Akihiko Hayashi

Phase diagram of La2NiO4+δ (0 ≤ δ ≤ 0.18): I. Phase at room temperature and phases transition above δ = 0.15

La2NiO4+δ was prepared with δ in the range of 0.00≤δ≤0.18. Oxygen stoichiometry, structure and phase transition were studied by TG-DTA and powder X-ray diffraction measurements. Five phases, i.e. orthorhombic Bmab (δ=0.00), pseudo-tetragonal (δ=0.02), orthorhombic (0.05<δ<0.09), tetragonal I4/mmm (0.1<δ<0.15) and orthorhombic Fmmm (δ≥0.15), appeared in the sequence of δs with intervals of coexistence at room temperature. The orthorhombic phase (Fmmm) showed a first-order phase transition to a tetragonal phase (I4/mmm) with increasing transition temperatures from 320 K for δ=0.15 to 430 K for δ=0.18. A tentative phase diagram of La2NiO4+δ is also given.

Phase diagram of La2NiO4+δ (0 ≤ δ ≤ 0.18) II. Thermodynamics of excess oxygen, phase transitions (0.06 ≤ δ < 0.11) and phase segregation (0.03 ≤ 6 < 0.06)

Abstract Oxygen nonstoichiometry and structural phase transitions in La 2 NiO 4+δ were studied by thermogravimetry (TG) and in-situ X-ray powder diffraction. Doubly charged O 2− ions are proposed as the species of the excess oxygen from thermodynamical analysis of TG data. A transition from tetragonal 14/mmm to orthorhombic Fmmm phase was found in the wide range of 0.06 ≤ δ 2 NiO 4+δ is proposed for 0 ≤ δ ≤ 0.18. The character of the phase transitions is discussed in the light of the temperature dependence of the orthorhombicity in each orthorhombic phase.

Cu Nuclear Spin-Spin Relaxation in Tl2Ba2CuO6+δ

We have measured the temperature ( T ) dependence of the Gaussian decay rate 1/ T G associated with the 63 Cu nuclear spin-spin relaxation of Tl 2 Ba 2 CuO 6+δ ( T c ∼85 K) by the NMR spin-echo technique. 1/ T G increases from T c up to T sp ∼120 K and decreases monotonically in a Curie-Weiss-like law above T sp . In spite of the nonmonotonic T -dependence of (1/ T G ) 2 [∼χ( Q )], we have found that (1/ T G ) 2 scales to 1/ T 1 T and hence 1/ T 1 ∝ T χ( Q ) is valid above T c . This result indicates that the antiferromagnetic spin-fluctuation theory for the two-dimensional itinerant electron system holds for the present overdoped monolayered Cu oxide without a dynamical gap.

Structural and superconducting properties of oxygenated La2−χMχCuO4+y (M=Sr, Ba; O≦χ<0.1)

Abstract Oxygen stoichiometry, superconductivity and structural properties of La 2−χ M χ CuO 4+ y (M=Sr, Ba; 0≦χ P O 2 =360 atm at 600°C were studied by TG-DTA, electrical resistivity, magnetic susceptibility and X-ray diffraction measurements. All oxygenated samples showed superconducivity. The phase separation into two phases with different orthorhombic strain was observed in the samples with 0≦χ≦0.03 under ambient atmosphere, and the phase with the smaller orthorhombic strain was the superconducting one. The superconductivity and the phase separation in this system appear to be related to the average valence of Cu ions and the structural properties, respectively, which are affected by both oxygen nonstoichiometry and substitution.

Magnetic structure of the 4,4,4′,4′,5,5,5′,5′-octamethyl-2,2′-m-phenylenebis(4,5-dihydroimidazol-1-oxyl 3-oxide) biradical: quantum spin effect of S= 1 species associated with structural change

A peculiar magnetic structure has been found in the crystal of a stable organic biradical molecule with S = 1,4,4,4′,4′,5,5,5′,5′-octamethyl-2,2′-m-phenylenebis(4,5-dihydroimidazol-1-oxyl 3-oxide)(m-BNN). A high-field magnetization process up to 40 T at 1.8 K exhibits a two-fold saturation process. Powder X-ray diffraction reveals the breakdown of the translational equivalence between the biradical dimers at low temperature. From these results, it is concluded that there is a four-fold quasi-degeneracy associated with a singlet pair formation between two S= 1/2 spins in only half the biradical dimers in the crystal.

Successive structural phase transitions in stoichiometric La2NiO4 observed by X-ray diffraction

Abstract Phase transition in La2NiO4+δ near stoichiometric oxygen content has been studied between 10 and 800 K by powder X-ray diffraction. In addition to a second-order transition from a high-temperature tetragonal (HTT) phase into a low-temperature orthorhombic (LTO1) phase at 700 K on cooling, a transition into another orthorhombic (LTO2) phase was observed at 75 K. The LTO2 phase developed into domains large enough for X-ray powder diffraction. We present evidence that the LTO2 phase is apparently an orthorhombic one and the second transition is of first order. The nature of phase transition in La2MO4 (M = Ni, Cu) is also discussed in relation to the temperature dependence of the orthorhombicity.

Evidence for Temperature-Dependent Antiferromagnetic Correlation Length in High-Tc Cu Oxides

We report the temperature( T )-dependent anisotropy r (≡ 63 W a b / 63 W c c ) of the planar 63 Cu nuclear spin-lattice relaxation rates 63 W α (α= a b , c c ) in the high- T c Cu oxides, YBa 2 Cu 4 O 8 , YBa 2 Cu 3 O 6.96 and Tl 2 Ba 2 CuO 6+δ , from around T c up to about 300 K. We discuss within the spin-fluctuation model that the anisotropy r is dependent only on the antiferromagnetic correlation length ξ and that its T -dependence for the lightly-doped material is different from that of the over-doped ones. We provide the T -dependent anisotropy r as direct evidence of the T -dependent ξ .

Cu NMR and NQR Study of CuS

We report Cu NMR and NQR studies of the cupric sulfide CuS from 4.2 K to room temperature. We have assigned the site for two distinct 63 Cu NMR lines based on an abrupt change of the Cu NMR spectrum at 55 K due to a structural phase transition for both the powder and the oriented CuS. We found that the Cu nuclear spin-lattice relaxation rate 1/ T 1 is magnetic in origin and highly anisotropic, and that the temperature dependence of 1/ T 1 T is similar to that of the uniform magnetic susceptibility χ 0 ( T ). The orbital contribution to the relaxation rate and the nature of the electron spin correlation at the Cu sites are discussed.

NMR Study of Anomalous CDW Behaviors in a Layered Copper Sulfide, K3Cu8S6

The NMR method has been applied to investigate the electronic states of K 3 Cu 8 S 6 , a copper sulfide with a layered structure. This compound has been known to show anomalous CDW behaviors such as the metal-CDW-metal transitions at T c1 (153 K) and T c2 (55 K), respectively. We measured the temperature dependences of the Knight shifts for the four Cu sites to investigate mainly the nature of the metallic state below T c2 . As a result, it has turned out that the CDW state remains even below T c2 although the material shows a metallic character, and that the Knight shifts for all the Cu sites do not change at T c2 . For these results, we propose as one of the possible interpretations that the state below T c2 is a metallic state with the CDW but no Fermi surface, i.e. the metallic state without the Pauli paramagnetism.

NMR study of the spin dynamics in full over-doped Tl2Ba2CuO y (T c=0 K)

The temperature dependence and the anisotropy of the Knight shift and the spin-lattice relaxation time (T1) at the Cu and the O sites of the full over-doped Tl2Ba2CuOy(Tc=0 K: Tl2201L) were measured. The Knight shift values at the Cu and the o sites in the CuO2 plane are larger in Tl2201L compared to those in Tl2Ba2CuOy(Tc=85 K: Tl2201M). This indicates the increase of the spin part of the Knight shift due to the over-doping. Based on the comparison between theT1 in Tl2201L and Tl2201M, the enhancement of the antiferromagnetic correlation still remains in the whole over-doped region.