Materials perspectives on achieving pyrochlore-based quantum spin liquid ground states

Exotic magnetic systems often stem from frustration caused by competing interactions. The epitome of magnetically frustrated system is incarnated by the quantum spin liquids. These systems are expected to evade magnetic ordering or freezing down to absolute zero Temperature. Magnetic correlations are expected to be strong and can lead to quantum entanglement over large scales as well as the emergence of exotic excitations. Stabilizing such a phase in a rare-earth pyrochlore oxide yields a so-called quantum spin ice resulting from the coherent superposition of ‘two-in-two-out’ spin ice configurations, reminiscent of the arrangement of hydrogen atoms in water ice. In some rare-earth pyrochlore oxides, magnetic rare earth ions experience a strong uniaxial anisotropy as well as frustrated ferromagnetic interactions, two of the main ingredients stabilizing a spin ice state. In particular, Ho2Ti2O7 and Dy2Ti2O7 have been thoroughly studied and are now two well established examples of classical spin ices. Owing to a reduced dipolar interaction, similar materials hosting smaller magnetic moments are thought to be ideal systems to search for quantum spin ices states. Such a state is favored by an Hamiltonian composed of a leading but not exceedingly large Ising term, forming the ’ice’ state, accompanied by smaller but non-negligible transverse terms allowing quantum fluctuations to tunnel through equivalent spin ice configurations. Additionally, other properties such as multipolar degrees of freedom, structural disorder or low-lying crystal-electric field levels are expected to have significant contributions to the magnetic ground state. Cerium-based pyrochlores are promising quantum spin liquids candidates, which received little attention until recently due to their demanding synthesis. Using thermodynamic measurements together with a detailed structural and crystal-electric field analysis, we show that both Ce2Sn2O7 and Ce2Hf2O7 display magnetic moments with an Ising character and no sign of spin ordering or freezing despite signs of magnetic correlations at low temperatures. Through various neutron experiments, we demonstrate that the compounds form a peculiar octupolar quantum spin ice state, where dominant octupolar correlations conspire to form a coherent ‘ice-like’ phase from which fractional excitations emerge. High resolution neutron backscattering spectroscopy shows convinc- ing agreement with the theoretical predictions of the quantum electrodynamics emerging from a quantum spin ice phase. Disorder is known to have a significant influence on spin liquids, typically leading to spin freezing or ordering. In particular, pyrochlore oxides containing non-Kramers rare earths are very sensitive to structural disorder, a specificity that was theoretically proposed as a way to stabilize a quantum spin liquid phase. We hereby present the study of three cases where structural disorder plays a prominent role in the low temperature magnetic properties. We start with the case of Tb2Hf2O7, a strongly disordered yet very promising spin liquid candidate with a puzzling low temperature correlated state. A detailed structural analysis, coupled with a point charge model, yields a qualitative understanding of the single-ion properties of this compound and provides new insights into the mechanisms behind its low temperature behavior. We then perform a comparative structural study of two praseodymium-based compounds, Pr2Hf2O7 and Pr2Zr2O7, which display distinct magnetic ground states in spite of their structural and chemical proximity. Finally, we show how the control over the structural disorder via chemical substitution allows the tuning of the magnetic behavior of spin ice state observed in Ho2Ti2O7

Fractional matter coupled to the emergent gauge field in a quantum spin ice

International audienceElectronic spins can form long-range entangled phases of condensed matter named quantum spin liquids. Their existence is conceptualized in models of two- or three-dimensional frustrated magnets that evade symmetry-breaking order down to zero temperature. Quantum spin ice (QSI) is a theoretically well-established example described by an emergent quantum electrodynamics, with excitations behaving like photon and matter quasiparticles. The latter are fractionally charged and equivalent to the `spinons' emerging from coherent phases of singlets in one dimension, where clear experimental proofs of fractionalization exist. However, in frustrated magnets it remains difficult to establish consensual evidence for quantum spin liquid ground states and their fractional excitations. Here, we use backscattering neutron spectroscopy to achieve extremely high resolution of the time-dependent magnetic response of the candidate QSI material Ce$_2$Sn$_2$O$_7$. We find a gapped spectrum featuring a threshold and peaks that match theories for pair production and propagation of fractional matter excitations (spinons) strongly coupled to a background gauge field. The observed peaks provide evidence for a QSI through spectroscopic signatures of space-time symmetry fractionalization, while the threshold behavior corroborates the regime of strong light-matter interaction predicted for the emergent universe in a QSI

Fractional matter coupled to the emergent gauge field in a quantum spin ice

International audienceElectronic spins can form long-range entangled phases of condensed matter named quantum spin liquids. Their existence is conceptualized in models of two- or three-dimensional frustrated magnets that evade symmetry-breaking order down to zero temperature. Quantum spin ice (QSI) is a theoretically well-established example described by an emergent quantum electrodynamics, with excitations behaving like photon and matter quasiparticles. The latter are fractionally charged and equivalent to the `spinons' emerging from coherent phases of singlets in one dimension, where clear experimental proofs of fractionalization exist. However, in frustrated magnets it remains difficult to establish consensual evidence for quantum spin liquid ground states and their fractional excitations. Here, we use backscattering neutron spectroscopy to achieve extremely high resolution of the time-dependent magnetic response of the candidate QSI material Ce$_2$Sn$_2$O$_7$. We find a gapped spectrum featuring a threshold and peaks that match theories for pair production and propagation of fractional matter excitations (spinons) strongly coupled to a background gauge field. The observed peaks provide evidence for a QSI through spectroscopic signatures of space-time symmetry fractionalization, while the threshold behavior corroborates the regime of strong light-matter interaction predicted for the emergent universe in a QSI

Crystal-field states and defect levels in candidate quantum spin ice Ce2_{2}Hf2_{2}O7_{7}

International audienceWe report the synthesis of powder and single-crystal samples of cerium pyrohafnate and their characterization using neutron diffraction, thermogravimetry and x-ray absorption spectroscopy. We evaluate the amount of nonmagnetic Ce4+ defects and use this result to interpret the spectrum of crystal-electric field transitions observed using inelastic neutron scattering. The analysis of these single-ion transitions indicates the dipole-octupole nature of the ground-state doublet and a significant degree of spin-lattice coupling. The single-ion properties calculated from the crystal-electric field parameters obtained spectroscopically are in good agreement with bulk magnetic susceptibility data down to about 1 K. Below this temperature, the behavior of the magnetic susceptibility indicates a correlated regime without showing any sign of magnetic long-range order or freezing down to 0.08 K. We conclude that Ce2Hf2O7 is another candidate to investigate exotic correlated states of quantum matter, such as the octupolar quantum spin ice recently argued to exist in the isostructural compounds Ce2Sn2O7 and Ce2Zr2O7

Dipolar-octupolar correlations and hierarchy of exchange interactions in Ce2_2Hf2_2O7_7

We investigate the correlated state of Ce$_2$Hf$_2$O$_7$ using neutron scattering, finding signatures of correlations of both dipolar and octupolar character. A dipolar inelastic signal is also observed, as expected for spinons in a quantum spin ice (QSI). Fits of thermodynamic data using exact diagonalization methods indicate that the largest interaction is an octupolar exchange, with a strength roughly twice as large as other terms. This hierarchy of exchange interactions - far from a perturbative regime but still in the octupolar QSI phase - rationalises observations in neutron scattering, which illustrate the multipolar nature of degrees of freedom in Ce$^{3+}$ pyrochlores.Comment: 6 pages, 3 figure

Dipolar-octupolar correlations and hierarchy of exchange interactions in Ce2_2Hf2_2O7_7

6 pages, 3 figuresWe investigate the correlated state of Ce$_2$Hf$_2$O$_7$ using neutron scattering, finding signatures of correlations of both dipolar and octupolar character. A dipolar inelastic signal is also observed, as expected for spinons in a quantum spin ice (QSI). Fits of thermodynamic data using exact diagonalization methods indicate that the largest interaction is an octupolar exchange, with a strength roughly twice as large as other terms. This hierarchy of exchange interactions - far from a perturbative regime but still in the octupolar QSI phase - rationalises observations in neutron scattering, which illustrate the multipolar nature of degrees of freedom in Ce$^{3+}$ pyrochlores

Dipolar-octupolar correlations and hierarchy of exchange interactions in Ce2_2Hf2_2O7_7

6 pages, 3 figuresWe investigate the correlated state of Ce$_2$Hf$_2$O$_7$ using neutron scattering, finding signatures of correlations of both dipolar and octupolar character. A dipolar inelastic signal is also observed, as expected for spinons in a quantum spin ice (QSI). Fits of thermodynamic data using exact diagonalization methods indicate that the largest interaction is an octupolar exchange, with a strength roughly twice as large as other terms. This hierarchy of exchange interactions - far from a perturbative regime but still in the octupolar QSI phase - rationalises observations in neutron scattering, which illustrate the multipolar nature of degrees of freedom in Ce$^{3+}$ pyrochlores

Emergent electronic landscapes in a novel valence-ordered nickelate with tri-component nickel coordination

The metal-hydride-based topochemical reduction process has produced novel thermodynamically unstable phases across various transition metal oxide series with unusual crystal structures and non-trivial ground states. Here, by such an oxygen (de-) intercalation method we synthesis a novel samarium nickelate with ordered nickel valences associated with tri-component coordination configurations. This structure, with a formula of Sm$_{9}$Ni$_{9}$O$_{22}$ as revealed by four-dimensional scanning transmission electron microscopy, emerges from the intricate planes of {303}$_{\text{pc}}$ ordered apical oxygen vacancies. X-ray spectroscopy measurements and ab-initio calculations show the coexistence of square-planar, pyramidal and octahedral Ni sites with mono-, bi- and tri-valences. It leads to an intense orbital polarization, charge-ordering, and a ground state with a strong electron localization marked by the disappearance of ligand-hole configuration at low-temperature. This new nickelate compound provides another example of previously inaccessible materials enabled by topotactic transformations and presents a unique platform where mixed Ni valence can give rise to exotic phenomena

Valence-Ordered Thin-Film Nickelate with Tricomponent Nickel Coordination Prepared by Topochemical Reduction

International audienceThe metal-hydride-based "topochemical reduc-tion" process has produced several thermodynamically unstable phases across various transition metal oxide series with unusual crystal structures and nontrivial ground states. Here, by such an oxygen (de-)intercalation method we synthesis a samarium nickelate with ordered nickel valences associated with tri-component coordination configurations. This structure, with a formula of Sm 9 Ni 9 O 22 as revealed by four-dimensional scanning transmission electron microscopy (4D-STEM), emerges from the intricate planes of {303} pc ordered apical oxygen vacancies. X-ray spectroscopy measurements and ab initio calculations show the coexistence of square planar, pyramidal, and octahedral Ni sites with mono-, bi-, and tri-valences. It leads to an intense orbital polarization, charge-ordering, and a ground state with a strong electron localization marked by the disappearance of ligand-hole configuration at low temperature. This nickelate compound provides another example of previously inaccessible materials enabled by topotactic transformations and presents an interesting platform where mixed Ni valence can give rise to exotic phenomena