1,956 research outputs found
An automated and versatile ultra-low temperature SQUID magnetometer
We present the design and construction of a SQUID-based magnetometer for
operation down to temperatures T = 10 mK, while retaining the compatibility
with the sample holders typically used in commercial SQUID magnetometers. The
system is based on a dc-SQUID coupled to a second-order gradiometer. The sample
is placed inside the plastic mixing chamber of a dilution refrigerator and is
thermalized directly by the 3He flow. The movement though the pickup coils is
obtained by lifting the whole dilution refrigerator insert. A home-developed
software provides full automation and an easy user interface.Comment: RevTex, 10 pages, 10 eps figures. High-resolution figures available
upon reques
Unconventional antiferromagnetic correlations of the doped Haldane gap system YBaNiZnO
We make a new proposal to describe the very low temperature susceptibility of
the doped Haldane gap compound YBaNiZnO. We propose a new
mean field model relevant for this compound. The ground state of this mean
field model is unconventional because antiferromagnetism coexists with random
dimers. We present new susceptibility experiments at very low temperature. We
obtain a Curie-Weiss susceptibility as expected
for antiferromagnetic correlations but we do not obtain a direct signature of
antiferromagnetic long range order. We explain how to obtain the ``impurity''
susceptibility by subtracting the Haldane gap contribution to
the total susceptibility. In the temperature range [1 K, 300 K] the
experimental data are well fitted by . In the temperature range [100 mK, 1 K] the experimental data are
well fitted by , where increases with
. This fit suggests the existence of a finite N\'eel temperature which is
however too small to be probed directly in our experiments. We also obtain a
maximum in the temperature dependence of the ac-susceptibility which
suggests the existence of antiferromagnetic correlations at very low
temperature.Comment: 19 pages, 17 figures, revised version (minor modifications
Classical and nonclassical randomness in quantum measurements
The space of positive operator-valued measures on the Borel sets of a compact
(or even locally compact) Hausdorff space with values in the algebra of linear
operators acting on a d-dimensional Hilbert space is studied from the
perspectives of classical and non-classical convexity through a transform
that associates any positive operator-valued measure with a certain
completely positive linear map of the homogeneous C*-algebra
into . This association is achieved by using an operator-valued integral
in which non-classical random variables (that is, operator-valued functions)
are integrated with respect to positive operator-valued measures and which has
the feature that the integral of a random quantum effect is itself a quantum
effect. A left inverse for yields an integral representation,
along the lines of the classical Riesz Representation Theorem for certain
linear functionals on , of certain (but not all) unital completely
positive linear maps . The extremal and
C*-extremal points of the space of POVMS are determined.Comment: to appear in Journal of Mathematical Physic
Decomposition of time-covariant operations on quantum systems with continuous and/or discrete energy spectrum
Every completely positive map G that commutes which the Hamiltonian time
evolution is an integral or sum over (densely defined) CP-maps G_\sigma where
\sigma is the energy that is transferred to or taken from the environment. If
the spectrum is non-degenerated each G_\sigma is a dephasing channel followed
by an energy shift. The dephasing is given by the Hadamard product of the
density operator with a (formally defined) positive operator. The Kraus
operator of the energy shift is a partial isometry which defines a translation
on R with respect to a non-translation-invariant measure.
As an example, I calculate this decomposition explicitly for the rotation
invariant gaussian channel on a single mode.
I address the question under what conditions a covariant channel destroys
superpositions between mutually orthogonal states on the same orbit. For
channels which allow mutually orthogonal output states on the same orbit, a
lower bound on the quantum capacity is derived using the Fourier transform of
the CP-map-valued measure (G_\sigma).Comment: latex, 33 pages, domains of unbounded operators are now explicitly
specified. Presentation more detailed. Implementing the shift after the
dephasing is sometimes more convenien
Non-equilibrium quasi-stationary states in a magnetized plasma
International audienceNon-equilibrium quasi-stationary states resulting from curvature driven interchange instabilities and drift-wave instabilities in a low beta, weakly ionized, magnetized plasma are investigated in the context of laboratory experiments in a toroidal configuration. Analytic modelling, numerical simulations and experimental results are discussed with emphasis on identifying the unstable modes and understanding the physics of anomalous particle and energy fluxes and their linkage to self-organized pressure profiles
Density functional theory calculations and vibrational spectroscopy on iron spin-crossover compounds
Iron complexes with a suitable ligand field undergo spin-crossover (SCO),
which can be induced reversibly by temperature, pressure or even light.
Therefore, these compounds are highly interesting candidates for optical
information storage, for display devices and pressure sensors. The SCO
phenomenon can be conveniently studied by spectroscopic techniques like Raman
and infrared spectroscopy as well as nuclear inelastic scattering, a technique
which makes use of the M\"ossbauer effect. This review covers new developments
which have evolved during the last years like, e.g. picosecond infrared
spectroscopy and thin film studies but also gives an overviewon newtechniques
for the theoretical calculation of spin transition phenomena and vibrational
spectroscopic data of SCO complexes
Generalized Bell Inequality Experiments and Computation
We consider general settings of Bell inequality experiments with many
parties, where each party chooses from a finite number of measurement settings
each with a finite number of outcomes. We investigate the constraints that Bell
inequalities place upon the correlations possible in a local hidden variable
theories using a geometrical picture of correlations. We show that local hidden
variable theories can be characterized in terms of limited computational
expressiveness, which allows us to characterize families of Bell inequalities.
The limited computational expressiveness for many settings (each with many
outcomes) generalizes previous results about the many-party situation each with
a choice of two possible measurements (each with two outcomes). Using this
computational picture we present generalizations of the Popescu-Rohrlich
non-local box for many parties and non-binary inputs and outputs at each site.
Finally, we comment on the effect of pre-processing on measurement data in our
generalized setting and show that it becomes problematic outside of the binary
setting, in that it allows local hidden variable theories to simulate maximally
non-local correlations such as those of these generalised Popescu-Rohrlich
non-local boxes.Comment: 16 pages, 2 figures, supplemental material available upon request.
Typos corrected and references adde
Optimal discrimination of quantum operations
We address the problem of discriminating with minimal error probability two
given quantum operations. We show that the use of entangled input states
generally improves the discrimination. For Pauli channels we provide a complete
comparison of the optimal strategies where either entangled or unentangled
input states are used.Comment: 4 pages, no figure
Decoherence and Entanglement Dynamics in Fluctuating Fields
We study pure phase damping of two qubits due to fluctuating fields. As
frequently employed, decoherence is thus described in terms of random unitary
(RU) dynamics, i.e., a convex mixture of unitary transformations. Based on a
separation of the dynamics into an average Hamiltonian and a noise channel, we
are able to analytically determine the evolution of both entanglement and
purity. This enables us to characterize the dynamics in a concurrence-purity
(CP) diagram: we find that RU phase damping dynamics sets constraints on
accessible regions in the CP plane. We show that initial state and dynamics
contribute to final entanglement independently.Comment: 10 pages, 5 figures, added minor changes in order to match published
versio
Domain Wall Spin Dynamics in Kagome Antiferromagnets
We report magnetization and neutron scattering measurements down to 60 mK on
a new family of Fe based kagome antiferromagnets, in which a strong local spin
anisotropy combined with a low exchange path network connectivity lead to
domain walls intersecting the kagome planes through strings of free spins.
These produce unfamiliar slow spin dynamics in the ordered phase, evolving from
exchange-released spin-flips towards a cooperative behavior on decreasing the
temperature, probably due to the onset of long-range dipolar interaction. A
domain structure of independent magnetic grains is obtained that could be
generic to other frustrated magnets.Comment: 5 pages, 4 figure
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