4,899 research outputs found

    Dephasing due to background charge fluctuations

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    In quantum computation, quantum coherence must be maintained during gate operation. However, in physical implementations, various couplings with the environment are unavoidable and can lead to a dephasing of a quantum bit(qubit). The background charge fluctuations are an important dephasing process, especially in a charge qubit system. We examined the dephasing rate of a qubit due to random telegraph noise. Solving stochastic differential equations, we obtained the dephasing rate of a qubit constructed of a coupled-dot system; we applied our results to the charge Josephson qubit system. We examined the dephasing rates due to two types of couplings between the coupled-dot system and the background charge, namely, fluctuation in the tunnel coupling constant and fluctuation in the asymmetric bias. For a strong coupling condition, the dephasing rate was inversely proportional to the time constant of the telegraph noise. When there is fluctuation in the tunnel coupling constant, Gaussian decay occurs in the initial regime. We also examined the rate of dephasing due to many impurity sites. For a weak coupling condition with fluctuation in the asymmetric bias, the obtained dephasing rate coincided with that obtained by the perturbation method using the spectral weight of a boson thermal bath, which is proportional to the inverse of the frequency.Comment: 10 pages, 6 figures, RevTeX, to be published in Phys. Rev.

    USp(2k) Matrix Model: F Theory Connection

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    We present a zero dimensional matrix model based on USp(2k)USp(2k) with supermultiplets in symmetric, antisymmetric and fundamental representations. The four dimensional compactification of this model naturally captures the exact results of Sen \cite{Sen} in FF theory. Eight dynamical and eight kinematical supercharges are found, which is required for critical string interpretation. Classical vacuum has ten coordinates and is equipped with orbifold structure. We clarify the issue of spacetime dimensions which FF theory represented by this matrix model produces.Comment: 11 pages, Latex: interpretation as large T^{6}/Z^{2} IIB orientifold added, the final version to appear in Progress of Theoretical Physic

    Current-induced persistent magnetization in a relaxorlike manganite

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    A single crystal of 7% Fe-doped (La0.7_{0.7}Pr0.3_{0.3})0.65_{0.65}Ca0.35_{0.35}MnO3_3 shows up as a typical relaxor ferromagnet, where ferromagnetic metallic and charge-orbital-ordered insulating clusters coexist with controllable volume fraction by external stimuli. There, the persistent ferromagnetic metallic state can be produced by an electric-current excitation as the filamentary region, the magnetization in which is increased by ~0.4μB\mu_{\rm B} per Mn. A clear distinction from the current heating effect in a magnetic field, which conversely leads to a decrease in ferromagnetic fraction, enables us to bi-directionally switch both the magnetization and resistance by applying the voltages with different magnitudes.Comment: 4 pages, 3 figure

    Influence of length and measurement geometry on magnetoimpedance in La0.7Sr0.3MnO3

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    We show that ac magnetoresistance at room temperature in La0.7Sr0.3MnO3 is extremely high (= 47% in H = 100 mT, f = 3-5 MHz), and magnetic field dependence of reactance exhibits a double peak behavior. However, magnitudes of the ac magnetoresistance and magnetoreactance for a fixed length of the sample (li) decrease with decreasing separation (lv) between voltage probes unlike the dc magnetoresistance. On the contrary, change in li has a negligible influence on magnetoimpedance when lv is fixed. Our results indicate that high frequency electrical transport is sensitive to local variations in the magnetic permeability.Comment: 12 pages, 3 figure