167,941 research outputs found
Magnetoresistance and spin-transfer torque in magnetic tunnel junctions
We comment on both recent progress and lingering puzzles related to research
on magnetic tunnel junctions (MTJs). MTJs are already being used in
applications such as magnetic-field sensors in the read heads of disk drives,
and they may also be the first device geometry in which spin-torque effects are
applied to manipulate magnetic dynamics, in order to make nonvolatile magnetic
random access memory. However, there remain many unanswered questions about
such basic properties as the magnetoresistance of MTJs, how their properties
change as a function of tunnel-barrier thickness and applied bias, and what are
the magnitude and direction of the spin-transfer-torque vector induced by a
tunnel current.Comment: 37 pages, 2 figures. Contribution to a collection of "Current
Perspectives" articles on spin transfer torque now available in the Journal
of Magnetism and Magnetic Material
Stiffness Analysis Of Multi-Chain Parallel Robotic Systems
The paper presents a new stiffness modelling method for multi-chain parallel
robotic manipulators with flexible links and compliant actuating joints. In
contrast to other works, the method involves a FEA-based link stiffness
evaluation and employs a new solution strategy of the kinetostatic equations,
which allows computing the stiffness matrix for singular postures and to take
into account influence of the external forces. The advantages of the developed
technique are confirmed by application examples, which deal with stiffness
analysis of a parallel manipulator of the Orthoglide famil
Hund's metals, explained
A possible practical definition for a Hund's metal is given, as a metallic
phase - arising consistently in realistic simulations and experiments in
Fe-based superconductors and other materials - with three features: large
electron masses, high-spin local configurations dominating the paramagnetic
fluctuations and orbital-selective correlations. These features are triggered
by, and increase with the proximity to, a Hund's coupling-favored Mott
insulator that is realized for half-filled conduction bands. A clear crossover
line is found where these three features get enhanced, departing from the Mott
transition at half filling and extending in the interaction/doping plane,
between a normal (at weak interaction and large doping) and a Hund's metal (at
strong interaction and small doping). This phenomenology is found identically
in models with featureless bands, highlighting the generality of this physics
and its robustness by respect to the details of the material band structures.
Some analytical arguments are also given to gain insight into these defining
features. Finally the attention is brought on the recent theoretical finding of
enhanced/diverging electronic compressibility near the Hund's metal crossover,
pointing to enhanced quasiparticle interactions that can cause or boost
superconductivity or other instabilities.Comment: Lecture prepared for the Autumn School on Correlated Electrons, 25-29
September 2017, Juelich. To appear on: E. Pavarini, E. Koch, R. Scalettar,
and R. Martin (eds.) The Physics of Correlated Insulators, Metals, and
Superconductors Modeling and Simulation Vol. 7 Forschungszentrum Juelich,
2017, ISBN 978-3-95806-224-5 http://www.cond- mat.de/events/correl1
Optimization of three-dimensional micropost microcavities for cavity quantum electrodynamics
This article presents a detailed analysis, based on the first-principles
finite-difference time-domain method, of the resonant frequency, quality factor
(Q), mode volume (V), and radiation pattern of the fundamental (HE11) mode in a
three-dimensional distributed-Bragg-reflector (DBR) micropost microcavity. By
treating this structure as a one-dimensional cylindrical photonic crystal
containing a single defect, we are able to push the limits of Q/V beyond those
achievable by standard micropost designs, based on the simple rules established
for planar DBR microcavities. We show that some of the rules that work well for
designing large-diameter microposts (e.g., high-refractive index contrast) fail
to provide high-quality cavities with small diameters. By tuning the
thicknesses of mirror layers and the spacer, the number of mirror pairs, the
refractive indices of high and low refractive index regions, and the cavity
diameter, we are able to achieve Q as high as 10^4, together with a mode volume
of 1.6 cubic wavelengths of light in the high-refractive-index material. The
combination of high Q and small V makes these structures promising candidates
for the observation of such cavity quantum electrodynamics phenomena as strong
coupling between a quantum dot and the cavity field, and single-quantum-dot
lasing.Comment: 11 pages, 8 figure
Reducing Cascading Failure Risk by Increasing Infrastructure Network Interdependency
Increased coupling between critical infrastructure networks, such as power
and communication systems, will have important implications for the reliability
and security of these systems. To understand the effects of power-communication
coupling, several have studied interdependent network models and reported that
increased coupling can increase system vulnerability. However, these results
come from models that have substantially different mechanisms of cascading,
relative to those found in actual power and communication networks. This paper
reports on two sets of experiments that compare the network vulnerability
implications resulting from simple topological models and models that more
accurately capture the dynamics of cascading in power systems. First, we
compare a simple model of topological contagion to a model of cascading in
power systems and find that the power grid shows a much higher level of
vulnerability, relative to the contagion model. Second, we compare a model of
topological cascades in coupled networks to three different physics-based
models of power grids coupled to communication networks. Again, the more
accurate models suggest very different conclusions. In all but the most extreme
case, the physics-based power grid models indicate that increased
power-communication coupling decreases vulnerability. This is opposite from
what one would conclude from the coupled topological model, in which zero
coupling is optimal. Finally, an extreme case in which communication failures
immediately cause grid failures, suggests that if systems are poorly designed,
increased coupling can be harmful. Together these results suggest design
strategies for reducing the risk of cascades in interdependent infrastructure
systems
Ising superconductivity and magnetism in NbSe
Recent studies on superconductivity in NbSe have demonstrated a large
anisotropy in the superconducting critical field when the material is reduced
to a single monolayer. Motivated by this recent discovery, we use density
functional theory (DFT) calculations to quantitatively address the
superconducting properties of bulk and monolayer NbSe. We demonstrate that
NbSe is close to a ferromagnetic instability, and analyze our results in
the context of experimental measurements of the spin susceptibility in
NbSe. We show how this magnetic instability, which is pronounced in a
single monolayer, can enable sizeable singlet-triplet mixing of the
superconducting order parameter, contrary to contemporary considerations of the
pairing symmetry in monolayer NbSe, and discuss approaches as to how this
degree of mixing can be addressed quantitatively within our DFT framework. Our
calculations also enable a quantitative description of the large anisotropy of
the superconducting critical field, using DFT calculations of monolayer
NbSe in the normal stateComment: 13 pages, 6 figure
Impedence Control for Variable Stiffness Mechanisms with Nonlinear Joint Coupling
The current discussion on physical human robot
interaction and the related safety aspects, but also the interest
of neuro-scientists to validate their hypotheses on human motor
skills with bio-mimetic robots, led to a recent revival of tendondriven
robots. In this paper, the modeling of tendon-driven
elastic systems with nonlinear couplings is recapitulated. A
control law is developed that takes the desired joint position
and stiffness as input. Therefore, desired motor positions are
determined that are commanded to an impedance controller.
We give a physical interpretation of the controller. More importantly,
a static decoupling of the joint motion and the stiffness
variation is given. The combination of active (controller) and
passive (mechanical) stiffness is investigated. The controller
stiffness is designed according to the desired overall stiffness.
A damping design of the impedance controller is included in
these considerations. The controller performance is evaluated
in simulation
- âŠ