Collective nature of spin excitations in superconducting cuprates probed by resonant inelastic X-ray scattering.

We used resonant inelastic x-ray scattering (RIXS) with and without analysis of the scattered photon polarization, to study dispersive spin excitations in the high temperature superconductor YBa_{2}Cu_{3}O_{6+x} over a wide range of doping levels (0.1≤x≤1). The excitation profiles were carefully monitored as the incident photon energy was detuned from the resonant condition, and the spin excitation energy was found to be independent of detuning for all x. These findings demonstrate that the largest fraction of the spin-flip RIXS profiles in doped cuprates arises from magnetic collective modes, rather than from incoherent particle-hole excitations as recently suggested theoretically [Benjamin et al. Phys. Rev. Lett. 112, 247002 (2014)]. Implications for the theoretical description of the electron system in the cuprates are discussed.

Exchange biasing single molecule magnets: coupling of TbPc2 to antiferromagnetic layers.

We investigate the possibility to induce exchange bias between single molecule magnets (SMM) and metallic or oxide antiferromagnetic substrates. Element-resolved X-ray magnetic circular dichroism measurements reveal, respectively, the presence and absence of unidirectional exchange anisotropy for TbPc(2) SMM deposited on antiferromagnetic Mn and CoO layers. TbPc(2) deposited on Mn thin films present magnetic hysteresis and a negative horizontal shift of the Tb magnetization loop after field cooling, consistent with the observation of pinned spins in the Mn layer coupled parallel to the Tb magnetic moment. Conversely, molecules deposited on CoO substrates present paramagnetic magnetization loops with no indication of exchange bias. These experiments demonstrate the ability of SMM to polarize the pinned uncompensated spins of an antiferromagnet during field-cooling and realize metal-organic exchange-biased heterostructures using antiferromagnetic pinning layers.

Antiferromagnetically spin polarized oxygen observed in magnetoelectric TbMn2O5.

We report the direct measurement of antiferromagnetic spin polarization at the oxygen sites in the multiferroic TbMn2O5, through resonant soft x-ray magnetic scattering. This supports recent theoretical models suggesting that the oxygen spin polarization is key to the magnetoelectric coupling mechanism. The spin polarization is observed through a resonantly enhanced diffraction signal at the oxygen K edge at the commensurate antiferromagnetic wave vector. Using the fdmnes code we have accurately reproduced the experimental data. We have established that the resonance arises through the spin polarization on the oxygen sites hybridized with the square based pyramid Mn3+ ions. Furthermore we have discovered that the position of the Mn3+ ion directly influences the oxygen spin polarization.

Nature of the magnetic order and origin of induced ferroelectricity in TbMnO3.

The magnetic structures which endow TbMnO(3) with its multiferroic properties have been reassessed on the basis of a comprehensive soft x-ray resonant scattering (XRS) study. The selectivity of XRS facilitated separation of the various contributions (Mn L(2) edge, Mn 3d moments; Tb M(4) edge, Tb 4f moments), while its variation with azimuth provided information on the moment direction of distinct Fourier components. When the data are combined with a detailed group theory analysis, a new picture emerges of the ferroelectric transition at 28 K. Instead of being driven by the transition from a collinear to a noncollinear magnetic structure, as has previously been supposed, it is shown to occur between two noncollinear structures.

The simultaneous measurement of energy and linear polarization of the scattered radiation in resonant inelastic soft x-ray scattering.

Resonant Inelastic X-ray Scattering (RIXS) in the soft x-ray range is an element-specific energy-loss spectroscopy used to probe the electronic and magnetic excitations in strongly correlated solids. In the recent years, RIXS has been progressing very quickly in terms of energy resolution and understanding of the experimental results, but the interpretation of spectra could further improve, sometimes decisively, from a full knowledge of the polarization of incident and scattered photons. Here we present the first implementation, in a high resolution soft-RIXS spectrometer used to analyze the scattered radiation, of a device allowing the measurement of the degree of linear polarization. The system, based on a graded W/B4C multilayer mirror installed in proximity of the CCD detector, has been installed on the AXES spectrometer at the ESRF (European Synchrotron Radiation Facility); it has been fully characterized and it has been used for a demonstration experiment at the Cu L3 edge on a high-Tc superconducting cuprate. The loss in efficiency suffered by the spectrometer equipped with this test facility was a factor 17.5. We propose also a more advanced version, suitable for a routine use on the next generation of RIXS spectrometers and with an overall efficiency up to 10%.

ID20: a beamline for magnetic and resonant X-ray scattering investigations under extreme conditions.

A new experimental station at ESRF beamline ID20 is presented which allows magnetic and resonant X-ray scattering experiments in the energy range 3-25 keV to be performed under extreme conditions. High magnetic field up to 10 T, high pressure up to 30 kbar combined with low temperatures down to 1.5 K are available and experiments can be performed at the M-edges of actinide elements, L-edges of lanthanides and K-edges of transition metals.

Charge density wave dislocation as revealed by coherent x-ray diffraction.

Coherent x-ray diffraction experiments have been performed on high quality crystals of the charge density wave (CDW) system K0.3MoO3. The satellite reflections associated with the CDW have been measured as a function of the 20-microm-diameter beam position. For some positions, regular fringes have been observed. We show that this observation is consistent with the presence of a single CDW dislocation. Beyond charge density wave systems, this experiment shows that coherent x-ray diffraction is a suitable tool to probe topological defects embedded in the bulk.

X-ray resonant scattering study of the quadrupolar order in UPd3.

Quadrupolar ordering in a 5f electron system has been observed directly for the first time, using x-ray scattering techniques. In UPd (3) at low temperatures satellite peaks appear at (1,0,l) (orthorhombic notation) with l odd and even. Both sets of peaks show a resonant enhancement of the scattering at the M(IV) edge of U. At resonance, the dominant scattering of the l odd peaks occurs in the unrotated polarization channel, whereas for l even a significant rotated component is found. These results are discussed in terms of possible structures of the antiferroquadrupolar phases.

Dynamics of phason fluctuations in the i-AlPdMn quasicrystal.

We report on the study of the dynamics of long wavelength phason fluctuations in the i-AlPdMn icosahedral phase using coherent x-ray scattering. When measured with a coherent x-ray beam, the diffuse intensity due to phasons presents strong fluctuations or speckles pattern. From room temperature to 500 degrees C the speckle pattern is time independent. At 650 degrees C the time correlation of the speckle pattern exhibits an exponential time decay, from which a characteristic time tau is extracted. We find that tau is proportional to the square of the phason wavelength, which demonstrates that phasons are collective diffusive modes in quasicrystals, in agreement with theoretical predictions.