9,765 research outputs found

    AC transport through a resonant level between ferromagnetic electrodes

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    We report the investigation of the spin-valve effect through a resonant level between a ferromagnetic electrode in the presence of an external ac bias. We use the current conserving and gauge invariant theory developed by Büttiker to calculate the dynamic conductance. Specifically, we have calculated the tunneling magnetoresistance (TMR) ratio as a function of various system parameters such as the angle between magnetization of the left and right leads, ac frequency, and the Fermi energy. We found that the TMR ratio can be modulated by ac frequency. At large frequency, the TMR ratio can be negative.published_or_final_versio

    A spin injector

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    We theoretically put forward a spin injector, which consists of a three-terminal ferromagnetic-metal (FM) nonmagnetic-semiconductor (NS)-superconductor (SC) mesoscopic hybrid system. This device can inject not only the spin-up current but also the pure spin current into the NS lead. The crossed Andreev reflection plays a key role in this device. Such a spin injector may be realized within the reach of the present-day technology. © 2004 American Institute of Physics.published_or_final_versio

    Giant enhancement of dynamic conductance in molecular devices

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    We report theoretical investigations of dynamic conductance of molecular systems in the metal-molecule-metal device configuration. The quantum coherent ac transport may be mediated by resonant states extending the entire molecule, or mediated by localized states within the molecule itself. The latter is characterized by tiny features in the dc conductance, but the dissipative part of dynamic conductance can be enhanced by several orders of magnitude as the ac frequency is increased. This phenomenon can be understood from an analytical model.published_or_final_versio

    Collateral Formation After Repeated Transient Dearterialization of the Rat Liver

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    Hepatic artery ligation is used for the palliation of patients with malignant liver tumours. Collaterals are developed rapidly and could to some extent explain why the growth is affected for only a short period. With intermittent dearterialization, collaterals seem to be avoided and possibly a more extended effect should be expected. The most efficient period of dearterialization to avoid collaterals was studied in this experiment. Five groups of rats were treated with daily repeated transient dearterializations for 0 (n = 3), 60 (n = 6), 120 (n = 6), 180 (n = 6) and 240 minutes (n = 6) respectively for 5 days and compared to another group (n = 3) that was permanently dearterialized. After treatment, celiac angiograms were obtained. All hepatic arteries were reliably occluded and patent after 5 days of daily blockades in all but two rats. There were no collaterals demonstrable on the angiograms in the first four groups after 5 days of intermittent obstruction of the arterial blood flow to the liver. After 240 minutes of dearterialization as well as after collaterals developed and were clearly demonstrated on the angiograms after six days. Liver enzymes were normal even after 4 hours of dearterialization. Repeated occlusions of the hepatic artery was reliably achieved with the implantable minioccluder. Repeated, transient dearterializations for 1, 2 or 3 hours could be performed without development of collaterals and without damage to the liver

    Spin pump in the presence of a superconducting lead

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    We present a theoretical analysis of a spin pump in the presence of a superconducting lead. The spin pump is facilitated by a rotating magnetic field which provides a spin flip mechanism and hence can generate a spin current without an accompanying charge current. Using a nonequilibrium Green's function method, we obtain a general solution of the pumped charge current and spin current in both the adiabatic and non-adiabatic regimes. The numerical results for the charge current and spin current are presented as we vary different system parameters such as the gate voltage, the external magnetic field, and the pumping frequency. We find that for a quantum dot with a single resonant level in line with the Fermi energy of the left normal lead, a pure spin current is generated by a rotating magnetic field at any frequency. We have identified two kinds of photon-assisted processes which dominate at low pumping frequencies and high pumping frequencies, respectively.published_or_final_versio

    Time constant of heart rate recovery after low level exercise as a useful measure of cardiovascular fitness

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    In this study we aimed to establish the usefulness of the time constant of heart rate recovery (Tr) in the evaluation of cardiovascular fitness. 15 male subjects exercised on recumbent bicycle at three different workloads (75W, 100W 125W) where R-R intervals were monitored to determine Tr. In order to find the maximal oxygen uptake (V̇O2max) of each subject, oxygen consumption rate (V̇O2) was recorded throughout the treadmill exercise (10km/h). Based on V̇O2max' we classified the subjects into two groups: the "fit" group and the "unfit" group. We found a significant difference in Tr between these two groups only existed when the workload was 75W (p ≤ 0.01) and only at this workload did the R-R intervals achieve stability during the 5 minutes of exercise. Furthermore, we found the cut-off value for predicting cardiovascular fitness at this workload was 55 seconds, with an associated sensitivity of 85.7% and specificity of 87.5%. © 2006 IEEE

    The influence of portal deviation on the effect of repeat dearterializations of a transplantable adenocarcinoma to the rat liver

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    As liver tumours receive some of their blood supply from the portal vein, we wanted to illustrate the influence of portal blood flow in combination with dearterialization in the treatment of liver tumours. Forty male, inbred Wistar/Furth rats with an adenocarcinoma transplanted to the liver were treated with various inflow occlusions repeated daily for 5 days. Deviation of the portal blood flow alone with an end-side porto-caval shunt did not alter the tumour growth (p = 0.089). Thirty min of repeat dearterializations was potentiated by portal deviation so that tumour growth was delayed (p = 0.004). However, repeat dearterializations for 60 min in portal deviated rats induced irreversible liver damage and all rats died in a few days. Repeated dearterializations for 60 minutes alone retarded the tumour growth as efficiently (p = 0.007). Simultaneous occlusion of the hepatic artery and the portal vein for 30 minutes with a side-side porto-caval shunted (total devascularization) did not affect tumour growth (p = 0.154). Liver aminotransferases (ASAT and ALAT) were substantially increased following dearterialization for 30 min in rats with either an end-side or a side-side porto-caval shunt. Dearterialization for 60 min in rats with end-side porto-caval shunts gave a further release of ASAT and ALAT. In conclusion, portal deviation did not augment the therapeutic benefit of repeat dearterializations for the treatment of this experimental liver tumour. Repeat dearterializations alone seemed to be a feasible and efficient therapy for liver tumours

    Energy loss in perturbative QCD

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    We review the properties of energetic parton propagation in hot or cold QCD matter, as obtained in recent works. Advances in understanding the energy loss - collisional and radiative - are summarized, with emphasis on the latter: it features very interesting properties which may help to detect the quark-gluon plasma produced in heavy ion collisions. We describe two different theoretical approaches, which lead to the same radiated gluon energy spectrum. The case of a longitudinally expanding QCD plasma is investigated. The energy lost by a jet with given opening angle is calculated in view of making predictions for the suppression (quenching) of hard jet production. Phenomenological implications for the difference between hot and cold matter are discussed. Numerical estimates of the loss suggest that it may be significantly enhanced in hot compared to cold matter.Comment: 49 pages latex file with 11 embedded PS figures. Uses ar.sty (included), one equation revised. submitted to Annual Review of Nuclear and Particle Scienc

    Spin-polarized transport through a quantum dot: Anderson model with on-site Coulomb repulsion

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    We report on a theoretical analysis of transport characteristics of a spin-valve system formed by a quantum dot connecting to two ferromagnetic electrodes whose magnetic moments are oriented at an angle θ with respect to each other. We pay special attention to the effects of a finite on-site Coulomb repulsion U. Using the Keldysh nonequilibrium Green’s functions we derived a formula for the current in general terms of bias, temperature, and the parameters θ,U. We have studied the local density of states and nonlinear conductance of this device in the Kondo regime at different polarization angle θ. Our results suggest that the Kondo peaks in the local density of states and in the conductance can be modulated by θ.published_or_final_versio