Evolution of ( La 1 - y Pr y ) 0.7 Ca 0.3 MnO 3 crystal structure with A-cation size, temperature, and oxygen isotope substitution

The atomic structure of ( La _{1 - y} Pr _y ) _0.7 Ca _0.3 MnO ₃ compound with 0.5≤ y ≤1 has been systematically studied by neutron powder diffraction in the temperature range from 15 to 293 K. For composition with y = 0.75, the structural analysis was performed on two samples, one containing the natural mixture of oxygen isotopes and the other one 75% enriched by ¹⁸ O. The room temperature structural characteristics of the series, including cell volume, average Mn-O bond distance, and average Mn-O-Mn bond angle, are the linear functions of the < r _A >. Temperature dependencies of these parameters are quite smooth, except for the point T = T _FM , where a jump like changes occur. The isotope enriched samples have been found identical in crystal and magnetic structure down to the temperature of transition of the sample with ¹⁶ O into the metallic ferromagnetic phase. It confirms that different transport and magnetic properties of the samples with ¹⁶ O and ¹⁸ O at low temperature are driven by the different oxygen atoms dynamics solely. Temperature dependencies of the CO and AFM diffraction peak intensities and of the peak widths for compositions close to the metal-insulator boundary ( y ≈ 0.75) indicate the macroscopically phase separated AFM-dielectric + FM-metallic state below T _FM . Received 28 April 2000     

Ferromagnetism in lacunar perovskite manganites Pr 0.7 Sr 0.3 - x x MnO 3 and Pr 0.7 - x x Sr 0.3 MnO 3

Deficiency effects in the A site upon the structural, magnetic and electrical properties in the lacunar perovskite manganite oxides Pr_0.7Sr_{0.3-x x}MnO₃ ( 0 ? x ? 0.3) and Pr_{0.7-x x}Sr_0.3MnO₃ ( 0 ? x ? 0.23) have been investigated. This study focuses on the different parameters which govern the magnetic and electrical properties in such samples. The powder X-ray diffraction patterns for all samples could be indexed either with a rhombohedral perovskite structure and R c space group for x ? 0.2 in strontium deficient samples and for x ? 0.1 for praseodymium deficient ones. For other values of x the samples could be indexed in the orthorhombic structure with Pbnm space group. Magnetic and electrical investigations show that praseodymium and strontium vacancies do not have similar effects on the lacunar compounds. Magnetization measurements versus temperature show that all our samples exhibit a magnetic transition when the temperature decreases. All the praseodymium deficient samples exhibit a paramagnetic-ferromagnetic transition when the temperature decreases while the strontium deficient ones exhibit this transition only for low x values. The magnetic transition temperature shifts to lower values as the strontium deficiency increases (from 265 K for x = 0 to 90 K for x = 0.3) and to higher values with the praseodymium deficiency increase (from 265 K for x = 0 to 315 for x = 0.23). Resistivity measurements as a function of temperature show a semiconducting-metallic transition for all x values in the praseodymium lacunar samples and only for low x values ( 0 ? x ? 0.1) in the strontium lacunar ones when the temperature decreases. Received 12 April 2000 and Received in final form 8 January 2001     

Size and concentration effects in the optical properties of alloyed ( Au x Ag 1 - x ) n and core-shell ( Ni x Ag 1 - x ) n embedded clusters

Optical properties of mixed clusters (Au_xAg _{1 - x} ) _n and (Ni_xAg _{1 - x} ) _n , produced by laser vaporization and embedded in an alumina matrix, are reported. The size effects are investigated for different concentrations (x = 0.25, 0.5 and 0.75) in the diameter range 2-4 nm. For alloyed clusters (Au_xAg _{1 - x} ) _n of a given size an almost linear evolution of the surface plasmon frequency ω _s with the concentration is observed (between those of pure gold and pure silver clusters). Moreover the blue-shift and the damping of the resonance with decreasing size is all the more important as the gold concentration in the particles increases. Such results are in agreement with theoretical calculations carried out in the frame of the time-dependent local-density-approximation (TDLDA) including an inner skin of ineffective screening and the porosity of the matrix. The optical response of (Ni_xAg _{1 - x} ) _n clusters exhibits a surface plasmon resonance in the same spectral range as the one observed for pure silver clusters, but considerably damped and broadened. For a given mean cluster size 3.0 nm, a blue-shift of the resonance is observed when increasing the nickel concentration (between x = 0.25 and x = 0.75). The results are in good qualitative agreement with classical predictions in the dipolar approximation, assuming a core-shell geometry. Received 21 November 2000     

Some experimental and theoretical studies on the Mn magnetic moment in Y ( Ni 1 - x Mn x ) 2 B 2 C

The magnetic moment of the Mn impurities was obtained from magnetization measurements of Y ( Ni _{1 - x} Mn _x ) ₂ B ₂ C as a function of the concentration x less than 0.15. Using the coherent potential approximation and starting from 3 d density of states, obtained from the first principles calculations, the magnetic moments are obtained within a two sublattice model. For adequately estimated values of the Coulomb interactions U, the position of the energy level of Mn and adopting values for the intersublattice hybridization term, a qualitative agreement with the observed experimental data is obtained. Received 23 April 2001 and Received in final form 29 October 2001     

Phase transitions in Sr 0.61 Ba 0.39 Nb 2 O 6 :Ce 3+ : I. Susceptibility of clusters and domains

The complex dielectric susceptibility of Sr _0.61 Ba _0.39 Nb ₂ O ₆ :Ce ³⁺ (SBN61:Ce) has been measured at frequencies and temperatures before and after poling. The relaxor behaviour with large polydispersivity observed above the ferroelectric phase transition temperature, T _c = 360 and 340 K for x (Ce) = 0 and 0.0066, respectively, is perfectly modeled within the framework of Chamberlin's dynamically correlated domain approach. Below T _c the dynamic nanodomain state crosses over into a ferroelectric state with polydispersive domain wall dynamics at very low frequencies. Presumably SBN61:Ce belongs to the three-dimensional random field Ising rather than to the dipole glass universality class. Received 1 October 1999     

Polarized infrared reflectivity study of an oriented ceramic of Bi 2 Sr 2 Ca 2 Cu 3 O 10 + δ (Bi-2223)

The reflectivity spectra of an oriented ceramic of Bi-2223 has been investigated by polarized infrared reflectivity spectroscopy in the energy range 0.005-2.2 eV. It is shown that the data for the polarization parallel to the c axis cannot be fitted with a one-component Drude or extended-Drude model. The conductivity spectrum is then obtained from the best fit of a “double-damping Drude” model to reflectivity spectra, itself derived from the factorized form of the dielectric function, and by a Kramers-Kronig inversion as well. The data and their analysis give a new insight of the 2D character of the system. Received 26 April 2001 and Received in final form 28 August 2001     

Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments in Au/ La 2 - x Sr x CuO 4 point-contact junctions

We present new Au/La_2-xSr_xCuO₄ (LSCO) point-contact conductance measures as a function of voltage and temperature in samples with 0.08 ? x ? 0.2. Andreev reflection features disappear at about the bulk T _c , giving no evidence of gap for T > T _c . The fit of the normalized conductance at any T < T _c supports a (s + d)-wave symmetry of the gap, whose dominant low-T s component follows the T _c ( x ) curve in contrast with recent angle-resolved photoemission spectroscopy and quasiparticle tunneling data. These results prove the separation between pseudogap and phase-coherence superconducting gap in LSCO at x 0.2. Received 14 June 2001     

Phase transitions in Sr 0.61 Ba 0.39 Nb 2 O 6 :Ce 3+ : II. Linear birefringence studies of spontaneous and precursor polarization

The linear birefringence (LB) of Sr _0.61-x Ba _0.39 Nb ₂ O ₆ :Ce ³⁺ x (SBN61:Ce) has been measured as a function of temperature within the range of . Large tails have been observed above the ferroelectric phase transition temperatures T _c = 350, 328, 320 and 291 K for the concentrations x = 0, 0.0066, 0.0113 and 0.0207, respectively. Within an Ornstein-Zernike analysis the critical exponents , and are determined. It suggests that pure SBN61 belongs to the 3D Ising universality class. Doping with Ce ³⁺ ions, which seem to act as random fields, enhances the relaxor properties. The critical exponents and of SBN61:Ce shift against those of the three-dimensional random-field Ising model. Received 1 October 1999     

Optical study of RE 1 + x Ba 2 - x Cu 3 O 6 (RE = Nd, Sm) aaand YBa 2 Cu 3 O 6 in the mid infrared range

Infrared reflectance, and transmission measurements as well as Raman scattering have been used to study the RE_1+xBa_2-xCu₃O₆ (RE = Nd, Sm) and YBa₂Cu₃O₆ absorption bands in the 1100-1500 cm^-1 infrared range as a function of temperature and beam polarization. In addition to two-phonon absorption between 1100 and 1170 cm^-1, we observe excitations around 1400 cm^-1, occurring in oxygen rich enclosures within the samples, and assign them to an excitation involving two-phonons plus the 270 cm^-1 local mode related to Cu-O broken chains. Thus, the previously reported possible magnetic origin of the 1436 cm^-1 sharp absorption band in YBa₂Cu₃O₆ is contested. Received 14 February 2001 and Received in final form 12 April 2001     

Superconductivity of SrTiO

Superconducting SrTiO _{3 - δ} was obtained by annealing single crystalline SrTiO₃ samples in ultra high vacuum. An analysis of the V ( I ) characteristics revealed very small critical currents I _c which can be traced back to an unavoidable doping inhomogeneity. R ( T ) curves were measured for a range of magnetic fields B at I ≪ I _c , thereby probing only the sample regions with the highest doping level. The resulting curves B _c2 ( T ) show upward curvature, both at small and strong doping. These results are discussed in the context of bipolaronic and conventional superconductivity with Fermi surface anisotropy. We conclude that the special superconducting properties of SrTiO _{3 - δ} can be related to its Fermi surface and compare this finding with properties of the recently discovered superconductor MgB₂. Received 4 December 2002 / Received in final form 10 March 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: jourdan@uni-mainz.de     

Ground-state properties of LiV2O4 and Li1-xZnx(V1-yTiy)2O4

We present susceptibility, microwave resistivity, NMR and heat-capacity results for Li_1-xZn_x(V_1-yTi_y)₂O₄ with 0 ? x ? 0.3 and 0 ? y ? 0.3. For all doping levels the susceptibility curves can be fitted with a Curie-Weiss law. The paramagnetic Curie-Weiss temperatures remain negative with an average value close to that of the pure compound Θ≈ - 36 K. Spin-glass anomalies are observed in the susceptibility, heat-capacity and NMR measurements for both type of dopants. From the temperature dependence of the spin-lattice relaxation rate we found critical-dynamic behavior in the Zn doped compounds at the freezing temperatures. For the Ti-doped samples two successive freezing transitions into disordered low-temperature states can be detected. The temperature dependence of the heat capacity for Zn-doped compounds does not resemble that of canonical spin glasses and only a small fraction of the total vanadium entropy is frozen at the spin-glass transitions. For pure LiV₂O₄ the spin-glass transition is completely suppressed. The temperature dependence of the heat capacity for LiV₂O₄ can be described using a nuclear Schottky contribution and the non-Fermi liquid model, appropriate for a system close to a spin-glass quantum critical point. Finally an ( x / y , T )-phase diagram for the low-doping regime is presented. Received 16 March 2001 and Received in final form 30 October 2001     

Ageing of the avalanche angle in immersed granular matter

We have studied the temperature dependent resistivity ρ( T ) of La_2-xSr_xCuO₄ epitaxial thin films in the doping range 0.045 ⩽ x ⩽ 0.25 in pulsed magnetic fields up to 50 T. The zero-field resistivity ρ( T ) of these samples in the pseudogap regime, can be scaled onto one single universal curve in a broad temperature range by using a linear transformation of both temperature and resistivity. The high field data ρ( T ) reveal a metal to insulator transition (MIT) at low temperatures, well into the overdoped regime. For samples having k _F l < 1, with k_F the Fermi wave vector and l the mean free path, this low temperature insulating behavior of the resistivity is described by the variable range hopping conductivity (VRH). For samples with k _F l > 1, the divergence follows ρ( T ) ∼ ln (1/ T ) or a power law, depending upon the Sr-content. We further found that the residual conductivity at the minimum in ρ( T ), appearing due to the MIT, follows a linear behavior with respect to the Sr-content. It is argued that the unusual MIT in compounds with k _F l > 1, is most probably associated with the pseudogap and the behavior of charge stripes at low temperatures. Received 4 January 2002 / Received in final form 7 May 2002 Published online 14 October 2002 RID="a" ID="a"e-mail: liesbet.weckhuysen@fys.kuleuven.ac.be     

Study of : pronounced differences between the and ground states

We present electrical resistivity and specific heat measurements of alloys on the Rh rich side of the phase diagram of the Ce(Rh_1-xPd_x)₂Si₂ system. We compare these results with those obtained at intermediate and low Rh concentrations. The analysis of the concentration and temperature dependence of the entropy and of the scaling behaviour of C _el ( T ) and ρ( T ) clearly confirm a separation of the magnetic phase diagram into two regions: the region x ≤0.3, showing a concentration independent characteristic temperature for the 4 f-electrons with T ₀ ≈ 45 K, while for x > 0.3, T₀ decreases to T ₀ ( x = 1) ≈ 15 K. At low Pd-content, T_N decreases very rapidly from T _N = 36 K in pure CeRh₂Si₂ to T _N = 18 K at x = 0.1. With higher Pd concentration T_N stabilizes at T _N ≈ 15 K whereas the magnitude of the anomalies in C _el ( T ) and in the susceptibility around T_N are further reduced and disappear at x ≈ 0.3. This differs from the behavior found on the Pd-rich side, where T_N decreases continuously to zero with increasing Rh content. The pronounced differences observed between both phase boundaries and the drastic effect of doping on the Rh rich side suggest an itinerant character in CeRh₂ Si₂, in contrast with the localized character of CePd₂Si₂. Further evidence for the itinerant character of CeRh₂Si₂ is given by the ρ( T ) dependence observed for x ≤0.3, which scales with ρ( T ) of the prototype itinerant compound YCo₂. Received 31 December 2001 / Received in final form 6 July 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: berisso@cab.cnea.gov.ar     

Neutron diffraction study of the magnetic ordering in the series R 2 BaNiO 5 (R = Rare Earth)

A neutron diffraction study, as a function of temperature, of the title compounds is presented. The whole family (space group Immm, a ≈ 3.8?, b ≈ 5.8?, c ≈ 11.3?) is structurally characterised by the presence of flattened NiO₆ octahedra that form chains along the a-axis, giving rise to a strong Ni-O-Ni antiferromagnetic interaction. Whereas for Y-compound only strong 1D correlations exist above 1.5 K, presenting the Haldane gap characteristic of 1D AF chain with integer spin, 3D AF ordering is established simultaneously for both R and Ni sublattices at temperatures depending on the rare earth size and magnetic moment. The magnetic structures of R₂BaNiO₅ ( R = Nd, Tb, Dy, Ho, Er and Tm) have been determined and refined as a function of temperature. The whole family orders with a magnetic structure characterised by the temperature-independent propagation vector = (1/2, 0, 1/2). At 1.5 K the directions of the magnetic moments differ because of the different anisotropy of the rare earth ions. Except for Tm and Yb (which does not order above 1.5 K), the magnetic moment of the R³⁺ cations are close to the free-ion value. The magnetic moment of Ni²⁺ is around 1.4 , the strong reduction with respect to the free-ion value is probably due to a combination of low-dimensional quantum effects and covalency. The thermal evolution of the magnetic structures from T _N down to 1.5 K is studied in detail. A smooth re-orientation, governed by the magnetic anisotropy of R³⁺, seems to occur below and very close to T _N in some of these compounds: the Ni moment rotates from nearly parallel to the a-axis toward the c-axis following the R moments. We demonstrate that for setting up the 3D magnetic ordering the R-R exchange interactions cannot be neglected. Received 19 July 2001     

Preparation, magnetoresistance and magnetocaloric effect of two-layered perovskite La 2.5- x K 0.5+ x Mn 2 O 7+δ

Polycrystalline two-layered perovskite La_2.5-xK_0.5+xMn₂O _{7 + δ} (0 < x < 0.5) samples have been prepared by a modified sol-gel method and their magnetoresistance and magnetocaloric effects have been studied. A large deviation between the metal-insulator (MI) transition temperature (T _ρ ) and the magnetic transition temperature (T_C) is observed. Large magnetoresistance (MR) effects with Δρ/ρ of ∼40% at 12 kOe are obtained in wide temperature ranges. The maximum of the magnetic entropy change peaks at its Curie temperature (T_C), far above its MI transition temperature (T _ρ ). The large magnetic entropy change (∼1.4 J/kg.K) is obtained in the sample La_2.5-xK_0.5+xMn₂O _{7 + δ} (x = 0.35) upon 10 kOe applied magnetic field. Received 2 May 2002 / Received in final form 1st October 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: wzhong@ufp.nju.edu.cn     

Superconducting and magnetic properties of La3−x In Cex

Measurements of the critical temperature, T_c(x), the critical field, H_c2(x,T) and the susceptibility, χ (T), as a function de la concentration, x, in the system La₃₋x Ce_xIn, are compared with the theories of Muller-Hartmann and Zittsrtz, and Abrikosov and Gor'kov. The H_c2(x, T = 0) and χ(T) measurements indicate the appearance of short range antiferromagnetic order around x = 0.04 which is probably responsible for the anomalous behavior of T_c(x).     

Merged beam study of the associative ionisation ( C + , N + ? and ? O + )+ O -

Total cross-sections have been measured for the associative ionisation of C ⁺ + O ^- , N ⁺ + O ^- and O ⁺ + O ^- by means of a merged-beam set-up operating with keV beams. These original measurements might be relevant to the understanding of some astrophysical objects or laboratory-made plasmas (flames and etching plasmas). The magnitude of these cross-sections is particularly large whatever the associating system, as these are in the range of 1×10 ^-14 cm² at thermal energies. Their behaviour as a function of energy significantly differs from one system to another, and is characterised by the Wigner law at low energy, and a rapid fall-off at higher energy due to competition with non-associative ionisation processes. Received 10 December 2001 and Received in final form 12 March 2002     

Absence of magnetic ordering in low dimensional mixed magnetic Mn1−xNixCl2·H2O

A new mixed magnet, Mn_1−xNi_xCl₂·H₂O, is examined by dc magnetization and susceptibility measurements across the entire composition range. The pure components are quasi-one-dimensional Heisenberg antiferromagnets ordering at 2.17 K (Mn) and 5.6₅ K (Ni) due to weaker interchain exchange supplementing the dominant exchange along MCl₂MCl₂M… chemical and structural chains. High temperature magnetic susceptibilities yield Curie and Weiss constants in χ_M=C/(T−θ). C(x) is linear but θ(x) displays curvature, which is analyzed to show that unlike-ion exchange is ferromagnetic and similar in size to like-ion. Most notable is the absence of antiferromagnetic susceptibility maxima down to 1.6 K from x=0.10 to 0.95. For x=0.05 a susceptibility maximum appears, with T_max almost 20% lower than in the pure Mn component but T_c reduced by 2%. The size of the susceptibility is enhanced by admixture, the effect of disrupted antiferromagnetic tendencies. Magnetization isotherms evolve with composition. Larger values of magnetization, under the same measuring conditions, occur for mixtures than for pure components, consistent with frustration, which weakens antiferromagnetic alignment tendencies. The competing ferromagnetic (Ni) and antiferromagnetic (Mn) intrachain interactions, along with disorder and low dimensional characteristics, presumably lead to the absence of magnetic order over a remarkably broad composition range.     

Magnetic properties of undoped YBa 2 Cu 3 O 6 + x system

In the present paper, we study the magnetic properties of bilayer cuprate antiferromagnets. In order to evaluate the expressions for spin-wave dispersion, sublattice magnetization, Néel temperature and the magnetic contribution to the specific heat, the double time Green's function technique has been employed in the random phase approximation (RPA). The spin wave dispersion curve for a bilayer antiferromagnetic system is found to consist of one acoustic and one optic branch. The “optical magnon gap” has been attributed solely to the intra-bilayer exchange coupling (J _⊥ ) as its magnitude does not change significantly with the inter-bilayer exchange coupling (J_z). However J_z is essential to obtain the acoustic mode contribution to the magnetization. The numerical calculations show that the Néel temperature (T _N ) of the bilayer antiferromagnetic system increases with the J_z and a small change in J_z gives rise to a large change in the Néel temperature of the system. The magnetic specific heat of the system follows a T² behaviour but in the presence of J_z it varies faster than T². Received 13 July 2000 and Received in final form 14 May 2001     

Hopping conductivity and activated transport in InxGa1-xAs quantum wells

We present measurements of the diagonal R_xx and off-diagonal R_xy magnetoresistance under quantum Hall conditions on several high electron mobility transistors (HEMT) based on In_xGa_1-xAs quantum wells. From the magnetoresistance tensor we obtain the longitudinal conductivity σ _xx . We study the transport mechanisms near the σ _xx minima at temperatures ranging between 2 K and 35 K; activated transport is the dominant mechanism for temperatures above 7 K while variable range hopping conductivity is significant for lower temperatures. We show that electron-electron correlations should be taken into account to explain the conductivity vs temperature behaviour below 5 K. Finally, we study the behaviour of the localization length as a function of Landau level filling and obtain a critical exponent γ = 3.45±0.15. Received 6 June 2001 and Received in final form 16 October 2001