203 research outputs found
Flux-vector model of spin noise in superconducting circuits: Electron versus nuclear spins and role of phase transition
Superconducting Quantum Interference Devices (SQUIDs) and other
superconducting circuits are limited by intrinsic flux noise with spectral
density with whose origin is believed to be due to
spin impurities. Here we present a theory of flux noise that takes into account
the vectorial nature of the coupling of spins to superconducting wires. We
present explicit numerical calculations of the flux noise power (spectral
density integrated over all frequencies) for electron impurities and lattice
nuclear spins under several different assumptions. The noise power is shown to
be dominated by surface electron spins near the wire edges, with bulk lattice
nuclear spins contributing % of the noise power in aluminum and niobium
wires. We consider the role of electron spin phase transitions, showing that
the spin-spin correlation length (describing e.g. the average size of
ferromagnetic spin clusters) greatly impacts the scaling of flux noise with
wire geometry. Remarkably, flux noise power is exactly equal to zero when the
spins are polarized along the flux vector direction, forming what we call a
poloidal state. Flux noise is non-zero for other spin textures, but gets
reduced in the presence of correlated ferromagnetic fluctuations between the
top and bottom wire surfaces, where the flux vectors are antiparallel. This
demonstrates that engineering spin textures and/or inter-surface correlation
provides a method to reduce flux noise in superconducting devices.Comment: New version accepted in PRB. Contains new discussion about the
poloidal stat
Effect of an inhomogeneous external magnetic field on a quantum dot quantum computer
We calculate the effect of an inhomogeneous magnetic field, which is
invariably present in an experimental environment, on the exchange energy of a
double quantum dot artificial molecule, projected to be used as a 2-qubit
quantum gate in the proposed quantum dot quantum computer. We use two different
theoretical methods to calculate the Hilbert space structure in the presence of
the inhomogeneous field: the Heitler-London method which is carried out
analytically and the molecular orbital method which is done computationally.
Within these approximations we show that the exchange energy J changes slowly
when the coupled dots are subject to a magnetic field with a wide range of
inhomogeneity, suggesting swap operations can be performed in such an
environment as long as quantum error correction is applied to account for the
Zeeman term. We also point out the quantum interference nature of this slow
variation in exchange.Comment: 12 pages, 4 figures embedded in tex
Dangling-bond spin relaxation and magnetic 1/f noise from the amorphous-semiconductor/oxide interface: Theory
We propose a model for magnetic noise based on spin-flips (not
electron-trapping) of paramagnetic dangling-bonds at the
amorphous-semiconductor/oxide interface. A wide distribution of spin-flip times
is derived from the single-phonon cross-relaxation mechanism for a
dangling-bond interacting with the tunneling two-level systems of the amorphous
interface. The temperature and frequency dependence is sensitive to three
energy scales: The dangling-bond spin Zeeman energy delta, as well as the
minimum (E_min) and maximum (E_max) values for the energy splittings of the
tunneling two-level systems. We compare and fit our model parameters to a
recent experiment probing spin coherence of antimony donors implanted in
nuclear-spin-free silicon [T. Schenkel {\it et al.}, Appl. Phys. Lett. 88,
112101 (2006)], and conclude that a dangling-bond area density of the order of
10^{14}cm^{-2} is consistent with the data. This enables the prediction of
single spin qubit coherence times as a function of the distance from the
interface and the dangling-bond area density in a real device structure. We
apply our theory to calculations of magnetic flux noise affecting SQUID devices
due to their Si/SiO_2 substrate. Our explicit estimates of flux noise in SQUIDs
lead to a noise spectral density of the order of 10^{-12}Phi_{0}^{2} {Hz}^{-1}
at f=1Hz. This value might explain the origin of flux noise in some SQUID
devices. Finally, we consider the suppression of these effects using surface
passivation with hydrogen, and the residual nuclear-spin noise resulting from a
perfect silicon-hydride surface.Comment: Final published versio
Electrical control of magnon propagation in multiferroic BiFeO3 films
The spin wave spectra of multiferroic BiFeO3 films is calculated using a
phenomenological Landau theory that includes magnetostatic effects. The lowest
frequency magnon dispersion is shown to be quite sensitive to the angle between
spin wave propagation vector and the Neel moment. Since electrical switching of
the Neel moment has recently been demonstrated in this material, the
sensitivity of the magnon dispersion permits direct electrical switching of
spin wave propagation. This effect can be used to construct spin wave logical
gates without current pulses, potentially allowing reduced power dissipation
per logical operation
Comment on "Ferroelectrically Induced Weak Ferromagnetism by Design", C. Fennie, PRL 100, 167203 (2008)
The question of how ferroelectric polarization is coupled to magnetism in
magnetoelectric multiferroics, in which both types of order are simultaneously
present, is of considerable scientific and practical interest. A recent Letter
\cite{fennie} presents an analysis of the important ``ABO'' class of
perovskite multiferroics. This Letter argues that antiferromagnetic
multiferroics with magnetic ions on the B site, such as the well-studied
room-temperature multiferroic bismuth ferrite (A=Bi, B=Fe), cannot show linear
magnetoelectric coupling of the form .
Here is polarization and and are
antiferromagnetic and ferromagnetic moments. The conclusion of Ref.
\onlinecite{fennie} is that only materials with magnetic A-site have this
coupling. This Comment presents a compact analysis of magnetoelectric coupling
in the ABO multiferroics. We show that the argument of Ref.
\onlinecite{fennie} does forbid if the final low-symmetry phase
contains only one distortion that, like , breaks all inversion
symmetries. In reality, there are multiple distortions in this symmetry class,
and cross-terms generate . Our analysis gives simple conclusions about
existence and optimization of magnetoelectric coupling in ABO materials.Comment: Published version (4 pages, including supplementary information
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