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We report the observation of a novel nonlinear optical response from the prototypical three-dimensional topological insulator Bi$_2$Se$_3$ through the process of high-order harmonic generation. We find that the generation efficiency increases as the laser polarization is changed from linear to elliptical, and it becomes maximum for circular polarization. With the aid of a microscopic theory and a detailed analysis of the measured spectra, we reveal that such anomalous enhancement encodes the characteristic topology of the band structure that originates from the interplay of strong spin-orbit coupling and time-reversal symmetry protection. Our study reveals a new platform for chiral strong-field physics and presents a novel, contact-free, all-optical approach for the spectroscopy of topological insulators. The implications are in ultrafast probing of topological phase transitions, light-field driven dissipationless electronics, and quantum computation., 21 pages, 5 figures

We study ultrafast x-ray diffraction on the charge density wave (CDW) of ${\mathrm{SmTe}}_{3}$ using an x-ray free-electron laser. The high momentum and time resolution afforded by the x-ray laser enabled capturing fine wave-vector and time-dependent features of the CDW that originate from fast (in time) and sharp (in real space) variations of the CDW lattice distortion, which we attribute to an inversion of the order parameter. These domain inversions occur near the surface and are caused by the short penetration depth of the near-infrared pump with the wavelength centered at 800 nm, resulting in CDW domain walls perpendicular to the sample surface. These domain walls break the CDW long-range order on the scale of the x-ray probe depth, controlled experimentally by the x-ray incidence angle and suppress the diffraction intensity of the CDW for times much longer than the $\ensuremath{\sim}1\text{\ensuremath{-}}\mathrm{ps}$ recovery of the electronic gap observed in time and angle-resolved photoemission spectroscopy. We model the spatial and temporal dependences of the order parameter using a simple Ginzburg-Landau model with all the parameters obtained from the published literature. We find reasonable agreement between the calculated and the measured diffraction across the momentum, time, fluence, and incidence angle dependence without adjusting any parameters. We reconstruct the spatial and temporal dependences of the lattice order parameter and find that at long times, depending on the pump fluence, multiple domain walls remain at distances of a few nanometers from the surface.

A two-bunch scheme to generate high-power, high-repetition-rate THz radiation for pump–probe experiments at LCLS II is studied. The two-bunch beam dynamics, the THz wiggler and radiation are described, as well as the transport system bringing the THz pulses from the wiggler to the X-ray experimental hall., Experiments using a THz pump and an X-ray probe at an X-ray free-electron laser (XFEL) facility like the Linac Coherent Light Source II (LCLS II) require frequency-tunable (3 to 20 THz), narrow bandwidth (∼10%), carrier-envelope-phase-stable THz pulses that produce high fields (>1 MV cm−1) at the repetition rate of the X-rays and are well synchronized with them. In this paper, a two-bunch scheme to generate THz radiation at LCLS II is studied: the first bunch produces THz radiation in an electromagnet wiggler immediately following the LCLS II undulator that produces X-rays from the second bunch. The initial time delay between the two bunches is optimized to compensate for the path difference in THz transport. The two-bunch beam dynamics, the THz wiggler and radiation are described, as well as the transport system bringing the THz pulses from the wiggler to the experimental hall.

ZrTe$_5$ has been of recent interest as a potential Dirac/Weyl semimetal material. Here, we report the results of experiments performed via in-situ 3D double-axis rotation to extract the full $4\pi$ solid angular dependence of the transport properties. A clear anomalous Hall effect (AHE) was detected for every sample, with no magnetic ordering observed in the system to the experimental sensitivity of torque magnetometry. Interestingly, the AHE takes large values when the magnetic field is rotated in-plane, with the values vanishing above $\sim 60$ K where the negative longitudinal magnetoresistance (LMR) also disappears. This suggests a close relation in their origins, which we attribute to Berry curvature generated by the Weyl nodes., Comment: 7 pages, 4 figures

Non-equilibrium conditions may lead to novel properties of materials with broken symmetry ground states not accessible in equilibrium as vividly demonstrated by non-linearly driven mid-infrared active phonon excitation. Potential energy surfaces of electronically excited states also allow to direct nuclear motion, but relaxation of the excess energy typically excites fluctuations leading to a reduced or even vanishing order parameter as characterized by an electronic energy gap. Here, using femtosecond time- and angle-resolved photoemission spectroscopy, we demonstrate a tendency towards transient stabilization of a charge density wave after near-infrared excitation, counteracting the suppression of order in the non-equilibrium state. Analysis of the dynamic electronic structure reveals a remaining energy gap in a highly excited transient state. Our observation can be explained by a competition between fluctuations in the electronically excited state, which tend to reduce order, and transiently enhanced Fermi surface nesting stabilizing the order., Whilst excited electronic states may exhibit unique non-equilibrium behavior, order is inhibited by fluctuations. Here, the authors use femtosecond photoemission spectroscopy to demonstrate transient stabilization of charge density wave order in rare earth tritellurides after near-infrared excitation.

Cuprate superconductors host a multitude of low-energy optical phonons. Using time- and angle-resolved photoemission spectroscopy, we study coherent phonons in Bi$_{2}$Sr$_{2}$Ca$_{0.92}$Y$_{0.08}$Cu$_{2}$O$_{8+\delta}$. Sub-meV modulations of the electronic band structure are observed at frequencies of $3.94\pm 0.01$ and $5.59\pm 0.06$ THz. For the dominant mode at 3.94 THz, the amplitude of the band energy oscillation weakly increases as a function of momentum away from the node. Theoretical calculations allow identifying the observed modes as CuO$_{2}$-derived $A_{1g}$ phonons. The Bi- and Sr-derived $A_{1g}$ modes which dominate Raman spectra in the relevant frequency range are absent in our measurements. This highlights the mode-selectivity for phonons coupled to the near-Fermi-level electrons, which originate from CuO$_{2}$ planes and dictate thermodynamic properties., Comment: 7 pages, 3 figures

We report an optically induced reduction of the $f\text{\ensuremath{-}}d$ hybridization in the prototypical heavy-fermion compound ${\mathrm{YbRh}}_{2}{\mathrm{Si}}_{2}$. We use femtosecond time- and angle-resolved photoemission spectroscopy to monitor changes of spectral weight and binding energies of the Yb $4f$ and Rh $4d$ states before the lattice temperature increases after pumping. Overall, the $f\text{\ensuremath{-}}d$ hybridization decreases smoothly with increasing electronic temperature up to $\ensuremath{\sim}250\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ but changes slope at $\ensuremath{\sim}100\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. This temperature scale coincides with the onset of coherent Kondo scattering and with thermally populating the first excited crystal electrical field level. Extending previous photoemission studies, we observe a persistent $f\text{\ensuremath{-}}d$ hybridization up to at least $\ensuremath{\sim}250\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, which is far larger than the coherence temperature defined by transport but in agreement with the temperature dependence of the noninteger Yb valence. Our data underlines the distinction of probes accessing spin and charge degrees of freedom in strongly correlated systems.

We present a combined ultrafast optical pump-probe and ultrafast x-ray diffraction measurement of the CDW dynamics in SmTe$_3$ at 300 K. The ultrafast x-ray diffraction measurements, taken at the Linac Coherent Light Source reveal a $\sim 1.55$ THz mode that becomes overdamped with increasing fluence. We identify this oscillation with the lattice component of the amplitude mode. Furthermore, these data allow for a more clear identification of the frequencies present in the optical pump-probe data. In both, reflectivity and diffraction, we observe a crossover of the response from linear (for small displacements) to quadratic in the amplitude of the order parameter displacement. Finally, a time-dependent Ginzburg-Landau model captures the essential features of the experimental observations., Comment: 7 pages, 5 figures

In the past decade, topological materials have been continuously attracting the interest of the condensed-matter physics community because of their unique band structures and transport properties. Recently, ZrTe${}_{5}$ is becoming a promising platform to study topological phase transitions, as it could possibly be a 3D Dirac semimetal, a 3D weak topological insulator (TI), or a 3D strong TI, which are distinguished by whether there is a finite band gap and whether there is a topological surface state (TSS). This paper performs a systematic high-momentum-resolution photoemission study on ZrTe${}_{5}$ using 6 eV photon energy. The conduction and valence bands near $\mathrm{\ensuremath{\Gamma}}$ are measured with a gap between 18 and 29 meV. The gap size is smaller than former studies. This work also examines the spectral difference at different photon energies, which is attributed to final-state effects and the 3D nature of the band structure. By doing so, the authors confirm the gap is not due to some specific out-of-plane momentum, and conclude that there is no TSS. The final-state interpretation also reconciles the discrepancies of previous studies regarding the existence of the TSS. Hence, the results are consistent with ZrTe${}_{5}$ being a 3D weak topological insulator.

We investigate the lattice coupling to the spin and charge orders in the striped nickelate, ${\mathrm{La}}_{1.75}{\mathrm{Sr}}_{0.25}{\mathrm{NiO}}_{4}$, using time-resolved resonant x-ray scattering. Lattice-driven dynamics of both spin and charge orders are observed when the pump photon energy is tuned to that of an ${E}_{u}$ bond- stretching phonon. We present a likely scenario for the behavior of the spin and charge order parameters and its implications using a Ginzburg-Landau theory.

We study the microscopic origins of photocurrent generation in the topological insulator Bi$_2$Se$_3$ via time- and angle-resolved photoemission spectroscopy. We image the unoccupied band structure as it evolves following a circularly polarized optical excitation and observe an asymmetric electron population in momentum space, which is the spectroscopic signature of a photocurrent. By analyzing the rise times of the population we identify which occupied and unoccupied electronic states are coupled by the optical excitation. We conclude that photocurrents can only be excited via resonant optical transitions coupling to spin-orbital textured states. Our work provides a microscopic understanding of how to control photocurrents in systems with spin-orbit coupling and broken inversion symmetry.

We study optimally doped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{0.92}{\mathrm{Y}}_{0.08}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212) using angle-resolved two-photon photoemission spectroscopy. Three spectral features are resolved near 1.5, 2.7, and 3.6 eV above the Fermi level. By tuning the photon energy, we determine that the 2.7-eV feature arises predominantly from unoccupied states. The 1.5- and 3.6-eV features reflect unoccupied states whose spectral intensities are strongly modulated by the corresponding occupied states. These unoccupied states are consistent with the prediction from a cluster perturbation theory based on the single-band Hubbard model. Through this comparison, a Coulomb interaction strength $U$ of 2.7 eV is extracted. Our study complements equilibrium photoemission spectroscopy and provides a direct spectroscopic measurement of the unoccupied states in cuprates. The determined Coulomb $U$ indicates that the charge-transfer gap of optimally doped Bi2212 is 1.1 eV.

BiTeCl is a topological insulator with strong inversion asymmetry, which exhibits bulk charge polarization and pyroelectricity. Such a long-sought topological insulator paves the way for applications involving natural p–n junctions and spintronics.

The dynamics of an order parameter's amplitude and phase determines the collective behaviour of novel states emerged in complex materials. Time- and momentum-resolved pump-probe spectroscopy, by virtue of its ability to measure material properties at atomic and electronic time scales and create excited states not accessible by the conventional means can decouple entangled degrees of freedom by visualizing their corresponding dynamics in the time domain. Here, combining time-resolved femotosecond optical and resonant x-ray diffraction measurements on striped La1.75Sr0.25NiO4, we reveal unforeseen photo-induced phase fluctuations of the charge order parameter. Such fluctuations preserve long-range order without creating topological defects, unlike thermal phase fluctuations near the critical temperature in equilibrium10. Importantly, relaxation of the phase fluctuations are found to be an order of magnitude slower than that of the order parameter's amplitude fluctuations, and thus limit charge order recovery. This discovery of new aspect to phase fluctuation provides more holistic view for the importance of phase in ordering phenomena of quantum matter., Comment: 23 pages, 6 figures. Published version can be found at Nature Communications 3, 838 (2012)

We report on new hidden states of matter created under femtosecond timescale non-equilibrium conditions in dichalcogenides and complex oxides, focusing on their origin, optical and electronic control, relaxation mechanisms and applications leading to ultrafast optomemristors.

4 páginas, 4 figuras.-- et al., Quasiparticle lifetimes in metals as described by Fermi-liquid theory are essential in surface chemistry and determine the mean free path of hot carriers. Relaxation of hot electrons is governed by inelastic electron–electron scattering, which occurs on femtosecond timescales owing to the large scattering phase space competing with screening effects. Such lifetimes are widely studied by time-resolved two-photon photoemission, which led to understanding of electronic decay at surfaces. In contrast, quasiparticle lifetimes of metal bulk and films are not well understood because electronic transport leads to experimental lifetimes shorter than expected theoretically. Here, we lift this discrepancy by investigating Pb quantum-well structures on Si(111), a two-dimensional model system. For electronic states confined to the film by the Si bandgap we find quantitative agreement with Fermi-liquid theory and ab initio calculations for bulk Pb, which we attribute to efficient screening. For states resonant with Si bands, extra decay channels open for electron transfer to Si, resulting in lifetimes shorter than expected for bulk. Thereby we demonstrate that for understanding electronic decay in nanostructures coupling to the environment is essential, and that even for electron confinement to a few ångströms Fermi-liquid theory for bulk can remain valid., This work has been funded by the Deutsche Forschungsgemeinschaft through BO 1823/2 and by the Ministerio de Ciencia y Tecnologia (grant FIS2007-66711-C02-01). P.S.K. acknowledges support by the International Max-Planck Research School `Complex Surfaces in Material Science'.

We study optimally doped Bi-2212 ($T_\textrm{c}=96$~K) using femtosecond time- and angle-resolved photoelectron spectroscopy. Energy-resolved population lifetimes are extracted and compared with single-particle lifetimes measured by equilibrium photoemission. The population lifetimes deviate from the single-particle lifetimes in the low excitation limit by one to two orders of magnitude. Fundamental considerations of electron scattering unveil that these two lifetimes are in general distinct, yet for systems with only electron-phonon scattering they should converge in the low-temperature, low-fluence limit. The qualitative disparity in our data, even in this limit, suggests that scattering channels beyond electron-phonon interactions play a significant role in the electron dynamics of cuprate superconductors., Comment: 5 pages, 3 figures, accepted by Physical Review Letters

We characterize the topological insulator Bi$_2$Se$_3$ using time- and angle- resolved photoemission spectroscopy. By employing two-photon photoemission, a complete picture of the unoccupied electronic structure from the Fermi level up to the vacuum level is obtained. We demonstrate that the unoccupied states host a second, Dirac surface state which can be resonantly excited by 1.5 eV photons. We then study the ultrafast relaxation processes following optical excitation. We find that they culminate in a persistent non-equilibrium population of the first Dirac surface state, which is maintained by a meta-stable population of the bulk conduction band. Finally, we perform a temperature-dependent study of the electron-phonon scattering processes in the conduction band, and find the unexpected result that their rates decrease with increasing sample temperature. We develop a model of phonon emission and absorption from a population of electrons, and show that this counter-intuitive trend is the natural consequence of fundamental electron-phonon scattering processes. This analysis serves as an important reminder that the decay rates extracted by time-resolved photoemission are not in general equal to single electron scattering rates, but include contributions from filling and emptying processes from a continuum of states., Comment: In Press: Journal of Electron Spectroscopy & Related Phenomena (special issue on 2-photon photoemission spectroscopy)

We characterize the occupied and unoccupied electronic structure of the topological insulator Bi2Se3 by one-photon and two-photon angle-resolved photoemission spectroscopy and slab band structure calculations. We reveal a second, unoccupied Dirac surface state with similar electronic structure and physical origin to the well-known topological surface state. This state is energetically located 1.5 eV above the conduction band, which permits it to be directly excited by the output of a Ti:sapphire laser. This discovery demonstrates the feasibility of direct ultrafast optical coupling to a topologically protected, spin-textured surface state.

The femtosecond dynamics of the Fermi surface of DyTe3 and its band structure are investigated by time- and angle-resolved photoemission spectroscopy. We directly monitor the ultrafast collapse of the charge density wave gap within 200 fs.

© 2016 AIP Publishing LLC. We developed a table-top vacuum ultraviolet (VUV) laser with 113.778 nm wavelength (10.897 eV) and demonstrated its viability as a photon source for high resolution angle-resolved photoemission spectroscopy (ARPES). This sub-nanosecond pulsed VUV laser operates at a repetition rate of 10 MHz, provides a flux of 2 × 1012photons/s, and enables photoemission with energy and momentum resolutions better than 2 meV and 0.012 Å-1, respectively. Space-charge induced energy shifts and spectral broadenings can be reduced below 2 meV. The setup reaches electron momenta up to 1.2 Å-1, granting full access to the first Brillouin zone of most materials. Control over the linear polarization, repetition rate, and photon flux of the VUV source facilitates ARPES investigations of a broad range of quantum materials, bridging the application gap between contemporary low energy laser-based ARPES and synchrotron-based ARPES. We describe the principles and operational characteristics of this source and showcase its performance for rare earth metal tritellurides, high temperature cuprate superconductors, and iron-based superconductors.

Using femtosecond time- and angle-resolved photoemission spectroscopy, we investigated the nonequilibrium dynamics of the topological insulator Bi2Se3. We studied p-type Bi2Se3, in which the metallic Dirac surface state and bulk conduction bands are unoccupied. Optical excitation leads to a metastable population at the bulk conduction band edge, which feeds a nonequilibrium population of the surface state persisting for >10 ps. This unusually long-lived population of a metallic Dirac surface state with spin texture may present a channel in which to drive transient spin-polarized currents.

We investigated ultrafast energy relaxation in unoccupied quantum well states of Pb/Si(111) with femtosecond time-resolved two-photon photoemission. Decay rates of 6pz quantum well states are compatible with Fermi liquid theory if inter-subband scattering is considered.

We present time- and angle-resolved photoemission spectroscopy measurements on the charge density wave system CeTe$_{3}$. Optical excitation transiently populates the unoccupied band structure and reveals a gap size of 2$\Delta$ = 0.59 eV. The occupied Te-5p band dispersion is coherently modified by three modes at $\Omega_{1}$ = 2.2 THz, $\Omega_{2}$ = 2.7 THz and $\Omega_{3}$ = 3 THz. All three modes lead to small rigid energy shifts whereas $\Delta$ is only affected by $\Omega_{1}$ and $\Omega_{2}$. Their spatial polarization is analyzed by fits of a transient model dispersion and DFT frozen phonon calculations. We conclude that the modes $\Omega_{1}$ and $\Omega_{2}$ result from in-plane ionic lattice motions, which modulate the charge order, and that $\Omega_{3}$ originates from a generic out-of-plane $A_{1g}$ phonon. We thereby demonstrate how the rich information from trARPES allows identification of collective modes and their spatial polarization, which explains the mode-dependent coupling to charge order., Comment: 5 pages, 4 figures

The advances in femtosecond laser techniques facilitate the investigation of ultrafast electron dynamics at surfaces directly in the time-domain. We employ time-resolved two-photon-photoemission (2PPE) spectroscopy to study the electron dynamics of the unoccupied electronic states in hexafluorobenzene (C6F6) on Cu(1 1 1) serving as a model system for charge transfer across organic–metal interfaces. Our coverage-dependent study reveals a lifetime of the lowest unoccupied molecular resonance of 7 fs for a single monolayer (ML) which increases to 37 fs above 3 ML coverage. We find that the population build-up of the excited state is delayed by a characteristic time of about 10 fs with respect to the exciting laser pulse. By angle-resolved 2PPE spectroscopy, the mechanism of the delayed population rise is identified as intraband relaxation in the adsorbate band structure. The actual electron-transfer to the metal substrate occurs through interband scattering between the molecular resonance and substrate states on comparable timescales. Therefore the present study demonstrates that relaxation of hot electrons at molecule–metal interfaces include—even in the presence of strong electronic molecule–substrate interaction—also decay channels within the adlayer.