arXiv subjects

Julio F. Fernandez

Publications and source records attributed to Julio F. Fernandez.

3 recordsLinked to original sources

arXiv2010

Evidence against an Almeida-Thouless line in disordered systems of Ising dipoles

By tempered Monte Carlo simulations, an Almeida-Thouless (AT) phase-boundary line in site-diluted Ising spin systems is searched for. Spins interact only through dipolar fields and occupy a small fraction of lattice sites. The spin-glass susceptibility of these systems and of the Sherrington-Kirkpatrick model are compared. The correlation length as a function of system size and temperature is also studied. The results obtained are contrary to the existence of an AT line.

cond-mat.stat-mech↗

arXiv2003

Reply to ``Comment on `Tunnel Window's Imprint on Dipolar Field Distributions' ''

Stamp and Tupitsyn (ST) have recently posted a Comment (P.C.E. Stamp and I.S. Tupitsyn, arXiv:cond-mat/0211413) on a Letter of ours (J.J. Alonso and J.F. Fernandez, Phys. Rev. Lett. v87, 097205 (2001)). They claim that, in our Letter, ``Short-time relaxation was caused by a simple noise field, acting uniformly over an `energy window' of width 2*delta h_{hf} (to simulate the fluctuating nuclear spin bias)''. We see no basis for this statement.

cond-mat↗

arXiv2001

Coherence in magnetic quantum tunneling

Currently, spin tunneling at very low temperatures is assumed to proceed as an incoherent sequence of events that take place whenever a bias field h(t) that varies randomly with time t becomes sufficiently small, as in Landau-Zener transitions. We study the behavior of a suitably defined coherence time t_p. Coherence effects become significant when t_p >= t_h$, where t_h is the correlation time for h(t). The theory of tunneling of Prokof'ev and Stamp (PS), which rests on the assumption that t_p <= t_h, is extended beyond this constraint. It is shown, both analytically and numerically, that t_p >= t_h when t_h*dh <= h, where dh is the rms deviation of h. Equations that give t_p and the tunneling rate as a function of t_h*dh both for t_h*dh>=h, where the theory of PS hold, and for t_h*dh <= h, where it does not, are derived.

cond-mat.mes-hall↗