Disentangling multipole resonances through a full x-ray polarization analysis

Complete polarization analysis applied to resonant x-ray scattering at the Cr K-edge in K2CrO4 shows that incident linearly polarized x-rays can be converted into circularly polarized x-rays by diffraction at the Cr pre-edge (E = 5994 eV). The physical mechanism behind this phenomenon is a subtle interference effect between purely dipole (E1-E1) and purely quadrupole (E2-E2) transitions, leading to a phase shift between the respective scattering amplitudes. This effect may be exploited to disentangle two close-lying resonances that appear as a single peak in a conventional energy scan, in this way allowing to single out and identify the different multipole order parameters involved.Comment: 6 pages, 6 figure

High-field irreversible moment reorientation in the antiferromagnet Fe1.1_{1.1}Te

Magnetization measurements have been performed on single-crystalline Fe$_{1.1}$Te in pulsed magnetic fields $\mathbf{H}\perp\mathbf{c}$ up to 53 T and temperatures from 4.2 to 65 K. At $T=4.2$ K, a non-reversible reorientation of the antiferromagnetic moments is observed at $\mu_0H_R=48$ T as the pulsed field is on the rise. No anomaly is observed at $H_R$ during the fall of the field and, as long as the temperature is unchanged, during both rises and falls of additional field pulses. The transition at $H_R$ is reactivated if the sample is warmed up above the N\'{e}el temperature $T_N\simeq60$ K and cooled down again. The magnetic field-temperature phase diagram of Fe$_{1.1}$Te in $\mathbf{H}\perp\mathbf{c}$ is also investigated. We present the temperature dependence of $H_R$, as well as that of the antiferromagnetic-to-paramagnetic borderline $H_c$ in temperatures above 40 K.Comment: 5 pages, 4 figure

Magnetic ordering in GdNi2B2C revisited by resonant x-ray scattering: evidence for the double-q model

Recent theoretical efforts aimed at understanding the nature of antiferromagnetic ordering in GdNi2B2C predicted double-q ordering. Here we employ resonant elastic x-ray scattering to test this theory against the formerly proposed, single-q ordering scenario. Our study reveals a satellite reflection associated with a mixed-order component propagation wave vector, viz., (q_a,2q_b,0) with q_b = q_a approx= 0.55 reciprocal lattice units, the presence of which is incompatible with single-q ordering but is expected from the double-q model. A (3q_a,0,0) wave vector (i.e., third-order) satellite is also observed, again in line with the double-q model. The temperature dependencies of these along with that of a first-order satellite are compared with calculations based on the double-q model and reasonable qualitative agreement is found. By examining the azimuthal dependence of first-order satellite scattering, we show the magnetic order to be, as predicted, elliptically polarized at base temperature and find the temperature dependence of the "out of a-b plane" moment component to be in fairly good agreement with calculation. Our results provide qualitative support for the double-q model and thus in turn corroborate the explanation for the "magnetoelastic paradox" offered by this model.Comment: 8 pages, 5 figures. Submitted to Phys. Rev.

Mean-field model of the ferromagnetic ordering in the superconducting phase of ErNi_2B_2C

A mean-field model explaining most of the details in the magnetic phase diagram of ErNi_2B_2C is presented. The low-temperature magnetic properties are found to be dominated by the appearance of long-period commensurate structures. The stable structure at low temperatures and zero field is found to have a period of 40 layers along the a direction, and upon cooling it undergoes a first-order transition at T_C = 2.3 K to a different 40-layered structure having a net ferromagnetic component of about 0.4 mu_B/Er. The neutron-diffraction patterns predicted by the two 40-layered structures, above and below T_C, are in agreement with the observations of Choi et al.Comment: 4 pages, 3 figures (Revtex4

Lockin to Weak Ferromagnetism in TbNi2B2C and ErNi2B2C

This article describes a model in which ferromagnetism necessarily accompanies a spin-density-wave lockin transition in the borocarbide structure provided the commensurate phase wave vector satisfies Q = (m/n)a* with m even and n odd. The results account for the magnetic properties of TbNi2B2C, and are also possibly relevant also for those of ErNi2B2C.Comment: 4 page

Direct observation of the high magnetic field effect on the Jahn-Teller state in TbVO4

We report the first direct observation of the influence of high magnetic fields on the Jahn-Teller (JT) transition in TbVO4. Contrary to spectroscopic and magnetic methods, X-ray diffraction directly measures the JT distortion; the splitting between the (311)/(131) and (202)/(022) pairs of Bragg reflections is proportional to the order parameter. Our experimental results are compared to mean field calculations, taking into account all possible orientations of the grains relative to the applied field, and qualitative agreement is obtained.Comment: 11 pages, 4 figures, submitted to Phys. Rev. Let

X-ray diffraction microscopy based on refractive optics

A formalism is presented for dark-field X-ray microscopy using refractive optics. The new technique can produce three-dimensional maps of lattice orientation and axial strain within millimetre-sized sampling volumes and is particularly suited toin situstudies of materials at hard X-ray energies. An objective lens in the diffracted beam magnifies the image and acts as a very efficient filter in reciprocal space, enabling the imaging of individual domains of interest with a resolution of 100 nm. Analytical expressions for optical parameters such as numerical aperture, vignetting, and the resolution in both direct and reciprocal spaces are provided. It is shown that the resolution function in reciprocal space can be highly anisotropic and varies as a function of position in the field of view. Inserting a square aperture in front of the objective lens facilitates disjunct and space-filling sampling, which is key for three-dimensional reconstruction and analysis procedures based on the conservation of integrated intensity. A procedure for strain scanning is presented. Finally the formalism is validated experimentally at an X-ray energy of 17 keV.</jats:p

Formal Proof of SCHUR Conjugate Function

The main goal of our work is to formally prove the correctness of the key commands of the SCHUR software, an interactive program for calculating with characters of Lie groups and symmetric functions. The core of the computations relies on enumeration and manipulation of combinatorial structures. As a first "proof of concept", we present a formal proof of the conjugate function, written in C. This function computes the conjugate of an integer partition. To formally prove this program, we use the Frama-C software. It allows us to annotate C functions and to generate proof obligations, which are proved using several automated theorem provers. In this paper, we also draw on methodology, discussing on how to formally prove this kind of program.Comment: To appear in CALCULEMUS 201

Polarization analysis of K-edge resonant x-ray scattering of germanium

The polarization of K-edge resonant scattering at the space group ``forbidden'' (0 0 6) reflection of Ge was measured as function of the azimuthal angle, psi. The experimental results are compared to model calculations based on symmetry analysis of the resonant scattering tensors.Comment: 4 pages, 3 figures. Submitted to Phys. Rev. B Rapid Communications V2: Updated after referee report