A new family of magnetoelectric materials: A2M4O9(A = Ta, Nb; M = Mn, Co)

The existence of the magnetoelectric (ME) effect in the transition metal niobates and tantalates A₂M₄O₉(A = Ta, Nb; M = Mn, Co) has been confirmed by the observation of the electrically induced magnetic moment in these materials. Both parallel and prependicular ME susceptibilities were measured as a function of temperature in powder specimens. The transition temperatures were found to be 110 ±1 (Nb₂Mn₄O₉), 103 ±1 (Ta₂Mn₄O₉), 27.0 ±0.5 (Nb₂Co₄O₉), and 20.6 ±0.5°K (Ta₂Co₄O₉) from the ME measurements.     

Electrons and holes in undoped Nd2CuO4

Nd₂CuO_4±δ is the non-superconducting prototype of the Re_2−xM_xCuO₄t−y family (Re=Pr, Nd, Sm and M=Ceor Th) of n-type oxide superconductors. Four-probe DC conductivity, EMF in P(O₂) gradient, and thermopower measurements have been used to characterise its electric transport and defect structure between 300 and 900°C and between 5×10⁻⁴ and 1 atm oxygen partial pressure.

The results show that Nd₂CuO_4±δ can be oxygen under-stoichiometric (with n-type conductivity), near-stoichiometric, and over-stoichiometric (with p-type conductivity) in different T, P(O₂) ranges.     

Re-entrant ferrimagnetism in TbMn6Ge6

The magnetic ordering of the hexagonal compound TbMn₆Ge₆ has been studied by neutron diffraction at various temperatures between 1.8 and 500 K. In almost the whole magnetically ordered regime the magnetic structures are incommensurate with the crystal lattice. The corresponding wave vector q₁ = (0,0,q_z) is strongly temperature dependent and decreases from q_z = 0.1307 r.l.u. at 1.8 K to q_z = 0.091 r.l.u. at 410 K. The low-temperature range (T < 85 K) is characterised by a triple ferrimagnetic spiral structure with net moment along the c-direction. At T_t = 85 K the net moment disappears and the magnetic ordering changes into a flat spiral structure that probably persists up to the magnetic ordering temperature, T_N = 450 K. Before reaching the magnetic ordering temperature, however, an additional ferrimagnetic component perpendicular to the c-direction develops, marking the onset of re-entrant ferrimagnetism. The resulting structure described by two propagation vectors corresponds to a distorted spiral. Various models are discussed. The observed re-entrant ferrimagnetism is in agreement with results of previously reported magnetic measurements.     

Synthesis and characterization of HgBa2Can−1CunO2n+2+δ (n = 1, 2, and 3)

We have successfully prepared the first three members of the mercury-based superconducting compounds Hg--- Ba₂Ca_n−1Cu_nO_2n+2+δ, namely Hg---1201, Hg---1212 and Hg---1223 with high purity and very good quality. T he influence of the synthesis parameters is studied in detail. Using the sealed quartz tube method, very simple procedures are found to ensure a 100% reproducibility of nearly 100% pure Hg---1201 and 85–90% Hg---1212 and Hg---1223. Oxygen annealing of the sample Hg---1201 at 300°C for 18 h results in an enhancement of its critical temperature up to 97 K. The symmetry of the first and second members is tetragonal with lattice parameters a = 3.8831 (1) Å, C = 9.5357 (2) Å, and A = 3.8624 (1) Å, C = 12.7045 (2) Å, respectively. X-ray diffraction lines of Hg---1223 can be indexed in a tetragonal cell with a = 3.8564 (1) Å and C = 15.8564 (9) Å as well as in an orthorhombic cell with lattice parameters a = 5.4537 (1) Å, B = 5.4247 (1) Å, and C = 15.8505 (7) Å.     

Effective activation energy Ue(T,J,H) in textured Bi1.84Pb0.4Sr2Ca2Cu3Oy silver-clamped thick films

We have measured the resistivity of textured Bi_1.84Pb_0.4Sr₂Ca₂Cu₃O_y silver-clamped thick films as a function of temperature, current density ranging from 10 to 1×10³ A/cm² and magnetic field up to 0.3 T. We find that the effective activation energy U_e follows U_e(T,J,H)=U₀(1−T/T_p)^mln(J_c0/J)H⁻ with m=1.75 for Hab-plane and 2.5 for Hc-axis and =0.76 for Hab and 0.97 for Hc, for the current density regime above 100 A/cm², where T_p is a function of applied magnetic field and current density. This result suggests the effective activation energy U_e be correlated with the temperature, current density and magnetic field. The possible dissipative mechanisms responsible for the temperature, current density and magnetic field dependence of the effective activation energy are discussed.     

Synthesis and structural characterisation of YSr2Cu3−xMxO7±y (M = Fe, Ti, Al, Co, Ga, Pb; 0.5 < x < 1), and the non-existence of the parent phase YSr2Cu3O7

The possibility of synthesising phases of general composition YSr₂Cu_3−xM_xO_7±y has been examined for a wide variety of metals M. Single phase tetragonal products (typically a = 3.83 Å, c = 11.5 Å) can be synthesised provided that the concentration of M is sufficiently large (x > 0.5), and M has a preference for the Cu(1) sites (the four-coordinate sites) of the YBa₂Cu₃O₇, structure. Neutron diffraction has demonstrated that a high degree of disorder exists in the (001) planes containing the M cations and, in particular, significant oxygen displacements are found. The non-existence of the unsubstituted variant YSr₂Cu₃O₇ is explained in terms of the high degree of compressive stress on the Cu(1) sites which could exist in such a material.     

Solid-state electrochemical CO2 sensor by coupling lithium ion conductor (Li2CO3-Li3PO4-Asl2O3) with oxide ion-electron mixed conductor (La0.9Sr0.1MnO3)

A solid-state electrochemical cell of the type O₂(air), PtLa_0.9Sr_0.1MnO₃/Li₂CO₃(+5 mol% Li₃PO₄ + 6 mol% Al₂O₃)/Au, CO₂, O₂ was composed to determine CO₂ concentration, where Li₂CO₃, a lithium ion conductor, was used as an electrolyte, and the perovskite-type oxide (La_0.9Sr_0.1MnO₃)/O₂-electrode as a reference electrode. The electromotive force (EMF) of the cell was found to be proportional to the logarithm of CO/₂ partial pressure in CO₂/O₂/N₂ gas mixtures at temperatures between 300 and 400 °C. The EMF responded to changes of CO₂ partial pressure within 1 min at 400 °C. The sensitivity to CO₂ of this cell was not affected by coexistence of O₂, and the EMF remained constant after the first 15 days. The mechanism for sensing CO₂ is discussed.     

The crystallographic symmetries of single BaPb1−xBixO3 crystals grown from BaCO3-PbO2-Bi2O3 solutions

Single crystals of Perovskite-type oxide BaPb_1−xBi_xO₃ are grown from BaCO₃-PbO₂-Bi₂O₃ solutions which are weighed in two kinds of mixing ratios: X/2 mol % BaCO₃ − (100−X) mol % PbO₂ − X/2 mol % Bi₂O₃ and (10+X/2) mol % BaCO₃ − (90−X) mol % PbO₂ − X/2 mol % Bi₂O₃ These room temperature crystal structures are examined by using an X-ray powder diffraction method. The crystals grown from X/2−(100−X)−X/2 mol % solutions are orthorhombic at room temperature, while the structures are tetragonal with crystals grown from relatively Ba rich and Pb poor ( (10+X/2)−(90−X)−X/2 mol % ) solutions. This result indicates that the difference in the mixing ratio of the initial materials brings about a drastic structural change. The orthorhombic and the tetragonal crystals of x0.25 exhibit superconducting transition at 10K and 12K, respectively. The transition temperature in the latter is 2K higher than the former. In the light of this result, some difference between orthorhombic and tetragonal crystals is considered to influence superconductivity in this system.     

Measurement and analysis of the infrared reflection spectrum of semiconducting SnS

The infrared reflection spectrum of p-type SnS single crystals was measured between 2 and 25μ. The observed dispersion was ascribed to the combined effects of lattice vibrations and free holes. An analysis gave for the index of refraction n₀ = 3.6 ± 0.1, the dielectric constant = 19.5 ± 2, and the effective charge on the atoms e^* = 0.7e₀.

The effective mass of holes is isotropic in the plane perpendicular to the c-axis, with m^* = 0.20m₀; the effective mass parallel to the c-axis is much larger: m^* m₀.     

Superparamagnetic iron oxide particles and positive enhancement for myocardial perfusion studies assessed by subsecond T1-weighted MRI

Superparamagnetic iron oxide particles (SPIOs) are usually referred to as T₂ MR contrast agents, reducing signal intensity (SI) on T₂-weighted MR images (negative enhancement). This study reports the original use of SPIOs as T₁-enhancing contrast agents, primarily assessed in vitro, and then applied to an in vivo investigation of a myocardial perfusion defect. Using a strongly T₁-weighted subsecond MR sequence with SPIOs intravenous (IV) bolus injection, MR imaging of myocardial vascularization after reperfusion was performed, on a dog model of coronary occlusion followed by reperfusion. Immediately after the intravenous bolus injection of 20 μmol/kg of SPIOs, a positive signal intensity enhancement was observed respectively, in the right and left ventricular cavity and in the nonischemic left myocardium. Moreover, compared to normal myocardium, the remaining ischemic myocardial region (anterior wall of the left ventricle) appeared as a lower and delayed SI enhancing area (cold spot). Mean peak SIE in the nonischemic myocardium (posterior wall) was significantly higher than in the ischemic myocardium (anterior wall) (110 ± 23% vs. 74 ± 22%, Mann-Whitney test < 1%, n₁ = 6, n₂ − n₁ = 0, U > 2). In conclusion, the T₁ effect of SPIOs at low dose, during their first intravascular distribution, suggests their potential use as positive markers to investigate the regional myocardial blood flow and some perfusion defects such as the “no-reflow phenomenon”.     

K2NiF4型Sr2CrO4的磁电性质研究

以SrO和CrO₂为原料, 在高温高压的条件下直接反应生成纯相的K₂NiF₄结构的Sr₂CrO₄多晶样品. 结构用粉末X射线衍射及GSAS精修表征. 磁化率测试显示样品存在一个弱的反铁磁相变, 奈尔温度为T_N=95 K. 在奈尔温度以上, 磁化率随温度的变化遵循居里-外斯定律. 对样品进行了电阻测试, 结果显示了样品的绝缘特性.     

Systematic thermopower measurements of the thallium cuprates Tl(Ba,Sr)2Cam−1CumO2m+3−δ and Tl2Ba2Cam−1CumO2m+4+δ

The thermoelectric power (TEP) S versus temperature has been systematically investigated for several series of the superconducting cuprates Tl(Ba,Sr)₂Ca_m−1Cu_mO_2m+3−δ (m = 2, 3) and Tl₂Ba₂Ca_m−1Cu_mO_2m+4+δ (m = 1, 2, 3). The consideration of the S(T_c) curves allows two important points to be found evidence for. The first one deals with the fact that all these superconducting thallium cuprates are systematically overdoped whatever T_c, and whatever the number of Cu or Tl layers; no underdoped superconducting cuprate could be obtained. The second point shows that there exist two classes of Tl cuprates: the weakly overdoped cuprates that exhibit a T_{c max} ≥ 100 K (all the triple copper layer cuprates and the 2212 cuprates) and those which can be heavily doped that exhibit a T_{c max} ≤ 90 K (the 2201 and the 1212 cuprates). The different behavior of thallium cuprates compared to YBa₂Cu₃O_7−δ and to bismuth cuprates is discussed.     

Dilatometry of the high-temperature proton conductor Ba3Ca1.18Nb1.82O9 − δ

Firstly, the production of solid bulk specimens of the proton conductor Ba₃Ca_1.18Nb_1.82O_{9 − gd} (BCN18) of known water content up to [H^.] = n_H/n_Ba = 0.16 is described. Secondly, measuring the length change of such samples versus water content [H^.] demonstrated that the sample lengths increased linearly with a slope of (Δl/l)/[H^.] = (2.13 ±0.07) × 10⁻². Thirdly, the density of bulk samples was found to decrease linearly with water content [H^.]. This decrease was in good agreement with the above values derived from the length change. Fourthly, high temperature dilatometry showed that samples reach the thermodynamic solubility values in water vapor atmospheres only at temperatures above 700 °C. Two time scales were observed for the time-dependence of the elongation upon exposure to water vapor. A fast process occurred within minutes above 700 °C, a slow one took several hours. The fast one was identified as the chemical diffusion of the diffusion pair H^. and V_o^. which is required for the water uptake and loss of BCN18. The chemical diffusivity of water is described by the parameters D₀ = (0.022 ± 0.002) cm²/s and ΔH* = (0.79 ± 0.05) eV.     

Entropy of planar random surfaces on the lattice

We study planar random surfaces on a hypercubic lattice in two and three dimensions by Monte Carlo techniques. Our data are consistent with the formula n₀(A;C) A^b₀A, where n₀(A;C) is the number of planar random surfaces with area A and boundary C. We find b₀ = −1.4 ± 0.2, = 5.31 ± 0.03 (for d = 2) and b₀ = −1.5 ± 0.2, = 7.13 ± 0.05 (for d = 3). The values of b₀ disagree with those obtained from the Polyakov string model.     

Neutron diffraction study of magnetic ordering in ErMn2Si2, ErMn2Ge2 and ErFe2Si2

Neutron diffraction study of polycrystalline compounds ErMn₂Si₂, ErMn₂Ge₂ and ErFe₂Si₂ was performed in the temperature range between 1.8 and 293 K. All compounds have tetragonal, ThCr₂Si₂-type crystal structure. The antiferromagnetic collinear structure of ErMn₂Si₂ and ErMn₂Ge₂ at both RT and LNT, consists of a sequence + - + - of ferromagnetic layers of Mn atoms. The magnetic moment of an Mn atom (≈2μ_B) is parallel to the c-axist. At low temperatures (LHT and lower), the ferromagnetic ordering within the Er sublattice is observed. The magnetic moment (μ_Er ≈ 9μ_B) is perpendicular to the c-axis. From the temperature dependence of the intensities of the magnetic peaks, the following values for the Curie temperatures were obtained: (10±5) K for ErMn₂Si₂ and (8.5±3) K for ErMn₂Ge₂. For ErFe₂Si₂ a collinear antiferromagnetic structure of the + - - + type was found, the magnetic unit cell consisting of the chemical one, doubled along the c-axis.     

Low-temperature phase and magnetic interactions in FCC Fe-Cr-Ni alloys

The low-temperature (5 K < T < 300 K) magnetic properties of a set of nine isostructural fcc Fe-Cr-Ni (Fe ≈ 68 at %, Cr ≈ 20 at%, Ni ≈ 9 at%) alloys were studied by SQUID magnetometry, neutron diffraction and ultrasonic techniques. Type-1 antiferromagnetic (AF) ordering was observed below the Néel temperature, T_N. The dc susceptibility, χ(T), did not exhibit a simple Curie-Weiss dependence. Above T_N, atemperature independent component χ₀ was observed, i.e., χ(T) = χ₀ + C / (itT + θ. T_N was systematically influenced by the lattice parameter, a, decreasing from (47.9 ± 0.5) K to (35.0 ± 0.5) K as a increased by only 0.25%. The average magnetic moment of ≈ 0.6μ_B obtained from neutron scattering was lower than the ≈ 1 μ_B obtained from the SQUID data. Mean field estimates of antiferromagnetic nearest-neighbors exchange interaction (J₁) and ferromagnetic second-nearest-neighbors interaction (J₂) indicate that |J₂/J₁|≈ 1.5. We believe that this is evidence of the RKKY interaction, and self-consistently argue that only the external d electrons are responsible for the localized average moment. This may mean that s-d hybridization of the external electrons is weak in these alloys.     

Duree de vie et depolarisation par collisions avec l''helium des niveaux 5P3/2 Et 5P1/2 de l''ion Cd

The lifetimes of the Cd⁺ 5²P_3/2 and 5²P_1/2 states have been measured by the Hanle effect. The Cd⁺ ions are produced in a d.c. discharge in cadmium vapor, with helium as buffer gas. The results are: τ(5²P_3/2) = (2.60±0.20) ×10⁻⁹sec, and τ(5²P_1/2) = (3.05 ± 0.13) × 10⁻⁹sec.

We measured also the cross sections for the destruction of the orientation in the 5²P_1/2Cd⁺ state (<5Ų), of the orientation (18±10Ų) and of the alignment (46±10Ų) in the 5²P_3/2 state due to collisions with the helium atoms.     

Spectral intensities in the ν1 band of NH3

Intensities have been measured for individual transitions in the Q and R branches of the ν₁ band of NH₃ using a difference-frequency laser spectrometer. The data yield an integrated band strength of S⁰_v=219.36±1.03 cm^-2/MPa at 297 K, corresponding to a transition moment of μ_v = 8.535(20) × 10^-32 C·m, and a Herman-Wallis correction factor, (1 + _jm)², where _j = 0.0209(20). The intensities of a few lines for K 7 were noticeably perturbed by a perpendicular Coriolis interaction with 2ν₄ (E, L = 2), so were excluded from the fit. A small sample of ν₃ band lines occurring in the ν₁ band scans also yields a rough estimate of the ν₃ band intensity with evident irregular perturbations.     

Rigorous pion-pion scattering lengths from threshold πN → ππN data

A new evaluation of the universal ππ scattering length relation is used to extract the ππ s-wave scattering lengths from threshold pion production data. Previous work has shown that the chiral perturbation series relating threshold pion production to ππ scattering lengths appears to converge well only for the isospin-2 case, giving a₂ = −0.031 ± 0.007m_π⁻¹. A model-independent and data-insensitive universal curve then implies a₀ = 0.235 ± 0.03m_π⁻¹ for the isospin-0 scattering length.     

Proton spectroscopy of human stroke: Assessment of transverse relaxation times and partial volume effects in single volume STEAM MRS

Proton T₂ relaxation times were measured in 13 stroke patients and 13 aged-matched normal subjects at 2.1 T. Spectra were acquired from an 8-cc volume using the STEAM sequence with echo times (TE) of 30.4 ms and 270.0 ms and repetition time of 2.8 s. Transverse relaxation times were estimated using two-point calculations. Percentage volume of infarct in the STEAM voxel was measured on spin-echo MRI encompassing the infarct and correlated with the peak amplitude of N-acetylated compounds (NA). T₂ values of NA, creatine, and choline resonances showed no significant difference between patients and controls. T₂ for lactate in patients was 780 ± 257 ms, respectively (mean ± SE, n = 7). In stroke patients, high inverse correlation was found between the absolute NA signal and partial volume of normal brain contributing to each spectrum (p < .001, r = 0.97). Together with unchanged T₂, this suggests that NAA largely disappears from infarcted tissue within 24 hr postinfarct.