8,682 research outputs found

    Effective charging energy for a regular granular metal array

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    We study the Ambegaokar-Eckern-Sch\"{o}n (AES) model for a regular array of metallic grains coupled by tunnel junctions of conductance gg and calculate both paramagnetic and diamagnetic terms in the Kubo formula for the conductivity. We find analytically, and confirm by numerical path integral Monte Carlo methods, that for 0<g<40<g<4 the conductivity obeys an Arrhenius law σ(T)∌exp⁥[−E∗(g)/T]\sigma(T)\sim\exp[-E^{*}(g)/T] with an effective charging energy E∗(g)E^{*} (g) when the temperature is sufficiently low, due to a subtle cancellation between T2T^2 inelastic-cotunneling contributions in the paramagnetic and diamagnetic terms. We present numerical results for the effective charging energy and compare the results with recent theoretical analyses. We discuss the different ways in which the experimentally observed σ(T)∌exp⁥[−T0/T]\sigma(T)\sim\exp[-\sqrt{T_{0}/T}] law could be attributed to disorder.Comment: 5 pages, 3 figures, ReVTeX; added estimates of effective charging energies and discussion of effects of disorde

    "Exact" Algorithm for Random-Bond Ising Models in 2D

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    We present an efficient algorithm for calculating the properties of Ising models in two dimensions, directly in the spin basis, without the need for mapping to fermion or dimer models. The algorithm gives numerically exact results for the partition function and correlation functions at a single temperature on any planar network of N Ising spins in O(N^{3/2}) time or less. The method can handle continuous or discrete bond disorder and is especially efficient in the case of bond or site dilution, where it executes in O(L^2 ln L) time near the percolation threshold. We demonstrate its feasibility on the ferromagnetic Ising model and the +/- J random-bond Ising model (RBIM) and discuss the regime of applicability in cases of full frustration such as the Ising antiferromagnet on a triangular lattice.Comment: 4.2 pages, 5 figures, accepted for publication in Phys. Rev. Let

    High power (60mW) single frequency erbium:ytterbium codoped fiber laser

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    The characteristics of a high power Er3+:Yb3+ single frequency fiber laser pumped at 980nm are reported. The device gives 60mW output power with RIN 10MHz and linewidth 500kHz. At low output powers (&lt; 30mW) the slope efficiency is as high as 25%, falling to 12% at higher powers, the saturation behaviour is related to a bottleneck effect due to the finite Yb-Er transfer rate. Improved performance can be obtained using new fibers with an increased rare-earth concentration which show negligible signs of erbium clustering

    Noise Predictions for STM in Systems with Local Electron Nematic Order

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    We propose that thermal noise in local stripe orientation should be readily detectable via STM on systems in which local stripe orientations are strongly affected by quenched disorder. Stripes, a unidirectional, nanoscale modulation of electronic charge, are strongly affected by quenched disorder in two-dimensional and quasi-two-dimensional systems. While stripe orientations tend to lock to major lattice directions, dopant disorder locally breaks rotational symmetry. In a host crystal with otherwise C4C_4 rotational symmetry, stripe orientations in the presence of quenched disorder map to the random field Ising model. While the low temperature state of such a system is generally a stripe glass in two dimensional or strongly layered systems, as the temperature is raised, stripe orientational fluctuations become more prevalent. We propose that these thermally excited fluctuations should be readily detectable in scanning tunneling spectroscopy as {\em telegraph noise} in the high voltage part of the local I(V)I(V) curves. We predict the spatial, temporal, and thermal evolution of such noise, including the circumstances under which such noise is most likely to be observed. In addition, we propose an in-situ test, amenable to any local scanning probe, for assessing whether such noise is due to correlated fluctuations rather than independent switchers.Comment: 8 pages, 8 figure
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