2024-11-16

Group members awarded for "Excellent Doctoral Thesis"!

Yanyan Shangguan, graduated from our group in 2023, was awarded "Excellent Doctoral Thesis" by the University, with Thesis entitled "Neutron Scattering Investigations on Quantum Magnets Na₃Ni₂BiO₆ and Na₂Cu₂TeO₆". As her advisor, Prof. Wen was awarded "Advisor of Excellent Doctoral Thesis" accordingly. Congratulations to them and the group!

2024-10-28

Absence of Altermagnetic Spin Splitting Character in Rutile Oxide RuO2

Our research group, in collaboration with Professor Dawei Shen from the University of Science and Technology of China, has published a PRL paper on RuO₂, a candidate material of altermagnetism [Phys. Rev. Lett. 133, 176401 (2024)]. 

Altermagnetism, a novel magnetic ordering state described based on spin space groups rather than magnetic space groups, has recently been theoretically predicted and has garnered significant attention. It is characterized by unconventional antiferromagnetic-like magnetic order with zero net magnetization, while exhibiting ferromagnetic-like-split energy bands. These features endow altermagnets with great potential for applications in spintronic devices. 

RuO₂ was previously considered to be one of the most promising candidate materials for altermagnetism. In this work, we investigate the electronic structure of RuO₂ to explore whether it is an altermagnet or not. We grew high-quality RuO₂ single crystals. Utilizing spin- and angle-resolved photoemission spectroscopy, our collaborators find that the band structure of RuO₂ indeed aligns with the non-magnetic ground state. Meanwhile, we identify anomalous in-plane polarization of the low-lying bulk bands that contradict the d-wave symmetry predicted for altermagnetism. These findings challenge the altermagnetic order previously proposed for RuO₂, prompting a reevaluation of its magnetic properties.

2024-06-08

Spin and lattice dynamics in the van der Waals antiferromagnet MnPSe3

Our group published a paper entitled "Spin and lattice dynamics in the van der Waals antiferromagnet MnPSe₃" in Physical Review B [Phys. Rev. B 109, 224411 (2024)]. MnPSe₃ is a member of the antiferromagnetic van der Waals family MPX₃ (where M can be Fe, Mn, Co, or Ni, and X can be S or Se). In our study, we performed inelastic neutron scattering measurements on single-crystal samples of MnPSe₃ to investigate the spin dynamics and determine the effective spin model. The magnon bands observed are well-described by a spin model that includes a Heisenberg term with three intraplane exchange parameters (J₁=−0.73 meV, J₂=−0.014 meV, J₃=−0.43 meV) and one interplane exchange parameter (J_c=−0.054 meV), along with an easy-plane single-ion anisotropy term (D=−0.035 meV). Additionally, we observed the intersection of magnon and phonon bands. However, in the intersecting region, no anomalous spectral features indicating the formation of magnon-phonon hybrid excitations were detected. This work provides comprehensive characterizations on the spin and lattice dynamics in this material.

2024-04-24

Possible gapless quantum spin liquid behavior in the triangular-lattice Ising antiferromagnet PrMgAl11O19

Our group published a paper entitled “Possible gapless quantum spin liquid behavior in the triangular-lattice Ising antiferromagnet PrMgAl₁₁O₁₉” in PRB [Phys. Rev. B 109, 165143 (2024)] in collaboration with the group led by our group alumni Prof. Zhen Ma at Hubei Normal University. Quantum spin liquids (QSLs) represent a novel state where spins are highly entangled but do not order even at zero temperature due to strong quantum fluctuations. Such a state is mostly studied in Heisenberg models defined on geometrically frustrated lattices. In this work, we turn to a new triangular-lattice antiferromagnet PrMgAl₁₁O₁₉, in which the interactions are believed to be of Ising type. Magnetic susceptibility measured with an external field along the c axis is two orders of magnitude larger than that with a field in the a-b plane, showcasing an ideal easy-axis behavior. Meanwhile, there is neither magnetic phase transition nor spin freezing observed down to 1.8 K. Ultralow-temperature specific heat measured down to 50 mK does not capture any phase transition either, but a hump at 4.5 K, below which the magnetic specific heat exhibits a quasiquadratic temperature dependence that is consistent with a Dirac QSL state. Inelastic neutron scattering experiments reveal a gapless broad continuum of low-energy magnetic excitation at the base temperature 55 mK, in favor of the realization of a gapless QSL. These results provide a scarce example for the QSL behaviors observed in an Ising-type magnet, which can serve as a promising platform for future research on QSL physics based on an Ising model.

2024-01-30

Microscopic origin of the spin-reorientation transition in the kagome topological magnet TbMn6Sn6

Our group published a paper with the same title in Physical Review B, reporting the observation of magnetic excitations in a kagome topological magnet TbMn₆Sn₆ [Phys. Rev. B 109, 014434 (2024)]. The collaborative effort involved researchers from our School of Physics, as well as those from Nanjing University of Posts and Telecommunications, and Zhejiang University.

TbMn₆Sn₆, a correlated topological magnet featuring a Mn-based kagome lattice, exhibits a fascinating Chern gap opening at the Dirac point at low temperatures. In this study, we employed neutron scattering techniques to delve into the spin-reorientation transition in TbMn₆Sn₆. We find the coexistence of two Tb modes at 200 K, which can be understood using a model based on the SU(N) spin-wave theory. This model takes into account the temperature-dependent evolution of the ground state Tb 4f orbitals, influenced by factors such as crystalline electric field, single-ion anisotropy, and exchange interactions between Tb and Mn ions. The interplay leads to a change of the Tb 4f ground state and drives the spin-reorientation transition in TbMn₆Sn₆. Notably, our work highlights the similarity between the coupling of localized Tb 4f moments and itinerant Mn d electrons with the widely-discussed Kondo effect in heavy fermion systems where the Kondo physics is purely dominated by f electrons.

Building on our recent exploration of the Kondo effect in d-electron systems [Phys. Rev. X 12, 011022(2022)], this study broadens the applicability of Kondo physics to various classes of condensed matter systems. The intricate interplay of factors in TbMn₆Sn₆ successfully explain the spin-reorientation transition and underscores the versatility of Kondo physics in diverse condensed matter phenomena.

2023-12-31

Happy New Year！

Happy New Year to all!

Cheers to a brand new year filled with endless possibilities and exciting adventures!

2023-09-30

Topological magnon polarons in Fe2Mo3O8

Our group published a paper entitled “Direct observation of topological magnon polarons in a multiferroic material” in Nature Communications [Nat. Commun. 14, 6093 (2023)] in collaboration with Prof. Jian-Xin Li’s group at our School of Physics. Magnons and phonons, quanta of spin waves and lattice vibrations respectively, constitute two fundamental collective excitations in ordered magnets. When these two entities are strongly coupled, intriguing collective excitations known as magnon polarons can emerge. They possess hybrid magnonic and phononic signatures, and are responsible for many exotic spintronic and magnonic phenomena. They can provide a phonon-involved way to generate and manipulate spin currents carried by magnons thanks to their hybrid nature, signifying promising potentials in spintronics technology. Despite long-term sustained experimental efforts in chasing for magnon polarons, direct spectroscopic evidence of their existence is hardly observed. In this work, we report the direct observation of magnon polarons using neutron spectroscopy on a multiferroic Fe₂Mo₃O₈. We unambiguously identify two distinct hallmarks of magnon polarons: the appearance of a gap at the nominal intersection of the original magnon and phonon bands, along with mixing, interconverting and reversing between the magnonic and phononic components. We attribute the formation of magnon polarons to the strong magnon-phonon coupling induced by Dzyaloshinskii-Moriya interaction. Intriguingly, we find that the band-inverted magnon polarons are topologically nontrivial. These results uncover exotic elementary excitations arising from the magnon-phonon coupling, and offer a new route to topological states by considering hybridizations between different types of fundamental excitations. Our work, in conjunction with our recent publication in Nature Physics [Nat. Phys. 19, 1868 (2023)], underscores the emergence of novel phenomena driven by the strong magnon-phonon coupling. More details can be found here.

2023-09-26

Our group’s second paper in Nature Physics

Our group published a paper entitled “A one-third magnetization plateau phase as evidence for the Kitaev interaction in a honeycomb-lattice antiferromagnet” in Nature Physics [Nat. Phys. 19, 1883 (2023)] in collaboration with Prof. Jian-Xin Li’s group at our School of Physics, and Prof. Wei Li’s group at Institute of Theoretical Physics, CAS. 

Frustration plays an essential role in quantum magnets. The frustration-induced quantum fluctuation could avoid the formation of ordered magnetic ground states, and lead to magnetically disordered phases such as the quantum spin liquids. On the other hand, the quantum fluctuation can lift the degeneracy of the ground state and select a specific spin state within a finite range of external magnetic field; this gives rise to an exotic magnetization plateau phase in which the magnetization is a fraction of its saturation value. Since frustration is a prerequisite for the fractional magnetization plateau phase, whether it will occur in a genuine honeycomb lattice in which geometrical frustration as that occurs in triangular and Kagome lattices is absent, and how to understand it if it does occur, remain outstanding questions. In this work, we report comprehensive thermodynamic and neutron scattering measurements on high-quality single crystals of the spin-1 honeycomb-lattice antiferromagnet Na₃Ni₂BiO₆. We show that the magnetization curve has a definite plateau at 1/3 of the saturation magnetization between 5.2 and 7.4 T at 2 K. By performing elastic neutron scattering measurements, we obtain complete contour maps for the magnetic Bragg peaks in the (H, K, 0) plane in zero field and in the plateau phase. By comparing experimental results with calculations, we propose the microscopic magnetic configuration of the 1/3 magnetization plateau phase to be a zero-up-zero-down-up-up (○↑○↓↑↑) ferrimagnetic state. Our density-functional-theory and tensor-network calculations show that a minimal model with Heisenberg exchange couplings J, a bond-dependent anisotropic Kitaev interaction K and a single-ion anisotropy term D can well explain the experimental observations. In particular, the Kitaev interaction, which was demonstrated to exist in α-RuCl₃ in our early work [Phys. Rev. Lett. 118, 107203 (2017)], leads to the exchange frustration and stabilizes the one-third-plateau phase. This work not only extends the study of the fractional magnetization plateau phase to honeycomb-lattice compounds which conventionally do not exhibit geometrical frustrations, but also expands the territory of quantum magnets that host Kitaev physics from S = 1/2 to higher-spin systems. More details can be found here.

2023-09-15

Paper on Fe2Mo3O8 published in Nature Physics

Our group published a paper entitled “Fluctuation-enhanced phonon magnetic moments in a polar antiferromagnet” in Nature Physics [Nat. Phys. 19, 1868 (2023)] in collaboration with Prof. Qi Zhang’s group at our School of Physics. 

Phonons are the quasiparticles of collective lattice excitations that may carry finite angular momenta, but commonly exhibit negligible magnetic moments. A large phonon magnetic moment enables the direct mutual control of magnetic orders and lattice motions, and could be applied to develop spin–phononic devices. Despite reports of large phonon magnetic moments in some non-magnetic and paramagnetic systems, such phenomena have not yet been discovered in magnetically ordered systems. Furthermore, the roles of many-body correlations and fluctuations in phonon magnetism remain unclear. In this work, combining magneto-Raman spectroscopy and inelastic neutron scattering measurements on a polar antiferromagnet Fe₂Mo₃O₈, we show that a pair of low-lying chiral phonons carry large magnetic moments (0.11μ_B). Additionally, we observe a sixfold enhancement in the phonon magnetic moment in the vicinity of the Néel temperature. A microscopic model based on the coupling between phonons and both magnons and paramagnons accounts for the experimental observations. These findings present a new paradigm in the study of phonon magnetism, where the many-body correlations, instead of single-particle interactions, play key roles. It further establishes Fe₂Mo₃O₈ as an ideal platform for exploring phonon magnetism and developing hybrid phononic and spintronic devices. More details can be found here.

2023-05-21

Celebrating Our Achievers: Dr. Yanyan Shangguan, Aspiring Physicist, and Xiaoxue Zhao, Future High School Educator

We are thrilled to announce the successful thesis defenses of two of our distinguished students, Dr. Yanyan Shangguan and Xiaoxue Zhao, on May 19th.

Dr. Yanyan Shangguan has reached a significant milestone in her academic journey by successfully defending her PhD thesis. Her unwavering dedication, rigorous efforts, and intellectual prowess throughout her doctoral studies have led her to this momentous achievement. Her groundbreaking research in physics has deepened our understanding of the subject and promises to contribute further in the field as she continues her research journey. 

Just as commendable is Xiaoxue Zhao's successful Master's thesis defense. Her dedication, resilience, and exceptional scholarship throughout her Master's studies is truly inspiring. Her future as a high school teacher will undoubtedly be enriched by this accomplishment, and we anticipate that she will inspire a new generation of learners with her passion for education. 

Join us in congratulating Dr. Yanyan Shangguan and Xiaoxue Zhao on these notable accomplishments! We eagerly anticipate their future contributions to the worlds of physics and education, respectively.

2023-04-27

Signatures of a gapless quantum spin liquid in a Kitaev material Na3Co2-xZnxSbO6

Our group published a paper entitled “Signatures of a gapless quantum spin liquid in a Kitaev material Na₃Co_2-xZn_xSbO₆” in PRB [Phys. Rev. B 107, 165143 (2023)] in collaboration with the group led by our group alumni Prof. Zhen Ma at Hubei Normal University. In our recent work, we have demonstrated the significant role of Zn doping in suppressing the magnetic order and inducing quantum paramagnetic behaviors in a 3d based Kitaev magnet Na₂Co₂TeO₆ [Phys. Rev. Mater. 7, 014407 (2023)]. In this work, we show results associated with a gapless quantum spin liquid (QSL) in another Kitaev cobaltate Na₃Co_2-xZn_xSbO₆. X-ray diffraction characterizations reveal no structural transition but quite tiny changes on the lattice parameters over our substitution range 0≤x≤0.4. Magnetic susceptibility and specific heat results both show that antiferromagnetic (AFM) transition temperature is continuously suppressed with increasing Zn content x and neither long-range magnetic order nor spin freezing is observed when x≥0.2. More importantly, a linear term of the specific heat representing fermionic excitations is captured below 5 K in the magnetically disordered regime, as opposed to the C_m∝T³ behavior expected for bosonic excitations in the AFM state. These results indicate the presence of gapless fractional excitations in the samples with no magnetic order, evidencing a potential QSL state induced by doping in a Kitaev system.

2023-03-10

Prof. Wen received the “Outstanding Reviewer Awards” from IOP Publishing

Prof. Wen was recognized as an Outstanding Reviewer for Chinese Physics Letters for the year 2022 and granted the “Outstanding Reviewer Awards” from IOP Publishing. Congratulations!

2023-01-20

Suppression of the antiferromagnetic order by Zn doping in a possible Kitaev material Na2Co2TeO6

Our group published a paper entitled “Suppression of the antiferromagnetic order by Zn doping in a possible Kitaev material Na₂Co₂TeO₆” in PRM [Phys. Rev. Mater. 7, 014407 (2023)] in collaboration with the group led by our group alumni Prof. Zhen Ma at Hubei Normal University. A 3d based honeycomb cobaltate Na₂Co₂TeO₆ has attracted enormous attention due to the proposed proximity to the Kitaev spin-liquid state as its 4d/5d counterparts. In this work, we partially substitute magnetic Co²⁺ with nonmagnetic Zn²⁺ in Na₂Co₂TeO₆ in an extensive range and perform structural, magnetic, and thermodynamic studies to investigate the doping evolution of the magnetic ground states. X-ray diffractions reveal no structural transition but only minor changes on the lattice parameter c over a wide substitution range 0≤x≤1.5. Magnetic susceptibility and specific heat measurements both show a suppression of long-range magnetic order with increasing zinc content. After x∼1.0, it develops into a spin-glass state with short-range order, which is rapidly supplanted by a magnetically disordered state when x≥1.3. These results explicitly track the evolution process of the magnetic ground states and establish a magnetic phase diagram of Na₂Co_2−xZn_xTeO₆. Zn doping may serve as a feasible way to enhance quantum fluctuations and induce quantum paramagnetic behaviors that may provide insights about the Kitaev physics.

2022-12-31

Happy New Year!

Happy New Year to all! Wish all have a wonderful 2023! 

2022-11-21

Works on α-RuCl3 won "Best Poster Prize" at the CPS Fall Meeting!

Earlier this year, our group published two papers reporting evidence of the fractional excitations for the Kitaev quantum-spin-liquid candidate α-RuCl₃ in Chinese Physics Letters as Express Letters [CPL 39, 057501 (2022); CPL 39, 027501 (2022)]. Based on these results, Kejing Ran, former PhD student of the group and now working at ShanghaiTech, gave a poster presentation at the 2022 Chinese Physical Society's Fall Meeting held in Shenzhen. The poster entitled “Evidence for Magnetic Fractional Excitations in a Kitaev Quantum-Spin-Liquid Candidate” was awarded the "Best Poster Prize". Congratulations to Kejing and the group!

2022-09-27

Group members awarded for "Excellent Doctoral Thesis"!

Song Bao, graduated from our group in 2021, was awarded "Excellent Doctoral Thesis" by the University, with Thesis entitled "Spin and Lattice Dynamics of the Magnetic Topological Materials Cu₃TeO₆ and Fe_3-xGeTe₂".  As his advisors, Prof. Wen and our long-term collaborator, Prof. Li, were awarded "Advisor of Excellent Doctoral Thesis" accordingly. Congratulations to them and the group!

2022-08-23

Congratulations to Prof. Wen for being granted the Distinguished Young Scholars Fund (国家杰青) by NSFC

On August, 9th, Prof. Wen was officially notified by National Science Foundation of China (NSFC) that he was granted "The National Science Fund for Distinguished Young Scholars"（国家杰出青年科学基金）. Congratulations to him for this great achievement! This is also the accomplishment of the whole Neutron Scattering Group, and a wonderful outcome of the sustained efforts of all the former and present group members. With this generous funding support (4 million RMB) from NSFC, the group will work harder and more creatively on “Neutron Spectroscopy Study on Quantum Magnetism”, and make more breakthroughs by discovering novel quantum magnets and finding exotic elementary excitations, using the state-of-the-art neutron spectroscopy as well as thermal conductivity measurements.

2022-06-22

Enhanced low-energy magnetic excitations evidencing the Cu-induced localization in the Fe-based superconductor Fe0.98Te0.5Se0.5

Our group published a paper entitled “Enhanced low-energy magnetic excitations evidencing the Cu-induced localization in the Fe-based superconductor Fe_0.98Te_0.5Se_0.5” in PRB [Phys. Rev. B 105, 245129 (2022)]. Understanding the doping effect is key to the understanding of the interplay between superconductivity and magnetism in iron-based superconductors. In this work, we have performed inelastic neutron scattering measurements on optimally doped Fe_0.98Te_0.5Se_0.5 and 10% Cu-doped Fe_0.88Cu_0.1Te_0.5Se_0.5 to investigate the doping effects on the spin excitations in the whole energy range up to 300 meV. It is found that substitution of Cu for Fe enhances the low-energy spin excitations (≤100 meV), especially around the (0.5, 0.5) point, and leaves the high-energy magnetic excitations intact. In contrast to the expectation that Cu with spin 1/2 will dilute the magnetic moments contributed by Fe with a larger spin, we find that the 10% Cu doping enlarges the effective fluctuating moment from 2.85 to 3.13 μ_B/Fe, although there is no long- or short-range magnetic order around (0.5, 0.5) and (0.5, 0). The presence of enhanced magnetic excitations in the 10% Cu doped sample which is in the insulating state indicates that the magnetic excitations must have some contributions from the local moments, reflecting the dual nature of the magnetism in iron-based superconductors. We attribute the substitution effects to the localization of the itinerant electrons induced by Cu dopants. These results also indicate that the Cu doping does not act as electron donor as in a rigid-band shift model, but more as scattering centers that localize the system, enhance the local moments and suppress the superconductivity.

2022-04-10

Neutron Spectroscopy Evidence for a Possible Magnetic-Field-Induced Gapless Quantum-Spin-Liquid Phase in a Kitaev Material α-RuCl3

Our group published a paper entitled “Neutron Spectroscopy Evidence for a Possible Magnetic-Field-Induced Gapless Quantum-Spin-Liquid Phase in a Kitaev Material α-RuCl₃” in CPL as Express Letter [Chin. Phys. Lett. 39, 057501 (2022)]. So far, α-RuCl₃ has been the most promising Kitaev quantum-spin-liquid (QSL) candidate, but 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. In this work, we explore the evolution of the magnetic excitations of α-RuCl₃ under an in-plane magnetic field, by carrying out inelastic neutron scattering measurements on high-quality single crystals. We find that on the verge of the critical field 7.5 T, the continuum representing the fractional excitations near the Γ point still exists, while the spin-wave excitations near the M point associated with the zigzag magnetic order vanish, which indicates the emergence of a pure QSL state. By following the gap evolution with field, we establish a three-zone phase diagram, containing a low-field gapped zigzag order phase, an intermediate-field gapless QSL, and a high-field gapped partially polarized state. These results demonstrate that an in-plane magnetic field can drive α-RuCl₃ into a long-sought QSL state near the critical field.

2022-02-03

Neutron Spectroscopy Evidence on the Dual Nature of Magnetic Excitations in a van der Waals Metallic Ferromagnet Fe2.72GeTe2

Our group published a paper entitled “Neutron Spectroscopy Evidence on the Dual Nature of Magnetic Excitations in a van der Waals Metallic Ferromagnet Fe_2.72GeTe₂” in PRX [Phys. Rev. X 12, 011022 (2022)]. Magnetism has been a long and mysterious issue in condensed matter physics. Its understanding can usually be divided into two opposite camps: the local-moment or itinerant picture. But in the intermediate range where both local moments and itinerant electrons are present, the nature of magnetism remains elusive. Here, by performing inelastic neutron scattering on a van der Waals metallic ferromagnet Fe_2.72GeTe₂, which can sustain tunable room-temperature ferromagnetism down to the monolayer limit, we found the spin excitations are composed of a dispersive mode at low energies and a columnlike continuum at high energies, resulting from local moments and itinerant electrons, respectively. Moreover, we also found that the low-energy spin waves at 100 K are more coherent than those at 4 K, which is evidence of the weakening of the Kondo screening at high temperatures. These results unambiguously demonstrate the coexistence of local moments and itinerant electrons and the Kondo effect between these two components in Fe_2.72GeTe₂. These findings shed light on the understanding of magnetism in transition-metal compounds. More details can be found here.

2021-12-31

Happy New Year!

Happy New Year to all! Wish all have a wonderful 2022! 

2021-12-31

Disorder-induced broadening of the spin waves in the triangular-lattice quantum spin liquid candidate YbZnGaO4

Our group published a paper entitled “Disorder-induced broadening of the spin waves in the triangular-lattice quantum spin liquid candidate YbZnGaO₄” in PRB [Phys. Rev. B 104, 224433 (2021)]. Disorder is important in the study of quantum spin liquids (QSLs), but as we demonstrated previously, its role on the spin dynamics remains elusive [Phys. Rev. Lett. 120, 087201 (2018); Phys. Rev. B 102, 224415 (2020); Nat. Commun. 11, 5631 (2020)]. In this work, we explore the disorder effect by performing inelastic neutron scattering (INS) on the triangular-lattice QSL candidate YbZnGaO4 under a c-axis magnetic field. With an intermediate field of 2.5 T, the broad continuum of magnetic excitations measured at zero field becomes more smeared both in energy and momentum. When a field up to 10 T drives the system into a fully polarized state, we observe clear spin-wave excitations with a gap of 1.4 meV comparable to the bandwidth. However, different from the sharp and well-defined spin waves expected for a clean ferromagnetic state, the spectra exhibit strong broadening in energy and momentum. By considering the disorder effect arising from the random site mixing of nonmagnetic Zn²⁺ and Ga³⁺ ions, our classical Monte Carlo simulations can well reproduce the INS spectra both at zero and high fields. These results elucidate the critical role of disorder in broadening the magnetic excitation spectra and mimicking the spin-liquid features in frustrated quantum magnets.

2021-12-31

Evidence for Magnetic Fractional Excitations in a Kitaev Quantum-Spin-Liquid Candidate α-RuCl3

Our group published a paper entitled "Evidence for Magnetic Fractional Excitations in a Kitaev Quantum-Spin-Liquid Candidate α-RuCl₃" in CPL as Express Letter [Chin. Phys. Lett. 39, 027501 (2022)]. α-RuCl₃ is by far the most promising candidate for Kitaev quantum-spin-liquid (QSL) material. In 2017 and 2018, our group has determined the dominant Kitaev interaction and proposed the Kitaev-off-diagonal (Κ-Γ) model to describe the material, and tuned the material into the QSL phase by applying magnetic field [Phys. Rev. Lett. 118, 107203 (2017); Phys. Rev. Lett. 119, 227208 (2017); Phys. Rev. Lett. 120, 067202 (2018)]. However, there has not been concrete evidence for the fractional excitations resulting from the Kitaev QSL state so far. In this work, we present the first polarized inelastic neutron scattering study on α-RuCl₃ single crystals to explore the scattering continuum around the Γ point at the Brillouin zone center and obtain evidence for the magnetic fractional excitations. We find that there exist pure continuous magnetic excitations near the Γ point, which are robust against temperature, as opposed to the spin waves near the Μ point that vanish above T_N. In addition, by comparing the calculation results using Κ-Γ and Kitaev–Heisenberg models with the unpolarized neutron experimental data, we find that the Κ-Γ model with a ferromagnetic Κ = −7.2 meV and a comparable Γ = 5.6 meV can reproduce not only the spin-wave excitations near the Μ point, but also the continuous excitations near the Γ point. These results are evident that there exist exotic fractional excitations in α-RuCl₃ due to its proximity to the Kitaev QSL state, and further support the Κ-Γ model to be the minimal effective spin model in describing the system. More details can be found here.

2021-12-29

Evidence for strong correlations at finite temperatures in the dimerized magnet Na2Cu2TeO6

Our group published a paper entitled “Evidence for strong correlations at finite temperatures in the dimerized magnet Na₂Cu₂TeO₆” in PRB [Phys. Rev. B 104, 224430 (2021)]. Dimerized magnets forming alternating Heisenberg chains exhibit quantum coherence and entanglement and thus can find potential applications in quantum information and computation. However, magnetic systems typically undergo thermal decoherence at finite temperatures. In this work, we report comprehensive inelastic neutron scattering (INS) measurements on a dimerized magnet Na₂Cu₂TeO₆, which is identified to be an alternating antiferromagnetic-ferromagnetic (AFM-FM) chain compound. We find that the excited quasiparticles in Na₂Cu₂TeO₆ can counter thermal decoherence and maintain strong correlations at elevated temperatures. At low temperatures, our INS excitation spectra clearly identify Na₂Cu₂TeO₆ to be an alternating AFM-FM chain compound with weak but nonzero interchain coupling. More importantly, we find the energy scans show asymmetric line shapes at elevated temperatures different from conventional magnets, which is a manifestation of a strongly correlated state resulting from hard-core constraint and quasiparticle interactions. These results serve not only as evidence for strong correlations at finite temperatures in Na₂Cu₂TeO₆, but also for the universality of the strongly correlated state in a broad range of quantum magnetic systems.