Enhanced pinning and proliferation of matching effects in a superconducting film with a Penrose array of magnetic dots

The vortex dynamics in superconducting films deposited on top of a five-fold Penrose array of magnetic dots is studied by means of transport measurements. We show that in the low pinning regime (demagnetized dots) a few periodic and aperiodic matching features coexist. In the strong pinning regime (magnetized dots) a richer structure of unforeseen periodic and aperiodic vortex patterns appear giving rise to a clear enhancement of the critical current in a broader field range. Possible stable vortex configurations are determined by molecular dynamics simulations

Implementation of universal quantum gates based on nonadiabatic geometric phases

We propose an experimentally feasible scheme to achieve quantum computation based on nonadiabatic geometric phase shifts, in which a cyclic geometric phase is used to realize a set of universal quantum gates. Physical implementation of this set of gates is designed for Josephson junctions and for NMR systems. Interestingly, we find that the nonadiabatic phase shift may be independent of the operation time under appropriate controllable conditions. A remarkable feature of the present nonadiabatic geometric gates is that there is no intrinsic limitation on the operation time, unlike adiabatic geometric gates. Besides fundamental interest, our results may simplify the implementation of geometric quantum computation based on solid state systems, where the decoherence time may be very short.Comment: 5 pages, 2 figures; the version published in Phys. Rev. Let

Vortex rectification effects in films with periodic asymmetric pinning

We study the transport of vortices excited by an ac current in an Al film with an array of nanoengineered asymmetric antidots. The vortex response to the ac current is investigated by detailed measurements of the voltage output as a function of ac current amplitude, magnetic field and temperature. The measurements revealed pronounced voltage rectification effects which are mainly characterized by the two critical depinning forces of the asymmetric potential. The shape of the net dc voltage as a function of the excitation amplitude indicates that our vortex ratchet behaves in a way very different from standard overdamped models. Rather, as demonstrated by the observed output signal, the repinning force, necessary to stop vortex motion, is considerably smaller than the depinning force, resembling the behavior of the so-called inertia ratchets. Calculations based on an underdamped ratchet model provide a very good fit to the experimental data.Comment: 5 pages, 4 figure

WavePacket: A Matlab package for numerical quantum dynamics. III: Quantum-classical simulations and surface hopping trajectories

WavePacket is an open-source program package for numerical simulations in quantum dynamics. Building on the previous Part I [Comp. Phys. Comm. 213, 223-234 (2017)] and Part II [Comp. Phys. Comm. 228, 229-244 (2018)] which dealt with quantum dynamics of closed and open systems, respectively, the present Part III adds fully classical and mixed quantum-classical propagations to WavePacket. In those simulations classical phase-space densities are sampled by trajectories which follow (diabatic or adiabatic) potential energy surfaces. In the vicinity of (genuine or avoided) intersections of those surfaces trajectories may switch between surfaces. To model these transitions, two classes of stochastic algorithms have been implemented: (1) J. C. Tully's fewest switches surface hopping and (2) Landau-Zener based single switch surface hopping. The latter one offers the advantage of being based on adiabatic energy gaps only, thus not requiring non-adiabatic coupling information any more. The present work describes the MATLAB version of WavePacket 6.0.2 which is essentially an object-oriented rewrite of previous versions, allowing to perform fully classical, quantum-classical and quantum-mechanical simulations on an equal footing, i.e., for the same physical system described by the same WavePacket input. The software package is hosted and further developed at the Sourceforge platform, where also extensive Wiki-documentation as well as numerous worked-out demonstration examples with animated graphics are available

Field induced d_x^2-y^2+id_xy state in d-density-wave metals

We argue that the d_{xy} component of the order parameter can be generated to form the d_x^2-y^2+id_xy-density wave state by the external magnetic field. The driving force for this transition is the coupling of the magnetic field with the orbital magnetism. The fully gapped particle spectrum and the magnetically active collective mode of the condensate are discussed as a possible signature of the d+id' density wave state.Comment: 5 pages, 2 color figure

Radiation hardness qualification of PbWO_4 scintillation crystals for the CMS Electromagnetic Calorimeter

Ensuring the radiation hardness of PbWO_4 crystals was one of the main priorities during the construction of the electromagnetic calorimeter of the CMS experiment at CERN. The production on an industrial scale of radiation hard crystals and their certification over a period of several years represented a difficult challenge both for CMS and for the crystal suppliers. The present article reviews the related scientific and technological problems encountered

Time and frequency resolved spontaneous emission from supramolecular pheophorbide-a complexes: A mixed quantum classical computation

A mixed quantum classical methodology is utilized to compute the time and frequency resolved emission spectrum of a chromophore complex dissolved in ethanol. The single complex is formed by a butanediamine dendrimer to which pheophorbide-a molecules are covalently linked. The electronic excitations are described in a Frenkel-exciton model treated quantum mechanically and all nuclear coordinates are described classically by carrying out room-temperature MD simulations. Starting with the full quantum formula for the emission spectrum, it is translated to the mixed quantum classical case and used to compute time resolved spectra up to 2 ns. To account for radiative decay the chromophore complex excited-state dynamics have to be described in a density matrix theory. While the full emission spectrum only reflects excited-state decay the introduction of partial spectra allows to uncover details of excitation energy redistribution among the chromophores