We investigate the quantum phase transition in a one-dimensional chain of ultra-small superconducting grains, considering both the self-and junction capacitances. At zero temperature, the system is transformed into a two-dimensional system of classical vortices, where the junction capacitance introduces anisotropy in the interaction between vortices. This leads to the superconductor-insulator transition of the Berezinskii-Kosterlitz-Thouless type, as the ratios of the Josephson coupling energy to the charging energies are varied. It is found that the junction capacitance plays a role similar to that of dissipation and tends to suppress quantum fluctuations; nevertheless the insulator region survives even for arbitrarily large values of the junction capacitance.PACS Numbers: 74.50.+r, 67.40.Db Quantum phase transitions, which are induced by quantum fluctuations at zero temperature, are distinguished from classical phase transitions in several important respects; this has attracted much attention in recent years [1]. In particular, advances in fabrication techniques have made available arrays of ultra-small superconducting grains, where the charging energy dominates the Josephson coupling energy and accordingly, quantum fluctuation effects are of paramount importance. Such Josephson-junction arrays have become a prototype system displaying quantum phase transitions between the superconducting and the insulating phases. In the vicinity of the superconductor-insulator transition, the fluctuation effects depend crucially on the dimensionality of the system. In the case of two-dimensional (2D) arrays, rich effects of quantum fluctuations and resulting phase transitions have been examined for a rather general form of the capacitance matrix, although there still exist unsettled issues in the quantum regime, such as lowtemperature re-entrance [2][3][4]. On the other hand, onedimensional (1D) chains of Josephson junctions, where quantum fluctuations should be more important, have been studied mainly in the two limiting cases: the selfcharging model and the nearest-neighbor model where only nearest neighboring charges interact [5,6]. In the 1D system with only self-capacitance, the role of dissipation on the quantum phase transition [7] as well as the persistence current and voltage [8] has also been considered.This paper investigates the quantum phase transitions in general Josephson-junction chains with both the selfand junction capacitances. At zero temperature, the system is transformed into a two-dimensional system of classical vortices, where the junction capacitance introduces anisotropy in the interaction between vortices. This leads to the superconductor-insulator transition of the Berezinskii-Kosterlitz-Thouless (BKT) type [9,10], as the ratios of the Josephson coupling energy to the charging energies are varied. Interestingly, the junction capacitance here plays a role similar to that of dissipation and tends to suppress quantum fluctuations, enhancing superconductivity. However, the suppression is not strong...

The sintering and electrical characteristics of La‐modified Na1/2Bi1/2TiO3 (NBT) was investigated from a defect structure viewpoint. To reveal the role of cation vacancies, two series of ceramics, with different cation vacancies, were processed to compensate the excess positive charge of lanthanum ions. In a region of complete solid solution, the grain size of NBLT‐B {[(Na0.5Bi0.5)1−xLax]Ti1−0.25xO3} was smaller than that of NBLT‐A {[(Na0.5Bi0.5)1−1.5xLax]TiO3} and densification was enhanced more effectively in NBLT‐B. With the aid of thermoelectric power, electric conductivity, and electrotransport measurements, it was found that different sintering behaviors between NBLT‐A and NBLT‐B specimens were related to the change in the type of cation vacancies present and that lanthanum ion–cation vacancy pairs played an important role in reducing the grain growth and enhancing the densification process.

Changes in structure and phase transition behavior are investigated for two types of La-doped sodium bismuth titanate ceramics. When A-site vacancies are formed by La incorporation, the phase transition near 200 °C becomes pronounced. Hysteresis loop and ε(T) demonstrate that the structure above 200 °C is the incommensurate antiferroelectric phase. In contrast, B-site vacancies produced by La doping do not contribute to the incommensurate phase. The origin for the formation of the incommensurate antiferroelectric state between the rhombohedral ferroelectric phase and tetragonal paraelectric phase is discussed in a view of decoupling effects due to the A-site vacancies.

The underlying phenomenology of the crystallographic orientation dependence on ferroelectric fatigue behavior was investigated in rhombohedral Pb(Zn1∕3Nb2∕3)O3-5%PbTiO3 (PZN-5PT) crystals. It was recently found that an electric field (E field) application along the ⟨001⟩ direction of PZN-5PT crystal did not induce the fatigue to 105cycles of bipolar electric field cycling (switching), while the ferroelectric fatigue became evident from 103cycles of polarization switching along the ⟨111⟩ direction. In this study, the dependence of ferroelectric fatigue on the crystal orientation is explained by changes in internal stress, switching mechanisms, and domain configuration. The magnitude of the in-plane tensile strain was a maximum during a domain switching in ⟨111⟩ oriented crystals, resulting in the suppressed motion of domain boundaries in ⟨111⟩ oriented crystals. In addition to the stress, differences in domain switching mechanisms and domain boundary density for ⟨001⟩ and ⟨111⟩ oriented crystals contributed to the orientation dependence of ferroelectric fatigue. Sideways domain growth became dominant and domain width increased when the E field was applied along the ⟨111⟩ direction. Sideways domain growth of ⟨111⟩ oriented crystal swept oxygen vacancies of the domains during the consecutive switching process, leading to the effective accumulation of oxygen vacancies at the domain boundaries. Smaller domain boundary densities found in ⟨111⟩ oriented crystals, in comparison to ⟨100⟩ oriented crystals, also contributed to the increase in the density of accumulated oxygen vacancies at domain boundaries after sweeping oxygen vacancies of the domains, due to the impact of increased E-field cycling and cumulative switching. High in-plane tensile stress and a high concentration of oxygen vacancies at the domain boundaries due to sideways growth and small domain boundary density were suggested to pin the movement of domain boundaries and enhance the fatigue in ⟨111⟩ oriented crystals.

The domain configuration of 0.95Pb(Zn1/3Nb2/3)O3–0.05PbTiO3 single crystals was investigated as a function of the applied electric-field direction with respect to the crystallographic orientation. A normal ferroelectric domain structure, which was not observable in virgin crystals (not exposed to electric field), appeared upon the electric-field (E-field) induced transition from the relaxor state to the normal ferroelectric state. After E-field was applied along the polar 〈111〉 direction and removed, a band-shaped domain configuration appeared as a result of subsequent depolarization. The depoled 〈111〉 crystal consisted of predominantly (110) domain boundaries, which represents the minimum energy domain state in rhombohedral ferroelectrics. In contrast, crystals must transform into the phase with the symmetry lower than rhombohedral, i.e., monoclinic structure, by an E-field application along 〈001〉. Domain boundaries in 〈001〉 poled crystals were indexed to be (001), being a favorable state for monoclinic ferroelectrics. The dependence of the domain configuration upon the E-field application direction could be explained using the relationship between the mechanical compatibility with respect to the ferroelectric strain and the crystal symmetry of a domain. The residual strain on domain boundaries in 〈001〉 crystals may inhibit crystals to recover the rhombohedral structure, resulting in a monoclinic ferroelectric crystal even upon the removal of the E-field.