Dimer Coupling Energies of the Si(001) Surface.

The coupling energies between the buckled dimers of the Si(001) surface were determined through analysis of the anisotropic critical behavior of its order-disorder phase transition. Spot profiles in high-resolution low-energy electron diffraction as a function of temperature were analyzed within the framework of the anisotropic two-dimensional Ising model. The validity of this approach is justified by the large ratio of correlation lengths, ξ_{∥}^{+}/ξ_{⊥}^{+}=5.

Sauter-Schwinger Effect for Colliding Laser Pulses.

Via a combination of analytical and numerical methods, we study electron-positron pair creation by the electromagnetic field A(t,r)=[f(ct-x)+f(ct+x)]e_{y} of two colliding laser pulses. Employing a generalized Wentzel-Kramers-Brillouin approach, we find that the pair creation rate along the symmetry plane x=0 (where one would expect the maximum contribution) displays the same exponential dependence as for a purely time-dependent electric field A(t)=2f(ct)e_{y}. The prefactor in front of this exponential does also contain corrections due to focusing or defocusing effects induced by the spatially inhomogeneous magnetic field.

Phonon Pair Creation by Inflating Quantum Fluctuations in an Ion Trap.

Quantum theory predicts intriguing dynamics during drastic changes of external conditions. We switch the trapping field of two ions sufficiently fast to tear apart quantum fluctuations, i.e.

Avalanche of entanglement and correlations at quantum phase transitions.

We study the ground-state entanglement in the quantum Ising model with nearest neighbor ferromagnetic coupling J and find a sequential increase of entanglement depth d with growing J. This entanglement avalanche starts with two-point entanglement, as measured by the concurrence, and continues via the three-tangle and four-tangle, until finally, deep in the ferromagnetic phase for J = ∞, arriving at a pure L-partite (GHZ type) entanglement of all L spins. Comparison with the two, three, and four-point correlations reveals a similar sequence and shows strong ties to the above entanglement measures for small J.