817 research outputs found
Multiwalled carbon nanotube: Luttinger liquid or not?
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 environment-quantum-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.Comment: RevTeX, 4 pages, 2 EPS-figures, submitted to Phys. Rev. Let
Superconductor-insulator quantum phase transition in a single Josephson junction
The superconductor-to-insulator quantum phase transition in resistively
shunted Josephson junctions is investigated by means of path-integral Monte
Carlo simulations. This numerical technique allows us to directly access the
(previously unexplored) regime of the Josephson-to-charging energy ratios
E_J/E_C of order one. Our results unambiguously support an earlier theoretical
conjecture, based on renormalization-group calculations, that at T -> 0 the
dissipative phase transition occurs at a universal value of the shunt
resistance R_S = h/4e^2 for all values E_J/E_C. On the other hand,
finite-temperature effects are shown to turn this phase transition into a
crossover, which position depends significantly on E_J/E_C, as well as on the
dissipation strength and on temperature. The latter effect needs to be taken
into account in order to reconcile earlier theoretical predictions with recent
experimental results.Comment: 7 pages, 6 figure
Temperature Dependence of Zero-Bias Resistances of a Single Resistance-Shunted Josephson Junction
Zero-bias resistances of a single resistance-shunted Josephson junction are
calculated as a function of the temperature by means of the path-integral Monte
Carlo method in case a charging energy is comparable with a
Josephson energy . The low-temperature behavior of the zero-bias
resistance changes around , where is
a shunt resistance and . The temperature dependence of the
zero-bias resistance shows a power-law-like behavior whose exponent depends on
. These results are compared with the experiments on
resistance-shunted Josephson junctions
Effect of quantum noise on Coulomb blockade in normal tunnel junctions at high voltages
We have investigated asymptotic behavior of normal tunnel junctions at
voltages where even the best ohmic environments start to look like RC
transmission lines. In the experiments, this is manifested by an exceedingly
slow approach to the linear behavior above the Coulomb gap. As expected on the
basis of the quantum theory taking into account interaction with the
environmental modes, better fits are obtained using 1/sqrt{V}- than 1/V-
dependence for the asymptote. These results agree with the horizon picture if
the frequency-dependent phase velocity is employed instead of the speed of
light in order to determine the extent of the surroundings seen by the
junction.Comment: 9 pages, 4 figures, submitted to Phys. Rev.
Nanoelectronic thermometers optimised for sub-10 millikelvin operation
We report the cooling of electrons in nanoelectronic Coulomb blockade
thermometers below 4 mK. Above 7 mK the devices are in good thermal contact
with the environment, well isolated from electrical noise, and not susceptible
to self-heating. This is attributed to an optimised design that incorporates
cooling fins with a high electron-phonon coupling and on-chip electronic
filters, combined with a low-noise electronic measurement setup. Below 7 mK the
electron temperature is seen to diverge from the ambient temperature. 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.Comment: 11 pages, 4 figures. (Fixed fitted saturation T_e on p9
A sextupole ion beam guide to improve the efficiency and beam quality at IGISOL
The laser ion source project at the IGISOL facility, Jyvaskyla, has motivated
the development and construction of an rf sextupole ion beam guide (SPIG) to
replace the original skimmer electrode. The SPIG has been tested both off-line
and on-line in proton-induced fission, light-ion and heavy-ion induced
fusion-evaporation reactions and, in each case, has been directly compared to
the skimmer system. For both fission and light-ion induced fusion, the SPIG has
improved the mass-separated ion yields by a factor of typically 4 to 8.
Correspondingly, the transmission efficiency of both systems has been studied
in simulations with and without space charge effects. The transport capacity of
the SPIG has been experimentally determined to be 10^12 ions/s before space
charge effects start to take effect. A direct comparison with the simulation
has been made using data obtained via light-ion fusion evaporation. Both
experiment and simulation show an encouraging agreement as a function of
current extracted from the ion guide.Comment: Latex formatted, submitted to NIM B, 17 pages with 22 .eps figure
Evidence of 4He Crystallization via Quantum Tunneling at mK Temperatures
We have investigated creation of 4He crystals from the superfluid phase at the temperature range 2 mKâ1.0 K. Statistical nucleation-event distributions in overpressure were found to be broad, asymmetric, and temperature independent below 100 mK. Our statistical analysis agrees with a theoretical model suggesting that solid formation is driven by macroscopical quantum-mechanical fluctuations from a seed preexisting in a cavity on the wall.Peer reviewe
Facet Growth of 4He Crystals at mK Temperatures
We have investigated growth of c facets in good quality helium crystals with screw dislocation densities 0â20 cm exp â2 along the c axis. Three distinct regimes of growth were observed. One of them can be explained by spiral growth provided that kinetic energy of moving steps and their tendency to localization at large driving forces are taken into account. In the absence of screw dislocations we find burstlike growth unless the speed is less than 0.5 nm/s, in which case anomalous, intrinsic growth of facets is detected.Peer reviewe
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