26,862 research outputs found
Accurate calculation of resonances in multiple-well oscillators
Quantum--mechanical multiple--well oscillators exhibit curious complex
eigenvalues that resemble resonances in models with continuum spectra. We
discuss a method for the accurate calculation of their real and imaginary
parts
Monte Carlo study of the spin-glass phase of the site-diluted dipolar Ising model
By tempered Monte Carlo simulations, we study site-diluted Ising systems of
magnetic dipoles. All dipoles are randomly placed on a fraction x of all L^3
sites of a simple cubic lattice, and point along a given crystalline axis. For
x_c< x<=1, where x_c = 0.65, we find an antiferromagnetic phase below a
temperature which vanishes as x tends to x_c from above. At lower values of x,
we find an equilibrium spin-glass (SG) phase below a temperature given by k_B
T_{sg} = x e_d, where e_d is a nearest neighbor dipole-dipole interaction
energy. We study (a) the relative mean square deviation D_q^2 of |q|, where q
is the SG overlap parameter, and (b) xi_L/L, where xi_L is a correlation
length. From their variation with temperature and system size, we determine
T_{sg}. In the SG phase, we find (i) the mean values and decrease
algebraically with L as L increases, (ii) double peaked, but wide,
distributions of q/ appear to be independent of L, and (iii) xi_L/L rises
with L at constant T, but extrapolations to 1/L -> 0 give finite values. All of
this is consistent with quasi-long-range order in the SG phase.Comment: 15 LaTeX pages, 15 figures, 3 tables. (typos fixed in Appendix A
Cotunneling theory of inelastic STM spin spectroscopy
We propose cotunneling as the microscopic mechanism that makes possible
inelastic electron spectroscopy of magnetic atoms in surfaces for a wide range
of systems, including single magnetic adatoms, molecules and molecular stacks.
We describe electronic transport between the scanning tip and the conducting
surface through the magnetic system (MS) with a generalized Anderson model,
without making use of effective spin models. Transport and spin dynamics are
described with an effective cotunneling Hamiltonian in which the correlations
in the magnetic system are calculated exactly and the coupling to the
electrodes is included up to second order in the tip-MS and MS-substrate. In
the adequate limit our approach is equivalent to the phenomenological Kondo
exchange model that successfully describe the experiments . We apply our method
to study in detail inelastic transport in two systems, stacks of Cobalt
Phthalocyanines and a single Mn atom on CuN. Our method accounts both, for
the large contribution of the inelastic spin exchange events to the conductance
and the observed conductance asymmetry.Comment: 12 pages, 6 figure
Reply to Comment on "Magnetization Process of Single Molecule Magnets at Low Temperatures"
This is the reply to a Comment by I.S.Tupitsyn and P.C.E. Stamp (PRL
v92,119701 (2004)) on a letter of ours (J.F.Fernandez and J.J.Alonso, PRL v91,
047202 (2003)).Comment: 2 LaTeX pages, 1 eps figure. Submitted to PRL on 20 October 200
Understanding open-charm mesons
We present a theoretical framework that accounts for the new and
mesons measured in the open-charm sector. These resonances are
properly described if considered as a mixture of conventional wave
quark-antiquark states and four-quark components. The narrowest states are
basically wave quark-antiquark mesons, while the dominantly four-quark
states are shifted above the corresponding two-meson threshold. We study the
electromagnetic decay widths as basic tools to scrutiny their nature.Comment: 4 pages, to appear in the proceedings of 9th International Workshop
on Meson Production, Properties and Interaction (Meson2006), Kracow (Poland),
9 - 13 June 200
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