10 research outputs found
Quality of Variational Trial States
Besides perturbation theory (which clearly requires the knowledge of the
exact unperturbed solution), variational techniques represent the main tool for
any investigation of the eigenvalue problem of some semibounded operator H in
quantum theory. For a reasonable choice of the employed trial subspace of the
domain of H, the lowest eigenvalues of H usually can be located with acceptable
precision whereas the trial-subspace vectors corresponding to these eigenvalues
approximate, in general, the exact eigenstates of H with much less accuracy.
Accordingly, various measures for the accuracy of the approximate eigenstates
derived by variational techniques are scrutinized. In particular, the matrix
elements of the commutator of the operator H and (suitably chosen) different
operators with respect to degenerate approximate eigenstates of H obtained by
variational methods are proposed as new criteria for the accuracy of
variational eigenstates. These considerations are applied to precisely that
Hamiltonian for which the eigenvalue problem defines the well-known spinless
Salpeter equation. This bound-state wave equation may be regarded as (the most
straightforward) relativistic generalization of the usual nonrelativistic
Schroedinger formalism, and is frequently used to describe, e.g., spin-averaged
mass spectra of bound states of quarks.Comment: LaTeX, 7 pages, version to appear in Physical Review
Time ordering and counting statistics
The basic quantum mechanical relation between fluctuations of transported
charge and current correlators is discussed. It is found that, as a rule, the
correlators are to be time-ordered in an unusual way. Instances where the
difference with the conventional ordering matters are illustrated by means of a
simple scattering model. We apply the results to resolve a discrepancy
concerning the third cumulant of charge transport across a quantum point
contact.Comment: 19 pages, 1 figure; inconsequential mistake and typos correcte
Semi-Relativistic Hamiltonians of Apparently Nonrelativistic Form
We construct effective Hamiltonians which despite their apparently
nonrelativistic form incorporate relativistic effects by involving parameters
which depend on the relevant momentum. For some potentials the corresponding
energy eigenvalues may be determined analytically. Applied to two-particle
bound states, it turns out that in this way a nonrelativistic treatment may
indeed be able to simulate relativistic effects. Within the framework of hadron
spectroscopy, this lucky circumstance may be an explanation for the sometimes
extremely good predictions of nonrelativistic potential models even in
relativistic regions.Comment: 20 pages, LaTeX, no figure