Coherent tunnelling across a quantum point contact in the quantum Hall regime.

The unique properties of quantum hall devices arise from the ideal one-dimensional edge states that form in a two-dimensional electron system at high magnetic field. Tunnelling between edge states across a quantum point contact (QPC) has already revealed rich physics, like fractionally charged excitations, or chiral Luttinger liquid. Thanks to scanning gate microscopy, we show that a single QPC can turn into an interferometer for specific potential landscapes. Spectroscopy, magnetic field and temperature dependences of electron transport reveal a quantitatively consistent interferometric behavior of the studied QPC. To explain this unexpected behavior, we put forward a new model which relies on the presence of a quantum Hall island at the centre of the constriction as well as on different tunnelling paths surrounding the island, thereby creating a new type of interferometer. This work sets the ground for new device concepts based on coherent tunnelling.

Transport inefficiency in branched-out mesoscopic networks: an analog of the Braess paradox.

We present evidence for a counterintuitive behavior of semiconductor mesoscopic networks that is the analog of the Braess paradox encountered in classical networks. A numerical simulation of quantum transport in a two-branch mesoscopic network reveals that adding a third branch can paradoxically induce transport inefficiency that manifests itself in a sizable conductance drop of the network. A scanning-probe experiment using a biased tip to modulate the transmission of one branch in the network reveals the occurrence of this paradox by mapping the conductance variation as a function of the tip voltage and position.

Effects of chronic elevation of atrial natriuretic peptide and free fatty acid levels in the induction of type 2 diabetes mellitus and insulin resistance in patients with mitral valve disease.

BACKGROUND AND AIMS: The relationship between atrial natriuretic peptide (ANP), increased free fatty acid (FFA) and insulin resistance in patients with mitral valve disease (MVD), a group characterised by elevated atrial pressure and increased ANP levels, is not defined. The present study was performed to evaluate, in MVD patients, the relationship between increased ANP and FFA levels and insulin resistance and the role of mitral valve replacement/repair in ameliorating these metabolic alterations. Conversely, coronary heart disease (CHD) patients were evaluated before and after coronary artery bypass grafting (CABG), since they are known to be insulin resistant in the presence of chronic FFA increase. METHODS AND RESULTS: Fifty MVD patients and 55 CHD patients were studied before and 2 months after surgery and compared with 166 normal subjects. Before surgery, 56% of MVD patients had impaired glucose tolerance or newly diagnosed type 2 diabetes after a standard oral glucose load and this percentage decreased to 46% after surgery. In CHD, impaired glucose tolerance (IGT) or newly diagnosed type 2 diabetic patients were 67% of patients before and after CABG. In MVD, left atrial (LA) volume, ANP, FFA incremental area and insulin levels were higher and Insulin Sensitivity (IS) index significantly reduced while after surgery, LA volume, ANP and FFA significantly decreased and IS index significantly improved. In CHD, insulin resistance and hyperinsulinaemia were present both before and after surgery with increased tumour necrosis factor (TNF)-α and interleukin (IL)-6 levels. CONCLUSION: In MVD, a higher degree of abnormal glucose tolerance and insulin resistance are associated to increased levels of ANP and FFA, while these metabolic alterations are improved by mitral valve replacement/repair surgery. Clinical Trial.gov registration number NCT 00520962.

Phonon- and surface-roughness-limited mobility of gate-all-around 3C-SiC and Si nanowire FETs.

We present numerical simulations of gate-all-around (GAA) 3C-SiC and Si nanowire (NW) field effect transistors (FETs) using a full quantum self-consistent Poisson-Schrödinger algorithm within the non-equilibrium Green's function (NEGF) formalism. A direct comparison between Si and 3C-SiC device performances sheds some light on the different transport properties of the two materials. Effective mobility extraction has been performed in a linear transport regime and both phonon- (PH) and surface-roughness-(SR) limited mobility values were computed. 3C-SiC FETs present stronger acoustic phonon scattering, due to a larger deformation potential, resulting in lower phonon-limited mobility values. Although Si NW devices reveal a slightly better electrostatic control compared to 3C-SiC ones, SR-limited mobility shows a slower degradation with increasing charge density for 3C-SiC devices. This implies that the difference between Si and 3C-SiC device mobility is reduced at large gate voltages. 3C-SiC nanowires, besides their advantages compared to silicon ones, present electrical transport properties that are comparable to the Si case.

Scanning gate microscopy of quantum rings: effects of an external magnetic field and of charged defects.

We study scanning gate microscopy (SGM) in open quantum rings obtained from buried semiconductor InGaAs/InAlAs heterostructures. By performing a theoretical analysis based on the Keldysh-Green function approach we interpret the radial fringes observed in experiments as the effect of randomly distributed charged defects. We associate SGM conductance images with the local density of states (LDOS) of the system. We show that such an association cannot be made with the current density distribution. By varying an external magnetic field we are able to reproduce recursive quasi-classical orbits in LDOS and conductance images, which bear the same periodicity as the Aharonov-Bohm effect.

Imaging electron wave functions inside open quantum rings.

Combining scanning gate microscopy (SGM) experiments and simulations, we demonstrate low temperature imaging of the electron probability density |Psi|(2)(x,y) in embedded mesoscopic quantum rings. The tip-induced conductance modulations share the same temperature dependence as the Aharonov-Bohm effect, indicating that they originate from electron wave function interferences. Simulations of both |Psi|(2)(x,y) and SGM conductance maps reproduce the main experimental observations and link fringes in SGM images to |Psi|(2)(x,y).

Relation between left ventricular function and oxidative stress in patients undergoing bypass surgery.

OBJECTIVE: To determine whether preoperative left ventricular ejection fraction (LVEF) is related to the degree of myocardial oxidative stress during bypass surgery in man. DESIGN: Observational study. SETTING: Tertiary care centre. PATIENTS AND INTERVENTIONS: 31 patients (LVEF range was 20% to 68%) undergoing elective coronary bypass surgery with blood cardioplegic reperfusion were studied. Arterial and coronary sinus blood was collected before aortic cross clamping (T0) and at 0 (T1), 15 (T2), and 30 (T3) minutes after unclamping. Transmural left ventricular biopsies were also obtained from 15 patients at T0 and at T1. MAIN OUTCOME MEASURES: Glutathione and adenine nucleotides were measured in myocardial biopsies, while coronary sinus-artery differences for glutathione, nucleotides, and products of lipid peroxidation were calculated from blood specimens. Creatine kinase (myocardial band; CK-MB) was measured in plasma at four and 12 hours after operation. RESULTS: Myocardial glutathione and adenine nucleotides were correlated (p < 0.02) with preoperative LVEF both at T0 (r = 0.909 and 0.672) and T1 (r = 0.603 and 0.605). Oxidised glutathione released from the heart during reperfusion was inversely correlated with LVEF (r = -0.448, -0.466, and -0461 at T1, T2, and T3, p < 0.01), while reduced glutathione (r = 0.519 and 0.640 at T1 and T2) and glutathione redox ratio (r = 0.647, 0.714, 0.645, and 0.702 at T0, T1, T2, and T3) showed a direct correlation (p < 0.01). Lipid peroxidation at T1 was negatively related to LVEF (r = -0.492). CK-MB was also negatively related to LVEF (r = -0.440 at 4 h and -0.462 at 12 h). CONCLUSIONS: The capacity to counterbalance oxidative burst following ischaemia and reperfusion appears to be related to the functional ability of the heart.

Role of leucocytes in free radical production during myocardial revascularisation.

OBJECTIVE: To evaluate the role of leucocytes in free radical production in patients with depressed or normal ejection fraction undergoing coronary bypass. DESIGN: Two randomised control trials. SETTING: Tertiary care centre. PATIENTS AND INTERVENTIONS: In the first study, 22 patients with ejection fractions of < or = 40% received blood cardioplegic reperfusion with (n = 11) or without (n = 11) leucocyte depletion. In the second study, 22 patients with ejection fractions > or = 45% received either leucocyte depleted (n = 11) or blood cardioplegia (n = 11). MAIN OUTCOME MEASURES: Glutathione, hypoxanthine, and lipid peroxidation products were measured in coronary sinus blood and plasma before aortic cross clamping and at 0, 15, and 30 minutes after unclamping. Haemodynamic variables and creatine kinase MB isoenzymes were monitored on the first postoperative day. Comparison between treatments was performed on difference (delta) between measurements at time 0 and at baseline, and on slopes obtained by fitting measurements after unclamping with a linear regression model. RESULTS: At unclamping no difference in delta for plasma glutathione redox ratio (oxidised/total glutathione, %) was observed between treated and control groups with low ejection fraction (delta = 16 (SD 8.39) and 24 (7.0) redox ratio %, respectively). Baseline value recovery rate (redox ratio %/min) was significantly faster in treated v control patients (slope -0.912 (0.380) v -0.158 (0.200), P < 0.005, respectively). Cardiac index showed a trend to greater improvement in the treated group (slope 0.04 (0.03) v 0.003 (0.002) 1/min/m2/h, P < 0.02, treated v controls, respectively). In patients with normal ejection fraction, leucocyte depletion did not result in significant improvement v controls. CONCLUSIONS: Leucocyte depletion seems to provide benefit only in patients with left ventricular dysfunction.

Myocardial protection with and without leukocyte depletion: a comparative study on the oxidative stress.

We tested the hypothesis that controlled reperfusion with leukocyte-depleted blood could improve myocardial protection by reducing the oxidative stress in patients undergoing myocardial revascularization. Thirty-four patients receiving antegrade/retrograde blood cardioplegia were divided into: group A: 11 patients with ejection fractions (EF) less than 35%, treated with leukocyte-depleted controlled blood reperfusion, group B: 11 patients with EF less than 35% in whom no leukocyte depletion was performed, group C: 6 patients with EF more than 45% treated as group A and group D: 6 patients with EF more than 45% without leukocyte depletion. To asses the oxidative stress, we evaluated total, total oxidized (GSSX), and reduced glutathione (GSH) in coronary sinus plasma, immediately before cross-clamping the aorta (T0), and at 0 (T1), 15 (T2) and 30 (T3) min after unclamping it. In groups A and B a significant shift towards oxidation of redox status of glutathione (GSH/GSSX) at T1 vs T0 was observed. Glutathione redox ratio remained low in group B while in group A it returned to the basal value at T2 with a significant difference from group B at T2 and T3. No differences were observed between groups C and D. In conclusion, our data show that leukocyte-depleted reperfusion can afford a better myocardial protection in patients with left ventricular dysfunction, while it seems unnecessary in patients with normal EF.

Myocardial revascularization with bilateral internal thoracic artery in patients with left main disease: an incremental risk?

Although the long-term patency of the internal thoracic artery (ITA) has been well proved, there is still some concern about its preoperative performance. We considered 80 patients with left main disease (mean age 60.2 years) who underwent coronary artery bypass grafting in our institute from March 1988 to September 1992. Patients with left main disease were divided into 2 groups: group I-38 patients receiving only ITA grafts on the left coronary system and group II-42 patients having a single ITA graft together with saphenous vein grafts on the left coronary system. No patients in group I received a saphenous graft on the left coronary system and three patients with right coronary artery involvement received total arterial myocardial revascularization with the use of the inferior epigastric artery. Perioperative complications in group I and group II patients were, respectively: myocardial necrosis in 2 (6.9%) and 3 (8.8%), use of intraaortic balloon pump in 2 (6.9%) and 2 (5.9%). No death occurred in either group. In our experience, the use of bilateral ITA grafts in patients with left main stenosis was not related to an incremental risk. We conclude that left main disease should not be considered as counterindication to the extensive use of arterial conduits.

Mitral valve prosthetic endocarditis: development of left ventricular-coronary sinus fistula following replacement.

We report the history and course of a patient in whom a left ventricular-coronary sinus fistula developed following mitral valve replacement due to prosthetic endocarditis. Six months after the intervention the patient suddenly presented with deterioration of her symptoms, holosystolic murmur and signs of congestive heart failure. Transesophageal echocardiography showed a left-to-right shunt but did not show its exact location. At surgery, exploration of the right atrium revealed a left ventricular-coronary sinus communication due to discontinuation of the left ventricular free wall next to the coronary sinus; repair of the defect was successfully performed by direct suture. The postoperative course was uneventful and the patient recovered quickly. This case is reported to stress that debridement of the mitral annulus and removal of an old prosthesis must be very carefully performed and to facilitate the diagnosis of this rare but severe complication of repeated mitral valve replacement.