🔬 materials science

Tuning Dirac-Rashba and Double Dirac Cone Surface States of Topological Crystalline Insulator Pb $_{1-x}$ Sn $_{x}$ Se by Transition Metal Adsorbate

This study demonstrates that depositing submonolayer transition metals on Pb $_{1-x}$ Sn $_{x}$ Se surfaces allows for the systematic tuning of Dirac and Rashba surface states, where polar (111) surfaces exhibit controllable Rashba splitting up to 3.5 eV·Å due to symmetry breaking and doping, while nonpolar (001) surfaces preserve inversion symmetry and instead show dephasing of double Dirac cones.

Original authors: Bartłomiej Turowski, Wojciech Brzezicki, Ondřej Caha, Rafał Rudniewski, Natalia Olszowska, Jacek Kołodziej, Marta Aleszkiewicz, Tomasz Wojciechowski, Tomasz Wojtowicz, Timo Hyart, Gunther Springholz

Published 2026-08-27

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CC BY 4.0

Original authors: Bartłomiej Turowski, Wojciech Brzezicki, Ondřej Caha, Rafał Rudniewski, Natalia Olszowska, Jacek Kołodziej, Marta Aleszkiewicz, Tomasz Wojciechowski, Tomasz Wojtowicz, Timo Hyart, Gunther Springholz, Valentine V. Volobuev

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where electricity flows not just as a current of charge, but as a stream of spinning particles, each carrying a tiny magnetic compass needle. In this realm, the direction of the spin is locked to the direction of motion, a phenomenon that could revolutionize how we store and process information. This is the promise of a special class of materials called topological insulators. Inside these materials, electrons behave like ordinary insulators, refusing to flow, but on their very surface, they conduct electricity with a unique, protected freedom. These surface electrons are robust; they resist scattering and maintain their spin orientation, making them ideal candidates for the next generation of ultra-efficient electronic devices. However, to harness this potential, scientists need to understand how these delicate surface states react when they meet other materials, particularly magnetic metals, which are often used to control electron spin.

A team of researchers has now taken a closer look at this interaction using a specific type of topological insulator made from a mixture of lead, tin, and selenium. They focused on two different crystal faces of this material: one that is polar, meaning it has a distinct electrical imbalance across its layers, and another that is non-polar and more symmetrical. By depositing tiny, sub-atomic layers of magnetic metals like iron and manganese onto these surfaces, they watched how the electronic landscape changed in real time. Using a powerful technique that shoots light at the material to knock electrons loose and map their energy and momentum, the scientists were able to visualize exactly how the surface electrons responded to the new magnetic neighbors.

The results revealed a striking difference depending on which face of the material was exposed. On the polar surface, the arrival of the magnetic metal atoms did something remarkable. It caused the surface electrons to split into two distinct paths, a phenomenon known as Rashba splitting, where the electrons' spin directions separate based on their momentum. Even more surprisingly, the original topological surface states did not disappear or get blocked; instead, they coexisted with these new split states. The researchers found that by simply adjusting how much metal they deposited, they could tune the strength of this splitting over a vast range, reaching values as high as 3.5 eV·Å. This giant splitting is significant because it suggests a powerful way to manipulate the flow of spin and charge, a key requirement for advanced spintronic devices. The team determined that this effect was not just a simple result of the electric field created by the metal atoms, but a complex interplay involving the atoms' own internal spin-orbit coupling and their orbital angular momentum, which essentially describes how the electrons swirl around the atomic nucleus.

In contrast, when the same metals were deposited on the non-polar surface, the outcome was entirely different. Because this surface maintains a high degree of symmetry, the magnetic atoms could not break the necessary conditions to create the Rashba splitting. Instead of splitting, the electrons on this surface simply moved closer together in momentum space. The two distinct cones of electron states, which usually sit apart, began to merge slightly as the metal coverage increased. This showed that the symmetry of the crystal face acts as a gatekeeper, deciding whether the dramatic splitting effect can occur or if the electrons will simply shift their positions.

Throughout these experiments, the topological surface states remained intact and gapless, meaning they did not develop the energy gaps that magnetic materials often create in other systems. This resilience is crucial, as it proves that the unique properties of these materials can survive the introduction of magnetic elements. The study confirms that by carefully choosing the crystal orientation and the type of metal adsorbate, scientists can engineer the surface of these materials to either generate massive spin-splitting effects or to subtly tune the separation of electron states. These findings provide a versatile platform for future devices that rely on the precise control of electron spin, offering a clear path toward more efficient and powerful quantum technologies.

Technical Summary: Tuning Dirac-Rashba and Double Dirac Cone Surface States of Topological Crystalline Insulator Pb $_{1-x}$ Sn $_x$ Se by Transition Metal Adsorbate

Problem Statement
Topological insulators (TIs) and topological crystalline insulators (TCIs) offer unique electronic properties, such as helical spin textures and spin-momentum locking, which are critical for spin-charge conversion and spin-orbitronic devices. However, the electronic band structure at the interface between a TCI and a magnetic metal (TI/MM) remains poorly understood. Specifically, the mechanisms governing the interconversion efficiency and the response of topological surface states (TSS) to uncompensated magnetic moments from adjacent materials require further investigation. While previous studies have explored TSS in isolation or with non-magnetic dopants, the systematic mapping of how submonolayer transition metal (TM) adsorbates modify the surface electronic structure of TCIs—particularly regarding the coexistence of Dirac and Rashba states—has not been fully elucidated. Furthermore, the potential for inducing magnetic gaps or quantum anomalous Hall effects via surface magnetism in TCIs remains experimentally elusive.

Methodology
The study employs a combined experimental and theoretical approach using the TCI material Pb $_{1-x}$ Sn $_x$ Se.

Sample Preparation: High-quality 1- $\mu$ m-thick Pb $_{1-x}$ Sn $_x$ Se epilayers with Sn concentrations ( $x_{Sn}$ ) of 0.25 and 0.30 were grown via molecular beam epitaxy (MBE) on (111) BaF $_2$ and (001) KCl substrates. Moderate Bi doping was used to tune the Fermi level. Samples were either capped with amorphous Se for transport or transferred under ultra-high vacuum (UHV) to preserve surface integrity.
Adsorbate Deposition: Submonolayer coverages (0.05–0.6 ML) of transition metals Fe and Mn were deposited in-situ on the clean surfaces at room temperature.
Experimental Characterization: Angle-resolved photoemission spectroscopy (ARPES) was performed at the SOLARIS synchrotron (Kraków) using photon energies of 15–90 eV. Measurements were conducted at 80 K to ensure the samples were in the topological phase. The study utilized 2D curvature analysis to resolve band dispersions and core-level spectroscopy to verify surface composition and chemical shifts.
Theoretical Modeling: A tight-binding (TB) model based on the SnTe family of TCIs was developed. The model incorporated a surface potential generated by charge filling of the TSS, solved via the Poisson equation with Thomas-Fermi screening. This approach allowed for the simulation of the electrostatic effects of TM adsorbates on the surface band structure without assuming magnetic ordering.

Key Results
The study reveals distinct, orientation-dependent modifications to the surface electronic structure induced by TM adsorbates:

Polar (111) Surface: Coexistence of Dirac and Rashba States
- Observation: On the (111) surface, the deposition of Fe and Mn induces a strong Rashba-type interaction. The topological surface states (TSS) coexist with spin-split Rashba surface states (RSS), forming a "Dirac-Rashba" spectrum.
- Rashba Parameter Tuning: The Rashba parameter ( $\alpha_R$ ) is tunable over a wide range from 0 to 3.5 eV $\cdot$ \AA, depending on the TM type and coverage. $\alpha_R$ increases with coverage and saturates at 0.2–0.4 ML.
- Fermi Level Behavior: In p-type samples, Fe deposition causes a pronounced upward shift of the Fermi level ( $E_F$ ). In n-type samples, $E_F$ remains relatively stable, yet $\alpha_R$ continues to increase, suggesting that the giant Rashba splitting is not solely driven by electrostatic band bending.
- Mechanism: The coexistence is attributed to the filling of TSS by charge from the TM adsorbates. Theoretical calculations confirm that TSS and RSS can coexist without hybridization if the surface potential minimum is sufficiently distant from the surface. The giant splitting is enhanced by the interplay of atomic spin-orbit coupling (SOC) and orbital angular momentum (OAM), supported by strong circular dichroism observed in ARPES.
- Magnetic Gap: Despite the presence of magnetic moments, no magnetic gap opens at the Dirac point within the experimental temperature range (8–80 K), indicating the robustness of the TSS against the specific magnetic proximity effects induced here.
Non-Polar (001) Surface: Suppression of Rashba Splitting
- Observation: On the (001) surface, which exhibits a double Dirac cone (DC) structure, TM deposition does not induce Rashba splitting. This is because the (001) surface preserves inversion symmetry (due to alternating cation/anion layers), which suppresses the Rashba effect.
- Momentum-Space Contraction: Instead of splitting, the adsorbates induce a reduction in the momentum-space separation between the two Dirac cones. This is attributed to surface charge imbalance and the dephasing of the wavefunctions associated with the double Dirac cones.
- Fermi Level Shift: Similar to the (111) surface, an upward shift in $E_F$ is observed, but the primary structural change is the contraction of the double Dirac cone separation rather than the emergence of new spin-split bands.

Significance and Claims
The paper claims that these findings establish the transition metal/TCI interface as a versatile platform for engineering surface electronic states.

Dirac-Rashba Hybridization: The work demonstrates that giant Rashba splittings can be achieved and tuned in TCIs, creating a robust Dirac-Rashba spectrum where the inner Rashba branches remain pinned to the TSS. This configuration is highlighted as particularly attractive for spintronic applications due to its potential for enhancing and tuning spin-charge interconversion efficiency.
Symmetry Dependence: The study underscores the critical role of crystal symmetry, showing that while the polar (111) surface allows for the coexistence of TSS and RSS, the non-polar (001) surface preserves inversion symmetry, leading to different phenomena (dephasing and momentum contraction) rather than Rashba splitting.
Robustness of TSS: The persistence of gapless TSS in the presence of magnetic adsorbates challenges the expectation of immediate magnetic gap opening, suggesting that precise tuning of the Fermi level to the Dirac point or higher Curie temperatures may be required to induce 2D surface ferromagnetism in these systems.
Mechanism of Giant Splitting: The results suggest that the giant Rashba effect in these systems arises from a combination of surface potential gradients, strong SOC, and OAM contributions, rather than electrostatic band bending alone.

The authors conclude that this model system provides a foundation for investigating interfacial spin-orbit coupling and magnetic proximity effects, offering a pathway for the development of advanced spin-orbitronic devices.

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