Phase-selective orbital-charge conversion in $\mathrm{MoTe_2}$

Technical Summary: Phase-selective orbital-charge conversion in MoTe₂

Problem and Motivation
Two-dimensional transition metal dichalcogenides (TMDs) are recognized for their potential in spin-orbitronics due to strong spin-orbit coupling and broken inversion symmetry. However, their orbital transport properties remain largely unexplored. MoTe₂ is a specific system of interest due to its polymorphism, existing in semiconducting (2H), metallic (1T′), and topological (Td) phases. While previous studies have demonstrated controllable phase transitions via external perturbations, the fundamental relationship between structural phase control and spin/orbital transport in scalable, wafer-compatible films remains unclear. The authors investigate whether the structural phase of MoTe₂, specifically the transition between 2H and 1T′, governs its ability to convert angular momentum (spin or orbital) into charge currents.

Methodology
The study employs DC magnetron sputtering to grow MoTe₂ thin films on Yttrium Iron Garnet (YIG) substrates, capped with Platinum (Pt) to prevent oxidation. This approach is chosen for its potential for wafer-scale fabrication. The investigation utilizes a multi-faceted characterization strategy:

• Structural and Vibrational Analysis: Grazing-incidence X-ray diffraction (GIXRD) and Raman spectroscopy are used to identify crystalline phases (2H vs. 1T′) across varying film thicknesses.
• Electronic Structure: Scanning tunneling microscopy and spectroscopy (STM/STS) probe the local density of states (DOS) and band gaps. These experimental findings are corroborated by Density Functional Theory (DFT) calculations.
• Transport Measurements: Spin-pumping driven by ferromagnetic resonance (SP-FMR) is utilized to detect angular-momentum-to-charge conversion. The experimental heterostructures (YIG/Pt/MoTe₂/Pt) allow for the isolation of contributions from the inverse spin Hall effect (ISHE), inverse spin Rashba-Edelstein effect (ISREE), and inverse orbital Rashba-Edelstein effect (IOREE).
• Theoretical Modeling: First-principles linear-response calculations based on the Kubo-Bastin formalism are performed to compute spin and orbital Rashba-Edelstein susceptibilities for semi-infinite 2H and 1T′ MoTe₂ slabs.

Key Results

1. Thickness-Driven Phase Transition: The study identifies a critical thickness crossover near 4.5 nm.
   • Ultrathin Films (< 4.5 nm): Stabilize in the metallic 1T′ phase. Raman spectra show characteristic 1T′ modes ($B_g$, $A_u$, $A_g$), and STS reveals a nearly gapless, semimetallic electronic structure.
   • Thicker Films (> 4.5 nm): Stabilize in the semiconducting 2H phase. Raman spectra exhibit 2H modes ($E_{1g}$, $E_{2g}^1$), and STS confirms a finite band gap of approximately 1.03 eV.
2. Orbital-to-Charge Conversion: SP-FMR measurements reveal that while neither phase exhibits a measurable spin-to-charge conversion (ISHE/ISREE), the metallic 1T′ phase generates a significant additional transverse charge-conversion signal. This signal is absent in the semiconducting 2H phase and in control samples where MoTe₂ blocks spin flux to the Pt layer.
3. Theoretical Correlation: First-principles calculations confirm that the 1T′ phase possesses a large orbital Rashba-Edelstein response near the Fermi level, dominated by orbital contributions rather than spin. Conversely, the 2H phase shows a strong suppression of both spin and orbital responses due to its band gap. The calculations further indicate that these responses are concentrated at the outermost layer, consistent with an interfacial mechanism.

Key Contributions

• Discovery of Phase-Dependent Orbitronics: The paper demonstrates that the orbital-to-charge conversion efficiency in MoTe₂ is intrinsically linked to its structural phase. Efficient conversion is observed exclusively in the metallic 1T′ phase, while the semiconducting 2H phase suppresses this effect.
• Scalable Phase Engineering: The authors establish that sputtering can intrinsically stabilize the 1T′ phase in ultrathin films without post-growth processing (such as electrostatic doping or strain engineering), offering a pathway for wafer-scale phase control.
• Orbital vs. Spin Dominance: The work highlights that in the 1T′ phase, the orbital Rashba-Edelstein effect is the dominant mechanism for charge conversion, overshadowing spin-based effects, which are negligible in both phases under the experimental conditions.

Significance and Claims
The authors claim that this study establishes MoTe₂ as a tunable platform for orbitronics, where orbital currents can be controlled via structural engineering (specifically film thickness). The findings suggest that the metallic 1T′ phase hosts efficient orbital-to-charge conversion, a property that is naturally suppressed in the semiconducting 2H phase. The research underscores the importance of the electronic structure (gapless vs. gapped) in determining transport properties. The authors position these results as a step toward designing orbital-based spintronic components using 2D materials with structural polymorphism, noting that the scalability of the sputtering technique is advantageous for device integration. They suggest future work could explore dynamic phase switching for real-time control, but the current paper focuses on the static, thickness-driven stabilization of these phases.