Experimental evidence for the Coulomb blockade of Cooper pair tunneling and Bloch oscillations in single Josephson junctions

We report on measurements of the current-voltage characteristics of ultrasmall Josephson junctions. The junctions were made of either Al or Pb alloy, and the leads connecting the junctions to the outside world were high resistance thin film microstrips fabricated on the chip very close to the junction. The high frequency impedance of these leads was sufficiently large to enable the observation in a single Josephson junction of the Coulomb blockade of Cooper pair tunneling, as well as evidence for the time correlation of Cooper pair tunneling (Bloch Oscillations).     

Coulomb blockade and coherent single-cooper-pair tunneling in single Josephson junctions

We have measured the current-voltage characteristics of small-capacitance single Josephson junctions at low temperatures ( T< or =0.04 K), where the strength of the coupling between the single junction and the electromagnetic environment was controlled with one-dimensional arrays of dc SQUIDs. We have clearly observed Coulomb blockade of Cooper-pair tunneling and even a region of negative differential resistance, when the zero-bias resistance of the SQUID arrays is much higher than the quantum resistance h/e(2) approximately 26 kOmega. The negative differential resistance is evidence of coherent single-Cooper-pair tunneling in the single Josephson junction.     

Dynamic properties of a Josephson junction balanced comparator with Coulomb blockade

The dynamics of a Josephson junction balanced comparator with Coulomb blockade has been analyzed. An expression for the time resolution in the case of a linearly increasing gating voltage pulse has been derived with regard to the Bloch inductance. It has been shown that the time resolution depends on the Bloch inductance of small Josephson junctions. Estimates have confirmed the feasibility of a subpicosecond time resolution for balance Josephson comparators with Coulomb blockade.     

Coulomb blockade in triangular lateral small-size quantum dots

An AlGaAs/GaAs lateral quantum dot of triangular shape with a characteristic size L<100 nm containing less than ten electrons was studied. Single-electron oscillations of the conductance G of this dot were measured at G<e²/h. When going from Ge²/h to G≈0.5e²/h, a decrease was found not only in the amplitude but also in the period of the oscillations. A calculation of the 3D-electrostatics demonstrated that this effect is due to a change in the dot size produced by control voltages.     

Quasiparticle and Cooper pair tenneling in small capacitance Josephson junctions

The tunneling of single electrons in small capacitance tunnel junctions is influenced by charging effects and by the fluctuations of the elecromagnetic environment. We study the effect of an external circuit with arbitrary impedance on the tunneling of quasiparticles and Cooper pairs in voltage driven Josephson junctions. We present results at finite temperatures and also consider an acdriven system.     

Interplay of spin accumulation and Coulomb blockade in double ferromagnetic junctions

Single-electron tunneling in a double junction in which a nonmagnetic metallic island is separated from two ferromagnetic electrodes by tunnel barriers, is analysed theoretically in the sequential tunneling regime and for an arbitrary intrinsic spin relaxation time on the island. It is shown that nonequilibrium spin polarization of the island results in tunnel magnetoresistance due to rotation of the electrode magnetizations from antiparallel to parallel alignment. It is also shown that discrete charging of the island gives rise to a fine structure in the voltage dependence of the island spin polarization and tunnel magnetoresistance     

Cooper pairs and the gas-liquid transition

The question whether the gas-liquid phase transition occurs in the many-Cooper pair system is studied. When the idea of Yang-Lee zeros is applied to the grand partition function equivalent to the BCS model, its impossibility is proved.     

Coherent Cooper pair tunneling in systems of Josephson junctions: Effects of quasiparticle tunneling and of the electromagnetic environment

Small capacitance tunnel junctions show single electron effects and, in the superconducting state, the coherent tunneling of Cooper pairs. We study these effects in a system of two Josephson junctions, driven by a voltage source with a finite impedance. Novel features show up in theI?V characteristics, in particular pronounced structures at subgap voltages. These are due to Cooper pair tunneling, combined with tunneling of quasiparticles or with excitation of the electromagnetic environment.     

Noise properties of SQUIDs with Superconductor-Semiconductor-Superconductor proximity effect Josephson junctions

SQUIDs using Superconductor-Semiconductor-Superconductor (SSmS) proximity effect Josephson junctions were prepared and noise measurements were carried out. Since SSmS junctions are basic elements of Josephson field effect transistors (JoFETs), information about dynamic properties of JoFETs can be gained in this way. A planar geometry was used for the SSmS junctions, with a single crystalline silicon wafer acting as both, substrate and proximity layer. Rf- and dc-SQUIDs could be realized. When the SQUIDs were operated in a flux locked loop, flux noise values comparable to conventional tunnel junction SQUIDs were measured.