Steve Kevan

Rashba effect at the surfaces of rare-earth metals and their monoxides

We present a systematic study of the Rashba-type spin–orbit interaction at the (0001) surfaces of rare-earth metals and their surface monoxides, specifically of Tb metal and the O/Tb, O/Lu and O/Y surfaces. By means of photoemission experiments and ab initio band-structure calculations, we uncover the influence of this interaction on the surface electronic structure. In turn, the dramatic impact of the charge-density distribution of the surface/interface states on the strength of the Rashba-type spin splitting is demonstrated. We discuss the Rashba effect at magnetic and non-magnetic rare-earth surfaces, and compare with cases where it is negligible. The difference between the Rashba effect and magnetic linear dichroism in photoemission is pointed out to help avoid possible confusion in connection with the simultaneous appearance of these two effects at a magnetic surface.

Orbital domain dynamics in a doped manganite

We explore a number of novel effects near the orbital-order phase transition in a half-doped manganite, Pr0.5Ca0.5MnO3. 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.

DISPERSION COMPENSATION HIGH-RESOLUTION ELECTRON ENERGY-LOSS SPECTROMETER FOR TIME-RESOLVED SURFACE STUDIES

A new high‐resolution electron energy‐loss spectrometer based on the dispersion compensation (DC) concept has been designed, built, and tested. The parallel processing inherent in DC has allowed us to attain signal levels two orders of magnitude in excess of conventional designs while operating at comparable resolution. This is consistent with expectations based on simple model calculations. The resolving power of the spectrometer is not as good as theoretically predicted probably due to uncontrolled beam angle effects. Initial tests on a variety of systems are presented. We analyze the strengths and weaknesses of the design and suggest future improvements.

Effect of structural incoherence on the low‐angle diffraction pattern of synthetic multilayer materials

The sequential, layering techniques used to prepare multilayer materials result in significant structural incoherence due to deviations from the intended thicknesses within an elemental layer and local deviations from the average due to islanding of the depositing elements during deposition. We demonstrate that if the domain size of the structural incoherence is large compared with the wavelength of the scattering radiation, the structural incoherence manifests itself in the low‐angle diffraction pattern by attenuating the intensity of the subsidiary maxima relative to the Bragg maxima. We also show that the subsidiary maxima in the low‐angle diffraction pattern of a multilayer result from incomplete destructive interference from all of the interfaces, not just from the top and bottom surface of the film. A technique for incorporating structural incoherence when modeling the diffraction pattern of a multilayer structure is presented. The ability of this model to simulate the experimental diffraction pattern of an iron‐silicon multilayer is demonstrated.

Mapping spatial and field dependence of magnetic domain memory by soft X‐ray speckle metrology

The occurrence of magnetic domain memory has been observed in ferromagnets, either induced by structural defects or by exchange couplings. Being able to quantify the amount of memory as a function of length scale, field and temperature is both of fundamental and technological importance. A technique has been refined to statistically quantify the magnetic domain memory in ferromagnetic thin films by using coherent soft-X-ray scattering metrology. This technique, based on cross-correlating magnetic speckle patterns, provides a unique way to map out the behavior of domain memory. Here, the details of our correlation method and the necessary treatment of the X-ray scattering images to extract spatial and field dependences in the memory information are reviewed. The resulting correlation maps, measured on [Co/Pd]IrMn multilayers, show how magnetic domain memory evolves at various spatial scales, as a function of the field magnitude throughout magnetization cycles, but also as a function of field cycling and of temperature. This technique can easily be applied to a wide variety of systems presenting memory effects, in soft and hard matter, and also to dynamical studies.

Brightness of micronozzle helium source

We have measured the brightness of several helium free jet sources. Five converging nozzles with diameters between 0.6 and 5μm, and three tube nozzles with diameters between 2 and 10μm were studied at stagnation temperatures of 77 and 300K and at stagnation pressures depending on nozzle size from 350to17000kPa. Smaller nozzles produced higher brightness beams with values approaching 1028(ssrm2)−1. At low-temperature quantum effects on the helium collision cross section significantly decreased the source brightness. We explore the possibility of producing even higher brightness sources with smaller diameter.

Many body effects at surfaces and interfaces

Abstract The past two decades have experienced the development of angle-resolved photoemission to the point that it can now provide crucial information on an energy scale that is relevant to many key problems in condensed matter physics. At the same time, our ability to prepare and characterize surfaces, interfaces, and thin films has improved to the extent that many exotic phases can be produced with a high degree of perfection. The combination of these two developments suggests many interesting experiments can be performed that probe the coupling between charge, lattice, and spin degrees of freedom in the context of surface and interface physics. In this paper we briefly review a few groundbreaking experiments that have initiated this effort. We also speculate on future developments in this area.

Field mapping and temperature dependence of magnetic domain memory induced by exchange couplings

Strong magnetic domain memory is achieved in (Co/Pd)IrMn exchange-biased ferromagnetic thin films when zero-field-cooled (ZFC) below their blocking temperature TB. By mapping out the amount of memory throughout the entire magnetization cycle, from nucleation to saturation, at different temperatures below and above TB, we discover how microscopic morphological changes in the magnetic domain patterns correlate with the macroscopic magnetic hysteresis, in the presence or absence of exchange couplings. Our unique inter-field correlation maps show that in the ZFC state, the film exhibits the highest amount of domain memory, exceeding 90%, when domain patterns are compared at the same field value, in the coercive region of the magnetization loop. However, domain patterns also cross-correlate surprisingly well when measured at different field values, on a wide field range centered about the coercive region. The shape and symmetry of the correlation maps provide further insights into the microscopic morphological changes in the domain patterns and the amount of reversibility in the reversal process, at the nanoscale.

Electron–phonon coupling in W(110)-(1×1)H

Abstract Recently, we reported high resolution angle-resolved photoemission measurements that indicated unusually strong coupling between adsorbate vibrations and a surface-localized electron state in the system W(110)-(1×1)H. We have now extended these measurements to probe the electron–phonon coupling strength as a function of position on the relevant Fermi contour. We find that the strength of the coupling is strongly dependent on Fermi wave vector, with the electron–phonon coupling constant λ varying between zero and ∼0.8. Qualitatively, the strength of the coupling is related to the position of the surface band in the projected bulk band gap, and therefore to the degree of surface localization of the surface state wave function. Such effects should play an important role in all adsorbate vibronic interactions, since the vibrational modes are generally highly localized to the outermost surface layer. We discuss these results in relation to the surface phonon anomalies observed on this surface.

A contactless photoconductance technique for the identification of impact ionization

A new, contactless, microwave photoconductance based technique for the direct measurement of the spectral dependence of free carrier generation efficiency in semiconductors is described and demonstrated. The technique is applied to the search for hetero-junction assisted impact ionization (HAII) with promising initial results. A strong photo-dielectric effect is also revealed for a ZnS:i-Si interface demonstrating the ability of the technique to characterize results other than the enhanced photoconductivity we seek. Such characterization will help to inform the next step in hetero-junction preparation.