Microwave absorption studies of MgB2 superconductor

Microwave absorption studies have been carried out on MgB₂ superconductor using a standard X-band EPR spectrometer. The modulated low-field microwave absorption signals recorded for polycrystalline (grain size ∼ 10 μm) samples suggested the absence of weak-link character. The field dependent direct microwave absorption has been found to obey a ✓H dependence with two different slopes, which indicated a transition from strongly pinned lattice to flux flow regime.     

Anisotropic transport and optical properties of superconducting (La1-xSrx)2CuO4 single crystal thin films

An experimental study on the resistivity, Hall coefficient, upper critical field, and optical reflection and transmission characteristics is described for single-crystalline superconducting (La_1-xSr_x)₂CuO₄ thin films epitaxially grown on SrTiO₃ single crystal substrates by rf magnetron sputtering. These properties exhibit pronounced anisotropy between the c-axis direction and the Cu-O plane. The characteristics are consistently explained from the point of view of the two-diemensional (2D) electronic structure. The most noteworthy implication of the experimental results is that the transport along the c-axis is far from an itinerant picture. This leads to the idea that the superconductivity in the series of cuprate materials is caused by the 2D electronic system arising from the Cu-O planes, and some interactions between them, which is not possibly due to itinerant carriers.     

ELECTROCHEMICAL OXIDATION OF La2CuO4 SINGLE CRYSTALS

Bulk superconducting La₂CuO_4+δ single crystals are obtained by using electrochemical intercalation technique from the as-grown insulating samples. Oxidation is carried out by constant current I=10μA at temperature T=70℃ and room temperature, respectively. Structure and magnetic properties are studied by low-temperature X-ray diffraction and susceptibility measurements. A superconducting phase with T_c of 19K and δ-0.12 can be attributed to the formation of oxygen clusters. Room temperature oxidation is inhomogeneous: two superconducting phases with T_c1 of 24K and T_c2 of 8K and an antiferromagnetic phase are coexisting in the crystal. It is found that the appearance of T_c in this system has the "step" tendency.     

薄膜厚度对La1.85Sr0.15CuO4薄膜结构和超导电性的影响

使用直流同轴磁控溅射法,在SrTiO₃(STO)衬底上成功制备出c取向的La_1.85Sr_0.15CuO₄(LSCO)超导薄膜.通过电输运测量系统和X射线衍射仪研究了薄膜厚度对LSCO(x＝0.15)薄膜电学性质和晶体结构的影响.实验证明随着膜厚增加,(006)衍射峰的半高宽(Full Width at Half Maximum,FWHM)逐渐减小,薄膜的取向性增强,与此同时,薄膜的超导转变温 关键词： 1.85Sr_0.15CuO₄薄膜')" href="#">La_1.85Sr_0.15CuO₄薄膜 超导电性 晶体结构     

High T c superconductors as ionic metals and the role of phonons in high T c superconductivity

We have investigated a number of zone center and zone boundary (X-point) modes in La₂CuO₄ using the full potential Linearized Augmented Plane Wave (LAPW) method and find excellent agreement with experiment for phonon frequencies, showing that the local density approximation (LDA) is an excellent approximation for the total energies, charge density, and static density response in high T _c superconductors. Results are compared with those of a non-empirical ionic model for La₂CuO₄ and YBa₂Cu₃O₇. Self-consistent calculations show that the ionic nature of these materials leads to large changes in the potential with atomic displacements. The ionic contributions to the electron-phonon matrix elements, which were neglected in previous studies, are large and possibly sufficient to explain high T _c .     

Effect of heat treatment on electronic phase in underdoped La2-xSrxCuO4 single crystal

Superconducting La1.937Sr0.063CuO4 crystals grown by the travelling-solvent floating-zone technique were thermally treated under various temperatures and oxygen pressures for moderately adjusting the oxygen content. The response of intrinsic electronic property of the crystals to the change of hole density in La2-xSrxCuO4 in the vicinity of the magic doping of x= 1/16 （= 0.0625） is studied in detail by magnetic measurements under various fields up to 1 T. It is found that when the superconducting critical temperature （Tc） increases with the oxygen content, there appears also a new subtle electronic state that can be detected from the differential curves of diamagnetic susceptibility dx/dT of the crystal sample. In contrast with the intrinsic state, the new subtle electronic state is very fragile under the magnetic fields. Our results indicate that a moderate change in oxygen doping does not significantly modify the intrinsic electronic state originally existing at the magic doping level.     

Low temperature synthesis and superconducting properties of polycrystalline T′-(La,Sm)2CuO4

La_2−xSm_xCuO_4+δ with the Nd₂CuO₄ structure was synthesized by precipitation from fused alkaline hydroxide, CsOH, at 400 °C. The as-sintered polycrystalline samples showed no diamagnetic signal, but after post-annealing in vacuum of ≈10⁻⁸ atm at 650–700 °C, the samples with x = 0.1 and 0.3 exhibited superconductivity at T_c on = 25 K. The volume fraction of the superconducting phase estimated from the slope of the Zero-Field-Cooled magnetization data was over 30% for x = 0.3, which confirmed that the La_1.7Sm_0.3CuO_4+δ is a bulk superconductor.     

X-ray study of the low-temperature phase transition in K2CoCl4

Precise lattice parameter measurements and intensity measurements of selected main and satellite reflections of K₂CoCl₄ have been performed in the temperature range 100 to 300 K in the vicinity of the low-temperature phase transition (commensurate-commensurate phase transition, T _c = 142 K). A broadening of the FWHM for the h01 reflections was observed below 142 K which suggests a transition from an orthorhombic phase to a monoclinic phase.     

Magnetism and superconductivity in ErNi2B2C

We have performed a series of neutron diffraction experiments from the magnetic order and the vortex lattice in single crystal ErNi₂B₂C. The incommensurate magnetic structure develops additional even harmonics below the ‘ferromagnetic’ ordering temperature, T _F of 2.3 K. This feature and the existence of rods of diffuse scattering suggest the development of ferromagnetic microdomain walls. The magnetic structure is very sensitive to the application of a magnetic field with changes in modulation vector and harmonic content. Studies of the vortex lattice show the presence of a 45° reorientation transition and a distorted hexagonal to square transition as a function of applied field. Further distortions of the vortex lattice occur at T _N, but no changes are seen at T _F.     

Microscopic distortion and order parameter in langbeinite K2Cd2(SO4)3

The structural data on the orthorhombic phase of K₂Cd₂(SO₄)₃ at four different temperatures of Abraham et al. (1978) [J. Chem. Phys., 68, 1926] is reanalyzed in terms of frozen symmetry modes. The eigenvector of the primary distortion present in the orthorhombic phase is derived. The temperature variation of the amplitude of the order parameter is determined and compared with those of the amplitude of the frozen secondary distortion and the amplitude of the orthorhombic strain. In contrast with some previous literature, it is shown that, within experimental accuracy, all these magnitudes exhibit the correlation expected from a conventional Landau model.     

Synthesis and electrical properties of Ln<Subscript>2</Subscript>CuO<Subscript>4+</Subscript><Subscript><Emphasis Type="Italic">δ</Emphasis></Subscript> (Ln: Nd or La) mixed conductor sputter deposited coatings

Nd₂CuO₄ and La₂CuO₄ are potential candidates as cathode material for intermediate temperature-solid oxide fuel cells. Nd–Cu and La–Cu oxides were co-sputtered on rotating substrates from metallic La, Nd and Cu targets in the presence of a reactive argon–oxygen gas mixture. Structural and chemical features of these films have been determined by X-ray diffraction and energy-dispersive spectroscopy. Their electrical resistivity was measured using the four-point probe method. As-deposited Nd–Cu based coatings are amorphous and, after annealing, crystallise in K₂NiF_{4+ δ} -type structure for Nd/Cu atomic ratio of 2, with more or less Nd₂O₃ or CuO, depending on whether Nd or Cu is in excess, respectively. As-deposited La–Cu based coatings are also amorphous and crystallise in La₂CuO₄ for La/Cu >2 or in LaCuO_3±δ perovskite-type structure when Cu is in excess. The electrical measurements show a clear relation between resistivity and structure of the coatings. After annealing, crystallised neodymium-based coatings show higher resistivity than lanthanum-based ones. Finally, LaCuO_3±δ exhibit higher resistivity than La₂CuO₄. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Reduction dependence of superconductivity in the end-member T′ cuprates

We have recently achieved superconductivity in T′-RE₂CuO₄ (RE: Pr, Nd, Sm, Eu, and Gd), using epitaxial thin films by metal organic decomposition. The key recipes to achieve superconductivity are low-P_O2 firing and subsequent vacuum reduction to minimize the amount of impurity oxygen atoms, which are very harmful to high-T_c superconductivity. In this article, we report our investigation on the reduction dependence of superconductivity of T′-RE₂CuO₄. For thin films, the amount of remnant O_ap atoms is difficult to evaluate but we propose that one good measure for this may be the c-axis lattice constant, which tells us whether the reduction is insufficient or excessive.     

Phase transition in Cu2+-doped RbH2 PO4 as observed by EPR

The electron paramagnetic resonance (EPR) spectra of Cu²⁺-doped RbH₂ PO₄ at elevated temperatures indicate a phase transition at 358 K. The EPR-silent state at this temperature is attributed to a so-called polymeric phase transition. After the transition when the temperature is lowered to 293 K, the EPR signal does not appear; therefore, the transition is irreversible. This result seems to be in agreement with the other observations. The EPR spectra for the sample indicate the presence of two sites for Cu²⁺, and the values of EPR parameters are in accord with the literature on Cu²⁺-doped single crystals. Any other phase transitions could not to be observed at low temperatures down to 113 K.     

Influence of La2O3 on the Thermal Stability of Polybenzoxazine: Effect of Substituted Groups

Four kinds of benzoxazine monomers were synthesized, using phenol and aniline, 4-methylaniline, 4-nitroaniline, or 3-nitroaniline as starting materials. La₂O₃ was incorporated into the benzoxazines to prepare polybenzoxazine composites. The polybenzoxazines and their composites were analyzed through Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), and TGA–FTIR. The results revealed that La₂O₃ can improve the thermal stability of aniline-based polybenzoxazine (Pa) and 4-methylaniline-based polybenzoxazine (Pa-pC) due to the delay of evaporation of the amine compounds during their thermal degradation process. However, La₂O₃ did not influence the thermal stability of polybenzoxazines based on 4-nitroaniline (Pa-pN) or 3-nitroaniline (Pa-mN), mainly resulting from the introduction of the nitro group remarkably changing the thermal degradation processes of Pa-pN and Pa-mN, producing a great deal of CO₂ and H₂O in their degradation gases, while amine and phenolic compounds gases were hardly detected.     

First-order transition switch-off of superconductivity in UGe2

The ac magnetic susceptibility, the thermal expansion and the magnetostriction were measured using a single crystalline sample of UGe₂ under pressure. We find that as the field exceeds a transition field, where the system transforms from a high pressure phase (P>P_X) into a low pressure phase (P<P_X), the superconductivity steeply collapses within a transient region arising from the pressure inhomogeneity. We also estimate an accurate value of P_X in zero temperature limit and a pressure distribution around it. Using thus estimated quantity, we argue that the superconductivity (in zero magnetic field) below the critical pressure is not intrinsic but extrinsic due to the pressure inhomogeneity.     

An investigation of paddle wheel Cu2(μ2-O2CCH3)4 for gas molecule adsorptions

Metal organic frameworks (MOFs) have been well-known and extensively researched due to the high storage /good selectivity for gas molecules. Herein, the structures and electron paramagnetic resonance (EPR) spectra for dicopper paddle wheel MOF compound (Cu₂(µ₂-O₂CCH₃)₄ with various gas molecule are theoretically investigated by density functional theory (DFT) calculations. The adsorption energies and isotherms (including pure gas molecules and the mixed ones) are calculated for the gas molecules interacting with the unsaturated Cu₂(µ₂-O₂CCH₃)₄. Both quantities exhibit the roughly consistent orders (e.g. H₂S?>?NH₃?>?CO₂?>?CO?>?H₂O?>?N₂?>?NO?>?H₂ for isotherms and H₂S?>?NH₃?>?N₂?>?CO₂?>?NO?>?H₂O?>?H₂?>?CO for adsorption energies), possibly suggesting that this material may act as a potential adsorbent of these gas molecules. The catalytic property of Cu₂(µ₂-O₂CCH₃)₄ for oxidation of CO and NO into non-toxic molecules and splitting of H₂O into H₂ and O₂ in the solvent condition are uniformly discussed. Simulation of Grand Canonical Monte Carlo (GCMC) in MS 8.0 and calculations in Langmuir model reveal that Cu₂(µ₂-O₂CCH₃)₄ has good selectivity for CH₄ in natural gas (CH₄/CO₂/N₂) and SO₂ in fog (SO₂/NO/NO₂/H₂O/O₂), which would exhibit potential environmentally friendly applications.     

Effects on structure and superconductivity for La and Ca codoping in YBa2Cu3Oy cuprates

The nominal composition of Y_0.8Ca_0.2Ba_2−xLa_xCu₃O_y (YBLCO) cuprates with x≤0.50 has been synthesized by the standard solid state reaction technique. X-ray diffraction and the resistivity measurements are used to characterize the structure and the superconductivity of YBLCO cuprates. There is no structural phase transition in the whole doping range. The dependencies of the lattice constants and some other structural parameters on the content of La for the samples YBLCO with x≤0.20 are different than those for the samples with x≥0.25. The zero resistance temperature T_c0 increases with the increase of the content of La in YBLCO as x≤0.20, and decreases as x≥0.25. We compared these results with those of Nd-doped Y_0.8Ca_0.2Ba_2−zNd_zCu₃O_y cuprates. It seems that T_c0 is related to the structural parameters due to Ca and La codoping in YBLCO.     

New compounds with incommensurate phases: Cs2CdI4 and Cs2ZnI4

Abstract

In this paper the results of birefringence studies and of optical observations in polarized light in a wide temperature region are presented for crystals Cs₂CdI₄ and Cs₂ZnI₄. There is the following sequence of phases: commensurate orthorhombic Pnma ? incommensurate ? monoclinic ferroelastic P2₁/n ? triclinic ferroelastic PT. A correlation was observed between the peculiarities of birefringence and NQR spectra temperature dependence. An assumption is made, that in Cs₂ZnI₄ crystal in a broad pre-transition region (T - T _i = 100 K) precursor clusters exist, which manifest themselves as coexistence of NQR spectra of two phases and as deviation of birefringence from the linear temperature dependence (“tail”).