Zurek-Kibble Mechanism for the Spontaneous Vortex Formation in Nb−Al/Alox/NbNb-Al/Al_{ox}/Nb Josephson Tunnel Junctions: New Theory and Experiment

New scaling behavior has been both predicted and observed in the spontaneous production of fluxons in quenched $Nb-Al/Al_{ox}/Nb$ annular Josephson tunnel junctions as a function of the quench time, $\tau_{Q}$. The probability $f_{1}$ to trap a single defect during the N-S phase transition clearly follows an allometric dependence on $\tau_{Q}$ with a scaling exponent $\sigma = 0.5$, as predicted from the Zurek-Kibble mechanism for {\it realistic} JTJs formed by strongly coupled superconductors. This definitive experiment replaces one reported by us earlier, in which an idealised model was used that predicted $\sigma = 0.25$, commensurate with the then much poorer data. Our experiment remains the only condensed matter experiment to date to have measured a scaling exponent with any reliability.Comment: Four pages, one figur

New Experiments for Spontaneous Vortex Formation in Josephson Tunnel Junctions

It has been argued by Zurek and Kibble that the likelihood of producing defects in a continuous phase transition depends in a characteristic way on the quench rate. In this paper we discuss an improved experiment for measuring the Zurek-Kibble scaling exponent $\sigma$ for the production of fluxons in annular symmetric Josephson Tunnel Junctions. We find $\sigma \simeq 0.5$. Further, we report accurate measurements of the junction gap voltage temperature dependence which allow for precise monitoring of the fast temperature variations during the quench.Comment: 12 pages, 5 figures, submitted to Phys. Rev.

Switching between dynamic states in intermediate-length Josephson junctions

The appearance of zero-field steps (ZFS’s) in the current-voltage characteristics of intermediate-length overlap-geometry Josephson tunnel junctions described by a perturbed sine-Gordon equation (PSGE) is associated with the growth of parametrically excited instabilities of the McCumber background curve (MCB). A linear stability analysis of a McCumber solution of the PSGE in the asymptotic linear region of the MCB and in the absence of magnetic field yields a Hill’s equation which predicts how the number, locations, and widths of the instability regions depend on the junction parameters. A numerical integration of the PSGE in terms of truncated series of time-dependent Fourier spatial modes verifies that the parametrically excited instabilities of the MCB evolve into the fluxon oscillations characteristic of the ZFS’s. An approximate analysis of the Fourier mode equations in the presence of a small magnetic field yields a field-dependent Hill’s equation which predicts that the major effect of such a field is to reduce the widths of the instability regions. Experimental measurements on Nb-NbxOy-Pb junctions of intermediate length, performed at different operating temperatures in order to vary the junction parameters and for various magnetic field values, verify the physical existence of switching from the MCB to the ZFS’s. Good qualitative, and in many cases quantitative, agreement between analytic, numerical, and experimental results is obtained

Enterprise Governance in Transition – A Stakeholder Perspective

Mature market economies have thrived on very diverse systems of corporate governance. Transition economies in Central and Eastern Europe have entered the market economy with a special historical inheritance, and critical political decisions of key institutions that have a bearing on the new systems of governance evolving in the region. In this paper, I use an analytical stakeholder approach (different from a normative approach) to identify how the specific conditions in countries in transition have influenced the evolution of specific governance structures, and how this influences the workability of the system. I employ a broad definition of enterprise governance that incorporates fixed, residual and appropriated rights among a broad range of different stakeholders. The governance system is a function of the markets that the firm operates in, by state regulation, and by other specific firm and stakeholder conditions. Based on this definition, I analyse some general determinants behind the governance structure in a market economy, focussing on the distribution of rights among stake- holders. Governance systems in Western countries are used as a benchmark to explore the specific conditions for governance structures in economies in transition. Governance structures changed over different stages in the transition process, with privatisation being the single most important determining factor. Consequently, the role of different stakeholders varies across countries, and has evolved considerably over time. The specific conditions of the transition process have favoured insider ownership mostly in the form of management ownership, but in some cases, also broader employee ownership. However, the relative strength of insiders, especially employees, has declined considerably in later stages of the transition