Light-Driven Raman Coherence as a Nonthermal Route to Ultrafast Topology Switching in a Dirac Semimetal

Abstract

A grand challenge underlies the entire field of topology-enabled quantum logic and information science: how to establish topological control principles driven by quantum coherence and understand the time-dependence of such periodic driving? Here we demonstrate a THz pulse-induced phase transition in Dirac materials that is periodically driven by vibrational coherence due to excitation of the lowest Raman-active mode. Above a critical field threshold, there emerges a longlived metastable phase with unique Raman coherent phonon-assisted switching dynamics, absent for optical pumping. The switching also manifest itself by non-thermal spectral shape, relaxation slowing down near the Lifshitz transition where the critical Dirac point (DP) occurs, and diminishing signals at the same temperature that the Berry curvature induced Anomalous Hall Effect varnishes. These results, together with first-principles modeling, identify a mode-selective Raman coupling that drives the system from strong to weak topological insulators, STI to WTI, with a Dirac semimetal phase established at a critical atomic displacement controlled by the phonon pumping. Harnessing of vibrational coherence can be extended to steer symmetry-breaking transitions, i.e., Dirac to Weyl ones, with implications on THz topological quantum gate and error correction applications.

Light quantum control of persisting Higgs modes in iron-based superconductors

“…We characterize the THz quantum quench coherent dynamics directly in the time domain [31][32][33] (Methods) by measuring the responses to two phase-locked THz pulses as differential field transmission of the weak THz probe field ΔE/E 0 (blue circles, Fig. 1d) for THz pump field, E pump = 56 kV/cm and as a function of pump-probe time delay Δt pp .…”

Light control of surface–bulk coupling by terahertz vibrational coherence in a topological insulator

“…Our data for the bulk conducting state clearly shows the roadmap and it will be interesting to extend the demonstrated coherent-driving scheme to bulk insulating samples and other systems including semimetals and semiconductors to increase coherent transport. 11,12,31 We expect two major changes with elevated temperature: (1) decrease of coherent phonon amplitude for the same pump field and (2) much stronger surface-bulk coupling, assisted by the population of incoherent phonons. Figure 5 shows these expected behaviors.…”

“…This effect of coherent phonons is in stark contrast to that of incoherent phonons from thermal excitations, which diminishes topological enhancement of surface transport via random lattice scattering, heating, surface-bulk charge transfer and coupling. Ultrafast phononics has been explored as a new avenue to manipulate properties of superconductors, 8 oxides, 9,10 semimetal 11 and photovoltaic semiconductors. 12 Although experimental realizations of coherent phonon generation have been explored in TIs, 7,13,14 the influence of such vibrational coherence on the surface helical transport has not been observed until now.…”

Photocurrent-driven transient symmetry breaking in the Weyl semimetal TaAs

“…Topological materials, like the newly discovered 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 [1][2][3][4], we demonstrate here an all-electronic mechanism based on photocurrent generation. Using second-harmonic generation spectroscopy as a sensitive probe of symmetry changes [5,6], we observe an ultrafast breaking of time-reversal and spatial symmetries following femtosecond optical excitation in the prototypical type-I Weyl semimetal TaAs.…”