The synthesis of materials with well-controlled composition and structure improves our understanding of their intrinsic electrical transport properties. Recent developments in atomically controlled growth have been shown to be crucial in enabling the study of new physical phenomena in epitaxial oxide heterostructures. Nevertheless, these phenomena can be infl uenced by the presence of defects that act as extrinsic sources of both doping and impurity scattering. Control over the nature and density of such defects is therefore necessary to fully understand the intrinsic materials properties and exploit them in future device technologies. Here, it is shown that incorporation of a strontium copper oxide nano-layer strongly reduces the impurity scattering at conducting interfaces in oxide LaAlO 3 -SrTiO 3 (001) heterostructures, opening the door to high carrier mobility materials. It is proposed that this remote cuprate layer facilitates enhanced suppression of oxygen defects by reducing the kinetic barrier for oxygen exchange in the hetero-interfacial fi lm system. This design concept of controlled defect engineering can be of signifi cant importance in applications in which enhanced oxygen surface exchange plays a crucial role.
We present tunneling data from optimally-doped, superconducting BaFe1.86Co0.14As2 and its parent compound, BaFe2As2. In the superconductor, clear coherence-like peaks are seen across the whole field of view, and their analysis reveals nanoscale variations in the superconducting gap value, ∆. The average magnitude of 2∆ is ∼7.4 kBTc, which exceeds the BCS weak coupling value for either s-or d-wave superconductivity. The characteristic length scales of the deviations from the average gap value, and of an anti-correlation discovered between the gap magnitude and the zero bias conductance, match well with the average separation between the Co dopant ions in the superconducting FeAs planes. The tunneling spectra themselves possess a peak-dip-hump lineshape, suggestive of a coupling of the superconducting electronic system to a well-defined bosonic mode of energy 4.7 kBTC , such as the spin resonance observed recently in inelastic neutron scattering.
%hat we believe to be the first experimental results have been obtained on strong magnetic xray dichroism in the M4, 5 absorption spectra of magnetically ordered rare-earth materials, in accordance with recent predictions.The feasibility of using x-rays to determine the magnetic structure of magnetically ordered materials by magnetic dichroism has recently been predicted theoretically. ' Strong magnetic x-ray dichroism (MXD) is expected in the Mq, q absorption edge structure of rare-earth-metal compounds. Polarized synchrotron radiation can therefore be used to reveal information on the local rare-earth-metal magnetic moments in solids, thin films, and surfaces. In this Brief Report we will give what we believe is the first experimental proof of this effect.The M4, q absorption in rare-earth-metal compounds shows good agreement with the atomic Hartree-Fock calculations for the transitions from the 4f"(J) Hund's rule ground state to the manifold of 3d 4f"+'(J') final states.Although in x-ray absorption spectroscopy (XAS) hundreds of excited levels may be involved, one can distinguish between three different types of excitations, namely for J' -J -1, 0, and 1.In the presence of a magnetic field the 2J+1 degenerate ground state 4f"(J) splits into sublevels MJ -J, . . . , +J. The relative population of these sublevels depends on the temperature. That the polarization vector of the x rays has a drastic effect on the spectrum can be seen from the simple case where only M -J is populated (T 0 K). With the polarization direction parallel to the magnetization only the hM 0 transitions are allowed.The transitions J' -J -1 will then vanish, because the M' -Jsublevel is not present in the J' J -l state.Here, we will illustrate in more detail the use of MXD on terbium iron garnet (TbIG), which has a rather complicated magnetic structure. The rhombohedral (or trigonal distorted cubic) cell of ferrimagnetic rare-earth-metal iron garnets (R3Fe50t2) contains eight formula units. 5 The Fe + ions occupy the 24 d (tetrahedron) and the 16 a (octahedron) positions. The larger R + ions occupy the 6 c (dodecahedral) positions. Below the Weel temperature the spins of the 24 d and 16 a irons are ordered antiparallel along the [111]axis. The rare-earth-metal moments couple antiferromagnetically to the net iron moment. By symmetry the 6 c sites can be divided into c~, c2, c3 and cI c2 c3 where C2 (c2 ) and C3 (c3 ) are obtained from the c~(ct ) by rotation around the trigonal [111]axis. From neutron diffraction at 4.2 K the Tb moments in TbIG are known to form a double umbrella structure. 7s The magnetic moments on c~and cI are both in the (011) plane, having angles P 30. 79' and P' -28. 07' with the trigonal axis, and absolute values m 8.18ptt and m' 8.90ptt, respectively. Above 4.2 K the values of P, P', m and rn' are expected to decrease. In our experiment, a single crystal of TbIG was mounted on a rotatable helium-flow cryostat in an ultrahigh vacuum of -10 ' Torr. The temperature at the surface of the sample was 55+'5 K. A CosSm pe...
We elucidate the termination surface of cleaved single crystals of the BaFe 2−x Co x As 2 and Fe y Se 1−x Te x families of the high-temperature iron-based superconductors. By combining scanning tunneling microscopic data with low-energy electron diffraction we prove that the termination layer of the BaFe 2 As 2 systems is a remnant of the Ba layer, which exhibits a complex diversity of ordered and disordered structures. The observed surface topographies and their accompanying superstructure reflections in electron diffraction depend on the cleavage temperature. In stark contrast, Fe y Se 1−x Te x possesses only a single termination structure-that of the tetragonally ordered Se 1−x Te x layer.
We have realized a two dimensional permanent magnetic lattice of Ioffe-Pritchard microtraps for ultracold atoms. The lattice is formed by a single 300 nm magnetized layer of FePt, patterned using optical lithography. Our magnetic lattice consists of more than 15000 tightly confining microtraps with a density of 1250 traps/mm 2 . Simple analytical approximations for the magnetic fields produced by the lattice are used to derive relevant trap parameters. We load ultracold atoms into at least 30 lattice sites at a distance of approximately 10 µm from the film surface. The present result is an important first step towards quantum information processing with neutral atoms in magnetic lattice potentials.
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