P. Santini - Academia.edu (original) (raw)
Papers by P. Santini
Physical Review Letters, 2002
We report the results of resonant X-ray scattering experiments performed at the Np M4,5 edges in ... more We report the results of resonant X-ray scattering experiments performed at the Np M4,5 edges in NpO2. Below T0 = 25 K, the development of long-range order of Np electric quadrupoles is revealed by the growth of superlattice Bragg peaks. The electronic transition is not accompanied by any measurable crystallographic distortion, either internal or external, so the symmetry of the system remains cubic. The polarization and azimuthal dependence of the intensity of the resonant peaks is well reproduced assuming anisotropic tensor susceptibility (ATS) scattering from a triple-q longitudinal antiferroquadrupolar structure. Electric quadrupole order in NpO2 could be driven by the ordering at T0 of magnetic octupoles of Γ5 symmetry, splitting the Np ground state quartet and leading to a singlet ground state with zero dipole magnetic moment. PACS numbers: 75.25.+z,75.10.-b
Physical Review Letters, 2006
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.
Physica B Condensed Matter
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
Reviews of Modern Physics, 2009
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.
Physical Review Letters, 2010
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.
Physical Review Letters, 2007
Using inelastic neutron scattering and applied fields up to 11.4 T, we have studied the spin dyna... more Using inelastic neutron scattering and applied fields up to 11.4 T, we have studied the spin dynamics of the Cr 7 Ni antiferromagnetic ring in the energy window 0.05-1.6 meV. We demonstrate that the external magnetic field induces an avoided crossing (anticrossing) between energy levels with different total-spin quantum numbers. This corresponds to quantum oscillations of the total spin of each molecule. The inelastic character of the observed excitation and the field dependence of its linewidth indicate that molecular spins oscillate coherently for a significant number of cycles. Precise signatures of the anticrossing are also found at higher energy, where measured and calculated spectra match very well.
![Research paper thumbnail of Quantum Magneto-Oscillations in a Supramolecular Mn(II)-[3×3] Grid](https://attachments.academia-assets.com/44586559/thumbnails/1.jpg)
](Physical Review Letters, 2004
The magnetic grid molecule Mn(II)-[3 × 3] has been studied by high-field torque magnetometry at 3... more The magnetic grid molecule Mn(II)-[3 × 3] has been studied by high-field torque magnetometry at 3 He temperatures. At fields above 5 T, the torque vs. field curves exhibit an unprecedented oscillatory behavior. A model is proposed which describes these magneto oscillations well. PACS numbers: 33.15.Kr, 75.10.Jm Among the known magneto-oscillatory effects, the de Haas-van Alphen (dHvA) effect in metals (and the group of effects related to it) is the most prominent example, having had a formative influence on our modern picture of solid state physics . The dHvA effect originates from a quantization of closed electron orbits into Landau levels in a magnetic field, so that the density of states at the Fermi energy exhibits a markedly oscillatory evolution as a function of magnetic field. Currently, it finds application in a wide range of materials, e.g. heavy fermion compounds [2], low-dimensional organic metals [3], hightemperature superconductors [4], two-dimensional electron gases , or in the recently discovered superconductor MgB 2 [6].
Physical Review Letters, 2000
ABSTRACT
Physical Review B, 2006
Inelastic neutron scattering is used to study excited spin multiplets and low-energy phonons in t... more Inelastic neutron scattering is used to study excited spin multiplets and low-energy phonons in the Fe 8 nanomagnet. The energies of the three lowest-lying excited multiplets are determined. In addition, we find weakly dispersive optical phonons with energies lower than the anisotropy splitting of the ground S = 10 spin multiplet. We show that these optical phonons may produce sharp minima in the field dependence of the relaxation time. The positions of the calculated minima agree with those that had been inferred from frequencydependent specific heat and ac-susceptibility measurements.
Physical Review B, 2005
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.
Inelastic neutron scattering has been applied to the study of the spin dynamics of Cr-based antif... more Inelastic neutron scattering has been applied to the study of the spin dynamics of Cr-based antiferromagnetic octanuclear rings where a finite total spin of the ground state is obtained by substituting one Cr 3+ ion (s = 3/2) with Zn (s = 0), Mn (s = 5/2) or Ni (s = 1) di-cations. Energy and intensity measurements for several intra-multiplet and inter-multiplet magnetic excitations allow us to determine the spin wavefunctions of the investigated clusters. Effects due to the mixing of different spin multiplets have been considered. Such effects proved to be important to correctly reproduce the energy and intensity of magnetic excitations in the neutron spectra. On the contrary to what is observed for the parent homonuclear Cr8 ring, the symmetry of the first excited spin states is such that anticrossing conditions with the ground state can be realized in the presence of an external magnetic field. Heterometallic Cr7M wheels are therefore good candidates for macroscopic observations of quantum effects.
Physical Review B, 2004
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
Physical Review B, 2011
We report the results of inelastic neutron scattering experiments performed with triple-axis spec... more We report the results of inelastic neutron scattering experiments performed with triple-axis spectrometers to investigate the low-temperature collective dynamics in the ordered phase of uranium dioxide. The results are in very good agreement with the predictions of mean-field random phase approximation calculations, emphasizing the importance of multipolar superexchange interactions. By comparing neutron scattering intensities in different polarization channels and at equivalent points in different Brillouin zones, we show the mixed magnetovibrationalquadrupolar character of the observed excitations. The high-energy resolution afforded by the cold triple-axis spectrometer allowed us to study in detail the magnon-phonon interaction giving rise to avoided crossings along the [00ξ ] reciprocal space direction.
Physical Review B, 2004
We have used element specific X-ray resonant scattering to investigate the M edge resonances in a... more We have used element specific X-ray resonant scattering to investigate the M edge resonances in a single crystal of U 0.75 Np 0.25 O 2 . Earlier neutron diffraction and Mössbauer studies had shown the presence of long-range AF order below T N = 19 K, with sizeable magnetic moment both on the U and the Np ions. RXS results confirm the presence of an ordered dipole magnetic moment on the Np ions, but also reveal the presence of an anti-ferro arrangement of the electric quadrupole moments on both U and Np ions, with the same propagation vector that defines the magnetic structure. From the azimuthal dependence of the intensities we are able to determine the exact configuration of the quadrupolar ordering. The intensities of Bragg peaks associated with magnetic dipole and electric quadrupole order have different temperature dependences. On cooling below T N , the magnetic dipole order develops faster on the uranium ions, with magnetic order on the Np ions increasing at a lower rate. At the same temperature, quadrupolar order on both the U and Np ions occurs along with an internal Jahn-Teller lattice distortion. PACS numbers: 75.25.+z, 75.10.-b
Physical Review B, 2005
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.
Physical Review B, 2008
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
Physical Review Letters, 2002
We report the results of resonant X-ray scattering experiments performed at the Np M4,5 edges in ... more We report the results of resonant X-ray scattering experiments performed at the Np M4,5 edges in NpO2. Below T0 = 25 K, the development of long-range order of Np electric quadrupoles is revealed by the growth of superlattice Bragg peaks. The electronic transition is not accompanied by any measurable crystallographic distortion, either internal or external, so the symmetry of the system remains cubic. The polarization and azimuthal dependence of the intensity of the resonant peaks is well reproduced assuming anisotropic tensor susceptibility (ATS) scattering from a triple-q longitudinal antiferroquadrupolar structure. Electric quadrupole order in NpO2 could be driven by the ordering at T0 of magnetic octupoles of Γ5 symmetry, splitting the Np ground state quartet and leading to a singlet ground state with zero dipole magnetic moment. PACS numbers: 75.25.+z,75.10.-b
Physical Review Letters, 2006
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.
Physica B Condensed Matter
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
Reviews of Modern Physics, 2009
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.
Physical Review Letters, 2010
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.
Physical Review Letters, 2007
Using inelastic neutron scattering and applied fields up to 11.4 T, we have studied the spin dyna... more Using inelastic neutron scattering and applied fields up to 11.4 T, we have studied the spin dynamics of the Cr 7 Ni antiferromagnetic ring in the energy window 0.05-1.6 meV. We demonstrate that the external magnetic field induces an avoided crossing (anticrossing) between energy levels with different total-spin quantum numbers. This corresponds to quantum oscillations of the total spin of each molecule. The inelastic character of the observed excitation and the field dependence of its linewidth indicate that molecular spins oscillate coherently for a significant number of cycles. Precise signatures of the anticrossing are also found at higher energy, where measured and calculated spectra match very well.
Physical Review Letters, 2004
The magnetic grid molecule Mn(II)-[3 × 3] has been studied by high-field torque magnetometry at 3... more The magnetic grid molecule Mn(II)-[3 × 3] has been studied by high-field torque magnetometry at 3 He temperatures. At fields above 5 T, the torque vs. field curves exhibit an unprecedented oscillatory behavior. A model is proposed which describes these magneto oscillations well. PACS numbers: 33.15.Kr, 75.10.Jm Among the known magneto-oscillatory effects, the de Haas-van Alphen (dHvA) effect in metals (and the group of effects related to it) is the most prominent example, having had a formative influence on our modern picture of solid state physics . The dHvA effect originates from a quantization of closed electron orbits into Landau levels in a magnetic field, so that the density of states at the Fermi energy exhibits a markedly oscillatory evolution as a function of magnetic field. Currently, it finds application in a wide range of materials, e.g. heavy fermion compounds [2], low-dimensional organic metals [3], hightemperature superconductors [4], two-dimensional electron gases , or in the recently discovered superconductor MgB 2 [6].
Physical Review Letters, 2000
ABSTRACT
Physical Review B, 2006
Inelastic neutron scattering is used to study excited spin multiplets and low-energy phonons in t... more Inelastic neutron scattering is used to study excited spin multiplets and low-energy phonons in the Fe 8 nanomagnet. The energies of the three lowest-lying excited multiplets are determined. In addition, we find weakly dispersive optical phonons with energies lower than the anisotropy splitting of the ground S = 10 spin multiplet. We show that these optical phonons may produce sharp minima in the field dependence of the relaxation time. The positions of the calculated minima agree with those that had been inferred from frequencydependent specific heat and ac-susceptibility measurements.
Physical Review B, 2005
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.
Inelastic neutron scattering has been applied to the study of the spin dynamics of Cr-based antif... more Inelastic neutron scattering has been applied to the study of the spin dynamics of Cr-based antiferromagnetic octanuclear rings where a finite total spin of the ground state is obtained by substituting one Cr 3+ ion (s = 3/2) with Zn (s = 0), Mn (s = 5/2) or Ni (s = 1) di-cations. Energy and intensity measurements for several intra-multiplet and inter-multiplet magnetic excitations allow us to determine the spin wavefunctions of the investigated clusters. Effects due to the mixing of different spin multiplets have been considered. Such effects proved to be important to correctly reproduce the energy and intensity of magnetic excitations in the neutron spectra. On the contrary to what is observed for the parent homonuclear Cr8 ring, the symmetry of the first excited spin states is such that anticrossing conditions with the ground state can be realized in the presence of an external magnetic field. Heterometallic Cr7M wheels are therefore good candidates for macroscopic observations of quantum effects.
Physical Review B, 2004
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
Physical Review B, 2011
We report the results of inelastic neutron scattering experiments performed with triple-axis spec... more We report the results of inelastic neutron scattering experiments performed with triple-axis spectrometers to investigate the low-temperature collective dynamics in the ordered phase of uranium dioxide. The results are in very good agreement with the predictions of mean-field random phase approximation calculations, emphasizing the importance of multipolar superexchange interactions. By comparing neutron scattering intensities in different polarization channels and at equivalent points in different Brillouin zones, we show the mixed magnetovibrationalquadrupolar character of the observed excitations. The high-energy resolution afforded by the cold triple-axis spectrometer allowed us to study in detail the magnon-phonon interaction giving rise to avoided crossings along the [00ξ ] reciprocal space direction.
Physical Review B, 2004
We have used element specific X-ray resonant scattering to investigate the M edge resonances in a... more We have used element specific X-ray resonant scattering to investigate the M edge resonances in a single crystal of U 0.75 Np 0.25 O 2 . Earlier neutron diffraction and Mössbauer studies had shown the presence of long-range AF order below T N = 19 K, with sizeable magnetic moment both on the U and the Np ions. RXS results confirm the presence of an ordered dipole magnetic moment on the Np ions, but also reveal the presence of an anti-ferro arrangement of the electric quadrupole moments on both U and Np ions, with the same propagation vector that defines the magnetic structure. From the azimuthal dependence of the intensities we are able to determine the exact configuration of the quadrupolar ordering. The intensities of Bragg peaks associated with magnetic dipole and electric quadrupole order have different temperature dependences. On cooling below T N , the magnetic dipole order develops faster on the uranium ions, with magnetic order on the Np ions increasing at a lower rate. At the same temperature, quadrupolar order on both the U and Np ions occurs along with an internal Jahn-Teller lattice distortion. PACS numbers: 75.25.+z, 75.10.-b
Physical Review B, 2005
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.
Physical Review B, 2008
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