arXiv subjects

Tongyang Guan

Publications and source records attributed to Tongyang Guan.

4 recordsLinked to original sources

arXiv2026

Quantitative Disentanglement of Terahertz Spin and Orbital Pumping in 3d Ferromagnetic Heterostructures

Spin and orbital pumping - the injection of spin and orbital angular momentum from a driven ferromagnet into an adjacent nonmagnetic layer - are fundamental processes underlying angular-momentum generation and transport in magnetic heterostructures. Femtosecond optical excitation extends these phenomena into the ultrafast regime, where spintronic terahertz emission spectroscopy (STES) detects picosecond angular-momentum currents in a contact-free manner through spin-to-charge and orbital-to-charge conversion. Microscopic theory predicts that orbital-pumping efficiency increases from Fe to Ni across the 3d series, yet whether these predictions hold under ultrafast excitation remains unclear. A central challenge is that spin and orbital currents are generated simultaneously and contribute additively to the same terahertz emission, preventing their quantitative separation. Here, we overcome this limitation by combining STES with wedge-sample thickness control in heterostructures whose nonmagnetic layers (Ta, W, and Nb) have spin Hall and orbital Hall angles of opposite signs. The two angular-momentum channels therefore exhibit distinct emission polarities and thickness dependences, enabling their quantitative decomposition. Systematic measurements on Fe, Co, and Ni heterostructures reveal that the orbital-pumping contribution increases progressively toward Ni, reaching several tens of percent of the spin-current contribution - far exceeding theoretical predictions. Even Fe generates a non-negligible orbital current that becomes essential in the thin-nonmagnetic-layer regime. The extracted orbital diffusion lengths are consistently shorter than spin diffusion lengths and increase with decreasing spin-orbit coupling strength of the nonmagnetic layer. These results establish a quantitative framework for ultrafast spin and orbital pumping in magnetic heterostructures.

cond-mat.mes-hall↗

arXiv2025

Evidence of Ultrashort Orbital Transport in Heavy Metals Revealed by Terahertz Emission Spectroscopy

The orbital angular momentum of electrons offers a promising, yet largely unexplored, degree of freedom for ultrafast, energy-efficient information processing. As the foundation of orbitronics, understanding how orbital currents propagate and convert into charge currents is essential - but remains elusive due to the challenge in disentangling orbital and spin dynamics in ultrathin films. Although orbital currents have been predicted to propagate over long distances in materials, recent theoretical studies argue that lattice symmetry may constrain their mean free paths (MFPs) to the scale of a single atomic layer. In this work, we provide the first direct experimental evidence for ultrashort orbital MFPs in heavy metals (HMs) - W, Ta, Pt - revealed by femtosecond terahertz emission spectroscopy. This is enabled by sub-nanometer-precision control of thin-film thickness using wedge-shaped HM|Ni heterostructures. By employing a multi-component terahertz-emission model, we quantitatively extract the orbital MFPs, consistently finding them shorter than their spin counterparts. Furthermore, control experiments rule out interfacial orbital-to-charge conversion as the dominant mechanism, confirming that the process is governed by the bulk inverse orbital Hall effect. Our findings resolve a central controversy in orbitronics and provide key insights into orbital transport and conversion mechanisms.

cond-mat.mtrl-sci↗

arXiv2023

Flexible generation of structured terahertz fields via programmable exchange-biased spintronic emitters

Structured light, particularly in the terahertz frequency range, holds considerable potential for a diverse range of applications. However, the generation and control of structured terahertz radiation pose major challenges. In this work, we demonstrate a novel programmable spintronic emitter that can flexibly generate a variety of structured terahertz waves. This is achieved through the precise and high-resolution programming of the magnetization pattern on the emitter surface, utilizing laser-assisted local field cooling of an exchange-biased ferromagnetic heterostructure. Moreover, we outline a generic design strategy for realizing specific complex structured terahertz fields in the far field. Our device successfully demonstrates the generation of terahertz waves with diverse structured polarization states, including spatially separated circular polarizations, azimuthal or radial polarization states, and a full Poincare beam. This innovation opens a new avenue for designing and generating structured terahertz radiations, with potential applications in terahertz microscopy, communication, quantum information, and light-matter interactions.

physics.optics↗

arXiv2023

Nonrelativistic and nonmagnetic control of terahertz charge currents via electrical anisotropy in RuO2 and IrO2

Precise and ultrafast control over photo-induced charge currents across nanoscale interfaces could lead to important applications in energy harvesting, ultrafast electronics, and coherent terahertz sources. Recent studies have shown that several relativistic mechanisms, including inverse spin-Hall effect, inverse Rashba-Edelstein effect and inverse spin-orbit-torque effect, can convert longitudinally injected spin-polarized currents from magnetic materials to transverse charge currents, thereby harnessing these currents for terahertz generation. However, these mechanisms typically require external magnetic fields and suffer from low spin-polarization rates and low efficiencies of relativistic spin-to-charge conversion. In this work, we present a novel nonrelativistic and nonmagnetic mechanism that directly utilizes the photo-excited high-density charge currents across the interface. We demonstrate that the electrical anisotropy of conductive oxides RuO2 and IrO2 can effectively deflect injected charge currents to the transverse direction, resulting in efficient and broadband terahertz radiation. Importantly, this new mechanism has the potential to offer much higher conversion efficiency compared to previous methods, as conductive materials with large electrical anisotropy are readily available, whereas further increasing the spin-Hall angle of heavy-metal materials would be challenging. Our new findings offer exciting possibilities for directly utilizing these photo-excited high-density currents across metallic interfaces for ultrafast electronics and terahertz spectroscopy.

physics.optics↗