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.
