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Field evolution of the magnetic structure and spin Hamiltonian in Cs$_2$RuO$_4$
Authors:
S. D. Nabi,
E. Ressouche,
D. G. Mazzone,
J. Lass,
R. Sibille,
Z. Yan,
S. Gvasaliya,
A. Zheludev
Abstract:
We report neutron diffraction under applied magnetic fields and complementary zero-field neutron spectroscopy measurements on Cs$_2$RuO$_4$. Previous work [S. D. Nabi et al., Phys. Rev. B 112, 134436 (2025)] identified a spin-flop-like transition accompanied by a quantum critical point within the ordered phase, attributed to strong frustration between alternating single-ion anisotropy planes. Here…
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We report neutron diffraction under applied magnetic fields and complementary zero-field neutron spectroscopy measurements on Cs$_2$RuO$_4$. Previous work [S. D. Nabi et al., Phys. Rev. B 112, 134436 (2025)] identified a spin-flop-like transition accompanied by a quantum critical point within the ordered phase, attributed to strong frustration between alternating single-ion anisotropy planes. Here, we quantitatively confirm the predicted field evolution of the magnetic structure using neutron diffraction. Furthermore, analysis of the excitation spectrum within an SU(3) spin-wave framework resolves previously undetermined parameters of the minimal spin Hamiltonian and lifts the associated degeneracies.
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Submitted 11 August, 2026; v1 submitted 22 May, 2026;
originally announced May 2026.
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Coherent spin waves in a maximal entropy phase
Authors:
Arnau Romaguera,
Eugenio Paris,
Elizabeth Skoropata,
Stefano Agrestini,
Mirian Garcia-Fernandez,
Marisa Medarde,
Noah Schnitzer,
Lopa Bhatt,
Berit H. Goodge,
Yun Yen,
Matthias Krack,
Michael Schüler,
Romain Sibille,
Tom Fennell,
Daniel G. Mazzone,
Jakob Lass,
Ellen Fogh,
Anirudha Ghosh,
Marco Caputo,
Carlos William Galdino,
Zhijia Zhang,
Thorsten Schmitt,
Milan Radovic,
Luc Patthey,
Hiroki Ueda
, et al. (2 additional authors not shown)
Abstract:
In solids, disorder is conventionally regarded as detrimental to coherence. It typically localizes and dampens collective excitations, as exemplified by Anderson localization or the broadening of magnetic modes in systems lacking long-range order. While high-entropy materials are specifically designed to harness disorder and stabilize homogeneous mixed-phase structures that can display unique prop…
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In solids, disorder is conventionally regarded as detrimental to coherence. It typically localizes and dampens collective excitations, as exemplified by Anderson localization or the broadening of magnetic modes in systems lacking long-range order. While high-entropy materials are specifically designed to harness disorder and stabilize homogeneous mixed-phase structures that can display unique properties, this same disorder is nonetheless expected to preclude the formation of coherent magnetic excitations. To test the limits of this picture, we selected the antiferromagnetic system YBaCuFeO5, as it features two distinct transition metal atoms with significantly different magnetic moments, rendering its spin dynamics exceptionally sensitive to local atomic ordering. Combining resonant inelastic x-ray scattering and linear spin wave theory, we reveal a surprising paradox: YBaCuFeO5 exhibits an unexpected, entropy-driven mixed phase, in which disorder, rather than reducing the lifetime of the collective excitations, favors coherence. In this mixed phase, the spin waves remain dispersive, markedly distinct from those expected for an ordered ground state, and exhibit well-defined acoustic and optical branches separated by a large optical gap. These results demonstrate that in entropy-stabilized magnets, disorder can favor coherent collective modes previously thought to be exclusive to low-entropy systems.
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Submitted 28 April, 2026; v1 submitted 26 April, 2026;
originally announced April 2026.
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Field-direction sensitivity of Kondo hybridization in UTe$_2$
Authors:
Thomas Halloran,
Gicela Saucedo Salas,
Sylvia K. Lewin,
J. A. Rodriguez-Rivera,
Colin L. Sarkis,
Jakob Lass,
Daniel G. Mazzone,
Marc Janoschek,
Nicholas P. Butch
Abstract:
Neutron scattering experiments on the spin-triplet superconductor UTe$_2$ have established that the dominant low-energy magnetic response is along Brillouin zone boundaries, resembling the magnetic susceptibility of narrow-gap interband excitations. We report a study of the sensitivity of these excitations to magnetic field along the crystallographic $\hat{a}$-axis. Up to fields of $μ_0 H$=13 T, t…
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Neutron scattering experiments on the spin-triplet superconductor UTe$_2$ have established that the dominant low-energy magnetic response is along Brillouin zone boundaries, resembling the magnetic susceptibility of narrow-gap interband excitations. We report a study of the sensitivity of these excitations to magnetic field along the crystallographic $\hat{a}$-axis. Up to fields of $μ_0 H$=13 T, the maximal inelastic neutron spectral weight increases in energy transfer, with a pronounced increase in $d\hbarω_{peak}/dH$ near $μ_0 H$=7 T. This behavior parallels the field and temperature dependent features of the electrical resistivity that are associated with Kondo hybridization. Our measurements suggest that $\hat{a}$-axis fields near $μ_0 H$=7~T induce a change in the hybridization between heavy $f$-electrons and the bare conduction band.
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Submitted 17 March, 2026;
originally announced March 2026.
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Magnetic phase diagram and spin Hamiltonian of antiferromagnet Cs$_2$CoI$_4$
Authors:
S. D. Nabi,
L. Facheris,
V. Romerio,
V. Kocsis,
K. Yu. Povarov,
D. Sheptyakov,
J. Lass,
D. G. Mazzone,
H. Kikuchi,
T. Masuda,
S. A. Barnett,
D. R. Allan,
Z. Yan,
S. Gvasaliya,
A. Zheludev
Abstract:
We report comprehensive thermodynamic and neutron scattering measurements on the $S$ = 3/2 antiferromagnet Cs$_2$CoI$_4$, a member of the thoroughly studied family of frustrated magnets Cs$_2MX_4$ ($M$ = Cu, Co, Ru, $X$ = Br, Cl, I, O). Unlike previously studied members, Cs$_2$CoI$_4$ undergoes a structural phase transition, for which we determine the low-temperature crystallographic structure. Th…
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We report comprehensive thermodynamic and neutron scattering measurements on the $S$ = 3/2 antiferromagnet Cs$_2$CoI$_4$, a member of the thoroughly studied family of frustrated magnets Cs$_2MX_4$ ($M$ = Cu, Co, Ru, $X$ = Br, Cl, I, O). Unlike previously studied members, Cs$_2$CoI$_4$ undergoes a structural phase transition, for which we determine the low-temperature crystallographic structure. The resulting symmetry reduction strongly affects both the magnetic exchange interactions and single-ion anisotropy. Despite the large parameter space, we propose a minimal magnetic Hamiltonian that reasonably captures the observed excitation spectrum, analyzed using extended SU(4) linear spin-wave theory.
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Submitted 29 May, 2026; v1 submitted 9 March, 2026;
originally announced March 2026.
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Accelerating discovery across scientific disciplines through reproducible workflows with AiiDAlab
Authors:
Aliaksandr V. Yakutovich,
Daniel Hollas,
Edan Bainglass,
Jusong Yu,
Corsin Battaglia,
Miki Bonacci,
Lucas Fernandez Vilanova,
Stephan Henne,
Anders Kaestner,
Michel Kenzelmann,
Graham Kimbell,
Jakob Lass,
Fabio Lopes,
Daniel G. Mazzone,
Andres Ortega-Guerrero,
Xing Wang,
Nicola Marzari,
Carlo A. Pignedoli,
Giovanni Pizzi
Abstract:
With ever-increasing computational capabilities, robust and automated research workflows have become essential for orchestrating large numbers of interdependent simulations. However, significant technical expertise is still required to configure execution environments, define calculation inputs, interpret outputs, and manage the complexity of parallel code execution on remote machines. To address…
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With ever-increasing computational capabilities, robust and automated research workflows have become essential for orchestrating large numbers of interdependent simulations. However, significant technical expertise is still required to configure execution environments, define calculation inputs, interpret outputs, and manage the complexity of parallel code execution on remote machines. To address these challenges, we developed AiiDAlab, a Jupyter-based web platform powered by the AiiDA computational infrastructure that provides a framework for managing and automating computational workflows while ensuring reproducibility through full provenance tracking. Through a collection of open-source user-friendly applications, AiiDAlab enables scientists to set up, execute, and analyze complex computational workflows without interacting directly with the underlying technical details, allowing them to focus on their research questions. In this paper, we discuss how AiiDAlab has matured over the past few years, expanding beyond computational materials science and its AiiDA origins. We present recent developments towards integrating with electronic laboratory notebooks (ELNs) for FAIR-compliant data management, adoption in large-scale facilities for secure access to experimental data and analytical tools, and applications in educational settings. Together with community-driven efforts to simplify onboarding, improve access to computational resources, and support large-scale data workflows, these advancements position AiiDAlab as a powerful platform for accelerating scientific discovery and fostering collaboration across disciplines.
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Submitted 26 May, 2026; v1 submitted 18 December, 2025;
originally announced December 2025.
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Magnetic and phononic dynamics in the two-ladder quantum magnet (C5H9NH3)2CuBr4
Authors:
J. Philippe,
F. Elson,
T. Arh,
S. Sanz,
M. Metzelaars,
D. W. Tam,
O. K. Forslund,
O. Shliakhtun,
C. Jiang,
J. Lass,
M. D. Le,
J. Ollivier,
P. Bouillot,
T. Giamarchi,
M. Bartkowiak,
D. G. Mazzone,
P. Kögerler,
M. Månsson,
A. M. Läuchli,
Y. Sassa,
M. Janoschek,
B. Normand,
G. Simutis
Abstract:
In quantum magnetic materials it is common to observe both static and dynamic lattice effects on the magnetic excitation spectrum. Less common is to find that the magnetic correlations have a significant impact on the phonon spectrum. Can such an interplay occur in a structurally soft system with comparable elastic and magnetic energy scales? Here we study the metal-organic material (C5H9NH3)2CuBr…
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In quantum magnetic materials it is common to observe both static and dynamic lattice effects on the magnetic excitation spectrum. Less common is to find that the magnetic correlations have a significant impact on the phonon spectrum. Can such an interplay occur in a structurally soft system with comparable elastic and magnetic energy scales? Here we study the metal-organic material (C5H9NH3)2CuBr4 (Cu-CPA), in which an explanation of the low-lying excitations depends crucially on a full understanding of both the spin and lattice subsystems. We report high-resolution neutron spectroscopy enabled by large, deuterated single-crystals that reveal how both sectors are affected by the recently discovered structural phase transition. By measuring over several Brillouin zones, we disentangle the vibrational contribution to the spectrum in order to obtain an accurate estimate of the quasi-one-dimensional magnetic signal. The low-energy magnetic excitations are dominated by two gaps, $Δ$ b = 0.41 meV and $Δ$ a = 0.55 meV, which contribute with equal intensity ratios, confirming that Cu-CPA realizes a two-ladder spin Hamiltonian, and we deduce the magnetic interaction parameters of both ladders. The phonon spectrum contains a highly localized mode at an anomalously low-energy around 2 meV. This characteristic frequency drops by approximately 5 percent as magnetic correlations become established with decreasing temperature, and we connect this behavior with the location and structure of the cyclopentylammonium rings.
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Submitted 28 October, 2025;
originally announced October 2025.
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Refined spin Hamiltonian on the Cairo pentagonal lattice of Bi2Fe4O9
Authors:
Emma Y. Lenander,
Frida B. Nielsen,
Jakob Lass,
Ursula B. Hansen,
Kristine M. L. Krighaar,
Asbjørn Preuss,
Tobias Weber,
Mechthild Enderle,
Henrik Jacobsen,
Uwe Stuhr,
Ryoichi Kajimoto,
Mitsutaka Nakamura,
Manfred Burianek,
Andrea Kirsch,
Henrik M. Rønnow,
Kim Lefmann,
Pascale P. Deen
Abstract:
The frustrated magnet Bi2Fe4O9 has been reported to exhibit complex spin dynamics coexisting with conventional spin wave excitations. The magnetic Fe3+ (S = 5/2) ions are arranged into a distorted two-dimensional Cairo pentagonal lattice with weak couplings between the layers, developing long-ranged non-collinear antiferromagnetic order below 245 K. In order to enable studies and modelling of the…
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The frustrated magnet Bi2Fe4O9 has been reported to exhibit complex spin dynamics coexisting with conventional spin wave excitations. The magnetic Fe3+ (S = 5/2) ions are arranged into a distorted two-dimensional Cairo pentagonal lattice with weak couplings between the layers, developing long-ranged non-collinear antiferromagnetic order below 245 K. In order to enable studies and modelling of the complex dynamics close to TN, we have reexamined the magnetic excitations across the complete energy scale (0 < E < 90 meV) at 10 K. We discover two distinct gaps, which can be explained by introducing, respectively, easy axis and easy plane anisotropy on the two unequivalent Fe-sites. We develop a refined spin Hamiltonian that accurately accounts for the dispersion of essentially all spin-wave branches across the full spectral range, except around 40 meV, where a splitting and dispersion are observed. We propose that this mode is derived from phonon hybridization. Polarisation analysis shows that the system has magnetic anisotropic fluctuations, consistent with our model. A continuum of scattering is observed above the spin wave branches and is found to principally be explained by an instrumental resolution effect. The full experimental mapping of the excitation spectrum and the refined spin Hamiltonian provides a foundation for future quantitative studies of spin waves coexisting with unconventional magnetic fluctuations in this frustrated magnet found at higher temperatures.
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Submitted 8 October, 2025; v1 submitted 6 October, 2025;
originally announced October 2025.
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Magnetic Field Dependence of the Spin Fluctuations in CeCu$_{5.8}$Ag$_{0.2}$
Authors:
X. Boraley,
A. D. Christianson,
J. Lass,
C. Balz,
M. Bartkowiak,
Ch. Niedermayer,
J. M. Lawrence,
L. Poudel,
D. G. Mandrus,
F. Ronning,
M. Janoschek,
D. G. Mazzone
Abstract:
Quantum phase transitions are among the most intriguing phenomena that can occur when the electronic ground state of correlated metals are tuned by external parameters such as pressure, magnetic field or chemical substitution. Such transitions between distinct states of matter are driven by quantum fluctuations, and can give rise to macroscopically coherent phases that are at the forefront of cond…
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Quantum phase transitions are among the most intriguing phenomena that can occur when the electronic ground state of correlated metals are tuned by external parameters such as pressure, magnetic field or chemical substitution. Such transitions between distinct states of matter are driven by quantum fluctuations, and can give rise to macroscopically coherent phases that are at the forefront of condensed matter research. However, the nature of the critical fluctuations, and thus the fundamental physics controlling many quantum phase transitions, remain poorly understood in numerous strongly correlated metals. Here we study the model material CeCu$_{5.8}$Ag$_{0.2}$ to gain insight into the implications of critical fluctuations originating from different regions in reciprocal space. By employing an external magnetic field along the crystallographic $a$- and $c$-axis as auxiliary tuning parameter we observe a pronounced anisotropy in the suppression of the quantum critical fluctuations, reflecting the spin anisotropy of the long-range ordered ground state at larger silver concentration. Coupled with the temperature dependence of the quantum critical fluctuations, these results suggest that the quantum phase transition in CeCu$_{5.8}$Ag$_{0.2}$ is driven by three-dimensional spin-density wave fluctuations.
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Submitted 30 January, 2026; v1 submitted 12 September, 2025;
originally announced September 2025.
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Spin-flop-like transition as quantum critical point in Cs$_2$RuO$_4$
Authors:
S. D. Nabi,
M. Zhu,
K. Yu. Povarov,
D. G. Mazzone,
J. Lass,
Y. Wu,
Z. Yan,
S. Gvasaliya,
A. Zheludev
Abstract:
We report thermodynamic, neutron diffraction, and inelastic neutron scattering measurements on Cs$_2$RuO$_4$, a member of the celebrated family of frustrated magnets Cs$_2$MX$_4$ (M = Cu, Co, X = Br, Cl). Unlike the previously studied members, it is based on $4d$ transition metal ions with $S=1$. Mapping out the $H-T$ magnetic phase diagram reveals an unusual continuous spin-flop-like phase transi…
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We report thermodynamic, neutron diffraction, and inelastic neutron scattering measurements on Cs$_2$RuO$_4$, a member of the celebrated family of frustrated magnets Cs$_2$MX$_4$ (M = Cu, Co, X = Br, Cl). Unlike the previously studied members, it is based on $4d$ transition metal ions with $S=1$. Mapping out the $H-T$ magnetic phase diagram reveals an unusual continuous spin-flop-like phase transition associated with a quantum critical point within the antiferromagnetically ordered phase. A quantitative analysis of the complex magnetic excitation spectrum measured in zero field allows us to derive a model magnetic Hamiltonian for this compound. Its main feature is a frustration of magnetic anisotropy on a level that is much higher than in any of the previously studied species. This frustration naturally explains the peculiar phase transition observed.
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Submitted 21 October, 2025; v1 submitted 26 July, 2025;
originally announced July 2025.
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Long-range magnetic interactions in Nd$_2$PdSi$_3$ and the formation of skyrmion phases in centrosymmetric metals
Authors:
Viviane Peçanha-Antonio,
Zhaoyang Shan,
Michael Smidman,
Juba Bouaziz,
Bachir Ouladdiaf,
Iurii Kibalin,
Marie Helene Lemee,
Christian Balz,
Jakob Lass,
Daniel A. Mayoh,
Geetha Balakrishnan,
Julie B. Staunton,
Davashibhai Adroja,
Andrew T. Boothroyd
Abstract:
We present an extensive X-ray and neutron scattering study of the structure and magnetic excitations of Nd$_2$PdSi$_3$, a sister compound of Gd$_2$PdSi$_3$ which was recently found to host a skyrmion lattice phase despite its centrosymmetric crystal structure. Dispersive magnetic excitations were measured throughout the Brillouin zone and modeled to determine the magnetic interactions between Nd i…
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We present an extensive X-ray and neutron scattering study of the structure and magnetic excitations of Nd$_2$PdSi$_3$, a sister compound of Gd$_2$PdSi$_3$ which was recently found to host a skyrmion lattice phase despite its centrosymmetric crystal structure. Dispersive magnetic excitations were measured throughout the Brillouin zone and modeled to determine the magnetic interactions between Nd ions. Our analysis reveals that the magnetic interactions in this system extend over large distances and are significantly affected by a crystallographic superstructure formed by ordering of the Pd and Si atoms. The results suggest that the mechanism for the skyrmion phase formation in this family of materials, specifically Gd$_2$PdSi$_3$, is through the long-range RKKY interactions rather than short-range triangular-lattice frustration.
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Submitted 5 January, 2026; v1 submitted 14 April, 2025;
originally announced April 2025.
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Bond-dependent interactions and ill-ordered state in the honeycomb cobaltate BaCo$_2$(AsO$_4$)$_2$
Authors:
A. Devillez,
J. Robert,
E. Lhotel,
R. Ballou,
C. Cavenel,
F. Denis Romero,
Q. Faure,
H. Jacobsen,
J. Lass,
D. G. Mazzone,
U. Bengaard Hansen,
M. Enderle,
S. Raymond,
S. De Brion,
V. Simonet,
M. Songvilay
Abstract:
The ground state and Hamiltonian of the honeycomb lattice material BaCo$_{2}$(AsO$_{4}$)$_{2}$ hosting magnetic Co$^{2+}$, have been debated for decades. The recent proposal for anisotropic bond-dependent interactions in such honeycomb cobaltates has raised the prospect of revisiting its Hamiltonian in the context of Kitaev physics. To test this hypothesis, we have combined magnetization, ac-susce…
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The ground state and Hamiltonian of the honeycomb lattice material BaCo$_{2}$(AsO$_{4}$)$_{2}$ hosting magnetic Co$^{2+}$, have been debated for decades. The recent proposal for anisotropic bond-dependent interactions in such honeycomb cobaltates has raised the prospect of revisiting its Hamiltonian in the context of Kitaev physics. To test this hypothesis, we have combined magnetization, ac-susceptibility and neutron scattering measurements on a BaCo$_{2}$(AsO$_{4}$)$_{2}$ single-crystal, together with advanced modeling. Our experimental results highlight a collinear magnetic ground state with intrinsic disorder associated to an average incommensurate propagation vector. Monte Carlo simulations and linear spin wave calculations were performed to obtain a spin model compatible with this unusual ground state, the dispersion of magnetic excitations and a magnetization plateau under magnetic field. We thus show that bond-dependent anisotropic interactions, including Kitaev-like interactions, are necessary to account for the puzzling properties of this long-explored material, and are hence a general ingredient in the cobaltates.
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Submitted 28 March, 2025;
originally announced March 2025.
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Microscopic Origin of Reduced Magnetic Order in a Frustrated Metal
Authors:
X. Boraley,
O. Stockert,
J. Lass,
R. Sibille,
Ø. S. Fjellvåg,
S. H. Moody,
A. M. Läuchli,
V. Fritsch,
D. G. Mazzone
Abstract:
Although magnetic frustration in metals provides a promising avenue for novel quantum phenomena, their microscopic interpretation is often challenging. Here we use the face-centered cubic intermetallic HoInCu$_4$ as model material to show that Hamiltonians neglecting the charge degree of freedom are appropriate for frustrated metals possessing low density of states at the Fermi surface. Through ne…
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Although magnetic frustration in metals provides a promising avenue for novel quantum phenomena, their microscopic interpretation is often challenging. Here we use the face-centered cubic intermetallic HoInCu$_4$ as model material to show that Hamiltonians neglecting the charge degree of freedom are appropriate for frustrated metals possessing low density of states at the Fermi surface. Through neutron scattering techniques we determine matching magnetic exchange interactions in the paramagnetic and field-polarized states using an effective spin-1 Heisenberg Hamiltonian, for which we identify antiferromagnetic nearest and next-nearest neighbour interactions $J_1$ and $J_2$ that are close to the critical ratio $J_2$/$J_1$ = 1/2. The study further provides evidence that spin-wave theory fails to predict the low-energy spin dynamics in the antiferromagnetic zero-field state, which is dominated by overdamped magnetic excitations. We conclude that the low-energy fluctuations arise from quantum fluctuations, accounting for the missing moment of the strongly renormalized magnetic long-range order.
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Submitted 24 July, 2025; v1 submitted 12 February, 2025;
originally announced February 2025.
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Field-Induced Magnon Decay, Magnon Shadows, and Roton-like Excitations in the Honeycomb Antiferromagnet YbBr$_3$
Authors:
J. A. Hernández,
A. A. Eberharter,
M. Schuler,
J. Lass,
D. G. Mazzone,
R. Sibille,
S. Raymond,
K. W. Krämer,
B. Normand,
B. Roessli,
A. M. Läuchli,
M. Kenzelmann
Abstract:
Although the search for quantum many-body phenomena in magnetic materials has a strong focus on highly frustrated systems, even unfrustrated quantum magnets show a multitude of unconventional phenomena in their spin excitation spectra. YbBr$_3$ is an excellent realization of the $S = 1/2$ antiferromagnetic Heisenberg model on the honeycomb lattice, and we have performed detailed spectroscopic expe…
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Although the search for quantum many-body phenomena in magnetic materials has a strong focus on highly frustrated systems, even unfrustrated quantum magnets show a multitude of unconventional phenomena in their spin excitation spectra. YbBr$_3$ is an excellent realization of the $S = 1/2$ antiferromagnetic Heisenberg model on the honeycomb lattice, and we have performed detailed spectroscopic experiments with both unpolarized and polarized neutrons at all applied magnetic fields up to saturation. We observe extensive excitation continua, which cause strong renormalization and the decay of single magnons at higher fields, while coherent features include field-induced ``shadows'' of the single magnons and the spectacular emergence of a roton-like excitation. To guide and interpret our experiments, we performed systematic calculations by the method of cylinder matrix-product states that provide quantitative agreement with the neutron scattering data and a qualitative benchmark for the spectral signatures of strong quantum fluctuations even in the absence of magnetic frustration.
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Submitted 7 July, 2025; v1 submitted 23 December, 2024;
originally announced December 2024.
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Absence of altermagnetic magnon band splitting in MnF$_2$
Authors:
Vincent C. Morano,
Zeno Maesen,
Stanislav E. Nikitin,
Jakob Lass,
Daniel G. Mazzone,
Oksana Zaharko
Abstract:
Altermagnets are collinear compensated magnets in which the magnetic sublattices are related by rotation rather than translation or inversion. One of the quintessential properties of altermagnets is the presence of split chiral magnon modes. Recently, such modes have been predicted in MnF$_2$. Here, we report inelastic neutron scattering results on an MnF$_2$ single-crystal along high-symmetry Bri…
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Altermagnets are collinear compensated magnets in which the magnetic sublattices are related by rotation rather than translation or inversion. One of the quintessential properties of altermagnets is the presence of split chiral magnon modes. Recently, such modes have been predicted in MnF$_2$. Here, we report inelastic neutron scattering results on an MnF$_2$ single-crystal along high-symmetry Brillouin zone paths for which the magnon splitting is expected. Within the resolution of our measurement, we do not observe the predicted splitting. The inelastic spectrum is well-modeled using $J_1, ~J_2, ~J_3$ nearest-neighbor exchange interactions with weak uniaxial anisotropy. These interactions have higher symmetry than the crystal lattice, while the interactions predicted to produce the altermagnetic splitting are negligibly small. Therefore, the two magnon modes appear to be degenerate over the entire Brillouin zone and the spin dynamics of MnF$_2$ is indistinguishable from a classical Néel antiferromagnet. Application of magnetic field causes a Zeeman splitting of the magnon modes close to the $\mathrmΓ$ point. Even if chiral magnon modes are allowed by altermagnetic symmetry, the splitting in real materials such as MnF$_2$ can be negligibly small.
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Submitted 4 December, 2024;
originally announced December 2024.
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Connection between f-electron correlations and magnetic excitations in UTe2
Authors:
Thomas Halloran,
Peter Czajka,
Gicela Saucedo Salas,
Corey Frank,
Chang-Jong Kang,
J. A. Rodriguez-Rivera,
Jakob Lass,
Daniel G. Mazzone,
Marc Janoschek,
Gabi Kotliar,
Nicholas P. Butch
Abstract:
The detailed anisotropy of the low-temperature, low-energy magnetic excitations of the candidate spin-triplet superconductor UTe$_2$ is revealed using inelastic neutron scattering. The magnetic excitations emerge from the Brillouin zone boundary at the high symmetry $Y$ and $T$ points and disperse along the crystallographic $\hat{b}$-axis. In applied magnetic fields to at least $μ_0 H=11$~T along…
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The detailed anisotropy of the low-temperature, low-energy magnetic excitations of the candidate spin-triplet superconductor UTe$_2$ is revealed using inelastic neutron scattering. The magnetic excitations emerge from the Brillouin zone boundary at the high symmetry $Y$ and $T$ points and disperse along the crystallographic $\hat{b}$-axis. In applied magnetic fields to at least $μ_0 H=11$~T along the $\hat{c}-$axis, the magnetism is found to be field-independent in the $(hk0)$ plane. The scattering intensity is consistent with that expected from U$^{3+}$/U$^{4+}$ $f$-electron spins with preferential orientation along the crystallographic $\hat{a}$-axis, and a fluctuating magnetic moment of 2.3(7) $μ_B$. These characteristics indicate that the excitations are due to intraband spin excitons arising from $f$-electron hybridization.
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Submitted 6 September, 2024; v1 submitted 26 August, 2024;
originally announced August 2024.
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(C$_5$H$_9$NH$_3$)$_2$CuBr$_4$: a metal-organic two-ladder quantum magnet
Authors:
J. Philippe,
F. Elson,
M. P. N. Casati,
S. Sanz,
M. Metzelaars,
O. Shliakhtun,
O. K. Forslund,
J. Lass,
T. Shiroka,
A. Linden,
D. G. Mazzone,
J. Ollivier,
S. Shin,
M. Medarde,
B. Lake,
M. Mansson,
M. Bartkowiak,
B. Normand,
P. Kögerler,
Y. Sassa,
M. Janoschek,
G. Simutis
Abstract:
Low-dimensional quantum magnets are a versatile materials platform for studying the emergent many-body physics and collective excitations that can arise even in systems with only short-range interactions. Understanding their low-temperature structure and spin Hamiltonian is key to explaining their magnetic properties, including unconventional quantum phases, phase transitions, and excited states.…
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Low-dimensional quantum magnets are a versatile materials platform for studying the emergent many-body physics and collective excitations that can arise even in systems with only short-range interactions. Understanding their low-temperature structure and spin Hamiltonian is key to explaining their magnetic properties, including unconventional quantum phases, phase transitions, and excited states. We study the metal-organic coordination compound (C$_5$H$_9$NH$_3$)$_2$CuBr$_4$ and its deuterated counterpart, which upon its discovery was identified as a candidate two-leg quantum ($S = 1/2$) spin ladder in the strong-leg coupling regime. By growing large single crystals and probing them with both bulk and microscopic techniques, we deduce that two previously unknown structural phase transitions take place between 136 K and 113 K. The low-temperature structure has a monoclinic unit cell giving rise to two inequivalent spin ladders. We further confirm the absence of long-range magnetic order down to 30 mK and discuss the implications of this two-ladder structure for the magnetic properties of (C$_5$H$_9$NH$_3$)$_2$CuBr$_4$.
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Submitted 6 September, 2024; v1 submitted 12 April, 2024;
originally announced April 2024.
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Characterizing the diffuse continuum excitations in the classical spin liquid $h$-YMnO$_3$
Authors:
Jakob Lass,
Emma Y. Lenander,
Kristine M. L. Krighaar,
Tara N. Tošić,
Dharmalingam Prabhakaran,
Pascale P. Deen,
Sofie Holm-Janas,
Kim Lefmann
Abstract:
We extend previous inelastic neutron scattering results on the geometrically frustrated antiferromagnet hexagonal-YMnO$_3$, which has been suggested to belong to the class of classical spin liquids. We extend the energy transfer coverage of the diffuse signal up to 6.9 meV within a wide temperature range around the ordering temperature, $T_\mathrm{N}$. The two distinct diffuse signals in the a-b p…
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We extend previous inelastic neutron scattering results on the geometrically frustrated antiferromagnet hexagonal-YMnO$_3$, which has been suggested to belong to the class of classical spin liquids. We extend the energy transfer coverage of the diffuse signal up to 6.9 meV within a wide temperature range around the ordering temperature, $T_\mathrm{N}$. The two distinct diffuse signals in the a-b plane, the signal localized at $Γ$' and the scattering intensity connecting $Γ$' points over the M', are shown to be only weakly energy dependent. In addition, an external magnetic field of up to 10.5 T applied along c is shown to have no effect on the diffuse signal. In the orthogonal scattering plane, the signals are shown to be dependent on l only through the magnetic form factor, showing that the correlations are purely two-dimensional, and supporting its origin to be the frustrated Mn$^{3+}$ triangles. This result is corroborated by atomistic spin dynamics simulations showing similar scattering vector and temperature behaviours. Lastly, data for the spin wave scattering in the (h, 0, l) plane allow for a discussion of the magnetic ground state where better agreement is found between the data and an ordered structure of the $Γ_1$ or $Γ_3$ symmetry, albeit crystal electric field arguments dismisses the $Γ_1$ as possibility.
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Submitted 12 March, 2024;
originally announced March 2024.
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Magnetic ground state and perturbations of the distorted kagome Ising metal TmAgGe
Authors:
C. B. Larsen,
D. G. Mazzone,
N. Gauthier,
H. D. Rosales,
F. A. Gómez Albarracín,
J. Lass,
X. Boraley,
S. L. Bud'ko,
P. C. Canfield,
O. Zaharko
Abstract:
We present the magnetic orders and excitations of the distorted kagome intermetallic magnet TmAgGe. Using neutron single crystal diffraction we identify the propagation vectors $\bf{k}$ = (1/2 0 0) and $\bf{k}$ = (0 0 0) and determine the magnetic structures of the zero-field and magnetic field-induced phases for $H$ along the $a$ and [-1 1 0] crystal directions. We determine the experimental magn…
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We present the magnetic orders and excitations of the distorted kagome intermetallic magnet TmAgGe. Using neutron single crystal diffraction we identify the propagation vectors $\bf{k}$ = (1/2 0 0) and $\bf{k}$ = (0 0 0) and determine the magnetic structures of the zero-field and magnetic field-induced phases for $H$ along the $a$ and [-1 1 0] crystal directions. We determine the experimental magnetic field-temperature ($H$, $T$)-phase diagram and reproduce it by Monte-Carlo simulations of an effective spin exchange Hamiltonian for one distorted kagome layer. Our model includes a strong axial single-ion anisotropy and significantly smaller exchange couplings which span up to the third-nearest neighbours within the layer. Single crystal inelastic neutron scattering (INS) measurements reveal an almost flat, only weakly dispersive mode around 7 meV that we use alongside bulk magnetization data to deduce the crystal-electric field (CEF) scheme for the Tm$^{3+}$ ions. Random phase approximation (RPA) calculations based on the determined CEF wave functions of the two lowest quasi-doublets enable an estimation of the interlayer coupling that is compatible with the experimental INS spectra. No evidence for low-energy spin waves associated to the magnetic order was found, which is consistent with the strongly Ising nature of the ground state.
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Submitted 16 June, 2023;
originally announced June 2023.
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A microscopic Kondo lattice model for the heavy fermion antiferromagnet CeIn$_3$
Authors:
W. Simeth,
Z. Wang,
E. A. Ghioldi,
D. M. Fobes,
A. Podlesnyak,
N. H. Sung,
E. D. Bauer,
J. Lass,
J. Vonka,
D. G. Mazzone,
C. Niedermayer,
Yusuke Nomura,
Ryotaro Arita,
C. D. Batista,
F. Ronning,
M. Janoschek
Abstract:
Electrons at the border of localization generate exotic states of matter across all classes of strongly correlated electron materials and many other quantum materials with emergent functionality. Heavy electron metals are a model example, in which magnetic interactions arise from the opposing limits of localized and itinerant electrons. This remarkable duality is intimately related to the emergenc…
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Electrons at the border of localization generate exotic states of matter across all classes of strongly correlated electron materials and many other quantum materials with emergent functionality. Heavy electron metals are a model example, in which magnetic interactions arise from the opposing limits of localized and itinerant electrons. This remarkable duality is intimately related to the emergence of a plethora of novel quantum matter states such as unconventional superconductivity, electronic-nematic states, hidden order and most recently topological states of matter such as topological Kondo insulators and Kondo semimetals and putative chiral superconductors. The outstanding challenge is that the archetypal Kondo lattice model that captures the underlying electronic dichotomy is notoriously difficult to solve for real materials. Here we show, using the prototypical strongly-correlated antiferromagnet CeIn$_3$, that a multi-orbital periodic Anderson model embedded with input from ab initio bandstructure calculations can be reduced to a simple Kondo-Heisenberg model, which captures the magnetic interactions quantitatively. We validate this tractable Hamiltonian via high-resolution neutron spectroscopy that reproduces accurately the magnetic soft modes in CeIn$_3$, which are believed to mediate unconventional superconductivity. Our study paves the way for a quantitative understanding of metallic quantum states such as unconventional superconductivity.
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Submitted 5 January, 2024; v1 submitted 3 August, 2022;
originally announced August 2022.
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Nanocriticality in the magnetic phase transition of CoO nanoparticles
Authors:
Machteld E. Kamminga,
Jonas Okkels Birk,
Jari í Hjøllum,
Henrik Jacobsen,
Jakob Lass,
Thorbjørn L. Koch,
Niels B. Christensen,
Christof Niedermayer,
Lukas Keller,
Luise Theil Kuhn,
Elisabeth T. Ulrikkeholm,
Erik Brok,
Cathrine Frandsen,
Kim Lefmann
Abstract:
The universal theory of critical phase transitions describes the critical behavior at second-order phase transitions in infinitely large systems. With the increased contemporary interest in nanoscale materials, we investigated CoO nanoparticles by means of neutron scattering and found how the theory of critical phenomena breaks down in the nanoscale regime. Using CoO as a model system, we have ide…
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The universal theory of critical phase transitions describes the critical behavior at second-order phase transitions in infinitely large systems. With the increased contemporary interest in nanoscale materials, we investigated CoO nanoparticles by means of neutron scattering and found how the theory of critical phenomena breaks down in the nanoscale regime. Using CoO as a model system, we have identified a size-dependent nanocritical temperature region close to the antiferromagnetic phase transition where the magnetic correlation length of the nanoparticles converges to a constant value, which is significantly smaller than that of the saturated state found at low temperatures. This is in clear contrast to the divergence around $T_{\rm N}$ observed for bulk systems. Our findings of nanocriticality in the magnetic phase transition is of great importance for the understanding of phase transitions at the nanoscale.
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Submitted 30 May, 2022; v1 submitted 27 May, 2022;
originally announced May 2022.
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Spin Density Wave versus Fractional Magnetization Plateau in a Triangular Antiferromagnet
Authors:
L. Facheris,
K. Yu. Povarov,
S. D. Nabi,
D. G. Mazzone,
J. Lass,
B. Roessli,
E. Ressouche,
Z. Yan,
S. Gvasaliya,
A. Zheludev
Abstract:
We report an excellent realization of the highly non-classical incommensurate spin-density wave (SDW) state in the quantum frustrated antiferromagnetic insulator Cs$_2$CoBr$_4$. In contrast to the well-known Ising spin chain case, here the SDW is stabilized by virtue of competing planar in-chain anisotropies and frustrated interchain exchange. Adjacent to the SDW phase is a broad $m = 1/3$ magneti…
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We report an excellent realization of the highly non-classical incommensurate spin-density wave (SDW) state in the quantum frustrated antiferromagnetic insulator Cs$_2$CoBr$_4$. In contrast to the well-known Ising spin chain case, here the SDW is stabilized by virtue of competing planar in-chain anisotropies and frustrated interchain exchange. Adjacent to the SDW phase is a broad $m = 1/3$ magnetization plateau that can be seen as a commensurate locking of the SDW state into the up-up-down (UUD) spin structure. This represents the first example of long-sought SDW-UUD transition in triangular-type quantum magnets.
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Submitted 18 August, 2022; v1 submitted 22 April, 2022;
originally announced April 2022.
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Revealing three-dimensional quantum criticality by Sr-substitution in Han Purple
Authors:
Stephan Allenspach,
Pascal Puphal,
Joosep Link,
Ivo Heinmaa,
Ekaterina Pomjakushina,
Cornelius Krellner,
Jakob Lass,
Gregory S. Tucker,
Christof Niedermayer,
Shusaku Imajo,
Yoshimitsu Kohama,
Koichi Kindo,
Steffen Krämer,
Mladen Horvatić,
Marcelo Jaime,
Alexander Madsen,
Antonietta Mira,
Nicolas Laflorencie,
Frédéric Mila,
Bruce Normand,
Christian Rüegg,
Raivo Stern,
Franziska Weickert
Abstract:
Classical and quantum phase transitions (QPTs), with their accompanying concepts of criticality and universality, are a cornerstone of statistical thermodynamics. An exemplary controlled QPT is the field-induced magnetic ordering of a gapped quantum magnet. Although numerous "quasi-one-dimensional" coupled spin-chain and -ladder materials are known whose ordering transition is three-dimensional (3…
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Classical and quantum phase transitions (QPTs), with their accompanying concepts of criticality and universality, are a cornerstone of statistical thermodynamics. An exemplary controlled QPT is the field-induced magnetic ordering of a gapped quantum magnet. Although numerous "quasi-one-dimensional" coupled spin-chain and -ladder materials are known whose ordering transition is three-dimensional (3D), quasi-2D systems are special for several physical reasons. Motivated by the ancient pigment Han Purple (BaCuSi$_{2}$O$_{6}$), a quasi-2D material displaying anomalous critical properties, we present a complete analysis of Ba$_{0.9}$Sr$_{0.1}$CuSi$_{2}$O$_{6}$. We measure the zero-field magnetic excitations by neutron spectroscopy and deduce the magnetic Hamiltonian. We probe the field-induced transition by combining magnetization, specific-heat, torque and magnetocalorimetric measurements with low-temperature nuclear magnetic resonance studies near the QPT. By a Bayesian statistical analysis and large-scale Quantum Monte Carlo simulations, we demonstrate unambiguously that observable 3D quantum critical scaling is restored by the structural simplification arising from light Sr-substitution in Han Purple.
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Submitted 9 June, 2021; v1 submitted 11 March, 2021;
originally announced March 2021.
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MJOLNIR: A Software Package for Multiplexing Neutron Spectrometers
Authors:
Jakob Lass,
Henrik Jacobsen,
Daniel G. Mazzone,
Kim Lefmann
Abstract:
Novel multiplexing triple-axis neutron scattering spectrometers yield significant improvements of the common triple-axis instruments. While the planar scattering geometry keeps ensuring compatibility with complex sample environments, a simultaneous detection of scattered neutrons at various angles and energies leads to tremendous improvements in the data acquisition rate. Here we report on the sof…
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Novel multiplexing triple-axis neutron scattering spectrometers yield significant improvements of the common triple-axis instruments. While the planar scattering geometry keeps ensuring compatibility with complex sample environments, a simultaneous detection of scattered neutrons at various angles and energies leads to tremendous improvements in the data acquisition rate. Here we report on the software package MJOLNIR that we have developed to handle the resulting enhancement in data complexity. Using data from the new CAMEA spectrometer of the Swiss Spallation Neutron Source at the Paul Scherrer Institut, we show how the software reduces, visualises and treats observables measured on multiplexing spectrometers. The software package has been generalised to a uniformed framework, allowing for collaborations across multiplexing instruments at different facilities, further facilitating new developments in data treatment, such as fitting routines and modelling of multi-dimensional data.
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Submitted 29 July, 2020;
originally announced July 2020.
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Commissioning of the novel Continuous Angle Multi-Energy Analysis Spectrometer at the Paul Scherrer Institut
Authors:
Jakob Lass,
Henrik Jacobsen,
Kristine M. L. Krighaar,
Dieter Graf,
Felix Groitl,
Frank Herzog,
Masako Yamada,
Christian Kägi,
Raphael Müller,
Roman Bürge,
Marcel Schild,
Manuel S. Lehmann,
Alex Bollhalder,
Peter Keller,
Marek Bartkowiak,
Uwe Filges,
Urs Greuter,
Gerd Theidel,
Henrik M. Rønnow,
Christof Niedermayer,
Daniel G. Mazzone
Abstract:
We report on the commissioning results of the cold neutron multiplexing secondary spectrometer CAMEA (\textbf{C}ontinuous \textbf{A}ngle \textbf{M}ulti-\textbf{E}nergy \textbf{A}nalysis) at the Swiss Spallation Neutron Source (SINQ) at the Paul Scherrer Institut, Switzerland. CAMEA is optimized for an efficient data acquisition of scattered neutrons in the horizontal scattering plane, allowing for…
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We report on the commissioning results of the cold neutron multiplexing secondary spectrometer CAMEA (\textbf{C}ontinuous \textbf{A}ngle \textbf{M}ulti-\textbf{E}nergy \textbf{A}nalysis) at the Swiss Spallation Neutron Source (SINQ) at the Paul Scherrer Institut, Switzerland. CAMEA is optimized for an efficient data acquisition of scattered neutrons in the horizontal scattering plane, allowing for detailed and rapid mapping of low-energy excitations under extreme sample environment conditions.
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Submitted 13 March, 2023; v1 submitted 29 July, 2020;
originally announced July 2020.
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Classical spin liquid or extended critical range in h-YMnO$_3$?
Authors:
Sofie Janas,
Jakob Lass,
Ana-Elena Tutueanu,
Morten L. Haubro,
Christof Niedermayer,
Uwe Stuhr,
Guangyong Xu,
Dharmalingam Prabhakaran,
Pascale P. Deen,
Sonja Holm-Dahlin,
Kim Lefmann
Abstract:
Inelastic neutron experiments on the classical triangular-lattice geometrically frustrated antiferromagnet h-YMnO$_3$ reveal diffuse, gapless magnetic excitations present both below and far above the ordering temperature, $T_N$. The correlation length of the excitations increases as the temperature approaches zero, bearing strong resemblance to critical scattering. We model the scattering as criti…
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Inelastic neutron experiments on the classical triangular-lattice geometrically frustrated antiferromagnet h-YMnO$_3$ reveal diffuse, gapless magnetic excitations present both below and far above the ordering temperature, $T_N$. The correlation length of the excitations increases as the temperature approaches zero, bearing strong resemblance to critical scattering. We model the scattering as critical spin-spin correlations in a two-dimensional magnetic ground state, and we speculate that this may provide a general framework to understand features typically attributed to classical spin liquids.
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Submitted 5 June, 2020;
originally announced June 2020.
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Nature of the field-induced magnetic incommensurability in multiferroic Ni$_3$TeO$_6$
Authors:
J. Lass,
Ch. Røhl Andersen,
H. K. Leerberg,
S. Birkemose,
S. Toth,
U. Stuhr,
M. Bartkowiak,
Ch. Niedermayer,
Zhilun Lu,
R. Toft-Petersen,
M. Retuerto,
J. Okkels Birk,
K. Lefmann
Abstract:
Using single crystal neutron scattering we show that the magnetic structure Ni$_3$TeO$_6$ at fields above 8.6 T along the $c$ axis changes from a commensurate collinear antiferromagnetic structure with spins along c and ordering vector $Q_C$= (0 0 1.5), to a conical spiral with propagation vector $Q_{IC}$= (0 0 1.5$\pmδ$),$δ\sim$0.18, having a significant spin component in the ($a$,$b$) plane. We…
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Using single crystal neutron scattering we show that the magnetic structure Ni$_3$TeO$_6$ at fields above 8.6 T along the $c$ axis changes from a commensurate collinear antiferromagnetic structure with spins along c and ordering vector $Q_C$= (0 0 1.5), to a conical spiral with propagation vector $Q_{IC}$= (0 0 1.5$\pmδ$),$δ\sim$0.18, having a significant spin component in the ($a$,$b$) plane. We determine the phase diagram of this material in magnetic fields up to 10.5 T along $c$ and show the phase transition between the low field and conical spiral phases is of first order by observing a discontinuous jump of the ordering vector. $Q_{IC}$ is found to drift both as function of magnetic field and temperature. Preliminary inelastic neutron scattering reveals that the spin wave gap in zero field has minima exactly at $Q_{IC}$ and a gap of about 1.1 meV consisting with a cross-over around 8.6 T. Our findings excludes the possibility of the inverse Dzyaloshinskii-Moriya interaction as a cause for the giant magneto-electric coupling earlier observed in this material and advocates for the symmetric exchangestriction as the origin of this effect.
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Submitted 3 December, 2019; v1 submitted 30 September, 2019;
originally announced September 2019.