The 0.7 (2e 2 /h) conductance anomaly is studied in strongly confined, etched GaAs/GaAlAs quantum point contacts, by measuring the differential conductance as a function of source-drain and gate bias as well as a function of temperature. We investigate in detail how, for a given gate voltage, the differential conductance depends on the finite bias voltage and find a so-called self-gating effect, which we correct for. The 0.7 anomaly at zero bias is found to evolve smoothly into a conductance plateau at 0.85 (2e 2 /h) at finite bias. Varying the gate voltage the transition between the 1.0 and the 0.85 (2e 2 /h) plateaus occurs for definite bias voltages, which defines a gate voltage dependent energy difference ∆. This energy difference is compared with the activation temperature Ta extracted from the experimentally observed activated behavior of the 0.7 anomaly at low bias. We find ∆ = kBTa which lends support to the idea that the conductance anomaly is due to transmission through two conduction channels, of which the one with its subband edge ∆ below the chemical potential becomes thermally depopulated as the temperature is increased.PACS 73.61.-r, 73.23.-b

We demonstrate transport spectroscopy on bottom-up grown few-electron quantum dots in semiconductor nanowires. The dots are defined by InP double barrier heterostructures in InAs nanowires catalytically grown from nanoparticles. By changing the dot size, we can design devices ranging from single-electron transistors to few-electron quantum dots. In the latter case, electrons can be added one by one to the dots from 0 to ∼50 electrons while maintaining an almost constant charging energy, with addition spectra of the devices displaying shell structures as a result of spin and orbital degeneracies. The reduced dimensionality of the nanowire emitter gives rise to pronounced resonant tunneling peaks, where a gate can be used to control the peak positions.

Assessment of vascular properties is essential to diagnosis and follow-up and basic understanding of pathogenesis in brain tumors. In this study, a procedure is presented that allows concurrent estimation of cerebral perfusion, blood volume, and blood-brain permeability from dynamic T 1 -weighted imaging of a bolus of a paramagnetic contrast agent passing through the brain. The methods are applied in patients with brain tumors and in healthy subjects. Perfusion was estimated by model-free deconvolution using Tikhonov's method (gray matter/white matter/tumor: 72 ؎ 16/30 ؎ 8/56 ؎ 45 mL/100 g/min); blood volume (6 ؎ 2/4 ؎ 1/7 ؎ 6 mL/100 g) and permeability (0.9 ؎ 0.4/0.8 ؎ 0.3/3 ؎ 5 mL/100 g/min) were estimated by using Patlak's method and a two-compartment model. A corroboration of these results was achieved by using model simulation. In addition, it was possible to generate maps on a pixel-by-pixel basis of cerebral perfusion, cerebral blood volume, and blood-brain barrier permeability. The measurement of cerebral brain perfusion (CBF) and cerebral blood volume (CBV) is important when diagnosing and treating various conditions such as vascular, degenerative, and neoplastic diseases. These diseases are often associated with an increase in the blood-brain barrier (BBB) permeability. Despite the widespread use of MR perfusion imaging, a concurrent estimation of CBF, CBV, and permeability in the case of a deficient BBB has proved difficult to achieve. Dynamic contrast-enhanced T 1 -weighted MRI (DCE-MRI) was developed in the 1990s for estimating BBB leakage (1-3). A concurrent estimation of perfusion has not been possible and an estimation of the CBV has been problematic, as summarized in Johnson et al. (4). In the case of a deficient BBB, tracers for bolus tracking will leak into extravascular space. Hence, the volume of distribution (V d ) of the tracer can have contributions from both intra-and an extravascular space. Depending on the tracer kinetic model used, this can lead to CBF and CBV quantification errors. In particular, if CBV is estimated as the V d of the tracer, an overestimation will occur in areas with BBB leakage. In T* 2 susceptibility weighted contrast-enhanced MRI, the effect of BBB leakage may compromise the estimation of CBV, due to a mixing effect of T 1 and T* 2 relaxation (5). In addition, T* 2 susceptibility weighted contrast-enhanced MRI has inherent problems related to a differential effect of the contrast agent in blood and in tissue, resulting in an overestimation of CBF (6) and nonlinearity between concentration of the contrast agent and change of relaxation rate (7).The focus of the present study is on developing a methodology whereby a concurrent estimation of CBF, CBV, and the permeability can be obtained using DCE-MRI. Recently, it has been shown that it is possible to measure CBF in human subjects with no BBB leakage when using DCE-MRI (8). In the present study, we demonstrate that in both a simulation model and in healthy subjects and patients with a brain tumor that CBF, CBV, and...

These data show that migraine without aura is associated with dilatation of extra- and intracerebral arteries and that the headache location is associated with the location of the vasodilatation. Furthermore, contraction of extracerebral and not intracerebral arteries is associated with amelioration of headache. Collectively, these data suggest that vasodilatation and perivascular release of vasoactive substances is an integral mechanism of migraine pathophysiology.