53 research outputs found
Ab initio determination of the ionization potentials of DNA and RNA nucleobases
Quantum chemical high level ab initio coupled-cluster and multiconfigurational perturbation methods have been used to compute vertical and adiabatic ionization potentials of the five canonical DNA and RNA nucleobases: uracil, thymine, cytosine, adenine, and guanine. Several states of their cations have been also calculated. The present results represent a systematic compendium of these magnitudes, establishing theoretical reference values at a level not reported before, calibrating computational strategies, and guiding the assignment of the features in the experimental photoelectron [email protected] [email protected]
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FTIR spectra and conformations of 2′-deoxyuridine in Kr matrices
The Fourier transform infrared spectra in the range 4000–200 cm⁻¹ of pyrimidine nucleoside 2'-deoxyuridine
(dU) have been obtained in the low temperature inert Kr matrices. For the first time, instead of a
usual flat mirror, a low temperature one-coordinate retroreflector was used as the matrix substrate. Owing to
this, the matrix setup is insensitive to dip angle vibrations of the cryostat and is favourable to work with thinner
matrix layers. Of two syn-conformers with dU_s1 and dU_s2 (stabilized by the intramolecular hydrogen
bond O5'H…O2), only dT_s2 conformer with C2'-endo structure of the ribose ring was uniquely quenched.
The height of the interconversion barrier of the minor syn-conformer dU_s1 was estimated to be below 0.7
kcal/mole. It was shown that the energy relaxation of impurities in Kr is slower than in Ar matrices
Point-contact spectroscopy of the antiferromagnetic superconductor HoNi2B2C in the normal and superconducting state
Point-contact (PC) spectroscopy measurements on antiferromagnetic (AF)
(T_N=5.2K) HoNi2B2C single crystals in the normal and two different
superconducting (SC) states (T_c=8.5K and T_c^*=5.6K<T_c, with 2\Delta/kT_c^*=3.9. The strong coupling
Eliashberg analysis of the low-temperature SC phase with T_c^*=5.6K =T_N,
coexisting with the commensurate AF structure, suggests a sizable value of the
EPI constant \lambda_s=0.93. We also provide strong support for the recently
proposed by us ''Fermi surface (FS) separation'' scenario for the coexistence
of magnetism and superconductivity in magnetic borocarbides, namely, that the
superconductivity in the commensurate AF phase survives at a special (nearly
isotropic) FS sheet without an admixture of Ho 5d states. Above T_c^* the SC
features in the PC characteristics are strongly suppressed pointing to a
specific weakened SC state between T_c* and T_c.Comment: 11 pages, 8 figs, to be published in PRB, Vol.75, Iss.2
Connective neck evolution and conductance steps in hot point contacts
Dynamic evolution of the connective neck in Al and Pb mechanically
controllable break junctions was studied during continuous approach of
electrodes at bias voltages V_b up to a few hundred mV. A high level of power
dissipation (10^-4 - 10^-3 W) and high current density (j > 10^10 A/cm^2) in
the constriction lead to overheating of the contact area, electromigration and
current-enhanced diffusion of atoms out of the "hot spot". At a low electrode
approach rate (10 - 50 pm/s) the transverse dimension of the neck and the
conductance of the junction depend on V_b and remain nearly constant over the
approach distance of 10 - 30 nm. For V_b > 300 mV the connective neck consists
of a few atoms only and the quantum nature of conductance manifests itself in
abrupt steps and reversible jumps between two or more levels. These features
are related to an ever changing number of individual conductance channels due
to the continuous rearrangement in atomic configuration of the neck, the
recurring motion of atoms between metastable states, the formation and breaking
of isolated one-atom contacts and the switching between energetically
preferable neck geometries.Comment: 21 pages 10 figure
Giant parametric amplification of the nonlinear response in a single crystal of beryllium in a quantizing magnetic field
The nonlinear response to an ac magnetic field with and without parametric pumping was studied
experimentally on a rod-like beryllium single crystal at low temperatures in a quantizing magnetic
field applied parallel to the hexagonal axis. At low temperatures (≤ 3 K) giant parametric
amplification of the nonlinear response is observed in narrow windows near de Haas– van Alphen
(dHvA) beating antinodes where the amplitude of the magnetic oscillations is maximal. This effect
is of threshold character with respect to the amplitude of the dHvA oscillations and occurs as a result
of a diamagnetic phase transition of a strongly correlated electron gas into nonuniform state
with formation of Condon domain structure. It is shown that the nonuniform phase appears periodically
in a magnetic field. The components of rectified signal field reproduce the envelope of H–T
critical curves and can be used for reconstruction of diamagnetic phase diagrams
Inelastic Light Scattering From Correlated Electrons
Inelastic light scattering is an intensively used tool in the study of
electronic properties of solids. Triggered by the discovery of high temperature
superconductivity in the cuprates and by new developments in instrumentation,
light scattering both in the visible (Raman effect) and the X-ray part of the
electromagnetic spectrum has become a method complementary to optical
(infrared) spectroscopy while providing additional and relevant information.
The main purpose of the review is to position Raman scattering with regard to
single-particle methods like angle-resolved photoemission spectroscopy (ARPES),
and other transport and thermodynamic measurements in correlated materials.
Particular focus will be placed on photon polarizations and the role of
symmetry to elucidate the dynamics of electrons in different regions of the
Brillouin zone. This advantage over conventional transport (usually measuring
averaged properties) indeed provides new insights into anisotropic and complex
many-body behavior of electrons in various systems. We review recent
developments in the theory of electronic Raman scattering in correlated systems
and experimental results in paradigmatic materials such as the A15
superconductors, magnetic and paramagnetic insulators, compounds with competing
orders, as well as the cuprates with high superconducting transition
temperatures. We present an overview of the manifestations of complexity in the
Raman response due to the impact of correlations and developing competing
orders. In a variety of materials we discuss which observations may be
understood and summarize important open questions that pave the way to a
detailed understanding of correlated electron systems.Comment: 62 pages, 48 figures, to appear in Rev. Mod. Phys. High-resolution
pdf file available at http://onceler.uwaterloo.ca/~tpd/RMP.pd
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