Susumu Ogawa

Ultrafast interferometric pump–probe correlation measurements in systems with broadened bands or continua

Ultrafast (femtosecond) interferometric pump–probe techniques can be used to measure rates of population and quantum phase decay in complicated media such as liquids and solids. However, the levels probed in such systems are often inhomogeneously broadened or are part of a continuum of states. The use of broadband ultrafast lasers thus results in multiple levels being excited and detected. The inherent averaging that is due to this effect can alter the measured coherent response, thus affecting the information that can be retrieved on the phase decay. The importance of these effects is considered for the representative case of two-photon photoemission from metals. The effects of (i) continuum excitation; (ii) excitation from the Fermi level, i.e., a spectral step function; (iii) excitation from broadened levels with a finite width; and (iv) photoelectron energy analyzer resolution are determined.

The role of Auger decay in hot electron excitation in copper

The role of different excitation mechanisms in two-photon photoemission measurements of the hot electron population dynamics in copper is considered. The effective hot electron lifetimes derived from two-pulse correlation measurements with ∼3.1–3.8 eV, 50 fs laser pulse excitation are different depending on whether the hot electrons are generated by interband d→sp or intraband sp→sp excitation (S. Pawlik, M. Bauer, M. Aeschlimann, Surf. Sci. 377–379 (1997) 206). A proposed explanation is that the latter process actually occurs by the Auger recombination of long-lived d-band holes resulting in complex hot electron population dynamics involving this delayed generation process and decay by the electron–electron scattering [E. Knoesel, A. Hotzel, M. Wolf, Phys. Rev. B 57 (1998) 12812]. This proposal is tested by simulation of interferometric two-pulse correlation measurements on the low index surfaces of copper (Cu(111), (100), and (110)) by the optical Bloch equations. The lower limit for the d-hole lifetime due to the Auger recombination of 24±3 fs for modeling of how this generation process affects the hot electron population kinetics is established from the d-hole decoherence measurements at the X5 point. Optical Bloch equation fits of the data show that at most <10% of hot electrons at 1.4 eV are generated through a secondary generation mechanism, therefore Auger recombination cannot explain the anomalous hot electron population dynamics.

Femtosecond dynamics of hot-electron relaxation in Cu(110) and Cu(100)

Hot-electron relaxation dynamics due to electron-electron scattering at Cu(110) and Cu(100) surfaces are measured with < 10 fs time resolution by two-photon time-resolved photoemission spectroscopy. Comparison of experimental population decay rates for hot electrons with 1.3-3.2 eV energy above the Fermi level with those calculated by the Fermi liquid theory shows significant disagreement. The experimental rates are on average ∼ 5 times slower, have a different energy dependence than predicted by theory, and depend on the crystal face of Cu. The time scales for hot-electron thermalization measured here are important for understanding and controlling hot-electron-induced chemistry at metal surfaces.

Realization of a micrometre-scale spin-wave interferometer

The recent development of spin dynamics opens perspectives for various applications based on spin waves, including logic devices. The first important step in the realization of spin-wave-based logics is the manipulation of spin-wave interference. Here, we present the experimental realization of a micrometre-scale spin-wave interferometer consisting of two parallel spin-wave waveguides. The spin waves propagate through the waveguides and the superposition or interference of the electrical signals corresponding to the spin waves is measured. A direct current flowing through a metal wire underneath one of the spin-wave waveguides affects the propagation properties of the corresponding spin wave. The signal of constructive or destructive interference depends on the magnitude and direction of the applied direct current. Thus, the present work demonstrates a unique manipulation of spin-wave interference.

Energy relaxation and dephasing times of excited electrons in Bi2Sr2CaCu2O8+δ from interferometric 2-photon time-resolved photoemission

Abstract The electron dynamics inBi 2 Sr 2 CaCu 2 O 8+δ are investigated with interferometric 2-photon time-resolved photoemission. By decomposing the interferometric 2-pulse correlation into oscillating and non-oscillating components energy relaxation times can be obtained for excited electrons above the Fermi level, T 1 , and dephasing times of electron-hole pairs, T 2 . The procedure to determine T 1 and T 2 is discussed and first results on the electron energy relaxation time in a high temperature superconductor are presented.

Scalability of Spin Accumulation Sensor

In this work, we investigated a scalability of spin accumulation signal for various device sizes. We found that the spin accumulation signal is enhanced by shrinking Cu wire width and thickness. Moreover we estimate the signal to noise ratio (SNR) of a spin accumulation head structure by using the size dependence of spin accumulation signal. The SNR increases with reducing nonmagnetic wire width and thickness, which are corresponding to a track width and a gap width, respectively. And it has the maximum value around 2 ~ 3 Tbit/inch2 resolution. From these results, it has a possibility that spin accumulation effect is applicable to a new read head for high density hard disk drive.

Two-photon photoemission spectroscopy at clean and oxidized Cu(110) and Cu(100) surfaces

Two-photon photoemission spectra of Cu(110) and Cu(100) are measured using ∼ 3.2 eV excitation laser with ∼ 20 fs pulse width. The spectra are measured as a function of laser polarization, excitation energy, bias voltage, and oxygen coverage. They are discussed in terms of the bulk and surface electronic structure of Cu and hot electron relaxation dynamics, and compared to previous nanosecond laser studies. In addition, three-photon photoemission through the n = 1 image potential state on Cu(100) is observed.

Effect of palladium particle size on the appearance of superstructure of graphite in palladium/graphite model catalyst

Scanning tunneling microscopy images of palladium particles supported on highly oriented pyrolytic graphite as a model catalyst in ultra-high vacuum have been observed. We found superstructures on graphite lattice due to electronic interaction between palladium particles and graphite in the vicinity of small two-dimensional palladium particles (lateral size <2 nm, height <0.5 nm). However, such superstructures could not be observed near larger three-dimensional palladium particles (lateral size ∼4 nm, height ∼2 nm). The results indicate the importance of not only the size but also the dimension of metal particles in interaction between palladium and graphite, the nature of which can be interpreted by the difference in electronic properties of atomic and bulk palladium. This has important implications to the understanding of metal-support electronic interaction and its effect on the surface catalytic reactivity of supported metal catalysts.

Voltage-induced magnetization dynamics in CoFeB/MgO/CoFeB magnetic tunnel junctions

Recent progress in magnetic tunnel junctions (MTJs) with a perpendicular easy axis consisting of CoFeB and MgO stacking structures has shown that magnetization dynamics are induced due to voltage-controlled magnetic anisotropy (VCMA), which will potentially lead to future low-power-consumption information technology. For manipulating magnetizations in MTJs by applying voltage, it is necessary to understand the coupled magnetization motion of two magnetic (recording and reference) layers. In this report, we focus on the magnetization motion of two magnetic layers in MTJs consisting of top layers with an in-plane easy axis and bottom layers with a perpendicular easy axis, both having perpendicular magnetic anisotropy. According to rectified voltage (Vrec) measurements, the amplitude of the magnetization motion depends on the initial angles of the magnetizations with respect to the VCMA direction. Our numerical simulations involving the micromagnetic method based on the Landau-Lifshitz-Gilbert equation of motion indicate that the magnetization motion in both layers is induced by a combination of VCMA and transferred angular momentum, even though the magnetic easy axes of the two layers are different. Our study will lead to the development of voltage-controlled MTJs having perpendicular magnetic anisotropy by controlling the initial angle between magnetizations and VCMA directions.

Propagation of nonlinearly generated harmonic spin waves in microscopic stripes

We report on the experimental study of the propagation of nonlinearly generated harmonic spin waves in microscopic CoFeB stripes. Using an all electrical technique with coplanar waveguides, we find that two kinds of spin waves can be generated by nonlinear frequency multiplication. One has a non-uniform spatial geometry and thus requires appropriate detector geometry to be identified. The other corresponds to the resonant fundamental propagative spin waves and can be efficiently excited by double- or triple-frequency harmonics with any geometry. Nonlinear excited spin waves are particularly efficient in providing an electrical signal arising from spin wave propagation.