the two interfaces improves the matching of surface modes, thereby increasing the evanescent-wave transmission coefficient and producing a much higher spectral peak.

A third configuration—bilayer orthogonal strain, where one sheet is strained along the x-direction and the other along the y-direction at equal magnitude—produces an intermediate outcome. Under compression, the normalized coefficient fluctuates only slightly around unity, with a variation amplitude of approximately 0.08, indicating that strain modulation in orthogonal directions partially cancels out. Under large tensile strain, the coefficient climbs to 4.13 at +3 percent and 20.58 at +4 percent, with the spectral peak reaching 1.83 × 10^−14. The team attributes this enhancement primarily to the y-axis tensile layer, whose strengthened low-frequency dielectric response dominates the coupling within the heat-transfer band. However, because the x-strained layer’s dielectric response and surface-mode dispersion do not fully match those of the y-strained layer, the energy transmission coefficient is enhanced but falls short of the bilayer co-directional y-axis tensile level, demonstrating that the mismatch of strain-modified dielectric responses limits synergistic coupling between the two interfaces.

The strain range investigated here—±4 percent—was deliberately chosen to remain within the elastic deformation regime. Previous first-principles studies have shown that free-standing χ3 borophene possesses good dynamical and mechanical stability, with in-plane Young’s moduli of 180 to 210 newtons per meter along both lattice directions, and prior strain-engineering work suggests deformation up to roughly ±6 percent remains mechanically sustainable. All compressive strain cases converged successfully in geometry optimization without structural collapse or severe out-of-plane instability, lending confidence that the predicted enhancements are physically realizable rather than artifacts of an unstable model.

The implications reach well beyond borophene itself. Near-field radiative heat transfer already underpins emerging technologies including near-field thermophotovoltaics, thermal rectification, localized heat dissipation of nanostructures, and near-field thermal microscopy, and functional devices such as thermal switches, thermal diodes, and thermal transistors have enabled precise heat-flow management at the micro- and nanoscale. Previous strain-engineering efforts in two-dimensional materials—up to 73 percent modulation in black phosphorus and more than 60 percent reduction in thermal conductance in bilayer graphene—demonstrated the principle, but the order-of-magnitude switching behavior predicted here for borophene represents a substantial leap in modulation depth. The authors note that their work provides a theoretical basis for developing efficient thermal management devices based on borophene, and that carefully designing the direction and magnitude of strain enables precise control over near-field heat transport.

Caveats remain. The study is based on an ideal freestanding borophene model; in practical applications, substrates may alter the dielectric response and surface-mode dispersion, while defects, vacancies, and surface contamination may introduce additional scattering or damping. The mechanical and thermal stability of borophene under large tensile strain, as well as the experimental control of strain and nanoscale gaps, must be addressed in future work. Borophene was first synthesized on silver substrates in 2015, and its excellent electronic properties have already driven interest in metal-ion batteries, gas sensing, and micro/nanoelectronics; its outstanding optical properties enable near-field thermal radiation capability that surpasses conventional metals and metal oxides. If strain-tunable borophene devices can be fabricated with the precision the theory demands, the result could be a new class of nanoscale thermal switches in which a few percent of mechanical deformation flips heat flow by two orders of magnitude—a capability that would reshape how engineers manage heat in the smallest devices ever built.

Subject of Research: Strain-tunable near-field radiative heat transfer in borophene sheets

Article Title: Regulation of near-field radiative heat transfer between two-dimensional borophene sheets under uniaxial strain based on first-principles calculations

Article References: Huang, H., & Cheng, Q. (2026). Regulation of near-field radiative heat transfer between two-dimensional borophene sheets under uniaxial strain based on first-principles calculations. Frontiers in Thermal Engineering, 6, Article 1864229. https://doi.org/10.3389/fther.2026.1864229

Image Credits: AI Generated

DOI: 10.3389/fther.2026.1864229

Keywords: borophene, near-field radiative heat transfer, mechanical strain, first-principles calculations, thermal management, two-dimensional materials, Regulation, near-field, radiative, heat, transfer, between

The research, led by Philipp de Vrese of the Max Planck Institute for Meteorology together with Victor Brovkin, Moritz Günther, Sarah M. Kang, Thomas Raddatz, Wim Thiery and Yi Yao, addresses one of the most stubborn gaps in climate accounting. Land-use and land-cover change is the only major anthropogenic climate driver whose hydrological contribution is not fully quantified in the assessments of the Intergovernmental Panel on Climate Change. The reason is technical but consequential: changes in surface hydrology, whether from agricultural irrigation, deforestation or afforestation, alter the partitioning of energy between latent and sensible heat, and these non-radiative effects are thought to rival the radiative effects of surface albedo changes in magnitude. If they do, the standard radiative forcing framework, which underpins how climate drivers from carbon dioxide to aerosols are compared, would break down for land-surface change.

The key insight behind the new work is a difference in how surface properties shape regional and global climate. Changes in surface albedo modify Earth’s heat content directly, because they change the amount of solar radiation the planet absorbs at the top of the atmosphere. Changes in surface hydrology, by contrast, merely redistribute the available energy between evaporation and direct heating of the air. Without a direct effect on the planet’s overall energy budget, the spatially integrated climate impact of hydrological change must arise through indirect mechanisms whose temperature effects need not align with the local response. The team set out to test whether these indirect pathways ultimately even out, leaving a radiative signature that scales linearly with planetary albedo.

To do so, the researchers ran idealized simulations with the Max Planck Institute’s Earth system model, MPI-ESM, in which they perturbed either surface albedo or water availability across three extensive latitudinal bands: the tropics between 10 degrees south and 10 degrees north, the subtropics between 15 and 35 degrees north, and the mid-latitudes between 40 and 60 degrees north. The perturbations were deliberately severe. Surface albedo was increased by 0.15, comparable to a dramatic brightening of the land, while water availability was boosted by maintaining the thin skin reservoir of water on the surface at 80 percent of its maximum capacity, effectively eliminating moisture constraints on evaporation. Each simulation ran for 50 years under pre-industrial greenhouse gas concentrations, starting from an equilibrium control experiment, so that the climate response could be isolated cleanly from internal variability.

The analysis of the surface energy balance revealed a striking pattern. Locally, the two perturbation types behaved as expected: albedo increases cooled the surface by reducing absorbed shortwave radiation, while hydrological perturbations produced their strongest local cooling through enhanced evaporation. But when the team examined the zonally averaged fluxes, the picture changed fundamentally. The increase in latent heat flux from added evaporation was offset by opposing shifts in the sensible heat flux, because evaporative cooling lowers the surface temperature while condensational heating higher in the atmosphere warms the air, jointly reducing the surface-atmosphere temperature gradient that drives turbulent exchange. Where turbulent fluxes did increase, the resulting energy deficit at the surface was compensated by enhanced atmospheric counterradiation. In every simulation, the only surface energy-balance component consistently linked to the cooling was a decrease in net surface shortwave radiation, a purely radiative process.

The mechanism connecting hydrological change to shortwave radiation turned out to be clouds. The atmosphere has a finite water-holding capacity, so when evapotranspiration rises, precipitation must rise to match it, increasing cloudiness. This effect is amplified by the reduction in the sensible heat flux, which cools the atmosphere and raises relative humidity, further promoting cloud formation through enhanced lower-troposphere stability. The resulting clouds, particularly optically thick low-altitude clouds, reflect additional sunlight back to space. For albedo increases, the cloud response is more nuanced: cooling lowers saturation vapor pressure, drying the atmosphere and reducing shortwave absorption by water vapor, which increases surface insolation, while the spatially varying cloud response can either amplify or diminish the change in surface brightness. In both cases, however, the decisive shifts in the surface energy balance are radiative, mediated by cloud radiative forcing.

The global temperature response confirmed the hypothesis with remarkable precision. Observations and reanalysis data place the slope between top-of-atmosphere outgoing longwave radiation and surface temperature, the so-called lw-TOARSTR, at between 1.9 and 2.2 watts per square meter per kelvin. In the simulations, none of the clear-sky TOARSTRs deviated from this range by more than 10 percent, and there was no systematic difference between albedo and hydrological perturbations or between perturbations applied in different latitudes. The all-sky shortwave TOARSTR averaged 2.5 watts per square meter per kelvin, implying a surface temperature response of roughly 0.4 kelvin per watt per square meter change in top-of-atmosphere net shortwave radiation. In other words, the spatially integrated biophysical climate impact of land-surface change scales approximately linearly with the resulting shift in planetary albedo, regardless of whether the perturbation is radiative or hydrological in origin.

Notably, while non-radiative effects strongly influence local temperatures, their net contribution to the global-mean signal proved negligible. The simulations also showed that remote effects are largely independent of the perturbation type, arising mainly through changes in meridional heat transport. Except for the tropical albedo case, all perturbations cooled the boreal region, the Arctic Ocean and the North Atlantic, while most northern-hemisphere perturbations warmed the southern border of the Sahel and South Asia through a southward shift of the Intertropical Convergence Zone. A distinctive warming pattern along North America’s west coast stemmed from changes in the intensity and position of the Aleutian Low. Even the tropical albedo experiment, in which the initial brightening triggered an extreme decline in precipitation across the Amazon and Congo basins, reducing tree cover and cloud cover so that more shortwave radiation was ultimately absorbed, produced global TOARSTRs consistent with the other experiments, suggesting that synergistic effects between simultaneous surface changes are of minor importance for the global temperature response.

Because the global response depends chiefly on regional cloud-cover characteristics, the team built a simplified estimation scheme based on local shifts in surface brightness and evapotranspiration combined with region-specific sensitivity parameters derived from low-altitude cloud statistics. Applied to their own simulations, including combined brightness and hydrological perturbations and changes in surface roughness, as well as to multi-model experiments from the Land-Use and Irrigation Model Intercomparison Projects, the scheme reproduced the simulated responses with high agreement. Temperature estimates deviated from the simulated values by less than a third of the latter, and the agreement was even better for combined perturbations, since the biases partly offset each other. The approach also captured roughness changes well, indicating that, like hydrological change, roughness acts primarily through shifts in the Bowen ratio and cloudiness.

The findings offer straightforward answers to longstanding questions. They show that the radiative forcing framework can be extended to all aspects of land-surface change if the rapid cloud response is treated as a radiative forcing rather than a climate feedback, as is already done for cloud adjustments to aerosols. Such treatment could also resolve the apparent difference between climate sensitivity to land-use change and to atmospheric carbon dioxide. Perhaps most strikingly, the results explain why agricultural irrigation has profound local impacts yet only a minor influence on global mean temperature: irrigated areas are predominantly located in arid regions, where local evaporative cooling may be strongest but the cloud sensitivity to changes in evapotranspiration is comparatively low. Applying the same amount of water in the boreal region, with weaker evaporative cooling but substantially higher cloud sensitivity, would induce a global cooling several times larger. The authors note that terrestrial water management therefore offers some potential as a climate-mitigation tool, though they caution that their investigation relies on a single Earth system model, and the sensitivity parameters describing cloud effects are highly model-dependent, with inter-model variability in cloud feedback remaining a key source of uncertainty. The general shifts in the surface energy balance, however, agree well with those produced by other models, and the TOARSTRs are consistent with observation-based estimates, suggesting the applicability of the key findings is neither limited to the model used nor to the idealized scenarios investigated.

Subject of Research: Global climate impact of land-surface change driven by radiative effects

Article Title: Global climate impact of land-surface change driven by radiative effects

Article References: de Vrese, P., Brovkin, V., Günther, M., Kang, S. M., Raddatz, T., Thiery, W., & Yao, Y. (2026). Global climate impact of land-surface change driven by radiative effects. Nature Communications, 17(1), Article 10454. https://doi.org/10.1038/s41467-026-77834-0

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77834-0

Keywords: Global, climate, impact, land-surface, change, driven, radiative, effects, scientific research