arXiv subjects

Shixun Cao

Publications and source records attributed to Shixun Cao.

At least 19 recordsLinked to original sources

arXiv2026

Observation of Magnetic-Anisotropy Crossover and High-Temperature Skyrmions in the Dirac Magnet Fe3Ge with a Distorted Kagome Lattice

Topological materials that simultaneously host robust high-temperature skyrmions and nontrivial electronic band structures have attracted tremendous interest owing to their distinctive advantages for both fundamental research and prospective technological applications. Here, we report the observation of robust skyrmions in the Dirac kagome magnet Fe3Ge, which exhibits a high Curie temperature of ~ 650 K. At room temperature, Fe3Ge shows a large intrinsic anomalous Hall conductivity of ~ 380 {\Omega}-1cm-1, originating from its nontrivial electronic band topology. Systematic magnetization measurements reveal a spin reorientation transition at ~ 375 K, indicating a crossover from easy-plane to easy-axis magnetic anisotropy. Below the spin reorientation temperature, a large topological Hall effect is observed, arising from microscopic noncoplanar spin structures. Lorentz transmission electron microscopy shows that mesoscopic skyrmions are stabilized in the easy-axis magnetic anisotropy regime and persist over an exceptionally wide temperature window of 375-650 K, far exceeding that of most previously reported skyrmion-hosting materials. These results establish Fe3Ge as a promising platform for exploring diverse topological properties, with strong potential for advancing future high-temperature spintronic applications, ranging from next-generation information storage to logic computing devices.

cond-mat.mtrl-sci↗

arXiv2026

Optomagnetic non-thermal modification of the ferromagnetic resonance

We investigate the photoinduced shift of the ferromagnetic resonance (FMR) frequency in magnets caused by the inverse Cotton-Mouton effect (ICME) under linearly polarized light. Using a Lagrangian description of magnetization dynamics, we derive the equations of motion, and obtain analytical expressions for the resonance frequency in both in-plane and out-of-plane equilibrium configurations. The theory shows that the FMR frequency depends on the polarization angle and propagation direction of light, with ICME producing a frequency shift that can dominate over thermal effects. The analytical results agree well with numerical simulations and with available experimental data for bismuth-substituted yttrium iron garnet, enabling estimation of the ICME contribution. These findings demonstrate that linearly polarized light can be used to control ferromagnetic resonance through magneto-optical effects.

cond-mat.mtrl-sci↗

arXiv2026

Emergence of multiple quasi-ferromagnetic magnon modes induced by strong magnetoelastic coupling in $TmFeO_3$ single crystal

We investigate the magnetization dynamics of $TmFeO_3$ single crystals across the spin-reorientation phase transition using broadband microwave absorption spectroscopy up to 87.5 GHz. Temperature- and magnetic-field-dependent antiferromagnetic resonance measurements reveal the characteristic softening of the quasi-ferromagnetic (q-FM) resonance mode at the $\Gamma_2\rightarrow\Gamma_{24}$ and $\Gamma_{24}\rightarrow\Gamma_4$ transition points. The finite magnon gap observed at the transition points reflects the strong magnetoelastic coupling. In addition to the uniform q-FM mode, multiple magnon modes appear in the intermediate $\Gamma_{24}$ phase, separated by approximately 0.5--2 GHz and exhibiting similar field and temperature dependence. These additional modes are attributed to nonuniform spin-wave excitations arising from the periodic magnetic domain structure present in the intermediate phase and their hybridization with acoustic phonons mediated by strong magnetoelastic coupling. Our results demonstrate that the spin-reorientation transition in $TmFeO_3$ provides a natural platform for generating multiple hybridized magnon modes, offering new opportunities for tunable magnonic excitations in rare-earth orthoferrites.

cond-mat.mtrl-sci↗

arXiv2026

Electrically Accessible Metamagnetic Transition via a Doping-Induced Low-Energy Magnetic State in Antiferromagnetic Insulator RFeO3

Low-energy antiferromagnetic phase transitions offer an appealing platform for low-power spintronic functionalities, yet their direct electrical access in insulating antiferromagnets remains challenging, particularly in the low-field regime where subtle Neeel vector reorientations dominate. Here, we demonstrate that targeted rare-earth-site engineering enables an electrically accessible metamagnetic transition in the insulating orthoferrite Ho0.5Dy0.5FeO3. By combining the distinct spin-reorientation sequences of DyFeO3 and HoFeO3, Dy substitution stabilizes a dual spin-reorientation pathway, hosting an intermediate state with a reduced energy barrier. This low-energy antiferromagnetic state can be tuned into the weak-ferromagnetic state under low magnetic fields. The critical field decreases with increasing temperature, providing a favorable window for functional manipulation. Both longitudinal and transverse spin Hall magnetoresistance channels exhibit clear and reproducible signatures of the metamagnetic transitions. Owing to the enhanced sensitivity of the transverse channel, additional low-field features are resolved, reflecting the projection of the Neel vector onto the spin-accumulation direction. Electrical transport measurements correlate directly with the magnetically determined phase boundaries, establishing a purely electrical access to low-energy phase transitions and to illustrate a viable pathway for exploring low-power spin dynamics in insulating oxide antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2025

Observation of cooperative strong coupling between optical phonon and crystal-field excitations in a pseudo Jahn-Teller system

Cooperative interactions between localized electronic excitations and crystal lattice are central to the emergence of complex structural phases in materials. However, the scaling relations governing these collective behaviors remain largely unexplored. Here, using magneto-Raman spectroscopy, we report the direct observation of the strongly coupled optical phonon and non-degenerate crystal-field excitations (CFEs) in ErFeO3. By independently tuning the effective population of Jahn-Teller-active erbium ions through temperature and chemical dilution with Jahn-Teller-inactive yttrium ions, we identify the coupling strength varies linearly with the square root of electronic excitations population. Notably, Y-doping reveals the hybridization gap reduces significantly faster than predicted by density scaling alone, indicating phonon coherence is essential for establishing this cooperative interaction. Our findings highlight the role of optical phonons in mediating short-range interactions that drive cooperative Jahn-Teller effect, evidencing the pathway for tailoring electronic and vibrational properties of Jahn-Teller materials through population control.

cond-mat.mtrl-sci↗

arXiv2025

Altermagnetic magnon transport in the \textit{d}-wave altermagnet \ch{LuFeO3}

Altermagnets exhibit a spin-split band structure despite having zero net magnetization, leading to special magnonic properties such as anisotropic magnon lifetimes and field-free spin transport. Here, we present a direct experimental demonstration of non-local magnon transport in the \textit{d}-wave altermagnet \ch{LuFeO3}, using both spin Seebeck and spin Hall effect-based injection and detection. We observe a non-local spin signal at zero magnetic field when the transport is along an altermagnetic direction, but not for transport along other directions. The observed sign reversal between two distinct altermagnetic directions in the spin Seebeck response demonstrates the altermagnetic nature of the magnon transport. In contrast, when transport is aligned along or perpendicular to the easy axis, both the first-harmonic signal and the sign-reversal effect vanish, consistent with symmetry-imposed suppression. These findings are supported by atomistic spin dynamics simulations, as well as linear spin wave theory calculations, which explain how our altermagnetic system hosts anisotropic spin Seebeck transport. Our results provide direct evidence of direction-dependent magnon splitting in altermagnets and highlight their potential for field-free magnonic spin transport, offering a promising pathway for low-power spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Weak low-temperature ferromagnetism and linear magnetoresistance in Lu$_{0.75}$Fe$_6$Sn$_6$ with a disordered HfFe$_6$Ge$_6$-type structure

We report the synthesis of Lu$_{0.75}$Fe$_6$Sn$_6$ single crystals with a Fe-kagome lattice using a self-flux method. The crystal structure, magnetic, thermodynamic and electrical transport properties were investigated. Structure refinement reveals that Lu$_{0.75}$Fe$_6$Sn$_6$ has a HfFe$_6$Ge$_6$-type structure as the major framework intergrown with a CoSn-type structure, leading to a vacancy of 25% on the Lu-site and disorder on the Sn-site. It exhibits a significant magnetic anisotropy with weak ferromagnetism in the ab-plane below 40 K and antiferromagnetic behavior along the c-axis. The weak ferromagnetism is due to the canted antiferromagnetism with magnetic moment deviating from the $c$-axis to the ab-plane. Besides, an anisotropic non-saturated linear magnetoresistance is also observed in Lu$_{0.75}$Fe$_6$Sn$_6$, probably resulting from the structural disorder in the sample.

cond-mat.mtrl-sci↗

arXiv2024

Anisotropic transport properties and topological Hall effect in the annealed kagome antiferromagnet FeGe

Electron correlation often gives birth to various orders in quantum materials. Recently, a strongly correlated kagome antiferromagnet FeGe is discovered to undergo a charge density wave transition inside the A-type antiferromagnetic state, providing an opportunity to explore the interplay between charge order and magnetism. Here, we reported the observation of anisotropic resistivity and Hall effect, along with a topological Hall effect, in the annealed FeGe crystals. As the current flows along the \emph{ab}-plane, the temperature dependence of $\rho_{ab}$ exhibits a distinct resistivity loop related to a first-order transition at $T_{cdw}$. The applied magnetic fields do not alter $T_{cdw}$ but can induce a spin-flop transition at $H_{sf}$. Consequently, a field-induced large topological Hall effect is observed in the canting antiferromagnetic (CAFM) state below $T_{cant}$, which is possibly attributed to the non-trivial spin texture during the spin-flop process. Whereas, as current is parallel to \emph{c}-axis, both the field-induced transitions in $\rho_{c}$ and $\chi_{c}$ disappear. Instead, the Hall resistivity in the annealed FeGe significantly exhibits a deviation from the linear field-dependent. These findings provide valuable insight into revealing the interplay among magnetism, charge order and topology in the kagome magnets.

cond-mat.str-el↗

arXiv2024

Terahertz stimulated parametric downconversion of a magnon mode in an antiferromagnet

In condensed matter systems, interactions between collective modes offer avenues for nonlinear coherent manipulation of coupled excitations and quantum phases. Antiferromagnets, with their inherently coupled magnon modes, provide a promising platform for nonlinear control of microscopic spin waves and macroscopic magnetization. However, nonlinear magnon-magnon interactions have been only partially elaborated, leaving key gaps in the prospects for potential ultrahigh-bandwidth magnonic signal processing. Here, we use a pair of intense terahertz pulses to sequentially excite two distinct coherent magnon modes in an antiferromagnet and find that the magnon mode with a lower frequency undergoes amplification when the higher-frequency mode is driven. We unveil the nonlinear excitation pathways of this stimulated parametric downconversion process by using polarization-selective two-dimensional terahertz spectroscopy. Our work provides fundamental insights into nonlinear magnonics in antiferromagnets, laying the groundwork for forthcoming spintronic and magnonic devices based on nonlinear magnon-magnon interactions.

cond-mat.mtrl-sci↗

arXiv2024

Annealing-induced long-range charge density wave order in magnetic kagome FeGe: fluctuations and disordered structure

Charge density wave (CDW) in kagome materials with the geometric frustration is able to carry unconventional characteristics. Recently, a CDW has been observed below the antiferromagnetic order in kagome FeGe, in which magnetism and CDW are intertwined to form an emergent quantum ground state. However, the CDW is only short-ranged and the structural modulation originating from it has yet to be determined experimentally. Here we realize a long-range CDW order by post-annealing process, and resolve the structure model through single crystal x-ray diffraction. Occupational disorder of Ge resulting from short-range CDW correlations above $T_\mathrm{CDW}$ is identified from structure refinements. The partial dimerization of Ge along the $c$ axis is unveiled to be the dominant distortion for the CDW. Occupational disorder of Ge is also proved to exist in the CDW phase due to the random selection of partially dimerized Ge sites. Our work provides useful insights for understanding the unconventional nature of the CDW in FeGe.

cond-mat.str-el↗

arXiv2024

Charge density wave with suppressed long-range structural modulation in canted antiferromagnetic kagome FeGe

Kagome lattice can host abundant exotic quantum states such as superconductivity and charge density wave (CDW). Recently, successive orders of A-type antiferromagnetism (AFM), CDW and canted AFM have been manifested upon cooling in kagome FeGe. However, the mechanism of CDW and interaction with magnetism remains unclear. Here we investigate the evolution of CDW with temperature across the canted AFM by single-crystal x-ray diffraction, scanning tunneling microscope (STM) and resonant elastic x-ray scattering (REXS). Interestingly, CDW-induced superlattice reflections become weak after the canted AFM, although long-range CDW order is still detectable by STM and REXS. We uncover a novel long-range CDW order with suppressed structural modulation, likely due to the competition for the underlying crystal structure between CDW and canted AFM. Additionally, occupational modulations of Ge1 in the kagome plane and displacive modulations of all atoms were extracted. The results confirm Ge dimerization along the c axis and suggest a dynamic transformation between different CDW domains.

cond-mat.str-el↗

arXiv2024

Observation of the Magnonic Dicke Superradiant Phase Transition

Two-level atoms coupled with single-mode cavity photons are predicted to exhibit a quantum phase transition when the coupling strength exceeds a critical value, entering a phase in which atomic polarization and photonic field are finite even at zero temperature and without external driving. However, this phenomenon, the superradiant phase transition (SRPT), is forbidden by a no-go theorem due to the existence of the diamagnetic term in the Hamiltonian. Here, we present spectroscopic evidence for a magnonic SRPT in ErFeO$_3$, where the role of the photonic mode (two-level atoms) in the photonic SRPT is played by an Fe$^{3+}$ magnon mode (Er$^{3+}$ spins). The absence of the diamagnetic term in the Fe$^{3+}$-Er$^{3+}$ exchange coupling ensures that the no-go theorem does not apply. Terahertz and gigahertz magnetospectroscopy experiments revealed the signatures of the SRPT -- a kink and a softening, respectively, of two spin-magnon hybridized modes at the critical point.

quant-ph↗

arXiv2023

Large Energy Shifts of Crystal-field Excitations in Erbium Orthoferrite Driven by Internal Magnetic Fields

Due to the complex interactions between rare-earth elements and transition metals, as well as between themselves, rare-earth transition-metal oxides are likely to exhibit highly intriguing and novel magnetic structures and dynamic behaviours. Rare-earth elements in these compounds frequently demonstrate unusual behaviours in their crystal-field (CF) excitations, which necessitate thorough studies for in-depth comprehensions. When cooling from 10 K to 1.5 K through the magnetic ordering temperature of $Er^{3+}$ at 4.1 K, we observed a significant energy shift of the low-lying CF excitation of $Er^{3+}$ in erbium orthoferrite ($ErFeO_3$) from 0.32 meV to 0.75 meV utilizing the inelastic neutron-scattering technique. A sound CF model was proposed for $Er^{3+}$ in $ErFeO_3$ by fitting to the observed CF excitation peaks, which enables to explain all the observed experimental results in a very consistent manner. According to the model, the ground crystal field level of $Er^{3+}$, which corresponds to the lowest Kramers doublet supposed to be at zero energy transfer, has been shifted by the internal magnetic fields induced by both $Er^{3+}$ and $Fe^{3+}$ spin orders below and above the $Er^{3+}$ ordering temperature, respectively. Additional measurements in various magnetic fields offer compelling evidence in favour of this hypothesis. The measured external field dependence of the CF excitation energy led to a derivation of the internal field of $Er^{3+}$ as 0.54 T, which is strongly corroborated by theoretical modelling. Additionally, the g-factor for the $Er^{3+}$ ground state in $ErFeO_3$ shows an exceptionally significant anisotropy.

cond-mat.str-el↗

arXiv2023

Giant shifts of crystal-field excitations in ErFeO3 driven by internal magnetic fields

Due to the complex interactions between rare-earth elements and transition metals, as well as/or themselves, rare-earth transition-metal oxides are likely to exhibit highly intriguing and novel magnetic structures and dynamic behaviours. Rare-earth elements in these compounds frequently demonstrate unusual behaviours in their crystal-field (CF) excitations, which necessitate thorough research for in-depth comprehensions. When cooling from 10 K to 1.5 K via the magnetic ordering temperature of Er3+ at 4.1 K, we observed a significant energy shift of the low-lying CF excitation of Er3+ in ErFeO3 from 0.35 meV to 0.75 meV utilizing the inelastic neutron-scattering technique. A sound CF model was proposed for Er3+ in ErFeO3 by fitting to the observed CF excitation peaks, which enables to explain all the observed experimental results in a very consistent manner. According to the model, the ground crystal field level of Er3+, which corresponds to the lowest Kramers doublet supposed to be at zero energy, has been shifted by the internal magnetic fields induced by both Er3+ and Fe3+ spin orders below and above the Er3+ ordering temperature, respectively. Additional measurements in various magnetic fields offer compelling evidence in favour of this hypothesis. The measured external field dependence of the CF excitation energy led to the derivation of the internal field of Er3+ as 0.33 meV, which is strongly corroborated by theoretical modelling. Additionally, the effective g-factor for Er3+ in ErFeO3 showed an exceptionally significant anisotropy.

cond-mat.str-el↗

arXiv2023

Terahertz field-driven magnon upconversion in an antiferromagnet

Tailored light excitation and nonlinear control of lattice vibrations have emerged as powerful strategies to manipulate the properties of quantum materials out of equilibrium. Generalizing the use of coherent phonon-phonon interactions to nonlinear couplings among other types of collective modes would open unprecedented opportunities in the design of novel dynamic functionalities in solids. For example, the collective excitations of magnetic order -- magnons -- can carry information with little energy dissipation, and their coherent and nonlinear control would provide an attractive route to achieve collective-mode-based information processing and storage in forthcoming spintronics and magnonics. Here, we discover that intense terahertz (THz) fields can initiate processes of magnon upconversion mediated by an intermediate magnetic resonance. By using a suite of advanced spectroscopic tools, including a newly demonstrated two-dimensional (2D) THz polarimetry technique enabled by single-shot detection, we unveil the unidirectional nature of coupling between distinct magnon modes of a canted antiferromagnet. Calculations of spin dynamics further suggest that this coupling is a universal feature of antiferromagnets with canted magnetic moments. These results demonstrate a route to inducing desirable energy transfer pathways and THz-induced coupling between coherent magnons in solids and pave the way for a new era in the development of ultrafast control of magnetism.

cond-mat.mtrl-sci↗

arXiv2023

Quantum Simulation of an Extended Dicke Model with a Magnetic Solid

The Dicke model describes the cooperative interaction of an ensemble of two-level atoms with a single-mode photonic field and exhibits a quantum phase transition as a function of light--matter coupling strength. Extending this model by incorporating short-range atom--atom interactions makes the problem intractable but is expected to produce new phases. Here, we simulate such an extended Dicke model using a crystal of ErFeO$_3$, where the role of atoms (photons) is played by Er$^{3+}$ spins (Fe$^{3+}$ magnons). Through magnetocaloric effect and terahertz magnetospectroscopy measurements, we demonstrated the existence of a novel atomically ordered phase in addition to the superradiant and normal phases that are expected from the standard Dicke model. Further, we elucidated the nature of the phase boundaries in the temperature--magnetic-field phase diagram, identifying both first-order and second-order phase transitions. These results lay the foundation for studying multiatomic quantum optics models using well-characterized many-body condensed matter systems.

quant-ph↗

arXiv2022

Magnetically Tuned Continuous Transition from Weak to Strong Coupling in Terahertz Magnon Polaritons

Depending on the relative rates of coupling and dissipation, a light-matter coupled system is either in the weak- or strong-coupling regime. Here, we present a unique system where the coupling rate continuously increases with an externally applied magnetic field while the dissipation rate remains constant, allowing us to monitor a weak-to-strong coupling transition as a function of magnetic field. We observed a Rabi splitting of a terahertz magnon mode in yttrium orthoferrite above a threshold magnetic field of ~14 T. Based on a microscopic theoretical model, we show that with increasing magnetic field the magnons transition into magnon polaritons through an exceptional point, which will open up new opportunities for in situ control of non-Hermitian systems.

cond-mat.mes-hall↗

arXiv2022

Spin and charge density waves in the quasi-one-dimensional KMn6Bi5

AMn6Bi5 materials (A = Na, K, Rb and Cs) consisting of unique Mn-cluster chains emerge as a new family of superconductors with the suppression of their antiferromagnetic (AFM) order under high pressures. Here, we report transverse incommensurate spin density waves (SDWs) for the Mn atoms with a propagating direction along the chain axes as a ground state for KMn6Bi5 by single crystal neutron diffraction. The SDWs have a refined amplitude of ~2.46 Bohr magnetons for the Mn atoms in the pentagons and ~0.29 Bohr magnetons with a large standard deviation for Mn atoms in the center between the pentagons. AFM dominate both the nearest-neighbor Mn-Mn interactions within the pentagon and next-nearest-neighbor Mn-Mn interactions out of the pentagon (along the propagating wave). The SDWs exhibit both local and itinerant characteristics probably formed by a cooperative interaction between local magnetic exchange and conduction electrons. A significant magnetoelastic effect during the AFM transition, especially along the chain direction, has been demonstrated by temperature-dependent x-ray powder diffraction. Single crystal x-ray diffraction below the AFM transition revealed satellite peaks originating from charge density waves along the chain direction with a q-vector twice as large as the SDW one, pointing to a strong real space coupling between them. Our work not only manifests a fascinating interplay among spin, charge, lattice and one dimensionality to trigger intertwined orders in KMn6Bi5 but also provides important piece of information for the magnetic structure of the parent compound to understand the mechanism of superconductivity in this new family.

cond-mat.str-el↗