arXiv subjects

Emanuele Longo

Publications and source records attributed to Emanuele Longo.

6 recordsLinked to original sources

arXiv2026

Probing the quantum metric of 3D topological insulators

The surface states of 3D topological insulators possess geometric structures that imprint distinctive signatures on electronic transport. A prime example is the Berry curvature, which controls, for instance, electric frequency doubling via its higher order moments. In addition to the Berry curvature, topological surface states are expected to exhibit a nontrivial quantum metric, which plays a key role in governing nonlinear magnetotransport. However, its manifestation has yet to be experimentally observed and controlled in 3D topological insulators. Here, we provide evidence for a nonlinear response activated by the quantum metric of the topological surface states of Sb$_2$Te$_3$. We measure a time-reversal odd, nonlinear magnetoresistance that is independent from the temperature and the scattering time below 30 K, and is thus of intrinsic geometrical origin. This quantum metric magnetoresistance can be controlled by tuning the contributions of the top and bottom topological surface states by voltage gating. Our measurements thus demonstrate the existence and tunability of quantum geometry-induced transport in topological phases of matter and provide strategies for designing novel functionalities in topological devices.

cond-mat.mes-hall↗

arXiv2025

Atomic-scale Origin of Interfacial Dzyaloshinskii--Moriya Interaction in Pt/Fe/Au

Recent results show remarkable interfacial--Dzyaloshinskii--Moriya interaction (i-DMI) in magnetic heterostructures composed of relatively thick Fe thin films ($\gg 1$ nm). To investigate its origin, we present a thorough magnetic characterization of the SiO$_2$/${}^{57}\mathrm{Fe}$($t$)/Au and SiO$_2$/Pt/${}^{57}\mathrm{Fe}$($t$)/Au heterostructures ($t = 1.6$ - $2.2$ nm). An i-DMI constant of $D_s^{\mathrm{Pt}} = 0.43\,\mathrm{mJ\,m^{-2}}$ is extracted for the Pt/Fe/Au system, as probed by wavevector-resolved Brillouin light scattering spectroscopy. Ferromagnetic resonance and conversion electron Mössbauer spectroscopy indicate that significant surface anisotropy is present in Pt/Fe/Au, and that an interfacial Fe fraction ($\sim18$) exhibits tilted magnetization ($\sim25^\circ$ out of plane) and an enhanced hyperfine field, correlating with the stronger i-DMI.

cond-mat.mtrl-sci↗

arXiv2025

Transition Metal Dichalcogenide 1T$'$-MoTe$_2$ Nanoscale Films as Spin Pumping Platforms

Transition metal dichalcogenides (TMDs) have emerged as a promising class of materials for spintronics, with the aim of promoting efficient spin-charge conversion (SCC) in TMD/ferromagnet (FM)-based devices. The MoTe$_2$ semimetal with distorted orthorhombic crystal structure in the 1T$'$ phase has gathered particular attention due to its high spin-orbit coupling and reconfigurability as a type-II Weyl semimetal close to room temperature. Here, we report on the role of chemically grown 1T$'$-MoTe$_2$ thin films in inducing SCC in 1T$'$-MoTe$_2$/FM heterostructures as measured at room temperature. Ferromagnetic resonance (FMR) and electrically detected spin-pumping FMR measurements performed on 1T$'$-MoTe$_2$/Co/Au and 1T$'$-MoTe$_2$/Au/Co/Au heterostructures reveal a spin-mixing conductance value of up to $\sim1.6 \times 10^{20}~\mathrm{m^{-2}}$ and a spin Hall angle of $1.7\%$. These findings position MoTe$_2$ thin films as a competitive spin-charge conversion option compared to other functional materials (e.g., heavy metals, topological insulators), highlighting their potential for future applications in spintronic devices.

cond-mat.mtrl-sci↗

arXiv2025

High accuracy Spin Hall Effect Induced Spin Accumulation detection in MOKE Measurements

Charge to spin (orbital) momentum conversion phenomena enclose great potential for advancing applications in spin/orbitronics. Although current-induced magnetic moment accumulation is crucial both for fundamental understanding and practical applications, direct quantifications are scarce. Optical polarization measurements, namely magneto-optical Kerr rotation (MOKE) ($θ_K$), have been used to get direct evidence of magnetic accumulation perpendicular to a current flow density ($J$) in late transition metals (Pt) as well as in light transition elements (Ti, Cr), and used to conclude evidence of spin or orbital momentum accumulation. However, discrepancies of the reported $θ_K/J$ values, exceeding one order of magnitude, together with early claims that conventional MOKE experiments were not a suitable tool, is prompting revisions of methods and results. Here, we report on a new methodology for MOKE measurements that solves known bottlenecks. We obtain a sensitivity of $(354 \pm 27)$ nV/nrad and use the designed protocol to measure $|θ_K^S/J| = (7.92 \pm 1.94)$ nrad/$(10^7$ A/cm$^2)$ and $|θ_K^P/J| = (6.89 \pm 1.74)$ nrad/$(10^7$ A/cm$^2)$ in a 50 nm thick Pt bar for S and P polarized incident light, respectively.The extracted value of $|θ_K^S/J|$ is significantly smaller, about a 7-fold reduction, than previous results on a nominally identical device. Given that differences in the microstructure of Pt films cannot account for such large discrepancy, this implies that experimental procedures and models should be revised accordingly.

cond-mat.mtrl-sci↗

arXiv2025

Terahertz time-domain signatures of the inverse Edelstein effect in topological-insulator|ferromagnet heterostructures

Three-dimensional topological insulators possess topologically protected surface states with spin-momentum locking, which enable spin-charge-current interconversion (SCI) by the inverse Edelstein effect (IEE). However, it remains experimentally challenging to separate the surface-related IEE from the bulk-type inverse spin Hall effect (ISHE). Here, we search for distinct time-domain signatures of the two SCI phenomena in a $\mathcal{F}$|TI model stack of a ferromagnetic-metal layer $\mathcal{F}$ (Co and Fe) and a topological-insulator layer TI (Bi$_2$Te$_3$, SnBi$_2$Te$_4$ and Bi$_{1-x}$Sb$_x$ with $x$ = 0.15 and 0.3), where the focus is on Bi$_2$Te$_3$. A femtosecond laser pulse serves to induce a transient spin voltage $μ_s^{\mathcal{F}}$ in $\mathcal{F}$ and, thus, drive an ultrafast spin current out of $\mathcal{F}$. SCI results in a transverse charge current with a sheet density $I_c$ that is detected by sampling the emitted terahertz electric field. Analysis of the dynamics of $I_c(t)$ vs time $t$ relative to $μ_s^{\mathcal{F}}(t)$ reveals two components with distinct time scales: (i) a quasi-instantaneous response and (ii) a longer-lived response with a relaxation time of 270 fs, which is independent of the chosen $\mathcal{F}$ material. Component (i) is consistently ascribed to the ISHE. In contrast, we interpret component (ii) as a signature of interfacial spin accumulation and the IEE at the $\mathcal{F}$/Bi$_2$Te$_3$ interface, with a fraction of $< 10^{-2}$ of the incident spins participating. This assignment is fully consistent with respect to its dynamics and magnitude. We rate other possible signal contributions, such as spin trapping in intermediate states, as less likely. Our results show that the femtosecond dynamics of photocurrents provide important insights into the mechanisms of spin transport and SCI in $\mathcal{F}$|TI stacks.

cond-mat.mes-hall↗

arXiv2021

Large spin-to-charge conversion at room temperature in extended epitaxial Sb2Te3 topological insulator chemically grown on Silicon

Spin-charge interconversion phenomena at the interface between magnetic materials and topological insulators (TIs) are attracting enormous interest in the research effort towards the development of fast and ultra-low power devices for the future information and communication technology. We report a large spin-to-charge conversion efficiency in Au/Co/Au/Sb2Te3/Si(111) heterostructures based on Sb2Te3 TIs grown by metal organic chemical vapor deposition on 4 inches Si(111) substrates. By conducting room temperature spin pumping ferromagnetic resonance, we measure an inverse Edelstein Effect length λIEE up to 0.75 nm, a record value for 3-dimensional chalcogenide-based TIs heterostructures. Our results open the path toward the use of chemical methods to produce TIs on large area Si substrates and characterized by highly performing spin-charge conversion, thus marking a milestone toward future technology-transfer.

cond-mat.mtrl-sci↗