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.

Local superconducting density of states of ErNi2B2C

We present local tunnelling microscopy and spectroscopy measurements at low temperatures in single crystalline samples of the magnetic superconductor ErNi2B2C. The electronic local density of states shows a striking departure from s-wave BCS theory with a finite value at the Fermi level, which amounts to half of the normal phase density of states.Comment: 9 pages, 3 figure

Field Dependence of the Superconducting Basal Plane Anisotropy of TmNi2B2C

The superconductor TmNi2B2C possesses a significant four-fold basal plane anisotropy, leading to a square Vortex Lattice (VL) at intermediate fields. However, unlike other members of the borocarbide superconductors, the anisotropy in TmNi2B2C appears to decrease with increasing field, evident by a reentrance of the square VL phase. We have used Small Angle Neutron Scattering measurements of the VL to study the field dependence of the anisotropy. Our results provide a direct, quantitative measurement of the decreasing anisotropy. We attribute this reduction of the basal plane anisotropy to the strong Pauli paramagnetic effects observed in TmNi2B2C and the resulting expansion of vortex cores near Hc2.Comment: 8 pages, 6 figures, 1 tabl

An imaging vector magnetograph for the next solar maximum

Researchers describe the conceptual design of a new imaging vector magnetograph currently being constructed at the University of Hawaii. The instrument combines a modest solar telescope with a rotating quarter-wave plate, an acousto-optical tunable prefilter as a blocker for a servo-controlled Fabry-Perot etalon, CCD cameras, and on-line digital image processing. Its high spatial resolution (1/2 arcsec pixel size) over a large field of view (5 by 5 arcmin) will be sufficient to significantly measure, for the first time, the magnetic energy dissipated in major solar flares. Its millisecond tunability and wide spectral range (5000 to 7000 A) enable nearly simultaneous vector magnetic field measurements in the gas-pressure-dominated photosphere and magnetically-dominated chromosphere, as well as effective co-alignment with Solar-A's X ray images. Researchers expect to have the instrument in operation at Mees Solar Observatory (Haleakala) in early 1991. They have chosen to use tunable filters as wavelength-selection elements in order to emphasize the spatial relationships between magnetic field elements, and to permit construction of a compact, efficient instrument. This means that spectral information must be obtained from sequences of images, which can cause line profile distortions due to effects of atmospheric seeing

Superconductivity in Dense MgB2MgB_2 Wires

$MgB_2$ becomes superconducting just below 40 K. Whereas porous polycrystalline samples of $MgB_2$ can be synthesized from boron powders, in this letter we demonstrate that dense wires of $MgB_2$ can be prepared by exposing boron filaments to $Mg$ vapor. The resulting wires have a diameter of 160 ${\mu}m$, are better than 80% dense and manifest the full $\chi = -1/4{\pi}$ shielding in the superconducting state. Temperature-dependent resistivity measurements indicate that $MgB_2$ is a highly conducting metal in the normal state with $\rho (40 K)$ = 0.38 $\mu Ohm$-$cm$. Using this value, an electronic mean free path, $l \approx 600~\AA$ can be estimated, indicating that $MgB_2$ wires are well within the clean limit. $T_c$, $H_{c2}(T)$, and $J_c$ data indicate that $MgB_2$ manifests comparable or better superconducting properties in dense wire form than it manifests as a sintered pellet.Comment: Figures' layout fixe

Decoupling of the superconducting and magnetic (structural) phase transitions in electron-doped BaFe2As2

Study and comparison of over 30 examples of electron doped BaFe2As2 for transition metal (TM) = Co, Ni, Cu, and (Co/Cu mixtures) have lead to an understanding that the suppression of the structural/antiferromagnetic phase transition to low enough temperature in these compounds is a necessary condition for superconductivity, but not a sufficient one. Whereas the structural/antiferromagnetic transitions are suppressed by the number of TM dopant ions (or changes in the c-axis) the superconducting dome exists over a limited range of values of the number of electrons added by doping (or values of the {a/c} ratio). By choosing which combination of dopants are used we can change the relative positions of the upper phase lines and the superconducting dome, even to the extreme limit of suppressing the upper structural and magnetic phase transitions without the stabilization of low temperature superconducting dome

Thermal expansion and effect of pressure on superconductivity in CuxTiSe2

We report measurements of thermal expansion on a number of polycrystalline CuxTiSe2 samples corresponding to the parts of x - T phase diagram with different ground states, as well as the pressure dependence of the superconducting transition temperature for samples with three different values of Cu-doping. Thermal expansion data suggest that the x - T phase diagram may be more complex than initially reported. T_c data at elevated pressure can be scaled to the ambient pressure CuxTiSe2 phase diagram, however, significantly different scaling factors are needed to accommodate the literature data on the charge density wave transition suppression under pressure

Imaging Orbital-selective Quasiparticles in the Hund's Metal State of FeSe

Strong electronic correlations, emerging from the parent Mott insulator phase, are key to copper-based high temperature superconductivity (HTS). By contrast, the parent phase of iron-based HTS is never a correlated insulator. But this distinction may be deceptive because Fe has five active d-orbitals while Cu has only one. In theory, such orbital multiplicity can generate a Hund's Metal state, in which alignment of the Fe spins suppresses inter-orbital fluctuations producing orbitally selective strong correlations. The spectral weights $Z_m$ of quasiparticles associated with different Fe orbitals m should then be radically different. Here we use quasiparticle scattering interference resolved by orbital content to explore these predictions in FeSe. Signatures of strong, orbitally selective differences of quasiparticle $Z_m$ appear on all detectable bands over a wide energy range. Further, the quasiparticle interference amplitudes reveal that $Z_{xy}<Z_{xz}<<Z_{yz}$, consistent with earlier orbital-selective Cooper pairing studies. Thus, orbital-selective strong correlations dominate the parent state of iron-based HTS in FeSe.Comment: for movie M1, see http://www.physik.uni-leipzig.de/~kreisel/osqp/M1.mp4, for movie M2, see http://www.physik.uni-leipzig.de/~kreisel/osqp/M2.mp4, for movie M3, see http://www.physik.uni-leipzig.de/~kreisel/osqp/M3.mp4, for movie M4, see http://www.physik.uni-leipzig.de/~kreisel/osqp/M4.mp4, for movie M5, see http://www.physik.uni-leipzig.de/~kreisel/osqp/M5.mp

Systematic effects of carbon doping on the superconducting properties of Mg(B1−x_{1-x}Cx_x)2_2

The upper critical field, $H_{c2}$, of Mg(B$_{1-x}$C$_x$)$_2$ has been measured in order to probe the maximum magnetic field range for superconductivity that can be attained by C doping. Carbon doped boron filaments are prepared by CVD techniques, and then these fibers are then exposed to Mg vapor to form the superconducting compound. The transition temperatures are depressed about $1 K/$ C and $H_{c2}(T=0)$ rises at about $5 T/$ C. This means that 3.5% C will depress $T_c$ from $39.2 K$ to $36.2 K$ and raise $H_{c2}(T=0)$ from $16.0 T$ to $32.5 T$. Higher fields are probably attainable in the region of 5% C to 7% C. These rises in $H_{c2}$ are accompanied by a rise in resistivity at $40 K$ from about $0.5 \mu \Omega cm$ to about $10 \mu \Omega cm$. Given that the samples are polycrystalline wire segments, the experimentally determined $H_{c2}(T)$ curves represent the upper $H_{c2}(T)$ manifold associated with $H\perp c$