arXiv subjects

John Q. Xiao

Publications and source records attributed to John Q. Xiao.

At least 19 recordsLinked to original sources

arXiv2026

Autonomous Quantum Transport Measurements of 2D Semiconductors by an AI Agent

Artificial-intelligence (AI) agents are beginning to enter experimental laboratories, automating experiments and accelerating scientific discovery. Herein, we introduce an AI-driven workflow in which an AI agent performs multi-step, multi-day quantum transport measurements end-to-end. Specifically, given brief instructions, the agent starts by planning the multi-step measurements, then safely operates the cryogenic instruments, analyzes the data, and concludes with a final report. We demonstrate this AI workflow on multiple monolayer and bilayer MoS2 devices. Through autonomous measurement campaigns lasting up to six days, the agent determined the conduction-band spin-orbit coupling energy in monolayer MoS2, and mapped a layer- and valley-resolved phase diagram in bilayer MoS2. This experimental workflow is implemented through the FermiLink agent harness, which emphasizes instrumental safety and the reliability of the measurement and analysis. The framework is general and can be readily adapted to other types of experiments, representing a step toward self-driving laboratories.

cond-mat.mes-hall↗

arXiv2026

Competing ferromagnetic and antiferromagnetic interactions in non-altermagnetic Ru$_{1-x}$Cr$_{x}$O$_{2}$

We investigate the proposal of hole doping RuO$_{2}$ via alloying with Cr ions to induce altermagnetism. Thin film samples of Ru$_{1-x}$Cr$_{x}$O$_{2}$ ($0 \leq x \leq 0.28$) were prepared by reactive magnetron co-sputtering on TiO$_{2}$ substrates, with epitaxial nature verified by X-ray diffraction and composition depth profiles determined by X-ray photoelectron spectroscopy combined with Ar etching. Neutron diffraction measurements of samples with $x = 0$ and $x = 0.23$ at low temperatures show no evidence of long-range altermagnetic order with spins oriented along the $c$-axis or within the $ab$-plane. In contrast, temperature dependent susceptibility measurements in samples with $x \geq 0.16$ show a gradual downturn below $T\approx20~$K, suggestive of antiferromagnetic interactions, although this coexists with ferromagnetic hysteresis and remnant magnetization at $4~$K for samples with $x \geq 0.23$. Together, our magnetometry and neutron scattering measurements suggest the coexistence of antiferromagnetically coupled ferromagnetic CrO\textsubscript{2} clusters. This indicates that Cr ions do not hole dope Ru bands to induce an altermagnet state, but rather the holes remain localized to the Cr ions.

cond-mat.mtrl-sci↗

arXiv2026

Anomalous magneto-optical response at $\mathrm{RuO_2 / WSe_2}$ van der Waals interface

Ruthenium dioxide ($\mathrm{RuO_2}$) has been proposed as an altermagnetic candidate, although its magnetic ground state remains controversial. Here, we probe weak interfacial magnetic states at the surface of (001)-oriented $\mathrm{RuO_2}$ films using the magnetic proximity effect (MPE) in a van der Waals heterostructure consisting of monolayer tungsten diselenide ($\mathrm{WSe_2}$) atop $\mathrm{RuO_2}$. Temperature-dependent magneto-optical spectroscopy reveals an anomalous excitonic energy shift and a deviation from conventional Varshni behavior below 55 K that are absent in an encapsulated $\mathrm{WSe_2}$ control sample. The anomalous shift reverses sign upon field cooling with opposite magnetic field polarity, indicating a magnetic origin. Polarization-resolved measurements further show a nearly field-independent and fluctuating valley splitting in $\mathrm{WSe_2 / RuO_2}$ in strong contrast to the conventional linear Zeeman splitting observed in the control bare $\mathrm{WSe_2}$ sample. These results suggest that the valley states are governed predominantly by interfacial exchange fields associated with weak surface magnetic states in $\mathrm{RuO_2}$, which do not produce a conventional linear Zeeman response within the applied magnetic field range. Importantly, this approach enables direct optical probing of emergent surface magnetism without introducing an additional ferromagnetic layer, positioning MPE-based optical probing as a tool for investigating weak surface magnetism and offering new possibilities for studying magnetic materials with controversial magnetic states.

cond-mat.mtrl-sci↗

arXiv2026

Broken intrinsic symmetry induced magnon-magnon coupling in synthetic ferrimagnets

Synthetic antiferromagnets offer rich magnon energy spectra in which optical and acoustic magnon branches can hybridize. Here, we demonstrate a broken intrinsic symmetry induced coupling of acoustic and optical magnons in a synthetic ferrimagnet consisting of two dissimilar antiferromagnetically interacting ferromagnetic metals. Two distinct magnon modes hybridize at degeneracy points, as indicated by an avoided level-crossing. The avoided level-crossing gap depends on the interlayer exchange interaction between the magnetic layers, which can be controlled by adjusting the non-magnetic interlayer thickness. A large avoided level crossing gap of 3.9 GHz is revealed, exceeding the coupling strength that is typically found in other magnonic hybrid systems based on a coupling of magnons with photons or magnons with phonons.

cond-mat.mtrl-sci↗

arXiv2025

Probing Magnetic Properties of RuO$_{2}$ Heterostructures Through the Ferromagnetic Layer

RuO$_{2}$ has been proposed as the prototypical altermagnetic material. However, several reports have recently questioned its intrinsic magnetic ordering, leading to conflicting findings, especially in thin film heterostructures pointing to possible interface effects being convoluted with supposed antiferromagnetic/altermagnetic signatures. Here, extensive magnetometry measurements were performed on two independently grown thin film heterostructures of RuO$_{2}$ interfaced with either NiFe or Fe acting as the ferromagnetic layer. Below about 15 K, both samples exhibit exchange bias fields when cooled to approximately 2 K in a $+$1 T field, and a spin transitional feature is observed around 31 K. Magneto-Raman measurements on RuO$_{2}$ thin films only reveal a magnon mode when there is a NiFe layer, suggesting that RuO$_{2}$ does not intrinsically possess long range magnetic ordering.. When in contact with a ferromagnet, RuO$_2$ displays effects that could be ascribed to antiferromagnetism. However, the lack of intrinsic magnon modes points toward possible diffusion between the layers or spin disorder at the interface as seen by density functional theory (DFT) calculations.

cond-mat.mtrl-sci↗

arXiv2025

Deciphering the origin of spin current in spintronic terahertz emitters and its imprint on their electromagnetic radiation via time-dependent density functional theory

Spin current flowing between femtosecond laser pulse (fsLP)-driven ferromagnetic metal and adjacent normal metal (NM) hosting strong spin-orbit coupling is invariably invoked to explain terahertz (THz) radiation believed to be emitted solely by NM layer. Despite being such a central concept, the microscopic origin of interlayer spin current remains vague. Here, we employ recently developed [A. Kefayati {\em et al.}, Phys. Rev. Lett. {\bf 133}, 136704 (2024)] time-dependent density functional theory plus Jefimenko equations approach to extract spin current between Co and NM=Pt or NM=W layer where Co is driven by fsLP responsible for its demagnetization, i.e., shrinking of its magnetization vector, $M^y(t)/M^y(t=0)<1$. By comparing time dependence of spin current with those of other relevant quantities, we find that: ({\em i}) spin current is generated by demagnetization dynamics because it {\em follows} closely $dM^y/dt$, thus it is an example of quantum pumping phenomenon that cannot be captured by phenomenological notions (such as ``spin voltage'') and related semiclassical transport theories; ({\em ii}) time dependence of pumped spin current {\em does not follow} closely that of charge current emerging within NM layer via spin-to-charge conversion mechanisms; ({\em iii}) THz emission can be governed by {\em both} charge current (i.e., its time derivative entering the Jefimenko equations) within Co layer or NM layer, but in different times frames. We also unravel a special case of NM=W where spin-to-charge conversion by the inverse spin Hall effect and its contribution to THz emission is suppressed, despite large spin Hall angle of W, because of localization of excited electrons onto the outer unfilled $d$-orbitals of W.

cond-mat.mes-hall↗

arXiv2025

Magnon spectrum of altermagnets beyond linear spin wave theory: Magnon-magnon interactions via time-dependent matrix product states vs. atomistic spin dynamics

The energy-momentum dispersion of magnons, as collective low-energy excitations of magnetic material, is computed from an effective quantum spin Hamiltonian but simplified via linearized Holstein-Primakoff transformations to describe noninteracting magnons. The dispersion produced by such linear spin wave theory (LSWT) is then plotted as ``sharp bands'' of infinitely long-lived quasiparticles. However, magnons are prone to many-body interactions with other quasiparticles -- such as electrons, phonons or other magnons -- which can lead to shifting (i.e., band renormalization) and broadening of ``sharp bands'' as the signature of finite quasiparticle lifetime. The magnon-magnon interactions can be particularly important in antiferromagnets (AFs), and, therefore, possibly in newly classified altermagnets sharing many features of collinear AFs. Here, we employ nonperturbative quantum many-body calculations, via time-dependent matrix product states (TDMPS), to obtain magnon spectral function for RuO$_2$ altermagnet whose effective quantum spin Hamiltonian is put onto 4-leg cylinder. Its upper band is shifted away from upper ``sharp band'' of LSWT, as well as broadened, which is explained as the consequence of {\em hybridization} of the latter with three-magnon continuum. This implies that two-magnon Raman scattering spectra {\em cannot} be computed from LSWT bands, which offers a litmus test for the relevance of magnon-magnon interactions. Finally, we employ atomistic spin dynamics (ASD) simulations, based on classical Landau-Lifshitz-Gilbert (LLG) equation, to obtain magnon spectrum at finite temperature and/or at a fraction of the cost of TDMPS calculations. Despite including magnon-magnon interactions via nonlinearity of LLG equation, ASD simulations {\em cannot} match the TDMPS-computed magnon spectrum, thereby signaling {\em nonclassical} effects harbored by AFs and altermagnets.

cond-mat.str-el↗

arXiv2024

Revisiting altermagnetism in RuO2: a study of laser-pulse induced charge dynamics by time-domain terahertz spectroscopy

Altermagnets are a recently discovered class of magnetic material with great potential for applications in the field of spintronics, owing to their non-relativistic spin-splitting and simultaneous antiferromagnetic order. One of the most studied candidates for altermagnetic materials is rutile structured RuO2. However, it has recently come under significant scrutiny as evidence emerged for its lack of any magnetic order. In this work, we study bilayers of epitaxial RuO2 and ferromagnetic permalloy (Fe19Ni81) by time-domain terahertz spectroscopy, probing for three possible mechanisms of laser-induced charge dynamics: the inverse spin Hall effect (ISHE), electrical anisotropic conductivity (EAC), and inverse altermagnetic spin-splitting effect (IASSE). We examine films of four common RuO2 layer orientations: (001), (100), (110), and (101). If RuO2 is altermagnetic, then the (100) and (101) oriented samples are expected to produce anisotropic emission from the IASSE, however, our results do not indicate the presence of IASSE for either as-deposited or field annealed samples. The THz emission from all samples is instead consistent with charge dynamics induced by only the relativistic ISHE and the non-relativistic and non-magnetic EAC, casting further doubt on the existence of altermagnetism in RuO2. In addition, we find that in the (101) oriented RuO2 sample, the combination of ISHE and EAC emission mechanisms produces THz emission which is tunable between linear and elliptical polarization by modulation of the external magnetic field.

cond-mat.mtrl-sci↗

arXiv2024

Electric polarization induced by magnons and magnon Nernst effects

Magnons offer a promising path toward energy-efficient information transmission and the development of next-generation classical and quantum computing technologies. However, methods to efficiently excite, manipulate, and detect magnons remain a critical need. Here, we show that magnons, despite their charge-neutrality, can induce electric polarization as a result of both their spin and orbital moments. We demonstrate this by calculating the electric polarization induced by magnons in two-dimensional (2D) honeycomb antiferromagnets. The electric polarization becomes finite when the Dzyaloshinskii-Moriya Interaction (DMI) is present and its magnitude can be increased by symmetries of the system. We illustrate this by computing and comparing the electric polarizations induced by the magnon Nernst effects in 2D materials with Néel and Zigzag ordering. Our findings show that in the Zigzag order, where the effect is dominated by the magnon orbital moment, the induced electric polarization is approximately three orders of magnitude greater than in the Néel phase. These findings reveal that electric fields could enable both detection and manipulation of magnons under certain conditions by leveraging their spin and orbital angular moment. They also suggest that the discovery or engineering of materials with substantial magnon orbital moments could lead to more practical use of magnons for future computing and information transmission device applications.

cond-mat.mes-hall↗

arXiv2024

Charge and spin current pumping by ultrafast demagnetization dynamics

The surprising discovery of ultrafast demagnetization -- where electric field of femtosecond laser pulse couples to electrons of a ferromagnetic (FM) layer causing its magnetization vector {\em to shrink while not rotating}, is also assumed to be accompanied by generation of spin current in the direction orthogonal to electric field. However, understanding of the microscopic origin of such spin current and how efficiently it can be converted into charge current, as the putative source of THz radiation, is lacking despite nearly three decades of intense studies. Here we connect the standard pumping phenomena driven by microwave precession of magnetization vector replacing periodic time-dependence of magnetization precession with nonperiodic time-dependence of demagnetization, as obtained from experiments on ultrafast-light-driven Ni layer. Applying time-dependent nonequilibrium Green's functions, able to evolve such setup with arbitrary time dependence, reveals how demagnetization dynamics pumps both charge and spin currents in directions both parallel and orthogonal to electric field of laser pulse, even in the absence of spin-orbit coupling and thereby induced spin-to-charge conversion mechanisms. Although pumped currents follow $dM_z/dt$ in some setups, this becomes obscured when NM layers are disconnected and pumped currents start to reflect from FM boundaries (as is the case of experimental setups). Finally, we use the Jefimenko equations to compute electromagnetic radiation by charge current pumped in disconnected setup during demagnetization, or later during its slow recovery, unraveling that radiated electric field only in the former time interval exhibits features in 0.1--30 THz frequency range probed experimentally or explored for applications of spintronic THz emitters.

cond-mat.mes-hall↗

arXiv2024

Diamond Micro-Chip for Quantum Microscopy

The nitrogen vacancy (NV) center in diamond is an increasingly popular quantum sensor for microscopy of electrical current, magnetization, and spins. However, efficient NV-sample integration with a robust, high-quality interface remains an outstanding challenge to realize scalable, high-throughput microscopy. In this work, we characterize a diamond micro-chip (DMC) containing a (111)-oriented NV ensemble; and demonstrate its utility for high-resolution quantum microscopy. We perform strain imaging of the DMC and find minimal detrimental strain variation across a field-of-view of tens of micrometer. We find good ensemble NV spin coherence and optical properties in the DMC, suitable for sensitive magnetometry. We then use the DMC to demonstrate wide-field microscopy of electrical current, and show that diffraction-limited quantum microscopy can be achieved. We also demonstrate the deterministic transfer of DMCs with multiple materials of interest for next-generation electronics and spintronics. Lastly, we develop a polymer-based technique for DMC placement. This work establishes the DMC's potential to expand the application of NV quantum microscopy in materials, device, geological, biomedical, and chemical sciences.

physics.app-ph↗

arXiv2022

Large Bilinear Magnetoresistance from Rashba Spin-Splitting on the Surface of a Topological Insulator

In addition to the topologically protected linear dispersion, a band-bending-confined two-dimensional electron gas with tunable Rashba spin-splitting (RSS) was found to coexist with the topological surface states on the surface of topological insulators (TIs). Here, we report the observation of large bilinear magnetoresistance (BMR) in Bi2Se3 films decorated with transition metal atoms. The magnitude of the BMR sensitively depends on the type and amount of atoms deposited, with a maximum achieved value close to those of strong Rashba semiconductors. Our first-principles calculations reproduce the quantum well states and reveal sizable RSS in all Bi2Se3 heterostructures with broken inversion symmetry. Our results show that charge-spin interconversion through RSS states in TIs can be fine-tuned through surface atom deposition and easily detected via BMR for potential spintronic applications.

cond-mat.mes-hall↗

arXiv2022

Phonon-mediated strong coupling between a three-dimensional topological insulator and a two-dimensional antiferromagnetic material

Van der Waals antiferromagnetic and topological insulator materials provide powerful platforms for modern optical, electronic, and spintronic devices applications. The interaction between an antiferromagnet (AFM) and a topological insulator (TI), if sufficiently strong, could offer emergent hybrid material properties that enable new functionality exceeding what is possible in any individual material constituent. In this work, we study strong coupling between THz excitations in a three dimensional (3D) topological insulator and a quasi-two dimensional (2D) antiferromagnetic material resulting in a new hybridized mode, namely a surface Dirac plasmon-phonon-magnon polariton. We find that the interaction between a surface Dirac plasmon polariton in the 3D TI and a magnon polariton in the 2D AFM is mediated by the phonon coupling in the 3D TI material. The coupling of phonons with an electromagnetic wave propagating in the 3D TI enhances the permittivity of the TI thin film in a way that results in a strong correlation between the dispersion of Dirac plasmon polaritons on the surfaces of the TI with the thickness of the TI. As a result, the dispersion of surface Dirac plasmon polaritons in the TI can be tuned toward resonance with the magnon polariton in the AFM material by varying the TI's thickness, thereby enhancing the strength of the coupling between the excitations in the two materials. The strength of this coupling, which results in the surface Dirac plasmon-phonon-magnon polariton, can be parameterized by the amplitude of the avoided-crossing splitting between the two polariton branches at the magnon resonance frequency...

cond-mat.mes-hall↗

arXiv2022

Observation of nonlinear planar Hall effect in magnetic insulator/topological insulator heterostructures

Interfacing topological insulators (TIs) with magnetic insulators (MIs) has been widely used to study the interaction between topological surface states and magnetism. Previous transport studies typically interpret the suppression of weak antilocalization or appearance of the anomalous Hall effect as signatures of magnetic proximity effect (MPE) imposed to TIs. Here, we report the observation of nonlinear planar Hall effect (NPHE) in Bi2Se3 films grown on MI thulium and yttrium iron garnet (TmIG and YIG) substrates, which is an order of magnitude larger than that in Bi2Se3 grown on nonmagnetic gadolinium gallium garnet (GGG) substrate. The nonlinear Hall resistance in TmIG/Bi2Se3 depends linearly on the external magnetic field, while that in YIG/Bi2Se3 exhibits an extra hysteresis loop around zero field. The magnitude of the NPHE is found to scale inversely with carrier density. We speculate the observed NPHE is related to the MPE-induced exchange gap opening and out-of-plane spin textures in the TI surface states, which may be used as an alternative transport signature of the MPE in MI/TI heterostructures.

cond-mat.mes-hall↗

arXiv2022

Above-room-temperature ferromagnetism in ultrathin van der Waals magnet

Two-dimensional (2D) magnetic van der Waals materials provide a powerful platform for studying fundamental physics of low-dimensional magnetism, engineering novel magnetic phases, and enabling ultrathin and highly tunable spintronic devices. To realize high quality and practical devices for such applications, there is a critical need for robust 2D magnets with ordering temperatures above room temperature that can be created via exfoliation. Here the study of exfoliated flakes of cobalt substituted Fe5GeTe2 (CFGT) exhibiting magnetism above room temperature is reported. Via quantum magnetic imaging with nitrogen-vacancy centers in diamond, ferromagnetism at room temperature was observed in CFGT flakes as thin as 16 nm. This corresponds to one of the thinnest room-temperature 2D magnet flakes exfoliated from robust single crystals, reaching a thickness relevant to practical spintronic applications. The Curie temperature Tc of CFGT ranges from 310 K in the thinnest flake studied to 328 K in the bulk. To investigate the prospect of high-temperature monolayer ferromagnetism, Monte Carlo calculations were performed which predicted a high value of Tc ~270 K in CFGT monolayers. Pathways towards further enhancing monolayer Tc are discussed. These results support CFGT as a promising platform to realize high-quality room-temperature 2D magnet devices.

cond-mat.mtrl-sci↗

arXiv2022

Surface plasmon-phonon-magnon polariton in a topological insulator-antiferromagnetic bilayer structure

We present a robust technique for computationally studying surface polariton modes in hybrid materials. We use a semi-classical model that allows us to understand the physics behind the interactions between collective excitations of the hybrid system and develop a scattering and transfer matrix method that imposes the proper boundary conditions to solve Maxwell equations and derive a general equation describing the surface polariton in a heterostructure consisting of N constituent materials. We apply this method to a test structure composed of a topological insulator (TI) and an antiferromagnetic material (AFM) to study the resulting surface Dirac plasmon-phonon-magnon polariton (DPPMP). We find that interactions between the excitations of the two constituents result in the formation of hybridized modes and the emergence of avoided-crossing points in the dispersion relations for the DPPMP. For the specific case of a Bi2Se3 TI material, the polariton branch with low frequency below 2 THz redshifts upon increasing the thickness of TI thin film, which leads to an upper bound on the thickness of the TI layer that will allow an observable signature of strong coupling and the emergence of hybridized states. We also find that the strength of the coupling between the TI and the AFM, which is parameterized by the amplitude of the avoided-crossing splitting between the two polariton branches at the magnon resonance frequency, depends on the magnitude of the magnetic dipole and the line width of the magnon in the AFM material as well as on the Fermi energy of Dirac plasmon in the TI. Finally, we predict that materials with extremely high quality, i.e. low scattering loss rate, are essential to achieve an experimentally-observable strong coupling between a TI and AFM.

cond-mat.mtrl-sci↗

arXiv2022

Revealing room temperature ferromagnetism in exfoliated Fe$_5$GeTe$_2$ flakes with quantum magnetic imaging

Van der Waals material Fe$_5$GeTe$_2$, with its long-range ferromagnetic ordering near room temperature, has significant potential to become an enabling platform for implementing novel spintronic and quantum devices. To pave the way for applications, it is crucial to determine the magnetic properties when the thickness of Fe5GeTe2 reaches the few-layers regime. However, this is highly challenging due to the need for a characterization technique that is local, highly sensitive, artifact-free, and operational with minimal fabrication. Prior studies have indicated that Curie temperature TC can reach up to close to room temperature for exfoliated Fe$_5$GeTe$_2$ flakes, as measured via electrical transport; there is a need to validate these results with a measurement that reveals magnetism more directly. In this work, we investigate the magnetic properties of exfoliated thin flakes of van der Waals magnet Fe$_5$GeTe$_2$ via a quantum magnetic imaging technique based on nitrogen vacancy diamond. Through imaging the stray fields, we confirm room-temperature magnetic order in Fe$_5$GeTe$_2$ thin flakes with thickness down to 7 units cell. The stray field patterns and their response to magnetizing fields with different polarities point to a perpendicular easy-axis anisotropy. Furthermore, we perform imaging at different temperatures and determine the Curie temperature of the flakes at Tc~300 K. These results provide the basis for realizing a room-temperature monolayer ferromagnet with Fe$_5$GeTe$_2$. This work also demonstrates that the imaging technique enables a rapid screening of multiple flakes simultaneously, thereby paving the way towards high throughput characterization of potential 2D magnets near room temperature.

cond-mat.mes-hall↗

arXiv2022

Direct probing of strong magnon-photon coupling in a planar geometry

We demonstrate direct probing of strong magnon-photon coupling using Brillouin light scattering spectroscopy in a planar geometry. The magnonic hybrid system comprises a split-ring resonator loaded with epitaxial yttrium iron garnet thin films of 200 nm and 2.46 $μ$m thickness. The Brillouin light scattering measurements are combined with microwave spectroscopy measurements where both biasing magnetic field and microwave excitation frequency are varied. The cooperativity for the 200 nm-thick YIG films is 4.5, and larger cooperativity of 137.4 is found for the 2.46 $μ$m-thick YIG film. We show that Brillouin light scattering is advantageous for probing the magnonic character of magnon-photon polaritons, while microwave absorption is more sensitive to the photonic character of the hybrid excitation. A miniaturized, planar device design is imperative for the potential integration of magnonic hybrid systems in future coherent information technologies, and our results are a first stepping stone in this regard. Furthermore, successfully detecting the magnonic hybrid excitation by Brillouin light scattering is an essential step for the up-conversion of quantum signals from the optical to the microwave regime in hybrid quantum systems.

cond-mat.mtrl-sci↗