400 research outputs found
Trimethylsulfonium Methanesulfonate
In the title compound, C3H9S+.CH3O3S-, a thermal decomposition product of dimethyl sulfoxide, both cation and anion lie on mirror planes. In the cation, the S atom lies 0.792 (2) Ã… out of the plane defined by the three C atoms, with S-C distances of 1.781 (2) and 1.786 (3) Ã…. In the anion, the S-O distances are 1.4556 (14) and 1.4646 (19) Ã…, and the S-C distance is 1.759 (3) Ã…
Electronic structure of nanoscale iron oxide particles measured by scanning tunneling and photoelectron spectroscopies
We have investigated the electronic structure of nano-sized iron oxide by
scanning tunnelling microscopy (STM) and spectroscopy (STS) as well as by
photoelectron spectroscopy. Nano particles were produced by thermal treatment
of Ferritin molecules containing a self-assembled core of iron oxide. Depending
on the thermal treatment we were able to prepare different phases of iron oxide
nanoparticles resembling gamma-Fe2O3, alpha-Fe2O3, and a phase which apparently
contains both gamma-Fe2O3 and alpha-Fe2O3. Changes to the electronic structure
of these materials were studied under reducing conditions. We show that the
surface band gap of the electronic excitation spectrum can differ from that of
bulk material and is dominated by surface effects.Comment: REVTeX, 6 pages, 10 figures, submitted to PR
Universal transport in 2D granular superconductors
The transport properties of quench condensed granular superconductors are
presented and analyzed. These systems exhibit transitions from insulating to
superconducting behavior as a function of inter-grain spacing.
Superconductivity is characterized by broad transitions in which the resistance
drops exponentially with reducing temperature. The slope of the log R versus T
curves turns out to be universaly dependent on the normal state film resistance
for all measured granular systems. It does not depend on the material, critical
temperature, geometry, or experimental set-up. We discuss possible physical
scenarios to explain these findings.Comment: 4 pages, 3 figure
Mechanistic Rationale for Ketene Formation During Dabbing and Vaping
Ketene is one of the most toxic vaping emissions identified to date. However, its high reactivity renders it relatively challenging to identify. In addition, certain theoretical studies have shown that realistic vaping temperature settings may betoo low to produce ketene. Each of these issues is addressed herein. First, an isotopically labeled acetate precursor is used for the identification of ketene with enhanced rigor in vaped aerosols. Second, discrepancies between theoretical and experimental findings are explained by accounting for the effects of aerobic (experimental) versus anaerobic (simulated and theoretical) pyrolysis conditions. This finding is also relevant to explaining the relatively low-temperature production of aerosol toxicants beyond ketene. Moreover, the study presented herein shows that ketene formation during vaping is not limited to molecules possessing a phenyl acetate substructure. This means that ketene emission during vaping, including from popular flavorants such as ethyl acetate, may be more prevalent than is currently known
Weak Localization Effect in Superconductors
We study the effect of weak localization on the transition temperatures of
superconductors using time-reversed scattered state pairs, and find that the
weak localization effect weakens electron-phonon interactions. With solving the
BCS equation, the calculated values for are in good agreement
with experimental data for various two- and three-dimensional disordered
superconductors. We also find that the critical sheet resistance for the
suppression of superconductivity in thin films does not satisfy the universal
behavior but depends on sample, in good agreement with experiments. but depends
on sample, in good agreement with experiments.Comment: 14 pages, Revtex, 5 ps figure
Infrared Studies of the Onset of Conductivity in Ultra-Thin Pb Films
In this paper we report the first experimental measurement of the infrared
conductivity of ultra-thin quenched-condensed Pb films. For dc sheet
resistances such that the ac conductance increases with
frequency but is in disagreement with the predictions of weak localization. We
attribute this behavior to the effects of an inhomogeneous granular structure
of these films, which is manifested at the very small probing scale of infrared
measurements. Our data are consistent with predictions of two-dimensional
percolation theory.Comment: Submitted to Physical Review Letter
Unusual Thermal Stability of a Site-Ordered MC60 Rocksalt Structure (M=K, Rb, or Cs)
X-ray diffraction and differential scanning calorimetry of MxC60, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit is an ordered rocksalt structure in which only the octahedral sites are occupied. The unusual high-T stability of this ordered phase is attributed to the entropy of molecular orientational disorder and/or thermal disorder of the alkali-metal ions within the octahedral sites. Unique to KxC60 with x≥1.4, we find at intermediate temperatures an fcc site-disordered lattice gas phase with random occupancy of tetrahedral and octahedral sites, which is thus isostructural with superconducting K3C60
Scaling of the superfluid density in high-temperature superconductors
A scaling relation \rho_s \simeq 35\sigma_{dc}T_c has been observed in the
copper-oxide superconductors, where \rho_s is the strength of the
superconducting condensate, T_c is the critical temperature, and \sigma_{dc} is
the normal-state dc conductivity close to T_c. This scaling relation is
examined within the context of a clean and dirty-limit BCS superconductor.
These limits are well established for an isotropic BCS gap 2\Delta and a
normal-state scattering rate 1/\tau; in the clean limit 1/\tau \ll 2\Delta, and
in the dirty limit 1/\tau > 2\Delta. The dirty limit may also be defined
operationally as the regime where \rho_s varies with 1/\tau. It is shown that
the scaling relation \rho_s \propto \sigma_{dc}T_c is the hallmark of a BCS
system in the dirty-limit. While the gap in the copper-oxide superconductors is
considered to be d-wave with nodes and a gap maximum \Delta_0, if 1/\tau >
2\Delta_0 then the dirty-limit case is preserved. The scaling relation implies
that the copper-oxide superconductors are likely to be in the dirty limit, and
that as a result the energy scale associated with the formation of the
condensate is scaling linearly with T_c. The a-b planes and the c axis also
follow the same scaling relation. It is observed that the scaling behavior for
the dirty limit and the Josephson effect (assuming a BCS formalism) are
essentially identical, suggesting that in some regime these two effects may be
viewed as equivalent. This raises the possibility that electronic
inhomogeneities in the copper-oxygen planes may play an important role in the
nature of the superconductivity in the copper-oxide materials.Comment: 8 pages with 5 figures and 1 tabl
Structure and Phase Transitions of the 6, 6-Cyclopropane Isomer of C_ {61} H_ {2}
We have used x-ray powder diffraction and differential scanning calorimetry to study the crystalline structures and thermal behavior of the 6,6-cyclopropane isomer of C61H2. At room temperature, the C61H2 cyclopropane molecules, like those of the 6,5-annulene isomer and C60O epoxide, are orientationally disordered and crystallize on a face-centered-cubic lattice such that their methylene groups are statistically disordered among the octahedral voids. Unlike 6,5−C61H2 and C60O, the low-temperature structure is not Pa3¯, but rather a low-symmetry orthorhombic lattice in which a≈
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