H. Sakata

Effect of purification on dielectric properties near the commensurate-incommensurate transition point of Rb2ZnCl4

Abstract Purification of Rb2ZnCl4 by recrystallization and zone melting is described in detail and the results of chemical analysis are given. Single crystals were grown from an aqueous solution which had been purified by repeated recrystallization. The thermal hysteresis of the dielectric constant in the purified crystal was reduced to 0.2 K from 2.8 K in the unpurified crystal. The switching time of the field-induced commensurate-incommensurate transition was shortened significantly in the vicinity of Tc . These indicate the weakening of pinning in the purified crystal. The low relaxation frequency of domain wall motion in the commensurate phase starts to rise abruptly when the transition to the incommensurate phase begins. This is understood to reflect a process in which the isolated domain walls are incorporated into the discommensuration lattice of the incommensurate phase. The mathematical analysis of the ordinary recrystallization process and the reverse process is given in Appendix.

Influence of disorder on superconductivity in Bi2Sr2CaCu2Ox studied by low temperature scanning tunneling spectroscopy

Abstract The spatial distributions of superconducting energy gap 2 Δ p have been measured at 6 K in underdoped Bi 2 Sr 2 CaCu 2 O x by scanning tunneling spectroscopy. The 2 Δ p has been found to vary in the length scale of about 3 nm. In different regions of the cleaved surface, the average values of 2 Δ p are equal to each other, while the variances are different. This means that the superconducting properties in Bi 2 Sr 2 CaCu 2 O x are not determined only by the carrier concentration. To characterize the superconductivity of Bi 2 Sr 2 CaCu 2 O x , it is necessary to introduce a new parameter of microscopic degree of disorder, which is defined as a ratio of the variance to the average for the distribution of 2 Δ p .

Local electronic density of states near steps on the c-plane of Bi2Sr2CaCu2Ox studied using low temperature scanning tunneling spectroscopy and microscopy

The local electronic density of states near steps on the c-plane of Bi 2 Sr 2 CaCu 2 O x running along the [110] direction in the CuO 2 plane was measured by low temperature scanning tunneling spectroscopy and microscopy. The electronic states were found to be insulating near a step edge on the upper terrace and an insulator-superconductor boundary was formed 7 nm away from the step edge. The superconducting tunneling spectra on the upper terrace depend on the distance from the boundary and the tunneling conductance inside the superconducting gap increases in the region of 10 nm from the boundary. It is speculated that the changes of the superconducting spectra are due to the boundary effects of d x2-y2 -wave superconductors.

Studies of boundary effects and symmetry of Cooper pairing in Bi2Sr2CaCu2Ox by LT-STS

Abstract The local density of states of quasi-particles, N(E,r), around columnar defects generated by bombardment of high-energy heavy ions parallel to the c-axis in Bi2Sr2CaCu2Ox (Bi-2212) and 2H-NbSe2 has been studied by scannning tunneling spectroscopy at 4.2xa0K and compared. The regions of columnar defects have been found to be insulating and circular superconductor–insulator boundaries have been studied. In Bi-2212 the N(E,r) has exhibited the increase at E=0 meV and the decrease at E∼35 meV at [1xa01xa00] and [ 1 xa01xa00] directions in CuO2 square network and seems to show the pattern with four-fold symmetry, while in 2H -NbSe2 such things have not been observed. This is new evidence of the presence of dx2–y2 component in the order parameter of Bi-2212.

Imaging of vortex lattice in single crystal YNi2B2C by STM

Abstract The vortex lattice in YNi 2 B 2 C has been observed through the measurements of the local quasi-particle density of states with low temperature scanning tunneling microscopy and spectroscopy. The vortex lattice has shown the hexagonal to the square lattice transition around 0.1T under the magnetic field applied along the c-axis. On the other hands, under the magnetic field along the a-axis, the transition has occurred at much higher magnetic field of 1.0T. We have also observed that the vortex lattice gets disordered above 2.0T.

Nucleation and growth of antistripples in the initial stage of domain pattern coarsening in sodium nitrite

Abstract The process of domain pattern coarsening in a nearly one-dimensional kink system in ferroelectric sodium nitrite, NaNO2, has been studied by means of an etching technique. Starting from a domain pattern of which the average domain width is 0.6 μm, the pattern coarsens initially through nucleation and growth of antistripples. The antistripples are elongated remarkably along the ferroelectric b-axis like a dagger, thus reducing the depolarization energy. Frequently the antistripples have a complicated shape in the bcplane with many thorns projecting toward the b-surface. Nucleation of antistripples does no more occur between walls separated by more than 2 μm. In the late stage the pattern coarsens through the backward motion of the fronts of wedge-shaped domains which were produced through combined growth processes of antistripples. The pattern retains its nearly one-dimensional character until the average domain width exceeds 30 μm.

Polarization reversal process in K2ZnCl4 studied by the nematic liquid crystal technique

Abstract The nematic liquid crystal technique has been applied to the study of the polarization reversal process in the commensurate-ferroelectric phase of K2ZnCl4 where there are six types of domains distinguished by the phase of the order parameter ψ = n π/3 (n = 1, 2,…, 6). A single domain crystal undergoes polarization reversal through the sidewise expansion of domains nucleated at the predetermined locations in the bulk or at the crystal edges. The phase of the primary order parameters differs by φ between the switched and unswitched regions. In addition we found that domain walls serve as sources to create DCs in the commensurate-to-incommensurate transition.

Mechanism of domain pattern coarsening in ferroelectric sodium nitrite

Abstract The mechanism of domain pattern coarsening in a nearly one-dimensional kink system in ferroelectric sodium nitrite, NaNO2, has been studied by means of an etching technique. Starting from a domain pattern of which the average domain width is 0.6 μm, the pattern coarsens initially through nucleation and growth of antistripples. However, nucleation of antistripples does no more occur between walls separated by more than 2 μm. In the late stage the pattern coarsens through the backward motion of the fronts of wedge-shaped domains and the average domain width increases in proportion to the square root of time. Throughout the whole process, the domain size distribution functions are well scaled and have a shape resembling the Poisson distribution with a cutoff for narrow widths. The structure function of the pattern shows a peak at a non-zero wavevector.

Kinetic process of formation of discommensuration lattice in the first-order commensurate-incommensurate transition

Abstract The stability of the discommensuration lattice (DC lattice) and the kinetic process of the first-order commensurate-incommensurate transition (C-IC transition) have been studied theoretically and experimentally. At an early stage of the C-to-IC transition, the crystal is gradually covered with the clusters of DCs. After the crystal is covered with a DC lattice, its lattice spacing narrows to the final equilibrium value. At an early stage of the IC-to-C transition, the DC lattice remains to cover the crystal and only its lattice spacing widens. After the spacing has widened to the threshold value, the commensurate phase comes to appear and finally covers the whole crystal. This peculiar feature of the phase transition arises from the nature of the order parameter of the incommensurate system.

Kinetic process of the commensurate-incommensurate phase transition in K2SeO4

Abstract The kinetic process of the commensurate-incommensurate phase transition in K2SeO4 has been studied by the measurements of the time evolutions of polarization and dielectric constant during the field-induced transition. The time evolutions are qualitatively similar to those observed for Rb2ZnCl4, and can be explained by our theory of the time evolution of discommensuration lattice during the commensurate-incommensurate transition.