Light-induced hexatic state in a layered quantum material.

The tunability of materials properties by light promises a wealth of future applications in energy conversion and information technology. Strongly correlated materials such as transition metal dichalcogenides offer optical control of electronic phases, charge ordering and interlayer correlations by photodoping. Here, we find the emergence of a transient hexatic state during the laser-induced transformation between two charge-density wave phases in a thin-film transition metal dichalcogenide, 1T-type tantalum disulfide (1T-TaS2). Introducing tilt-series ultrafast nanobeam electron diffraction, we reconstruct charge-density wave rocking curves at high momentum resolution. An intermittent suppression of three-dimensional structural correlations promotes a loss of in-plane translational order caused by a high density of unbound topological defects, characteristic of a hexatic intermediate. Our results demonstrate the merit of tomographic ultrafast structural probing in tracing coupled order parameters, heralding universal nanoscale access to laser-induced dimensionality control in functional heterostructures and devices.

Chirality-Driven Orbital Angular Momentum and Circular Dichroism in CoSi.

Chiral crystals and molecules were recently predicted to form an intriguing platform for unconventional orbital physics. Here, we report the observation of chirality-driven orbital textures in the bulk electronic structure of CoSi, a prototype member of the cubic B20 family of chiral crystals. Using circular dichroism in soft x-ray angle-resolved photoemission, we demonstrate the formation of a bulk orbital-angular-momentum texture and monopolelike orbital-momentum locking that depends on crystal handedness. We introduce the intrinsic chiral circular dichroism, icCD, as a differential photoemission observable and a natural probe of chiral electron states. Our findings render chiral crystals promising for spin-orbitronics applications.

Bipolaronic Nature of the Pseudogap in Quasi-One-Dimensional (TaSe4)2I Revealed via Weak Photoexcitation.

The origin of the pseudogap in many strongly correlated materials has been a longstanding puzzle. Here, we present experimental evidence that many-body interactions among small Holstein polarons, i.e., the formation of bipolarons, are primarily responsible for the pseudogap in (TaSe4)2I. After weak photoexcitation of the material, we observe the appearance of both dispersive (single-particle bare band) and flat bands (single-polaron sub-bands) in the gap by using time- and angle-resolved photoemission spectroscopy. Based on Monte Carlo simulations of the Holstein model, we propose that the melting of pseudogap and emergence of new bands originate from a bipolaron to single-polaron crossover. We also observe dramatically different relaxation times for the excited in-gap states in (TaSe4)2I (∼600 fs) compared with another 1D material Rb0.3MoO3 (∼60 fs), which provides a new method for distinguishing between pseudogaps induced by polaronic or Luttinger-liquid many-body interactions.

Photon-shot-noise-limited transient absorption soft X-ray spectroscopy at the European XFEL.

Femtosecond transient soft X-ray absorption spectroscopy (XAS) is a very promising technique that can be employed at X-ray free-electron lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here, a dedicated setup for soft X-rays available at the Spectroscopy and Coherent Scattering (SCS) instrument at the European X-ray Free-Electron Laser (European XFEL) is presented. It consists of a beam-splitting off-axis zone plate (BOZ) used in transmission to create three copies of the incoming beam, which are used to measure the transmitted intensity through the excited and unexcited sample, as well as to monitor the incoming intensity. Since these three intensity signals are detected shot by shot and simultaneously, this setup allows normalized shot-by-shot analysis of the transmission. For photon detection, an imaging detector capable of recording up to 800 images at 4.5 MHz frame rate during the FEL burst is employed, and allows a photon-shot-noise-limited sensitivity to be approached. The setup and its capabilities are reviewed as well as the online and offline analysis tools provided to users.

Coherent modulation of the electron temperature and electron-phonon couplings in a 2D material.

Ultrashort light pulses can selectively excite charges, spins, and phonons in materials, providing a powerful approach for manipulating their properties. Here we use femtosecond laser pulses to coherently manipulate the electron and phonon distributions, and their couplings, in the charge-density wave (CDW) material 1T-TaSe2 After exciting the material with a femtosecond pulse, fast spatial smearing of the laser-excited electrons launches a coherent lattice breathing mode, which in turn modulates the electron temperature. This finding is in contrast to all previous observations in multiple materials to date, where the electron temperature decreases monotonically via electron-phonon scattering. By tuning the laser fluence, the magnitude of the electron temperature modulation changes from ∼200 K in the case of weak excitation, to ∼1,000 K for strong laser excitation. We also observe a phase change of π in the electron temperature modulation at a critical fluence of 0.7 mJ/cm2, which suggests a switching of the dominant coupling mechanism between the coherent phonon and electrons. Our approach opens up routes for coherently manipulating the interactions and properties of two-dimensional and other quantum materials using light.

Survival of Floquet-Bloch States in the Presence of Scattering.

Floquet theory has spawned many exciting possibilities for electronic structure control with light, with enormous potential for future applications. The experimental demonstration in solids, however, remains largely unrealized. In particular, the influence of scattering on the formation of Floquet-Bloch states remains poorly understood. Here we combine time- and angle-resolved photoemission spectroscopy with time-dependent density functional theory and a two-level model with relaxation to investigate the survival of Floquet-Bloch states in the presence of scattering. We find that Floquet-Bloch states will be destroyed if scattering-activated by electronic excitations-prevents the Bloch electrons from following the driving field coherently. The two-level model also shows that Floquet-Bloch states reappear at high field intensities where energy exchange with the driving field dominates over energy dissipation to the bath. Our results clearly indicate the importance of long scattering times combined with strong driving fields for the successful realization of various Floquet phenomena.

Yu-Shiba-Rusinov States in the Charge-Density Modulated Superconductor NbSe2.

NbSe2 is a remarkable superconductor in which charge-density order coexists with pairing correlations at low temperatures. Here, we study the interplay of magnetic adatoms and their Yu-Shiba-Rusinov (YSR) bound states with the charge density order. Exploiting the incommensurate nature of the charge-density wave (CDW), our measurements provide a thorough picture of how the CDW affects both the energies and the wave functions of the YSR states. Key features of the dependence of the YSR states on adsorption site relative to the CDW are explained by model calculations. Several properties make NbSe2 a promising substrate for realizing topological nanostructures. Our results will be important in designing such systems.

Orbital- and k_{z}-Selective Hybridization of Se 4p and Ti 3d States in the Charge Density Wave Phase of TiSe_{2}.

We revisit the enduring problem of the 2×2×2 charge density wave (CDW) order in TiSe_{2}, utilizing photon energy-dependent angle-resolved photoemission spectroscopy to probe the full three-dimensional high- and low-temperature electronic structure. Our measurements demonstrate how a mismatch of dimensionality between the 3D conduction bands and the quasi-2D valence bands in this system leads to a hybridization that is strongly k_{z} dependent. While such a momentum-selective coupling can provide the energy gain required to form the CDW, we show how additional "passenger" states remain, which couple only weakly to the CDW and thus dominate the low-energy physics in the ordered phase of TiSe_{2}.

Collapse of long-range charge order tracked by time-resolved photoemission at high momenta.

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 motion. This technique is particularly useful in the study and in the control 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 freedom. 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 energy. 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-TiSe(2 )(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 systems, 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.

Suppression of the Charge Density Wave State in Two-Dimensional 1T-TiSe2 by Atmospheric Oxidation.

Two-dimensional (2D) metallic transition-metal dichalcogenides (TMDCs), such as 1T-TiSe2 , have recently emerged as unique platforms for exploring their exciting properties of superconductivity and the charge density wave (CDW). 2D 1T-TiSe2 undergoes rapid oxidation under ambient conditions, significantly affecting its CDW phase-transition behavior. We comprehensively investigate the oxidation process of 2D TiSe2 by tracking the evolution of the chemical composition and atomic structure with various microscopic and spectroscopic techniques and reveal its unique selenium-assisting oxidation mechanism. Our findings facilitate a better understanding of the chemistry of ultrathin TMDCs crystals, introduce an effective method to passivate their surfaces with capping layers, and thus open a way to further explore the functionality of these materials toward devices.

Ultrafast Metamorphosis of a Complex Charge-Density Wave.

Modulated phases, commensurate or incommensurate with the host crystal lattice, are ubiquitous in solids. The transition between such phases involves formation and rearrangement of domain walls and is generally slow. Using ultrafast electron diffraction, we directly record the photoinduced transformation between a nearly commensurate and an incommensurate charge-density-wave phase in 1T-TaS(2). The transformation takes place on the picosecond time scale, orders of magnitude faster than previously observed for commensurate-to-incommensurate transitions. The transition speed and mechanism can be linked to the peculiar nanoscale structure of the photoexcited nearly commensurate phase.

Quantum phase transition from triangular to stripe charge order in NbSe2.

The competition between proximate electronic phases produces a complex phenomenology in strongly correlated systems. In particular, fluctuations associated with periodic charge or spin modulations, known as density waves, may lead to exotic superconductivity in several correlated materials. However, density waves have been difficult to isolate in the presence of chemical disorder, and the suspected causal link between competing density wave orders and high-temperature superconductivity is not understood. Here we used scanning tunneling microscopy to image a previously unknown unidirectional (stripe) charge-density wave (CDW) smoothly interfacing with the familiar tridirectional (triangular) CDW on the surface of the stoichiometric superconductor NbSe(2). Our low-temperature measurements rule out thermal fluctuations and point to local strain as the tuning parameter for this quantum phase transition. We use this quantum interface to resolve two longstanding debates about the anomalous spectroscopic gap and the role of Fermi surface nesting in the CDW phase of NbSe(2). Our results highlight the importance of local strain in governing phase transitions and competing phenomena, and suggest a promising direction of inquiry for resolving similarly longstanding debates in cuprate superconductors and other strongly correlated materials.

Real-time observation of non-equilibrium phonon-electron energy and angular momentum flow in laser-heated nickel.

Identifying the microscopic nature of non-equilibrium energy transfer mechanisms among electronic, spin, and lattice degrees of freedom is central to understanding ultrafast phenomena such as manipulating magnetism on the femtosecond timescale. Here, we use time- and angle-resolved photoemission spectroscopy to go beyond the often-used ensemble-averaged view of non-equilibrium dynamics in terms of quasiparticle temperature evolutions. We show for ferromagnetic Ni that the non-equilibrium electron and spin dynamics display pronounced variations with electron momentum, whereas the magnetic exchange interaction remains isotropic. This highlights the influence of lattice-mediated scattering processes and opens a pathway toward unraveling the still elusive microscopic mechanism of spin-lattice angular momentum transfer.

Observation of time-reversal symmetry breaking in the band structure of altermagnetic RuO2.

Altermagnets are an emerging elementary class of collinear magnets. Unlike ferromagnets, their distinct crystal symmetries inhibit magnetization while, unlike antiferromagnets, they promote strong spin polarization in the band structure. The corresponding unconventional mechanism of time-reversal symmetry breaking without magnetization in the electronic spectra has been regarded as a primary signature of altermagnetism but has not been experimentally visualized to date. We directly observe strong time-reversal symmetry breaking in the band structure of altermagnetic RuO2 by detecting magnetic circular dichroism in angle-resolved photoemission spectra. Our experimental results, supported by ab initio calculations, establish the microscopic electronic structure basis for a family of interesting phenomena and functionalities in fields ranging from topological matter to spintronics, which are based on the unconventional time-reversal symmetry breaking in altermagnets.

Electronic structure, adsorption geometry, and photoswitchability of azobenzene layers adsorbed on layered crystals.

Mono- and multilayers of the molecular photoswitch azobenzene were adsorbed on two layered transition-metal dichalcogenides, semiconducting HfS2 and metallic TiTe2, at temperatures of 80-120 K and investigated in situ using valence-band and core-level photoelectron spectroscopy as well as near-edge X-ray absorption fine structure spectroscopy. The spectroscopic results indicate similar growth modes on the two substrates. In the monolayer systems, the azobenzene molecules tend to lie flat on the surface with average tilt angles of <15°, whereas the multilayer systems show a larger average tilt angle of 35-45°, depending on substrate surface conditions. The chemical environment of azobenzene, as investigated by XPS, does not change significantly from mono- to multilayers suggesting weak adsorbate-substrate coupling for the molecular layer that forms the interface with the substrate. Irradiation with ultraviolet light with a wavelength of 365 nm leads to a partial rearrangement of the adsorbed azobenzene molecules with a trans-to-cis conversion of up to 35%.

Unconventional electronic phase transition in SnBi2Te4: role of anomalous thermal expansion.

We propose SnBi2Te4to be a novel topological quantum material exhibiting temperature (T) mediated transitions between rich electronic phases. Our combined theoretical and experimental results suggest that SnBi2Te4goes from a low-Tsemimetallic phase to a high-T(room temperature) insulating phase via an intermediate metallic phase. Single crystals of SnBi2Te4are characterized by various experimental probes including synchrotron based x-ray diffraction, magnetoresistance, Hall effect, Seebeck coefficient and magnetization. X-ray diffraction data confirms an anomalous thermal expansion of the unit cell volume below ∼100 K, which significantly affects the bulk band structure and hence the transport properties. Simulated surface states are found to be topologically robust with varyingT. This indirectly supports the experimentally observed paramagnetic singularity in the entireT-range. The proposed coexistence of such rich phases is a rare occurrence, yet it facilitates a fertile ground to tune them in a material driven by structural changes.

Large Orbital Moment of Two Coupled Spin-Half Co Ions in a Complex on Gold.

The magnetic properties of transition-metal ions are generally described by the atomic spins of the ions and their exchange coupling. The orbital moment, usually largely quenched due the ligand field, is then seen as a perturbation. In such a scheme, S = 1/2 ions are predicted to be isotropic. We investigate a Co(II) complex with two antiferromagnetically coupled 1/2 spins on Au(111) using low-temperature scanning tunneling microscopy, X-ray magnetic circular dichroism, and density functional theory. We find that each of the Co ions has an orbital moment comparable to that of the spin, leading to magnetic anisotropy, with the spins preferentially oriented along the Co-Co axis. The orbital moment and the associated magnetic anisotropy is tuned by varying the electronic coupling of the molecule to the substrate and the microscope tip. These findings show the need to consider the orbital moment even in systems with strong ligand fields. As a consequence, the description of S = 1/2 ions becomes strongly modified, which have important consequences for these prototypical systems for quantum operations.

Precursor region with full phonon softening above the charge-density-wave phase transition in 2H-TaSe2.

Research on charge-density-wave (CDW) ordered transition-metal dichalcogenides continues to unravel new states of quantum matter correlated to the intertwined lattice and electronic degrees of freedom. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of the canonical CDW compound 2H-TaSe2 complemented by angle-resolved photoemission spectroscopy and density functional perturbation theory. Our results rule out the formation of a central-peak without full phonon softening for the CDW transition in 2H-TaSe2 and provide evidence for a novel precursor region above the CDW transition temperature TCDW, which is characterized by an overdamped phonon mode and not detectable in our photoemission experiments. Thus, 2H-TaSe2 exhibits structural before electronic static order and emphasizes the important lattice contribution to CDW transitions. Our ab-initio calculations explain the interplay of electron-phonon coupling and Fermi surface topology triggering the CDW phase transition and predict that the CDW soft phonon mode promotes emergent superconductivity near the pressure-driven CDW quantum critical point.