323,716 research outputs found
On the Weak Measurement of Velocity in Bohmian Mechanics
No full textIn a recent article (Wiseman in New J. Phys. 9:165, 2007), Wiseman has proposed the use of so-called weak measurements for the determination of the velocity of a quantum particle at a given position, and has shown that according to quantum mechanics the result of such a procedure is the Bohmian velocity of the particle. Although Bohmian mechanics is empirically equivalent to variants based on velocity formulas different from the Bohmian one, and although it has been proven that the velocity in Bohmian mechanics is not measurable, we argue here for the somewhat paradoxical conclusion that Wiseman's weak measurement procedure indeed constitutes a genuine measurement of velocity in Bohmian mechanics. We reconcile the apparent contradictions and elaborate on some of the different senses of measurement at play here. © Springer Science+Business Media, LLC 2008
Band structure of a two-dimensional ferromagnetic antidot lattice
No full textThe spin wave band structure of a two-dimensional square array of NiFe circular
antidots having diameter of 120 nm and periodicity of 800 nm has been investigated by
using Brillouin light scattering technique and micromagnetic calculations based on the
dynamical matrix method [1]. The external magnetic field was applied in the plane and
perpendicularly to the transferred wave vector. Extended and localized spin modes
having a propagative nature were found. Opening of bandgaps is interpreted in terms of
Bragg diffraction of spin waves from the antidot lattice and this effect is explained by
studying the behaviour of the internal field as shown in Fig.1. The mean internal field is
larger along the vertical rows of antidots and smaller between the antidots (see panel (a)
for extended modes and (c) for localized modes). By developing an analytical model
according to which the mean internal field is represented by means of a rectangular step
function characterized by a region 1 corresponding to vertical rows of antidots and a
region 2 between the antidots (see panels (b) and (d)), the relevant scattering potential
for Bragg reflection is not provided by the holes themselves, but by the concomitant
internal field inhomogeneity between holes [2]. This is in contrast to antidots in
photonics and electronics where the back-reflection is directly caused by the presence of
holes. The research leading to these results has received funding from the European
Community's Seventh Framework Programme (FP7/2007-2013) under Grant
Agreement n228673 (MAGNONICS).
[1] L. Giovannini, F. Montoncello, and F. Nizzoli, Phys. Rev. B 75, 024416 (2007).
[2] R. Zivieri, S. Tacchi, F. Montoncello, L. Giovannini, F. Nizzoli, M. Madami, G.
Gubbiotti, G. Carlotti, S. Neusser, G. Duerr, and D. Grundler, Phys. Rev. B 83, (2012)
Quantum Hamiltonians and Stochastic Jumps
No full textWith many Hamiltonians one can naturally associate a psi(2)-distributed Markov process. For nonrelativistic quantum mechanics, this process is in fact deterministic, and is known as Bohmian mechanics. For the Hamiltonian of a quantum field theory, it is typically a jump process on the configuration space of a variable number of particles. We define these processes for regularized quantum field theories, thereby generalizing previous work of John S. Bell [3] and of ourselves [11]. We introduce a formula expressing the jump rates in terms of the interaction Hamiltonian, and establish a condition for finiteness of the rates
On the Global Existence of Bohmian Mechanics
No full textWe show that the particle motion in Bohmian mechanics, given by the solution of an ordinary differential equation, exists globally: For a large class of potentials the singularities of the velocity field and infinity will not be reached in finite time for typical initial values. A substantial part of the analysis is based on the probabilistic significance of the quantum flux. We elucidate the connection between the conditions necessary for global existence and the self-adjointness of the Schrodinger Hamiltonian
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