Berry Curvature Enhanced Nonlinear Photogalvanic Response of Type-II Weyl Cone

Abstract: The experimental manifestation of topological effects in bulk materials under ambient conditions, especially those with practical applications, has attracted enormous research interest. Recent discovery of Weyl semimetal provides an ideal material platform for such endeavors. The Berry curvature in a Weyl semimetal becomes singular at the Weyl node, creating an effective magnetic monopole in the k-space. A pair of Weyl nodes carry quantized effective magnetic charges with opposite signs, and therefore, opposite chirality. Although Weyl-point-related signatures such as chiral anomaly and non-closing surface Fermi arcs have been detected through transport and ARPES measurements, direct experimental evidence of the effective k-space monopole of the Weyl nodes has so far been lacking. In this work, signatures of the singular topology in a type-II Weyl semimetal TaIrTe4 is revealed in the photo responses, which are shown to be directly related to the divergence of Berry curvature. As a result of the divergence of Berry curvature at the Weyl nodes, TaIrTe4 exhibits unusually large photo responsivity of 130.2 mA/W with 4-{\mu}m excitation in an unbiased field effect transistor at room temperature arising from the third-order nonlinear optical response. The room temperature mid-IR responsivity is approaching the performance of commercial HgCdTe detector operating at low temperature, making Type-II Weyl semimetal TaIrTe4 of practical importance in terms of photo sensing and solar energy harvesting. Furthermore, the high shift photocurrent response at the Weyl cones is found to enhance the circularly polarized galvanic response from Weyl cones with opposite chirality, which opens new experimental possibilities for studying and controlling the chiral polarization of Weyl Fermions through an in-plane DC electric field in addition to the optical helicities.

Berry Curvature Enhanced Nonlinear Photo Response in Type-II Weyl Semimetals

In the study entitled "Berry Curvature Enhanced Nonlinear Photo Response of Type-II Weyl Semimetals," the investigation centers around the unique properties of TaIrTe₄, a type-II Weyl semimetal, with respect to its photo response characteristics stemming from its singular Berry curvature at the Weyl nodes. This work emphasizes the nonlinear optical phenomena influenced by the topological aspects inherent in type-II Weyl semimetals, particularly highlighting the experimental detection of high photo responsivity and the potential practical implications for optoelectronic devices.

The Berry curvature, divergent at Weyl nodes in the momentum space, acts on the nonlinear optoelectronic responses. TaIrTe₄ exhibits a photo responsivity of 130.2 mA/W with 4-μm excitation at room temperature through third-order nonlinear optical processes. Such high responsivity at room temperature signifies substantial progress in photodetector technology, potentially rivaling commercial HgCdTe detectors that operate at significantly lower temperatures. This positions TaIrTe₄ as a material of potential interest for applications in photo sensing and energy harvesting technologies. The experimental results in this work demonstrate the correlation between the singular Berry curvature and enhanced responses like shift currents, which are explored in depth theoretically.

The observed phenomena further extend to circularly polarized galvanic responses in Weyl semimetals, suggesting a boost in experimental control of chiral polarization properties of Weyl Fermions. This is possibly achieved through the material's unique optical selection rules and is accentuated by the Berry curvature singularity. The CPGE measurements reinforce the chiral nature of the injection processes, attributed to the role of Berry curvature, in conjunction with third-order nonlinear optical tensors.

From a theoretical standpoint, the analysis incorporates density-functional theory (DFT) calculations to simulate the optical response and elucidate the role of Berry-phase-related quantities in determining the nonlinear optical outcomes. The findings underscore that Weyl nodes, metaphorically described as k-space magnetic monopoles, significantly contribute to optical and transport phenomena due to their topological characteristics.

For practical implications, the research suggests that experimental techniques leveraging the enhanced Berry curvature should continue developing, particularly concerning type-II Weyl semimetals' application in mid-infrared detection. The giant shift current observed might be optimized further through adjustment of the material's symmetry and doping characteristics, facilitating broader wavelength applicability and greater control over chiral currents beyond conventional methodologies.

Moving forward, this work could stimulate further research in nonlinear optics and the quantum properties of materials, advancing potential applications in optoelectronics. The correlation between enhanced Berry curvature and optical response illuminates new directions for exploring other quantum systems where topology could play a pivotal role in device performance.

Overall, the paper advances our understanding of nonlinear optical phenomena in type-II Weyl semimetals, opening pathways for enhanced photonic technology and broadened exploration of topological effects in condensed matter physics.