[Paper Review] Berry Curvature Enhanced Nonlinear Photogalvanic Response of Type-II Weyl Cone

Junchao Ma, Qiangqiang Gu+8 more|arXiv (Cornell University)|Jun 22, 2018

Topological Materials and Phenomena35 references73 citations

TL;DR

The paper demonstrates that Berry curvature singularities at Type-II Weyl nodes in TaIrTe4 enhance nonlinear photogalvanic responses, yielding large room-temperature photoresponsivity driven by third-order nonlinear optics.

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-μm 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.

Motivation & Objective

• Motivate the search for topological signatures in bulk materials under ambient conditions.
• Investigate how Berry curvature singularities at Weyl nodes affect nonlinear optical responses.
• Quantify the photoresponse and assess potential device applications for sensing and solar energy.
• Show how Weyl node chirality and Berry curvature influence current generation under optical excitation.

Proposed method

• Analyze TaIrTe4 as a Type-II Weyl semimetal with singular Berry curvature at Weyl nodes.
• Measure photoresponse under mid-IR, 4-μm excitation in an unbiased field-effect transistor.
• Attribute observed responses to divergence of Berry curvature near Weyl nodes.
• Identify third-order nonlinear optical contributions as the mechanism for the response.
• Assess circularly polarized galvanic response linked to Weyl cone chirality.

Experimental results

Research questions

• RQ1 How does Berry curvature divergence at Weyl nodes influence nonlinear photogalvanic effects in Type-II Weyl semimetals?
• RQ2 Can TaIrTe4 exhibit large room-temperature photoresponsivity driven by third-order nonlinear processes?
• RQ3 Do Weyl node chiralities enhance circularly polarized photocurrents under in-plane electric fields?
• RQ4 What is the practicality of TaIrTe4 for photodetection and solar energy harvesting at mid-IR wavelengths?

Key findings

• Unusually large photoresponsivity of 130.2 mA/W with 4-μm excitation at room temperature in an unbiased FET.
• Responsivity arises from third-order nonlinear optical response.
• Observed response mirrors the Berry curvature divergence at Weyl nodes.
• High shift photocurrent at Weyl cones enhances circularly polarized galvanic response between opposite chiralities.
• Mid-IR room-temperature performance approaches that of HgCdTe detectors operated at low temperature, implying practical sensing potential.