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Using light to control transient phases in quantum materials is an emerging route to engineer new properties and functionality, with both thermal and non-thermal phases observed out of equilibrium. Transient phases are expected to be heterogeneous, either through photo-generated domain growth or by generating topological defects, and this impacts the dynamics of the system. However, this nanoscale heterogeneity has not been directly observed. Here we use time- and spectrally resolved coherent X-ray imaging to track the prototypical light-induced insulator-to-metal phase transition in vanadium dioxide on the nanoscale with femtosecond time resolution. We show that the early-time dynamics are independent of the initial spatial heterogeneity and observe a 200 fs switch to the metallic phase. A heterogeneous response emerges only after hundreds of picoseconds. Through spectroscopic imaging, we reveal that the transient metallic phase is a highly orthorhombically strained rutile metallic phase, an interpretation that is in contrast to those based on spatially averaged probes. Our results demonstrate the critical importance of spatially and spectrally resolved measurements for understanding and interpreting the transient phases of quantum materials.The intermediate states in photo-excited phase transitions are expected to be inhomogeneous. However, ultrafast X-ray imaging shows the early part of the metal–insulator transition in VO2 is homogeneous but then becomes heterogeneous.

Multi-dimensional interferometry of high harmonic generation is demonstrated using an inline Gouy interferometer. The rich data sets acquired with this technique, coupled with its inherent stability, enable robust phase analysis to be performed, allowing the delay in harmonic emission from a range of atomic and molecular gases to be measured relative to harmonic emission from argon with single-digit attosecond precision. Delays of − 78 ± 6  as, − 25 ± 6  as, 30 ± 3  as and 60 ± 3  as relative to emission from argon were measured for Xe, Kr, CO 2 and N 2 , respectively. The scheme can easily be incorporated in other high harmonic experiments, such as tomographic imaging of molecular orbitals, and provides an experimentally simple route towards probing ultrafast dynamics in molecular systems.

Fermi surfaces are essential for predicting, characterizing and controlling the properties of crystalline metals and semiconductors. Angle-resolved photoemission spectroscopy (ARPES) is the only technique directly probing the Fermi surface by measuring the Fermi momenta (k F ) from energy- and angular distribution of photoelectrons dislodged by monochromatic light. Existing apparatus is able to determine a number of k F -vectors simultaneously, but direct high-resolution 3D Fermi surface mapping remains problematic. As a result, no such datasets exist, strongly limiting our knowledge about the Fermi surfaces. Here we show that using a simpler instrumentation it is possible to perform 3D-mapping within a very short time interval and with very high resolution. We present the first detailed experimental 3D Fermi surface as well as other experimental results featuring advantages of our technique. In combination with various light sources our methodology and instrumentation offer new opportunities for high-resolution ARPES in the physical and life sciences. The Fermi surface is related to the energy distribution of electrons in a solid, and governs physical properties of metals and semiconductors. A new type of angle-resolved photoemission spectroscopy, probing the Fermi surface and combining short recording time with high resolution, is now presented.

Ultrafast laser pump-probe methods allow chemical reactions to be followed in real time, and have provided unprecedented insight into fundamental aspects of chemical reactivity. While evolution of the electronic structure of the system under study is evident from changes in the observed spectral signatures, information on rearrangement of the nuclear framework is generally obtained indirectly. Disentangling contributions to the signal arising from competing photochemical pathways can also be challenging. Here we introduce the new technique of three-dimensional covariance-map Coulomb explosion imaging, which has the potential to provide complete three-dimensional information on molecular structure and dynamics as they evolve in real time during a gas-phase chemical reaction. We present first proof-of-concept data from recent measurements on CF 3 I. Our approach allows the contributions from competing fragmentation pathways to be isolated and characterised unambiguously, and is a promising route to enabling the recording of ‘molecular movies’ for a wide variety of gas-phase chemical processes. Ultrafast laser pump-probe methods allow chemical reactions to be followed in real-time, but to directly show the rearrangement of nuclear frameworks is challenging. Here an improved real-time-resolved 3D covariance-map Coulomb explosion imaging technique for ultrafast gas-phase reactions is presented.

To pinpoint the electronic and structural mechanisms that affect intrinsic and extrinsic performance limits of 2D material devices, it is of critical importance to resolve the electronic properties on the mesoscopic length scale of such devices under operating conditions. Herein, angle‐resolved photoemission spectroscopy with nanoscale spatial resolution (nanoARPES) is used to map the quasiparticle electronic structure of a twisted bilayer graphene device. The dispersion and linewidth of the Dirac cones associated with top and bottom graphene layers are determined as a function of spatial position on the device under both static and operating conditions. The analysis reveals that microscopic rotational domains in the two graphene layers establish a range of twist angles from 9.8° to 12.7°. Application of current and electrostatic gating lead to strong electric fields with peak strengths of 0.75 V/μm at the rotational domain boundaries in the device. These proof‐of‐principle results demonstrate the potential of nanoARPES to link mesoscale structural variations with electronic states in operating device conditions and to disentangle such extrinsic factors from the intrinsic quasiparticle dispersion. Nanoscale angle‐resolved photoemission spectroscopy is applied to map the electronic structure of a twisted bilayer graphene device during the application of current and an electrostatic gate voltage. Rotational domains are found to strongly affect the measured linewidth of the top and bottom Dirac cones and lead to a spatially inhomogeneous electric field during operation of the device.

Twisted bilayer MoTe_{2} (tMoTe_{2}) is an emergent platform for exploring exotic quantum phases driven by the interplay between nontrivial band topology and strong electron correlations. Direct experimental access to its momentum-resolved electronic structure is essential for uncovering the microscopic origins of the correlated topological phases therein. Here, we report angle-resolved photoemission spectroscopy measurements of tMoTe_{2}, revealing pronounced twist-angle-dependent band reconstruction shaped by orbital character, interlayer coupling, and moiré potential modulation. Density functional theory captures the qualitative evolution, yet underestimates key energy scales across twist angles, highlighting the importance of electronic correlations. Notably, the hole effective mass at the K point exhibits a nonmonotonic dependence on twist angle, peaking near 2°, consistent with band flattening at the magic angle predicted by continuum models. Via electrostatic gating and surface dosing, we further visualize the evolution of electronic structure versus doping, enabling direct observation of the conduction band minimum and confirm tMoTe_{2} as a direct band gap semiconductor. These results establish a spectroscopic foundation for modeling and engineering emergent quantum phases in this moiré platform.

Controlling the electronic structure of two-dimensional materials using the combination of twist angle and electrostatic doping is an effective means to induce emergent phenomena. In bilayer graphene with an interlayer twist angle near the magic angle, the electronic dispersion is strongly modified by a manifold of hybridizing moiré Dirac cones leading to flat band segments with strong electronic correlations. Numerous technical challenges arising from spatial inhomogeneity of interlayer interactions, twist angle and device functionality have so far limited momentum-resolved electronic structure measurements of these systems to static conditions. Here, we present a detailed characterization of the electronic structure exhibiting miniband dispersions for twisted bilayer graphene, near the magic angle, integrated in a functional device architecture using micro-focused angle-resolved photoemission spectroscopy. The optimum conditions for visualizing the miniband dispersion are determined by exploiting the spatial resolution and photon energy tunability of the light source and applied to extract a hybridization gap size of \\((0.14 \\pm 0.03)\\)~eV and flat band segments extending across a moiré mini Brillouin zone. \\textit{In situ} electrostatic gating of the sample enables significant electron-doping, causing the conduction band states to shift below the Fermi energy. Our work emphasizes key challenges in probing the electronic structure of magic angle bilayer graphene devices and outlines conditions for exploring the doping-dependent evolution of the dispersion that underpins the ability to control many-body interactions in the material.

Heterostructures composed of the intrinsic magnetic topological insulator MnBi\\(_2\\)Te\\(_4\\) and its non-magnetic counterpart Bi\\(_2\\)Te\\(_3\\) host distinct surface electronic band structures depending on the stacking order and exposed termination. Here, we probe the ultrafast dynamical response of MnBi\\(_2\\)Te\\(_4\\) and MnBi\\(_4\\)Te\\(_7\\) following near-infrared optical excitation using time- and angle-resolved photoemission spectroscopy, and disentangle surface from bulk dynamics based on density functional theory slab calculations of the surface-projected electronic structure. We gain access to the out-of-equilibrium charge carrier populations of both MnBi\\(_2\\)Te\\(_4\\) and Bi\\(_2\\)Te\\(_3\\) surface terminations of MnBi\\(_4\\)Te\\(_7\\), revealing an instantaneous occupation of states associated with the Bi\\(_2\\)Te\\(_3\\) surface layer followed by carrier extraction into the adjacent MnBi\\(_2\\)Te\\(_4\\) layers with a laser fluence-tunable delay of up to 350 fs. The ensuing thermal relaxation processes are driven by phonon scattering with significantly slower relaxation times in the magnetic MnBi\\(_2\\)Te\\(_4\\) septuple layers. The observed competition between interlayer charge transfer and intralayer phonon scattering demonstrates a method to control ultrafast charge transfer processes in MnBi\\(_2\\)Te\\(_4\\)-based van der Waals compounds.