Probing the Yb3+^{3+} spin relaxation in Y0.98_{0.98}Yb0.02_{0.02}Ba2_{2}Cu3_{3}Ox_{x} by Electron Paramagnetic Resonance

The relaxation of Yb$^{3+}$ in YBa$_{2}$Cu$_{3}$O$_{x}$ ($6<x<7$) was studied using Electron Paramagnetic Resonance (EPR). It was found that both electronic and phononic processes contribute to the Yb$^{3+}$ relaxation. The phononic part of the relaxation has an exponential temperature dependence, which can be explained by a Raman process via the coupling to high-energy ($\sim$500 K) optical phonons or an Orbach-like process via the excited vibronic levels of the Cu$^{2+}$ ions (localized Slonczewski-modes). In a sample with a maximum oxygen doping $x$=6.98, the electronic part of the relaxation follows a Korringa law in the normal state and strongly decreases below $T_{c}$. Comparison of the samples with and without Zn doping proved that the superconducting gap opening is responsible for the sharp decrease of Yb$^{3+}$ relaxation in YBa$_{2}$Cu$_{3}$O$_{6.98}$. It was shown that the electronic part of the Yb$^{3+}$ relaxation in the superconducting state follows the same temperature dependence as $^{63}$Cu and $^{17}$O nuclear relaxations despite the huge difference between the corresponding electronic and nuclear relaxation rates.Comment: 8 pages, 6 figure

Geometric quantum gates with superconducting qubits

We suggest a scheme to implement a universal set of non-Abelian geometric transformations for a single logical qubit composed of three superconducting transmon qubits coupled to a single cavity. The scheme utilizes an adiabatic evolution in a rotating frame induced by the effective tripod Hamiltonian which is achieved by longitudinal driving of the transmons. The proposal is experimentally feasible with the current state of the art and could serve as a first proof of principle for geometric quantum computing.Comment: 7 pages, 5 figure

Butterfly hysteresis loop and dissipative spin reversal in the S=1/2, V15 molecular complex

Time resolved magnetization measurements have been performed on a spin 1/2 molecular complex, so called V$_{15}$. Despite the absence of a barrier, magnetic hysteresis is observed over a timescale of several seconds. A detailed analysis in terms of a dissipative two level model is given, in which fluctuations and splittings are of same energy. Spin-phonon coupling leads to long relaxation times and to a particular "butterfly" hysteresis loop.Comment: LaTeX/RevTeX, 3 figures.Approved for publication in PR

Electrical read-out of the local nuclear polarization in the quantum Hall effect

It is demonstrated that the now well-established `flip-flop' mechanism of spin exchange between electrons and nuclei in the quantum Hall effect can be reversed. We use a sample geometry which utilizes separately contacted edge states to establish a local nuclear spin polarization --close to the maximum value achievable-- by driving a current between electron states of different spin orientation. When the externally applied current is switched off, the sample exhibits an output voltage of up to a few tenths of a meV, which decays with a time constant typical for the nuclear spin relaxation. The surprizing fact that a sample with a local nuclear spin polarization can act as a source of energy and that this energy is well above the nuclear Zeeman splitting is explained by a simple model which takes into account the effect of a local Overhauser shift on the edge state reconstruction.Comment: Submitted to Phys. Rev. Let

Lattice study of the infrared behavior of QCD Green's functions in Landau gauge

We summarize the current status of our numerical results for the gluon and ghost propagators and for the Kugo-Ojima confinement parameter in quenched SU(3) lattice Landau gauge theory. The data for the propagators are compared to our results obtained in the case of full QCD, simulated using two flavors of dynamical clover-improved Wilson fermions. We demonstrate that the infrared behavior of the ghost propagator is consistent with the Kugo-Ojima confinement criterion. Explicit violation of reflection positivity by the gluon propagator is shown. Additionally, we present results of a running coupling constant both at low and at large momenta.Comment: 7 pages, 8 figures, talk presented at Lattice2006 (Confinement and Topology

A model for the phase separation controlled by doping and the internal chemical pressure in different cuprate superconductors

In the framework of a two-band model, we study the phase separation regime of different kinds of strongly correlated charge carriers as a function of the energy splitting between the two sets of bands. The narrow (wide) band simulates the more localized (more delocalized) type of charge carriers. By assuming that the internal chemical pressure on the CuO$_2$ layer due to interlayer mismatch controls the energy splitting between the two sets of states, the theoretical predictions are able to reproduce the regime of phase separation at doping higher than 1/8 in the experimental pressure-doping-$T_c$ phase diagram of cuprates at large microstrain as it appears in overoxygenated La$_2$CuO$_4$.Comment: 8 pages, 5 figures, submitted to Phys. Rev.

The Landau gauge gluon and ghost propagators in 4D SU(3) gluodynamics in large lattice volumes

We present recent results of the Landau gauge gluon and ghost propagators in SU(3) pure gauge theory at Wilson \beta=5.7 for lattice sizes up to 80^4 corresponding to physical volumes up to (13.2 fm)^4. In particular, we focus on finite-volume and Gribov copy effects. We employ a gauge fixing method that combines a simulated annealing algorithm with finalizing overrelaxation. We find the gluon propagator for the largest volumes and at q^2 ~ 0.01 GeV^2 to become flat. Although not excluded by our data, there is still no clear indication of a gluon propagator tending towards zero in the zero-momentum limit. New data for the ghost propagator are reported, too.Comment: 7 pages, 3 figures, poster presented at Lattice-2007, Regensburg, July 30 - August 4, 2007, 1 figure replace

Landau Gauge Gluon and Ghost Propagators from Lattice QCD

We report on recent numerical computations of the Landau gauge gluon and ghost propagators as well as of the ghost-gluon vertex function in pure SU(3) Yang-Mills theory and in full QCD on the lattice. Special emphasis is paid to the low momentum region. In particular, we present new data for the gluon propagator at momenta below 300 MeV. We also discuss different systematic effects as there are finite-size, lattice discretization and Gribov copy but also unquenching effects. A MOM-scheme running coupling \alpha_s(q^2) based on the ghost-gluon vertex is calculated and found to decrease for momenta below 550 MeV, even though the renormalization constant of the vertex deviates only weakly from being constant.Comment: Talk at the Workshop IRQCD '06, Rio de Janeiro, June 2006, given by M. M{\"u}ller-Preussker; 8 pages, 9 figures, a few wordings made more precise, figures put into equal style, missing legends adde