1,223 research outputs found
A geometry for optimizing nanoscale magnetic resonance force microscopy
We implement magnetic resonance force microscopy (MRFM) in an experimental
geometry, where the long axis of the cantilever is normal to both the external
magnetic field and the RF microwire source. Measurements are made of the
statistical polarization of H in polystyrene with negligible magnetic
dissipation, gradients greater than T/m within 100 nm of the magnetic
tip, and rotating RF magnetic fields over 12 mT at 115 MHz. This geometry could
facilitate the application of nanometer-scale MRFM to nuclear species with low
gyro-magnetic ratios and samples with broadened resonances, such as In spins in
quantum dots.Comment: 4 pages, 5 figure
Boundary between the thermal and statistical polarization regimes in a nuclear spin ensemble
As the number of spins in an ensemble is reduced, the statistical uctuations
in its polarization eventually exceed the mean thermal polarization. This
transition has now been surpassed in a number of recent nuclear magnetic
resonance experiments, which achieve nanometer-scale detection volumes. Here,
we measure nanometer- scale ensembles of nuclear spins in a KPF6 sample using
magnetic resonance force microscopy. In particular, we investigate the
transition between regimes dominated by thermal and statistical nuclear
polarization. The ratio between the two types of polarization provides a
measure of the number of spins in the detected ensemble
Antiferromagnetic s-d exchange coupling in GaMnAs
Measurements of coherent electron spin dynamics in
Ga(1-x)Mn(x)As/Al(0.4)Ga(0.6)As quantum wells with 0.0006% < x < 0.03% show an
antiferromagnetic (negative) exchange bewteen s-like conduction band electrons
and electrons localized in the d-shell of the Mn2+ impurities. The magnitude of
the s-d exchange parameter, N0 alpha, varies as a function of well width
indicative of a large and negative contribution due to kinetic exchange. In the
limit of no quantum confinement, N0 alpha extrapolates to -0.09 +/- 0.03 eV
indicating that antiferromagnetic s-d exchange is a bulk property of GaMnAs.
Measurements of the polarization-resolved photoluminescence show strong
discrepancy from a simple model of the exchange enhanced Zeeman splitting,
indicative of additional complexity in the exchange split valence band.Comment: 5 pages, 4 figures and one action figur
Spin dynamics in electrochemically charged CdSe quantum dots
We use time-resolved Faraday rotation to measure coherent spin dynamics in
colloidal CdSe quantum dots charged in an electrochemical cell at room
temperature. Filling of the 1Se electron level is demonstrated by the bleaching
of the 1Se-1S3/2 absorption peak. One of the two Lande g-factors observed in
uncharged quantum dots disappears upon filling of the 1Se electron state. The
transverse spin coherence time, which is over 1 ns and is limited by
inhomogeneous dephasing, also appears to increase with charging voltage. The
amplitude of the spin precession signal peaks near the half-filling potential.
Its evolution at charging potentials without any observable bleaching of the
1Se-1S3/2 transition suggests that the spin dynamics are influenced by
low-energy surface states.Comment: 4 pages, 4 figure
Force-detected nuclear double resonance between statistical spin polarizations
We demonstrate nuclear double resonance for nanometer-scale volumes of spins
where random fluctuations rather than Boltzmann polarization dominate. When the
Hartmann-Hahn condition is met in a cross-polarization experiment, flip-flops
occur between two species of spins and their fluctuations become coupled. We
use magnetic resonance force microscopy to measure this effect between 1H and
13C spins in 13C-enriched stearic acid. The development of a cross-polarization
technique for statistical ensembles adds an important tool for generating
chemical contrast in nanometer-scale magnetic resonance.Comment: 14 pages, 4 figure
Nuclear spin relaxation induced by a mechanical resonator
We report on measurements of the spin lifetime of nuclear spins strongly
coupled to a micromechanical cantilever as used in magnetic resonance force
microscopy. We find that the rotating-frame correlation time of the statistical
nuclear polarization is set by the magneto-mechanical noise originating from
the thermal motion of the cantilever. Evidence is based on the effect of three
parameters: (1) the magnetic field gradient (the coupling strength), (2) the
Rabi frequency of the spins (the transition energy), and (3) the temperature of
the low-frequency mechanical modes. Experimental results are compared to
relaxation rates calculated from the spectral density of the magneto-mechanical
noise.Comment: 4 pages, 4 figure
Force-detected nuclear magnetic resonance: Recent advances and future challenges
We review recent efforts to detect small numbers of nuclear spins using
magnetic resonance force microscopy. Magnetic resonance force microscopy (MRFM)
is a scanning probe technique that relies on the mechanical measurement of the
weak magnetic force between a microscopic magnet and the magnetic moments in a
sample. Spurred by the recent progress in fabricating ultrasensitive force
detectors, MRFM has rapidly improved its capability over the last decade. Today
it boasts a spin sensitivity that surpasses conventional, inductive nuclear
magnetic resonance detectors by about eight orders of magnitude. In this review
we touch on the origins of this technique and focus on its recent application
to nanoscale nuclear spin ensembles, in particular on the imaging of nanoscale
objects with a three-dimensional (3D) spatial resolution better than 10 nm. We
consider the experimental advances driving this work and highlight the
underlying physical principles and limitations of the method. Finally, we
discuss the challenges that must be met in order to advance the technique
towards single nuclear spin sensitivity -- and perhaps -- to 3D microscopy of
molecules with atomic resolution.Comment: 15 pages & 11 figure
Measurement of statistical nuclear spin polarization in a nanoscale GaAs sample
We measure the statistical polarization of quadrupolar nuclear spins in a
sub-micrometer (0.6 um^3) particle of GaAs using magnetic resonance force
microscopy. The crystalline sample is cut out of a GaAs wafer and attached to a
micro-mechanical cantilever force sensor using a focused ion beam technique.
Nuclear magnetic resonance is demonstrated on ensembles containing less than 5
x 10^8 nuclear spins and occupying a volume of around (300 nm)^3 in GaAs with
reduced volumes possible in future experiments. We discuss how the further
reduction of this detection volume will bring the spin ensemble into a regime
where random spin fluctuations, rather than Boltzmann polarization, dominate
its dynamics. The detection of statistical polarization in GaAs therefore
represents an important first step toward 3D magnetic resonance imaging of
III-V materials on the nanometer-scale.Comment: 20 pages, 6 figures, 1 supplementary fil
Quantum information storage and state transfer based on spin systems
The idea of quantum state storage is generalized to describe the coherent
transfer of quantum information through a coherent data bus. In this universal
framework, we comprehensively review our recent systematical investigations to
explore the possibility of implementing the physical processes of quantum
information storage and state transfer by using quantum spin systems, which may
be an isotropic antiferromagnetic spin ladder system or a ferromagnetic
Heisenberg spin chain. Our studies emphasize the physical mechanisms and the
fundamental problems behind the various protocols for the storage and transfer
of quantum information in solid state systems.Comment: 11 pages, 9 figures, Review article on the quantum spin based quantum
information processing, to appear the special issue of Low Temperature
Physics dedicated to the 70-th anniversary of creation of concept
"antiferromagnetism" in physics of magnetis
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