Real-time observation of the coherent transition to a metastable emergent state in $1T\ensuremath{-}\mathrm{Ta}{\mathrm{S}}_{2}$

Real-time observation of the coherent transition to a metastable emergent state in $1 T - Ta S_{2}$

Jan Ravnik, Igor Vaskivskyi, Tomaz Mertelj, and Dragan Mihailovic

Phys. Rev. B 97, 075304 – Published 12 February 2018

Abstract

The transition to a hidden metastable state in $1 T - Ta S_{2}$ is investigated in real time using coherent time-resolved femtosecond spectroscopy. Relying on spectral differences between phonon modes in the equilibrium states and in the metastable state, and temperature-tuning the metastable state lifetime, we perform stroboscopic measurements of the electronic response and switching of coherent oscillation frequency through the transition. Very fast coherent switching of the collective-mode frequency is observed ( $\sim 400 fs$ ), comparable to the electronic time scale $\sim 300 fs$ . A slower, 4.7-ps process is attributed to lattice relaxation. The observations are described well by a fast electronic band structure transformation into the metastable state, consistent with a topological transition.

Received 4 October 2017
Revised 12 January 2018

DOI:https://doi.org/10.1103/PhysRevB.97.075304

Physics Subject Headings (PhySH)

Charge density waves Critical phenomena Metal-insulator transition Optical phonons Specific phase transitions Topological phases of matter

Transition metal dichalcogenides

Coherent control Femtosecond laser spectroscopy Laser techniques Transient absorption spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jan Ravnik¹, Igor Vaskivskyi^1,2, Tomaz Mertelj^1,2, and Dragan Mihailovic^1,2

¹Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
²CENN Nanocenter, Jamova 39, SI-1000 Ljubljana, Slovenia

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Vol. 97, Iss. 7 — 15 February 2018

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Figure 1
(a) The structure of $1 T - Ta S_{2}$ . (b) Schematic phase diagram of $1 T - Ta S_{2}$ upon cooling, warming, and after photoexcitation. (c) A schematic diagram of the atomic displacements corresponding to the dominant breathing “amplitude mode” in the C state.
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Figure 2
(a) Time-resolved reflectivity of $1 T - Ta S_{2}$ in different states, which show exponential quasiparticle decay with superimposed oscillatory response from coherent excitation of collective modes and (b) their fast Fourier transforms. Dots are the experimental data; thick lines are fits with the DECP model [18]. Thin lines are individual oscillators used for the DECP fit, with only the oscillatory part plotted. The transition from the C to the H state results in a shift of intensity to a lower frequency (dashed vertical lines). In the NC and T states, peaks at even lower frequencies appear and become more prominent with higher temperature. (c) Temperature dependence of the collective-mode frequencies combined from different techniques [9, 19, 20] shown in the legend. (d) Temperature dependence of the mode frequencies. The size of the symbols and color intensity of the streaks represent the intensity of the corresponding modes in the C, H, NC, and T phases in blue, red, cyan, and light cyan, respectively.
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Figure 3
(a) Schematic diagram of the optical three-pulse experiment. (b,c) Three-pulse P-D-Pr traces for D pulse fluence adjusted above ( $F_{D} = 1.5 mJ / c m^{2}$ ) and below ( $F_{D} = 0.7 mJ / c m^{2}$ ) switching threshold, respectively. Dashed line shows the time of the D pulse arrival. Black curve is a single trace for selected $τ_{P - D}$ . (d) Standard two-pulse pump-probe trace with above-threshold pump pulses ( $F_{P} = 1.5 mJ / c m^{2}$ ). The reflectivity of the sample possesses fast rise and double-exponent relaxation. (e) Detailed analysis of the P-D-Pr trace for $τ_{P - D} = 2.6 ps$ (black) and extracted oscillatory response only (red dots) with FM sine fit (red line). The frequency profile used for the fit is shown in purple. Bottom panel: the same data zoomed in around $τ_{P - p} = τ_{P - D}$ . Dashed lines are sine fits before (blue) and after (green) the D pulse arrival using constant frequency $ω_{C}$ and $ω_{H}$ , respectively. The narrow region, in which the phase shift occurs and both fits diverge from data, is highlighted. (f) Estimation of electron and lattice temperature after the D pulse arrival using a two-temperature model for $F_{D} = 1.5 mJ / c m^{2}$ [1].
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Figure 4
(a) Time evolution of anharmonic potential used for simulations with Ginzburg-Landau theory. Starting from global minimum on free-energy surface (1) the D pulse drives the system to the high-symmetry state (2), from which it thermalizes via intermediate states (3) to the new metastable state, which corresponds to the local minimum on the equilibrium free-energy surface (4). (b) Simulated three-pulse spectrum using a Ginzburg-Landau model (a). (c) Evolution of quadratic potential used for the topological model. Starting from the ground state (1), the system follows to the transient state with high concentration of discommensurations (2), which then decrease to the new, nonzero value (3) [1]. (d,e) Results of simulations with model from (c) for $F_{D}$ above and below the switching threshold, respectively.
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Physical Review B

covering condensed matter and materials physics

Real-time observation of the coherent transition to a metastable emergent state in $1 T - Ta S_{2}$

Jan Ravnik, Igor Vaskivskyi, Tomaz Mertelj, and Dragan Mihailovic

Phys. Rev. B 97, 075304 – Published 12 February 2018

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Physical Review B

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Real-time observation of the coherent transition to a metastable emergent state in 1T−TaS2

Jan Ravnik, Igor Vaskivskyi, Tomaz Mertelj, and Dragan Mihailovic

Phys. Rev. B 97, 075304 – Published 12 February 2018

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Real-time observation of the coherent transition to a metastable emergent state in $1 T - Ta S_{2}$