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In the past triennium members of the Commission 20 have been very active in studying positions and motions of minor planets, comets and satellites including rings by observational and theoretical investigations, as it is described in this report In fact observers have been producing tremendous amount of astrometric data, much more than we could imagine twenty years ago, when we heard many complains and appeals to observers for needs of more observations, particularly, for minor planets and satellites. Technically, several new devices to detect faint objects and to measure their positions more effectively, have been developed. Theoreticians have faced many interesting and important problems to explain observational facts with several powerful methods developed recently.
The charge - transfer reaction between protons and oxygen atoms is critical to the chemistry of the oxygen family; the corresponding rate was evaluated by Field and Steigman (1971) on the assumption of orbiting collisions and statistical probability distribution among the levels. We re-examine this reaction, including the fine-structure excitation process, basing our analysis on a careful description of the different potential curves arising from the O-H+ and 0+-H systems and on the evaluation of the coupling responsible for the transitions.
Quantum wells created from nanostructured transition metal oxides offer unique possibilities for creating and manipulating quantum states of matter, including novel superconductors, high Curie temperature magnets, controllable metal-insulator transitions, and new topological states. This article explores what is known and conjectured about confined electronic states in oxide quantum wells. Theoretical challenges are reviewed, along with issues arising in the creation of oxide quantum wells. Examples from the current experimental state of the art are summarized, open questions are discussed, and prospects for the future are outlined. The key roles of epitaxial strain and proximity effects are emphasized.
Here we present the installation and successful commissioning of an L'-band Annular Groove Phase Mask (AGPM) coronagraph on VLT/NACO. The AGPM is a vector vortex coronagraph made from diamond subwavelength gratings tuned to the L' band. The vector vortex coronagraph enables high contrast imaging at very small inner working angle (here 0″.09, the diffraction limit of the VLT at L'), potentially being the key to a new parameter space. During technical and science verification runs, we discovered a late-type companion at two beamwidths from an F0V star (Mawet et al. 2013), and imaged the inner regions of β Pictoris down to the previously unexplored projected radius of 1.75 AU. The circumstellar disk was also resolved from ≃ 1″ to 5″ (see J. Milli et al., these proceedings). These results showcase the potential of the NACO L-band AGPM over a wide range of spatial scales.
We report studies on La1−xSrxMnO3 /SrTiO3/ La1−xSrxMnO3 (x=0.33) trilayer junctions made using 90° off-axis sputtering. Both (110) NdGaO3 and (001) (LaAlO3)0.3-(Sr2AlTaO6)0.7 (LSAT) are used for substrates. Optical lithography is used for junction formation. These sputtered trilayers show improved junction resistance uniformity over trilayers made using laser ablation. A magnetoresistance of ∼100% is observed for junctions on LSAT with 30 Å barrier at 13 K and around 100 Oe. The shape of junction magnetoresistance vs. field depends both on barrier thickness and on substrate type, suggesting that both inter-layer coupling and substrateinduced-strain play a role in determining the junction's micromagnetic state. These results indicate better junction interfaces can be obtained for manganite trilayer junctions by 90° off-axis sputtering.
Transport and structural properties of the colossal magnetoresistance pyrochlore TI2Mn2O7 are studied as a function of applied pressure up to ∼20GPa. This allows us to probe the effect of structural changes on the ferromagnetic transition and the transport properties. We observe a non-monotonous pressure dependence of the ferromagnetic transition temperature. We correlate this unusual variation with the structural parameters that, according to electronic band calculations, are key in controlling the properties of these materials.
We discuss the heteroepitaxial growth of La1−xSrxMnO3 films and CaTiO3 insulating barriers by molecular beam epitaxy. We find that the surface morphology and residual resistivity of the manganite electrodes is critically dependent on the film stoichiometry. The most important parameter is the concentration of La+Sr (cubic perovskite A-site cations) to that of Mn (B-site cation). If La+Sr is supplied in slight excess, the films grow with atomically flat surfaces, but the residual resistivity at 4.2K is high (as high as 6500 µΩ-cm), and Curie temperature (Tc) low (<300 K). If Mn is supplied in slight excess, the films have high Tc (370 K) and residual resistivity (35 µΩ-cm) better than bulk single crystal values, but the surface is no longer atomically flat. There appears to be a very narrow region of phase space where it is possible to have low resistivity, high Tc films with atomically flat surfaces. This is precisely where one would like to place heterostructure devices.
We have performed depth profile analyses of the lattice parameters in epitaxial thin films of La1−xCaxMno3 (LCMO), where x = 0.33 or 0.3, to understand the evolution of strain relaxation processes in these materials. The analyses were done using Grazing Incidence X-ray Scattering (GIXS) on films of different thicnesses on two different substrates, (100) oriented LaAlO3 (LAO), with a lattice mismatch of ∼2% and (110) oriented NGO, with a lattice mismatch of less than 0.1%. Films grown on LAO can exhibit up to three in-plane strained lattice constants, corresponding to a slight orthorhombic distortion of the crystal, as well as near-surface and columnar lattice relaxation. As a function of film thickness, a crossover from a strained film to a mixture of strained and relaxed regions in the film occurs in the range of 700 Å. The structural evolution at this thickness coincides with a change in the resistivity curve near the metalinsulator transition. The in-plane compressive strain has a range of 0.2 – 1.5%, depending on the film thickness for filsm in the range of 400 - 1500 A.
Lattice distortions, be they in the form of chemical and hydrostatic pressure in bulk or lattice mismatch between film and substrate, have significant effects on the transport as well as the magnetic properties of colossal magnetoresistance (CMR) materials. We summarize here our results on tensilely and compressively strained La0.7Sr0.3MnO3 (LSMO) thin films that indicate the important role of lattice distortions due to the lattice mismatch between the film and substrate. The strain due to lattice distortions can be used to tune the magnetic domain structure, magnetization, magnetic anisotropy and magnetotransport of LSMO thin films.
A careful analysis of high-resolution transmission electron microscopy images from La0.8Ca0.2MnO3 thin films indicates that the images can not explained based on either the classical Pnma structure or its monoclinic distortion. A cation ordered structure is proposed which could be responsible for the significantly higher Tc of the film (298K) compared with the bulk material of the same composition (190K).
We have synthesized ceramics of A2FeReO6 double-perovskites A2FeReO6 (A=Ba, Ca). Structural characterizations indicate a cubic structure with a=8.0854(1) Å for Ba2FeReO6 and a distorted monoclinic symmetry with a=5.396(1) Å, b=5.522(1) Å, c=7.688(2) Å and β=90.4° for Ca2FeReO6. The barium compound is metallic from 5K to 385K, i.e. no metal-insulator transition has been seen up to 385K, and the calcium compound is semiconducting from 5K to 385K. Magnetization measurements show a ferrimagnetic behavior for both materials, with Tc =315 K for Ba2FeReO6 and above 385K for Ca2FeReO6. At 5K we observed, only for Ba2FeReO6, a negative magnetoresistance of 10% in a magnetic field of 5T. Electrical, magnetic and thermal properties are discussed and compared to those of the analogous compounds Sr2Fe(Mo,Re)O6 recently studied.
The properties of a new cubic perovskite phase of thin-film BaFexTi1−xO3 (0.5 ≤x≤0.75) are reported. This material is novel because the corresponding bulk compounds have hexagonal structure for comparable x. The films, grown by pulsed laser deposition on MgO and SrTiO3 substrates, are magnetic (ferro- or ferri-, with Tc > 500°C) and ferroelectric (Tc ∼ 200-300°C).
We have studied magnetoresistance of a series of La1−xSrxMnO3 and La1−x−yCaxSryMnO3 samples, for which structural and ferromagnetic transformation temperatures are in close proximity. On cooling in zero magnetic field, we observe a rapid increase of resistivity just above TC for La1−xSrxMnO3 samples with x < 0.1425 and x ≤ 0.1725 due to the O*-O' and R-O* - structural phase transformations, respectively. This increase is followed by a rapid decrease due to the ferromagnetic transition. The applied magnetic field significantly shifts the ferromagnetic transition to higher temperatures and suppresses the structure-related resistivity increase. We show that a combination of structural and ferromagnetic transitions gives rise to an enhancement of the negative magnetoresistance due to strong spin-lattice coupling. By choosing a proper composition, the enhancement can be optimized to appear in relatively low magnetic fields. A proper selection of Sr and Ca contents in La1−x−yCaxSryMnO3 and preparation conditions leads to an enhancement of the magnetoresistance effect at room temperature.
We have grown epitaxial thin films of metastable four-layered hexagonal (4H) BaRuO3 on (111) SrTiO3 by 90° off-axis sputtering techniques. X-ray diffraction and transmission electron microscopy experiments reveal that the films are single crystals of c-axis 4H structures with an inplane epitaxial arrangement of BaRuO3  // SrTiO3 . Smooth multilayer growth has been observed in these films with a step height equaling the size of half unit cell. In-plane resistivity of the films is metallic, with a room temperature value of about 810µΩ-cm and slightly curved temperature dependence. Their magnetic susceptibility is paramagnetic. The metastable layered compounds can be very useful for understanding new solid-state phenomena and novel device applications.
The traveling wave (TW) method has been utilized to investigate the transport mechanism in paramagnetic-insulator state of La0.75Sr0.11Ca0.14MnO3 films. The drift mobility of the films increased from 2.5 × 10−2 cm2/Vs at 310 K to about 9.2 × 10−2 cm2/Vs at 400 K. The Arrhenius behaviors of the conductivity and drift mobility indicate that the transport process in manganites above the Curie temperature is dominated by the thermally assisted hopping of small polarons.
The evolution of structural, electrical and magnetic properties with the isovalent chemical substitution of Ca2+ into the Sr2+ sites in new series of two-dimensional La1.2(Srl.8−XCax)Mn2O7 compounds (x = 0 ∼ 1.8) and three-dimensional La0.6(Sr0.4−xCax)MnO3 compounds (x = 0 ∼ 0.4) are investigated. The highest magnetoresistance (MR) ratios [ρ(0) - ρ(H) / ρ(0)] of 52 % (H = 1.5 T) at 102 K and 13 % (H = 1.5 T) at 210 K were observed for the x = 0.4 samples in La1.2(Sr1.8−xCax Mn2O7 and La0.6(Sr0.4−xCax)MnO3, respectively. The Curie temperatures (Tc) decreased from 135 K to 102 K and 370 K to 270 K for x = 0 to 0.4 in La1.2(Sr1.8−xCax)Mn2O7 and La0.6(Sr0.4−xCax)MnO3, respectively. The compositional dependence of the structural variation has been found in La0.6(Sr0.4−xCax)MnO3. Our results confirm that the dimensionality as well as ionic size plays an important role in controlling the colossal magnetoresistance in manganites.
The specific heat of a polycrystalline sample of MnO was measured from T ≈ 1 K to T ≈ 400 K using two different experimental apparatuses at zero applied pressure. Features revealed by the data include a hyperfine contribution due to the Mn nuclei, a T2 temperature dependence at low temperatures due to the type-II antiferromagnetic magnon contribution, and a sharp but well defined antiferromagnetic transition (TN = 117.7095 K) that is clearly second order in nature. The critical exponent, α, deduced from the transition is consistent with a two dimensional Ising model. The specific heat of MnO is also compared with recent results on the type-A antiferromagnet LaMnO3.
We have measured atomic pair distribution functions (PDF) of La1−xCaxMnO3 using high energy x-ray diffraction. This approach yields accurate PDFs with very high real-space resolution. It also avoids potential pitfalls from the more usual neutron measurements that magnetic scattering is present in the measurement, that the neutron scattering length of manganese is negative leading to partial cancellation of PDF peaks, and that inelasticity effects might distort the resulting PDF We have used this to address the following questions which do not have a satisfactory answer: (1) What are the amplitudes and natures of the local Jahn-Teller and polaronic distortions in the CMR region. (2) Is the ground-state of the ferromagnetic metallic phase delocalized or polaronic. (3) As one moves away from the ground-state, by raising temperature or decreasing doping, towards the metal insulator transition, how does the state of the material evolve?
Two-dimensional electron gases (2DEGs) based on conventional semiconductors such as Si or GaAs have played a pivotal role in fundamental science and technology. The high mobilities achieved in 2DEGs enabled the discovery of the integer and fractional quantum Hall effects and are exploited in high-electron-mobility transistors. Recent work has shown that 2DEGs can also exist at oxide interfaces. These electron gases typically result from reconstruction of the complex electronic structure of the oxides, so that the electronic behavior of the interfaces can differ from the behavior of the bulk. Reports on magnetism and superconductivity in oxide 2DEGs illustrate their capability to encompass phenomena not shown by interfaces in conventional semiconductors. This article reviews the status and prospects of oxide 2DEGs.