Sub-gap spectroscopy of thermally excited quasiparticles in a Nb contacted carbon nanotube quantum dot

We present electronic transport measurements of a single wall carbon nanotube quantum dot coupled to Nb superconducting contacts. For temperatures comparable to the superconducting gap peculiar transport features are observed inside the Coulomb blockade and superconducting energy gap regions. The observed temperature dependence can be explained in terms of sequential tunneling processes involving thermally excited quasiparticles. In particular, these new channels give rise to two unusual conductance peaks at zero bias in the vicinity of the charge degeneracy point and allow to determine the degeneracy of the ground states involved in transport. The measurements are in good agreement with model calculations.Comment: 5 pages, 4 figure

Bistability in superconducting rings containing an inhomogeneous Josephson junction

We investigate the magnetic response of a superconducting Nb ring containing a ferromagnetic PdNi Josephson junction and a tunnel junction in parallel. A doubling of the switching frequency is observed within certain intervals of the external magnetic field. Assuming sinusoidal current-phase relations of both junctions our model of a dc-SQUID embedded within a superconducting ring explains this feature by a sequence of current reversals in the ferromagnetic section of the junction in these field intervals. The switching anomalies are induced by the coupling between the magnetic fluxes in the two superconducting loops.Comment: 5 pages, 4 figure

Universality of the Kondo effect in quantum dots with ferromagnetic leads

We investigate quantum dots in clean single-wall carbon nanotubes with ferromagnetic PdNi-leads in the Kondo regime. In most odd Coulomb valleys the Kondo resonance exhibits a pronounced splitting, which depends on the tunnel coupling to the leads and an external magnetic field $B$, and only weakly on gate voltage. Using numerical renormalization group calculations, we demonstrate that all salient features of the data can be understood using a simple model for the magnetic properties of the leads. The magnetoconductance at zero bias and low temperature depends in a universal way on $g \mu_B (B-B_c) / k_B T_K$, where $T_K$ is the Kondo temperature and $B_c$ the external field compensating the splitting.Comment: 4 pages, 4 figure

Spintronic magnetic anisotropy

An attractive feature of magnetic adatoms and molecules for nanoscale applications is their superparamagnetism, the preferred alignment of their spin along an easy axis preventing undesired spin reversal. The underlying magnetic anisotropy barrier --a quadrupolar energy splitting-- is internally generated by spin-orbit interaction and can nowadays be probed by electronic transport. Here we predict that in a much broader class of quantum-dot systems with spin larger than one-half, superparamagnetism may arise without spin-orbit interaction: by attaching ferromagnets a spintronic exchange field of quadrupolar nature is generated locally. It can be observed in conductance measurements and surprisingly leads to enhanced spin filtering even in a state with zero average spin. Analogously to the spintronic dipolar exchange field, responsible for a local spin torque, the effect is susceptible to electric control and increases with tunnel coupling as well as with spin polarization.Comment: 6 pages with 4 figures + 26 pages of Supplementary Informatio

Non‐contrast‐enhanced MRI of the pulmonary blood volume using two‐compartment‐modeled T1‐relaxation

Purpose: To introduce a novel technique, based on a two‐compartment model and nonselective inversion recovery (TCIR) for the non‐contrast‐enhanced evaluation of the fractional pulmonary blood volume (fPBV). Materials and Methods: Ten healthy volunteers and one patient with focal lung destruction underwent examination with conventional magnetic resonance imaging (MRI) and TCIR. The reproducibility of TCIR was evaluated statistically, analyzing three consecutive measurement series. In order to evaluate the sensitivity of TCIR, the influence of gravitation on fPBV values along the anterior–posterior direction was assessed. Therefore, two transverse parameter maps of each volunteer in supine and prone position were acquired and analyzed. A comparison of patient images from TCIR‐MRI, dynamic contrast‐enhanced (DCE) MRI, and contrast‐enhanced computed tomography (CT) was performed visually. Results: The statistical evaluation showed significant similarity within the volunteer group, proving reproducibility. The detected slope of the fPBV values in anterior–posterior direction for both supine and prone position demonstrated the sensitivity of TCIR to the gravitational effect on the pulmonary blood distribution. The comparison between CT, DCE‐, and TCIR‐MRI patient datasets showed high similarity in dimension and location of the pathological part of the lung parenchyma. Conclusion: The introduced TCIR‐technique is able to provide reproducible maps of the fPBV without the application of intravenous contrast media

Iterative reconstruction for few-view grating-based phase-contrast CT —An in vitro mouse model

The aim of this work is to investigate the improvement of image quality in few-view grating-based phase-contrast computed tomography (PCCT) applications via compressed sensing (CS) inspired iterative reconstruction on an in vitro mouse model. PCCT measurements are performed on a grating-based PCCT setup using a high-brilliance synchrotron source and a conventional tube source. The sampling density of the data is reduced by a factor of up to 20 and iteratively reconstructed. It is demonstrated that grating-based PCCT intrinsically meets the major conditions for a successful application of CS. Contrast fidelity and the reproduction of details is presented in all reconstructed objects. The feasibility of the iterative reconstruction on data generated with a conventional X-ray source is illustrated on a fluid phantom and a mouse specimen, undersampled by a factor of up to 20