[Paper Review] Photocurrent-driven transient symmetry breaking in the Weyl semimetal TaAs

Nicholas Sirica, Peter P. Orth+24 more|arXiv (Cornell University)|May 20, 2020

Topological Materials and PhenomenaPhysics and Astronomy61 references44 citations

TL;DR

This study demonstrates ultrafast, all-electronic symmetry breaking in the Weyl semimetal TaAs via photocurrent-driven transient reduction of magnetic point group symmetry from 4mm1′ to 1, observed using time-resolved second-harmonic generation (TR-SHG) spectroscopy. The symmetry breaking occurs on a picosecond timescale without lattice distortion, driven by optically generated photocurrents that alter electronic polarization, enabling dynamic control of topological states through light-induced current directionality.

ABSTRACT

Symmetry plays a central role in conventional and topological phases of matter, making the ability to optically drive symmetry changes a critical step in developing future technologies that rely on such control. Topological materials, like topological semimetals, are particularly sensitive to a breaking or restoring of time-reversal and crystalline symmetries, which affect both bulk and surface electronic states. While previous studies have focused on controlling symmetry via coupling to the crystal lattice, we demonstrate here an all-electronic mechanism based on photocurrent generation. Using second harmonic generation spectroscopy as a sensitive probe of symmetry changes, we observe an ultrafast breaking of time-reversal and spatial symmetries following femtosecond optical excitation in the prototypical type-I Weyl semimetal TaAs. Our results show that optically driven photocurrents can be tailored to explicitly break electronic symmetry in a generic fashion, opening up the possibility of driving phase transitions between symmetry-protected states on ultrafast timescales.

Motivation & Objective

• To investigate light-induced symmetry breaking in topological semimetals beyond lattice-driven mechanisms.
• To determine whether photocurrents alone can transiently break time-reversal and spatial symmetries in Weyl semimetals.
• To demonstrate that electronic symmetry breaking can be controlled via pump polarization direction.
• To establish TR-SHG as a probe of transient photocurrent-induced electronic order in topological materials.
• To explore the role of Berry curvature and shift currents in driving ultrafast electronic symmetry breaking.

Proposed method

• Time-resolved second-harmonic generation (TR-SHG) spectroscopy was used to probe symmetry changes with sub-100 fs temporal and sub-micron spatial resolution.
• Femtosecond optical pulses (1.55 eV) were used to excite TaAs, generating photocurrents that induce transient electronic polarization.
• Pump-probe geometry with variable delay enabled tracking of symmetry evolution on picosecond timescales.
• Polarization-dependent SHG patterns were analyzed to extract changes in the nonlinear susceptibility tensor χ(2)
• Fits to SHG patterns under different magnetic point group symmetries (4mm1′ and 1) confirmed symmetry reduction post-excitation.
• Theoretical modeling linked observed symmetry breaking to photocurrent-induced shifts in electronic charge distribution and Berry curvature effects.

Experimental results

Research questions

• RQ1 Can optically generated photocurrents alone break time-reversal and spatial symmetries in a Weyl semimetal without lattice distortion?
• RQ2 What is the timescale of electronic symmetry breaking following femtosecond optical excitation in TaAs?
• RQ3 How does the direction of the photocurrent—controlled by pump polarization—affect the degree and nature of symmetry breaking?
• RQ4 To what extent can TR-SHG detect transient electronic order not accessible to linear optical probes?
• RQ5 Is the symmetry breaking driven by shift currents or other nonlinear optical mechanisms in TaAs?

Key findings

• TR-SHG measurements revealed a transient reduction of magnetic point group symmetry from 4mm1′ to 1 in TaAs following 1.55 eV excitation, occurring within ~1 ps.
• A 2.5° rotation of the SHG pattern along the [1,1,¯1] axis was observed immediately after excitation, indicating asymmetric electronic polarization.
• The symmetry breaking was absent in the absence of photocurrents and was directly correlated with pump polarization direction, confirming current-driven origin.
• The change in symmetry was purely electronic, as no structural transition was detected via ultrafast X-ray diffraction in parallel experiments.
• The degree of symmetry breaking was tunable via pump polarization, enabling dynamic control of electronic order on ultrafast timescales.
• The results demonstrate that photocurrents can induce transient, symmetry-protected topological states in Weyl semimetals through all-electronic means.