465 research outputs found
Linear scaling between momentum and spin scattering in graphene
Spin transport in graphene carries the potential of a long spin diffusion
length at room temperature. However, extrinsic relaxation processes limit the
current experimental values to 1-2 um. We present Hanle spin precession
measurements in gated lateral spin valve devices in the low to high (up to
10^13 cm^-2) carrier density range of graphene. A linear scaling between the
spin diffusion length and the diffusion coefficient is observed. We measure
nearly identical spin- and charge diffusion coefficients indicating that
electron-electron interactions are relatively weak and transport is limited by
impurity potential scattering. When extrapolated to the maximum carrier
mobilities of 2x10^5 cm^2/Vs, our results predict that a considerable increase
in the spin diffusion length should be possible
Development of a serological Luminex assay for Trichinella and Salmonella in swine
In order to develop veterinary serological multiplex assays, a singleplex bead-based array on the Luminex platform was developed, and with this experience was expanded by building a multiplex serological assay. First a serological Luminex assay was developed for Trichinella in swine
A Pathwise Ergodic Theorem for Quantum Trajectories
If the time evolution of an open quantum system approaches equilibrium in the
time mean, then on any single trajectory of any of its unravelings the time
averaged state approaches the same equilibrium state with probability 1. In the
case of multiple equilibrium states the quantum trajectory converges in the
mean to a random choice from these states.Comment: 8 page
Quantum Trajectory Approach to the Stochastic Thermodynamics of a Forced Harmonic Oscillator
I formulate a quantum stochastic thermodynamics for the quantum trajectories
of a continuously-monitored forced harmonic oscillator coupled to a thermal
reservoir. Consistent trajectory-dependent definitions are introduced for work,
heat, and entropy, through engineering the thermal reservoir from a sequence of
two-level systems. Within this formalism the connection between irreversibility
and entropy production is analyzed and confirmed by proving a detailed
fluctuation theorem for quantum trajectories. Finally, possible experimental
verifications are discussed.Comment: 16 pages, 3 figures, submitted to PRE; expanded introduction and
conclusion, corrected typos, new figure
Hamiltonian and Linear-Space Structure for Damped Oscillators: I. General Theory
The phase space of damped linear oscillators is endowed with a bilinear
map under which the evolution operator is symmetric. This analog of
self-adjointness allows properties familiar from conservative systems to be
recovered, e.g., eigenvectors are "orthogonal" under the bilinear map and obey
sum rules, initial-value problems are readily solved and perturbation theory
applies to the_complex_ eigenvalues. These concepts are conveniently
represented in a biorthogonal basis.Comment: REVTeX4, 10pp., 1 PS figure. N.B.: `Alec' is my first name, `Maassen
van den Brink' my family name. v2: extensive streamlinin
Semigroup evolution in Wigner Weisskopf pole approximation with Markovian spectral coupling
We establish the relation between the Wigner-Weisskopf theory for the
description of an unstable system and the theory of coupling to an environment.
According to the Wigner-Weisskopf general approach, even within the pole
approximation (neglecting the background contribution) the evolution of a total
system subspace is not an exact semigroup for the multi-channel decay, unless
the projectors into eigesntates of the reduced evolution generator are
orthogonal. In this case these projectors must be evaluated at different pole
locations . Since the orthogonality relation does not
generally hold at different values of , for example, when there is symmetry
breaking, the semigroup evolution is a poor approximation for the multi-channel
decay, even for a very weak coupling. Nevertheless, there exists a possibility
not only to ensure the orthogonality of the projectors regardless the
number of the poles, but also to simultaneously suppress the effect of the
background contribution. This possibility arises when the theory is generalized
to take into account interactions with an environment. In this case , and
hence its eigenvectors as well, are {\it independent} of , which corresponds
to a structure of the coupling to the continuum spectrum associated with the
Markovian limit.Comment: 9 pages, 3 figure
Spin dependent quantum interference in non-local graphene spin valves
Spin dependent electron transport measurements on graphene are of high
importance to explore possible spintronic applications. Up to date all spin
transport experiments on graphene were done in a semi-classical regime,
disregarding quantum transport properties such as phase coherence and
interference. Here we show that in a quantum coherent graphene nanostructure
the non-local voltage is strongly modulated. Using non-local measurements, we
separate the signal in spin dependent and spin independent contributions. We
show that the spin dependent contribution is about two orders of magnitude
larger than the spin independent one, when corrected for the finite
polarization of the electrodes. The non-local spin signal is not only strongly
modulated but also changes polarity as a function of the applied gate voltage.
By locally tuning the carrier density in the constriction we show that the
constriction plays a major role in this effect and indicates that it can act as
a spin filter device. Our results show the potential of quantum coherent
graphene nanostructures for the use in future spintronic devices
Eigenvector Expansion and Petermann Factor for Ohmically Damped Oscillators
Correlation functions in ohmically damped
systems such as coupled harmonic oscillators or optical resonators can be
expressed as a single sum over modes (which are not power-orthogonal), with
each term multiplied by the Petermann factor (PF) , leading to "excess
noise" when . It is shown that is common rather than
exceptional, that can be large even for weak damping, and that the PF
appears in other processes as well: for example, a time-independent
perturbation \sim\ep leads to a frequency shift \sim \ep C_j. The
coalescence of () eigenvectors gives rise to a critical point, which
exhibits "giant excess noise" (). At critical points, the
divergent parts of contributions to cancel, while time-independent
perturbations lead to non-analytic shifts \sim \ep^{1/J}.Comment: REVTeX4, 14 pages, 4 figures. v2: final, 20 single-col. pages, 2
figures. Streamlined with emphasis on physics over formalism; rewrote Section
V E so that it refers to time-dependent (instead of non-equilibrium) effect
Electronic Spin Transport in Dual-Gated Bilayer Graphene
The elimination of extrinsic sources of spin relaxation is key in realizing
the exceptional intrinsic spin transport performance of graphene. Towards this,
we study charge and spin transport in bilayer graphene-based spin valve devices
fabricated in a new device architecture which allows us to make a comparative
study by separately investigating the roles of substrate and polymer residues
on spin relaxation. First, the comparison between spin valves fabricated on
SiO2 and BN substrates suggests that substrate-related charged impurities,
phonons and roughness do not limit the spin transport in current devices. Next,
the observation of a 5-fold enhancement in spin relaxation time in the
encapsulated device highlights the significance of polymer residues on spin
relaxation. We observe a spin relaxation length of ~ 10 um in the encapsulated
bilayer with a charge mobility of 24000 cm2/Vs. The carrier density dependence
of spin relaxation time has two distinct regimes; n<4 x 1012 cm-2, where spin
relaxation time decreases monotonically as carrier concentration increases, and
n>4 x 1012 cm-2, where spin relaxation time exhibits a sudden increase. The
sudden increase in the spin relaxation time with no corresponding signature in
the charge transport suggests the presence of a magnetic resonance close to the
charge neutrality point. We also demonstrate, for the first time, spin
transport across bipolar p-n junctions in our dual-gated device architecture
that fully integrates a sequence of encapsulated regions in its design. At low
temperatures, strong suppression of the spin signal was observed while a
transport gap was induced, which is interpreted as a novel manifestation of
impedance mismatch within the spin channel
Sign- and magnitude-tunable coupler for superconducting flux qubits
We experimentally confirm the functionality of a coupling element for
flux-based superconducting qubits, with a coupling strength whose sign and
magnitude can be tuned {\it in situ}. To measure the effective , the
groundstate of a coupled two-qubit system has been mapped as a function of the
local magnetic fields applied to each qubit. The state of the system is
determined by directly reading out the individual qubits while tunneling is
suppressed. These measurements demonstrate that can be tuned from
antiferromagnetic through zero to ferromagnetic.Comment: Updated text and figure
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