3,667 research outputs found
Are current-induced forces conservative?
The expression for the force on an ion in the presence of current can be
derived from first principles without any assumption about its conservative
character. However, energy functionals have been constructed that indicate that
this force can be written as the derivative of a potential function. On the
other hand, there exist compelling specific arguments that strongly suggest the
contrary. We propose physical mechanisms that invalidate such arguments and
demonstrate their existence with first-principles calculations. While our
results do not constitute a formal resolution to the fundamental question of
whether current-induced forces are conservative, they represent a substantial
step forward in this direction.Comment: 4 pages, 4 Figures, submitted to PR
Infinite dimensional Lie algebras in 4D conformal quantum field theory
The concept of global conformal invariance (GCI) opens the way of applying
algebraic techniques, developed in the context of 2-dimensional chiral
conformal field theory, to a higher (even) dimensional space-time. In
particular, a system of GCI scalar fields of conformal dimension two gives rise
to a Lie algebra of harmonic bilocal fields, V_m(x,y), where the m span a
finite dimensional real matrix algebra M closed under transposition. The
associative algebra M is irreducible iff its commutant M' coincides with one of
the three real division rings. The Lie algebra of (the modes of) the bilocal
fields is in each case an infinite dimensional Lie algebra: a central extension
of sp(infty,R) corresponding to the field R of reals, of u(infty,infty)
associated to the field C of complex numbers, and of so*(4 infty) related to
the algebra H of quaternions. They give rise to quantum field theory models
with superselection sectors governed by the (global) gauge groups O(N), U(N),
and U(N,H)=Sp(2N), respectively.Comment: 16 pages, with minor improvements as to appear in J. Phys.
Narrow structure in the coherent population trapping resonances in rubidium and Rayleigh scattering
The measurement of the coherent-population-trapping (CPT) resonances in
uncoated Rb vacuum cells has shown that the shape of the resonances is
different in different cells. In some cells the resonance has a complex shape -
a narrow Lorentzian structure, which is not power broadened, superimposed on
the power broadened CPT resonance. The results of the performed investigations
on the fluorescence angular distribution are in agreement with the assumption
that the narrow structure is a result of atom interaction with Rayleigh
scattering light. The results are interesting for indication of the vacuum
cleanness of the cells and building of magnetooptical sensors
Four Dimensional CFT Models with Rational Correlation Functions
Recently established rationality of correlation functions in a globally
conformal invariant quantum field theory satisfying Wightman axioms is used to
construct a family of soluble models in 4-dimensional Minkowski space-time. We
consider in detail a model of a neutral scalar field of dimension 2. It
depends on a positive real parameter c, an analogue of the Virasoro central
charge, and admits for all (finite) c an infinite number of conserved symmetric
tensor currents. The operator product algebra of is shown to coincide
with a simpler one, generated by a bilocal scalar field of
dimension (1,1). The modes of V together with the unit operator span an
infinite dimensional Lie algebra whose vacuum (i.e. zero energy lowest
weight) representations only depend on the central charge c. Wightman
positivity (i.e. unitarity of the representations of ) is proven to be
equivalent to .Comment: 28 pages, LATEX, amsfonts, latexsym. Proposition 2.3, and Conjecture
in Sec. 6 are revised. Minor errors are correcte
Elliptic Thermal Correlation Functions and Modular Forms in a Globally Conformal Invariant QFT
Global conformal invariance (GCI) of quantum field theory (QFT) in two and
higher space-time dimensions implies the Huygens' principle, and hence,
rationality of correlation functions of observable fields (see Commun. Math.
Phys. 218 (2001) 417-436; hep-th/0009004). The conformal Hamiltonian has
discrete spectrum assumed here to be finitely degenerate. We then prove that
thermal expectation values of field products on compactified Minkowski space
can be represented as finite linear combinations of basic (doubly periodic)
elliptic functions in the conformal time variables (of periods 1 and )
whose coefficients are, in general, formal power series in
involving spherical functions of the "space-like"
fields' arguments. As a corollary, if the resulting expansions converge to
meromorphic functions, then the finite temperature correlation functions are
elliptic. Thermal 2-point functions of free fields are computed and shown to
display these features. We also study modular transformation properties of
Gibbs energy mean values with respect to the (complex) inverse temperature
(). The results are used to obtain the
thermodynamic limit of thermal energy densities and correlation functions.Comment: LaTex. 56 pages. The concept of global conformal invariance set in a
historical perspective (new Sect. 1.1 in the Introduction), references added;
minor corrections in the rest of the pape
A theoretical investigation of ferromagnetic tunnel junctions with 4-valued conductances
In considering a novel function in ferromagnetic tunnel junctions consisting
of ferromagnet(FM)/barrier/FM junctions, we theoretically investigate multiple
valued (or multi-level) cell property, which is in principle realized by
sensing conductances of four states recorded with magnetization configurations
of two FMs; that is, (up,up), (up,down), (down,up), (down,down). To obtain such
4-valued conductances, we propose FM1/spin-polarized barrier/FM2 junctions,
where the FM1 and FM2 are different ferromagnets, and the barrier has spin
dependence. The proposed idea is applied to the case of the barrier having
localized spins. Assuming that all the localized spins are pinned parallel to
magnetization axes of the FM1 and FM2, 4-valued conductances are explicitly
obtained for the case of many localized spins. Furthermore, objectives for an
ideal spin-polarized barrier are discussed.Comment: 9 pages, 3 figures, accepted for publication in J. Phys.: Condens.
Matte
Influence of Magnetism on Phonons in CaFe2As2 Via Inelastic X-ray Scattering
In the iron pnictides, the strong sensitivity of the iron magnetic moment to
the arsenic position suggests a significant relationship between phonons and
magnetism. We measured the phonon dispersion of several branches in the high
temperature tetragonal phase of CaFe2As2 using inelastic x-ray scattering on
single-crystal samples. These measurements were compared to ab initio
calculations of the phonons. Spin polarized calculations imposing the
antiferromagnetic order present in the low temperature orthorhombic phase
dramatically improve agreement between theory and experiment. This is discussed
in terms of the strong antiferromagnetic correlations that are known to persist
in the tetragonal phase.Comment: 4 pages, 3 figures; added additional information and references about
spin fluctuation
Towards Integrability of Topological Strings I: Three-forms on Calabi-Yau manifolds
The precise relation between Kodaira-Spencer path integral and a particular
wave function in seven dimensional quadratic field theory is established. The
special properties of three-forms in 6d, as well as Hitchin's action
functional, play an important role. The latter defines a quantum field theory
similar to Polyakov's formulation of 2d gravity; the curious analogy with
world-sheet action of bosonic string is also pointed out.Comment: 31 page
Genes encoding for intestinal mucus adhesion factors in lactobacillus mucosae strains isolated from brazilian goat milk.
Molecular biology can be outstanding tool in the selection of probiotic candidates, based on the detection of genetic factors related to the beneficial properties of studied lactic acid bacteria (LAB). Surface proteins with mucus-binding capacity mediate the adherence of LAB to the intestine and are desirable feature for probiotic bacteria, as they can increase persistence and ability of LAB to colonize the intestine. In order to investigate mucus adhesion properties, the presence of genes encoding for adhesion proteins were investigated in three Lactobacillus mucosae strains isolated from goat milk.Poster abstract PO2-SB-205
High-bias stability of monatomic chains
For the metals Au, Pt and Ir it is possible to form freely suspended
monatomic chains between bulk electrodes. The atomic chains sustain very large
current densities, but finally fail at high bias. We investigate the breaking
mechanism, that involves current-induced heating of the atomic wires and
electromigration forces. We find good agreement of the observations for Au
based on models due to Todorov and coworkers. The high-bias breaking of atomic
chains for Pt can also be described by the models, although here the parameters
have not been obtained independently. In the limit of long chains the breaking
voltage decreases inversely proportional to the length.Comment: 7 pages, 5 figure
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