arXiv subjects

Shufan Cheng

Publications and source records attributed to Shufan Cheng.

10 recordsLinked to original sources

arXiv2026

Sizable Ligand-Mediated Bond-Dependent Interactions in a Spin-1 Triangular Antiferromagnet NiI$_2$

The bond-dependent anisotropic Kitaev interactions are the key for the Kitaev model, which has attracted intense interest for its potential to host quantum-spin-liquid states and fractional excitations. However, experimental realizations of such interactions remain scarce. Here, we investigate the magnetic excitations of NiI$_2$, a van der Waals magnet with spin $S=1$. By combining inelastic neutron scattering, magnetization measurements, magnetic structure analysis, first-principles calculations, and linear-spin-wave simulations, we identify a minimal model that features substantial Kitaev and off-diagonal $\Gamma$ interactions, which together stabilize the canted magnetic ground state and open a gap in the spin-wave spectrum. Notably, these interactions arise from strong spin-orbit coupling on the ligand ions, despite the quenched orbital moment of the magnetic Ni$^{2+}$ ions. Our results provide compelling experimental evidence for the ligand-driven Kitaev mechanism. This demonstrates a concrete pathway to generating strong bond-dependent anisotropy in systems where the magnetic ions themselves have weak spin-orbit coupling, thereby substantially broadening the range of potential Kitaev materials.

cond-mat.str-el↗

arXiv2026

Magnetic Signature of Chiral Phonons Revealed by Neutron Spectroscopy in Ferrimagnetic Fe$_{1.75}$Zn$_{0.25}$Mo$_3$O$_8$

Lattice vibrations can carry angular momentum and magnetic moments under broken inversion or time-reversal symmetry, forming so-called chiral phonons. While such excitations have been explored in nonmagnetic systems via optical probes, their direct detection in magnetic materials and coupling to spin excitations remain largely unexplored. Here, using neutron spectroscopy, sensitive to both nuclear and magnetic scattering, we reveal the magnetic signature of chiral phonons in ferrimagnetic Fe$_{1.75}$Zn$_{0.25}$Mo$_3$O$_8$ with Curie temperature $T_{\rm C}\sim49$ K. Below $T_{\rm C}$, we observe enhanced magnetic scattering of phonons at small momenta, arising from strong magnon-phonon coupling. In addition, out-of-plane intensity modulation, phonon mode splitting, and field-induced Zeeman shifts are observed, all closely associated with the ferrimagnetic order. These features vanish above $T_{\rm C}$, where phonon spectra are dominated by nuclear scattering. These observations demonstrate the existence of chiral phonons carrying substantial magnetic moments that directly contribute to magnetic scattering, and establish neutron spectroscopy as a powerful, momentum-resolved probe of their magnetic character.

cond-mat.str-el↗

arXiv2025

Disorder-induced spin excitation continuum and spin-glass ground state in the inverse spinel CuGa$_2$O$_4$

Spinel-structured compounds serve as prototypical examples of highly frustrated systems, and are promising candidates for realizing the long-sought quantum spin liquid (QSL) state. However, structural disorder is inevitable in many real QSL candidates and its impact remains a topic of intense debate. In this work, we conduct comprehensive investigations on CuGa$_2$O$_4$, a spinel compound with significant structural disorder, focusing on its thermodynamic properties and spectroscopic behaviors. No long-range magnetic order is observed down to $\sim$80 mK, as evidenced by magnetic susceptibility, specific heat and elastic neutron scattering measurements. More intriguingly, inelastic neutron scattering experiments reveal a broad gapless continuum of magnetic excitations around the Brillouin zone boundary, resembling the magnetic excitation spectra expected for a QSL. Nevertheless, a spin-freezing transition at $T_{\rm{f}} \approx $ 0.88 K is identified from the cusp in the dc susceptibility curves, where a bifurcation between zero-field-cooling and field-cooling curves occurs. Furthermore, ac susceptibility measurements show a peak close to $T_{\rm{f}}$ at low frequency, which shifts to higher temperature with increasing frequency. These results are evident that CuGa$_2$O$_4$ has spin-glass ground state, consistent with the establishment of short-range order inferred from the specific heat measurements. Collectively, these results illustrate the crucial role of disorder in defining the excitation spectrum out of the disordered ground state. Our findings shed light onto the broader class of AB$_2$O$_4$ spinels and advance our understanding of the spin dynamics in magnetically disordered systems.

cond-mat.str-el↗

arXiv2025

Significant Chiral Magnetotransport Magnified by Multiple Weyl Nodes

The intertwining of magnetism with topology is known to give rise to exotic quantum phenomena. Here, we explore the magnetotransport properties of NdAlSi, a magnetic Weyl semimetal that spontaneously breaks inversion and time-reversal symmetries and hosts a large number of Weyl nodes. We observe a significant negative magnetoresistance, which we attribute to the chiral anomaly associated with multiple Weyl nodes. Remarkably, the extracted chiral coefficient reaches approximately $52~\mathrm{m\Omega}^{-1}~\mathrm{m}^{-1}~\mathrm{T}^{-2}$, larger than many other topological materials. Additionally, we observe an exotic anomalous Hall effect with an out-of-sync behavior, where the anomalous Hall resistance does not exactly follow the field dependence of the magnetization, in contrast to that in conventional ferromagnets. These rich quantum transport phenomena, driven by the interplay between magnetism and Weyl nodes, establish NdAlSi as a prime platform for exploring the intricate topological behaviors of magnetic Weyl semimetals.

cond-mat.mes-hall↗

arXiv2025

Magnetic Interactions in the Polar Ferrimagnet with a Bipartite Structure

The polar magnets A$_2$Mo$_3$O$_8$ (A=Fe, Mn, Co, and Ni) feature a bipartite structure, where the magnetic A$^{2+}$ ions occupy two different sites with octahedral and tetrahedral oxygen coordinations. This bipartite structure provides a platform for the emergence of nontrivial magnetoelectric (ME) effects and intriguing excitation behaviors, and thus creates significant research interest. In this study, we conduct inelastic neutron scattering measurements on single crystals of Mn$_2$Mo$_3$O$_8$, an L-type ferrimagnet in the A$_2$Mo$_3$O$_8$ family, to investigate its spin dynamics. The obtained magnetic excitation spectra reveal two distinct magnon dispersions corresponding to the octahedral and tetrahedral spins in Mn$_2$Mo$_3$O$_8$. These magnon bands can be well described by a spin Hamiltonian including Heisenberg and single-ion anisotropy terms. Employing our effective spin model, we successfully reproduce the unusual temperature dependence of the L-type ferrimagnetic susceptibility through self-consistent mean-field theory. This research reveals the significance of the bipartite structure in determining the excitation properties of the polar magnets $\rm{A_{2}Mo_{3}O_{8}}$ and provides valuable insights into the spin dynamics of L-type ferrimagnets.

cond-mat.str-el↗

arXiv2024

Absence of altermagnetic spin splitting character in rutile oxide RuO$_2$

Rutile RuO$_2$ has been posited as a potential $d$-wave altermagnetism candidate, with a predicted significant spin splitting up to 1.4 eV. Despite accumulating theoretical predictions and transport measurements, direct spectroscopic observation of spin splitting has remained elusive. Here, we employ spin- and angle-resolved photoemission spectroscopy to investigate the band structures and spin polarization of thin-film and single-crystal RuO$_2$. Contrary to expectations of altermagnetism, our analysis indicates that RuO$_2$'s electronic structure aligns with those predicted under non-magnetic conditions, exhibiting no evidence of the hypothesized spin splitting. Additionally, we observe significant in-plane spin polarization of the low-lying bulk bands, which is antisymmetric about the high-symmetry plane and contrary to the $d$-wave spin texture due to time-reversal symmetry breaking in altermagnetism. These findings definitively challenge the altermagnetic order previously proposed for rutile RuO$_2$, prompting a reevaluation of its magnetic properties.

cond-mat.mtrl-sci↗

arXiv2024

Spin and lattice dynamics of a van der Waals antiferromagnet MnPSe$_3$

Antiferromagnetic van der Waals family $\rm \textit{M}P\textit{X}_{3}\ (M=Fe,\ Mn,\ Co,\text{ and}\ Ni; X=S\text{ and}\ Se)$ have attracted significant research attention due to the possibility of realizing long-range magnetic order down to the monolayer limit. Here, we perform inelastic neutron scattering measurements on single crystal samples of MnPSe$_3$, a member of the $\rm \textit{M}P\textit{X}_{3}$ family, to study the spin dynamics and determine the effective spin model. The excited magnon bands are well characterized by a spin model, which includes a Heisenberg term with three intraplane exchange parameters ($J_{1}=-0.73$~meV, $J_{2}=-0.014$~meV, $J_{3}=-0.43$~meV) and one interplane parameter ($J_{c}=-0.054$~meV), and an easy-plane single-ion anisotropy term ($D=-0.035$~meV). Additionally, we observe the intersection of the magnon and phonon bands but no anomalous spectral features induced by the formation of magnon-phonon hybrid excitations at the intersecting region. We discuss possible reasons for the absence of such hybrid excitations in MnPSe$_3$.

cond-mat.str-el↗

arXiv2023

Magnon Damping as a Probe of Kondo Coupling in Magnetically Ordered Systems

In $d$-electron systems, there can also be intricate interplay between Kondo coupling and magnetic interactions as that in $f$-electron systems, but the underlying mechanism remains elusive. Here, using inelastic neutron scattering, we investigate the temperature evolution of the low-energy spin waves (magnons) in a metallic van der Waals ferromagnet Fe$_{3-x}$GeTe$_{2}$, and observe that the magnon damping diverges at both low and high temperatures and exhibits a minimum at an intermediate temperature. These behaviours are described by a formula that combines logarithmic and power-law terms, representing the dominant contributions from Kondo coupling and thermal fluctuations, respectively. These findings can be explained by considering electron-magnon scattering of spin-flip type within the ferromagnetic Kondo-Heisenberg lattice model, distinct from the original Kondo effect which only considers the coupling between itinerant electrons and isolated impurity spins. These results unveil the intriguing interplay between itinerant electrons and spin waves in metallic 3$d$-electron systems with magnetic order, and provide magnon damping as a new effective probe of Kondo coupling in metallic quantum magnets, thereby opening new avenues for exploring Kondo physics from the magnon perspective.

cond-mat.str-el↗

arXiv2023

Observation of a 1/3 Magnetisation Plateau Phase as Evidence for the Kitaev Interaction in a Honeycomb-Lattice Antiferromagnet

Fractional magnetisation plateaus, in which the magnetisation is pinned at a fraction of its saturated value within a range of external magnetic field, are spectacular macroscopic manifestations of the collective quantum behaviours. One prominent example of the plateau phase is found in spin-1/2 triangular-lattice antiferromagnets featuring strong geometrical frustration, and is often interpreted as quantum-fluctuation-stabilised state in magnetic field via the "order-by-disorder" mechanism. Here, we observe an unprecedented 1/3 magnetisation plateau between 5.2 and 7.4 T at 2 K in a spin-1 antiferromagnet Na$_3$Ni$_2$BiO$_6$ with a honeycomb lattice, where conventionally no geometrical frustration is anticipated. By carrying out elastic neutron scattering measurements, we propose the spin structure of the plateau phase to be an unusual partial spin-flop ferrimagnetic order, transitioning from the zigzag antiferromagnetic order in zero field. Our theoretical calculations show that the plateau phase is stabilised by the bond-anisotropic Kitaev interaction. These results provide a new paradigm for the exploration of rich quantum phases in frustrated magnets and exotic Kitaev physics in high-spin systems.

cond-mat.str-el↗

arXiv2022

Neutron spectroscopy evidence for a possible magnetic-field-induced gapless quantum-spin-liquid phase in a Kitaev material $α$-RuCl$_3$

As one of the most promising Kitaev quantum-spin-liquid (QSL) candidates, $α$-RuCl$_3$ has received a great amount of attention. However, its ground state exhibits a long-range zigzag magnetic order, which defies the QSL phase. Nevertheless, the magnetic order is fragile and can be completely suppressed by applying an external magnetic field. Here, we explore the evolution of magnetic excitations of $α$-RuCl$_3$ under an in-plane magnetic field, by carrying out inelastic neutron scattering measurements on high-quality single crystals. Under zero field, there exist spin-wave excitations near the $M$ point and a continuum near the $\mitΓ$ point, which are believed to be associated with the zigzag magnetic order and fractional excitations of the Kitaev QSL state, respectively. By increasing the magnetic field, the spin-wave excitations gradually give way to the continuous excitations. On the verge of the critical field $μ_0H_{\rm c}=7.5$ T, the former vanish and only the latter is left, indicating the emergence of a pure QSL state. By further increasing the field strength, the excitations near the $\mitΓ$ point become more intense. By following the gap evolution of the excitations near the $\mitΓ$ point, we are able to establish a phase diagram composed of three interesting phases, including a gapped zigzag order phase at low fields, possibly-gapless QSL phase near $μ_0H_{\rm c}$, and gapped partially polarized phase at high fields. These results demonstrate that an in-plane magnetic field can drive $α$-RuCl$_3$ into a long-sought QSL state near the critical field.

cond-mat.str-el↗