Electron spins play a significant role in shaping the properties in quantum materials. So how do we observe the spin texture of electronic bands in momentum space? And how does this aid our understanding in solids in addition to conventional ARPES? We are actively working to answer these questions by developing a high resolution spin-resolved ARPES setup that has the ability to visualize the spin texture in electronic band structures.
It is widely acknowledged that electron spin interactions play key roles in the electronic structure of quantum materials. However, the specific role it plays is not always well understood, and cannot be approached by using conventional ARPES.
• Topological materials
Topological materials with strong spin-orbit coupling host topologically nontrivial surface states that are characteristic in their spin-momentum locking features. There is a variety of topological material families, with interesting materials in each family. Examples include topological insulator Bi2Te3, Kondo insulator SmB6, crystalline insulator PbSnTe, magnetic Weyl semimetal Co3Sn2S2, candidate topological superconductor FeTe1-xSex and so on. The topological property of many of these materials are still under debate, hence the spin texture in the surface state of such materials would serve as evidence for revealing the role of spin-orbit coupling and determining whether the materials are actually topological.
• Noncentrosymmetric materials
Broken inversion symmetry, together with strong spin-orbit coupling (SOC), can give rise to spin-splitting effects, as commonly seen on the surface of materials like Antimony. Noncentrosymmetric materials with strong spin-orbit coupling, such as BiTeX (X=Cl,I,Br) are less common and are associated with novel Rashba spin-splitting in the bulk. Rashba spin-splitting is closely related to a wide variety of novel physical phenomena, such as anisotropic magnetoresistance, Majorana fermions, FFLO states in superconductors, and even spintronics applications in the absence of magnetic field. Thus the study of the spin textures in these materials is crucial.
• Strongly correlated materials
In strongly correlated materials, the behavior of their electrons cannot be taken into account separately. Moreover, their spins can interact with charge, orbital, and lattice degrees-of-freedom to give rise to a variety of phases. Examples include high Tc superconductors in the copper and iron-based families, as well as manganites that host colossal magnetoresistance. These phenomena cannot be fully understood without a complete picture of the spin degree-of-freedom.
• Low-dimension magnetic materials
Due to the reduced dimensionality, low dimensional magnets usually exhibit weak magnetic ordering compared to conventional bulk magnets. But magnetic materials with dimension smaller than 3 have been showing unconventional magnetic behaviors. One example is the study of Cr-based chalcogenides, where intrinsic ferromagnetic order has been found. The spin-resolved quantum phase transition from paramagnetic to ferromagnetic in 2D magnetic materials is therefore highly desired for understanding the exchange interactions in lower dimensions that give rise to the magnetic ordering.
In addition to equilibrium studies of quantum materials, the need for advanced spintronics applications also requires us to understand and manipulate the spin interactions in a variety of materials in the time domain.
Our spin ARPES system now uses 6eV laser as excitation and is working on integrating an 11eV laser as well. The system is completely home-built and is based on the Time of Flight (TOF) technique. Unlike conventional ARPES system which uses a hemisphere deflector for obtaining energy distributions, the TOF system is based on measuring the photoelectrons' velocity distributions. TOF technique gives us higher sensitivity and efficiency compared to a conventional Mott polarimeter attached to a hemisphere deflector
The basic schematic is very straightforward. The angular resolution is provided by the system's finite angular acceptance of the photoelectrons from the sample. And the energy resolution is given by the time between the pulse excitation and detection by the Multichannel Plate t. Thus the relation between kinetic energy and the flight time t is given by:
Our spin-resolved capability is built based on electron exchange scattering polarimetry. The system is based on the theory that the scattering cross-section is spin-dependent. Therefore, we scatter the emitted photoelectron from a polarized magnetic scattering target and collect intensity data termed Iup and Idown for up and down polarized magnetic target independently. The scattered intensity is proportional to the cross-section. The resulting polarization P of the photoelectrons emitted is thus given by:
Our spin-TOF follows the approach developed in collaboration with Chris Jozwiak, Zahid Hussain, and Alessandra Lanzara at Lawrence Berkeley National Lab .
The figure shows the spin-polarized surface states of Bi2Se3. Red and blue colors indicate opposite in-plane spin polarization. The spin-momentum locking feature of the topological surface state is clearly shown, where opposite momenta possess opposite spin polarizations.
 C. Jozwiak et al. Spin-polarized surface resonances accompanying topological surface state formation. Nat. Commun. 7,13143 (2016)