7 research outputs found
Exploiting environmental resonances to enhance qubit quality factors
We discuss dephasing times for a two-level system (including bias) coupled to
a damped harmonic oscillator. This system is realized in measurements on
solid-state Josephson qubits. It can be mapped to a spin-boson model with a
spectral function with an approximately Lorentzian resonance. We diagonalize
the model by means of infinitesimal unitary transformations (flow equations),
and calculate correlation functions, dephasing rates, and qubit quality
factors. We find that these depend strongly on the environmental resonance
frequency ; in particular, quality factors can be enhanced
significantly by tuning to lie below the qubit frequency .Comment: 5 psges, 5 figure
Nonequilibrium excitations in Ferromagnetic Nanoparticles
In recent measurements of tunneling transport through individual
ferromagnetic Co nanograins, Deshmukh, Gu\'eron, Ralph et al.
\cite{mandar,gueron} (DGR) observed a tunneling spectrum with discrete
resonances, whose spacing was much smaller than what one would expect from
naive independent-electron estimates. In a previous publication,
\cite{prl_kleff} we had suggested that this was a consequence of nonequilibrium
excitations, and had proposed a ``minimal model'' for ferromagnetism in
nanograins with a discrete excitation spectrum as a framework for analyzing the
experimental data. In the present paper, we provide a detailed analysis of the
properties of this model: We delineate which many-body electron states must be
considered when constructing the tunneling spectrum, discuss various
nonequilibrium scenarios and compare their results with the experimental data
of Refs. \cite{mandar,gueron}. We show that a combination of nonequilibrium
spin- and single-particle excitations can account for most of the observed
features, in particular the abundance of resonances, the resonance spacing and
the absence of Zeeman splitting.Comment: 13 pages, 10 figure
A Model for Ferromagnetic Nanograins with Discrete Electronic States
We propose a simple phenomenological model for an ultrasmall ferromagnetic
grain, formulated in terms of the grain's discrete energy levels. We compare
the model's predictions with recent measurements of the discrete tunneling
spectrum through such a grain. The model can qualitatively account for the
observed features if we assume (i) that the anisotropy energy varies among
different eigenstates of one grain, and (ii) that nonequilibrium spin
accumulation occurs.Comment: 4 pages, 2 figure
Flow equation renormalization of a spin-boson model with a structured bath
We discuss the dynamics of a spin coupled to a damped harmonic oscillator.
This system can be mapped to a spin-boson model with a structured bath, i.e.
the spectral function of the bath has a resonance peak. We diagonalize the
model by means of infinitesimal unitary transformations (flow equations),
thereby decoupling the small quantum system from its environment, and calculate
spin-spin correlation functions.Comment: 2 pages, to be published in Physica E, proceedings of the LT 200