Coexistence of ferroelectricity and ferromagnetism in tantalum clusters

The atomic and electronic structures of Ta(N) (N=2-23) clusters have been determined in the framework of pseudopotential density-functional calculations, based upon an unbiased global search with guided simulated annealing to an empirical potential. It is found that the ground-state structures of Ta(N) are very similar to those of Nb(N), showing no preference for the icosahedral growth. Also, a size- and structure-dependent ferroelectricity is found in these tantalum clusters. More importantly, it is found that the ferroelectricity and ferromagnetism can coexist in the homogeneous transition-metal cluster, offering a possibility to obtain a new type of "multiferroic" materials composed of the clusters. Finally, the far-infrared spectroscopy is suggested to be an efficient tool to distinguish the ferroelectric clusters.     

Size-reduction induced ferromagnetism and photo-magnetic effects in azobenzene-thiol-passivated gold nanoparticles

We have observed size-reduction induced ferromagnetism and significant photo-magnetic effects in azobezene-thiol-modified gold nanoparticles. When the particle size was reduced to 1.7 nm, ferromagnetism even at room temperature was observed while diamagnetism was dominating with the particle size of 5.0 nm. Furthermore, reducing the particles size to 1.7 nm also gave rise to photo-magnetic properties because the decrease in particle size provided free volume between each of the azobenzene ligands. This photo-magnetic effect could be attributed to photo-induced changes in the values of d-charge losses due to the photoisomerization of azo-ligands, which were accompanied by the inversion of surface dipole values to the opposite sign.     

Room-temperature ferromagnetism in pure ZnO nanoflowers

ZnO nanoflowers are synthesized by hydrothermal method. The morphology of ZnO is captured by SEM, TEM and HRTEM, which is composed of closely packed nanorods of about 100 nm in diameter and 1 μm in length. The ZFC/FC curves show superparamagnetic features. The abnormal increase in magnetization curves below 14 K comes from the isolated vacancy clusters with no interaction. The magnetic hysteresis at 300 K displays saturation state and confirms room-temperature ferromagnetism. While the magnetic hysteresis at 5 K shows nonsaturation state due to the enhanced effects of vacancy clusters. The O 1s XPS results can be fitted to three Gaussian peaks. The existence of medium-binding energy located at 531.16 eV confirms the deficiency of O ions at the surface of ZnO nanoflowers.     

Induced ferromagnetism in one-side semihydrogenated silicene and germanene

The intriguing electronic and magnetic properties of one-side semihydrogenated silicene and germanene are investigated by means of first-principles calculations. Both one-side semihydrogenated silicene and germanene are confirmed as dynamically stable in the ground state based on phonon-mode analysis. Moreover, we find that semihydrogenation from only one side causes localized and unpaired 3p (4p) electrons in the unhydrogenated Si (Ge) sites and then introduces ferromagnetism to silicene (germanene) sheet with no need for doping, cutting, or etching. One-side semihydrogenated silicene and germanene are both identified as semiconductors with direct energy gaps: their gap values obtained from the HSE06 functional are estimated to be 1.74 eV and 1.32 eV, much greater than the PBE-GGA results of 0.94 eV and 0.41 eV, respectively. From pristine to one-side semihydrogenated and then to fully hydrogenated systems, silicene and germanene change from metallic to magnetic semiconducting and then to nonmagnetic semiconducting. The hydrogenation process provides a novel method to tune the properties of silicene and germanene with unprecedented potentials for future nanoelectronics.     

YBCO在氮纯化过程中的催化氧化作用

高温超导材料YBa2Cu3O7-x(即YBCO)在加氢的普氮气流中,在100～300 ℃的温区里就能生成H2O和高纯N2. 系列的X-光谱和热重分析以及相应的氮纯度测量表明,它是一种很有应用前景的氧化催化剂. 实验明确给出130～150 ℃是它的最佳催化温区,还根据YBCO的等温特性曲线和Langmuir理论讨论了YBCO在氮纯化过程中催化氧化作用的机制.     

Chemical control of electronic structure and superconductivity in layered borides and borocarbides: understanding the absence of superconductivity in LixBC

The synthetic search for materials related to the 39 K superconductor MgB2 has been difficult. The most promising theoretical suggestion, hole doping of LiBC, does not lead to a new superconductor. We show here that a combination of density functional theory (DFT) calculations, materials synthesis, and structural characterization reveals the origin of the puzzling absence of superconductivity in Li1/2BC as a subtle change in the electronic structure driven by structural response to the introduction of holes. This indicates that the unique aspects of the electronic structure of MgB2 will be demanding to replicate in other systems.     

Proximity-induced superconductivity in carbon nanotubes

Single wall carbon nanotubes (SWNT) are model systems for the study of electronic transport in one-dimensional conductors. They are expected to exhibit strong electronic correlations and non-Fermi liquid behavior as suggested by recent experiments. The possibility to induce supercurrents through such molecular wires is a challenging question both for experimentalists and theoreticians. In this paper we show experimental evidence of induced superconductivity in a SWNT. This proximity effect is observed in a single 1 nm diameter SWNT, in individual cristalline ropes containing about 100 nanotubes and also on multiwalled tubes. These samples are suspended as strings between two superconducting electrodes (double layer Au-Re, Au-Ta or Sn film) at a distance varying between 100 and 2 000 nm. This allows their structural study in a transmission electron microscope. When their resistance is low enough, SWNT become superconducting with surprisingly high critical currents (in the micro-Ampere range for a single tube of normal state resistance 25 kΩ). This critical current, extensively studied as function of temperature and magnetic field, exhibits unusual features which are not observed in conventional Superconducting-Normal-Superconducting junctions and can be related to the strong 1D character of these samples. We also show evidence of a huge sensitivity of dc transport properties of the tubes to electromagnetic radiation in the radio-frequency range.     

Possible flat-band ferromagnetism in an organic polymer

Motivated from the flat-band form magnetism conceived in the physics of correlated electrons; we predict that a polymer of five-membered rings can exhibit band (i.e. itinerant) ferromagnetism. We have first identified that the right material having a flat one-electron dispersion should be polyaminotriazole, for which we have performed a generalized gradient approximation (GGA) calculation based on the spin density functional. The result shows that the ground state is indeed ferromagnetic when the flat band is made half-filled. From the structural optimization, we also show that the magnetism overcomes the Peierls instability, unlike conjugated polymers such as polyacetylene. We have also confirmed that the mechanism for the ferromagnetism is the flat-band ferromagnetism by comparing the first-principles wavefunction with those in the Hubbard model. Possibility of the bulk ferromagnetism for the three dimensional polyaminotriazole crystal with chemical carrier doping is also discussed.     

Exploring the Microstructure of YBCO Nanopowders

High temperature superconductor YBa2Cu3O7-π (YBCO) nanopowders were successfully prepared by a citrate pyrolysis technique. The phase formation and microstructure of the nanopowders were studied. The microstructure analysis shows that the obtained YBCO powders have very fine grains with a size around several tens of nanomaters. The suoerfine morphology reveals that the thermal agglomeration of the particles decreases with decreasing the annealing temperature and shortening the annealing time. In addition, the complete phase formation of YBCO depends upon the annealing temperature and the annealing time as well. So far, these nanopowders have been used to fabricate the melt-textured YBCO bulk materials.     

Nitrogen vacancy and ferromagnetism in Cr-doped GaN: First-principles calculations

Nitrogen defects and their effect on the ferromagnetism (FM) in Cr-doped GaN have been systematically investigated by first-principles. Four considered configurations including one N vacancy (V_N), single substitutional Cr, double substitutional Cr, and complex of Cr-V_N are all ferromagnetic. The lowest energy arrangements for double Cr-doped (or Cr-V_N) GaN occur at the nearest Cr-Cr (or Cr-V_N) distance. One Cr contributes 84.3% of the total magnetic moment (2.533 μ_B), while one Cr-Cr pair leads to 5.998 μ_B moment (more than twice moment of one Cr) by the strong d-d spin coupling, which is mediated by two Cr 3d states antiferromagnetically coupling with the “commonly shared” N 2p states, and driven by a double exchange mechanism. The V_N can enhance the FM by adding about 1 μ_B to the Cr moment but reduce the FM spin exchange interaction between the nearest Cr-Cr pairs, so experimentally, high-performance samples may be synthesized by controlling N pressure.     

YBCO薄膜的织构和孪晶的XRD分析

采用多元电子束物理气相沉积法在｛001｝ LAO衬底上制备了YBCO薄膜,采用X射线衍射分析了其取向和孪晶结构,结果表明,薄膜中除了8％C取向的四方相外；其余为正交相,而正交相中存在27％的a取向,3％的b取向以及62％的C取向,且C取向晶粒存在面内90°±0.81°＜110＞孪晶.四方到正交相变不完全引起四方相残留、...     

钙钍掺杂YBCO超导体中的载流子浓度

测量了钙钍掺杂的多晶ＹＢＣＯ超导体的低温霍尔系数ＲＨ、霍尔电阻率ρＨ及其随温度Ｔ变化的特性。结果表明，在该掺杂系统中，ＲＨ大体上正比于Ｔ－１。推算并分析了霍尔载流子浓度ｎＨ，发现以掺杂Ｃａ、Ｔｈ来取代Ｙ时，超导转变温度Ｔｃ、霍尔浓度ｎＨ及温度变化率ｄｎＨ／ｄＴ都随掺杂浓度ｘ的增加而降低。     

Compton scattering studies of 4f ferromagnetism

Ferromagnetism can be studied in Compton scattering experiments, which use high-energy, elliptically polarised synchrotron radiation. The basis of the method and its interpretation in terms of site-specific moments will be explained and illustrated by a series of examples of archetypal compounds. For example, the hexaborides exhibit a wide range of electronic properties. Data will be presented for the heavy Fermion system CeB₆ and the controversial ferromagnet Ca_1−xLa_xB₆. Undoped CeB₆ exhibits a delocalised spin moment, more characteristic of the 5d than the 4f orbital. Whereas the magnetism in the Ca_1−xLa_xB₆ system has been suggested to be an extrinsic effect arising from iron contamination, our data indicate that the magnetism is intrinsic and inconsistent with what would be expected for an iron 3d moment. This supports the proposal that this system is, indeed, a weak itinerant ferromagnet. In Ru₂SrGdCu₂0₈, we observe a combination of Gd 4f, Gd 5d and Ru 4d moments and the 5d moment appears to flip orientation below the Néel temperature.     

Nature of Stoner condition for metallic ferromagnetism

For proper applications of the Stoner condition, we describe the assumptions in its underlying theory and their implications that need to be considered in practice. The nature of Stoner exchange parameters is examined based on spin-polarization perturbational orbital theory and a connection has been made in understanding the parameters with both local spin density and Hartree-Fock approximations via a self-interaction-corrected local spin density (SIC-LSD) exchange-correlation functional.     

Fabrication of YBCO/CeO2/YBCO multilayer films through an all chemical solution deposition approach

In this paper, the growth characteristics and superconducting properties of YBCO/CeO₂/YBCO multilayer films derived from an all chemical solution deposition process layer by layer are investigated. X-ray diffraction results show that the c-axis epitaxial growth feature of the bottom YBa₂Cu₃O_7-δ (YBCO) film was perfectly inherited. Alternating epitaxial growth of YBCO/CeO₂/YBCO multilayer films was confirmed by high resolution transmission electron microscopy in combination with selected area electron diffraction, which will lay the groundwork for preparation of SIS-type Josephson junctions. In addition, nanosized BaCeO₃ particle rather than BaCeO₃ transition layer was observed in the interface between YBCO and CeO₂. The superconductivity measurement results show that the proposal method gives better superconducting properties only for high magnetic field and at low magnetic field it deteriorates the superconducting properties.     

Dumbbells of five-connected Ge atoms and superconductivity in CaGe3

CaGe(3) has been synthesized at high-pressure, high-temperature conditions. The atomic pattern comprises intricate germanium layers of condensed moleculelike dimers. Below T(c) = 6.8 K, type II superconductivity with moderately strong electron-phonon coupling is observed.     

Structural disorder, octahedral coordination and two-dimensional ferromagnetism in anhydrous alums

The crystal structures of the triangular lattice, layered anhydrous alums KCr(SO₄)₂, RbCr(SO₄)₂ and KAl(SO₄)₂ are characterized by X-ray and neutron powder diffraction (NPD) at temperatures between 1.4 and 773 K. The compounds all crystallize in the space group , with octahedral coordination of the trivalent cations. In all cases, small amounts of disorder in the stacking of the triangular layers of corner sharing MO₆ octahedra and SO₄ tetrahedra is seen, with the MO₆−SO₄ network rotated in opposite directions between layers. The electron diffraction study of KCr(SO₄)₂ supports this model, which on an average can be taken to imply trigonal prismatic coordination for the M³⁺ ions; as was previously reported for the prototype anhydrous alum, KAl(SO₄)₂. The temperature-dependent magnetic susceptibilities for ACr(SO₄)₂ (A=K, Rb, Cs) indicate the presence of predominantly ferromagnetic interactions. Low-temperature powder neutron diffraction reveals that the magnetic ordering is ferromagnetic in-plane, with antiferromagnetic ordering between planes below 3 K.