Stanford
Shen Laboratory

Spin ARPES

Electron spin plays a significant role in shaping the properties of quantum materials, yet its specific roles are often poorly understood and cannot be resolved by conventional ARPES, which is blind to the spin degree-of-freedom. How do we observe the spin texture of electronic bands in momentum space, and how does this further our understanding of solids beyond what can be learned from conventional ARPES? We answer these questions with a high-efficiency, home-built spin-resolved ARPES setup capable of imaging the spin and orbital textures of electronic band structures. By densely sampling the band structure, we uncover physics that is invisible to spin-integrated probes: subtle reversals of spin polarization, entangled spin-orbital textures, and the microscopic structure of surface-state wave functions.

The central premise of our work is that the spin texture of electronic bands is a direct window into the spin-orbit interactions that govern quantum materials-- and that resolving it complements or even overturns the idealized pictures inferred from spin-integrated data alone. We pursue this across several material families in which spin-orbit coupling and broken symmetry conspire to produce nontrivial behavior.

Topological and Rashba systems

Materials with strong spin-orbit coupling can host spin-momentum locked states that spin-resolved ARPES is uniquely positioned to investigate. In the noncentrosymmetric semiconductor BiTeCl, for example, we found that the idealized Rashba model breaks down precisely where it matters most: the in-plane spin polarization of the outermost surface band reverses sign near the Fermi level, a consequence of higher-order spin-orbit terms tied to the local crystal symmetry. Because this reversal occurs where the bands are otherwise featureless, it is invisible to conventional ARPES, providing a vivid demonstration of why spin resolution is essential.

Spin polarization of the noncentrosymmetric semiconductor BiTeCl. Red and blue denote opposite in-plane spin polarization, revealing the Rashba-split surface bands [Qu 2023]
Spin polarization of the noncentrosymmetric semiconductor BiTeCl. Red and blue denote opposite in-plane spin polarization, revealing the Rashba-split surface bands [Qu 2023].

Wavefunction reconstruction

Photoemission probes spin through matrix elements that also encode the orbital angular momentum of the underlying states, so a complete picture requires interpreting both together. By tracking spin polarization and orbital character as a function of momentum, we can reconstruct the surface-state wave function itself. In the magnetic topological insulator MnBi₂Te₄, this approach shed light on long-standing ambiguities in band assignments and revealed a new principle: the orbital composition of the wave function directly controls the size of the magnetic gap, with in-plane p orbitals acting to reduce it. This offers fresh insight into the much-debated gapless Dirac cone in these materials, and connects naturally to broader efforts to extract quantum-geometric quantities such as Berry curvature from photoemission.

Coupled spin-orbital texture of the magnetic topological insulator MnBi4Te7 [Han 2025].

Interpretation of matrix elements

Our work also involves a careful investigation of the photoemission matrix elements themselves, including those responsible for circular dichroism (CD). CD is frequently used as a proxy for orbital angular momentum and as an "easy" substitute for the experimentally demanding technique of spin-resolved ARPES. Our work has shown that the interpretation of CD requires great care, as it carries substantial contributions from the photoemission process that are unrelated to the intrinsic orbital angular momentum of the bands. Disentangling these effects sharpens our ability to extract intrinsic material physics from spin- and orbital-resolved data.

Instrumentation

We have a hands-on approach to instrumentation development. Our spin-ARPES system is completely home-built and based on Time-of-Flight (TOF) spectroscopy combined with exchange-scattering polarimetry for spin resolution.

Pulsed light photoemits electrons from the sample; photoelectrons travel down the time-of-flight column and are directed either to the spin-integrated detector or to the exchange-scattering spin detector.

A key advance is the implementation of an electrostatic deflector mode, which maps momentum space without mechanically rotating the sample. We validated this mode on reference samples and quantitatively benchmarked it against trajectory simulations in SIMION software.

Photoelectrons from the biased sample pass through the entrance aperture, are steered by the orthogonal dx and dy deflector electrodes, and are selected by the exit aperture — enabling momentum mapping without sample rotation [Han 2023].

Publications

X. Han, J. Qu, H. Tan, Z. Tao, N. M. Meyer, P. S. Kirchmann, Y. Guo, B. Yan, Z.-X. Shen, and J. A. Sobota. Reconstructing the Wave Function of Magnetic Topological Insulators MnBi₂Te₄ and MnBi₄Te₇ Using Spin-Resolved Photoemission. Physical Review X 15, 031022 (2025)

I. Sidilkover, Y. Yen, S. W. D'Souza, J. Schusser, A. Pulkkinen, C. R. Rotundu, M. Hashimoto, D. Liu, Z.-X. Shen, J. Minár, M. Schüler, H. Soifer, and J. A. Sobota. Reexamining circular dichroism in photoemission from a topological insulator. Physical Review Research 7, 033027 (2025)

J. Qu, X. Han, S. Sakamoto, C. J. Jia, J. Liu, H. Li, D. Guan, Y.-J. Zeng, M. Schüler, P. S. Kirchmann, B. Moritz, Z. Hussain, T. P. Devereaux, Z.-X. Shen, and J. A. Sobota. Reversal of spin-polarization near the Fermi level of the Rashba semiconductor BiTeCl. npj Quantum Materials 8, 13 (2023)

X. Han, J. Qu, S. Sakamoto, D. Liu, D. Guan, J. Liu, H. Li, C. R. Rotundu, N. Andresen, C. Jozwiak, Z. Hussain, Z.-X. Shen, and J. A. Sobota. Development of deflector mode for spin-resolved time-of-flight photoemission spectroscopy. Review of Scientific Instruments 94, 103906 (2023)