238 research outputs found
Wide-range optical studies on various single-walled carbon nanotubes: the origin of the low-energy gap
We present wide-range (3 meV - 6 eV) optical studies on freestanding
transparent carbon nanotube films, made from nanotubes with different diameter
distributions. In the far-infrared region, we found a low-energy gap in all
samples investigated. By a detailed analysis we determined the average
diameters of both the semiconducting and metallic species from the near
infrared/visible features of the spectra. Having thus established the
dependence of the gap value on the mean diameter, we find that the frequency of
the low energy gap is increasing with increasing curvature. Our results
strongly support the explanation of the low-frequency feature as arising from a
curvature-induced gap instead of effective medium effects. Comparing our
results with other theoretical and experimental low-energy gap values, we find
that optical measurements yield a systematically lower gap than tunneling
spectroscopy and DFT calculations, the difference increasing with decreasing
diameter. This difference can be assigned to electron-hole interactions.Comment: 9 pages, 8 figures, to be published in Physical Review B,
supplemental material attached v2: Figures 1, 7 and 8 replaced, minor changes
to text; v3: Figures 3, 4 and 5 replaced, minor changes to tex
Phase-slip avalanches in the superflow of He through arrays of nanopores
Recent experiments by Sato et al. [1] have explored the dynamics of He
superflow through an array of nanopores. These experiments have found that, as
the temperature is lowered, phase-slippage in the pores changes its character,
from synchronous to asynchronous. Inspired by these experiments, we construct a
model to address the characteristics of phase-slippage in superflow through
nanopore arrays. We focus on the low-temperature regime, in which the
current-phase relation for a single pore is linear, and thermal fluctuations
may be neglected. Our model incorporates two basic ingredients: (1) each pore
has its own random value of critical velocity (due, e.g., to atomic-scale
imperfections), and (2) an effective inter-pore coupling, mediated through the
bulk superfluid. The inter-pore coupling tends to cause neighbours of a pore
that has already phase-slipped also to phase-slip; this process may cascade,
creating an avalanche of synchronously slipping phases. As the temperature is
lowered, the distribution of critical velocities is expected to effectively
broaden, owing to the reduction in the superfluid healing length, leading to a
loss of synchronicity in phase-slippage. Furthermore, we find that competition
between the strength of the disorder in the critical velocities and the
strength of the inter-pore interaction leads to a phase transition between
non-avalanching and avalanching regimes of phase-slippage.
[1] Sato, Y., Hoskinson, E. Packard, R. E. cond-mat/0605660.Comment: 8 pages, 5 figure
Charge transfer excitons in optical absorption spectra of C60-dimers and polymers
Charge-transfer (CT) exciton effects are investigated for the optical
absorption spectra of crosslinked C60 systems by using the intermediate exciton
theory. We consider the C60-dimers, and the two (and three) molecule systems of
the C60-polymers. We use a tight-binding model with long-range Coulomb
interactions among electrons, and the model is treated by the Hartree-Fock
approximation followed by the single-excitation configuration interaction
method. We discuss the variations in the optical spectra by changing the
conjugation parameter between molecules. We find that the total CT-component
increases in smaller conjugations, and saturates at the intermediate
conjugations. It decreases in the large conjugations. We also find that the
CT-components of the doped systems are smaller than those of the neutral
systems, indicating that the electron-hole distance becomes shorter in the
doped C60-polymers.Comment: Figures should be requested to the autho
Local superfluid densities probed via current-induced superconducting phase gradients
We have developed a superconducting phase gradiometer consisting of two
parallel DNA-templated nanowires connecting two thin-film leads. We have ramped
the cross current flowing perpendicular to the nanowires, and observed
oscillations in the lead-to-lead resistance due to cross-current-induced phase
differences. By using this gradiometer we have measured the temperature and
magnetic field dependence of the superfluid density and observed an
amplification of phase gradients caused by elastic vortex displacements. We
examine our data in light of Miller-Bardeen theory of dirty superconductors and
a microscale version of Campbell's model of field penetration.Comment: 5 pages, 6 figure
Three-dimensional electronic instabilities in polymerized solid A1C60
The low-temperature structure of A1C60 (A=K, Rb) is an ordered array of
polymerized C60 chains, with magnetic properties that suggest a non-metallic
ground state. We study the paramagnetic state of this phase using
first-principles electronic-structure methods, and examine the magnetic
fluctuations around this state using a model Hamiltonian. The electronic and
magnetic properties of even this polymerized phase remain strongly three
dimensional, and the magnetic fluctuations favor an unusual three-dimensional
antiferromagnetically ordered structure with a semi-metallic electronic
spectrum.Comment: REVTeX 3.0, 10 pages, 4 figures available on request from
[email protected]
Alfvén Eigenmodes in shear reversed plasmas
Experiments on JT-60U and JET have shown that plasma configurations with shear reversal are prone to the excitation of unusual AlfvĂ©n eigenmodes by energetic particles. These modes emerge outside the TAE frequency gap, where one might expect them to be strongly damped. The modes often appear in bunches and they exhibit a quasi-periodic pattern of predominantly upward frequency sweeping (AlfvĂ©n Cascades) as the safety factor q changes in time. This work presents a theory that explains the key features of the observed unusual modes including their connection to TAEâs as well as the modifications of TAEâs themselves near the shear reversal point. The developed theory has been incorporated into a reduced numerical model and verified with full geometry codes. JET experimental data on AlfvĂ©n spectroscopy have been simulated to infer the mode numbers and the evolution of qmin in the discharge. This analysis confirms the values of q that characterize the internal transport barrier triggering in reversed shear plasmas
Vibrational spectra of C60C8H8 and C70C8H8 in the rotor-stator and polymer phases
C60-C8H8 and C70-C8H8 are prototypes of rotor-stator cocrystals. We present
infrared and Raman spectra of these materials and show how the rotor-stator
nature is reflected in their vibrational properties. We measured the
vibrational spectra of the polymer phases poly(C60C8H8) and poly(C70C8H8)
resulting from a solid state reaction occurring on heating. Based on the
spectra we propose a connection pattern for the fullerene in poly(C60C8H8),
where the symmetry of the C60 is D2h. On illuminating the C60-C8H8 cocrystal
with green or blue light a photochemical reaction was observed leading to a
similar product to that of the thermal polymerization.Comment: 26 pages, 8 figures, to appear in Journal of Physical Chemistry B 2nd
version: minor changes in wording, accepted version by journa
Structure and properties of the stable two-dimensional conducting polymer Mg5C60
We present a study on the structural, spectroscopic, conducting,
and
magnetic properties of Mg5C60, which is a two-dimensional (2D)
fulleride polymer. The polymer phase is stable up to the
exceptionally
high temperature of 823 K. The infrared and Raman studies
suggest the
formation of single bonds between the fulleride ions and
possibly
Mg-C-60 covalent bonds. Mg5C60 is a metal at ambient
temperature, as
shown by electron spin resonance and microwave conductivity
measurements. The smooth transition from a metallic to a
paramagnetic
insulator state below 200 K is attributed to Anderson
localization
driven by structural disorder
Operation of a superconducting nanowire quantum interference device with mesoscopic leads
A theory describing the operation of a superconducting nanowire quantum
interference device (NQUID) is presented. The device consists of a pair of
thin-film superconducting leads connected by a pair of topologically parallel
ultra-narrow superconducting wires. It exhibits intrinsic electrical
resistance, due to thermally-activated dissipative fluctuations of the
superconducting order parameter. Attention is given to the dependence of this
resistance on the strength of an externally applied magnetic field aligned
perpendicular to the leads, for lead dimensions such that there is essentially
complete and uniform penetration of the leads by the magnetic field. This
regime, in which at least one of the lead dimensions lies between the
superconducting coherence and penetration lengths, is referred to as the
mesoscopic regime. The magnetic field causes a pronounced oscillation of the
device resistance, with a period not dominated by the Aharonov-Bohm effect
through the area enclosed by the wires and the film edges but, rather, in terms
of the geometry of the leads, in contrast to the well-known Little-Parks
resistance of thin-walled superconducting cylinders. A theory, encompassing
this phenomenology, is developed through extensions, to the setting of parallel
superconducting wires, of the Ivanchenko-Zil'berman-Ambegaokar-Halperin theory
for the case of short wires and the Langer-Ambegaokar-McCumber-Halperin theory
for the case of longer wires. It is demonstrated that the NQUID acts as a probe
of spatial variations in the superconducting order parameter.Comment: 20 pages, 18 figure
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