Hydrogen-induced modification of the local charge density in MnSb$_2$Te$_4$

Technical Summary: Hydrogen-induced modification of the local charge density in MnSb2Te4

Problem Statement
Intrinsic magnetic topological insulators (MTIs), specifically the MnSb2Te4 (MST) system, exhibit complex electronic and magnetic properties highly sensitive to intrinsic defects, particularly Mn-Sb antisites. While hydrogenation has been established as a reversible method to tune the magnetic order and carrier density in MST, the microscopic mechanism by which hydrogen interacts with the lattice and modifies the electronic band structure has remained unobserved. Previous studies indicated that hydrogen intake reduces p-type carrier density and switches magnetic order from ferromagnetic (FM) to antiferromagnetic (AFM), but the specific spectroscopic changes in occupied and unoccupied states, and the role of stoichiometry in these changes, were not directly characterized.

Methodology
The authors investigated the effects of ionic hydrogen (H+) intake on FM (Mn0.93Sb2.07Te4) and AFM (Mn0.77Sb2.25Te4) MnSb2Te4 single crystals. Hydrogenation was performed via immersion in aqueous HCl for 46 and 93 hours, respectively. The study employed a comprehensive suite of photoemission techniques:

• Angle-Resolved Photoemission Spectroscopy (ARPES): Conducted at the CASSIOPEE beamline (SOLEIL) at 15–20 K to map the low-energy band structure and valence band dispersion.
• Resonant Photoemission Spectroscopy (resPES): Performed near the Mn L3 absorption edge to isolate Mn 3d orbital contributions and determine orbital character.
• Time-Resolved ARPES (trARPES): Executed at 130 K using a 1.55 eV pump and a 6.2 eV probe to investigate unoccupied states, carrier dynamics, and relaxation times.
• X-ray Photoelectron Spectroscopy (XPS): Utilized core-level spectroscopy (Mn 2p, Te 3d, Sb 3d, Te 4d, Sb 4d) to probe chemical environments and local bonding changes.

Key Results

1. Spectral Weight Redistribution: Hydrogenation does not induce a rigid shift of the bands toward lower binding energies (as seen in alkali metal doping), indicating that H+ is distributed into the bulk rather than accumulating at the surface. Instead, the process causes a local redistribution of spectral weight. Specifically, spectral weight transfers from deep binding energies (approx. 8.5 eV) toward bands near the Fermi level and the Mn 3d band, suggesting a modification of the hybridization between Mn 3d and Te/Sb p orbitals.
2. Orbital Specificity: ResPES data reveals that hydrogen primarily affects delocalized states involved in Te-Mn-Te hybridization, while the localized Mn 3d flat band at -3.2 eV remains largely unaffected. This supports the model that H+ binds to charged defects (acceptor-like sites) rather than occupying interstitial sites freely.
3. Stoichiometry-Dependent Response: The response of the conduction band to hydrogenation is strikingly different between FM and AFM samples.
   • In the FM sample, hydrogenation reduces the spectral weight at the conduction band minimum (E2) by approximately half.
   • In the AFM sample, the effect is reversed and more pronounced, showing a threefold increase in spectral weight at E2.
   • The authors attribute this divergence to the different concentrations of intrinsic defects (Mn-Sb antisites) in samples with varying Mn content.
4. Carrier Dynamics: Time-resolved measurements show that the band gap decreases slightly (by ~30 meV) in both samples upon hydrogenation. Recombination dynamics remain unchanged in the FM sample but slow down by approximately 1 ps in the AFM sample. This slowing is interpreted as a reduction in elastic scattering centers due to the passivation of intrinsic defects by hydrogen.
5. Core-Level Evidence: XPS confirms that hydrogen modifies the local chemical environment. The Mn 2p spectra, which consist of spin-orbit components and satellite peaks, show main components shifting to lower binding energy, indicating increased electron density around the Mn sites. The Te 3d features narrow, suggesting site-dependent modification of the Mn-Te bonding environment. The intensity of a component attributed to SbMn antisites decreases without an energy shift, implying passivation of these specific defects.

Significance and Claims
The paper claims to provide the first spectroscopic evidence of how hydrogen interacts with the intrinsic defects of a magnetic quantum material. The authors establish that hydrogen acts as a selective dopant that passivates charged defects (likely Mn or Sb vacancies and antisites) rather than acting as a simple bulk electron donor. This interaction locally reshapes the electronic structure by modifying the Sb-Te-Mn hybridization network.

The study highlights that the electronic response to hydrogenation is non-trivial and heavily dependent on the material's stoichiometry and defect landscape. By demonstrating that hydrogen can reversibly tune the spectral weight and carrier dynamics in MnSb2Te4, the work positions hydrogenation as a viable tool for engineering the electronic and magnetic properties of layered 2D magnetic materials, offering a pathway to control quantum phases through defect management.