236 research outputs found
Competition between local and nonlocal dissipation effects in two-dimensional quantum Josephson junction arrays
We discuss the local and nonlocal dissipation effects on the existence of the
global phase coherence transitions in two dimensional Josephson-coupled
junctions. The quantum phase transitions are also examined for various lattice
geometries: square, triangular and honeycomb. The T=0 superconductor-insulator
phase transition is analyzed as a function of several control parameters which
include self-capacitance and junction capacitance and both local and nonlocal
dissipation effects. We found the critical value of the nonlocal dissipation
parameter \alpha_{1} depends on a geometry of the lattice. The critical value
of the normal state conductance seems to be difficult to obtain experimentally
if we take into consideration different damping mechanisms which are presented
in real physical systems.Comment: accepted to Physica C Ref. No.: PHYSC-D-06-00244R
Macroscopic quantum tunneling and phase diffusion in a LaSrCuO intrinsic Josephson junction stack
We performed measurements of switching current distribution in a submicron
LaSrCuO (LSCO) intrinsic Josephson junction (IJJ) stack in a
wide temperature range. The escape rate saturates below approximately 2\,K,
indicating that the escape event is dominated by a macroscopic quantum
tunneling (MQT) process with a crossover temperature K. We
applied the theory of MQT for IJJ stacks, taking into account dissipation and
the phase re-trapping effect in the LSCO IJJ stack. The theory is in good
agreement with the experiment both in the MQT and in the thermal activation
regimes.Comment: 9 pages, 7 figure
Electrical transport properties of small diameter single-walled carbon nanotubes aligned on ST-cut quartz substrates
A method is introduced to isolate and measure the electrical transport
properties of individual single-walled carbon nanotubes (SWNTs) aligned on an
ST-cut quartz, from room temperature down to 2 K. The diameter and chirality of
the measured SWNTs are accurately defined from Raman spectroscopy and atomic
force microscopy (AFM). A significant up-shift in the G-band of the resonance
Raman spectra of the SWNTs is observed, which increases with increasing SWNTs
diameter, and indicates a strong interaction with the quartz substrate. A
semiconducting SWNT, with diameter 0.84 nm, shows Tomonaga-Luttinger liquid and
Coulomb blockade behaviors at low temperatures. Another semiconducting SWNT,
with a thinner diameter of 0.68 nm, exhibits a transition from the
semiconducting state to an insulating state at low temperatures. These results
elucidate some of the electrical properties of SWNTs in this unique
configuration and help pave the way towards prospective device applications
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
Chlamydomonas DYX1C1/PF23 is essential for axonemal assembly and proper morphology of inner dynein arms
Cytoplasmic assembly of ciliary dyneins, a process known as preassembly, requires numerous non-dynein proteins, but the identities and functions of these proteins are not fully elucidated. Here, we show that the classical Chlamydomonas motility mutant pf23 is defective in the Chlamydomonas homolog of DYX1C1. The pf23 mutant has a 494 bp deletion in the DYX1C1 gene and expresses a shorter DYX1C1 protein in the cytoplasm. Structural analyses, using cryo-ET, reveal that pf23 axonemes lack most of the inner dynein arms. Spectral counting confirms that DYX1C1 is essential for the assembly of the majority of ciliary inner dynein arms (IDA) as well as a fraction of the outer dynein arms (ODA). A C-terminal truncation of DYX1C1 shows a reduction in a subset of these ciliary IDAs. Sucrose gradients of cytoplasmic extracts show that preassembled ciliary dyneins are reduced compared to wild-type, which suggests an important role in dynein complex stability. The role of PF23/DYX1C1 remains unknown, but we suggest that DYX1C1 could provide a scaffold for macromolecular assembly
Superconductor-Insulator Transition in a Capacitively Coupled Dissipative Environment
We present results on disordered amorphous films which are expected to
undergo a field-tuned Superconductor-Insulator Transition.The addition of a
parallel ground plane in proximity to the film changes the character of the
transition.Although the screening effects expected from "dirty-boson" theories
are not evident,there is evidence that the ground plane couples a certain type
of dissipation into the system,causing a dissipation-induced phase
transition.The dissipation due to the phase transition couples similarly into
quantum phase transition systems such as superconductor-insulator transitions
and Josephson junction arrays.Comment: 4 pages, 4 figure
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
Anomalous carbon clusters in 4H-SiC/SiO2 interfaces
We investigated a metal-oxide-semiconductor interface of dry-oxidized (0001¯) 4H-SiC, which was known as the most electrically deteriorated SiC MOSFET, by electrically detected magnetic resonance (EDMR) and observed a signal with an isotropic g factor (2.0024) and magnetic-field angular dependent signal widths. Judging from the g factor, the signal comes from sp2-bonded carbon clusters. In addition, we found that the angular dependence of EDMR signal widths was caused by two-dimensional dipolar broadening with exchange interaction between electron spins. However, the density of electron spins or carbon clusters was 5.4 × 1013 cm−2, which was not high enough for exchange interaction. Therefore, we propose inhomogeneous distribution of carbon clusters in the interface. At the interface, π* peaks from sp2-bonded carbon atoms were detected by electron energy loss spectroscopy. Scanning the electron beams along the interface revealed uneven existence of the π* peaks, which also proved that the sp2-bonded carbon atoms were distributed inhomogeneously in the interface. In addition, we found the formation of sp2-bonded carbon clusters at 4H-SiC(0001¯)/SiO2 interfaces and interaction between π-conjugate electron spins on the carbon clusters by first principles calculation. Such carbon clusters generated electrically active states widely in the energy gap of 4H-SiC. The states result in the Fermi level pinning of the MOSFET
The Origin of the Charge Ordering and Its Relevance to Superconductivity in -(BEDT-TTF)X: The Effect of the Fermi Surface Nesting and the Distant Electron-Electron Interactions
The origin of the charge ordering in organic compounds -(BEDT-TTF) ((SCN), Tl,Rb,Co, Cs,Zn) is studied using an extended
Hubbard model. Calculating the charge susceptibility within random phase
approximation (RPA), we find that the charge
ordering observed at relatively high temperatures can be considered as a
consequence of a cooperation between the Fermi surface nesting, controlled by
the hopping integral in the direction, and the electron-electron
interactions, where the distant (next nearest neighbor) interactions that have
not been taken into account in most of the previous studies play an important
role.Mean field analysis at T=0 also supports the RPA results, and further
shows that in the charge ordered state, some portions of the Fermi
surface remain ungapped and are nested with a nesting vector close to the
modulation wave vector of the horizontal stripe ordering observed at low
temperatures in (SCN). We further study the possibility of
superconductivity by taking into account the distant off-site repulsions and
the band structure corresponding to I, in which superconductivity is
experimentally observed. We find that there is a close competition between
-wave-like singlet pairing and -wave-like triplet pairing due
to a cooperation between the charge and the spin fluctuations. The present
analysis provides a possible unified understanding of the experimental phase
diagram of the -(BEDT-TTF) family, ranging from a charge ordered
insulator to a superconductor.Comment: 13 pages, 18 figures (Figs.5,6,7,14,15,18 compressed using jpeg2ps
Chlamydomonas DYX1C1/PF23 is essential for axonemal assembly and proper morphology of inner dynein arms
Cytoplasmic assembly of ciliary dyneins, a process known as preassembly, requires numerous non-dynein proteins, but the identities and functions of these proteins are not fully elucidated. Here, we show that the classical Chlamydomonas motility mutant pf23 is defective in the Chlamydomonas homolog of DYX1C1. The pf23 mutant has a 494 bp deletion in the DYX1C1 gene and expresses a shorter DYX1C1 protein in the cytoplasm. Structural analyses, using cryo-ET, reveal that pf23 axonemes lack most of the inner dynein arms. Spectral counting confirms that DYX1C1 is essential for the assembly of the majority of ciliary inner dynein arms (IDA) as well as a fraction of the outer dynein arms (ODA). A C-terminal truncation of DYX1C1 shows a reduction in a subset of these ciliary IDAs. Sucrose gradients of cytoplasmic extracts show that preassembled ciliary dyneins are reduced compared to wild-type, which suggests an important role in dynein complex stability. The role of PF23/DYX1C1 remains unknown, but we suggest that DYX1C1 could provide a scaffold for macromolecular assembly
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