arXiv subjects

Tao Qu

Publications and source records attributed to Tao Qu.

10 recordsLinked to original sources

arXiv2025

Tuning Magneto-Optical Zero-Reflection via Dual-Channel Hybrid Magnonics

Multi-channel coupling in hybrid systems makes an attractive testbed not only because of the distinct advantages entailed in each constituent mode, but also the opportunity to leverage interference among the various excitation pathways. Here, via combined analytical calculation and experiment, we demonstrate that the phase of the magnetization precession at the interface of a coupled yttrium iron garnet(YIG)/permalloy(Py) bilayer is collectively controlled by the microwave photon field torque and the interlayer exchange torque, manifesting a coherent, dual-channel excitation scheme that effectively tunes the magneto-optic spectrum. The different torque contributions vary with frequency, external bias field, and types of interlayer coupling between YIG and Py, which further results in destructive or constructive interferences between the two excitation channels, and hence, selective suppression or amplification of the hybridized magnon modes.

cond-mat.mtrl-sci↗

arXiv2025

Pump-induced magnon anticrossing due to three-magnon splitting and confluence

We present a plausible mechanism for achieving magnon-magnon level repulsion spectrum originating from the oscillation between the splitting and confluence in a three-magnon scattering process. When a magnetostatic mode on a YIG sphere is pumped by a microwave signal near the magnon resonance frequency with an increasing amplitude, the generated magnon condensate at half of the pumping frequency exerts a back-action to the original magnon mode. Such a strong nonlinear coupling manifests a striking feature of a 'bending effect' of the magnetostatic mode spectra, akin to the anti-crossing observed in a strongly coupled magnon-photon system.

cond-mat.mes-hall↗

arXiv2024

Ultrafast dynamics of wavelength-sensitive magnons in unconventional compensated semiconducting antiferromagnet

Antiferromagnet is a promising candidate for the next generation spintronic devices, benefiting from its ultrafast dynamics and spontaneous zero stray field. However, the understanding of their ultrafast spin behaviors is lacking due to the challenges of controlling/detecting the quenched net magnetization. Unconventional compensated semiconducting antiferromagnets present strong time-reversal symmetry breaking, spin splitting in the momentum space, and suitable bandgap for optical control/detection. Thus, it is a powerful platform to uncover the ultrafast dynamics of antiferromagnets. Here, we show an exotic wavelength-dependent spin dynamic in the unconventional compensated semiconducting antiferromagnet {\alpha}-MnTe via time-resolved quadratic magneto-optical Kerr effect measurement, where the probing photon energy of the laser matches its bandgap. This direct excitation and detection of distinct magnon modes reveal varying spin behaviors and time characteristics in a broad temperature range. It originates from the spins triggered at different bands of electronic structures and is depicted in an energy transfer model among electrons, phonons, and magnons. Our study of exotic optical properties in this unconventional semiconducting antiferromagnet fulfills the missing information of spin evolution in the time domain and paves the way for its utilization in ultrafast spintronic devices.

physics.app-ph↗

arXiv2023

Giant Hall Switching by Surface-State-Mediated Spin-Orbit Torque in a Hard Ferromagnetic Topological Insulator

Topological insulators (TI) and magnetic topological insulators (MTI) can apply highly efficient spin-orbit torque (SOT) and manipulate the magnetization with their unique topological surface states with ultra-high efficiency. Here, we demonstrate efficient SOT switching of a hard MTI, V-doped (Bi,Sb)2Te3 (VBST) with a large coercive field that can prevent the influence of an external magnetic field. A giant switched anomalous Hall resistance of 9.2 $k\Omega$ is realized, among the largest of all SOT systems, which makes the Hall channel a good readout and eliminates the need to fabricate complicated magnetic tunnel junction (MTJ) structures. The SOT switching current density can be reduced to $2.8\times10^5 A/cm^2$. Moreover, as the Fermi level is moved away from the Dirac point by both gate and composition tuning, VBST exhibits a transition from edge-state-mediated to surface-state-mediated transport, thus enhancing the SOT effective field to $1.56\pm 0.12 T/ (10^6 A/cm^2)$ and the interfacial charge-to-spin conversion efficiency to $3.9\pm 0.3 nm^{-1}$. The findings establish VBST as an extraordinary candidate for energy-efficient magnetic memory devices.

cond-mat.mes-hall↗

arXiv2023

Structural tuning magnetism and topology in a magnetic topological insulator

To date, the most widely-studied quantum anomalous Hall insulator (QAHI) platform is achieved by dilute doping of magnetic ions into thin films of the alloyed tetradymite topological insulator (TI) (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ (BST). In these films, long-range magnetic ordering of the transition metal substituants opens an exchange gap $\Delta$ in the topological surface states, stabilizing spin-polarized, dissipationless edge channels with a nonzero Chern number $\mathcal{C}$. The long-range ordering of the spatially separated magnetic ions is itself mediated by electronic states in the host TI, leading to a sophisticated feedback between magnetic and electronic properties. Here we present a study of the electronic and magnetic response of a BST-based QAHI system to structural tuning via hydrostatic pressure. We identify a systematic closure of the topological gap under compressive strain accompanied by a simultaneous enhancement in the magnetic ordering strength. Combining these experimental results with first-principle calculations we identify structural deformation as a strong tuning parameter to traverse a rich topological phase space and modify magnetism in the magnetically doped BST system.

cond-mat.mes-hall↗

arXiv2022

Oscillations and confluence in three-magnon scattering of ferromagnetic resonance

We have performed a time-resolved and phase-sensitive investigation of three-magnon scattering of ferromagnetic resonance (FMR) over several orders of magnitude in excitation power. We observe a regime that hosts transient oscillations of the FMR magnon population, despite higher-order magnon interactions at large powers. Also at high powers, the scattering generates $180^\circ$ phase shifts of the FMR magnons. These phase shifts correspond to reversals in the three-magnon scattering direction, between splitting and confluence. These scattering reversals are most directly observed after removing the microwave excitation, generating coherent oscillations of the FMR magnon population much larger than its steady-state value during the excitation. Our model is in strong agreement with these findings. These findings reveal the transient behavior of this three-magnon scattering process, and the nontrivial interplay between three-magnon scattering and the magnons' phases.

cond-mat.mes-hall↗

arXiv2020

The interplay of large two-magnon ferromagnetic resonance linewidths and low Gilbert damping in Heusler thin films

We report on broadband ferromagnetic resonance linewidth measurements performed on epitaxial Heusler thin films. A large and anisotropic two-magnon scattering linewidth broadening is observed for measurements with the magnetization lying in the film plane, while linewidth measurements with the magnetization saturated perpendicular to the sample plane reveal low Gilbert damping constants of $(1.5\pm0.1)\times 10^{-3}$, $(1.8\pm0.2)\times 10^{-3}$, and $<8\times 10^{-4}$ for Co$_2$MnSi/MgO, Co$_2$MnAl/MgO, and Co$_2$FeAl/MgO, respectively. The in-plane measurements are fit to a model combining Gilbert and two-magnon scattering contributions to the linewidth, revealing a characteristic disorder lengthscale of 10-100 nm.

cond-mat.mtrl-sci↗

arXiv2019

Ultra-high frequency magnetic resonance through strain-spin coupling in perpendicular magnetic multi-layers

The interaction between strain and spin has received intensive attention in the scientific community due to its abundant physical phenomena and huge technological impact. Until now, there is no experimental report on ultra-high frequency magnetic resonance through the strain-spin coupling for any technologically relevant perpendicular magnetic material. Here we report the experimental detection of the acoustic strain waves that have a response time on the order of 10 picoseconds in perpendicular magnetic [Co/Pd]n multilayers via a femtosecond laser pulse excitation. Through direct measurements of acoustic strain waves, we observe an ultra-high frequency magnetic resonance up to 60 GHz in [Co/Pd]n multilayers. We further report a theoretical model of the strain-spin interaction. Our model reveals that the energy could be transferred efficiently from the strain to the spins and well explains the existence of a steady resonance state through exciting the spin system. The physical origins of the resonance between strain waves and magnetic precession and the requested conditions for obtaining magnetic resonance within thin magnetic films have also been discussed after thorough analysis. These combined results point out a potential pathway to enable an extremely high frequency (EHF) magnetic resonance through the strain-spin coupling.

cond-mat.mtrl-sci↗

arXiv2003

Reversibility Window, Aging, and Nanoscale Phase Separation in GexAsxS1-2x Bulk Alloy Glasses

The non-reversing enthalpy near Tg, DHnr, in bulk GexAsxS1-2x glasses is found to display a global minimum (~0) in the 0.11 < x < 0.15 range, the reversibility window. Furthermore, the DHnr term is found to age for glass compositions both below (x < 0.11) and above (x > 0.15) the window but not in the window. Glass compositions in the window are rigid but stress-free, those below the window are floppy, and those above the window are stressed-rigid. Raman scattering shows floppy and stressed rigid networks to consist in part of monomers. The latter aspect of structure narrows the width of the reversibility window and suppresses in part aging effects observed outside the window in contrast to those in the fully polymerized selenide counterparts.

cond-mat.mtrl-sci↗

arXiv2003

The Intermediate Phase in Ternary GexAsxSe1-2x Glasses

Melt-quenched AsxGexSe1-2x glasses over the composition range, 0 < x < 0.26, are examined in Raman scattering, T-modulated Differential Scanning Calorimetry (MDSC), and 119Sn Mossbauer spectroscopy measurements. The non-reversing enthalpy near Tg, DHnr(x), accessed from MDSC shows a global minimum (~ 0) in the xc(1) = 0.09 < x < xc(2) = 0.16 range, and increases by an order of magnitude both at x < xc(1) and at x > xc(2). Raman mode frequency of corner-sharing Ge(Se1/2)4 tetrahedra studied as a function of x, also shows three distinct regimes (or power-laws, p) that coincide with DHnr(x) trends. These regimes are identified with mechanically floppy (x < xc(1)), intermediate (xc(1) < x < xc(2)), and stressed-rigid (x > xc(2)) phases. The Raman elasticity power-law in the intermediate phase, p1 = 1.04(3), and in the stressed rigid phase, p2= 1.52(5), suggest effective dimensionalities of d = 2 and 3 respectively.

cond-mat.dis-nn↗