K. Nakajima

Incommensurate magnetic correlations in La2NiO4.125: Magnetic phase diagram and two-dimensional hole-order

The magnetic phase diagram of La2NiO4+δ is studied by magnetization and neutron scattering measurements. Three-dimensional (3d) magnetic order with an incommensurate Ni2+ spin modulation in the NiO2 plane appears beyond oxygen doping δ around 0.10. The sharpness of the magnetic phase transition in this phase dramatically depends on δ and the sharpest one is observed at δ≈0.125=18. In contrast to the carrier doping with δ<0.10, which degradates the 3D commensurate magnetic order, the Neel temperature (TN) of this phase is raised by the increase of oxygen doping. Above TN two-dimensional (2D)_incommensurate fluctuations dominate. The magnetic phase diagram and the commensurate-incommensurate phase transition of this system are discussed based on the 2D hole-order in the NiO2 plane.

Magnetic structure and weak ferromagnetism of La2NiO4+δ

Abstract Magnetic properties of La 2 NiO 4+δ have been studied from single crystals with different oxygen concentrations. For the stoichiometric sample a three-dimensional antiferromagnetic long range order develops below T N =325 K . In contrast to La 2 CuO 4 no peak but only a small kink appears at T N in the thermal evolution of the magnetic susceptibility which is not due to the spin canting out of the NiO 2 plane. A transition to the spin canted structure associated with a net weak ferromagnetic moment takes place at the structural phase transition to the low temperature tetragonal phase. For the nonstoichiometric sample two phases with and without spin canting coexist.

Oxygen doping and phase separation in La2NiO4+δ

Abstract Excess oxygen content δ in single crystals of La2NiO4+δ is controlled by accurate heat treatments under various oxygen partial pressures. Magnetization and neutron diffraction revealed that the doping of excess oxygen induces new phases. In addition to the stoichiometric phase (phase I: TN=328 K) three new phases, II(TN∼210 K), II(TN>180 K), and III(TN∼80 K) are observed in 0≤δ≤0.05. The possibility of a miscibility gap or oxygen ordering for the dilute excess oxygen is discussed.

Spin dynamics and spin correlations in the spinS=1 two-dimensional square-lattice Heisenberg antiferromagnet La2NiO4

The static and dynamic spin fluctuations in the spinS=1, two-dimensional (2D) square-lattice antiferromagnet La2NiO4 have been studied over a wide temperature range using neutron scattering techniques. The spin correlations in La2NiO4 exhibit a crossover from two- to three-dimensional (3D) behavior as the Néel temperature is approached from above. Critical slowing down of the low-energy spin fluctuations is also observed just aboveTN. The correlation length, ξ(T), and the static structure factor,S(0), have been measured and are compared with recent theoretical calculations for the quantum 2D Heisenberg antiferromagnet using microscopic parameters determined from previous spin-wave measurements. Good agreement for ξ(T) is found with the exact low-temperature result of Hasenfratz and Niedermeyer provided that 2πps is renormalized by ≈20% from the spin-wave value.

Terahertz response for bicrystal YBCO Josephson junctions

The high-frequency response of bicrystal YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// Josephson junctions for far infrared laser irradiation is presented. The junctions on silicon and MgO substrates exhibit a remarkable first order Shapiro step for the methanol line at 1.76 THz. dV/dI measurements reveal several steps corresponding to the methanol lines at 1.48, 1.76 and 2.53 THz. The line width of Josephson oscillation at these frequencies is estimated from the width of the Shapiro steps. It is suggested that the excess shot noise broadens the line width.

Universality class of magnetic phase transition in the triangular lattice antiferromagnet CsMnI3

Universality classes of successive magnetic phase transitions in the triangular lattice antiferromagnet CsMnI 3 are studied by measuring the critical exponents γ and ν using neutron scattering. The critical exponents of the successive transitions at T N1 =11.2 K and T N2 =8.2 K, which correspond to orderings of magnetic moments parallel and perpendicular to the c -axis, respectively, are determined to be γ // =1.12±0.07, ν // =0.59±0.03 ( T N1 ) and γ ⊥ =1.04±0.03, ν ⊥ =0.56±0.02 ( T N2 ). These values and the β exponents β // =0.32±0.01, β ⊥ =0.35±0.01 measured in our previous work do not agree with a theory which predicted the three dimensional XY universality for both the transitions. This is in contrast with the satisfactory agreement of critical exponents of CsMnBr 3 with a theory of the novel three-dimensional Z 2 ×S 1 universality. A uniform magnetization parallel to the c -axis in terms of the three sublattices in the c -plane is found to increase slowly below T N1 .

Dynamic Spin Properties of La1.98Sr0.02CuO4

Neutron scattering experiments have been performed to study the dynamic spin properties of La 1.98 Sr 0.02 CuO 4 , a crystal with just enough Sr to destroy three-dimensional long range magnetic order and induce spin-glass behavior at low temperatures. The q -integrated susceptibility (int ,{rm d}^{2}q_{rm 2D},{rm Im},chi _{rm 2D}(q_{rm 2D},,omega )) follows the scaling function of ω/ T suggested by Keimer et al. Below 3 meV, however, clear deviation from the scaling function is observed at low temperatures. This probably originates from a gap in the spin-wave-like excitations due to the out-of-plane anisotropy.

Intrinsic noise temperatures of YBa2Cu3O7−δ Josephson devices on bicrystal substrates and the upper frequency limit for their operation

Following a method proposed by Divin and Modovets [Sov. Tech. Phys. Lett. 9, 108 (1983)], we have measured at millimeter waveband the intrinsic noise temperatures TN of YBa2Cu3O7−δ Josephson junctions or dc superconducting quantum interference devices (SQUIDs) fabricated on SrTiO3, yttria‐stabilized ZrO2, or Si bicrystal substrates. Over wide ranges of physical temperatures TP and the junction’s normal resistance RN, it was found that TN follows TP pretty well. This indicates that the intrinsic noise in the devices is dominated by Johnson noise. TN was also measured in cases where there is external magnetic field applied, or where there is another microwave radiation like the local oscillator in a mixer. The magnetic field or microwave radiation does not seem to affect TN in any appreciable way. To estimate the high frequency performance of the junctions on Si bicrystal substrates, direct irradiation by a far infrared laser at 1.81 THz is carried out and the clear first Shapiro step is observed.

Field effects on the dielectric property of YBCO bicrystal grain boundary junctions

Electric field effects on YBCO grain boundary Josephson junctions combined with inverted MIS (Metal, Insulator, Superconductor) structures were studied. Current steps and hysteresis recognized in current-voltage curves were strongly affected by the field. The current steps were attributed to the self-excited resonances, i.e., Fiske steps. Both the effects were interpreted by the field dependence of the effective dielectric constant of the grain boundary which has a certain contribution from the large dielectric constant of SrTiO/sub 3/ substrates. The effective dielectric constant normalized with barrier thickness of the junction was estimated to be about 40 nm/sup -1/ for the zero gate voltage and was decreased by a factor of 2 with the electric field of /spl plusmn/16 kV/cm. This feature is assumed adequate for tuning the phase velocity in the bicrystal grain boundaries.<<ETX>>

Spin correlations in high-temperature superconductors

Abstract Recent neutron magnetic scattering experiments from single crystals of both undoped and doped cuprous “214” oxides are reviewed. The magnetic properties in the undoped crystals are essentially interpreted by the quantum two-dimensional Heisenberg antiferromagnetic model. The three-dimensional long-range order occurs because of small but finite interplanar magnetic coupling plus various anisotropies. The doping of charge modifies the spin correlations significantly as well as the spin fluctuations, which suggests close correlations between the charge and spin degrees of freedom.