Dynamics of a rod in a homogeneous/inhomogeneous frozen disordered medium: Correlation functions and non-Gaussian effects

We present molecular dynamics simulations of the motion of a single rigid rod in a disordered static 2d-array of disk-like obstacles. Two different configurations have been used for the latter: A completely random one, and which thus has an inhomogeneous structure, and an homogeneous ``glassy'' one, obtained from freezing a liquid of soft disks in equilibrium. Small differences are observed between both structures for the translational dynamics of the rod center-of-mass. In contrast to this, the rotational dynamics in the glassy host medium is strongly slowed down in comparison with the random one. We calculate angular correlation functions for a wide range of rod length $L$ and density of obstacles $\rho$ as control parameters. A two-step decay is observed for large values of $L$ and $\rho$, in analogy with supercooled liquids at temperature close to the glass transition. In agreement with the prediction of the Mode Coupling Theory, a time-length and time-density scaling is obtained. In order to get insight on the relation between the heterogeneity of the dynamics and the structure of the host medium, we determine the deviations from Gaussianity at different length scales. Strong deviations are obtained even at spatial scales much larger than the rod length. The magnitude of these deviations is independent of the nature of the host medium. This result suggests that the large scale translational dynamics of the rod is affected only weakly by the presence of inhomogeneities in the host medium.Comment: Published in AIP Conference Proceedings 708 (2004) 576-58

Logarithmic Relaxation in a Kinetically Constrained Model

We present Monte Carlo simulations in a modification of the north-or-east-or-front model recently investigated by Berthier and Garrahan [J. Phys. Chem. B 109, 3578 (2005)]. In this coarse-grained model for relaxation in supercooled liquids, the liquid structure is substituted by a three-dimensional array of cells. A spin variable is assigned to each cell, with values 0 or 1 denoting respectively unexcited and excited local states in a mobility field. Change in local mobility (spin flip) for a given cell is permitted according to kinetic constraints determined by the mobilities of neighboring cells. In this work we keep the same kinetic constraints of the original model, but we introduce two types of cells (denoted as "fast'' and "slow'') with very different rates for spin flip. As a consequence, fast and slow cells exhibit very different relaxation times for spin correlators. While slow cells exhibit standard relaxation, fast cells display anomalous relaxation, characterized by a concave-to-convex crossover in spin correlators by changing temperature or composition. At intermediate state points logarithmic relaxation is observed over three time decades. These results display striking analogies with dynamic correlators reported in recent simulations on a bead-spring model for polymer blends.Comment: Major changes. To be published in Journal of Chemical Physic

Unveiling the Effect of Magnetic Noise in the Coherence of Single-Molecule Quantum Processors

Quantum bits (qubits) constitute the most elementary building-blocks of any quantum technology, where information is stored and processed in the form of quantum superpositions between discrete energy levels. In particular, the fabrication of quantum processors is a key long-term goal that will allow us conducting specific tasks much more efficiently than the most powerful classical computers can do. Motivated by recent experiments in which three addressable spin qubits are defined on a potential single-molecule quantum processor, namely the [Gd(H2O)P5W30O110]12− polyoxometalate, we investigate the decohering effect of magnetic noise on the encoded quantum information. Our state-of-the-art model, which provides more accurate results than previous estimates, show a noticeable contribution of magnetic noise in limiting the survival timescale of the qubits. Yet, our results suggest that it might not be the only dephasing mechanism at play but other mechanisms, such as lattice vibrations and physical movement of magnetic nuclei, must be considered to understand the whole decoherence process

The 750 GeV Diphoton Excess as a First Light on Supersymmetry Breaking

One of the most exciting explanations advanced for the recent diphoton excess found by ATLAS and CMS is in terms of sgoldstino decays: a signal of low-energy supersymmetry-breaking scenarios. The sgoldstino, a scalar, couples directly to gluons and photons, with strength related to gaugino masses, that can be of the right magnitude to explain the excess. However, fitting the suggested resonance width, Gamma ~ 45 GeV, is not so easy. In this paper we explore efficient possibilities to enhance the sgoldstino width, via the decay into two Higgses, two Higgsinos and through mixing between the sgoldstino and the Higgs boson. In addition, we present an alternative and more efficient mechanism to generate a mass splitting between the scalar and pseudoscalar components of the sgoldstino, which has been suggested as an interesting alternative explanation to the apparent width of the resonance.Comment: 14 pages, 3 figure

Domain walls in supersymmetric QCD

We consider domain walls that appear in supersymmetric SU(N) with one massive flavour. In particular, for N > 3 we explicitly construct the elementary domain wall that interpolates between two contiguous vacua. We show that these solutions are BPS saturated for any value of the mass of the matter fields. We also comment on their large N limit and their relevance for supersymmetric gluodynamics.Comment: 4 pages, 1 figure, uses latex with hep99 class files. Presented at the International Europhysics Conference in High Energy Physics, Tampere (Finland) 15-21 July 199