Spin-polarized Electron Source Research Articles

Computational design of a material for high-efficiency spin-polarized electron source

The search for the highest quality and efficient spin-polarized electron source (SPES) is of fundamental importance in the investigation of spin-dependent phenomena in atomic, condensed-matter, nuclear, and particle physics. We propose a material, AgGaSe2 in the CuAu phase, as a high-quality SPES. We show that AgGaSe2 in the CuAu phase has an “ideal” band structure that allows the generation of electron beams with 100% polarization: a direct band gap with large and positive spin-orbit splitting and crystal-field splitting. Moreover, we predict that strain-free AgGaSe2 in CuAu phase can be obtained by epitaxial growth on ZnSe substrate.

High Luminescence Polarization of InGaAs–AlGaAs Strained Layer Superlattice Fabricated as a Photocathode of Spin-Polarized Electron Source

High circular luminescence polarization of 65% was observed in the InGaAs–AlGaAs strained layer superlattice which was fabricated as a spin-polarized electron source. The dependence of photoluminescence polarization on the excitation photon energy was well explained by the calculated band structure of the sample. The spin relaxation time and the lifetime of photo-generated electrons in the conduction band were determined from the experimental results of the time-resolved photoluminescence measurement. The spin polarization of conduction band electrons at the instant of excitation was determined to be 94% from the photoluminescence polarization, the lifetime and the spin relaxation time.

Spin-polarized electron sources with GaAs–GaAsP superlattices for surface analyses

A spin-polarized electron source has been developed using the GaAs–GaAsP superlattice. Polarized electrons are obtainable by irradiating semiconductors of zincblende lattice structures such as GaAs with circularly polarized photons. In the case of GaAs, polarization is restricted to under 50% because of the degeneracy of the heavy-hole and light-hole states in the valence band. The degeneracy is removed in superlattices by their periodic potential and the heavy-hole state of GaAs wells is lifted in the GaAs–GaAsP superlattice. By exciting electrons only from the heavy-hole state of GaAs wells to the conduction band, polarized electron beams are obtained, and the degree of polarization is as high as 80%. This high polarization is obtained in a wide wavelength range from 810 to 830 nm that is easily realized by conventional semi-conductor laser diodes. A quantum efficiency of 0.3% and a polarization of 80% have been obtained. The developed source has potential applications in surface characterization and inverse photoemission spectroscopy.

歪んだＧａＡｓ薄膜を用いたスピン偏極電子線源

歪んだＧａＡｓ薄膜を用いたスピン偏極電子線源

Some «Exotic» Aspects of Physics With Microtips

ABSTRACTThis paper focuses on some experiments done using field emitter arrays (FEAs) produced at CEA-LETI, experiments which are outside of the display or microwave amplifier contexts. Among various studies done at CEA-LETI with FEAs we have selected:- the search towards a spin polarized electron source,- a study on the spatial coherence of the electron beam emitted by a single emitter,- the incorporation of an optical command in a FEA and finally,- the application of a FEA to electron injection into rare gas liquids.

STM Study of a Thin GaAs Tip.

A spin sensitive tip is needed to realize the spin-polarized STM, which can resolve the spin-polarization of surface density of states. GaAs and its related compound semiconductors are well known as spin-polarized electron sources and are also candidates for the spin probe. We made a GaAs thin film tip by cleavage. Since the GaAs film is thin enough, circular polarized light can illuminate through the tip and efficiently excite spin-polarized electrons near its apex. Because an atomically resolved Si(111) 7×7 surface was observed with the tip, the tip is mechanically stable. Optical responses of the tip were studied with Fe thin films grown on a MgO(001) surface. Using this magnetic thin film, we demonstrated that spin-dependent tunneling could be detected with the tip.

新しいスピン偏極電子線源とその応用

新しいスピン偏極電子線源とその応用

Computer stabilized spin polarized electron source

A systematic optimization of the components of our polarized electron source has markedly improved its long term stability over earlier versions. The major factors shortening the lifetime of the source have been identified. The essential parts of the spin polarized electron source, such as the vacuum system, crystal holder, cleaving mechanism, caesium dispenser, oxygen admittance tube, and computer control activation features are discussed. The lifetime of the source now exceeds 500 h with a constant polarization of 28.5%.

Spin-resolved electron spectroscopy with highly polarized sources: Inverse photoemission from ferromagnets

We report on the use of recently developed spin-polarized electron sources with very high polarization for electron spectroscopy. In particular we present data of spin-resolved inverse photoemission from Fe(100) films excited by polarized electrons produced by a strained GaAsP negative electron affinity photocathode. This highly polarized source (beam polarization P=65%) allows a direct and almost complete decoupling of the majority and minority empty states in Fe(100), much better than the standard GaAs sources (P=20%–30%). The preparation and characterization of the strained photocathode is also discussed.

SPLEEM of Magnetic Surfaces and Layered Structures

Spin-polarized low-energy electron microscopy (SPLEEM) is a new approach to magnetic-surface imaging. Historically, it is an outgrowth of LEEM (low-energy electron microscopy), which has been successfully applied to the high-speed in situ imaging of microscopic surface structures under clean ultrahigh-vacuum (UHV) conditions. In terms of basic instrument design, in SPLEEM a spin-polarized electron source of the Pierce type replaces the LaB6 field-emission electron source used in all dedicated LEEM systems. A new, fully electrostatic version of LEEM has recently been developed, resulting in a more compact, flange-on-type instrument component, which can be integrated with standard UHV surface analytical systems. The main advantage of the new SPLEEM technique is an advanced electron-source design that allows the spin direction to be arbitrarily adjusted with respect to the sample surface (see the following).

Low energy Mott polarimetry of electrons from negative electron affinity photocathodes

We present data on the spin polarization P and quantum yield Y of electrons photoemitted from negative electron affinity semiconductors, including GaAs(100), GaAsP(100) alloy, and strained GaAs layer epitaxially grown on a GaAsP(100) buffer. Near photothreshold the following values for P(Y) are, respectively, obtained: 26% (2.5×10−2), 40% (1×10−3), and 60% (1.5×10−4). We describe in detail the apparatus used containing a low energy (10–25 keV) Mott polarimeter. The system, completely fitted in a small volume (∼104 cm3) ultrahigh vacuum chamber, is intended as a test facility for characterizing candidate photocathode materials for spin polarized electron sources.

Circular Polarization of Photoluminescence Excitation Spectra of Strained GaAs Layer

Measurements of photoluminescence (PL) spectra and photoluminescence excitation (PLE) spectra were made on strained GaAs samples fabricated as a photocathode of spin-polarized electron source. PL and PLE due to the interband transition of the strained GaAs layer showed large polarization in comparison with those of the GaAs substrate. The increase of the hand gap and the splitting of the valence band were observed in luminescence polarization spectra of PLE although they are not clear in the original spectra. The spin polarization of conduction band electrons in the case of resonant excitation was estimated to be 72% from luminescence polarizations of PL and PLE spectra.

Liquid phase epitaxial growth of elastically strained InGaAsP layers for spin-polarized electron sources

InGaAsPGaAs(111)B pseudomorphic layers have been grown with elastic strains up to 1% and bandgaps ranging from 1.4 to 1.8 eV by liquid phase epitaxy (LPE) for the first time. Splitting of the valence band up to 50 meV was observed in these films. Photoemitters with the negative electron affinity state have been prepared by CsO activation of the strained InGaAsP films in ultrahigh vacuum (UHV).

Compact apparatus for spin-polarized photocathode characterization

We describe a very compact apparatus suitable for investigating candidate photocathode materials for spin-polarized electron sources. Photocathode characterization is obtained by means of quantum yield and spin polarization measurements versus photon energy. As an example, data on negative-electron-affinity GaAs(100) are presented.

A high power lamp-pumped LNA laser with thermally tuned etalon

This report describes arc-lamp pumped, tunable, continuous-wave LNA laser with improved output efficiency, suitable for optical pumping of helium. Using thermal tuning of intracavity etalons instead of angle tuning reduces losses caused by walk-off effects and provides continuous frequency scanning without a significant decrease in laser power. We obtain an output power of 4 W in a linewidth of 2.5 GHz with the laser wavelength tuned to the 1.083 μm absorption line of metastable helium. This result represents a fourfold gain in efficiency compared with conventional tuning by tilting. The laser provides efficient polarization of helium by optical pumping in various applications, such as nuclear spin polarization of 3He and spin-polarized electron sources.

GaAsP spin-polarized electron source for a 300 keV accelerator

We report on a GaAsP spin polarized electron source designed to operate in high voltage terminals of electrostatic accelerators of several hundred kV. The small source of simple mechanical and electron-optical design can easily be connected to an accelerator tube. A stable beam of 35%–40% degree of polarization can be produced with a lifetime of several weeks. A measurement of the asymmetry in the emission of bremsstrahlung by transversely polarized electrons has been performed.

Angle-resolved inverse photoemission spectroscopy with longitudinally spin-polarized electrons

We have built and tested a spin-polarized electron source based on photoemission from negative electron affinity GaAs(100). It provides 5–10 μA of longitudinally spin-polarized electrons in the energy range 5–20 eV. Following the design of Erdman and Zipf, the electron optics are simple and consist of only five elements. Their voltages are all proportional to each other and are linearly driven by one ramping voltage. The spin-polarized electrons are used for angle-resolved inverse photoemission measurements from magnetized sample surfaces. Our first results obtained from Ni(110) mounted on a horseshoe magnet agree well with the literature. From these measurements the degree of spin polarization is determined to be 35%±5%. We also checked the spin polarization using an absorbed-current spin detector in combination with a 90° cylindrical deflector.

Spin-polarized photoemission from AlGaAs/GaAs heterojunction: A convenient highly polarized electron source

We analyze the operation of a spin-polarized electron source, consisting of a 100 Å GaAs cap on top of Al0.3Ga0.7As, excited at 300 or 120 K by a He-Ne laser. The cap allows easy activation to negative electron affinity while the alloy permits gap matching to the light source, and thus large electron spin polarization (30% at 300 K, 36% at 120 K). We compare yield curves, energy distribution curves, and polarized energy distribution curves obtained on samples with 100 and 1000 Å caps and on bulk GaAs. The X conduction minimum position in the alloy is also determined.

Band-gap engineering for new photonic and electronic devices

Bandgap engineering is a powerful technique for the design of new semiconductor materials and devices. Heterojunctions and modern growth techniques, such as molecular beam epitaxy, allow band diagrams with nearly arbitrary and continuous band-gap variations to be made. A new generation of devices with unique capabilities, ranging from solid-state photomultipliers to resonant tunneling transistors and spin polarized electron sources, is emerging from this approach.

GaAs/AlAs monolayer superlattices: A new candidate for a highly spin-polarized electron source

Using the results of an ab-initio self-consistent local density Dirac calculation of its electronic structure, we suggest that the GaAs/AlAs monolayer superlattice should be a good candidate for a high quality spin-polarized photoelectron source, giving a theoretical polarization of electrons of 100% together with favourable conditions for their extraction from the solid.