key: cord-0148756-8kwfiwp1
authors: Li, Weijian; Naik, Gururaj V.
title: Large optical tunability from charge density waves in 1T-TaS$_2$ under incoherent illumination
date: 2020-06-03
journal: nan
DOI: nan
sha: 3f826b14afb64e46b1dbc66872b3e6e224d60b46
doc_id: 148756
cord_uid: 8kwfiwp1

Strongly correlated materials possess a complex energy landscape and host many interesting physical phenomena, including charge density waves (CDWs). CDWs have been observed and extensively studied in many materials since their first discovery in 1972. Yet, they present ample opportunities for discovery. Here, we report a large tunability in the optical response of a quasi-2D CDW material, 1T-TaS$_2$, upon incoherent light illumination at room temperature. We show that the observed tunability is a consequence of light-induced rearrangement of CDW stacking across the layers of 1T-TaS$_2$. Our model, based on this hypothesis, agrees reasonably well with experiments suggesting that the interdomain CDW interaction is a vital knob to control the phase of strongly correlated materials.

Light-matter interaction in strongly correlated materials is interesting because light can significantly alter the free energy landscape of strong correlations resulting in many new physical phenomena. [1] [2] [3] Optical excitation can tip the balance between various competing forms of order leading to photo-induced metal-to-insulator transitions, 4-6 charge density waves, [7] [8] [9] [10] ferromagnetic/antiferromagnetic transitions, 11, 12 superconductivity, 13 and others. 14 Though light-induced changes in the lattice, electrical, and magnetic properties have been extensively studied before, optical properties remain much to be explored. In this work, we study the tunable optical properties of a strongly correlated material, 1T-TaS 2 , which supports charge density waves (CDWs) at room temperature.

Many chalcogenides and organic compounds support CDWs at low temperatures. 15 However, 1T-TaS 2 , 1T-TaSe 2 , and a few other lanthanide tellurides support CDWs at room temperature and are interesting for device applications. [16] [17] [18] [19] [20] Among them, 1T-TaS 2 is the only material that exhibits nearly commensurate CDW (NCCDW) phase at room temperature resulting in a large tunability in its electrical conductivity.

The tunability of electrical properties of 1T-TaS 2 is extensively studied in the past. 1T-TaS 2 was shown to exhibit nonlinear electrical conductance at room temperature, 21 and hysteresis behavior of its electric resistance. [22] [23] [24] Also, the CDW phase transition was demonstrated to be sensitive to pressure, 25, 26 strain, 27,28 thickness, 29,30 gate voltage, 31-33 and chemical doping. 34, 35 However, the optical properties of 1T-TaS 2 and its tunability remain unexplored. In this work, we study light-tunable optical properties of 1T-TaS 2 .

The charge order in 1T-TaS 2 manifests as a lattice reorganization where 12 Ta atoms surrounding a central Ta atom move slightly inwards forming a David-star structure (see Fig. 1a ). Groups of such David-stars called CDW domains exist in each layer of 1T-TaS 2 at room temperature. The relative position or stacking arrangement of such CDW domains across layers has been recently reported to impact the electronic bandstructure of 1T-TaS 2 significantly. [36] [37] [38] [39] Depending on the stacking, the material can be insulating or metallic along its c-axis. Such drastic dependence of the electronic properties on the CDW stacking configuration is unusual and forms the central theme of this work. Here, we observe a substantial change in the dielectric function of 1T-TaS 2 only in the c-axis upon incoherent white light illumination. We attribute the observed changes in the dielectric function to the stacking rearrangement of the CDW domains in 1T-TaS 2 .

We obtained crystalline 1T-TaS 2 thin film samples by mechanical exfoliation, as reported in our previous work. 40 The reflectance of a 180 nm thick film measured at three different wavelengths using an objective of 0.85 numerical aperture is shown in Fig. 1b as a function of illumination intensity. The 1T-TaS 2 film exhibits an intensity-dependent reflectance even at low illumination intensities of a few mW/cm 2 . For comparison, the peak intensity of 1.5AM

sun is approximately 100 mW/cm 2 .

Using a Fourier plane imaging spectrometer, we measure the reflection and transmission spectra of the 1T-TaS 2 film at incident angles ranging from 0 to 53 • . From this data, we extract the anisotropic dielectric function of 1T-TaS 2 in the visible (see SI). The in-plane and out-of-plane (along c-axis) dielectric functions denoted by ε o and ε e respectively are shown in Fig. 1c for low illumination. 1T-TaS 2 is strongly anisotropic and highly absorbing dielectric in the visible.

As we increase the incoherent illumination intensity up to 1000 mW/cm 2 , the in-plane dielectric function remains the same (see SI), but the out-of-plane dielectric function changes.

The real and imaginary parts of the intensity-dependent ε e are as shown in Fig. 1d is about a millisecond. Such a large lifetime for free carriers is not physical. Additionally, the dielectric constants changing only along c-axis suggests a mechanism that involves interlayer interaction. To further probe the underlying mechanism, we carry out time-resolved measurements. Light off Light on The observed change in reflectance (∆R) could be fitted well with a sum of two exponential functions, as indicated in Fig. 2b . The two time-constants each for light-on and light-off transients are plotted in Fig. 2c as a function of the peak pump intensity. The time-constants are nearly independent on intensity and suggest that there must be at least two mechanisms behind the observed ∆R. Since the smallest laser intensity in Fig. 2 is about an order of magnitude more than that of white light in Fig. 1 , photothermal effects could be expected as one of the mechanisms contributing to the ∆R in Fig. 2 Photothermal effects depend on the thermal conductivity of the substrate. Hence, measuring time-constants for 1T-TaS 2 on different substrates could elicit the photothermal effect. Recently, CDW stacking has been shown to strongly influence the band structure of 1T-TaS 2 , especially in the Γ−A direction. 36 Thus, we hypothesize that the change in the c-axis dielectric function of 1T-TaS 2 originates from the rearrangement of CDW stacking across layers. Using the three-level model, the steady-state fractions of CDW domains in the two stacking configurations could be computed as a function of illumination intensity. Fig. 4b plots the fraction of CDW domains in stacking |1 (n 1 ) as a function of incident light intensity.

Note that the equilibrium value of n 1 is approximated to zero because the energy gap be- tween the two staking levels is more than 5k B T at room temperature (previous discussion about Fig. 3b) . From the rate equations, at steady-state, n 1 = τ 10 τ 21 (1+Rτ 10 )

Rτ 20 τ 21 +τ 21+τ 20 . In the limit, τ 10 >> τ 21 and τ 10 >> τ 20 , n 1 N ≈ Rτ 10 2(1+Rτ 10 ) for R << 1/τ 21 . We set τ 21 = τ 20 =3 ns and τ 10 =0.38 µs. For the intensities used in this work, R << 1/τ 21 and thus the maximum value of n 1 is close to 50%. Also, n 1 as a function of R is determined by only one parameter, τ 10 . Since, our time-resolved experiments measured τ 10 , we expect the prediction of Fig. 4b to be quantitative.

The electronic bandstructure of 1T-TaS 2 along Γ−A has been predicted to depend on the CDW stacking order. 36 Hence, we associate each stacking configuration to a unique dielectric function along the c-axis. We calculate the c-axis dielectric function for any intensity of incident light by using the population fraction information from Fig. 4b in Maxwell-Garnet effective medium equation. 44 The lowest intensity of incoherent light used in Fig. 1 corresponds to n 1 ≈1% and hence may be considered as the c-axis dielectric function of ground state stacking. Since this ground state ε e is a two-Lorentzian curve, we expect the ε e corresponding to stacking |1 is also a two-Lorentzian curve. The parameters of the two Lorentzian oscillators corresponding to stacking |1 are obtained by fitting the effective permittivity from In conclusion, we discovered a new optical phenomenon in 1T-TaS 2 . Our work showed that low-intensity white light illumination could change the c-axis permittivity of 1T-TaS 2 by unity-order. Time-resolved measurements showed that the speed of change is a few MHz. We hypothesized that the stacking of CDW domains across layers could rearrange with illumi-nation and thereby lead to a change in c-axis optical constants. Our modeling results agreed well with experimental measurements upholding our hypothesis. This discovery proves that the electronic bandstructure depends on the CDW stacking order, and the stacking order may be controlled by light. Our work shows that stacking order is a new dimension in the phase diagram of strongly correlated materials and enriches opportunities for discovery. This work could lead to further development of low-power and fast-tunable optical materials and potentially revolutionize future imaging, display, and sensing applications.

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The authors thank Bharadwaj group at Rice University for their help in optical characterization, and also thank all Naik group members for useful discussion and help with preparing this manuscript.

The following files are available free of charge.The supporting information is available free of charge via the Internet at http://pubs.acs.org.• SI: Methods; Ordinary and extraordinary dielectric functions; Dielectric function extraction; Time-resolved measurements using an ultrafast laser; Photothermal effect; Coulomb energy.