[Paper Review] Intrinsic Fermi Surface Contribution to the Circular Photogalvanic Effect

Lingyuan Gao, Zachariah Addison+2 more|arXiv (Cornell University)|Nov 12, 2020

Topological Materials and Phenomena5 references4 citations

TL;DR

This paper identifies an intrinsic Fermi surface contribution to the circular photogalvanic effect in ultra-clean, time-reversal invariant metals, arising from coherent electronic transitions on the Fermi surface under circularly-polarized light. In chiral Weyl semimetals, this effect is linear in frequency at low energies, topologically quantized by the Weyl node charge, and comparable in magnitude to the recently discovered quantized photogalvanic effect.

ABSTRACT

We study the Fermi surface contribution to the nonlinear DC photocurrent at quadratic order in a spatially uniform optical field in the ultra-clean limit. In addition to injection and ballistic currents, we find that circularly-polarized light incident on a time-reversal invariant metallic system generates an intrinsic contribution to the bulk photogalvanic effect deriving from photoinduced electronic transitions on the Fermi surface. In velocity gauge, this contribution originates in both the coherent band off-diagonal and diagonal parts of the density matrix, describing respectively, the coherent wave function evolution and the carrier dynamics of an excited population. We derive a formula for the intrinsic Fermi surface contribution for a chiral Weyl semimetal. At low frequency, this response is proportional to the frequency of the driving field, with its sign determined by the topological charge of the Weyl nodes and with its magnitude being comparable to the recently discovered quantized circular photogalvanic effect.

Motivation & Objective

• To identify and characterize a previously unaccounted-for intrinsic contribution to the bulk photogalvanic effect originating from the Fermi surface in ultra-clean metallic systems.
• To clarify the role of coherent band off-diagonal and diagonal elements of the density matrix in generating this Fermi surface current.
• To derive a quantitative formula for the intrinsic Fermi surface contribution in chiral Weyl semimetals under circularly-polarized light.
• To determine the frequency dependence and topological origin of this photogalvanic response in the low-frequency regime.
• To compare the magnitude of this intrinsic Fermi surface current with the recently discovered quantized circular photogalvanic effect.

Proposed method

• Employing the velocity gauge formulation to describe the interaction of circularly-polarized light with electrons in a metallic system.
• Decomposing the density matrix into coherent wave function evolution (off-diagonal) and carrier population dynamics (diagonal) contributions.
• Applying the linear response formalism to compute the nonlinear DC photocurrent at quadratic order in the optical field.
• Focusing on the ultra-clean limit to isolate intrinsic, non-scattering contributions from the Fermi surface.
• Deriving an analytical expression for the photogalvanic current in a chiral Weyl semimetal model.
• Analyzing the low-frequency limit to extract the frequency-linear dependence and topological signature.

Experimental results

Research questions

• RQ1 What is the origin of the intrinsic Fermi surface contribution to the circular photogalvanic effect in time-reversal invariant metals?
• RQ2 How do coherent band off-diagonal and diagonal terms of the density matrix contribute to the photogalvanic current?
• RQ3 What is the frequency dependence of the intrinsic Fermi surface current in chiral Weyl semimetals?
• RQ4 How is the sign and magnitude of this current related to the topological charge of Weyl nodes?
• RQ5 How does the magnitude of this intrinsic Fermi surface current compare to the quantized circular photogalvanic effect?

Key findings

• An intrinsic Fermi surface contribution to the circular photogalvanic effect exists in ultra-clean, time-reversal invariant metals, arising from coherent electronic transitions on the Fermi surface.
• This contribution originates from both the off-diagonal (coherent evolution) and diagonal (population dynamics) parts of the density matrix in the velocity gauge formulation.
• In chiral Weyl semimetals, the low-frequency response is linear in the driving field frequency.
• The sign of the photogalvanic current is determined by the topological charge of the Weyl nodes.
• The magnitude of this intrinsic Fermi surface current is comparable to the recently discovered quantized circular photogalvanic effect.
• The response is purely intrinsic and survives in the ultra-clean limit, indicating a topologically protected origin.