Critical currents in weakly textured MgB2: Nonlinear transport in anisotropic heterogeneous media

A model for highly non-linear transport in heterogeneous media consisting of anisotropic particles with a preferred orientation is proposed and applied to the current transport in weakly textured magnesium diboride, MgB2. It essentially explains why, unlike in conventional superconductors, a significant macroscopic anisotropy of the critical currents can be induced by the preparation of MgB2 tapes. The field and angular dependence of the critical current is calculated for various degrees of texture and compared to experimental data

Changes in the polar vortex: Effects on Antarctic total ozone observations at various stations

October mean total column ozone data from four Antarctic stations form the basis for understanding the evolution of the ozone hole since 1960. While these stations show similar emergence of the ozone hole from 1960 to 1980, the records are divergent in the last two decades. The effects of long-term changes in vortex shape and location are considered by gridding the measurements by equivalent latitude. A clear eastward shift of the mean position of the vortex in October with time is revealed, which changes the fraction of ozone measurements taken inside/outside the vortex for stations in the vortex collar region. After including only those measurements made inside the vortex, ozone behavior in the last two decades at the four stations is very similar. This suggests that dynamical influence must be considered when interpreting and intercomparing ozone measurements from Antarctic stations for detecting ozone recovery and ozone-related changes in Antarctic climate

From Majorana Fermions to Topological Order

We consider a system consisting of a 2D network of links between Majorana fermions on superconducting islands. We show that the fermionic Hamiltonian modeling this system is topologically-ordered in a region of parameter space. In particular we show that Kitaev's toric code emerges in fourth-order perturbation theory. By using a Jordan-Wigner transformation we can map the model onto a family of signed 2D Ising models in a transverse field where the signs (FM or AFM) are determined by additional gauge bits. Our mapping allows an understanding of the non-perturbative regime and the phase transition to a non-topological phase. We discuss the physics behind a possible implementation of this model and argue how it can be used for topological quantum computation by adiabatic changes in the Hamiltonian.Comment: 4+4 pages, 5 figures. v2 has a new reference and a few new comments. In v3: yet another new reference and Supplementary Material is renamed Appendix. In v4: several typos are corrected, to appear in Phys. Rev. Let

Quantized conductance at the Majorana phase transition in a disordered superconducting wire

Superconducting wires without time-reversal and spin-rotation symmetries can be driven into a topological phase that supports Majorana bound states. Direct detection of these zero-energy states is complicated by the proliferation of low-lying excitations in a disordered multi-mode wire. We show that the phase transition itself is signaled by a quantized thermal conductance and electrical shot noise power, irrespective of the degree of disorder. In a ring geometry, the phase transition is signaled by a period doubling of the magnetoconductance oscillations. These signatures directly follow from the identification of the sign of the determinant of the reflection matrix as a topological quantum number.Comment: 7 pages, 4 figures; v3: added appendix with numerics for long-range disorde

Chapter 2: A New Crisis Mechanism for the Euro Area

The European debt crisis followed the US financial crisis with a delay of one and a half years. While its first signs were visible in November and December of 2009 when the rating agency Fitch downgraded Ireland and Greece, it culminated on 28 April 2010 when the intra-day interest rate for two-year Greek government bonds peaked at 38 percent. Since then capital markets have been extremely unstable, showing signs of distrust in the creditworthiness of the GIPS countries: Greece, Ireland, Portugal and Spain. The European Union reacted by preparing voluminous rescue plans that, at this writing (January 2011), have been resorted to by Greece and Ireland.

Exact Rotating Wave Approximation

The Hamiltonian of a linearly driven two-level system, or qubit, in the standard rotating frame contains non-commuting terms that oscillate at twice the drive frequency, $\omega$, rendering the task of analytically finding the qubit's time evolution nontrivial. The application of the rotating wave approximation (RWA), which is suitable only for drives whose amplitude, or envelope, $H_1(t)$, is small compared to $\omega$ and varies slowly on the time scale of $1/\omega$, yields a simple Hamiltonian that can be integrated relatively easily. We present a series of corrections to the RWA Hamiltonian in $1/\omega$, resulting in an effective Hamiltonian whose time evolution is accurate also for time-dependent drive envelopes in the regime of strong driving, i.e., for $|H_1(t)| \lesssim \omega$. By extending the Magnus expansion with the use of a Taylor series we introduce a method that we call the Magnus-Taylor expansion, which we use to derive a recurrence relation for computing the effective Hamiltonian. We then employ the same method to derive kick operators, which complete our theory for non-smooth drives. The time evolution generated by our kick operators and effective Hamiltonian, both of which depend explicitly on the envelope and its time derivatives, agrees with the exact time evolution at periodic points in time. For the leading Hamiltonian correction we obtain a term proportional to the first derivative of the envelope, which competes with the Bloch-Siegert shift.Comment: 29 pages, 4 figures, changes made to improve clarity, references adde

Chapter 1: The Macroeconomic Outlook

The structural problems brought to light by the financial crisis have largely remained in place or shifted from the private (banking) sector to the public sector. In the United States, despite increased saving, household debt remains high; their wealth position has deteriorated substantially due to the bursting of the house price bubble. The real estate sector has shrunk, and the financial sector has still not fully recovered.