The transient optical conductivity of photoexcited 1T-TaS2 is determined over
a three-order-of-magnitude frequency range. Prompt collapse and recovery of the
Mott gap is observed. However, we find important differences between this
transient metallic state and that seen across the thermally-driven
insulator-metal transition. Suppressed low-frequency conductivity, Fano phonon
lineshapes, and a mid-infrared absorption band point to polaronic transport.
This is explained by noting that the photo-induced metallic state of 1T-TaS2 is
one in which the Mott gap is melted but the lattice retains its low-temperature
symmetry, a regime only accessible by photo-doping.Comment: 10 pages, 4 figure

Berger, Helmuth

Cavalleri, Andrea

Dean, Nicky

Fausti, Daniele

Gasparov, Lev

Kaiser, Stefan

Petersen, Jesse

Tobey, Ra'anan I.

English

arXiv

The transient optical conductivity of photoexcited 1T−TaS2 is determined over a three-order-of-magnitude frequency range. Prompt collapse and recovery of the Mott gap is observed. However, we find important differences between this transient metallic state and that seen across the thermally driven insulator-metal transition. Suppressed low-frequency conductivity, Fano phonon line shapes, and a midinfrared absorption band point to polaronic transport. This is explained by noting that the photoinduced metallic state of 1T−TaS2 is one in which the Mott gap is melted but the lattice retains its low-temperature symmetry, a regime only accessible by photodoping

Dean, N.

Petersen, J.

Fausti, D.

Tobey, R.

Kaiser, S.

Gasparov, L.

Berger, H.

Cavalleri, A.

MPG.PuRe

Polaronic Conductivity in the Photoinduced Phase of 1T−TaS2

The transient optical conductivity of photoexcited 1T-TaS2 is determined over a three-order-of-magnitude frequency range. Prompt collapse and recovery of the Mott gap is observed. However, we find important differences between this transient metallic state and that seen across the thermally-driven insulator-metal transition. Suppressed low-frequency conductivity, Fano phonon lineshapes, and a mid-infrared absorption band point to polaronic transport. This is explained by noting that the photo-induced metallic state of 1T-TaS2 is one in which the Mott gap is melted but the lattice retains its low-temperature symmetry, a regime only accessible by photo-doping

Petersen, J. C.

Tobey, R. I.

Gasparov, L. V.

DESY Publication Database

Polaronic Conductivity in the Photoinduced Phase of $\mathrm{1T − TaS_2}$

The transient optical conductivity of photoexcited 1T-TaS2 is determined over a three-order-of-magnitude frequency range. Prompt collapse and recovery of the Mott gap is observed. However, we find important differences between this transient metallic state and that seen across the thermally driven insulator-metal transition. Suppressed low-frequency conductivity, Fano phonon line shapes, and a midinfrared absorption band point to polaronic transport. This is explained by noting that the photoinduced metallic state of 1T-TaS2 is one in which the Mott gap is melted but the lattice retains its low-temperature symmetry, a regime only accessible by photodoping

Infoscience - École polytechnique fédérale de Lausanne

Polaronic Conductivity in the Photoinduced Phase of 1T-TaS2

DESY

The transient optical conductivity of photoexcited 1T−TaS2 is determined over a three-order-of-magnitude frequency range. Prompt collapse and recovery of the Mott gap is observed. However, we find important differences between this transient metallic state and that seen across the thermally driven insulator-metal transition. Suppressed low-frequency conductivity, Fano phonon line shapes, and a midinfrared absorption band point to polaronic transport. This is explained by noting that the photoinduced metallic state of 1T−TaS2 is one in which the Mott gap is melted but the lattice retains its low-temperature symmetry, a regime only accessible by photodoping

Polaronic Conductivity in the Photoinduced Phase of 1T-TaS2N. Dean,1,* J. C. Petersen,1,2 D. Fausti,1,2 R. I. Tobey,1,3 S. Kaiser,2 L. V. Gasparov,4 H. Berger,5 and A. Cavalleri1,2,†1Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom2Max Planck Department for Structural Dynamics, University of Hamburg,Centre for Free Electron Laser Science, Notkestraße 85, 22607 Hamburg, Germany3Condensed Matter Physics and Material Sciences Department, Brookhaven National Laboratory, Upton, New York 11973, USA4Department of Chemistry and Physics, University of North Florida,4567 St. Johns Bluff Road, South Jacksonville, Florida 32224, USA5Institue of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland(Received 1 September 2010; published 4 January 2011)The transient optical conductivity of photoexcited 1T-TaS2 is determined over a three-order-of-magnitude frequency range. Prompt collapse and recovery of the Mott gap is observed. However, wefind important differences between this transient metallic state and that seen across the thermally driveninsulator-metal transition. Suppressed low-frequency conductivity, Fano phonon line shapes, and amidinfrared absorption band point to polaronic transport. This is explained by noting that the photo-induced metallic state of 1T-TaS2 is one in which the Mott gap is melted but the lattice retains itslow-temperature symmetry, a regime only accessible by photodoping.DOI: 10.1103/PhysRevLett.106.016401 PACS numbers: 71.45.Lr, 63.20.kd, 73.20.Mf, 78.47.jgLow-dimensional dichalcogenides exhibit a wealth ofcompeting phenomena, ranging from Mott and Peierlstransitions [1,2] to spin and charge density waves (CDW)[3,4] and superconductivity [5]. Many such materialsalso display photoinduced phase transitions, wherein lightpulses can create new transient states by driving ultrafastchanges in the macroscopic properties [6,7].1T-TaS2 is a quasi-two-dimensional transition metaldichalcogenide in which CDW behavior coexists withstrong electron-electron correlations [8–12]. At high tem-peratures 1T-TaS2 is metallic. Below 350 K, a periodiclattice distortion creates star-shaped clusters of tantalumatoms, causing the CDW to become nearly commensurateas domains of such clusters appear [5,10]. Below TC ¼180 K, as the CDW becomes fully commensurate with thelattice and a long-rangeffiffiffiffiffi13p  ffiffiffiffiffi13p superlattice structureemerges [9–11], the system becomes insulating. This struc-tural distortion reduces the bandwidth below a criticalvalue and leads to the opening of a 100-meV correlation(Mott) gap, with electrons localized at the star centers[4,9,12]. Mott localization explains the low-temperatureconductivity, which is lower than that expected from bandstructure [9].The general features of the photoinduced dynamics in1T-TaS2 were first revealed in all-optical studies [13–15],which highlighted important changes in the near-infraredoptical properties and the excitation of coherent CDWamplitude-mode oscillations. Time-resolved photoelectronspectroscopy experiments [7] have provided a more com-prehensive view of the physics, demonstrating the promptcollapse and recovery of the gap at k ¼ 0 on a time scaleshort compared to the period of the coherent amplitudemode. This was interpreted as ultrafast melting of the Mottinsulator, driven primarily by carrier temperature and onlyweakly influenced by the amplitude mode. Recent ultrafastelectron diffraction measurements substantiate this view,showing that photodoping does not trigger complete re-laxation of the structural distortion [16].Here, we set out to probe the electrodynamics of thistransient Mott-melted phase, using a combination of ultra-fast optical probes that cover three decades of frequency.In this way, we access a state of the solid in whichphotodoping has removed the correlation gap, while thelattice symmetry, and presumably the bandwidth, of thelow-temperature Mott insulator are only weakly perturbed.In our experiments, 1T-TaS2 was held in its low-temperature insulating phase at 15 K and excited with1:3 m-wavelength pulses (950 meV) from an opticalparametric amplifier. In a first set of measurements, low-energy transport was probed with time-resolved THzspectroscopy [17–19]. The time-dependent reflectivitywas referenced against that of the unperturbed sampleand of a gold-coated portion of the surface. This allowedfor the determination of the complex optical constants foreach time delay [20]. The time-dependent conductivitywas extracted by considering a thin photoexcited filmwith an unperturbed bulk volume beneath, thereby keepinginto account the mismatch in pump and probe penetrationdepths (45 nm and 7 m, respectively).Figure 1(a) shows the real part of the conductivity 1obtained in the low-temperature equilibrium phase andat several pump-probe time delays for a pump intensityof 550 J cm2. For comparison, the pump fluence rangeused in earlier photoemission studies on the photoinducedphase was restricted to intensities lower than 150 J cm2[7]. In the conductivity data presented here, Drude-likePRL 106, 016401 (2011) P HY S I CA L R EV I EW LE T T E R Sweek ending7 JANUARY 20110031-9007=11=106(1)=016401(4) 016401-1  2011 The American Physical Societybehavior is indicated by a flat response in1. The temporalevolution of 1 (1 THz) is plotted in Fig. 1(b). A promptincrease is followed by exponential decay back towardsequilibrium, reflecting changes in the Drude spectralweight indicative of the loss and recovery of the Mottgap [7]. However, the photoinduced change in 1 issignificantly smaller than that observed upon heating.Furthermore, the three low-lying phonon modes observed[11,12] remain essentially unscreened and 1 increasescontinuously with excitation fluence, as shown inFig. 1(c). All these observations are inconsistent with thelarger steplike increase in 1 and the complete screeningof the phonon modes observed at 220 K [12,21]. Thisindicates a transient metallic state with qualitatively differ-ent transport properties to those observed in the thermalmetallic state.Figure 2 shows the complex conductivity around the1.6 THz phonon resonance at various time delays. Wefind that the Lorentzian phonon line shapes of the insulatorbecome strongly asymmetric in the photoexcited state.The real part 1 exhibits a clear shift of spectral weightfrom the low- to the high-energy side of the mode, with adip developing below the central frequency. The imaginarypart 2 flattens on the high-energy side. A similar effect,though of smaller magnitude, is seen in the other twomodes within our measurement bandwidth.These asymmetries are well described by the Fanoeffect, a general spectroscopic feature which arises fromcoupling between a discrete resonance and a continuumof excitations [22]. Such coupling produces interferencearound the resonance, leading to a complex conductivityof the formð!Þ ¼ i0ðq iÞ2iþ!2 !200!1;where !0 and 0 are the mode central frequency andlinewidth, respectively, and q is the Fano asymmetry pa-rameter (1=q denotes a more asymmetric profile) [22,23].Fano effects in phonon line shapes have been observedin other correlated electron systems both statically [23,24]and dynamically [25,26]. Their presence indicates strongelectron-phonon coupling, and studying them can revealmuch useful information on the interactions in correlatedsystems [24,27].The gray (red) lines in Fig. 2 show a fit to the line shapesof the 1.6 THz mode as a function of pump-probe timedelay. We note that the asymmetry parameter and renor-malized phonon frequency, shown on the right-hand side ofFig. 2, reach their maximum alteration after around 2 ps.This is in apparent contrast with the prompt increase inlow-frequency conductivity of Fig. 1. However, it is to bestressed that the temporal resolutions of the two measure-ments are different. The extrapolated dc conductivity isfound by measurement of the broadband THz response,which can be determined on the subpicosecond time scale.On the other hand, reshaping of the phonon linewidthsinvolves only spectral features that extend over a fewhundred GHz bandwidth and, as such, cannot be detectedfaster than a few picoseconds.The decay back to equilibrium of the asymmetry pa-rameter and mode frequency exhibit the same temporaldependence as 1 in Fig. 1 and of the Mott gap measuredin Ref. [7]. Moreover, the sign of the asymmetry showsthat the phonons are interacting with a continuum centeredat higher energy, likely the new states appearing as theMott gap collapses [22,24].At higher photon energies, photoinduced reflectancechanges were measured with pulses from a second opticalparametric amplifier, continuously tunable from the100-meV Mott gap (12:4 m wavelength) to the visible.FIG. 2 (color online). Temporal evolution of the 1.6 THz modeafter excitation with 550 J cm2 pump pulse. Left-handpanels: Real and imaginary parts of the conductivity for differentpump-probe delays (black lines). Gray (red) lines are fits using aFano line shape. Right-hand panels: Time-dependent asymmetryparameter 1=q and renormalized mode frequency !0.FIG. 1 (color online). (a) Real part of the conductivity 1 asa function of pump-probe time delay for 550 J cm2 pumpintensity. Static conductivity in the low- and high-temperaturephases is shown in black for comparison (high-temperature datafrom [12]). (b) Transient behavior of 1 (1 THz) for differentpump intensities (lines are guides to the eye). (c) Intensitydependence of 1 (1 THz) at 0 ps time delay.PRL 106, 016401 (2011) P HY S I CA L R EV I EW LE T T E R Sweek ending7 JANUARY 2011016401-2Figure 3 shows typical time-domain traces obtained forthree different probe energies above the gap. The reflec-tance shows a sudden increase near zero time delay, fol-lowed by decay with superimposed 2.4-THz oscillationsfrom the coherent CDW amplitude mode [13–15].The magnitude of the reflectance change, the relativesize of the oscillations, and their lifetime all depend onthe probe photon energy, as illustrated in the inset of Fig. 3.As the probe energy is lowered towards the gap, R=Rincreases dramatically due to the shift of spectral weightinto the previously gapped region [7]. Conversely, thecoherent lattice vibrations influence the optical responseprogressively less. This suggests that the structural distor-tions are only weakly connected to the low-frequencyelectrodynamics, further substantiating a view in whichthe Mott gap is melted independently from the coherentCDW distortions.The photoinduced changes in reflectance from the THzto the visible are summarized in Fig. 4. The data were takenat 0 ps time delay, when theMott gap has just collapsed andthe reflectance changes, and hence the associated conduc-tivity changes, are maximal. Note that while in the THzrange the raw signal is small ( 1%) due to the largedifference between the pump and probe penetration depths,in the midinfrared region, where the extinction coefficientsare better matched, a pronounced increase in reflectanceis clearly visible already in the unprocessed data.To extract broadband time-dependent optical conductiv-ity in the absence of phase information at high photonenergies, we applied the following procedure. First, thestatic broadband reflectance at 30 K was fitted by modelingthe equilibrium optical conductivity with a series ofLorentzians, accounting for the phonons and electronictransition bands. Second, we analyzed the photoinducedchanges in reflectivity, which could be modeled by shifts ofspectral weight among the oscillators of the equilibrium fit.However, the midinfrared response required the addition ofa new resonance centered at 155 meV. This feature has noanalogue in any of the thermal phases. Its time dependencecan be seen in the reflectance transients of Fig. 3 whichgive it its structure, especially the trace at 120 meV, closeto the center of the absorption band. It appears promptlyupon photoexcitation and has decayed after almost 1 ps,indicating a strong connection to the loss and revival of theMott gap, which occurs on the same time scales.The lower panel of Fig. 4 shows the broadband conduc-tivity of the photoinduced phase at 0 ps time delay, asdetermined by this procedure. Three features emerge asespecially meaningful: reduced low-frequency conductivitycompared to the high-temperature phase, Fano line shapesfor the phonons, indicative of strong electron-lattice cou-pling, and a transient midinfrared resonance. The nature ofthis midinfrared band is consistent with that of polaronbands in other strongly correlated materials [28,29], andthe asymmetric reshaping in the phonon line shapes em-phasizes strong coupling between electrons and the latticecoinciding with the appearance of this new feature.From the combined observations above, then, the keyconclusion of our work is that transport in the photoin-duced phase is polaronic. Microscopically, charges that inFIG. 3 (color online). Pump-probe traces for three probephoton energies. Inset: Peak reflectivity changes (R=R) as afunction of probe photon energy (black dots, left-hand axis) andratio of oscillation amplitude to peak R=R (red circles, right-hand axis). In all cases the pump intensity was 750 J cm2.FIG. 4 (color online). Upper panel: Reflectance change imme-diately after photoexcitation (red dots). Gray (red) line is a fit tothe data (see text). Lower panel: Calculated conductivity imme-diately after photoexcitation [gray (red) line]. Dashed black lineis a Lorentzian representing a polaron band. In both panels,continuous black lines are the reflectance and conductivity of thethermal phases [12].PRL 106, 016401 (2011) P HY S I CA L R EV I EW LE T T E R Sweek ending7 JANUARY 2011016401-3the ground state were localized by electron correlations aremade mobile after photodoping, although electron-latticecoupling, which is still strong due to the presence of theCDW, imposes a dominant energy scale on their transport.These features belong to an exotic state of 1T-TaS2 that isonly accessible with photodoping.In summary, we have used optical spectroscopy over abroad frequency range to measure the transport propertiesof photoexcited 1T-TaS2. Our data clearly indicate theformation of a new state, with polaronic transport thatdescends directly from the separate influence that lighthas on electron correlations and on the lattice response.As such, the use of ultrabroadband time-resolved opticalspectroscopy emerges as an indispensable tool to under-stand the role of many interacting degrees of freedom intransient states of complex matter.*Corresponding author.n.dean1@physics.ox.ac.uk†Corresponding author.andrea.cavalleri@mpsd.cfel.de[1] P. Fazekas and E. Tosatti, Physica (Amsterdam) 99B, 183(1980).[2] S. van Smaalen, Acta Crystallogr. Sect. A 61, 51(2004).[3] G. Grüner, Density Waves in Solids (Perseus, Cambridge,MA, 1994).[4] F. Clerc, C. Battaglia, H. Cercellier, C. Monney, H.Berger, L. Despont, M.G. Garnier, and P. Aebi, J. Phys.Condens. Matter 19, 355002 (2007).[5] B. Sipos, A. F. Kusmartseva, A. Akrap, H. Berger, L.Forro, and E. Tutis, Nature Mater. 7, 960 (2008).[6] F. Schmitt et al., Science 321, 1649 (2008).[7] L. Perfetti, P. A. Loukakos, M. Lisowski, U. Bovensiepen,H. Berger, S. Biermann, P. S. Cornaglia, A. Georges, andM. Wolf, Phys. Rev. Lett. 97, 067402 (2006); L. Perfetti,P. A. Loukakos, M. Lisowski, U. Bovensiepen, M. Wolf,H. Berger, S. Biermann, and A. Georges, New J. Phys. 10,053019 (2008).[8] J. A. Wilson, F. J. DiSalvo, and S. Mahajan, Adv. Phys. 24,117 (1975).[9] P. Fazekas and E. Tosatti, Philos. Mag. B 39, 229(1979).[10] F. Clerc, C. Battaglia, M. Bovet, L. Despont, C. Monney,H. Cercellier, M.G. Garnier, P. Aebi, H. Berger, and L.Forro, Phys. Rev. B 74, 155114 (2006).[11] S. Uchida and S. Sugai, Physica (Amsterdam) 105B, 393(1981).[12] L. V. Gasparov, K. G. Brown, A. C. Wint, D. B. Tanner,H. Berger, G. Margaritondo, R. Gaal, and L. Forro, Phys.Rev. B 66, 094301 (2002).[13] J. Demsar, L. Forró, H. Berger, and D. Mihailovic, Phys.Rev. B 66, 041101 (2002).[14] Y. Toda, K. Tateishi, and S. Tanda, Phys. Rev. B 70,033106 (2004).[15] T. Onozaki, Y. Toda, S. Tanda, and R. Morita, Jpn. J. Appl.Phys. 46, 870 (2007).[16] M. Eichberger, H Schäfer, M. Krumova, M. Beyer, J.Demsar, H. Berger, G. Moriena, G. Sciaini, and R. J.Dwayne Miller, Nature (London) 468, 799 (2010).[17] M. C. Nuss, P.M. Mankiewich, M. L. O’Malley, E. H.Westerwick, and P. B. Littlewood, Phys. Rev. Lett. 66,3305 (1991).[18] M. C. Beard, G.M. Turner, and C.A. Schmuttenmaer,J. Appl. Phys. 90, 5915 (2001).[19] E. Knoesel, M. Bonn, J. Shan, and T. F. Heinz, Phys. Rev.Lett. 86, 340 (2001).[20] S. Nashima, O. Morikawa, K. Takata, and M. Hangyo,Appl. Phys. Lett. 79, 3923 (2001).[21] A. Thompson, R. F. Gamble, and J. F. Revelli, Solid StateCommun. 9, 981 (1971).[22] U. Fano, Phys. Rev. 124, 1866 (1961).[23] I. Kezsmarki, N. Hanasaki, K. Watanabe, S. Iguchi,Y. Taguchi, S. Miyasaka, and Y. Tokura, Phys. Rev. B73, 125122 (2006).[24] S. Lupi, M. Capizzi, P. Calvani, B. Ruzicka, P. Maselli,P. Dore, and A. Paolone, Phys. Rev. B 57, 1248 (1998).[25] M. Hase, J. Demsar, and M. Kitajima, Phys. Rev. B 74,212301 (2006).[26] J. D. Lee, J. Inoue, and M. Hase, Phys. Rev. Lett. 97,157405 (2006).[27] A. Damascelli, K. Schulte, D. van der Marel, and A.A.Menovsky, Phys. Rev. B 55, R4863 (1997).[28] S. Lupi, P. Maselli, M. Capizzi, P. Calvani, P. Giura, andP. Roy, Phys. Rev. Lett. 83, 4852 (1999).[29] S. Ciuchi and S. Fratini, Phys. Rev. B 77, 205127 (2008).PRL 106, 016401 (2011) P HY S I CA L R EV I EW LE T T E R Sweek ending7 JANUARY 2011016401-4

FAUSTI, DANIELE

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Polaronic conductivity in the photoinduced phase of 1T-TaS2

Polaronic conductivity in the photoinduced phase of 1T-TaS2

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