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

Summary

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.

Abstract

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.

Introduction

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 detector^2,3, 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

Protocol

1. Sample Mount and Installation

1. Cut single-crystal samples of Bi₂Se₃¹³ in an approximate size of 1 × 1 × 0.5 mm³ and use sliver-based epoxy to glue the sample to the sample holder.
2. 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.
3. Install the sample into the sample magazine in the load lock, and start the pump until the pressure of the load lock is lower than 1×10^-5 Pa.

2. Sample Cleaving

....

Results

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

Discussion

ARPES and SARPES techniques have been commonly used for studying electronic band structures through the band mapping and spin-detection^1,2. 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

Materials

List of materials used in this article

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