arXiv subjects

J. Ollivier

Publications and source records attributed to J. Ollivier.

At least 19 recordsLinked to original sources

arXiv2026

Vortex order in magnetic frustrated GeNi$_2$O$_4$ and GeCo$_2$O$_4$ spinels

In the search for new spin textures based on singular magnetic objects like Bloch-points or vortices, spinel compounds emerge as an interesting playground due to the interplay between magnetic anisotropy and complex interactions that extend well beyond first neighbors on a pyrochlore lattice. Based on an exploration of the exchange interaction phase diagrams of members of the Ge$B_2$O$_4$ family with $B$=Co and Ni, we show, using simultaneous modeling of inelastic neutron scattering measurements and single-crystal neutron diffraction data, that a 2-$k$ magnetic structure may be stabilized in these compounds. This leads to a short period spin vortex crystal, a variant induced by the magnetic anisotropy of the 3-$k$ Bloch-point structure predicted for isotropic spins. Our study rationalizes the formation of these multi-$k$ spin textures in frustrated antiferromagnets, as well as their anisotropy-dependent evolution.

cond-mat.str-el↗

arXiv2026

Quantum spin liquid on a 3D bipartite lattice of spin trimers stabilized by enhanced effective anisotropy

Quantum spin liquids (QSLs) represent highly entangled states of matter in which frustration-induced quantum fluctuations suppress any symmetry-breaking phase transition down to absolute zero, giving rise to fractionalized excitations and emergent gauge fields. Theoretically, bond anisotropy can stabilize QSLs even on bipartite lattices, as exemplified by the Kitaev honeycomb model; however, no material has so far been established to realize such a state as its true ground state. Here we identify the three-dimensional spin-trimer magnet KBa$_3$Ca$_4$Cu$_3$V$_7$O$_{28}$ as a promising candidate for a bipartite quantum spin liquid persisting to the lowest temperatures. Strongly coupled Cu$^{2+}$ trimers form effective pseudospin-1/2 degrees of freedom upon cooling, which in turn constitute a three-dimensional bipartite network. Bulk thermodynamic measurements, neutron scattering, $μ$SR, and NMR detect no spin freezing or symmetry-breaking phase transition down to 20 mK, but instead reveal a gapless dynamical ground state with algebraic spin autocorrelations. Complementary Monte Carlo and exact-diagonalization calculations show that this state is stabilized by a strong enhancement of effective anisotropy: a weak microscopic Cu-Cu exchange anisotropy of approximately 15 percent is generically amplified at the trimer level, producing effective pseudospin-pseudospin interaction anisotropies of 60 to 100 percent. Our results establish trimer-based networks as a promising platform for realizing anisotropy-stabilized quantum entangled states, even in three-dimensional bipartite systems with only weak microscopic anisotropy.

cond-mat.str-el↗

arXiv2026

About the origin of the magnetic ground state of Tb$_{2}$Ir$_{2}$O$_{7}$

Magnetic-rare-earth pyrochlore iridates exhibit a rich variety of unconventional phases, driven by the complex interactions within and between the rare-earth and the iridium sublattices. In this study, we investigate the peculiar magnetic state of Tb$_{2}$Ir$_{2}$O$_{7}$, where a component of the Tb$^{3+}$ moment orders perpendicular to its local Ising anisotropy axis. By means of neutron diffraction and inelastic neutron scattering down to dilution temperatures, complemented by specific heat measurements, we show that this intriguing magnetic state is fully established at 1.5 K and we characterize its excitation spectrum across a broad range of energies. Our calculations reveal that bilinear interactions between Tb$^{3+}$ ions subjected to the Ir molecular field capture several key features of the experiments, but need to be supplemented to fully reproduce the observed behavior.

cond-mat.str-el↗

arXiv2025

Thermodynamical study of N$_2$ clathrate hydrate from DFT calculations

Thermodynamic stability of N$_2$ clathrate hydrates in the sI and sII structures is investigated using density functional theory with several exchange-correlation functionals, explicitly accounting for composition (cage occupancies) and pressure at T = 0 K. Among the tested functionals, revPBE-D3(0) best reproduces experimental lattice parameters and bulk moduli B$_0$ . Energetic analyses confirm the strong impact of large cage double occupancy on sI, whereas the convex-hull results show that sI with single occupancy remains thermodynamically stable up to $\sim$ 0.8 GPa alongside sII with single occupancy. Increasing pressure then stabilizes sII with double occupancy, consistent with its larger large-cage volume and lower framework strain. These results provide a coherent, first-principles thermodynamic framework for N$_2$ hydrate stability and a baseline for finite-temperature extension.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic and phononic dynamics in the two-ladder quantum magnet (C5H9NH3)2CuBr4

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.

cond-mat.str-el↗

arXiv2025

Dynamic Competition between Cooper-Pair and Spin-Density-Wave Condensation

Quantum matter phases may co-exist microscopically even when they display competing tendencies. A fundamental question is whether such a competition can be avoided through the elimination of one phase while the other one condenses into the ground state. Here, we present a high-resolution neutron spectroscopy study of the low-energy spin excitations in the high-temperature superconductor La1.855Sr0.145CuO4. In the normal state, we find low-energy magnetic fluctuations at incommensurate reciprocal lattice positions where spin-density-wave order emerges at lower Sr concentration or at high magnetic fields. While these spin excitations are largely suppressed by the emergence of the superconducting spin gap, some low-energy magnetic fluctuations persist deep inside the superconducting state. We interpret this result in terms of a dynamic competition between superconductivity and magnetism, where superconductivity impedes the condensation of low-energy magnetic fluctuations through the formation of magnetically-mediated Cooper pairs.

cond-mat.supr-con↗

arXiv2025

Low energy phonons in single crystal ZrW$_{2}$O$_{8}$

ZrW$_{2}$O$_{8}$ is the prototypical example of a material exhibiting negative thermal expansion (NTE). It is now widely accepted that in ZrW$_{2}$O$_{8}$, and in many other framework materials exhibiting NTE, a collection of low energy phonon modes, as opposed to just one or two, are responsible for the anomalous thermal properties. However, quantitative verification and analysis of the density functional theory (DFT) calculations which underpin this proposal are still lacking. In particular, probing the low energy phonons directly throughout reciprocal space using inelastic neutron scattering, as opposed to other techniques which only probe the Brillouin zone center, is technically challenging and hence rarely done. Here we report inelastic neutron scattering measurements in a large number of Brillouin zones over a 400 K temperature range. We find excellent agreement between DFT calculations and experimental data at low temperature. However, the shifts in phonon modes predicted by DFT due to the reduction in lattice parameter (warming) are not observed. This is most likely due to counteractive anharmonic effects, which we verified using finite temperature molecular dynamics (MD) calculations. Notwithstanding, both DFT and MD results are consistent with NTE in ZrW$_{2}$O$_{8}$ arising from the tension effect, and by extension this explanation is supported by the neutron scattering results.

cond-mat.mtrl-sci↗

arXiv2025

From hierarchical triangular spin liquid to multi-$q$ spin texture in spinel GeFe$_2$O$_4$

Combining macroscopic measurements, neutron scattering and modeling, we identify in the GeFe$_2$O$_4$ spinel a correlated paramagnetic state resulting from the predominance of third-neighbor antiferromagnetic interactions. These interactions materialize 4 isolated families of triangular planes with 120$^{\circ}$ spins emerging from the underlying pyrochlore lattice. At lower temperatures, a phase transition occurs from this hierarchical spin liquid to a non-coplanar spin texture that is characterized by 6 propagating vectors. This unusual multi-$q$ order is triggered by the presence of weaker interactions up to the sixth neighbors. The system is remarkably successful in coupling the different triangular planes while maintaining their two-dimensional 120$^{\circ}$ order. Our study highlights the hierarchy of interactions involved in GeFe$_2$O$_4$, which is singular among spinel compounds since first-neighbor interactions are only a small fraction of the dominant third neighbor ones.

cond-mat.str-el↗

arXiv2025

Braided Ising spin-tube physics in a purported kagome magnet

The magnetic insulator Nd$_3$BWO$_9$ has been previously proposed to realize the highly frustrated breathing kagome lattice model. We report a combination of single-crystal neutron scattering studies and numerical simulations that debunk this interpretation. We show that it is the inter-plane couplings that determine the physics. To explain the exotic magnetism, we derive a simple one-dimensional Ising model composed of twisted triangular spin-tubes, i.e., triple braids of Ising spin chains with almost-orthogonal anisotropy frames and competing ferro-antiferromagnetic interactions. This model can account for the ground state, excitations, the numerous field-induced fractional magnetization plateau phases and incommensurate magnetic correlations at elevated temperatures. Nd$_3$BWO$_9$ constitutes a rare example where rich magnetic phenomena can be understood and simulated quantitatively in terms of a simple classical Hamiltonian.

cond-mat.str-el↗

arXiv2024

Anomalous quasielastic scattering contribution in the centrosymmetric multi-$\mathbf{q}$ helimagnet SrFeO$_3$

Centrosymmetric compounds which host three-dimensional topological spin structures comprise a distinct subclass of materials in which multiple-$\mathbf{q}$ magnetic order is stabilized by anisotropy and bond frustration in contrast to the more common path of antisymmetric exchange interactions. Here we investigate static and dynamic magnetic properties of the cubic perovskite SrFeO$_3$ $\unicode{x2013}$ a rare example of a centrosymmetric material hosting two types of topological spin textures: skyrmion- and hedgehog-lattice phases. Our detailed magnetization and dilatometry measurements describe the domain selection processes and phase transitions in SrFeO$_3$. Spin excitations are investigated using inelastic neutron scattering for all three zero-field phases. In the higher-temperature ordered phases, high-energy magnons increasingly lose coherence, so that spin fluctuations are dominated by a distinct quasielastic component at low energies. We anticipate that this could be generic to symmetric helimagnets in which the chiral symmetry is spontaneously broken by the magnetic order.

cond-mat.str-el↗

arXiv2024

(C$_5$H$_9$NH$_3$)$_2$CuBr$_4$: a metal-organic two-ladder quantum magnet

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$_4$.

cond-mat.str-el↗

arXiv2024

Reentrant multiple-$\mathbf{q}$ magnetic order and a "spin-cholesteric" phase in Sr$_3$Fe$_2$O$_7$

Spin-nematic and spin-smectic phases have been reported in magnetic materials, which break rotational symmetry while preserving translational symmetry along certain directions. However, until now the analogy to liquid crystals remained incomplete because no magnetic analog of cholesteric order was known. Here we show that the bilayer perovskite Sr$_3$Fe$_2$O$_7$, previously believed to adopt a simple single-$\mathbf{q}$ spin-helical order, hosts two distinct types of multi-$\mathbf{q}$ spin textures and the first "spin-cholesteric". Its ground state represents a novel multi-$\mathbf{q}$ spin texture with unequally intense spin modulations at the two ordering vectors. This is followed in temperature by the new "spin-cholesteric" phase with spontaneously broken chiral symmetry, in which the translational symmetry is broken only along one of the crystal directions while the weaker orthogonal modulation melts, giving rise to intense short-range dynamical fluctuations. Shortly before the transition to the paramagnetic state, vortex-crystal order spanned by two equivalent $\mathbf{q}$ vectors emerges. The "spin-cholesteric" phase completes the spin analogy with liquid crystals and renders Sr$_3$Fe$_2$O$_7$ a touchstone for studying transitions among multiple-$\mathbf{q}$ spin textures in a centrosymmetric host.

cond-mat.str-el↗

arXiv2023

Magnetic structure, excitations and field induced transitions in the honeycomb lattice $\rm{Er_2Si_2O_7}$

We investigate the magnetic properties of the monoclinic D-type $\rm{Er_2Si_2O_7}$ with a distorted honeycomb lattice using powder and single crystal neutron scattering techniques, as well as single crystal magnetisation measurements. The powder neutron diffraction shows that below the ordering temperature, $T_{\rm N}=1.85$ K, the compound forms a ${\bf q}=0$ antiferromagnetic structure with four sublattices. For $H \! \parallel \! a$, magnetisation measurements reveal a narrow, but clearly visible plateau at one third of the magnetisation saturation value. The plateau's stabilisation is accompanied by a significant increase of the magnetic unit cell, as the magnetic peaks with fractional indices are observed in single crystal neutron diffraction experiments. At low-temperatures, the inelastic neutron scattering measurements reveal the presence of low-energy dispersionless excitations. Their spectrum is sensitive to the applied field, it significantly softens on the magnetisation plateau, and demonstrates the behaviour expected for a non-collinear Ising antiferromagnet away from the plateau.

cond-mat.str-el↗

arXiv2023

Spin-wave dispersion and magnon chirality in multiferroic TbMnO3

Inelastic neutron scattering experiments combining time-of-flight and polarized techniques yield a comprehensive picture of the magnon dispersion in multiferroic TbMnO3 including the dynamic chirality. Taking into account only Mn3+ moments, spin-wave calculations including nearest-neighbor interactions, frustrating next-nearest neighbor exchange as well as single-ion anisotropy and antisymmetric terms describe the energy dispersion and the distribution of neutron scattering intensity in the multiferroic state very well. Polarized neutron scattering reveals strong dynamic chirality of both signs that may be controlled by external electric fields in the multiferroic phase. Also above the onset of long-range multiferroic order in zero electric field, a small inelastic chiral component can be inverted by an electric field. The microscopic spin-wave calculations fully explain also the dynamic chirality of magnetic excitations, which is imprinted by the static chirality of the multiferroic phase. The ordering of Tb3+ moments at lower temperature reduces the broadening of magnons but also renders the magnon dispersion more complex.

cond-mat.str-el↗

arXiv2023

Magnetic ground states and excitations in Zn-doped averieite -- a family of oxide-based $S=1/2$ kagome antiferromagnets

Spin-1/2 kagome materials have recently attracted a resurgence of interest as they are considered an ideal host of the quantum spin liquid (QSL) state, which can underpin functionality such as superconductivity. Here we report the first synthesis and characterization of a new oxide-based distorted $S=1/2$ kagome antiferromagnet (KAFM) in the Zn$_x$Cu$_{5-x}$(VO$_4$)$_{2}$O$_2$CsCl (termed Zn$\mathbf {_x}$) series, namely Zn$_2$-averievite, Zn$\mathbf {_2}$ ($x=2$). Using magnetometry, synchrotron diffraction and neutron scattering we demonstrate an evolution of ground states with $x$ in Zn$\mathbf {_x}$; from long-range magnetic order in averievite ($x=0$), via a spin-glass-like ground state in Zn$\mathbf {_1}$, to a quantum spin liquid (QSL) in Zn$\mathbf {_2}$ for which inelastic neutron scattering reveals a gapless continuum of excitations. Similar to archetypal $S = 1/2$ KAFMs herbertsmithite and SrCr$_{8.19}$Ga$_{3.81}$O$_{19}$ (SCGO), the dynamic magnetic susceptibility of Zn$\mathbf {_2}$ shows scaling behavior consistent with proximity to a quantum critical point. The results demonstrate that the new Zn$\mathbf {_2}$ material is an excellent test bed for achieving the elusive goal of charge carrier doping in QSL states of $S = 1/2$ KAFMs, in-line with previous theoretical studies.

cond-mat.str-el↗

arXiv2022

Spin-wave dynamics in the KCeS2 delafossite: A theoretical description of powder inelastic neutron-scattering data

Layered rare-earth delafossites $ARX_2$ with $R$ = Yb(III) or Ce(III) have received a lot of interest as potential hosts for a quantum spin-liquid ground state. Some systems of this family that show no long-range order down to the lowest measured temperatures, such as NaYbO$_2$, NaYbS$_2$, and NaYbSe$_2$, are presumed to be in a quantum spin-liquid state. However, other isostructural compounds are known to order antiferromagnetically at subkelvin temperatures. Among them, KCeS$_2$ exhibits stripe-$yz$ magnetic order in the triangular-lattice planes that sets in below 400 mK. Here we investigate the spin-wave spectrum of this ordered phase with powder inelastic neutron scattering and describe it using a model Hamiltonian obtained from first-principles calculations based on the complete $|J,m_J\rangle$ multiplet description of the Ce sites with an anisotropic nearest-neighbor exchange interaction. Combing the current understanding of the exchange interaction in the systems with the effects of texturing in the powder, we have been able to model the inelastic neutron scattering spectrum with high fidelity.

cond-mat.str-el↗

arXiv2022

From spin liquid to magnetic ordering in the anisotropic kagome Y-Kapellasite Y3Cu9(OH)19Cl8: a single crystal study

Y3Cu9(OH)19Cl8 realizes an original anisotropic kagome model hosting a rich magnetic phase diagram [M. Hering et al, npj Computational Materials 8, 1 (2022)]. We present an improved synthesis of large phase-pure single crystals via an external gradient method. These crystals were investigated in details by susceptibility, specific heat, thermal expansion, neutron scattering and local muSR and NMR techniques. At variance with polycristalline samples, the study of single crystals gives evidence for subtle structural instabilities at 33K and 13K which preserve the global symmetry of the system and thus the magnetic model. At 2.1K the compound shows a magnetic transition to a coplanar (1/3,1/3) long range order as predicted theoretically. However our analysis of the spin wave excitations yields magnetic interactions which locate the compound closer to the phase boundary to a classical jammed spin liquid phase. Enhanced quantum fluctuations at this boundary may be responsible for the strongly reduced ordered moment of the Cu2+, estimated to be 0.075muB from muSR.

cond-mat.str-el↗

arXiv2022

Magnetically ordered and kagome quantum spin liquid states in the Zn-doped claringbullite series

Neutron scattering measurements have been performed on deuterated powder samples of claringbullite and Zn-doped claringbullite (Zn$_x$Cu$_{4-x}$(OD)$_{6}$FCl). At low temperatures, claringbullite Cu$_4$(OD)$_{6}$FCl forms a distorted pyrochlore lattice with long-range magnetic order and spin-wave-like magnetic excitations. Partial Zn doping leads to the nominal ZnCu$_3$(OD)$_{6}$FCl compound, a geometrically frustrated spin-1/2 kagome antiferromagnet that shows no transition to magnetic order down to 1.5 K. The magnetic excitations form a gapless continuum, a signature of fractional excitations in a quantum spin liquid.

cond-mat.str-el↗