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The journal of physical chemistry letters 9(7), 1491 - 1496 (2018). doi:10.1021/acs.jpclett.8b00338, Understanding and controlling the spin-crossover properties of molecular complexes can be of particular interest for potential applications in molecular spintronics. Using near-edge X-ray absorption fine structure spectroscopy, we investigated these properties for a new vacuum-evaporable Fe(II) complex, namely [Fe(pypyr(CF$_3$)$_2$)$_2$(phen)] (pypyr = 2-(2′-pyridyl)pyrrolide, phen = 1,10-phenanthroline). We find that the spin-transition temperature, well above room temperature for the bulk compound, is drastically lowered for molecules arranged in thin films. Furthermore, while within the experimentally accessible temperature range (2 K < T < 410 K) the bulk material shows indication of neither light-induced excited spin-state trapping nor soft X-ray-induced excited spin-state trapping, these effects are observed for molecules within thin films up to temperatures around 100 K. Thus, by arranging the molecules into thin films, a nominal low-spin complex is effectively transformed into a spin-crossover complex., Published by ACS, Washington, DC

The layered van der Waals antiferromagnet MnBi$_2$Te$_4$ has been predicted to combine the band ordering of archetypical topological insulators such as Bi$_2$Te$_3$ with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of MnBi$_2$Te$_4$(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure., Revised version

Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two-dimensional transition-metal dichalcogenide $1T$-$\mathrm{Ta}{\mathrm{S}}_{2}$, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on timescales of electronic hopping $\ensuremath{\hbar}/J\ensuremath{\approx}14\text{ }\text{ }\mathrm{fs}$. Despite strong electron-phonon coupling, the dynamics can be explained by purely electronic effects captured by the single-band Hubbard model under the assumption of weak hole doping, in agreement with our static sample characterization. This sensitive interplay of static doping and vicinity to the metal-insulator transition suggests a way to modify doublon relaxation on the few-femtosecond timescale.

We investigate the perturbation and subsequent recovery of the correlated electronic ground state of the Mott insulator 1T-TaS 2 by means of femtosecond time-resolved photoemission spectroscopy in normal emission geometry. Upon an increase of near-infrared excitation strength, a considerable collapse of the occupied Hubbard band is observed, which reflects a quench of short-range correlations. It is furthermore found that these excitations are directly linked to the lifting of the periodic lattice distortion which provides the localization centers for the formation of the insulating Mott state. We discuss the observed dynamics in a localized real-space picture.

Physical review / B 96(24), 241408 (2017). doi:10.1103/PhysRevB.96.241408, $MoTe_2$ has recently been shown to realize in its low-temperature phase the type-II Weyl semimetal (WSM). We investigated by time- and angle- resolved photoelectron spectroscopy (tr-ARPES) the possible influence of the Weyl points on the electron dynamics above the Fermi level $E_F$, by comparing the ultrafast response of $MoTe_2$in the trivial and topological phases. In the low-temperature WSM phase, we report an enhanced relaxation rate of electrons optically excited to the conduction band, which we interpret as a fingerprint of the local gap closure when Weyl points form. By contrast, we find that the electron dynamics of the related compound $WTe_2$is slower and temperature independent, consistent with a topologically trivial nature of this material. Our results shows that tr-ARPES is sensitive to the small modifications of the unoccupied band structure accompanying the structural and topological phase transition of $MoTe_2$., Published by Inst., Woodbury, NY

A previous time-resolved optical study reported on a metastable hidden electronic state in 1T-TaS2, which is only accessible upon photoexcitation and created under non-equilibrium conditions [1]. The properties of such a state are distinct from those of any other state in the equilibrium phase diagram and it is possible to revert to the thermodynamic initial state either by illuminating with picosecond laser pulses or by applying other thermal erase procedures. In this work we show photoinduced switching to a metastable hidden state on the same material, and probe it by means of both static and time-resolved photoemission spectroscopy, thus having direct access to the electronic structure of the system. From our experimental findings and comparison with other studies, we conclude that we obtain partial switching, leading to a hidden state with persisting insulating nature but significant modifications in the electronic structure and CDW ordering.

Metal-ferroelectric interfaces form the basis of novel electronic devices. A key effect determining the device functionality is the bias-dependent change of the electronic energy-level alignment at the interface. Here, hard x-ray photoelectron spectroscopy (HAXPES) is used to determine the energy-level alignment at two metal-ferroelectric interfaces—Au versus SrRuO3 on the relaxor ferroelectric Pb(Mg1/3 Nb2/3 )0.72 Ti0.28 O3 (PMN-PT)- directly in situ as a function of electrical bias. The bias-dependent average shifts of the PMN-PT core levels are found to have two dominant contributions on the 0.1–1-eV energy scale: one depending on the metal electrode and the remanent electric polarization and the other correlated with electric-field-induced strain. Element-specific deviations from the average shifts are smaller than 0.1 eV and appear to be related to predicted dynamical charge variations in PMN-PT. In addition, the efficiency of ferroelectric polarization switching is shown to be reduced near the coercive field under x-ray irradiation. The results establish HAXPES as a tool for the in operando investigation of metal-ferroelectric interfaces and suggest electric-field-induced modifications of the polarizationdistribution as a novel way to control the barrier height at such interfaces.

Angle-resolved photoelectron spectroscopy (ARPES) is widely used to study the electronic structure of crystalline solids such as high-temperature superconductors, topological insulators and graphene-based materials. Time-resolved ARPES has opened the door to the study of the response of such electronic features on ultrafast timescales. Now Rohwer et al. add a new dimension. Using high photon energies, they are able to study ultrafast dynamics at high momenta, at which some of the most interesting fundamental phenomena occur. Applying the technique to the charge density wave material 1T-TiSe2, they obtain stroboscopic images of the electronic band structure at high momentum and show that atomic-scale periodic long-scale order collapses on a surprisingly short timescale of 20 femtoseconds. This work reveals rapid response times in photoinduced properties that could stimulate research into new types of ultrafast switching device. Angle-resolved photoemission spectroscopy (ARPES) is widely used to study the electronic structure of a wide range of correlated materials. Time-resolved ARPES allows the study of the response of such electronic features on ultrafast timescales; this paper now adds an exciting new dimension by using high photon energies that allow the study of ultrafast dynamics at high momenta, where often the most interesting fundamental phenomena occur. The technique is applied to the charge density wave material 1T-TiSe2 and it is shown with stroboscopic imaging of the electronic band structure at high momentum that atomic-scale periodic long-range order collapses on a surprisingly short timescale of 20 femtoseconds. Intense femtosecond (10−15 s) light pulses can be used to transform electronic, magnetic and structural order in condensed-matter systems on timescales of electronic and atomic motion1,2,3. This technique is particularly useful in the study4,5 and in the control6 of materials whose physical properties are governed by the interactions between multiple degrees of freedom. Time- and angle-resolved photoemission spectroscopy is in this context a direct and comprehensive, energy- and momentum-selective probe of the ultrafast processes that couple to the electronic degrees of freedom7,8,9,10. Previously, the capability of such studies to access electron momentum space away from zero momentum was, however, restricted owing to limitations of the available probing photon energy10,11. Here, using femtosecond extreme-ultraviolet pulses delivered by a high-harmonic-generation source, we use time- and angle-resolved photoemission spectroscopy to measure the photoinduced vaporization of a charge-ordered state in the potential excitonic insulator 1T-TiSe2 (refs 12, 13). By way of stroboscopic imaging of electronic band dispersions at large momentum, in the vicinity of the edge of the first Brillouin zone, we reveal that the collapse of atomic-scale periodic long-range order happens on a timescale as short as 20 femtoseconds. The surprisingly fast response of the system is assigned to screening by the transient generation of free charge carriers. Similar screening scenarios are likely to be relevant in other photoinduced solid-state transitions and may generally determine the response times. Moreover, as electron states with large momenta govern fundamental electronic properties in condensed matter systems14, we anticipate that the experimental advance represented by the present study will be useful to study the ultrafast dynamics and microscopic mechanisms of electronic phenomena in a wide range of materials.