234 research outputs found
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
Many-spin effects in inelastic neutron scattering and electron paramagnetic resonance of molecular nanomagnets
Many molecular magnetic clusters, such as single-molecule magnets, are
characterized by spin ground states with defined total spin S exhibiting
zero-field-splittings. In this work, the spectroscopic intensities of the
transitions within the ground-state multiplet are analyzed. In particular, the
effects of a mixing with higher-lying spin multiplets, which is produced by
anisotropic interactions and is neglected in the standard single-spin
description, are investigated systematically for the two experimental
techniques of inelastic neutron scattering (INS) and electron paramagnetic
resonance (EPR), with emphasis on the former technique. The spectroscopic
transition intensities are calculated analytically by constructing
corresponding effective spin operators perturbationally up to second order and
consequently using irreducible tensor operator techniques. Three main effects
of spin mixing are observed. Firstly, a pronounced dependence of the INS
intensities on the momentum transfer Q, with a typical oscillatory behavior,
emerges in first order, signaling the many-spin nature of the wave functions in
exchange-coupled clusters. Secondly, as compared to the results of a
first-order calculation, the intensities of the transitions within the spin
multiplet are affected differently by spin mixing. This allows one, thirdly, to
differentiate the higher-order contributions to the cluster magnetic anisotropy
which come from the single-ion ligand-field terms and spin mixing,
respectively. The analytical results are illustrated by means of the three
examples of an antiferromagnetic heteronuclear dimer, the Mn-[3 x 3] grid
molecule, and the single-molecule magnet Mn12.Comment: 18 pages, 3 figures, REVTEX4, to appear in PR
Decoherence times of universal two-qubit gates in the presence of broad-band noise
The controlled generation of entangled states of two quantum bits is a
fundamental step toward the implementation of a quantum information processor.
In nano-devices this operation is counteracted by the solid-state environment,
characterized by a broadband and non-monotonic power spectrum, often 1/f at low
frequencies. For single-qubit gates, incoherent processes due to fluctuations
acting on different time scales result in peculiar short- and long-time
behavior. Markovian noise gives rise to exponential decay with relaxation and
decoherence times, T1 and T2, simply related to the symmetry of the
qubit-environment coupling Hamiltonian. Noise with the 1/f power spectrum at
low frequencies is instead responsible for defocusing processes and algebraic
short-time behavior. In this paper, we identify the relevant decoherence times
of an entangling operation due to the different decoherence channels
originating from solid-state noise. Entanglement is quantified by concurrence,
which we evaluate in an analytic form employing a multi-stage approach. The
'optimal' operating conditions of reduced sensitivity to noise sources are
identified. We apply this analysis to a superconducting \sqrt{i-SWAP} gate for
experimental noise spectra.Comment: 35 pages, 11 figure
Itinerancy and Hidden Order in
We argue that key characteristics of the enigmatic transition at in indicate that the hidden order is a density wave formed within
a band of composite quasiparticles, whose detailed structure is determined by
local physics. We expand on our proposal (with J.A. Mydosh) of the hidden order
as incommnesurate orbital antiferromagnetism and present experimental
predictions to test our ideas. We then turn towards a microscopic description
of orbital antiferromagnetism, exploring possible particle-hole pairings within
the context of a simple one-band model. We end with a discussion of recent
high-field and thermal transport experiment, and discuss their implications for
the nature of the hidden order.Comment: 18 pages, 7 figures. v2 contains added referenc
Magnetic string contribution to hadron dynamics in QCD
Dynamics of a light quark in the field of static source (heavy-light meson)
is studied using the nonlinear Dirac equation, derived recently. Special
attention is paid to the contribution of magnetic correlators and it is found
that it yields a significant increase of string tension at intermediate
distances. The spectrum of heavy-light mesons is computed with account of this
contribution and compared to experimental and lattice data.Comment: 10 pages Revte
Hidden Orbital Order in
When matter is cooled from high temperatures, collective instabilities
develop amongst its constituent particles that lead to new kinds of order. An
anomaly in the specific heat is a classic signature of this phenomenon. Usually
the associated order is easily identified, but sometimes its nature remains
elusive. The heavy fermion metal is one such example, where the
order responsible for the sharp specific heat anomaly at has
remained unidentified despite more than seventeen years of effort. In
, the coexistence of large electron-electron repulsion and
antiferromagnetic fluctuations in leads to an almost incompressible
heavy electron fluid, where anisotropically paired quasiparticle states are
energetically favored. In this paper we use these insights to develop a
detailed proposal for the hidden order in . We show that
incommensurate orbital antiferromagnetism, associated with circulating currents
between the uranium ions, can account for the local fields and entropy loss
observed at the transition; furthermore we make detailed predictions for
neutron scattering measurements
A microscopic semiclassical confining field equation for lattice gauge theory in 2+1 dimensions
We present a semiclassical nonlinear field equation for the confining field
in 2+1--dimensional lattice gauge theory (compact QED). The equation is
derived directly from the underlying microscopic quantum Hamiltonian by means
of truncation. Its nonlinearities express the dynamic creation of magnetic
monopole currents leading to the confinement of the electric field between two
static electric charges. We solve the equation numerically and show that it can
be interpreted as a London relation in a dual superconductor.Comment: 21 pages, epsf postscript figures included, full postscript available
at ftp://ftp.th.physik.uni-frankfurt.de/pub/cbest/micro.ps.Z or
http://www.th.physik.uni-frankfurt.de/~cbest/pub.htm
Superconducting qubit manipulated by fast pulses: experimental observation of distinct decoherence regimes
A particular superconducting quantum interference device (SQUID)qubit,
indicated as double SQUID qubit, can be manipulated by rapidly modifying its
potential with the application of fast flux pulses. In this system we observe
coherent oscillations exhibiting non-exponential decay, indicating a non
trivial decoherence mechanism. Moreover, by tuning the qubit in different
conditions (different oscillation frequencies) by changing the pulse height, we
observe a crossover between two distinct decoherence regimes and the existence
of an "optimal" point where the qubit is only weakly sensitive to intrinsic
noise. We find that this behaviour is in agreement with a model considering the
decoherence caused essentially by low frequency noise contributions, and
discuss the experimental results and possible issues.Comment: 16 pages, 9 figure
A Modified View on Octocorals: Heteroxenia fuscescens Nematocysts Are Diverse, Featuring Both an Ancestral and a Novel Type
Cnidarians are characterized by the presence of stinging cells containing nematocysts, a sophisticated injection system targeted mainly at prey-capture and defense. In the anthozoan subclass Octocorallia nematocytes have been considered to exist only in low numbers, to be small, and all of the ancestral atrichous-isorhiza type. This study, in contrast, revealed numerous nematocytes in the octocoral Heteroxenia fuscescens. The study demonstrates the applicability of cresyl-violet dye for differential staining and stimulating discharge of the nematocysts. In addition to the atrichous isorhiza-type of nematocysts, a novel type of macrobasic-mastigophore nematocysts was found, featuring a shaft, uniquely comprised of three loops and densely packed arrow-like spines. In contrast to the view that octocorals possess a single type of nematocyst, Heteroxenia fuscescens features two distinct types, indicating for the first time the diversification and complexity of nematocysts for Octocorallia
ATP release during cell swelling activates a Ca2+-dependent Cl - Current by autocrine mechanism in mouse hippocampal microglia
Microglia cells, resident immune cells of the brain, survey brain parenchyma by dynamically extending and retracting their processes. Cl- channels, activated in the cellular response to stretch/swelling, take part in several functions deeply connected with microglia physiology, including cell shape changes, proliferation, differentiation and migration. However, the molecular identity and functional properties of these Cl- channels are largely unknown. We investigated the properties of swelling-activated currents in microglial from acute hippocampal slices of Cx3cr1+/GFP mice by whole-cell patch-clamp and imaging techniques. The exposure of cells to a mild hypotonic medium, caused an outward rectifying current, developing in 5-10 minutes and reverting upon stimulus washout. This current, required for microglia ability to extend processes towards a damage signal, was carried mainly by Cl- ions and dependent on intracellular Ca2+. Moreover, it involved swelling-induced ATP release. We identified a purine-dependent mechanism, likely constituting an amplification pathway of current activation: under hypotonic conditions, ATP release triggered the Ca2+-dependent activation of anionic channels by autocrine purine receptors stimulation. Our study on native microglia describes for the first time the functional properties of stretch/swelling-activated currents, representing a key element in microglia ability to monitor the brain parenchyma
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