J.-S. Zhou

Transport and magnetic properties of the superconducting La2CuO4+δ phases (0 < δ < 0.09) prepared by electrochemical oxidation

Abstract A preliminary phase diagram is given for the system La 2 CuO 4+δ , o ≤ δ ≤ 0.09, prepared electrochemically at room temperature. Room-temperature X-ray data distinguish two orthorhombic phase fields, O I for δ M for δ > 0.055. The orthorhombicity of O I decreases with increasing δ, that of O M increases with δ. A spinodal phase segregation into an oxygen-rich superconductive phase and an oxygen-poor antiferromagnetic phase occurs in the temperature interval 220K T ≤ T sp ; below 220 K, the mobility of the interstitial oxygen becomes too low for a further static phase segregation. The phase segregation causes an increase in resistivity and a decrease in the Seebeck coefficient with decreasing temperature in the interval 220 T T sp . A similar transport behavior occurs in the range T c T T ϱ ≤ 120 K in samples 0.02 ≤ δ ≤ 0.04; from comparisons with the La 2− x Sr x CuO 4 system, we suggest that at lower temperatures this behavior is the signature for a further dynamic hole segregation, induced by atomic displacements, into domains more rich in mobile holes within the superconductive phase. The diamagnetic AC susceptibility associated with this superconductivity is independent of the magnetic field below 0.01 mT. In the compositional range 0.06 ≤ δ ≤ 0.09, the Seebeck data indicate a nearly constant concentration of itinerant holes in the superconductive CuO 2 planes. We conclude that in the O M phase oxygen-oxygen interactions within the superconductive layers trap out at oxygen-atom clusters the excess mobile holes Δ P = 2 δ −0.12 from the CuO 2 planes. An H c1 = 64 mT was obtained for δ = 0.09. A step in the resistivity drop at T c signals a temperature interval T c T T on within which the superconductive pairs are confined to small domains. Whether these domains represent a second superconductive phase correlated to additional oxygen ordering that fails to develop with increasing δ or should be considered a pair-fluctuation phenomenon is not resolved.

Linear temperature dependence of resistivity and change in the Fermi surface at the pseudogap critical point of|[nbsp]|a|[nbsp]|high-Tc superconductor

Transport measurements in a high-temperature superconductor provide evidence that the so-called pseudogap phase ends at a quantum critical point located inside the superconducting dome in the phase diagram of cuprates.

Intra- versus intergranular low-field magnetoresistance of Sr2FeMoO6 thin films

Thin films of (001)-oriented Sr2FeMoO6 have been epitaxially deposited on LaAlO3 and SrTiO3 (001) substrates. Comparison of their transport and magnetic properties with those of polycrystalline ceramic samples shows a metallic versus semiconductor temperature dependence and a saturation magnetization Ms at 10 K of 3.2 μB/f.u. in the film as against 3.0 for a tetragonal polycrystalline sample. However, the Curie temperature TC≈389 K is reduced from 415 K found for the tetragonal ceramic, which lowers Ms at 300 K in the thin films to 2.0 μB/f.u. compared to 2.2 μB/f.u. in the ceramics. A Wheatstone bridge arrangement straddling a bicrystal boundary has been used to verify that spin-dependent electron transfer through a grain boundary is responsible for the low-field magnetoresistance found in polycrystalline samples below TC.

High-Pressure Sequence of Ba3NiSb2O9 Structural Phases: New S=1 Quantum Spin Liquids Based on Ni2+

Two new gapless quantum spin-liquid candidates with S = 1 (Ni(2+)) moments: the 6H-B phase of Ba(3)NiSb(2)O(9) with a Ni(2+)-triangular lattice and the 3C phase with a Ni(2/3)Sb(1/3)-three-dimensional edge-shared tetrahedral lattice were obtained under high pressure. Both compounds show no magnetic order down to 0.35 K despite Curie-Weiss temperatures θ(CW) of -75.5 (6H-B) and -182.5 K (3C), respectively. Below ~25 K, the magnetic susceptibility of the 6H-B phase saturates to a constant value χ(0) = 0.013 emu/mol, which is followed below 7 K by a linear-temperature-dependent magnetic specific heat (C(M)) displaying a giant coefficient γ = 168 mJ/mol K(2). Both observations suggest the development of a Fermi-liquid-like ground state. For the 3C phase, the C(M) perpendicular T(2) behavior indicates a unique S = 1, 3D quantum spin-liquid ground state.

Effect of tolerance factor and local distortion on magnetic properties of the perovskite manganites

The substitutions of rare earths for La on the magnetic properties of the perovskites La0.7−xRxA0.3MnO3 (0<x<0.7), R=Pr or Gd and A=Ca, Sr, or Ba have shown that small substitution of Pr slightly increases the coercive field Hc and magnetization M, and strongly improves the magnetoresistance (MR) while lowering Tc. On the other hand, the substitution of Gd lowers Hc, M and Tc, however, increases MR of the system La0.7−xGdxSr0.3MnO3 at temperature <Tc. Large differences in the A-site ionic radii rA of the AMnO3 perovskites proved detrimental. The optimal composition has been discussed for the half-metallic ferromagnet of a spin-switch device based on the manganese oxides.

Grain-boundary room-temperature low-field magnetoresistance in Sr2FeMoO6 films

Thin films of (001)-oriented Sr2FeMoO6 have been epitaxially deposited or LaAlO3 and SrTiO3 (001) substrates by pulsed laser deposition. The deposition conditions were optimized. Single-phase Sr2FeMoO6 was obtained in 100 mTorr 99.999% Ar gas at 825 °C. Transport and magnetic data showed a metallic temperature dependence and a saturation magnetization Ms at 10 K of 3.2μB/f.u. However, the Curie temperature TC≈380 K was reduced from 415 K found for tetragonal polycrystalline best ceramics, which lowers Ms at 300 K in the thin films to 1.5μB/f.u. compared to 2.2μB/f.u. in the ceramics. A low remanence was attributed to the presence of antiphase boundaries. A Wheatstone bridge arrangement straddling a bicrystal boundary was used to verify that spin-dependent electron transfer through a grain boundary and not an antiphase boundary is responsible for the low-field magnetoresistance found in polycrystalline samples below TC.

Enhanced pressure dependence of magnetic exchange in A2+[V2]O4 spinels approaching the itinerant electron limit

We report a systematic enhancement of the pressure dependence of T(N) in A(2+)[V(2)]O(4) spinels as the V-V separation approaches the critical separation for a transition to itinerant-electron behavior. An intermediate phase between localized and itinerant-electron behavior is identified in Zn[V(2)]O(4) and Mg[V(2)]O(4) exhibiting mobile holes as large polarons. Partial electronic delocalization, cooperative ordering of V-V pairs in Zn[V(2)]O(4) below T(s) approximately T(N) and dT(N)/dP<0, signals that lattice instabilities associated with the electronic crossover are a universal phenomenon.

Orbital ordering in orthorhombic perovskites

In addition to a cooperative MO6/2 site rotation, orthorhombic AMO3 perovskites with Pbnm space group have a universal, intrinsic MO6/2 site distortion consisting of an orthorhombic component and a reduction from 90° of the O–M–O bond angle α that subtends the octahedral-site edges parallel to the orthorhombic b axis. RMO3 perovskites (R = rare earth or Y, M = 3d-block transition metal) exhibit a maximum intrinsic orthorhombic site distortion at an R3+-ion radius IR ≈ 1.11 A and a reduction of α that increases with IR > 1.11 A. How the universal site distortions influence Jahn–Teller cooperative orbital orderings on the M cations is illustrated in four orthorhombic perovskite families: RTiO3, RVO3, RMnO3, and RNiO3.

Bond-length fluctuations in transition-metal oxoperovskites

Abstract Bond-length fluctuations in transition-metal oxoperovskites may give rise to two-phase fluctuations in what appears to be a single phase to a diffraction experiment. Orbital disorder at Jahn–Teller ions results in bond-length fluctuations that give 3D-ferromagnetic, vibronic Mn(III)–O–Mn(III) superexchange interactions and allow disproportionation into Mn(IV) and Mn(II) in LaMnO 3 ; where orbitally ordered and disordered phases coexist, an external magnetic field stabilizes the orbitally disordered, ferromagnetic phase relative to the orbitally ordered, antiferromagnetic phase. Spin–lattice interactions in the paramagnetic phase of charge-transfer compounds give bond-length fluctuations arising from the semicovalent component of the superexchange interactions. At the crossover from localized to itinerant electronic behavior, the coexistence of two-phase fluctuations has been demonstrated in both the single-valent R NiO 3 family ( R =rare-earth, A =alkaline-earth) and the mixed-valent R 0.5 A 0.5 MnO 3 perovskites. “Bad-metal” behavior is found to be associated with bond-length fluctuations.

Enhancement of room temperature magnetoresistance in double perovskite ferrimagnets

The double perovskite Sr2FeMoO6 is a half-metallic ferrimagnet exhibiting significant magnetoresistance (MR) at 300 K in a magnetic field H<2 kOe. A preliminary study of the influence of isovalent and aliovalent substitutions for Sr2+ on the magnitude of the room-temperature MR is reported. Of the polycrystalline samples, Sr1.9A0.1FeMoO6 (A=Ca, Ba, La or Pr), the best result was obtained with the Ba(0.1) sample, which showed a factor of 2 enhancement of the MR over that previously reported for Sr2FeMoO6.