[en] We report direct angle-resolved photoemission measurements of the coupling between the symmetric stretch vibrational mode of adsorbed hydrogen and a surface band on W(110). This coupling is manifested by the surface band being split into two branches at a binding energy comparable to the vibrational mode energy, as confirmed by observation of a dramatic hydrogen/deuterium isotope effect. The electron-phonon coupling parameter λ is found to be significantly larger than that for bulk W, and to be closely related to the degree of surface localization of the surface state wave function. (c) 2000 The American Physical Society

[en] Using angle-resolved photoemission, we have mapped the dispersion relations and Fermi contours for surface-localized electron states onto clean and hydrogen-covered Cr(110) surfaces. In particular, we have probed the relationship between hydrogen adsorption and the evolution of the spin density wave (SDW) periodicity in chromium thin films observed previously. We find qualitatively similar surface band dispersion relations to those on W(110) and Mo(110), although with a narrower bandwidth, broader spectral features, and a smaller impact from the spin-orbit interaction. We compare our results to existing first-principles calculations and find a significant disagreement for a surface band that produces a prominent surface Fermi contour. Upon hydrogen adsorption, the Fermi contour for a particular surface band becomes well nested at a wave vector that stabilizes a commensurate SDW. We suggest that a competition between commensurate two-dimensional (2D) and incommensurate 3D Fermi surface nesting plays an important role in the SDW energetics in thin Cr(110) films

[en] Emerging interest in utilizing the transverse coherence properties of thermal energy atomic and molecular beams motivates the development of ionization detectors with near unit detection efficiency and adequate spatial resolution to resolve interference fringes of submicron dimension. We demonstrate that a field ionization tip coupled to a charged particle detector meets these requirements. We have systematically studied the current-voltage relationship for field ionization of helium using tungsten tips in diffuse gas and in a supersonic helium beam. For all 16 tips used in this study, the dependence of ion current on voltage for tips of fixed radius was found to differ from that for tips held at constant surface electric field. A scaling analysis is presented to explain this difference. Ion current increased on average to the 2.8 power of voltage for a tip at fixed field and approximately fifth power of voltage for fixed radius for a liquid nitrogen cooled tip in room temperature helium gas. For the helium beam, ion current increased as 2.2 power of voltage with constant surface field. The capture region of the tips was found to be up to 0.1 μm² for diffuse gas and 0.02 μm² in the beam. Velocity dependence and orientation of tip to beam were also studied

[en] Using angle-resolved photoemission spectroscopy, we have measured the Fermi surface of V(110) films epitaxially grown on a W(110) substrate. We compare our results for thicker films to existing calculations and measurements for bulk vanadium and find generally very good agreement. For thinner films, we observe and analyse a diverse array of quantum well states that split and distort the Fermi surface segments. We have searched unsuccessfully for a thickness-induced topological transition associated with contact between the zone-centre jungle gym and zone-boundary hole ellipsoid Fermi surface segments. We also find no evidence for ferromagnetic splitting of any bands on this surface

[en] We report soft x-ray speckle metrology measurements of microscopic return point and complementary point memory in Co/Pd magnetic films having perpendicular anisotropy. We observe that the domains assemble into a common labyrinth phase with a period that varies by nearly a factor of two between initial reversal and fields near saturation. Unlike previous studies of similar systems, the ability of the film to reproduce its domain structure after magnetic cycling through saturation varies from loop to loop, from position to position on the sample, and with the part of the speckle pattern used in the metrology measurements. We report the distribution of memory as a function of field and discuss these results in terms of the reversal process.

[en] We map out the charge-spin density profile of magneto-electric La_2/3Sr_1/3MnO₃ (LSMO)–BiFeO₃ (BFO) heterostructure using soft x-ray resonant magnetic reflectivity. We show that the spatial extent of interface orbitals can extend over a few lattice periods even for an atomically sharp interface. While LSMO magnetization is depleted at the interface, BFO does develop a weak magnetic moment mostly near the interface, probably due to a proximity-induced charge-transfer process. Our study reveals that simultaneous control of electronic and magnetic interfaces is essential in realizing the potential of oxide devices. (paper)

[en] We explore a number of novel effects near the orbital-order phase transition in a half-doped manganite, Pr_0.5Ca_0.5MnO₃. To probe the unusual short-range orbital order in this system, we have performed coherent soft x-ray resonant scattering measurements in a Bragg geometry to measure dynamics. Near the transition temperature, we observe a small fluctuating component in the scattered signal that is correlated with three effects: a rapidly decreasing total signal and orbital domain size, as well as an abrupt onset of a broad background intensity that we attribute to the thermal production of correlated polarons. Our speckle results suggest that the transition is characterized by a competition between a pinned orbital domain topology that remains static and mobile domain boundaries that exhibit slow, temporal fluctuations

[en] Light’s capacity to carry angular momentum is integral to our knowledge of physics and ability to probe matter. In addition to spin, photons can occupy free-space orbital angular momentum eigenstates. Visible light orbital angular momentum is used in quantum information experiments, super-resolution microscopy, optical tweezers and angular momentum transfer to atoms in optical lattices. Soft X-ray orbital angular momentum applications, slowed by the lack of suitable optics and the rarity of coherent X-ray sources, could enable the direct alteration of atomic states through orbital angular momentum exchange, and methods to study the electronic properties of quantum materials. We have made soft X-ray diffractive optics that generate single Laguerre–Gauss modes, observed carrying up to 30$\mathrm{\hslash <!--\; ???\; -->}$ angular momentum per photon, or their superpositions. We also present Hermite–Gauss diffractive optics and a soft X-ray orbital angular momentum analyser. Finally, these tools could enable both the manipulation and finer characterization of topologically complex electronic matter, such as magnetic skyrmions.

[en] Chromium films offer an excellent system to study the impact of dimensional confinement on physical properties associated with the spin-density-wave (SDW) ground state observed in bulk materials. These properties are also of some technological importance since chromium is a common component of thin film magnetic structures. We prepared chromium (1 1 0) films of high crystalline quality on a W(1 1 0) substrate with a wedge-shaped thickness profile so that the impact of confinement can be systematically studied. We have characterized these films using a combination of low-energy electron diffraction and microscopy as well as high-resolution angle-resolved photoemission spectroscopy. We have probed the Fermi surface and the nesting vectors therein that are relevant to the SDW ground state. We find these to predict accurately the observed bulk SDW periodicity. We have also characterized the SDW periodicity in the film directly by measuring the splitting between backfolded bands, and we find that this periodicity deviates markedly from the bulk periodicity for thinner films at higher temperatures. We have systematically mapped the SDW incommensurability and phase diagram as a function of both film thickness and temperature. We find commensurate and incommensurate phases that are separated by nearly continuous transitions. Our results suggest a simple model to explain the delicate interplay between commensurate and incommensurate phases that involves a balance between SDW stabilization energy and surface and interface energetics

[en] We report a detailed experimental and theoretical study of the electronic structure of Mo_1−xRe_x random alloys. We have measured electronic band dispersions for clean and hydrogen-covered Mo_1−xRe_x(110) with x = 0–0.25 using angle-resolved photoemission spectroscopy. Our results suggest that the bulk and most surface electronic bands shift relative to the Fermi level systematically and approximately rigidly with Re concentration. We distinguish and quantify two contributions to these shifts: a raise of the Fermi energy and an increase of the overall bandwidth. Alloy bands calculated using the first-principles Korringa–Kohn–Rostoker coherent-potential-approximation method accurately predict both of these effects. As derived from the rigid band model, the Fermi energy shift is inversely related to the bulk density of states in this energy region. Using our results, we also characterize an electronic topological transition of the bulk Fermi surface and relate this to bulk transport properties. Finally, we distinguish effects beyond the rigid band approximation: a highly surface-localized state and a composition-dependent impact of the spin–orbit interaction. (paper)