Uncovering the nature of transient and metastable nonequilibrium phases in $1T\ensuremath{-}{\mathrm{TaS}}_{2}$

Uncovering the nature of transient and metastable nonequilibrium phases in $1 T - {TaS}_{2}$

Tanusree Saha, Arindam Pramanik, Barbara Ressel, Alessandra Ciavardini, Fabio Frassetto, Federico Galdenzi, Luca Poletto, Arun Ravindran, Primož Rebernik Ribič, and Giovanni De Ninno

Phys. Rev. B 108, 035157 – Published 31 July 2023

Abstract

Complex systems are characterized by strong coupling between different microscopic degrees of freedom. Photoexcitation of such materials can drive them into new transient and metastable hidden phases that may not have any counterparts in equilibrium. By exploiting femtosecond time- and angle-resolved photoemission spectroscopy, we probe the photoinduced transient phase and the recovery dynamics of the ground state in a complex material: the charge density wave (CDW)–Mott insulator $1 T − {TaS}_{2}$ . We reveal striking similarities between the band structures of the transient phase and the (equilibrium) structurally undistorted metallic phase, with evidence for the coexistence of the low-temperature Mott insulating phase and high-temperature metallic phase. Following the transient phase, we find that the restorations of the Mott and CDW orders begin around the same time. This highlights that the Mott transition is tied to the CDW structural distortion, although earlier studies have shown that the collapses of Mott and CDW phases are decoupled from each other. Interestingly, as the suppressed order starts to recover, a metastable phase emerges before the material recovers to the ground state. Our results demonstrate that it is the CDW lattice order that drives the material into this metastable phase, which is indeed a commensurate CDW–Mott insulating phase but with a smaller CDW amplitude. Moreover, we find that the metastable phase emerges only under strong photoexcitation ( $\sim 3.6$ mJ/ ${cm}^{2}$ ) and has no evidence when the photoexcitation strength is weak ( $\sim 1.2$ mJ/ ${cm}^{2}$ ).

Received 14 January 2023
Accepted 10 July 2023

DOI:https://doi.org/10.1103/PhysRevB.108.035157

Physics Subject Headings (PhySH)

Electronic structure

Complex materials Mott insulators

Time & angle resolved photoemission spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Tanusree Saha ^1,*, Arindam Pramanik², Barbara Ressel¹, Alessandra Ciavardini¹, Fabio Frassetto ³, Federico Galdenzi¹, Luca Poletto³, Arun Ravindran¹, Primož Rebernik Ribič⁴, and Giovanni De Ninno^1,4

¹Laboratory of Quantum Optics, University of Nova Gorica, 5270 Ajdovščina, Slovenia
²Department of Theoretical Physics, Tata Institute of Fundamental Research, Mumbai 400005, India
³CNR-Institute of Photonics and Nanotechnologies (CNR-IFN), 35131 Padova, Italy
⁴Elettra Sincrotrone Trieste, Strada Statale 14 km 163.5, 34149 Trieste, Italy

^*Corresponding author: tanusree.saha@student.ung.si

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Vol. 108, Iss. 3 — 15 July 2023

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Figure 1
(a) (Top) In-plane structural distortion in the CCDW phase of $1 T − {TaS}_{2}$ produces “Star-of-David” clusters having inequivalent “a,” “b,” and “c” Ta atoms. Red and blue dashed lines indicate the unit cells in the CCDW and unreconstructed phases, respectively. The arrows indicate the displacement of the Ta atoms from their initial positions. (Bottom) Brillouin zone in the unreconstructed (blue) and distorted (red) phases with the high-symmetry points $Γ, M$ , and $K$ . (b) A schematic of the pump-probe experimental geometry where the electric field $\vec{E}$ of s- and p-polarized pulses are indicated by blue (along the $y$ axis) and green (in the $xz$ plane) double-headed arrows, respectively.
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Figure 2
(a) Time evolution of the electronic band structure in $1 T − {TaS}_{2}$ around the $M$ point (along the $M K$ direction). The peak positions of the energy distribution curves (EDCs) have been plotted as a function of $k_{| |}$ at each pump-probe delay $Δ t$ . (b) ARPES snapshots acquired before and after ( $Δ t = + 300$ fs) photoexcitation. (c) Corresponding EDC stacking where the blue curve represents the EDC at $M$ . The black curves are guides to the eye for the band dispersion. (d) Comparison of the band dispersion before photoexcitation and in the transient state of the system, where there is an energy shift towards $E_{F}$ and the band is more dispersive. All the data correspond to a high pump fluence of 3.6 mJ/ ${cm}^{2}$ and the dashed lines in panels (b) and (c) indicate $E_{F}$ . Binding energy is abbreviated to B.E.
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Figure 3
(a) Temporal evolution of the EDCs at early pump-probe delays integrated over a $k_{| |}$ range of $\pm 0.1 Å^{- 1}$ around the $Γ$ point (along the $Γ M$ direction). (b) ARPES snapshots acquired before and after ( $Δ t = + 300$ fs) photoexcitation. (c) Corresponding EDC stacking where the blue curve denotes the EDC at $Γ$ . Smooth curves are guides to the eye to emphasize the change in the band dispersion around $Γ$ in the transient phase. (d) ARPES snapshots around $Γ$ taken at different delays using s-polarized probe pulses. (e) Corresponding EDC stacking. The smooth black line indicates the flat upper Hubbard band, and its dynamics is obtained by changing the probe polarization from horizontal (p-pol) to vertical (s-pol). The data acquired using p-polarized and s-polarized probe pulses correspond to pump fluences of 3.2 and 3.6 mJ/ ${cm}^{2}$ , respectively, and the dashed lines indicate $E_{F}$ . Binding energy is abbreviated to B.E.
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Figure 4
(a) Time evolution of the electronic band dispersion around the $M$ point (along the $M K$ direction). For each pump-probe delay $Δ t$ , the peak positions of the EDCs are plotted as a function of $k_{| |}$ . (b) ARPES snapshots acquired before and after ( $Δ t = + 3.5$ ps) photoexcitation. (c) Corresponding EDC stacking where the blue curve represents the EDC at $M$ . The black curves are guides to the eye for the band dispersion. (d) Comparison of the band dispersion before photoexcitation and in the metastable phase of the system. The energy shifts around the band minimum and maximum are indicated by arrows. (e) ARPES snapshots acquired before and after ( $Δ t = 3.5$ ps) photoexcitation around the $Γ$ point (along the $Γ M$ direction). (f) Temporal evolution of the EDCs at longer delays integrated over a $k_{| |}$ range of $\pm 0.1 Å^{- 1}$ around $Γ$ . All the data correspond to a high pump fluence of 3.6 mJ/ ${cm}^{2}$ and the dashed lines in panels (b), (c), and (e) denote $E_{F}$ . Binding energy is abbreviated to B.E.
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Figure 5
(a) ARPES snapshots of the Ta $5 d$ subband at 0.5 eV below $E_{F}$ along the $Γ M$ direction taken before and after photoexcitation: delay $Δ t = - 1$ ps (left), $Δ t = + 30$ ps at pump fluence 4.2 mJ/ ${cm}^{2}$ (middle), and $Δ t = + 3$ ps at pump fluence 1.2 mJ/ ${cm}^{2}$ (right). (b) Corresponding stacked EDCs representing the band dispersion. Smooth black curves are guides to the eye for the dispersion and dashed lines denote $E_{F}$ . (c) Peak positions of the EDCs plotted as a function of $k_{| |}$ at various time delays for high fluence, 4.2 mJ/ ${cm}^{2}$ . (d) The same for low fluence, 1.2 mJ/ ${cm}^{2}$ . The data at high fluence show the presence of a metastable phase. Binding energy is abbreviated to B.E.
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Physical Review B

covering condensed matter and materials physics

Uncovering the nature of transient and metastable nonequilibrium phases in $1 T - {TaS}_{2}$

Tanusree Saha, Arindam Pramanik, Barbara Ressel, Alessandra Ciavardini, Fabio Frassetto, Federico Galdenzi, Luca Poletto, Arun Ravindran, Primož Rebernik Ribič, and Giovanni De Ninno

Phys. Rev. B 108, 035157 – Published 31 July 2023

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Physical Review B

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Uncovering the nature of transient and metastable nonequilibrium phases in 1T−TaS2

Tanusree Saha, Arindam Pramanik, Barbara Ressel, Alessandra Ciavardini, Fabio Frassetto, Federico Galdenzi, Luca Poletto, Arun Ravindran, Primož Rebernik Ribič, and Giovanni De Ninno

Phys. Rev. B 108, 035157 – Published 31 July 2023

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Uncovering the nature of transient and metastable nonequilibrium phases in $1 T - {TaS}_{2}$