秋にシンポジウムで講演させていただきます。
「新規分光法による物性研究の潮流と展望」というセッションで
領域９と５の共同開催、
The stream and prospects in condensed matter physics using novel spectroscopy
という英語タイトルがつく英語セッションです。
講演演題はどれも面白そうです。
初っ端を務めつことになり
「高分解能光電子回折による局所電子物性研究への展開」というテーマを頂いていました。
要旨は次の通りです。
「原子軌道励起に由来する回折法：局所電子物性研究への展開 」
"Atomic-Orbital-Excited Diffraction\\ as Local Electronic Property Analysis Method"
Introduction:
The photoelectron from a localized core level is an excellent probe for element-specific atomic structure analysis.
Photoelectron diffraction provides information on the surrounding atomic configuration, which is recorded as forward focusing peaks (FFPs) at local interatomic directions and diffraction patterns in the photoelectron intensity angular distribution (PIAD).
Since PIAD differs by different surrounding atomic arrangement, emitter atom sites can be specified by their characteristic diffraction patterns.
By combining this diffraction technique with core level spectroscopy -- we call it diffraction spectroscopy,
the electronic property information of each atomic site structure can be investigated individually.
We have applied this method to study the electronic and magnetic structures of Ni thin film [1]. X-ray absorption and magnetic circular dichroism (XAS/XMCD) spectra for each atomic layer were separated. Surface and interior core-level shifts and magnetic moments were determined for each atomic layer individually around the spin reorientation transition.
In the valence PIAD at high kinetic energy, the quantum phenomena in initial states ({\em band dispersion}) and final states (photoelectron diffraction) are both well observed.
Making use of the site-specific photoelectron diffraction, we succeeded in measuring the valence band spectra from graphene edge region [2].
Element-resolved density of state (DOS) for compound crystals can be obtained analyzing corresponding FFP intensities from different sites.
New Phenomena:
Upon a core level excitation by circularly polarized light (CPL), angular momentum of light (helicity) is transferred to an emitted photoelectron, which can be confirmed by the parallax shift measurement of FFP direction in a stereograph of atomic arrangement [3].
The angular momentum of the emitted photoelectron is the summation of CPL helicity and the magnetic quantum number (MQN) of core hole.
The circular dichroism of photoelectron FFP rotation around the incident-light axis was also found in the case of the valence band photoelectrons with high kinetic energy. The MQN of valence electron for each energy band can be deduced from the FFP rotation [4].
Recently, we reported the observation and quantitative analysis of the angular momentum transfer from light to Auger electrons, instead of photoelectron [5].
We measured full hemisphere photoelectron and Auger electron intensity angular distributions (PIADs and AIADs) from a Cu(001) surface.
In the case of resonant Auger electron where the excited core electron is trapped at the conduction band, the circular dichroism contrast was clearly observed. Moreover, from the kinetic energy dependence of Auger electron FFP parallax shift, we found that the angular momentum is transferred to Auger electron most effectively in the case of the $^1S_0$ two-hole creation where two states involved have a same MQN but with opposite signs.
By combining Auger electron spectroscopy with FFP shift measurement at absorption threshold, element and MQN specific hole state can be generated in the valence band.
New Method and Prospects:
The resonant Auger electron emission by CPL excitation is an excellent way to polarize valence bands with a specific orbital magnetic momentum and a specific atomic number in a controlled fashion localized in space and time. This is useful for revealing the contribution of each atomic orbital to the electronic properties in compound crystals and epitaxial thin films.
In the case of magnetic materials, circular dichroism in the x-ray absorption intensity, i.e. XMCD, was observed in FFP intensity together with angular momentum transfer (parallax shift) effect. We show that MQN-resolved XMCD spectra can be obtained by using this phenomenon.
Copper is nonmagnetic, while nickel, which has one less electron than copper, is ferromagnetic.
If we can generate a valence hole with arbitrary atomic orbital character in the solid, excited states with new electronic properties are created, which can not be achieved by a simple thermal excitation.
The Auger electron emission processes that involve two valence electrons allow {\em e.g.} for the creation of a nickel-like $3d^84s^2$ atom in a copper matrix. As we show here the polarization of this $3d^84s^2$ impurity may be controlled by the light incidence and polarization of the light that creates the core hole for the Auger decay process.
Note that MQN-polarized hole state is spin-polarized as well owing to the spin-orbital interaction.
As a prospect, we propose a spin relaxation dynamics experiment at the specific atomic site by time- and spin-resolved analysis of corresponding MQN-polarized Auger electron.
Acknowledgements:
This work was performed at SPring-8 and Paul Scherrer Institute. The author deeply thank Dr.~Tomohiro Matsushita, Dr.~Tetsuya Nakamura, Dr.~Toyohiko Kinoshita, and Prof.~Hiroshi Daimon for their support in the experiments and for helpful discussions. This research was supported by Kakenhi Kiban(B), 25287075 2013 and JSPS Grant- in-Aid for Scientific Research on Innovative Areas ``3D Active-Site Science'': 26105007 2604.
References:
[1] F.M., et al., PRL 100 (2008) 207201.
[2] F.M., et al., J. Electron Spectrosc. Relat. Phenom. 195 (2014) 347.
[3] H. Daimon, PRL 86 (2001) 2034.
[4] F.M., et al., J. Phys. Soc. Jpn. 76 (2007) 013705.
[5] F.M., et al., PRL 114 (2015) 015501.
「新規分光法による物性研究の潮流と展望」というセッションで
領域９と５の共同開催、
The stream and prospects in condensed matter physics using novel spectroscopy
という英語タイトルがつく英語セッションです。
講演演題はどれも面白そうです。
初っ端を務めつことになり
「高分解能光電子回折による局所電子物性研究への展開」というテーマを頂いていました。
要旨は次の通りです。
「原子軌道励起に由来する回折法：局所電子物性研究への展開 」
"Atomic-Orbital-Excited Diffraction\\ as Local Electronic Property Analysis Method"
Introduction:
The photoelectron from a localized core level is an excellent probe for element-specific atomic structure analysis.
Photoelectron diffraction provides information on the surrounding atomic configuration, which is recorded as forward focusing peaks (FFPs) at local interatomic directions and diffraction patterns in the photoelectron intensity angular distribution (PIAD).
Since PIAD differs by different surrounding atomic arrangement, emitter atom sites can be specified by their characteristic diffraction patterns.
By combining this diffraction technique with core level spectroscopy -- we call it diffraction spectroscopy,
the electronic property information of each atomic site structure can be investigated individually.
We have applied this method to study the electronic and magnetic structures of Ni thin film [1]. X-ray absorption and magnetic circular dichroism (XAS/XMCD) spectra for each atomic layer were separated. Surface and interior core-level shifts and magnetic moments were determined for each atomic layer individually around the spin reorientation transition.
In the valence PIAD at high kinetic energy, the quantum phenomena in initial states ({\em band dispersion}) and final states (photoelectron diffraction) are both well observed.
Making use of the site-specific photoelectron diffraction, we succeeded in measuring the valence band spectra from graphene edge region [2].
Element-resolved density of state (DOS) for compound crystals can be obtained analyzing corresponding FFP intensities from different sites.
New Phenomena:
Upon a core level excitation by circularly polarized light (CPL), angular momentum of light (helicity) is transferred to an emitted photoelectron, which can be confirmed by the parallax shift measurement of FFP direction in a stereograph of atomic arrangement [3].
The angular momentum of the emitted photoelectron is the summation of CPL helicity and the magnetic quantum number (MQN) of core hole.
The circular dichroism of photoelectron FFP rotation around the incident-light axis was also found in the case of the valence band photoelectrons with high kinetic energy. The MQN of valence electron for each energy band can be deduced from the FFP rotation [4].
Recently, we reported the observation and quantitative analysis of the angular momentum transfer from light to Auger electrons, instead of photoelectron [5].
We measured full hemisphere photoelectron and Auger electron intensity angular distributions (PIADs and AIADs) from a Cu(001) surface.
In the case of resonant Auger electron where the excited core electron is trapped at the conduction band, the circular dichroism contrast was clearly observed. Moreover, from the kinetic energy dependence of Auger electron FFP parallax shift, we found that the angular momentum is transferred to Auger electron most effectively in the case of the $^1S_0$ two-hole creation where two states involved have a same MQN but with opposite signs.
By combining Auger electron spectroscopy with FFP shift measurement at absorption threshold, element and MQN specific hole state can be generated in the valence band.
New Method and Prospects:
The resonant Auger electron emission by CPL excitation is an excellent way to polarize valence bands with a specific orbital magnetic momentum and a specific atomic number in a controlled fashion localized in space and time. This is useful for revealing the contribution of each atomic orbital to the electronic properties in compound crystals and epitaxial thin films.
In the case of magnetic materials, circular dichroism in the x-ray absorption intensity, i.e. XMCD, was observed in FFP intensity together with angular momentum transfer (parallax shift) effect. We show that MQN-resolved XMCD spectra can be obtained by using this phenomenon.
Copper is nonmagnetic, while nickel, which has one less electron than copper, is ferromagnetic.
If we can generate a valence hole with arbitrary atomic orbital character in the solid, excited states with new electronic properties are created, which can not be achieved by a simple thermal excitation.
The Auger electron emission processes that involve two valence electrons allow {\em e.g.} for the creation of a nickel-like $3d^84s^2$ atom in a copper matrix. As we show here the polarization of this $3d^84s^2$ impurity may be controlled by the light incidence and polarization of the light that creates the core hole for the Auger decay process.
Note that MQN-polarized hole state is spin-polarized as well owing to the spin-orbital interaction.
As a prospect, we propose a spin relaxation dynamics experiment at the specific atomic site by time- and spin-resolved analysis of corresponding MQN-polarized Auger electron.
Acknowledgements:
This work was performed at SPring-8 and Paul Scherrer Institute. The author deeply thank Dr.~Tomohiro Matsushita, Dr.~Tetsuya Nakamura, Dr.~Toyohiko Kinoshita, and Prof.~Hiroshi Daimon for their support in the experiments and for helpful discussions. This research was supported by Kakenhi Kiban(B), 25287075 2013 and JSPS Grant- in-Aid for Scientific Research on Innovative Areas ``3D Active-Site Science'': 26105007 2604.
References:
[1] F.M., et al., PRL 100 (2008) 207201.
[2] F.M., et al., J. Electron Spectrosc. Relat. Phenom. 195 (2014) 347.
[3] H. Daimon, PRL 86 (2001) 2034.
[4] F.M., et al., J. Phys. Soc. Jpn. 76 (2007) 013705.
[5] F.M., et al., PRL 114 (2015) 015501.