Previous time-resolved Experiments

Many time-resolved studies have been performed on 1T-TaS₂, investigating the dynamics of single-particle and collective excitations [24,34,76,163,173,186–188] as well as CDW phase transitions [7,14,173,189–191]. A selection of these works will be briefly discussed in this section.

Figure 1.29: Transient reflectivity measurements for 1T-TaS2(a) and 2H-TaSe2(b) at different temperature above and belowT_C-NCandT_I-N, respectively. The signals are offset for clarity. The insets represent the phase diagrams of the corresponding bulk material. Reprinted figure with permission from Ref. [173] by Demsar et al.. Copyright 2002 by the American Physical Society.

In the work of Demsar et al. [173], the collective and single-particle excitations of 1T-TaS₂ (and 2H-TaSe2) were probed by time-resolved optical spectroscopy. This technique measures the transient reflectivity of the material and allows to trace the evolution of above-gap carriers after pulsed optical excitation. For temperatures ranging from 30 K up to 220 K, the transient reflection

∆R/Rexhibits typical features comprising a strong increase within few femtoseconds followed by a relaxation on a few-picosecond time scale and THz-oscillations around the average value. From these curves, characteristic time constants, oscillation frequencies and the gap size are obtained. The prominent oscillation observed in these measurements is associated with the CDW amplitude mode of the system whose damping is strongly dependent on the temperature. Specifically, for the highest temperature of 220 K, the amplitude oscillation is attenuated after 2-4 ps only (see Fig. 1.29).

Hellmannet al. [163] investigated the C-NC phase transition by means of femtosecond time-resolved core-level photoemission spectroscopy, which allows for measuring the atomic-site-specific charge-order dynamics of the charge-density wave in the low-temperature commensurate phase.

More specifically, the 4f core-level splitting of Ta is a direct measure for the CDW gap∆_CDW. Upon intense fs-laser illumination, the response consists of a subpicosecond reduction of the CDW-induced splitting and a partial recovery on a few-picosecond time scale into a quasiequilibrium state having a lifetime of more than 10 ps. The authors conclude that the two-step melting process is governed by, firstly, a quasi-instantaneous collapse of the charge order due to hot electrons and, secondly, melting of the long-range order of the C phase via energy transfer to the lattice. Moreover, they point out that the strong coupling of the charge density modulation and the periodic lattice distortion in the equilibrium state is suspended after photoexcitation for the time scale of electron-phonon thermalization.

In a seminal work, Eichbergeret al. [24] studied the structural changes of the NC phase using femtosecond electron diffraction experiments in transmission geometry with a temporal resolution of about 250 fs, and optical spectroscopy (see Fig. 1.30). They monitored the transient change of main and satellite diffraction reflexes, observing a rapid suppression of the periodic lattice distortion by about 20% on a timescale of about 250 fs and a subsequent recovery to a thermalized state in about 4 ps (see Fig. 1.30). In particular, while the satellite peak intensity strongly decreases, the main lattice reflex shows a prominent local maximum as the CDW amplitude is quenched. The authors interpret their results based on a common free-energy picture of broken-symmetry states that includes the rapid breakdown of the electronic modulation, the excitation of collective atomic motion during relaxation and the transition to a thermalized state.

In a comparative study, time-resolved ARPES enabled Hellmann et al. [34] to trace the full electronic band dispersion of 1T-TaS₂, 1T-TiSe₂ and intercalated Rb:1T-TaS₂ (see Fig. 1.31).

Figure 1.30: Time-dependent diffraction intensities after optical excitation of 1T-TaS2. (a-e) Snapshots of diffraction pattern segment showing main and satellite reflexes as well as the diffuse background for several time delays. (f) Temporal evolution of relative intensities. (g) Transient reflectivity change exhibiting distinct oscillation frequencies (inset). Reprinted by permission from Springer Nature Customer Service Centre GmbH [24], Copyright 2010.

Contrasting the length and excitation-density dependence of the gap melting times, the authors gain detailed insight into the interaction-dependent processes and provide a more reliable classification of the three insulators. The time-dependent results for the C phase of pristine 1T-TaS2 show the fast Mott and Peierls gap collapse within < 50 fs and ∼ 200 fs, respectively, and an oscillatory component during relaxation that is associated with the coherent amplitude-mode oscillations of the CDW (see Fig. 1.31), as previously measured by optical spectroscopy.

The selection of works presented gives a brief overview of the transient response of the material, including the electronic and structural degrees of freedom. The associated time scales and excited modes will serve as a reference for the investigation of the IC and NC phase using ULEED in Chapter3. Special emphasis lies on the relaxation dynamics which exhibit a long-lived structural

non-equilibrium scenario. While several other recent works studied the ultrafast structural dynamics of 1T-TaS2 [191, 192] and related CDW materials [38, 193–195] using high-energy electrons, ULEED provides a new complementary view on the structural dynamics at the surface.

Figure 1.31: Time-resolved electronic band structure for the layered charge-density wave compounds 1T -TaS2(C phase at 110 K) (a,e,j and d,h,l), 1T-TiSe2(b,f,j) and intercalated Rb:1T-TaS2(c,g,k). While the first column compares unpumped and pumped ARPES spectra, the second and third column show the temporal evolution of momentum-integrated spectra for two different fluences. Reprinted by permission from Springer Nature Customer Service Centre GmbH [34], Copyright 2012.

Microgun for ultrafast LEED

G. Storeck, S. Vogelgesang, M. Sivis, S. Schäfer, and C. Ropers Structural Dynamics4, 044024, April 2017

DOI: 10.1063/1.4982947

We present the design and fabrication of a micrometer-scale electron gun for the implementa-tion of ultrafast low-energy electron diffracimplementa-tion from surfaces. A multi-step process involving photolithography and focused-ion-beam nanostructuring is used to assemble and electrically contact the photoelectron gun, which consists of a nanotip photocathode in a Schottky geom-etry and an einzel lens for beam collimation. We characterize the low-energy electron pulses by a transient electric field effect and achieve pulse durations of 1.3 ps at an electron energy of 80 eV. First diffraction images in a backscattering geometry (at 50 eV electron energy) are shown.

2.1 Introduction

Ultrafast electron diffraction [196] and microscopy [2] are rapidly evolving tools for the study of structural dynamics. In recent years, ultrafast variants of numerous techniques employing electrons as structural and spectroscopic probes were developed, including high-energy electron diffraction [197,198], transmission electron microscopy, [5,199–204] and electron energy loss spectroscopy [205,206].

One of the particular benefits of electron beams is the high scattering cross-section facilitating surface-sensitive electron diffraction, for example, in reflection high-energy and low-energy electron diffraction (RHEED and LEED). Ultrafast RHEED was implemented early on in Refs. [207] and [208], and its temporal resolution has reached the few-picosecond to femtosecond domain in the

past few years [23,209]. However, because of its grazing incidence geometry, the real strength of RHEED is itsin-situcapability to characterize growth during epitaxy, rather than to obtain direct representations of the surface structure and symmetry. Some drawbacks of RHEED are enhanced volume contributions for stepped and imperfect surfaces and its restriction to map a limited angular fraction of reciprocal space. Ultrafast low-energy electron diffraction (ULEED), on the other hand, is highly desirable due to LEED’s outstanding ability to map atomic-scale surface structures [125] but has remained particularly challenging experimentally [210,211]. A main obstacle in the implementation of ULEED lies in achieving ultrashort electron probe pulses at low energies, which are extremely susceptible to pulse spreading in the propagation from the electron source to the sample [210,212]. Recently, employing nanoscale photocathodes [128,213–218] and minimized propagation distances, this limitation was overcome in a compact transmission ULEED setup for the study of structural dynamics in monolayers and ultrathin films [46]. In a related approach, ultrafast point-projection microscopy was developed [219–221] and applied in the imaging of charge dynamics [219]. Extending the ULEED methodology to a backscattering geometry would enable investigations of ultrafast structural processes at surfaces, but, in order to avoid shadowing of the backscattered diffraction pattern, this requires the development of miniaturized photoelectron sources of sufficiently small outer diameters.

Here, we present the implementation of a nanofabricated electron gun (hereafter referred to as the ’microgun’) facilitating ULEED. The microgun consists of a tungsten nanotip photoemitter embedded in a shielded micrometer-scale electrostatic lens assembly (total outer diameter of 80 µm; Fig. 2.1d). Utilizing this photoelectron source, we achieve a temporal resolution in electron projection imaging of 1.3 ps at an electron energy of only 80 eV and a source-sample distance of 400 µm. High-quality electron diffraction patterns are recorded in a backscattering geometry, demonstrating the high spatial coherence of the generated electron beam. This photoelectron gun combines ultrafast temporal resolution with high momentum resolution and ultimate surface sensitivity, promoting access to numerous ultrafast phenomena in the structural dynamics at surfaces.