Name: experimental methods for spin- and angle-resolved photoemission spectroscopy combined with polarization-variable laser
Uploaded: 2018-06-28T13:00:00
Description: Watch this Scientific Journal Video about Experimental Methods for Spin- and Angle-Resolved Photoemission Spectroscopy Combined with Polarization-Variable Laser at JoVE.com

We thank M. Nakayama, S. Toyohisa, A. Fukushima and Y. Ishida for supports to the experimental setup. We gratefully acknowledge funding from the JSPS Grantin-Aid for Scientific Research (B) through Project No. 26287061 and for Young Scientists (B) through Project No. 15K17675. This work was also supported by MEXT of Japan (Innovative Area &#34;Topological Materials Science,&#34; Grant No. 16H00979) and JSPS KAKENHI (Grant No. 16H02209)

1. Sample Mount and Installation
<ol>
	<li>Cut single-crystal samples of Bi2Se313 in an approximate size of 1 &#215; 1 &#215; 0.5 mm3 and use sliver-based epoxy to glue the sample to the sample holder.</li>
	<li>Paste the scotch tape on the sample surface. 
	NOTE: The scotch tape is used to cleave the sample in ultrahigh vacuum (UHV) chamber to obtain an atomically clean surface.</li>
	<li>Install the sample into the sample magazine in the load lock, and sta...

<ol>
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Instrum. 87 (5), 053111 (2016).</li><li>Shimojima, T., Okazaki, K., Shin, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Low-temperature+and+high-energy-resolution+laser+photoemission+spectroscopy.">Low-temperature and high-energy-resolution laser photoemission spectroscopy.</a> J. Phys. Soc. Jpn. 84 (7), 072001 (2015).</li><li>Augustine, S., Mathai, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Growth,+morphology,+and+microindentation+analysis+of+Bi2Se3,+Bi1.8In0.2Se3,+and+Bi2Se2.8Te0.2+single+crystals.">Growth, morphology, and microindentation analysis of Bi2Se3, Bi1.8In0.2Se3, and Bi2Se2.8Te0.2 single crystals.</a> Mater. Res. Bull. 36 (13), 2251-2261 (2001).</li><li>Hasan, M. Z., Kane, C. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Colloquium:+Topological+insulators.">Colloquium: Topological insulators.</a> Rev. Mod. Phys. 82 (4), 3045-3067 (2010).</li><li>Ando, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Topological+insulator+materials.">Topological insulator materials.</a> J. Phys. Soc. Jpn. 82 (10), 102011 (2013).</li><li>Zhang, H., Liu, C. -. X., Qi, X. -. L., Dai, X., Fang, Z., Zhang, S. -. 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C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spin-orbital+texture+in+topological+insulators.">Spin-orbital texture in topological insulators.</a> Phys. Rev. Lett. 111 (6), 066801 (2013).</li><li>Kuroda, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Coherent+control+over+three-dimensional+spin-polarization+for+the+spin-orbit+coupled+surface+state+of+Bi2Se3.">Coherent control over three-dimensional spin-polarization for the spin-orbit coupled surface state of Bi2Se3.</a> Phys. Rev. B. 94 (16), 165162 (2016).</li><li>Meier, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Interference+of+spin+states+in+photoemission+from+Sb/Ag(111)+surface+alloys.">Interference of spin states in photoemission from Sb/Ag(111) surface alloys.</a> J Phys-Condens Mat. 23 (7), 072207 (2011).</li><li>Dil, J. H., Meier, F., Osterwalder, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rashba-type+spin+splitting+and+spin+interference+of+the+Cu(111)+surface+state+at+room+temperature.">Rashba-type spin splitting and spin interference of the Cu(111) surface state at room temperature.</a> J Electron Spectrosc. 201, 42-46 (2015).</li><li>Yaji, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spin-dependent+quantum+interference+in+photoemission+process+from+spin-orbit+coupled+states.">Spin-dependent quantum interference in photoemission process from spin-orbit coupled states.</a> Nature Commun. 8, 14588 (2017).</li><li>Noguchi, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+mapping+of+spin+and+orbital+entangled+wave+functions+under+interband+spin-orbit+coupling+of+giant+Rashba+spin-split+surface+states.">Direct mapping of spin and orbital entangled wave functions under interband spin-orbit coupling of giant Rashba spin-split surface states.</a> Phys. Rev. B. 95 (6), 04111(R) (2017).</li></ol>

ARPES and SARPES techniques have been commonly used for studying electronic band structures through the band mapping and spin-detection1,2. In addition to these general advantages shown above, laser-SARPES based on orbital selection rule in optical dipole excitation can be employed as a novel technique for visualizing the spin-orbital coupling effect in the wavefunction and quantum spin interference. As demonstrated in Figure 9<stron...

Angle-resolved photoemission spectroscopy (ARPES) technique has developed into one of the most powerful tool to investigate quasiparticle band structures in solid states1. The most of attractive feature of ARPES is the capability for band mapping to characterize electronic states in energy and momentum space. Spin-resolved ARPES (SARPES), which is here equipped with spin-detectors, e.g. Mott detector2,3, further enables us to resolve the spin character of the observed band structures4. Since the Mott detector can measure the spin with two axes (x and z, or y and z), the combination of the two Mott detectors further allows one to obtain the spin orientation in three dimension4,5. For several decades, however, the SARPES experiments were suffered from their low efficiency (typically 1/10000 compared to that for spin-integrated ARPES measurement)3,4,5,6,7, which had limited the energy and angular-resolutions. Recently, the energy resolution of SARPES has been increased with a high-efficient spin detector based on exchange scattering, the so-called very-low-energy electron-diffraction (VLEED) detector7,8,9,10. With this detector, the data quality has been significantly improved and the data acquisition time has been shortened. Recently, SARPES has succeeded greatly to address spin-polarized electronic states and particularly spin-orbit coupling effect resulting in the spin texture of the surface bands7.
Here, we employ SARPES measurements with a polarization-variable vacuum ultraviolet laser light (laser-SARPES) and demonstrate the great advantages of this combined technique. Through the investigation on the spin-orbit coupled surface states in Bi2Se3, we present two capabilities of laser-SARPES. Firstly, due to the orbital selection rule of linearly polarized lasers in dipole transition regime, p- and s-polarized lights selectively excite a part of eigen-wavefunctions with different orbital symmetry. Such an orbital selective excitation is thereby available in SARPES, namely, orbital-selective SARPES. Secondly, three-dimensional (3D) spin-detection in SARPES shows the direction of the spin quantum axis and directly displays full information of the light-polarization dependence. In this protocol, we briefly describe a methodology to perform this state-of-the-art laser-SARPES technique to study the strong spin-orbit coupling effects.
Our laser-SARPES system is located at The Institute for Solid State Physics, The University of Tokyo11. The schematic drawing of our laser-SAPRES machine is shown in Figure 1. The polarization-variable 7-eV laser light12 illuminates the sample surface and the photoelectrons are emitted from the sample. The polarization of laser is automatically controlled by MgF2-based &#955;/2- and &#955;/4-waveplates to selectively use linear and circular polarizations. A hemispherical electron analyzer corrects the photoelectrons, and analyzes their kinetic energy (Ekin) and emission angle (&#952;x and &#952;y). The photoelectron intensities are mapped on the Ekin-&#952;x screen monitored by a CCD camera. This image is directly transformed into the energy band structure in reciprocal space.
For SARPES measurement, the photoelectrons with a specific emission angle and kinetic energy analyzed by the electron analyzer are&#160;guided to two VLEED-type spin detectors with a 90-degree photoelectron deflector and the photoelectron beams are focused onto two different targets of Fe(001)-p(1&#215;1) films terminated by oxygen. The photoelectrons reflected by the targets are detected in single channel detection by using a channeltron placed in each spin detector. The VLEED targets can be magnetized with Helmholtz-type electric coils which are arranged with orthogonal geometry with respect to each other. The magnetization direction is controlled by the bipolar condenser bank. The double VLEED spin detectors thereby enable us to analyze the spin-polarization vector of the photoelectron in three dimensions.

<table>
	<tbody>
		<tr>
			<td>DA30-L hemispherical analyzer</td>
			<td>ScientaOmicron</td>
			<td></td>
			<td>http://www.scientaomicron.com/en/products/353/1170</td>
		</tr>
		<tr>
			<td>Silver-based epoxy</td>
			<td>Epoxy Technology</td>
			<td>H20E</td>
			<td></td>
		</tr>
		<tr>
			<td>Sctoch tape</td>
			<td>3M</td>
			<td>801-1-18C</td>
			<td></td>
		</tr>
		<tr>
			<td>UHV valve</td>
			<td>VAT</td>
			<td>01034-KE01</td>
			<td></td>
		</tr>
		<tr>
			<td>linear/rotary feedthrough</td>
			<td>Ferrovac</td>
			<td>MD40</td>
			<td></td>
		</tr>
		<tr>
			<td>transfer rod</td>
			<td>UHV design</td>
			<td>PP series</td>
			<td></td>
		</tr>
		<tr>
			<td>wobble stick</td>
			<td>Ferrovac</td>
			<td>WM40</td>
			<td></td>
		</tr>
		<tr>
			<td>Paladin compact 355</td>
			<td>Coherent</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>half waveplate</td>
			<td>Kogakugiken</td>
			<td></td>
			<td>order made</td>
		</tr>
		<tr>
			<td>Bipolar condenser bank</td>
			<td>Tsuji electronics</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

experimental methods for spin- and angle-resolved photoemission spectroscopy combined with polarization-variable laser

The goal of this protocol is to present how to perform spin- and angle-resolved photoemission spectroscopy combined with polarization-variable 7-eV laser (laser-SARPES), and demonstrate a power of this technique for studying solid state physics. Laser-SARPES achieves two great capabilities. Firstly, by examining orbital selection rule of linearly polarized lasers, orbital selective excitation can be carried out in SAPRES experiment. Secondly, the technique can show full information of a variation of the spin quantum axis as a function of the light polarization. To demonstrate the power of the collaboration of these capabilities in laser-SARPES, we apply this technique for the investigations of spin-orbit coupled surface states of Bi2Se3. This technique affords to decompose spin and orbital components from the spin-orbit coupled wavefunctions. Moreover, as a representative advantage of using the direct spin detection collaborated with the polarization-variable laser, the technique unambiguously visualizes the light polarization dependence of the spin quantum axis in three-dimension. Laser-SARPES dramatically increases a capability of photoemission technique.

Before starting SARPES experiments, k positions need to be accurately determined for taking spin-resolved spectrum by using high statistic spin-integrated ARPES results with high energy- and angular-resolutions (protocol 5.1-5.5). This is demonstrated in Figure 7 where the ARPES results for a Bi2Se3 single crystal are presented. This material is known as a prototypical topological insulator with a spin-polarized surface states<s...

Watch this Scientific Journal Video about Experimental Methods for Spin- and Angle-Resolved Photoemission Spectroscopy Combined with Polarization-Variable Laser at JoVE.com

Experimental Methods for Spin- and Angle-Resolved Photoemission Spectroscopy Combined with Polarization-Variable Laser

Here, we combine polarization-variable 7-eV laser with spin- and angle-resolved photoemission technique to visualize the spin-orbital coupling effect in solid states.

The goal of this protocol is to present how to perform spin- and angle-resolved photoemission spectroscopy combined with polarization-variable 7-eV laser ...

Spin and angle-resolved photoemission spectroscopy, the so called SARPES, is a powerful technique to investigate solid state electron structures including their spin information. In our method, we combined this technique with the polarization-variable 7-eV laser which enabled us to determine the electron spin structures, very precisely. SARPES was generally performed with synchrotron radiation or noble-gas-discharge lamp.Compared to this method, using a polarization-variable 7-eV laser is a large improvement. The polarization with the laser beam can be easily controlled using the waveplate. By using this property, we can completely trace the behavior of electron spin, which is sensitive to the light polarization.The photoelectron spin reaction depends on the light polarization. And that shows many physical meanings such as, spin-orbit coupling and spin-interference. For this reason, our technique can be gradually used for studying condensed matter physics.To begin, cut single crystal samples of bismuth selenide, and use a silver-based epoxy to glue them to a sample holder. Once the epoxy has cured, place a piece of scotch tape onto the surface of the samples. Then, transfer the sample magazine into the ultrahigh vacuum load lock and start the pump until the pressure of the load lock is lower than one times ten to the negative five pascal.Next, open the ultrahigh vacuum valve between the load lock and the preparation chamber, and move the sample magazine from the load lock to the preparation chamber, using the feedthrough mechanism attached to the load lock chamber. Now, use a transfer rod to pick up the sample from the sample magazine and then place the sample magazine back into the load lock and close the ultrahigh vacuum valve. Wait until the pressure of the preparation chamber is below five times ten to the negative seven pascal, and then peel the scotch tape from the surface of the bismuth selenide using the wobble stick in the preparation chamber.This will cleave the sample, producing an atomically clean surface. Transfer the sample to the UHV measurement chamber. There, use a screwdriver to fix the sample to the main gonio stage.Then, move the stage to the measurement position and use a micrometer stage to precisely position the sample onto the focus of the spectrometer. Next, turn on the neodymium-doped yttrium orthovanadate laser, and allow it to warm up for ten to 15 minutes. Open the laser beam shutter and make sure that the laser passes through a potassium beryllium fluoroborate crystal to generate a second harmonic wave of 177 nanometers.Optimize the power of the seven electron volt laser, by changing the power of the 355 nanometer laser using a variable attenuator. First, open the analyzer control software. In the sequence menu, select setup and then choose ARPES configuration, and ARPES mapping, in the list, to perform Fermi surface mapping with the photoelectron deflector.Click on edit, and configure the Fermi surface mapping so that the emission angle ranges from 12 degrees to 12 degrees with the step size of 0.5 degrees and 49 for the number of steps. Once set, go back into the sequence menu and select run. The hemispherical analyzer has a electron deflector which enable us to map the Fermi surface without rotating the sample.Manually change the machine setup for spin and angle-resolved photoemission spectroscopy measurement. This includes changing both the analyzer entrance slit and the aperture size. In the software, select setup and choose spin configuration, and normal from the list of options and click okay.Next, select DA30 on the menu bar and navigate to control theta. This will open the setting panel for the DA30 angle configuration. In the panel, choose the emission angle for theta x to be negative six degrees, and for theta y to be zero degrees.Then, use a command prompt to apply the magnetic field by controlling the bipolar condenser bank. This will magnetize the v-lead's target in the position direction along the x, y, or z-axis. Once set, click on run, to take the intensity spectrum.Next, apply a magnetic field to magnetize the v-lead target in the negative direction along the axis and run the scan to take another intensity spectrum. When finished, calculate the spin-polarization and the spin-resolved spectra. Using the command prompt, power the step motor to precisely change the angle of the half waveplate in order to tune the light polarization of the seven electron volt laser.Then, take the spin-resolved spectra for each of the x, y and z-axes. Scan the spin-resolved spectra as a function of the light polarization, while varying the half waveplate angle from zero degrees to 102 degrees with the step size of three degrees. Bismuth selenide, is a prototypical topological insulator with spin-polarized surface states.The obtained Fermi surface map of the surface state in bismuth selenide, shows an almost circled shape. Then, perform band mapping along the high symmetry line. It shows the X-shaped energy dispersion, the so called Dirac cone.From these results, one can now choose the specific emission angle for the SARPES experiment. When taking a theta x equals negative six degrees and theta y equals zero degrees across negative kF of the surface band, the energy distribution curves for different magnetization directions, peak in intensity near the Fermi energy. The intensities in the two spectra are different near the peak.From the difference between their intensities, one can calculate the spin-polarization as a function of the energy and the spin-resolved spectra. Apparently, the up spin spectrum has a peak structure but the down spin spectra is relatively flat. This reflects the almost 100%spin-polarization.When taking the same measurement for a different axis and with different light polarization, the spin direction of the electrons and its polarization dependence are clearly visible. For p-polarization, the data clearly shows the positively 100%spin-polarization only along y and there is no spin-polarization along the x and z directions. Interestingly, when one switches the light polarization from p to s, the sign of spin-polarization also switches to the negatively 100%for s-polarization without x and z spin component.As a consequence of full spin information, the electron's spin rotation is completely unraveled in three dimension. We have demonstrated the power of the SARPES, with the polarization-variable laser, for starting spin-polarized state of bismuth selenide. This allows us to directly visualize electron spin and its variations, as a function of light polarizations.This technique can be easily applied for many other physical systems. Our experiment show that, even small changes in the light fields really can remodify the photoelectron spin in three dimension. How the sample is illuminated is very important.One must take care of the experimental geometry to correctly understand the data.

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