VI curves of resistively shunted single Josephson junctions with different capacitances and tunneling resistances are found to display a crossover between two types of VI curves: one without and another with a resistance bump (negative second derivative) at zero bias. The crossover corresponds to the dissipative phase transition (superconductor-insulator transition) at which macroscopic quantum tunneling delocalizes the Josephson phase and destroys superconductivity. Our measured phase diagram does not agree with the diagram predicted by the original theory, but does coincide with a theory that takes into account the accuracy of voltage measurements and thermal fluctuations.[S0031-9007 (98)08351-3] PACS numbers: 74.50. + r, 73.23.Hk A Josephson junction is a unique physical object on which one can test a great variety of important physical concepts of modern physics, such as macroscopic quantum tunneling of the phase, quantum mechanical coherence, Coulomb blockade, etc. An important place in this list is occupied by the so called dissipative phase transition (DPT), predicted for various systems [1-3].The physical origin of this transition is the suppression of macroscopic quantum tunneling of the phase by the interaction with a dissipative quantum-mechanical environment, described by the Caldeira-Leggett model. Macroscopic quantum tunneling destroys superconductivity of a junction, whereas suppression of tunneling restores Josephson current. Hence, this transition is often called a superconductor-insulator transition (SIT).The detection of DPT in a single Josephson junction is of principal importance since it is the simplest system where this transition is expected, without any risk of being masked by other physical processes, as is possible in more complicated systems such as regular or random Josephson junction arrays. Some evidence of DPT (SIT) in a single Josephson junction has already been reported [4], but only for the case of weak Josephson coupling. It has not been enough to trace the whole phase diagram, including the range of strong Josephson coupling where the theoretical predictions are especially intriguing.In this Letter, we present results of our measurements on R dV ͞dI vs I curves, for a variety of single small isolated Josephson junctions, shunted and unshunted, with different values of capacitance C and normal state tunneling resistance R T . We have detected a crossover between two types of RI curves with an essentially different behavior at small currents. Relating this crossover with the DPT, we were able to map out the whole phase diagram for a Josephson junction. The position of the observed phase boundary does not agree with that expected from the original theory. However, the theory, revised to take into account a finite accuracy of our voltage measurements (viz., the minimum voltage that we are able to detect), explains well the observed phase diagram. We also argue that the real signature of DPT is a modification of VI curves as observed in our experiment: the SIT, traditionally define...

We have measured IV curves of multiwalled carbon nanotubes using end contacts. At low voltages, the tunneling conductance obeys non-Ohmic power law, which is predicted both by the Luttinger liquid and the environment-quantum-fluctuation theories. However, at higher voltages we observe a crossover to Ohm's law with a Coulomb-blockade offset, which agrees with the environmentquantum-fluctuation theory, but cannot be explained by the Luttinger-liquid theory. From the high-voltage tunneling conductance we determine the transmission line parameters of the nanotubes.

Ultra-low-field MRI uses microtesla fields for signal encoding and sensitive superconducting quantum interference devices for signal detection. Similarly, modern magnetoencephalography (MEG) systems use arrays comprising hundreds of superconducting quantum interference device channels to measure the magnetic field generated by neuronal activity. In this article, hybrid MEG-MRI instrumentation based on a commercial whole-head MEG device is described. The combination of ultra-low-field MRI and MEG in a single device is expected to significantly reduce coregistration errors between the two modalities, to simplify MEG analysis, and to improve MEG localization accuracy. The sensor solutions, MRI coils (including a superconducting polarizing coil), an optimized pulse sequence, and a reconstruction method suitable for hybrid MEG-MRI measurements are described. The performance of the device is demonstrated by presenting ultra-low-field-MR images and MEG recordings that are compared with data obtained with a 3T scanner and a commercial MEG device.

Cooling nanoelectronic structures to millikelvin temperatures presents extreme challenges in maintaining thermal contact between the electrons in the device and an external cold bath. It is typically found that when nanoscale devices are cooled to ∼10 mK the electrons are significantly overheated. Here we report the cooling of electrons in nanoelectronic Coulomb blockade thermometers below 4 mK. The low operating temperature is attributed to an optimized design that incorporates cooling fins with a high electron–phonon coupling and on-chip electronic filters, combined with low-noise electronic measurements. By immersing a Coulomb blockade thermometer in the 3He/4He refrigerant of a dilution refrigerator, we measure a lowest electron temperature of 3.7 mK and a trend to a saturated electron temperature approaching 3 mK. This work demonstrates how nanoelectronic samples can be cooled further into the low-millikelvin range.

Transparent and conductive patterns of carboxyl functionalized single-walled carbon nanotubes (SWCNT-COOHs) and the composites of those with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) were deposited on various substrates by inkjet printing. For low print repetitions, the PEDOT-PSS/SWCNT-COOH composite patterns show enhanced conductance as compared to the corresponding PEDOT-PSS conductors. The results suggest a decreased percolation threshold for the printed composite since the nanotubes establish electrical interconnections between the separate PEDOT-PSS (conductive phase) islands being dispersed in the insulating PSS-phase. However, the interaction between PEDOT-PSS and SWCNTs becomes insignificant and the conductivity is not enhanced by the nanotubes, when the amount of PEDOT-PSS is sufficient to form a continuous conducting phase. Up to now, patterns having sheet resistivities as low as ~1 kΩ/ᮀ could be achieved. Though there is a trade-off between transparency and conductivity -we achieved highly transparent patterns (~90%) with a reasonably low resistivity of ~10 kΩ/ᮀ. The ink and printing method proposed here offer new alternatives of conventional transparent conductive materials based on either polymers or indium oxides; and pose scaleable production of cost-effective transparent electronics.