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Interactions between the different degrees of freedom form the basis of many manifestations of intriguing physics in condensed matter. In this respect, quantifying the dynamics of normal modes that themselves arise from these interactions and how they interact with other excitations is of central importance. Of the different types of coupling that are often important, spin-lattice coupling is relevant to several sub-fields of condensed matter physics; examples include spintronics, high-TC superconductivity, and topological materials. While theories of materials where spin-lattice coupling is relevant can sometimes be used to infer the magnitude and character of this interaction, experimental approaches that can directly measure it are rare and incomplete. Here we use time-resolved X-ray diffraction to directly access the spin-lattice coupling by measuring both ultrafast atomic motion and the associated spin dynamics following the excitation of a coherent electromagnon by an intense THz pulse in a multiferroic hexaferrite. Comparing the dynamics of the two different components, one striking outcome is the different phase shifts relative to the driving field. This phase shift provides insight into the excitation process of such a coupled mode. This direct observation of combined lattice and magnetization dynamics paves the way to access the mode-selective spin-lattice coupling strength, which remains a missing fundamental parameter for ultrafast control of magnetism and is relevant to a wide variety of correlated electron physics., Comment: 4 figures

The interaction between phonons and high-energy excitations of electronic origin in cuprates and their role in the superconducting mechanisms is still controversial. Here we use coherent vibrational time-domain spectroscopy together with density functional and dynamical mean-field theory calculations to establish a direct link between the c-axis phonon modes and the in-plane electronic charge excitations in optimally doped YBCO. The non-equilibrium Raman tensor is measured by means of the broadband 'coherent-phonon' response in pump-probe experiments and is qualitatively described by our model using DFT in frozen phonon approximation plus single band DMFT to account for the electronic correlations. The major outcome of our experimental and theoretical study is to establish the link between out-of-plane copper ions displacements and the in-plane electronic correlations, and to estimate at few unit cells the correlation length of the associated phonon mode. The approach introduced here could help revealing the complex interplay between fluctuations of different nature and spatial correlation in several strongly-correlated materials., Comment: 13 pages, 6 figures, 24 panels

Electronic structure of crystalline materials is their fundamental characteristic which is the basis of almost all their physical and chemical properties. Angle-resolved photoemission spectroscopy (ARPES) is the main experimental tool to study all electronic structure aspects with resolution in k-space. However, its application to three-dimensional (3D) materials suffers from a fundamental problem of ill-defined surface-perpendicular wavevector kz. Here, we achieve sharp definition of kz to enable precise navigation in 3D k space by pushing ARPES into the soft-X-ray photon energy range. Essential to break through the notorious problem of small photoexcitation cross-section was an advanced photon flux performance of our instrumentation. We explore the electronic structure of a transition metal dichalcogenide VSe2 which develops charge density waves (CDWs) possessing exotic 3D character. We experimentally identify nesting of its 3D Fermi surface (FS) as the precursor for these CDWs. Our study demonstrates an immense potential of soft-X-ray ARPES (SX-ARPES) to resolve various aspects of 3D electronic structure.

Atomically controlled crystal growth of thin films has established foundations of nanotechnology aimed at the development of advanced functional devices. Crystallization under non-equilibrium conditions allows engineering of new materials with their atomically-flat interfaces in the heterostructures exhibiting novel physical properties. From a fundamental point of view, knowledge of the electronic structures of thin films and their interfaces is indispensable to understand the origins of their functionality which further evolves into realistic device application. In view of extreme surface sensitivity of the conventional vacuum-ultraviolet (VUV) angle-resolved photoemission spectroscopy (ARPES), with a probing depth of several angstroms, experiments on thin films have to use sophisticated in-situ sample transfer systems to avoid surface contamination. In this Letter, we put forward a method to circumvent these difficulties using soft X-ray (SX) ARPES. A GaAs:Be thin film in our samples was protected by an amorphous As layer with an thickness of $\sim 1$ nm exceeding the probing depth of the VUV photoemission with photon energy $h\nu$ around 100 eV. The increase of the probing depth with increasing $h\nu$ towards the SX region has clearly exposed the bulk band dispersion without any surface treatment. Any contributions from potential interface states between the thin film and the amorphous capping layer has been below the detection limit. Our results demonstrate that SX-ARPES enables the observation of coherent three-dimensional band dispersion of buried heterostructure layers through an amorphous capping layer, breaking through the necessity of surface cleaning of thin film samples. Thereby, this opens new frontiers in diagnostics of authentic momentum-resolved electronic structure of protected thin-film heterostructures., Comment: 5 pages, 3 figures

Using a three-dimensional spin polarimeter we have gathered evidence for the interference of spin states in photoemission from the surface alloy Sb/Ag(111). This system features a small Rashba-type spin-splitting of a size comparable to the linewidth of the quasiparticles, thus causing an intrinsic overlap between states with orthogonal spinors. Besides a small spin polarization caused by the spin-splitting, we observe a large spin polarization component in the plane normal to the quantization axis provided by the Rashba effect. Strongly suggestive of coherent spin rotation, this effect is largely independent of the photon energy and photon polarization., Comment: 4 pages, 4 figures

Understanding the nature of electrical conductivity, superconductivity and magnetism between layers of oxides is of immense importance for the design of electronic devices employing oxide heterostructures. We demonstrate that resonant inelastic X-ray scattering can be applied to directly probe the carriers in oxide heterostructures. Our investigation on epitaxially grown LaAlO3/SrTiO3 superlattices unambiguously reveals the presence of both localized and delocalized Ti 3d carriers. These two types of carriers are caused by oxygen vacancies and electron transfer due to the polar discontinuity at the interface. This result allows explaining the reported discrepancy between theoretically calculated and experimentally measured carrier density values in LaAlO3/SrTiO3 heterostructures., Comment: 14 pages, 3 figures

The important source of interface conductivity in LaAlO3/SrTiO3 heterostructures, the Ti 3d carriers, is probed with resonant inelastic x-ray scattering at the Ti 2p(3/2) edge of epitaxially grown superlattices. We reveal unambiguously the generation of both localized and delocalized Ti 3d carriers as a result of the built-up heterointerface. Furthermore, we determine that the interface Ti3+O6 octahedra are orthorhombically distorted and quantify the crystal-field splitting energies. We argue that for as-grown superlattices, both types of Ti 3d carriers originate mainly from oxygen vacancies, whereas for fully oxidized samples they result from electronic reconstruction.

The resolution of angle-resolved photoelectron spectroscopy (ARPES) in three-dimensional (3D) momentum k is fundamentally limited by ill defined surface-perpendicular wave vector k(perpendicular to) associated with the finite photoelectron mean free path. Pushing ARPES into the soft-x-ray energy region sharpens the k(perpendicular to) definition, allowing accurate electronic structure investigations in 3D materials. We apply soft-x-ray ARPES to explore the 3D electron realm in a paradigm transition metal dichalcogenide VSe2. Essential to break through the dramatic loss of the valence band photoexcitation cross section at soft-x-ray energies is the advanced photon flux performance of our synchrotron instrumentation. By virtue of the sharp 3D momentum definition, the soft-x-ray ARPES experimental band structure and Fermi surface of VSe2 show a textbook clarity. We identify pronounced 3D warping of the Fermi surface and show that its concomitant nesting acts as the precursor for the exotic 3D charge-density waves in VSe2. Our results demonstrate the immense potential of soft-x-ray ARPES to explore details of 3D electronic structure.

International audience; Recently we found strong evidence in favor of a BCS-like condensation of excitons in 1T-TiSe2 [Cercellier , Phys. Rev. Lett. 99, 146403 (2007)]. Theoretical photoemission intensity maps have been generated by the spectral function calculated within the exciton condensate phase model and set against experimental angle-resolved photoemission spectroscopy data. The scope of this paper is to present the detailed calculations in the framework of this model. They represent an extension of the original excitonic insulator phase model of Jerome [Phys. Rev. 158, 462 (1967)] to three dimensional and anisotropic band dispersions. A detailed analysis of its properties and comparison with experiment is presented. Finally, the disagreement with density-functional theory is discussed.

The physics and chemistry of liquid solutions play a central role in science, and our understanding of life on Earth. Unfortunately, key tools for interrogating aqueous systems, such as infrared and soft X-ray spectroscopy, cannot readily be applied because of strong absorption in water. Here we use gas dynamic forces to generate free-flowing, sub-micron, liquid sheets which are 2 orders of magnitude thinner than anything previously reported. Optical, infrared and X-ray spectroscopies are used to characterize the sheets, which are found to be tunable in thickness from over 1 micron down to less than 20 nanometers, which corresponds to fewer than 100 water molecules thick. At this thickness, aqueous sheets can readily transmit photons across the spectrum, leading to potentially transformative applications in infrared, X-ray, electron spectroscopies and beyond. The ultrathin sheets are stable for days in vacuum, and we demonstrate their use at free-electron laser and synchrotron light sources.