We investigate the nature of the hidden order parameter in the ordered phase of NpO2, which had b... more We investigate the nature of the hidden order parameter in the ordered phase of NpO2, which had been identified with a staggered arrangement of Gamma5 magnetic multipoles. By analyzing the existing experimental data, we show that the most likely driving order parameter is not provided by octupoles, as usually assumed, but rather by the rank-5 triakontadipoles. Calculations of the coupled dynamics of spins, Gamma5 quadrupoles, and Gamma5 triakontadipoles in the ordered phase enable us to analyze the resulting structure of low-energy excitations. We show that the powder inelastic neutron scattering cross section should contain, in addition to the already-observed peak at 6.5 meV, a second weaker peak at about 14 meV.
Resonant X-ray scattering (RXS) experiments on NpO2 provide indications for a low-temperature pha... more Resonant X-ray scattering (RXS) experiments on NpO2 provide indications for a low-temperature phase where magnetic octupole order is established in absence of magnetic dipole order. The interpretation of the RXS findings has led to a controversy about the symmetry of the primary order parameter, which has been identified either with G2 or with G5 octupoles. A consequence of long-range octupole order is that the degeneracy of the G8 crystal field ground state is removed. G8 would be split into two doublets if G2 octupoles order, and into two singlets and one doublet if the order parameter has G5 symmetry. Here we present specific heat measurements between 0.7K and 300K supporting a singlet ground state in the ordered phase below 25.5 K.
Molecular nano-magnets (MNMs) are clusters made by a small number of exchange-coupled ions, arran... more Molecular nano-magnets (MNMs) are clusters made by a small number of exchange-coupled ions, arranged in regular crystals and forming a macroscopic collection of identical and non-interacting nano-sized magnetic units. Novel and interesting phenomena have been observed in these systems, suggesting the possibility of controlling quantum coherence on the mesoscopic scale. Precious information on the spin correlation and dynamics in MNMs
This article reviews the physics of multipolar interactions and multipolar order in f -electron s... more This article reviews the physics of multipolar interactions and multipolar order in f -electron systems, using the actinide dioxides as a paradigm. In the past few years, these apparently simple cubic compounds have been studied intensively, and many new phenomena have been discovered. Here the experimental results are discussed together with the current theoretical understanding of multipolar interactions.
In presence of active orbital degrees of freedom, elementary excitations around a broken-symmetry... more In presence of active orbital degrees of freedom, elementary excitations around a broken-symmetry state may include multipolar waves, but none of these exotic dispersive excitation branches has ever been identified. We show that quadrupolar waves constitute a major component of the dynamics of uranium dioxide in its magnetoquadrupolar ordered phase, and that many unexplained features in existing inelastic neutron scattering data, including a whole excitation branch, are associated with these propagating quadrupolar fluctuations. Our model permits us to separate the role of Jahn-Teller and superexchange mechanisms as sources of quadrupolar interactions.
ABSTRACT The anisotropy of magnetic fluctuations propagating along the symmetric directions Δ, Σ,... more ABSTRACT The anisotropy of magnetic fluctuations propagating along the symmetric directions Δ, Σ, and Γ in the ordered phase of uranium dioxide has been studied by applying the spherical neutron-spin polarimetry technique to inelastic neutron scattering. The dependence of fluctuation amplitudes and directions as a function of the quasiparticle wave vector q has been investigated along the main crystallographic directions. The observed behavior is discussed in relation to the spin dynamics of the 3-k transverse magnetic structure. The observed polarization behavior of the spin waves, and in particular the acoustic mode, is shown to be a natural consequence of the transverse 3-k magnetic structure.
We used inelastic neutron scattering to determine the microscopic spin Hamiltonian of a tetrairon... more We used inelastic neutron scattering to determine the microscopic spin Hamiltonian of a tetrairon(III) molecular cluster with trigonal symmetry, [Fe4(thme)2(dpm)6] , obtained from a parent compound with C2 symmetry by site-specific ligand substitution. Intra-multiplet excitations within the anisotropy split S=5 ground spin state, and inter-multiplet transitions toward the first S=4 excited states have been observed. The model spin Hamiltonian used
ABSTRACT The spin dynamics of a ring-shaped molecule comprising 10 iron(III) ions (s=5∕2) is stud... more ABSTRACT The spin dynamics of a ring-shaped molecule comprising 10 iron(III) ions (s=5∕2) is studied by inelastic neutron scattering. Exchange integrals and single-ion anisotropy parameters are determined by studying several intermultiplet excitations. Difficulties associated with the dimension of the Hilbert space are overcome by exploiting both the irreducible tensor operator technique and the Lanczos algorithm. By calculating the time correlation function of the Néel vector we show that the low-temperature spin dynamics of Fe10 is only approximately described by the semiclassical picture of tunneling of the Néel vector.
We have performed polarized inelastic neutron scattering experiments in the ordered phase of nept... more We have performed polarized inelastic neutron scattering experiments in the ordered phase of neptunium dioxide. The observation of magnetic scattering between 11 and 18 meV can be attributed to a transition between two states with the same expectation value of the electric quadrupole but opposite values of the magnetic triakontadipole. In contrast to resonant x-ray scattering, which detects the secondary
We investigate the nature of the hidden order parameter in the ordered phase of NpO2, which had b... more We investigate the nature of the hidden order parameter in the ordered phase of NpO2, which had been identified with a staggered arrangement of Gamma5 magnetic multipoles. By analyzing the existing experimental data, we show that the most likely driving order parameter is not provided by octupoles, as usually assumed, but rather by the rank-5 triakontadipoles. Calculations of the coupled dynamics of spins, Gamma5 quadrupoles, and Gamma5 triakontadipoles in the ordered phase enable us to analyze the resulting structure of low-energy excitations. We show that the powder inelastic neutron scattering cross section should contain, in addition to the already-observed peak at 6.5 meV, a second weaker peak at about 14 meV.
Resonant X-ray scattering (RXS) experiments on NpO2 provide indications for a low-temperature pha... more Resonant X-ray scattering (RXS) experiments on NpO2 provide indications for a low-temperature phase where magnetic octupole order is established in absence of magnetic dipole order. The interpretation of the RXS findings has led to a controversy about the symmetry of the primary order parameter, which has been identified either with G2 or with G5 octupoles. A consequence of long-range octupole order is that the degeneracy of the G8 crystal field ground state is removed. G8 would be split into two doublets if G2 octupoles order, and into two singlets and one doublet if the order parameter has G5 symmetry. Here we present specific heat measurements between 0.7K and 300K supporting a singlet ground state in the ordered phase below 25.5 K.
Molecular nano-magnets (MNMs) are clusters made by a small number of exchange-coupled ions, arran... more Molecular nano-magnets (MNMs) are clusters made by a small number of exchange-coupled ions, arranged in regular crystals and forming a macroscopic collection of identical and non-interacting nano-sized magnetic units. Novel and interesting phenomena have been observed in these systems, suggesting the possibility of controlling quantum coherence on the mesoscopic scale. Precious information on the spin correlation and dynamics in MNMs
This article reviews the physics of multipolar interactions and multipolar order in f -electron s... more This article reviews the physics of multipolar interactions and multipolar order in f -electron systems, using the actinide dioxides as a paradigm. In the past few years, these apparently simple cubic compounds have been studied intensively, and many new phenomena have been discovered. Here the experimental results are discussed together with the current theoretical understanding of multipolar interactions.
In presence of active orbital degrees of freedom, elementary excitations around a broken-symmetry... more In presence of active orbital degrees of freedom, elementary excitations around a broken-symmetry state may include multipolar waves, but none of these exotic dispersive excitation branches has ever been identified. We show that quadrupolar waves constitute a major component of the dynamics of uranium dioxide in its magnetoquadrupolar ordered phase, and that many unexplained features in existing inelastic neutron scattering data, including a whole excitation branch, are associated with these propagating quadrupolar fluctuations. Our model permits us to separate the role of Jahn-Teller and superexchange mechanisms as sources of quadrupolar interactions.
ABSTRACT The anisotropy of magnetic fluctuations propagating along the symmetric directions Δ, Σ,... more ABSTRACT The anisotropy of magnetic fluctuations propagating along the symmetric directions Δ, Σ, and Γ in the ordered phase of uranium dioxide has been studied by applying the spherical neutron-spin polarimetry technique to inelastic neutron scattering. The dependence of fluctuation amplitudes and directions as a function of the quasiparticle wave vector q has been investigated along the main crystallographic directions. The observed behavior is discussed in relation to the spin dynamics of the 3-k transverse magnetic structure. The observed polarization behavior of the spin waves, and in particular the acoustic mode, is shown to be a natural consequence of the transverse 3-k magnetic structure.
We used inelastic neutron scattering to determine the microscopic spin Hamiltonian of a tetrairon... more We used inelastic neutron scattering to determine the microscopic spin Hamiltonian of a tetrairon(III) molecular cluster with trigonal symmetry, [Fe4(thme)2(dpm)6] , obtained from a parent compound with C2 symmetry by site-specific ligand substitution. Intra-multiplet excitations within the anisotropy split S=5 ground spin state, and inter-multiplet transitions toward the first S=4 excited states have been observed. The model spin Hamiltonian used
ABSTRACT The spin dynamics of a ring-shaped molecule comprising 10 iron(III) ions (s=5∕2) is stud... more ABSTRACT The spin dynamics of a ring-shaped molecule comprising 10 iron(III) ions (s=5∕2) is studied by inelastic neutron scattering. Exchange integrals and single-ion anisotropy parameters are determined by studying several intermultiplet excitations. Difficulties associated with the dimension of the Hilbert space are overcome by exploiting both the irreducible tensor operator technique and the Lanczos algorithm. By calculating the time correlation function of the Néel vector we show that the low-temperature spin dynamics of Fe10 is only approximately described by the semiclassical picture of tunneling of the Néel vector.
We have performed polarized inelastic neutron scattering experiments in the ordered phase of nept... more We have performed polarized inelastic neutron scattering experiments in the ordered phase of neptunium dioxide. The observation of magnetic scattering between 11 and 18 meV can be attributed to a transition between two states with the same expectation value of the electric quadrupole but opposite values of the magnetic triakontadipole. In contrast to resonant x-ray scattering, which detects the secondary
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Papers by P. Santini