Crystal Asymmetry Drives Unexpected Light-Triggered Currents

Researchers have demonstrated a substantial transverse thermoelectric effect within the Weyl semimetal TaIrTe₄, offering a novel pathway towards enhanced photodetection. Morgan G. Blevins and Abhishek Mukherjee, both from the Department of Electrical Engineering and Computer Science at Massachusetts Institute of Technology, led the investigation in collaboration with Vivian J. Santamaria-Garcia from the School of Engineering and Sciences at Tecnológico de Monterrey, Thanh Nguyen working with colleagues at the Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, and Svetlana V. Boriskina from the Department of Mechanical Engineering, Massachusetts Institute of Technology, alongside Mingda Li and Xianglin Ji. This work confirms that anomalous photocurrents in TaIrTe₄ arise from its significant transverse thermoelectric effect, rather than nonlinear charge responses, and crucially, establishes a method for controlling and amplifying these currents through careful engineering of both the material’s crystalline orientation and its thermal environment. By demonstrating enhanced photocurrents via substrate engineering, this research provides a framework for developing broadband photodetection schemes with potential applications in wavefront sensing, beam positioning, and precise edge detection.

Scientists have discovered a novel way to harness heat from light in a unique material, potentially revolutionising photodetection technology. The ability to control and amplify this effect through careful material engineering offers a pathway towards more sensitive and versatile optical sensors, underpinning advances in areas ranging from imaging to beam manipulation.

Researchers have uncovered a new understanding of how light interacts with a complex material, potentially paving the way for more efficient infrared detectors and novel sensing technologies. The work centres on tantalum iridium telluride (TaIrTe₄), a topological semimetal exhibiting unusual optical properties, revealing that anomalous photocurrents originate not from nonlinear optical effects, but from a substantial transverse thermoelectric effect.

This effect arises from the material’s unique crystal structure and its anisotropic (direction-dependent) ability to conduct both electrons and heat. The study confirms that these photocurrents, observed under both visible and far-infrared illumination, are driven by temperature differences created by the light itself. By carefully manipulating the orientation of crystal edges relative to electrodes and controlling the thermal environment around the TaIrTe₄, researchers demonstrate precise control over the spatial distribution and intensity of these photocurrents.

Crucially, they show that strategically engineering the substrate on which the material sits can locally amplify the photocurrent signal, opening up possibilities for creating broadband photodetection schemes. Such devices could be used in applications requiring precise light manipulation, including wavefront sensing, accurate beam positioning, and high-resolution edge detection.

The findings challenge earlier interpretations of photocurrent generation in this class of materials and highlight the importance of considering thermoelectric effects in the design of future optoelectronic devices. The research details how TaIrTe₄’s layered, non-centrosymmetric crystal structure contributes to its anisotropic electrical conductivity, exhibiting p-type behaviour along one direction and n-type behaviour along another.

This anisotropy is fundamental to the observed transverse thermoelectric effect, where a temperature gradient generates an electrical current perpendicular to both the gradient and the material’s conductivity. The team employed scanning photocurrent microscopy and sophisticated modelling based on the Shockley-Ramo theory to substantiate the thermoelectric origin of the anomalous photocurrents.

Anisotropic Seebeck coefficients drive photocurrent generation and high transverse thermoelectric performance in tantalum iridium telluride

Measurements reveal a transverse thermoelectric figure of merit, zxyT, of approximately 1.5 × 10−3 at room temperature for TaIrTe4, comparable to the high thermoelectric performance recently observed in LaPt2B, demonstrating a substantial thermoelectric effect without requiring a magnetic bias. The study establishes a strong transverse photo- thermoelectric effect (PTE) as the origin of anomalous photocurrents in TaIrTe4, confirming that these currents stem from anisotropic Seebeck coefficients.

Device configurations with a crystal a-axis oriented 25.5° relative to one electrode and a crystal edge oriented 45° to the same electrode exhibit zero current at the electrodes while sustaining edge currents. Simulations, validated by experimental data, demonstrate that the weighting field gradient aligns with the photocurrent vector field at off-axis crystal edges, enabling net current flow between electrodes.

Conversely, at the a-axis edge, the photocurrent is perpendicular to the weighting field gradient, resulting in zero net current. Time-resolved measurements of the photocurrent response reveal a decay time, τ, of 32 ±1.6 microseconds at the electrode interface and 31 ±5.0 microseconds at the off-axis edge, indicating a rapid response time. Analysis of the dielectric function of TaIrTe4 in the visible and near-infrared regions shows approximate in-plane isotropy, with εA approximately equal to εB, consistent with previous observations.

Further investigation into the long-wave infrared (LWIR) response reveals significant polarization dependence, with calculations showing that absorption into the TaIrTe4 flake is higher for light polarized parallel to the b-axis, resulting in a larger temperature gradient at the edge for that polarization. Spectral photocurrent maps measured between 7.7 and 12μm confirm this, with the edge current for E||b exceeding that for E||a. The ratio of edge currents, |Iedge b| / |Iedge a|, is notably greater than one, demonstrating a clear polarization-dependent enhancement of the photocurrent signal.

Thermoelectric current mapping via three-dimensional anisotropic heat equation modelling

Scanning photocurrent microscopy (SPCM) served as the primary technique for mapping local currents. Full methodological details and precise material parameters are provided in Supplementary Note S5. Simulations, closely matching experimental SPCM maps, revealed that at off-axis edges, the resulting temperature gradient generates a transverse thermoelectric current component parallel to the weighting field, facilitating current collection.

Conversely, along the natural a-axis edge, the local photocurrent is oriented perpendicularly to the weighting field gradient, preventing net current collection. These observations strongly indicate that the dominant photocurrent mechanism is indeed a transverse thermoelectric effect. To further investigate the temporal dynamics, device performance was assessed with increasing chopping frequency, revealing signal roll-off at the edge and electrode interface when modulation speed exceeded the material’s thermal diffusion time.

Investigations into the long-wave infrared (LWIR) photocurrent in TaIrTe4 aimed to differentiate between photothermal (PTE) and nonlinear optical (BPVE) origins. Measurements were focused on the off-axis crystal edge of Device A, isolating the thermoelectric contribution by avoiding plasmonic effects from the electrodes. Rigorous coupled-wave analysis (RCWA) was employed to calculate the polarization-dependent electric field intensity and absorption, demonstrating that absorption is greater for light polarized along the b-axis in the LWIR spectral region due to the material’s highly anisotropic permittivity.

To predict the expected thermoelectric photocurrent, the temperature rise (∆T(x, y)) and thermal gradient (∇T(x, y)) were calculated using a two-dimensional heat equation, modelling the laser spot as a Gaussian heat source and accounting for the insulating crystal edge-air boundary. The calculated trend of ∇T as a function of laser wavelength predicted a larger gradient for light polarized along the b-axis, aligning with expectations for |Iedge a |/|Iedge b |.

Experimental measurements of polarization- and wavelength-dependent photocurrent maps under 7-12μm illumination confirmed this prediction at the off-axis crystal edge, demonstrating a larger photocurrent magnitude for light polarized along the b-axis. Analysis of prior studies revealed that thinner TaIrTe4 flakes (t ≤60nm) exhibit a reversed absorption trend, with Abs(E ∥a) > Abs(E ∥b), due to the dominance of Im[εA].

This thickness-dependent crossover in polarization response further supports the thermoelectric origin of the photocurrent and reconciles seemingly contradictory results in the literature. To further control the thermoelectric mechanism, the thermal environment of Device B was engineered by partially suspending a TaIrTe4 flake over a SiO2 step on a 300-nm-SiO2/Si substrate.

Thermoelectric effects control photocurrent amplification in Weyl semimetals

Scientists are increasingly adept at manipulating the subtle interplay between light, heat, and materials, and this work on tantalum iridium telluride demonstrates a particularly elegant form of control. For years, understanding anomalous photocurrents in Weyl semimetals has been hampered by the difficulty of disentangling different physical mechanisms, specifically, distinguishing between genuine nonlinear optical effects and the influence of temperature gradients.

The initial assumption of nonlinear charge currents proved misleading, obscuring the crucial role of thermoelectric phenomena. This research doesn’t simply confirm the thermoelectric origin of these photocurrents; it establishes a pathway for actively engineering them. By carefully considering the orientation of crystal edges relative to electrodes, and by manipulating the thermal environment of the material via substrate design, researchers have demonstrated a significant amplification of the spatial photocurrent response.

This level of control is vital, moving beyond mere observation towards the possibility of creating functional devices. However, the reliance on specific substrate configurations introduces a practical limitation. Scaling up these devices, or adapting them to different applications, will require further investigation into the robustness of this engineered thermal environment.

Moreover, while the potential for broadband photodetection, wavefront sensing, and edge detection is exciting, realising these applications demands addressing challenges in integrating these flakes into larger, more complex systems. Future work might explore alternative materials with similar thermoelectric properties but greater ease of fabrication, or investigate methods for creating more uniform and scalable thermal interfaces.