Successive jahn-teller phase transitions in Cs2PbCu(NO2)6

The sequential Jahn-Teller phase transitions in Cs₂PbCu(NO₂)₆ were studied by neutron and X-ray diffraction technique. In each phase we observed satellite reflections. The position of satellites in the reciprocal space is commonly given by $\text{k}{}_{\mathrm{<mtext>0</mtext>}}\text{= (}\text{1}\text{2}\text{1}\text{2}\text{ζ)}$ with $\text{0?ζ?0.5.}$ In Phase II and Phase IV, the values of ζ are definitely determined as ζ = 0 and ζ = 0.5 respectively. In Phase III, ζ is not definite within $\text{0.20?ζ?0.25}$ depending on samples, thermal cycling,etc. Phase III may be an incommensurate phase although there is the tendency that the value of ζ is locked in either at 0.20 ($\text{1}\text{5}$) or at 0.25 ($\text{1}\text{4}$).     

Successive Jahn-Teller transitions and the canted pseudospin state in K2PbCu(NO2)6

The successive phase transitions in K₂PbCu(NO₂)₆ has been studied by precise X-ray measurements. The unit cell in Phase III is doubled along each of the cubic principal axes. The experimental results can be interpreted as sequencial cooperative Jahn-Teller transitions where the higher transition is due to the coupling with the uniform distortion, while the lower transition is caused by the coupling with the uniform distortion, while the lower transition is caused by the coupling with the zone boundary phonon mode with $\text{k}{}_0\text{= (}\text{1}\text{2}\text{,}\text{1}\text{2}\text{,}\text{1}\text{2}$. The electronic state in Phase III is characterized as the “canted pseudospin” state.     

High-sensitivity measurement of metastable ion abundances

Measurement of metastable ion abundances over a dynamic range of 10⁵ is possible with a specially designed mass spectrometer.     

Island dynamics in the large-helical-device plasmas

In the Large Helical Device plasma discharges, the size of an externally imposed island with mode number ( n/m = 1/1) decreases substantially when the plasma is collisionless ( nu(*)< approximately 1) and the beta is finite ( > approximately 0.1%) at the island location. For the collisional plasmas with finite beta, on the other hand, the size of the island increases. However, there is a threshold in terms of the vacuum island size below which the island enlargement is not seen.     

Competition of static stripe and superconducting phases in La(1.48)Nd(0.4)Sr(0.12)CuO(4) controlled by pressure

We have investigated the pressure effect on T(c) and the Hall coefficient in the static stripe-ordered phase of La(1.48)Nd(0.4)Sr(0.12)CuO(4) crystal under hydrostatic pressure. We found a dramatic change of the Hall coefficient and an abrupt increase of T(c) at low pressure of about 0.1 GPa. The results are indicative of a transition from one- to two-dimensional charge transport, associated with the suppression of low-temperature-tetragonal (LTT) phase. From the uniaxial pressure measurements it turns out that the observed critical change is induced primarily due to the in-plane compression of the CuO(2) planes which would make the pinning potential of the LTT lattice distortions weaker.     

c-axis optical response in the static stripe ordered phase of the cuprates

The c-axis far-infrared reflectivity spectra in the superconducting state were investigated for La1.85-yNd(y)Sr0.15CuO4. The Josephson plasma edge rapidly shifts towards lower frequency with increasing y, and almost disappears when y exceeds a critical value ( y(c) approximately 0.12) above which the low-temperature-tetragonal (LTT) deformation switches on, stabilizing the spin/charge ordered stripe phase. In the vicinity of y(c), a "normal state" reentrant behavior was observed with lowering temperature below Tc. This clearly demonstrates that the static stripe order pinned by the LTT distortion suppresses the interlayer phase coherence.     

"Devil's staircase"-type phase transition in NaV2O5 under high pressure

The "devil's staircase"-type phase transition in the quarter-filled spin-ladder compound NaV2O5 has been discovered at low temperature and high pressure by synchrotron radiation x-ray diffraction. A large number of transitions are found to successively take place among higher-order commensurate phases with 2a x 2b x zc type superstructures. The observed temperature and pressure dependence of modulation wave number q(c), defined by 1/z, is well reproduced by the axial next nearest neighbor Ising model. The q(c) is suggested to reflect atomic displacements presumably coupled with charge ordering in this system.     

Variation of the stress intensity factor along the crack front of interacting semi-elliptical surface cracks

Summary  In this study, the interaction between two semi-elliptical co-planar surface cracks is considered when Poisson's ratio ν = 0.3. The problem is formulated as a system of singular integral equations, based on the idea of the body force method. In the numerical calculation, the unknown density of body force density is approximated by the product of a fundamental density function and a polynomial. The results show that the present method yields smooth variations of stress intensity factors along the crack front very accurately, for various geometrical conditions. When the size of crack 1 is larger than the size of crack 2, the maximum stress intensity factor appears at a certain point, β₁=177^∘, of crack 1. Along the outside of crack 1, that is at β₁=0∼90^∘, the interaction can be negligible even if the two cracks are very close. The interaction can be negligible when the two cracks are spaced in such a manner that their two closest points are separated by a distance exceeding the small crack's major diameter. The variations of stress intensity factor of a semi-elliptical crack are tabulated and charted. Received 30 August 1999; accepted for publication 22 February 2000