Welcome to the Neutron Scattering Group!

 

We are an experimental group utilizing neutron scattering technique combined with other probes such as X-ray scattering and transport to do cutting-edge research on quantum materials including: high-Tc superconductors, quantum spin liquids, and topological materials, etc. The scattering experiments are carried out in neutron and light sources overseas. At Nanjing University, we maintain a lab growing single crystals and performing characterizations. 


Please contact Prof. Wen by his email jwen@nju.edu.cn for any questions or collaboration opportunities. We look forward to your joining us!

Updates

 

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 Fe3-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 Fe0.98Te0.5Se0.5” 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 Fe0.98Te0.5Se0.5 and 10% Cu-doped Fe0.88Cu0.1Te0.5Se0.5 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 μB/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 Fe2.72GeTe2” 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 Fe2.72GeTe2, 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 Fe2.72GeTe2. 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 TN. 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.


 

2021-07-05

Topological magnon insulator spin excitations in the two-dimensional ferromagnet CrBr3

 

Our group published a paper entitled “Topological magnon insulator spin excitations in the two-dimensional ferromagnet CrBr3” as a Letter in PRB [Phys. Rev. B 102, L020402 (2021)]. Topological magnons are bosonic analogues of topological fermions in electronic systems. They have beenstudied extensively by theory but rarely realized by experiment. Here, by performing inelastic neutron scattering measurements on single crystals of a two-dimensional ferromagnet CrBr3, which was classified as Dirac magnon semimetal featured by the linear bands crossing at the Dirac points, we fully map out the magnetic excitation spectra, and reveal that there is an apparent gap of ∼3.5 meV between the acoustic and optical branches of the magnons at the K point. By collaborative efforts between experiment and theoretical calculations using a five orbital Hubbard model obtained from first-principles calculations to derive the exchange parameters, we find that a Hamiltonian with Heisenberg exchange interactions, next-nearest-neighbor Dzyaloshinskii-Moriya (DM) interaction, and single-ion anisotropy is more appropriate to describe the system. Calculations using the model show that the lower and upper magnon bands separated by the gap exhibit Chern numbers of ±1. These results indicate that CrBr3 is a topological magnon insulator, where the nontrivial gap is a result of the DM interaction.


 

2021-06-17

Group members received "Outstanding Oral Presentation Award"!

 

Song Bao and Zhengwei Cai received "Outstanding Oral Presentation Award" in the 5th Jiangsu Physical Society Spring Conference, with talks entitled "Coexistence and interplay of itinerant and local moments in the van der Waals metallic ferromagnet Fe3-xGeTe2", and "Neutron Scattering and Transport Studies on Topological Magnon Insulator CrBr3", respectively. Congratulations to them!

 

 

2021-06-06

​Congratulations to Song and Zhengwei for successfully defending their theses !

 

Song and Zhengwei successfully defended their PhD Theses on May 27th. Their outstanding works have been highly appreciated by the Defense Committee. They will be awarded the PhD degree soon. Congratulations, Dr. Bao and Dr. Cai! The group thank them for their great contributions and wish both of them a wonderful career!

 

2021-05-17

Prof. Wen was awarded "Jiangsu Distinguished Young Scholar in Physics"!

 

Jiangsu Physical Society (JSPS) awarded Prof. Wen "Jiangsu Distinguished Young Scholar in Physics" on July, 2020. Congratulations to him!

 

2021-01-08

Prof. Wen was awarded "Jiangsu Provincial Science and Technology Medal for Young Scientists"!

 

On January 7th, 2021, Prof. Wen received “Jiangsu Provincial Science and Technology Medal for Young Scientists " for his accomplishment in cutting-edge research in experimental condensed matter physics, dedication in teaching and educating students, and contributions to the public service. Congratulations to him! More details can be found here.

 

2020-12-24

Prof. Wen received the honor of "My Favorite Supervisor in Graduate Career"!

 

Prof. Wen was honorably named as "My Favorite Supervisor in Graduate Career" by the unversity for his dedication and excellency in supervising graduate students. Congratulations to him!

 

2020-12-16

Disorder-induced spin-liquid-like behavior in kagome-lattice compounds

 

Our group published a paper entitled “Disorder-induced spin-liquid-like behavior in kagome-lattice compounds” in PRB [Phys. Rev. B 102, 224415 (2020)]. Quantum spin liquids (QSLs) are a novel state of matter that is subject to extensive research. However, the relationship between the ubiquitous disorder and the QSL behaviors is still unclear. In this work, to identify the role of disorder in the QSL candidates, we choose Tm3Sb3Zn2O14 and its sister compound Tm3Sb3Mg2O14 with quantifiable disorder, that is, strong and weak disorder in the former and latter, respectively, and investigate how disorder affects the material’s magnetic properties. The comparative and comprehensive experimental studies with magnetic susceptibility, ultralow-temperature specific heat, and inelastic neutron scattering (INS) show spin-liquid-like features in both compounds. However, we find in Tm3Sb3Zn2O14, which has strong disorder resulting from the random mixing of the magnetic Tm3+ and nonmagnetic Zn2+, that the low-energy magnetic excitations observed in the specific heat and INS measurements are substantially enhanced compared to those of Tm3Sb3Mg2O14, which has much less disorder. We believe that the effective spins of the Tm3+ ions in the Zn2+/Mg2+ sites give rise to the low-energy magnetic excitations, and the amount of the random occupancy determines the excitation strength. These results provide direct evidence of the mimicry of a QSL caused by disorder.

 

2020-11-06

Evidence of the Berezinskii-Kosterlitz-Thouless Phase in a frustrated magnet

 

Our group published a paper entitled “Evidence of the Berezinskii-Kosterlitz-Thouless Phase in a frustrated magnet” in Nature Communications [Nat. Commun. 11, 5631 (2020)] in collaboration with Prof. Weiqiang Yu’s group at RUC, Prof. Wei Li’s group at Beihang, Prof. Ziyang Meng’s group at HKU/IOP, and Prof. Yang Qi at Fudan. The Berezinskii-Kosterlitz-Thouless (BKT) mechanism, one of the main contributions leading to the Nobel Prize in Physics 2016 for “theoretical discoveries of topological phase transitions and topological phases of matter”, was proposed in 2D magnetic systems in early 1970s by two of the Nobel laureates (together with late Vadim Berezinskii) whom the mechanism was named after. It constitutes the first example of topological phase transition beyond the Landau-Ginzburg paradigm of order parameter and symmetry breaking. Such a topological phase transition has long been sought yet undiscovered directly in magnetic materials. In this work, we pin down two transitions that bound a BKT phase in an ideal 2D frustrated magnet TmMgGaO4, via nuclear magnetic resonance under in-plane magnetic fields, which do not disturb the low-energy electronic states and allow BKT fluctuations to be detected sensitively. Moreover, by applying out-of-plane fields, we find a critical scaling behavior of the magnetic susceptibility expected for the BKT transition. The experimental findings can be explained by quantum Monte Carlo simulations applied on an accurate triangular-lattice Ising model of the compound which hosts a BKT phase. These results provide a concrete example for the BKT phase and offer an ideal platform for future investigations on the BKT physics in magnetic materials. More details can be found here.


 

2020-08-26

Group members awarded for "Excellent Doctoral Thesis"!

 

Kejing Ran and Jinghui Wang, both graduated from our group in 2019, have been awarded "Excellent Doctoral Thesis" by the University, with Theses entitled "Neutron Scattering Investigations on a Kitaev Quantum Spin Liquid Candidate α-RuCl3", and "Neutron Scattering Study on Unconventional Superconductors", respectively.  As their advisor, Prof. Wen was awarded  "Advisor of Excellent Doctoral Thesis" accordingly. Congratulations to them and the group!

 

 

2020-07-22

Congratulations to Yuan and Zhen for successfully defending their theses !

 

Yuan and Zhen successfully defended their PhD Theses on July 21st. Their outstanding works have been highly appreciated by the Defense Committee. They will be awarded the PhD degree soon. Congratulations, Dr. Gan and Dr. Ma! The group thank them for their great contributions and wish both of them a wonderful career!

 

2020-06-12

Evidence for magnon-phonon coupling in the topological magnet Cu3TeO6

 

Our group published a paper entitled "Evidence for magnon-phonon coupling in the topological magnet Cu3TeO6" in PRB [Phys. Rev. B 101, 214419 (2020)]. In this work, we report comprehensive results of thermodynamic and inelastic neutron scattering (INS) measurements on a cubic collinear antiferromagnet Cu3TeO6, which hosts topological magnons. We find the measured thermal conductivity deviates from a pure phononic model, indicating that there is a strong coupling between magnons and phonons. Our INS measurements find an additional mode located at about 16.8 meV, which exhibits a slight downward dispersion around the zone center and disappears above the TN. The dispersion and temperature dependence of this mode is inconsistent with it being either a pure magnon or a phonon. Instead, we attribute it to a magnon-polaron mode, collective excitations resulting from the hybridization between magnons and phonons. We consider the suppression of thermal conductivity and the emergence of the magnon-polaron mode to be evidence for magnon-phonon coupling in Cu3TeO6. Our work shows that Cu3TeO6 provides a rare platform to investigate the interplay between topological magnons and phonons in a three-dimensional collinear antiferromagnetic case.


 

2020-04-08

Spin dynamics of a magnetic Weyl semimetal Sr1−xMn1−ySb2

 

Our group published a paper entitled "Spin dynamics of a magnetic Weyl semimetal Sr1−xMn1−ySb2" in PRB [Physical Review B 101, 134408 (2020)]. In this work, we report comprehensive inelastic neutron scattering measurements (INS) on single crystals of Sr1−xMn1−ySb2, which have been well characterized by magnetization and magnetotransport measurements, both of which demonstrate that the material is a topologically nontrivial semimetal. The INS spectra can be well described by a localized Heisenberg model with SJ1 ∼ 28.0 meV, SJ2 ∼ 9.3 meV, a small SJc ∼ −0.1 meV, and SD ∼ −0.07 meV. Despite the coexistence of Weyl fermions and magnons, we find no clear evidence that the magnetic dynamics are influenced by the Weyl fermions in this material, possibly because that the Weyl fermions and magnons reside in separated layers, and the interlayer coupling is weak due to the quasi-two-dimensional nature of the material.

 

2019-06-27

Evidence for singular-phonon-induced nematic superconductivity in Sr0.1Bi2Se3

 

Our group published a paper entitled "Evidence for singular-phonon-induced nematic superconductivity in a topological superconductor candidate Sr0.1Bi2Se3" in Nature Communications [Nat. Commun. 10, 2802 (2019)]. In this work, by performing inelastic neutron scattering measurements on a superconducting single crystal of Sr0.1Bi2Se3, a prime candidate for realizing topological superconductivity, we find that there exist highly anisotropic phonons, with the linewidths of the acoustic phonons increasing substantially at long wavelengths, but only for those along the [001] direction. This observation indicates a large and singular electron-phonon coupling at small momenta, which we propose to give rise to the exotic p-wave nematic superconducting pairing. Therefore, we show MxBi2Se3 (M = Cu, Sr, Nb) to be example systems where electron-phonon interaction can induce more exotic superconducting pairing than the s-wave, consistent with the topological superconductivity. More details can be found here.

 

 

2019-05-20

Congratulations to Kejing and Jinghui for successfully defending their theses !

 

Kejing and Jinghui successfully defended their PhD Theses on May 20th. Their outstanding works have been highly appreciated by the Defense Committee. They will be awarded the PhD degree soon. They continue their further research at ShanghaiTech University. Congratulations, Dr. Ran and Dr. Wang! The group thank them for their great contributions and wish both of them a wonderful career!