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Unconventional photoinduced charge density wave dynamics in 2H−NbSe2

R. Venturini, A. Sarkar, P. Sutar, Z. Jagličić, Y. Vaskivskyi, E. Goreshnik, D. Mihailovic, and T. Mertelj
Phys. Rev. B 108, 235160 – Published 20 December 2023
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  • INTRODUCTION
  • METHODS
  • RESULTS
  • DISCUSSION
  • SUMMARY AND CONCLUSIONS
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    We investigated temperature (T)-dependent ultrafast near-infrared (NIR) transient reflectivity dynamics in coexisting superconducting (SC) and charge density wave (CDW) phases of layered 2H−NbSe2 using NIR and visible excitations. With visible pump-photon excitation (400 nm) we find a slow high-energy quasiparticle relaxation channel, which is present in all phases. In the CDW phase, we observe a distinctive transient response component, irrespective of the pump-photon energy. The component is marked by the absence of coherent amplitude mode oscillations and a relatively slow, picosecond rise time, which is different than in most of the typical CDW materials. In the SC phase, another tiny component emerges that is associated with optical suppression of the SC phase. The transient reflectivity relaxation in the CDW phase is dominated by phonon diffusive processes with an estimated low-T heat diffusion constant anisotropy of ∼30. Strong excitation of the CDW phase reveals a weakly nonthermal CDW order parameter (OP) suppression. Unlike CDW systems with a larger gap, where the optical OP suppression involves only a small fraction of phonon degrees of freedom, the OP suppression in 2H−NbSe2 is characterized by the excitation of a large number of phonon degrees of freedom and significantly slower dynamics.

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    • Received 5 September 2023
    • Revised 24 November 2023
    • Accepted 27 November 2023

    DOI:https://doi.org/10.1103/PhysRevB.108.235160

    ©2023 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Acoustic phononsCharge density wavesDynamical phase transitionsLattice thermal conductivitySuperconductivity
    1. Physical Systems
    Layered crystalsLow-temperature superconductorsTransition metal dichalcogenides
    1. Techniques
    PhotoexcitationTime-resolved reflection spectroscopyUltrafast femtosecond pump probe
    Condensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

    Authors & Affiliations

    R. Venturini1, A. Sarkar1, P. Sutar1, Z. Jagličić2,3, Y. Vaskivskyi1, E. Goreshnik4, D. Mihailovic1,5, and T. Mertelj1,5,*

    • 1Department of Complex Matter, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
    • 2Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jamova cesta 2, Ljubljana, Slovenia
    • 3Institute of Mathematics, Physics and Mechanics, Jadranska 19, Ljubljana, Slovenia
    • 4Dept. of Inorganic Chemistry and Technology, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
    • 5Center of Excellence for Nanoscience and Nanotechnology (CENN Nanocenter), Jamova 39, 1000 Ljubljana, Slovenia

    • *tomaz.mertelj@ijs.si

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    Issue

    Vol. 108, Iss. 23 — 15 December 2023

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    Images

    • Figure 1
      Figure 1

      Sample characterization. (a) Temperature dependence of the SQUID magnetic susceptibility shows the crystals are superconducting below 6.8 K. (b) A 4.2 K scanning tunneling microscope image (set point parameters: tip bias V=−50 mV, I= 160 pA) and (d) the corresponding Fourier transform showing the ∼3×3 CDW state. (c) The electronic band structure adapted from Ref. [25]. The red hatched region corresponds to the 1.55 eV photon accessible excitation range. The 3.1 eV photon accessible excitation range exceeds the plotted energy range.

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    • Figure 2
      Figure 2

      Temperature-dependent transient reflectivity. (a) Schematics of the two-pulse pump-probe experiment. (b) and (c) T-dependent reflectivity transients with 3.1 eV and 1.55 eV pump-photon energy, respectively. Note the logarithmic scale after breaks in (b) and (c). (d) High-FP transients reveal more clearly the coherent sound wave oscillations. The lines are fits discussed in the text.

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    • Figure 3
      Figure 3

      Fluence-dependent transient reflectivity at T=4 K. (a) FP-normalized reflectivity transients at the 3.1 eV pump-photon energy. Note that curve overlap indicates a linear scaling with FP. (b) The corresponding unnormalized transients. (c), (d) The same at 1.55 eV pump-photon energy, respectively. The increased noise at high FP is due to the pump scattering. (e) Three-pulse transients as a function of the driving-pulse fluence, FD, at FP=19µJ/cm2. The dashed region indicates the D-pulse induced suppression of the signal at FD=3.3µJ/cm2, while the shaded region corresponds to the amplitude readout pump-probe delay discussed in the text with the vertical dashed line representing the D-pulse arrival time. The schematics indicating the pulse sequence is shown in the inset.

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    • Figure 4
      Figure 4

      Selected F-dependent transient response parameters at T=4 K. (a) The fluence dependence of the signal amplitudes. The open symbols correspond to the three-pulse experiment. The lines are the saturation model fits discussed in text. (b) The rise time (full circles) and three-pulse signal suppression time (open circles) as function of F. The inset to (b) shows the assumed saturation functions discussed in text.

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    • Figure 5
      Figure 5

      Depth dependence of the probe kernel [Eq. (a3)] in comparison to the normalized P and D deposited energy density profiles. The model kernel-weighted two-pulse [Eq. (a6)] and three-pulse response [Eq. (a7)] at the experimental, FP/Fth∼5, and a subthreshold FD are shown with the thick lines. The hatched region corresponds to the three-pulse signal suppression [cf. dashed region in Fig. 3]. The kernel parameters were taken from Table 1. The phase shift, ϕ∼π/2, obtained from the fit is such, that the negative-sign kernel region is relatively deep in the sample, minimizing its contribution to the signal. In the inset the two-pulse (1.55-eV PPE) and three-pulse kernel-weighted local responses at an above-threshold fluence are compared. The contribution from the deeper region is suppresed in the three-pulse experiment.

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    • Figure 6
      Figure 6

      Selected T-dependent transient response parameters. (a) The normalized transient reflectivity amplitude as a function of T at two excitation fluences at 1.55 eV pump photon energy. (b) T dependence of the lowest pump-fluence transient reflectivity. The shaded region corresponds to the averaging region of the amplitude readout. (c) and (d) the rise time and diffusion time as a function of T, respectively.

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    • Figure 7
      Figure 7

      Examples of fits to the data at different excitation conditions. The lines correspond to various decay models discussed in text.

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