[Paper Review] Colossal Photovoltaic Effect Driven by the Singular Berry Curvature in a Weyl Semimetal

Gavin B. Osterhoudt, Laura K. Diebel+8 more|arXiv (Cornell University)|Dec 13, 2017

Topological Materials and Phenomena12 citations

TL;DR

This study demonstrates a colossal bulk photovoltaic effect (BPVE) in the Weyl semimetal TaAs at room temperature, driven by the singular Berry curvature near Weyl nodes. By tuning light polarization relative to crystal axes, the researchers link the giant BPVE directly to the non-trivial topology of Weyl fermions, achieving a response order of magnitude larger than previous measurements, enabling topological detection and energy conversion applications.

ABSTRACT

The non-trivial evolution of wavefunctions in momentum space leads to novel phases and responses. As with any topological property, a key challenge lies in finding experiments that identify the non-local characteristics. Indeed, revealing this Berry curvature often requires extreme conditions and/or surface sensitive probes. These rely on measuring the Berry phase acquired through a closed loop evolution in momentum space, or through the bulk-boundary correspondence leading to new states at the surface. Alternatively, various components of the electronic structure can be detected by the bulk photovoltaic effect (BPVE) where non-linearity rectifies light to dc currents. Weyl semimetals are particularly promising, as their non-degenerate, Dirac-like, chiral states produce diverging Berry curvature. Here we report a room temperature, colossal BPVE in the Weyl semimetal TaAs, which is an order of magnitude larger than any previous measurement. By varying the polarization of light with respect to the crystal axis, we establish the collosal BPVE resulting from the non-trivial topology of Weyl fermions, i.e. the diverging Berry curvature near the Weyl nodes. The BPVE in Weyl semiemtals we uncover can be exploited for detecting electron topology, broadband detectors, and solar/thermal energy conversion.

Motivation & Objective

• To identify and measure the bulk photovoltaic effect (BPVE) in a Weyl semimetal as a probe of non-trivial topology.
• To demonstrate that the diverging Berry curvature near Weyl nodes generates an enhanced BPVE.
• To establish a room-temperature, bulk-sensitive method for detecting electron topology without requiring extreme conditions or surface probes.
• To explore the potential of Weyl semimetals in broadband photodetection and solar/thermal energy conversion.

Proposed method

• Measurement of the bulk photovoltaic current in TaAs under varying light polarization relative to crystal axes.
• Use of polarization-dependent optical excitation to probe the momentum-space Berry curvature distribution.
• Exploitation of the non-degenerate, chiral electronic states in Weyl semimetals to enhance nonlinear photogeneration of dc current.
• Analysis of the BPVE response as a function of light helicity and crystal orientation to isolate topological contributions.
• Leveraging the bulk-boundary correspondence and non-trivial wavefunction evolution in momentum space to link observable current to Berry curvature singularities.

Experimental results

Research questions

• RQ1 Can the bulk photovoltaic effect serve as a robust, room-temperature probe of non-trivial topology in Weyl semimetals?
• RQ2 To what extent does the diverging Berry curvature near Weyl nodes enhance the BPVE in TaAs?
• RQ3 How does the polarization of incident light influence the photovoltaic response in a Weyl semimetal with broken inversion symmetry?
• RQ4 Can the BPVE in Weyl semimetals be harnessed for practical applications such as broadband detection or energy conversion?

Key findings

• A colossal bulk photovoltaic effect was observed in TaAs at room temperature, exceeding previous measurements by an order of magnitude.
• The BPVE magnitude strongly depends on the polarization of incident light relative to the crystal axis, confirming its origin in the chiral, non-degenerate electronic states.
• The giant BPVE is directly linked to the singular Berry curvature near Weyl nodes, providing direct evidence of non-trivial topology in the bulk.
• The effect persists under ambient conditions, making it suitable for practical applications without cryogenic or surface-sensitive requirements.
• The observed BPVE response is sufficiently large to enable detection of electron topology and potential use in solar/thermal energy conversion devices.