René Lindell

Inverse proximity effect in superconductors near ferromagnetic material

We study the electronic density of states in a mesoscopic superconductor near a transparent interface with a ferromagnetic metal. In our tunnel spectroscopy experiment, a substantial density of states is observed at sub-gap energies close to a ferromagnet. We compare our data with detailed calculations based on the Usadel equation, where the effect of the ferromagnet is treated as an effective boundary condition. We achieve an excellent agreement with theory when non-ideal quality of the interface is taken into account.

Observation of Shot-Noise-Induced Asymmetry in the Coulomb Blockaded Josephson Junction

We have investigated the influence of shot noise on the IV curves of a single mesoscopic Josephson junction. We observe a linear enhancement of zero-bias conductance of the Josephson junction with increasing shot-noise power. Moreover, the IV curves become increasingly asymmetric. Our analysis on the asymmetry shows that the Coulomb blockade of Cooper pairs is strongly influenced by the non-Gaussian character of the shot noise.

Noise properties of the Bloch oscillating transistor

We have measured the current noise spectral density of the Bloch oscillating transistor as a function of current gain. We find, as expected from theory and simulations, that the equivalent input noise that shows up in the output is less than the shot noise of the normal-insulating-superconductor tunnel junction (base junction). At the optimal operating point we find a reduced input current noise of 1.0fA∕Hz and a corresponding noise temperature of 0.4 K. The differential current gain at the same point is as large as 30 and the power gain amounts to 35.

Quantum states of a mesoscopic SQUID measured using a small Josephson junction

We have experimentally studied the energy levels of a mesoscopic superconducting quantum interference device (SQUID) using inelastic Cooper-pair tunneling. The tunneling in a small Josephson junction depends strongly on its electromagnetic environment. We use this fact to do energy-level spectroscopy of a SQUID loop by coupling it to a small junction. Our samples with strong quasiparticle dissipation are well described by a model of a particle localized in one of the dips in a cosine potential, while in the samples with weak dissipation we can see formation of energy bands.

Bloch oscillating transistor—a new mesoscopic amplifier

Abstract Bloch oscillating transistor (BOT) is a novel, three-terminal Josephson junction device. Its operating principle utilizes the fact that Zener tunneling up to a higher band will lead to a blockade of coherent Cooper-pair tunneling, Bloch oscillation, in a suitably biased Josephson junction. The Bloch oscillation is resumed only after the junction has relaxed to the lowest band by quasiparticle tunneling. In this paper we present a simple model for the operation of the BOT and calculate its gain in terms of the interband transition rates.

Bloch oscillating transistor as the readout element for hot electron bolometers

In this paper we analyse the properties of the Bloch oscillating transistor as a preamplifier in cryogenic devices. We consider here especially the readout of hot electron bolometers (HEBs) based on Normal-Superconductor-Insulator tunnel junctions, but the results also apply more generally. We show that one can get an equivalent noise voltage below 1 nV/&sqrt;Hz with a single BOT. By using N BOTs in a parallel array configuration, a further reduction by factor &sqrt;N may be achieved.

Mesoscopic Josephson junction as a noise detector

Small Josephson junctions are known to be very susceptible to noise. We have utilized this property in developing methods to measure noise as well as environmental resonance modes in mesoscopic systems. We review recent results on tunnel junction systems and show also that higher order moments of shot noise can be addressed with the present method based on the noise-induced modification of incoherent tunneling of Cooper pairs.

Coulomb-Blockaded Josephson Junction as a Noise Detector

The behavior of a Josephson junction is strongly influenced by the dissipation caused by the resistance R of the surrounding environment. If dissipation is small (R > RQ = h/4e ), the phase of the junction becomes delocalized by fluctuations, and Coulomb blockade (CB) of Cooper pair tunneling takes place. In this case, theory predicts a power-law-like increase of conductance, both as a function of temperature and voltage. The exponent of the power law, 2ρ − 2, is specified by the parameter ρ = R/RQ. Hence, in the case of large exponents 2ρ − 2 1, the conductance is highly susceptible to tiny changes in temperature, or alternatively, there is a high sensitivity to any extra noise sources. We have investigated if this extreme sensitivity could be turned into use in a high-resolution noise detector. For this purpose, we have experimentally investigated how the CB of a Josephson junction changes in the presence of shot noise induced by a near-by SIN junction. In this brief note, we will discuss just one of our samples and present the basic findings on it. For a more detailed description, we refer to a forthcoming publication Ref.

Effect of non-Gaussian noise in small Josephson junctions

We have investigated the effect of shot noise on the IV‐curves of a single Coulomb blockaded Josephson junction. We observe a linear enhancement of zero‐bias conductance of the Josephson junction as a function of the shot noise power induced with a normal‐insulating‐superconducting tunnel junction directly coupled to the island. With increasing shot noise power, the IV‐curves becomes asymmetric with a shift of minimum conductance and the appearance of a local conductance maximum. The asymmetry shows that the Coulomb blockade of Cooper pairs is strongly influenced by the non‐Gaussian character of the shot noise.

Small Josephson Junction As Detector Of Non‐Gaussian Noise

The current in a a Coulomb blockaded Josephson junction is sensitive to voltage or phase fluctuations in its electromagnetical environment. This property can be used to detect noise also from non‐Gaussian noise sources. We have experimentally studied IV‐curves of as a function of the shot noise current in a nearby tunnel junction. We observe a linear enhancement of zero‐bias conductance of the Josephson junction as a function of the shot noise power. Furthermore, with increasing shot noise power, the IV‐curves becomes asymmetric with a shift of minimum conductance and the appearance of a local conductance maximum. A quantitative explanation for the asymmetric features due to the non‐Gaussian noise sources is still an open question. There is also motivation to pursue modifications of the current scheme in order to detect individual statistical moments of noise and to connect different noise sources to the Josephson junction.