6,346 research outputs found
Redetermination of para-aminopyridine (fampridine, EL-970) at 150 K
The structure of fampridine (EL-970) or 4-aminopyridine, C₅H₆N₂, has been redetermined at 150 K. The room-temperature structure has been reported previously [Chao & Schempp (1977). Acta Cryst. B33, 1557-1564]. Pyramidalization at the amine N atom occurs in fampridine, with the N atom 0.133 (11) Å from the plane of the three C/H/H atoms to which it is bonded; the interplanar angle between the pyridyl ring and NH2 group is 21 (2)°. Aggregation in the solid state occurs by N-H...N and N-H...[pi](pyridine) interactions with N...N and N...[pi](centroid) distances of 2.9829 (18) and 3.3954 (15) Å, respectively; a C-H...[pi](pyridine) contact completes the intermolecular interactions [C...[pi](centroid) = 3.6360 (16) Å]
Electric field effect on superconductivity at complex oxide interfaces
We examine the enhancement of the interfacial superconductivity between
LaAlO and SrTiO by an effective electric field. Through the
breaking of inversion symmetry at the interface, we show that a term coupling
the superfluid density and an electric field can augment the superconductivity
transition temperature. Microscopically, we show that an electric field can
also produce changes in the carrier density by relating the measured
capacitance to the density of states. Through the electron-phonon induced
interaction in bulk SrTiO, we estimate the transition temperature.Comment: 7 Pages, Submitted to Physical Revie
Nearly free electrons in the layered oxide superconductor Ag5Pb2O6
We present first measurements of quantum oscillations in the layered oxide
superconductor
Ag5Pb2O6. From a detailed angular and temperature dependent study of the dHvA
effect we determine the electronic structure and demonstrate that the electron
masses are very light, m^* is approximately equalt to 1.2 m_e. The Fermi
surface we observe is essentially that expected of nearly-free electrons -
establishing
Ag5Pb2O6 as the first known example of a monovalent, nearly-free electron
superconductor at ambient pressure.Comment: 4 pages, 3 figure
Energy-resolved inelastic electron scattering off a magnetic impurity
We study inelastic scattering of energetic electrons off a Kondo impurity. If
the energy E of the incoming electron (measured from the Fermi level) exceeds
significantly the Kondo temperature T_K, then the differential inelastic
cross-section \sigma (E,w), i.e., the cross-section characterizing scattering
of an electron with a given energy transfer w, is well-defined. We show that
\sigma (E,w) factorizes into two parts. The E-dependence of \sigma (E,w) is
logarithmically weak and is due to the Kondo renormalization of the effective
coupling. We are able to relate the w-dependence to the spin-spin correlation
function of the magnetic impurity. Using this relation, we demonstrate that in
the absence of magnetic field the dynamics of the impurity spin causes the
electron scattering to be inelastic at any temperature. Quenching of the spin
dynamics by an applied magnetic field results in a finite elastic component of
the electron scattering cross-section. The differential scattering
cross-section may be extracted from the measurements of relaxation of hot
electrons injected in conductors containing localized spins.Comment: 15 pages, 9 figures; final version as published, minor changes,
reference adde
Omnigenity as generalized quasisymmetry
Any viable stellarator reactor will need to be nearly omnigenous, meaning the
radial guiding-center drift velocity averages to zero over time for all
particles. While omnigenity is easier to achieve than quasisymmetry, we show
here that several properties of quasisymmetric plasmas also apply directly or
with only minor modification to the larger class of omnigenous plasmas. For
example, concise expressions exist for the flow and current, closely resembling
those for a tokamak, and these expressions are explicit in that no magnetic
differential equations remain. A helicity (M,N) can be defined for any
omnigenous field, based on the topology by which |B| contours close on a flux
surface, generalizing the helicity associated with quasisymmetric fields. For
generalized quasi-poloidal symmetry (M=0), the bootstrap current vanishes,
which may yield desirable equilibrium and stability properties. A concise
expression is derived for the radial electric field in any omnigenous plasma
that is not quasisymmetric. The fact that tokamak-like analytical calculations
are possible in omnigenous plasmas despite their fully-3D magnetic spectrum
makes these configurations useful for gaining insight and benchmarking codes. A
construction is given to produce omnigenous B(theta, zeta) patterns with
stellarator symmetry.Comment: 37 pages, 8 figure
Spin-Orbital Entanglement and Violation of the Goodenough-Kanamori Rules
We point out that large composite spin-orbital fluctuations in Mott
insulators with orbital degeneracy are a manifestation of quantum
entanglement of spin and orbital variables. This results in a dynamical nature
of the spin superexchange interactions, which fluctuate over positive and
negative values, and leads to an apparent violation of the Goodenough-Kanamori
rules. [{\it Published in Phys. Rev. Lett. {\bf 96}, 147205 (2006).}]Comment: 4 pages, 2 figure
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Modeling software design diversity
Design diversity has been used for many years now as a means of achieving a degree of fault tolerance in software-based systems. Whilst there is clear evidence that the approach can be expected to deliver some increase in reliability compared with a single version, there is not agreement about the extent of this. More importantly, it remains difficult to evaluate exactly how reliable a particular diverse fault-tolerant system is. This difficulty arises because assumptions of independence of failures between different versions have been shown not to be tenable: assessment of the actual level of dependence present is therefore needed, and this is hard. In this tutorial we survey the modelling issues here, with an emphasis upon the impact these have upon the problem of assessing the reliability of fault tolerant systems. The intended audience is one of designers, assessors and project managers with only a basic knowledge of probabilities, as well as reliability experts without detailed knowledge of software, who seek an introduction to the probabilistic issues in decisions about design diversity
Fluid/solid transition in a hard-core system
We prove that a system of particles in the plane, interacting only with a
certain hard-core constraint, undergoes a fluid/solid phase transition
Natural orbital functional theory and pairing correlation effects in electron momentum density
Occupation numbers of natural orbitals capture the physics of strong electron
correlations in momentum space. A Natural Orbital Density Functional Theory
based on the antisymmetrized geminal product provides these occupation numbers
and the corresponding electron momentum density. A practical implementation of
this theory approximates the natural orbitals by the Kohn-Sham orbitals and
uses a mean-field approach to estimate pairing amplitudes leading to
corrections for the independent particle model. The method is applied to weakly
doped \mbox{La_2_4}.Comment: 9 pages, 3 figures. Review paper contribution for the special issue
(V.40, No.3 2014) of Fizika Nizkikh Temperatur on New Trends of Fermiology
(shorter version
Correlated Persistent Tunneling Currents in Glasses
Low temperature properties of glasses are derived within a generalized
tunneling model, considering the motion of charged particles on a closed path
in a double-well potential. The presence of a magnetic induction field B
violates the time reversal invariance due to the Aharonov-Bohm phase, and leads
to flux periodic energy levels. At low temperature, this effect is shown to be
strongly enhanced by dipole-dipole and elastic interactions between tunneling
systems and becomes measurable. Thus, the recently observed strong sensitivity
of the electric permittivity to weak magnetic fields can be explained. In
addition, superimposed oscillations as a function of the magnetic field are
predicted.Comment: 4 page
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