31,557 research outputs found
Electron-Transport Properties of Na Nanowires under Applied Bias Voltages
We present first-principles calculations on electron transport through Na
nanowires at finite bias voltages. The nanowire exhibits a nonlinear
current-voltage characteristic and negative differential conductance. The
latter is explained by the drastic suppression of the transmission peaks which
is attributed to the electron transportability of the negatively biased plinth
attached to the end of the nanowire. In addition, the finding that a voltage
drop preferentially occurs on the negatively biased side of the nanowire is
discussed in relation to the electronic structure and conduction.Comment: 4 pages, 6 figure
Decay of correlations in the dissipative two-state system
We study the equilibrium correlation function of the polaron-dressed
tunnelling operator in the dissipative two-state system and compare the
asymptoptic dynamics with that of the position correlations. For an Ohmic
spectral density with the damping strength , the correlation functions
are obtained in analytic form for all times at any and any bias. For ,
the asymptotic dynamics is found by using a diagrammatic approach within a
Coulomb gas representation. At T=0, the tunnelling or coherence correlations
drop as , whereas the position correlations show universal decay
. The former decay law is a signature of unscreened attractive
charge-charge interactions, while the latter is due to unscreened dipole-dipole
interactions.Comment: 5 pages, 5 figures, to be published in Europhys. Let
Anomalous Lattice Response at the Mott Transition in a Quasi-2D Organic Conductor
Discontinuous changes of the lattice parameters at the Mott metal-insulator
transition are detected by high-resolution dilatometry on deuterated crystals
of the layered organic conductor -(BEDT-TTF)Cu[N(CN)]Br.
The uniaxial expansivities uncover a striking and unexpected anisotropy,
notably a zero-effect along the in-plane c-axis along which the electronic
interactions are relatively strong. A huge thermal expansion anomaly is
observed near the end-point of the first-order transition line enabling to
explore the critical behavior with very high sensitivity. The analysis yields
critical fluctuations with an exponent 0.8 0.15
at odds with the novel criticality recently proposed for these materials
[Kagawa \textit{et al.}, Nature \textbf{436}, 534 (2005)]. Our data suggest an
intricate role of the lattice degrees of freedom in the Mott transition for the
present materials.Comment: 4 pages, 4 figure
Arithmetical Congruence Preservation: from Finite to Infinite
Various problems on integers lead to the class of congruence preserving
functions on rings, i.e. functions verifying divides for all
. We characterized these classes of functions in terms of sums of rational
polynomials (taking only integral values) and the function giving the least
common multiple of . The tool used to obtain these
characterizations is "lifting": if is a surjective morphism,
and a function on a lifting of is a function on such that
. In this paper we relate the finite and infinite notions
by proving that the finite case can be lifted to the infinite one. For -adic
and profinite integers we get similar characterizations via lifting. We also
prove that lattices of recognizable subsets of are stable under inverse
image by congruence preserving functions
Nonlinear current-induced forces in Si atomic wires
We report first-principles calculations of current-induced forces in Si
atomic wires as a function of bias and wire length. We find that these forces
are strongly nonlinear as a function of bias due to the competition between the
force originating from the scattering states and the force due to bound states.
We also find that the average force in the wire is larger the shorter the wire,
suggesting that atomic wires are more difficult to break under current flow
with increasing length. The last finding is in agreement with recent
experimental data.Comment: 4 figure
The challenges of blended learning using a media annotation tool
Blended learning has been evolving as an important approach to learning and teaching in tertiary education. This approach incorporates learning in both online and face-to-face modes and promotes deep learning by incorporating the best of both approaches. An innovation in blended learning is the use of an online media annotation tool (MAT) in combination with face-to-face classes. This tool allows students to annotate their own or teacher-uploaded video adding to their understanding of professional skills in various disciplines in tertiary education. Examination of MAT occurred in 2011 and included nine cohorts of students using the tool. This article canvasses selected data relating to MAT including insights into the use of blended learning focussing on the challenges of combining face-to-face and online learning using a relatively new online tool
Are there Local Minima in the Magnetic Monopole Potential in Compact QED?
We investigate the influence of the granularity of the lattice on the
potential between monopoles. Using the flux definition of monopoles we
introduce their centers of mass and are able to realize continuous shifts of
the monopole positions. We find periodic deviations from the -behavior of
the monopole-antimonopole potential leading to local extrema. We suppose that
these meta-stabilities may influence the order of the phase transition in
compact QED.Comment: 11 pages, 5 figure
On the Quantum Invariant for the Brieskorn Homology Spheres
We study an exact asymptotic behavior of the Witten-Reshetikhin-Turaev
invariant for the Brieskorn homology spheres by use of
properties of the modular form following a method proposed by Lawrence and
Zagier. Key observation is that the invariant coincides with a limiting value
of the Eichler integral of the modular form with weight 3/2. We show that the
Casson invariant is related to the number of the Eichler integrals which do not
vanish in a limit . Correspondingly there is a
one-to-one correspondence between the non-vanishing Eichler integrals and the
irreducible representation of the fundamental group, and the Chern-Simons
invariant is given from the Eichler integral in this limit. It is also shown
that the Ohtsuki invariant follows from a nearly modular property of the
Eichler integral, and we give an explicit form in terms of the L-function.Comment: 26 pages, 2 figure
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