Dynamic Scaling and Two-Dimensional High-Tc Superconductors

There has been ongoing debate over the critical behavior of two-dimensional superconductors; in particular for high Tc superconductors. The conventional view is that a Kosterlitz-Thouless-Berezinskii transition occurs as long as finite size effects do not obscure the transition. However, there have been recent suggestions that a different transition actually occurs which incorporates aspects of both the dynamic scaling theory of Fisher, Fisher, and Huse and the Kosterlitz-Thouless-Berezinskii transition. Of general interest is that this modified transition apparently has a universal dynamic critical exponent. Some have countered that this apparent universal behavior is rooted in a newly proposed finite-size scaling theory; one that also incorporates scaling and conventional two-dimensional theory. To investigate these issues we study DC voltage versus current data of a 12 angstrom thick YBCO film. We find that the newly proposed scaling theories have intrinsic flexibility that is relevant to the analysis of the experiments. In particular, the data scale according to the modified transition for arbitrarily defined critical temperatures between 0 K and 19.5 K, and the temperature range of a successful scaling collapse is related directly to the sensitivity of the measurement. This implies that the apparent universal exponent is due to the intrinsic flexibility rather than some real physical property. To address this intrinsic flexibility, we propose a criterion which would give conclusive evidence for phase transitions in two-dimensional superconductors. We conclude by reviewing results to see if our criterion is satisfied.Comment: 14 page

Dynamic scaling for 2D superconductors, Josephson junction arrays and superfluids

The value of the dynamic critical exponent $z$ is studied for two-dimensional superconducting, superfluid, and Josephson Junction array systems in zero magnetic field via the Fisher-Fisher-Huse dynamic scaling. We find $z\simeq5.6\pm0.3$, a relatively large value indicative of non-diffusive dynamics. Universality of the scaling function is tested and confirmed for the thinnest samples. We discuss the validity of the dynamic scaling analysis as well as the previous studies of the Kosterlitz-Thouless-Berezinskii transition in these systems, the results of which seem to be consistent with simple diffusion ($z=2$). Further studies are discussed and encouraged.Comment: 19 pages in two-column RevTex, 8 embedded EPS figure

Temperature and Frequency Dependence of Complex Conductance of Ultrathin YBa2Cu3O7-x Films: A Study of Vortex-Antivortex Pair Unbinding

We have studied the temperature dependencies of the complex sheet conductance of 1-3 unit cell (UC) thick YBa2Cu3O7-x films sandwiched between semiconducting Pr0.6Y0.4Ba2Cu3O7-x layers at high frequencies. Experiments have been carried out in a frequency range between: 2 - 30 MHz with one-spiral coil technique, 100 MHz - 1 GHz frequency range with a new technique using the spiral coil cavity and at 30 GHz by aid of a resonant cavity technique. The real and imaginary parts of the mutual-inductance between a coil and a film were measured and converted to complex conductivity by aid of the inversion procedure. We have found a quadratic temperature dependence of the kinetic inductance, L_k^-1(T), at low temperatures independent of frequency, with a break in slope at T^dc_BKT, the maximum of real part of conductance and a large shift of the break temperature and the maximum position to higher temperatures with increasing frequency. We obtain from these data the universal ratio T^dc_BKT/L_k^-1(T^dc_BKT) = 25, 25, and 17 nHK for 1-, 2- and 3UC films, respectively in close agreement with theoretical prediction of 12 nHK for vortex-antivortex unbinding transition. The activated temperature dependence of the vortex diffusion constant was observed and discussed in the framework of vortex-antivortex pair pinning. PACS numbers: 74.80.Dm, 74.25.Nf, 74.72.Bk, 74.76.BzComment: PDF file, 10 pages, 6 figures, to be published in J. Low Temp. Phys.; Proc. of NATO ARW: VORTEX 200

Finite-Size Effects and Dynamical Scaling in Two-Dimensional Josephson Junction Arrays

In recent years many groups have used Fisher, Fisher, and Huse (FFH) dynamical scaling to investigate and demonstrate details of the superconducting phase transition. Some attention has been focused on two dimensions where the phase transition is of the Kosterlitz-Thouless-Berezinskii (KTB) type. Pierson et al. used FFH dynamical scaling almost exclusively to suggest that the dynamics of the two-dimensional superconducting phase transition may be other than KTB-like. In this work we investigate the ability of scaling behavior by itself to yield useful information on the nature of the transition. We simulate current-voltage (IV) curves for two-dimensional Josephson junction arrays with and without finite-size-induced resistive tails. We find that, for the finite-size effect data, the values of the scaling parameters, specifically the transition temperature and the dynamical scaling exponent z, depend critically on the magnitude of the contribution that the resistive tails make to the IV curves. In effect, the values of the scaling parameters depend on the noise floor of the measuring system.Comment: 24 pages, 8 figures; submitted to Physical Review

Current Accounts in Europe: Implications of the External Imbalances for the Future of the Common Monetary Policy

The paper discusses the seriousness of current account imbalances in the last few decades in Europe, with a particular focus on the European Monetary Union. A closer look at the development of current accounts in European economies suggests the existence of some serious structural problems that might jeopardize economic performance of single countries, but even more importantly, of the entire monetary union. Although current account positions have been subject of numerous research projects till now, scarce interest has been offered regarding specifically the situation in the member states of the euro area and in the euro candidate countries. This lack of interest could be justified among others with the myopic conviction expressed in the literature that current account positions become irrelevant in a monetary union. Instead, there are conceptual reasons to be worried about external imbalances in a currency area, and particularly, in the current as well as potentially enlarged EMU

Financial Liberalization, Growth, Productivity and Capital Accumulation: The Case of European Integration

In the present contribution, we concentrate on the process of financial liberalization in a specific context of European economic and monetary integration. We implement de facto and de jure measures of financial liberalization and find that formal aspects of financial openness generate a strongly positive impact on economic growth and its sources, productivity growth and capital accumulation. Moreover, there is evidence of a positive contribution to the process stemming from the EU membership, while no substantial effect comes from the euro adoption. Finally, we investigate the effects from financial integration on country groups within the EU

The Role of Country-of-Origin Characteristics for Foreign Direct Investment and Technical Cooperation in Post-Reform India

The decisions of foreign investors on technical cooperation versus equity engagements and on the degree of ownership in FDI projects are likely to depend on their relative bargaining position vis-à-vis the host country. India provides an interesting case for analyzing the interplay between country-of origin-characteristics and host-country-characteristics and their respective effects on ownership decisions since opening up its economy to FDI in the early 1990s. We perform negative binominal regressions by making use of a unique dataset on about 24,500 technical cooperation and FDI projects by investors from 45 countries of origin over the 1991-2004 period. We find that relative market size, relative financial market development, relative risk, relative endowment of human capital and previous international experience significantly affect the type of engagement by foreign investors in post-reform India

Bilateral Exports from Euro Zone Countries to the US - Does Exchange Rate Variability Play a Role?

The financial crisis and the debt crisis in Europe lead to pronounced swings of the $/€-exchange rate. The influence of this exchange rate uncertainty on exports is neither theoretically nor empirically unambiguous. Therefore, this investigation tries to find out what effect exchange rate volatility has got on exports from eleven euro zone countries to the US. Our results suggest that if exchange rate volatility exerts a significant influence on exports, it is typically negative. Furthermore, exports of SITC categories 6 and 7 seem to be affected negatively most often