Scattering Theory of Photon-Assisted Electron Transport

The scattering matrix approach to phase-coherent transport is generalized to nonlinear ac-transport. In photon-assisted electron transport it is often only the dc-component of the current that is of experimental interest. But ac-currents at all frequencies exist independently of whether they are measured or not. We present a theory of photon-assisted electron transport which is charge and current conserving for all Fourier components of the current. We find that the photo-current can be considered as an up- and down-conversion of the harmonic potentials associated with the displacement currents. As an example explicit calculations are presented for a resonant double barrier coupled to two reservoirs and capacitively coupled to a gate. Two experimental situations are considered: in the first case the ac-field is applied via a gate, and in the second case one of the contact potentials is modulated. For the first case we show that the relative weight of the conduction sidebands varies with the screening properties of the system. In contrast to the non-interacting case the relative weights are not determined by Bessel functions. Moreover, interactions can give rise to an asymmetry between absorption and emission peaks. In the contact driven case, the theory predicts a zero-bias current proportional to the asymmetry of the double barrier. This is in contrast to the discussion of Tien and Gordon which, in violation of basic symmetry principles, predicts a zero-bias current also for a symmetric double barrier.Comment: 15 pages, 6 figures, REVTE

Charge Fluctuations in Quantum Point Contacts and Chaotic Cavities in the Presence of Transport

We analyze the frequency-dependent current fluctuations induced into a gate near a quantum point contact or a quantum chaotic cavity. We use a current and charge conserving, effective scattering approach in which interactions are treated in random phase approximation. The current fluctuations measured at a nearby gate, coupled capacitively to the conductor, are determined by the screened charge fluctuations of the conductor. Both the equilibrium and the non-equilibrium current noise at the gate can be expressed with the help of resistances which are related to the charge dynamics on the conductor. We evaluate these resistances for a point contact and determine their distributions for an ensemble of chaotic cavities. For a quantum point contact these resistances exhibit pronounced oscillations with the opening of new channels. For a chaotic cavity coupled to one channel point contacts the charge relaxation resistance shows a broad distribution between 1/4 and 1/2 of a resistance quantum. The non-equilibrium resistance exhibits a broad distribution between zero and 1/4 of a resistance quantum.Comment: 9 pages, two-column Revtex, 6 figures include

Current trends in cannulation and neuroprotection during surgery of the aortic arch in Europe†‡

OBJECTIVES To conduct a survey across European cardiac centres to evaluate the methods used for cerebral protection during aortic surgery involving the aortic arch. METHODS All European centres were contacted and surgeons were requested to fill out a short, comprehensive questionnaire on an internet-based platform. One-third of more than 400 contacted centres completed the survey correctly. RESULTS The most preferred site for arterial cannulation is the subclavian-axillary, both in acute and chronic presentation. The femoral artery is still frequently used in the acute condition, while the ascending aorta is a frequent second choice in the case of chronic presentation. Bilateral antegrade brain perfusion is chosen by the majority of centres (2/3 of cases), while retrograde perfusion or circulatory arrest is very seldom used and almost exclusively in acute clinical presentation. The same pumping system of the cardio pulmonary bypass is most of the time used for selective cerebral perfusion, and the perfusate temperature is usually maintained between 22 and 26°C. One-third of the centres use lower temperatures. Perfusate flow and pressure are fairly consistent among centres in the range of 10-15 ml/kg and 60 mmHg, respectively. In 60% of cases, barbiturates are added for cerebral protection, while visceral perfusion still receives little attention. Regarding cerebral monitoring, there is a general tendency to use near-infrared spectroscopy associated with bilateral radial pressure measurement. CONCLUSIONS These data represent a snapshot of the strategies used for cerebral protection during major aortic surgery in current practice, and may serve as a reference for standardization and refinement of different approache

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Risk factor analysis of 170 single-institutional contegra implantations in pulmonary position

BACKGROUND: The aim of this study is to evaluate risk factors affecting survival of Contegra grafts used in the pulmonary position. METHODS: One hundred seventy Contegra implanted (2001 to 2007) in the pulmonary position for replacement after a prior repair (90), Ross procedure (29), tetralogy of Fallot and variants (22), truncus arteriosus (13), Rastelli procedure (8), and miscellaneous (8) were reviewed. Median age was 107 (0.1 to 894) months. Follow-up was 96% complete with a median duration of 65 (7 to 98) months. RESULTS: There were 7 early and 6 late deaths (none Contegra related) with a survival of 92%±2% at 98 months. Thirty-four Contegra were replaced at a median duration of 43 (7 to 82) months. Eight of 28 balloon-dilated Contegra remain palliated at 49 (23 to 73) months. Multivariate analysis showed Contegra less than 16 mm (p<0.001; HR [hazard ratio] 0.07), high pulmonary pressure (p<0.001; HR 4), and prior operation era (p=0.006; HR 0.3) as independent risk factors for Contegra replacement. The freedom from replacement for Contegra less than 16 mm and 16 mm or greater were 48%±8% and 98%±2%; for presence and absence of high pulmonary pressure were 52%±11% and 88%±3% and for era 1 and 2 were 77%±5% and 88%±4% at 60 months, respectively. Twenty-three of 123 surviving Contegra have a mean Doppler gradient greater than 20 mm Hg. Contegra less than 16 mm, smaller age group, obstructive arborization, and era 1 were significantly associated with higher gradients. While 18 (15%) surviving Contegra have moderate or more regurgitation, 88 (72%) have no significant gradient or regurgitation. CONCLUSIONS: Contegra grafts integrate well into the body. Larger Contegra show predictable function with a survival approaching homografts over medium term. Better case selection as one ascends the learning curve tends to improve survival. With easy availability and predictable quality, Contegra grafts continue to be a promising complement to homografts

Heart transplantation in adolescent and adult patients with congenital heart disease: a case-control study

BACKGROUND: The number of adolescent and adult patients with congenital heart disease undergoing heart transplantation is increasing. We aimed to better define the characteristics of these patients and their survival after transplantation. METHODS: We describe a group of patients with end-stage heart failure owing to congenital heart disease undergoing heart transplantation at a single tertiary center and compare their short- and long-term survival with a group of matched controls with dilated cardiomyopathy and the entire cohort of transplanted patients at our center. RESULTS: Between 1985 and 2006, a total of 322 orthotopic heart transplantations were performed at our center. Thirteen patients (mean age, 27.5 years) had a diagnosis of congenital heart disease with a wide spectrum of lesions. The survival of these 13 patients was 85% at 30 days, 1, 5, and 10 years and 77% at 20 years, which did not differ significantly to the short- and long-term survival of the entire cohort of patients with heart transplantation and to the survival of age-matched controls with dilated cardiomyopathy. CONCLUSION: In our single-center experience, short- and long-term survival after heart transplantation in a selected, small group of patients with end-stage heart failure owing to congenital heart disease did not differ significantly compared with a group of age-matched controls and the entire cohort