arXiv subjects

Sylvain Massabeau

Publications and source records attributed to Sylvain Massabeau.

4 recordsLinked to original sources

arXiv2026

Thickness-Dependent Spintronic Terahertz Emission in MBE-Grown PtTe$_2$: From Semiconductor to Type-II Dirac Semimetal

Spintronic terahertz (THz) emitters have established themselves as among the most practical broadband THz sources available, yet their performance remains fundamentally limited by the spin Hall conductivity of the nonmagnetic conversion layer - a quantity that is fixed once the material is chosen. Here, we demonstrate that in PtTe$_2$, a type-II Dirac semimetal within the transition metal dichalcogenide family, this limitation can be circumvented by exploiting the dramatic thickness-driven electronic phase evolution of the material itself. Using molecular beam epitaxy to grow PtTe$_2$ films with single-monolayer precision from 1 to 20 ML, we show that the spintronic THz emission tracks the underlying electronic phase diagram directly: it is absent in the single-layer semiconducting phase, turns on sharply at the semimetal transition near 2 ML, and reaches a peak amplitude six times that of an equivalent Pt reference at 10 ML, before declining at larger thicknesses due to THz reabsorption in the increasingly metallic film. This non-monotonic behavior is inconsistent with a bulk inverse spin Hall mechanism and instead reflects a multi-channel spin-to-charge conversion process in which spin-momentum-locked topological surface states and a thickness-dependent interfacial Rashba splitting both contribute and strengthen as the type-II Dirac band structure develops. First-principles calculations of the interfacial spin accumulation reproduce the experimental trend quantitatively, confirming this physical picture. These findings introduce thickness engineering of van der Waals semimetals as a new and accessible route to optimizing spintronic THz emitters and spin-orbit torques in magnetic memories (SOT-MRAMs), with direct implications for the broader class of dimensionally tunable topological materials.

cond-mat.mtrl-sci↗

arXiv2026

Rashba engineering at van der Waals interfaces

Two-dimensional transition metal dichalcogenide (TMD) interfaces offer a versatile platform for studying emergent quantum phenomena and enabling novel device functionalities. When distinct TMD monolayers are stacked vertically or laterally stitched, their interfaces can exhibit unique electronic band alignments, giving rise to long-lived interlayer excitons, charge transfer effects, and moiré superlattices with correlated states. Here, we demonstrate that the interface between a large variety of two different epitaxially grown TMD monolayers controls the intensity and sign of the Rashba spin splitting, which is probed using THz spintronic emission. Optimized TMD heterobilayers, such as HfSe$_2$/PtSe$_2$, show enhanced THz emission that surpass the spin-to-charge conversion efficiency of bulk TMDs, confirming the presence of Rashba states with large spin splitting at the interface. By combining spin- and angle-resolved photoemission spectroscopy with density functional theory, we reveal that the electronic hybridization between the two different TMD monolayers gives rise to extended in-gap states with strong Rashba spin-orbit coupling. The choice of TMD layers enables to engineer the sign and strength of spin-to-charge conversion in van der Waals heterobilayers opening up perspectives to build efficient and tunable THz spintronic emitters.

cond-mat.mes-hall↗

arXiv2025

Phonon-tunable THz magnonic emission in multiferroic heterostructures

Collective excitations such as magnons and polar phonons provide natural access to the terahertz (THz) regime, but efficient generation and tunability remain elusive. Multiferroic BiFeO3 combines both orders at room temperature, offering a unique platform for narrowband THz emission. Here, we achieve efficient sub-bandgap optical rectification of coupled phonon-polaritons near 2 THz in bare epitaxial thin films. In Pt/BiFeO3 bilayers, we demonstrate that coupling the electromagnon branch with ultrafast strain waves, optically generated in Pt layers with various thicknesses, can produce tunable and narrowband emission between 0.4-0.8 THz. These results uncover the intertwined role of phonons, magnons, and magneto-acoustic dynamics in antiferromagnetic multiferroics, and establish these hybrid platforms as versatile engineered narrowband THz sources.

cond-mat.mes-hall↗

arXiv2025

Spin and Orbital Rashba response in ferroelectric polarized PtSe$_2$/MoSe$_2$/LiNbO$_3$ heterostructures

Recent studies using Terahertz Time-Domain Spectroscopy (THz-TDS) with spintronic emitters as a source have revealed distinct signatures of the Rashba effect. This effect, which arises from the breaking of inversion symmetry in low-dimensional materials, has been recently investigated in CoFeB/PtSe$_2$/MoSe$_2$/LiNbO$_3$-based heterostructures [S. Massabeau et al., APL Mater. 13, 041102, 2025 ]. The observed phenomena are at the source of the generated THz far-field emission, typically through mechanisms such as spin-to-charge conversion triggered by the absorption of ultrafast optical pulses. In this work, we employ first-principles simulations to quantify the Rashba effect at PtSe$_2$/MoSe$_2$/LiNbO$_3$ interfaces, expanding the traditional understanding of spin transport by incorporating the orbital degree of freedom. Moreover, we quantify the degree of control on the THz emission depending on the polarization direction of LiNbO$_3$. In order to achieve this, we analyze the accumulation of both spin and orbital components using linear response theory, revealing distinct behaviors. These findings are crucial for a deeper understanding of the physical processes governing angular momentum-to-charge conversion and THz emission. Moreover, they may provide broader insights into various experimental outcomes, including those related to spin-orbit torque.

cond-mat.mes-hall↗