853 research outputs found
Driven diffusive system with non-local perturbations
We investigate the impact of non-local perturbations on driven diffusive
systems. Two different problems are considered here. In one case, we introduce
a non-local particle conservation along the direction of the drive and in
another case, we incorporate a long-range temporal correlation in the noise
present in the equation of motion. The effect of these perturbations on the
anisotropy exponent or on the scaling of the two-point correlation function is
studied using renormalization group analysis.Comment: 11 pages, 2 figure
A Type II Singularity Avoidance Algorithm for Parallel Manipulators using Output Twist Screws
Parallel robots (PRs) are closed-chain manipulators with diverse applications
due to their accuracy and high payload. However, there are configurations
within the workspace named Type II singularities where the PRs lose control of
the end-effector movements. Type II singularities are a problem for
applications where complete control of the end-effector is essential.
Trajectory planning produces accurate movements of a PR by avoiding Type II
singularities. Generally, singularity avoidance is achieved by optimising a
geometrical path with a velocity profile considering singular configurations as
obstacles. This research presents an algorithm that avoids Type II
singularities by modifying the trajectory of a subset of the actuators. The
subset of actuators represents the limbs responsible for a Type II singularity,
and they are identified by the angle between two Output Twist Screws. The
proposed avoidance algorithm does not require optimisation procedures, which
reduces the computational cost for offline trajectory planning and makes it
suitable for online trajectory planning. The avoidance algorithm is implemented
in offline trajectory planning for a pick and place planar PR and a spatial
knee rehabilitation P
Magnetic Flux Periodic Response of Nano-perforated Ultrathin Superconducting Films
We have patterned a hexagonal array of nano-scale holes into a series of
ultrathin, superconducting Bi/Sb films with transition temperatures 2.65 K
5 K. These regular perforations give the films a phase-sensitive
periodic response to an applied magnetic field. By measuring this response in
their resistive transitions, , we are able to distinguish regimes in
which fluctuations of the amplitude, both the amplitude and phase, and the
phase of the superconducting order parameter dominate the transport. The
portion of dominated by amplitude fluctuations is larger in lower
films and thus, grows with proximity to the superconductor to
insulator transition.Comment: Revised title, abstract, text, figure
Energy Gap Induced by Impurity Scattering: New Phase Transition in Anisotropic Superconductors
It is shown that layered superconductors are subjected to a phase transition
at zero temperature provided the order parameter (OP) reverses its sign on the
Fermi-surface but its angular average is finite. The transition is regulated by
an elastic impurity scattering rate . The excitation energy spectrum,
being gapless at the low level of scattering, develops a gap as soon as the
scattering rate exceeds some critical value of .Comment: Revtex, 11 page
Weak Localization Effect in Superconductors
We study the effect of weak localization on the transition temperatures of
superconductors using time-reversed scattered state pairs, and find that the
weak localization effect weakens electron-phonon interactions. With solving the
BCS equation, the calculated values for are in good agreement
with experimental data for various two- and three-dimensional disordered
superconductors. We also find that the critical sheet resistance for the
suppression of superconductivity in thin films does not satisfy the universal
behavior but depends on sample, in good agreement with experiments. but depends
on sample, in good agreement with experiments.Comment: 14 pages, Revtex, 5 ps figure
Coulomb Zero-Bias Anomaly: A Semiclassical Calculation
Effective action is proposed for the problem of Coulomb blocking of
tunneling. The approach is well suited to deal with the ``strong coupling''
situation near zero bias, where perturbation theory diverges. By a
semiclassical treatment, we reduce the physics to that of electrodynamics in
imaginary time, and express the anomaly through exact conductivity of the
system and exact interaction. For the diffusive anomaly, we
compare the result with the perturbation theory of Altshuler, Aronov, and Lee.
For the metal-insulator transition we derive exact relation of the anomaly and
critical exponent of conductivity.Comment: 9 pages, RevTeX 3.
Admittance Controller Complemented with Real-time Singularity Avoidance for Rehabilitation Parallel Robots
Rehabilitation tasks demand robust and accurate trajectory-tracking
performance, mainly achieved with parallel robots. In this field, limiting the
value of the force exerted on the patient is crucial, especially when an
injured limb is involved. In human-robot interaction studies, the admittance
controller modifies the location of the robot according to the user efforts
driving the end-effector to an arbitrary location within the workspace.
However, a parallel robot has singularities within the workspace, making
implementing a conventional admittance controller unsafe. Thus, this study
proposes an admittance controller that overcomes the limitations of singular
configurations by using a real-time singularity avoidance algorithm. The
singularity avoidance algorithm modifies the original trajectory based on the
actual location of the parallel robot. The complemented admittance controller
is applied to a 4 degrees of freedom parallel robot for knee rehabilitation. In
this case, the actual location is measured by a 3D tracking system because the
location calculated by the forward kinematics is inaccurate in the vicinity of
a singularity. The experimental results verify the effectiveness of the
proposed admittance controller for safe knee rehabilitation exercise
Sustainable recovery of critical elements from seawater saltworks bitterns by integration of high selective sorbents and reactive precipitation and crystallisation: Developing the probe of concept with on-site produced chemicals and energy
The availability of raw mineral resources containing elements included in the Critical Raw Materials (CRMs) list is a growing concern for the European Union. Sea mining has been identified as a promising secondary source. In particular, brines obtained in solar saltworks (bitterns) contain relevant amounts of valuable CRMs such as Mg(II), B(III), other alkaline/alkaline earth metals (Rb(I), Cs(I), Sr(II)) and transition/post-transition elements (Co(II), Ga(III), Ge(IV)). However, the low concentration of some of these elements (µg/L) requires an effort to develop recovery routes that are sustainable and economically feasible where the required chemicals and energy are produced on-site from the saltworks bitterns (i.e. HCl and NaOH). Even the conventional recovery processes such as ion exchange, sorption and precipitation, which have proved to be competitive for metals recovery, are challenged in the case of Trace Elements (TEs). This work studies the recovery of TEs included in the CRMs list from saltworks bitterns after ion exchange processes. First, batch crystallisation and reactive precipitation were tested for some target elements in single-component solutions: Sr(II), Co(II), Ga(III), Ge(IV) and B(III). Then, the experiments were carried out with multi-component synthetic solutions assuming different scenarios of bittern streams coming out a selective extraction stage using sorption and ion exchange processes. The targeted elements were recovered except for Ge(IV), where alternative routes need to be evaluated, as its precipitation involves the use of tannic acid or sulphide solutions that could not be produced from the bitterns. However, a further concentration step would be necessary to achieve element concentrations closer to the mineral phases saturation. Moreover, model simulations were performed using the PHREEQC program, which provided a good prediction of the experimental trends obtained in most cases
Suppression of \bbox{T_c} in superconducting amorphous wires
The suppression of the mean field temperature of the superconducting
transition, , in homogeneous amorphous wires is studied. We develop a
theory that gives in situations when the dynamically enhanced Coulomb
repulsion competes with the contact attraction. The theory accurately describes
recent experiments on --suppression in superconducting wires, after a
procedure that minimizes the role of nonuniversal mechanisms influencing
is applied.Comment: RevTeX, 4 pages, 3 figure
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