Ferromagnetism and superconductivity are most fundamental phenomena in condensed-matter physics. Entailing opposite spin orders, they share an important conceptual similarity: disturbances in ...magnetic ordering in magnetic materials can propagate in the form of spin waves (magnons) while magnetic fields penetrate superconductors as a lattice of magnetic flux quanta (fluxons). Despite a rich choice of wave and quantum phenomena predicted, magnon–fluxon coupling has not been observed experimentally so far. Here, we clearly evidence the interaction of spin waves with a flux lattice in ferromagnet/superconductor Py/Nb bilayers. We demonstrate that, in this system, the magnon frequency spectrum exhibits a Bloch-like band structure that can be tuned by the biasing magnetic field. Furthermore, we observe Doppler shifts in the frequency spectra of spin waves scattered on a flux lattice moving under the action of a transport current in the superconductor.A spectral study on a ferromagnet/superconductor heterostructure reveals the interaction between the spin-wave excitations in a magnetically ordered system (magnons) and the magnetic flux quanta formed in a superconductor (fluxons).
Most of superconductors in a magnetic field are penetrated by a lattice of quantized flux vortices. In the presence of a transport current causing the vortices to cross sample edges, emission of ...electromagnetic waves is expected due to the continuity of tangential components of the fields at the surface. Yet, such a radiation has not been observed so far due to low radiated power levels and lacking coherence in the vortex motion. Here, we clearly evidence the emission of electromagnetic waves from vortices crossing the layers of a superconductor/insulator Mo/Si superlattice. The emission spectra consist of narrow harmonically related peaks which can be finely tuned in the GHz range by the dc bias current and, coarsely, by the in-plane magnetic field value. Our findings show that superconductor/insulator superlattices can act as dc-tunable microwave generators bridging the frequency gap between conventional radiofrequency oscillators and (sub-)terahertz generators relying upon the Josephson effect.
The interaction of (quasi)particles with a periodic potential arises in various domains of science and engineering, such as solid-state physics, chemical physics, and communication theory. An ...attractive test ground to investigate this interaction is represented by superconductors with artificial pinning sites, where magnetic flux quanta (Abrikosov vortices) interact with the pinning potential U(r) = U(r + R) induced by a nanostructure. At a combination of microwave and dc currents, fluxons act as mobile probes of U(r): The ac component shakes the fluxons in the vicinity of their equilibrium points which are unequivocally determined by the local pinning force counterbalanced by the Lorentz force induced by the dc current, linked to the curvature of U(r) which can then be used for a successful fitting of the voltage responses. A good correlation of the deduced dependences U(r) with the cross sections of the nanostructures points to that pinning is primarily caused by vortex length reduction. Our findings pave a new route to a non-destructive evaluation of periodic pinning in superconductor thin films. The approach should also apply to a broad class of systems whose evolution in time can be described by the coherent motion of (quasi)particles in a periodic potential.
In this paper we apply Conformal Prediction (CP) to the k-Nearest Neighbours Regression (k-NNR) algorithm and propose ways of extending the typical nonconformity measure used for regression so far. ...Unlike traditional regression methods which produce point predictions, Conformal Predictors output predictive regions that satisfy a given confidence level. The regions produced by any Conformal Predictor are automatically valid, however their tightness and therefore usefulness depends on the nonconformity measure used by each CP. In effect a nonconformity measure evaluates how strange a given example is compared to a set of other examples based on some traditional machine learning algorithm. We define six novel nonconformity measures based on the k-Nearest Neighbours Regression algorithm and develop the corresponding CPs following both the original (transductive) and the inductive CP approaches. A comparison of the predictive regions produced by our measures with those of the typical regression measure suggests that a major improvement in terms of predictive region tightness is achieved by the new measures.
We investigated the temperature dependence of the longitudinal and transverse conductivity of Y1−zPrzBa2Cu3O7−δ single crystals with different praseodymium contents. The correlation between the ...experimental results and the predictions of various theoretical models was analyzed. It is observed that the increase of the praseodymium concentration in Y1−zPrzBa2Cu3O7−δ single crystals leads to increased localization effects and suppression of the superconducting state. Interestingly, for single crystals with large praseodymium content the anisotropy of the normal electrical resistivity ρc ρab(T) is described by the universal 'law of 1 2' for thermally activated hopping conductivity. This is in contrast to the case for comparable crystals of YBa2Cu3O7−δ.
Perovskites and related oxides are a very important class of functional materials that exhibit a range of stoichiometries and crystal structures. The oxygen diffusion mechanisms and activation ...energies in these materials are highly influenced by the composition, stoichiometry and crystal structure. We review the significance of a range perovskite related oxides but focus on two characteristic materials: La
2NiO
4+
δ
and
LnBa
2Cu
3O
7−
δ
(where
Ln
=
rare-earth cations). La
2NiO
4+
δ
is considered as a cathode material for solid-oxide fuel cells, whereas
LnBa
2Cu
3O
7−
δ
is the archetypal superconducting oxides.
Solid oxide fuel cells (SOFCs) are a key component of the future energy landscape. Although there is considerable research on the physical properties and technology of classic oxide materials for ...electrode and electrolytes in SOFCs, the field is very active as new experimental and theoretical techniques are now available that can improve these systems. In the present review, we consider key systems such as perovskite-related materials, the impact of strain and interfaces and advanced concepts that can improve the properties of SOFC materials. In particular, we consider the oxygen diffusion properties of perovskite-related materials and focus on La2NiO4+δ and the double perovskites such as GdBaCo2O5.5. Then, we review the importance of interfaces and strain as a way to engineer defect processes. Finally, we consider advanced concepts to form designed structures that explore the effect of local high entropy on lattice stabilization.
The oxygen reduction reaction is an important reaction at the cathode in solid oxide fuel cells. Materials that exhibit high chemical and mechanical stability, high ionic and electronic conductivity, ...and are non-toxic are of great interest as cathodes for the reduction of oxygen. Here, we use density functional theory simulations to examine the efficacy of 12CaO·7Al2O3 and 12SrO·7Al2O3 electrides and their doped forms for the conversion of O2 gas to form O2− in their nanocages via encapsulation. Calculations show that encapsulation is exoergic in the un-doped electrides, and the formation of O2− is confirmed by the charge analysis. A stronger encapsulation is noted for C12A7 electride than the S12A7 electride. The C12A7 electride doped with B or Ga also exhibits exoergic encapsulation, but its encapsulation energy is slightly lower than that calculated for the un-doped C12A7 electride. There is an enhancement in the encapsulation for the S12A7 electride doped with B compared to its un-doped form. Doping of Ga in S12A7 electride exhibits only a very small change in the encapsulation with respect to its un-doped form. The present results can be of interest in the design of cathode material for solid oxide fuel cells.
We consider the following problem. At each point of discrete time the learner must make a prediction; he is given the predictions made by a pool of experts. Each prediction and the outcome, which is ...disclosed after the learner has made his prediction, determine the incurred loss. It is known that, under weak regularity, the learner can ensure that his cumulative loss never exceedscL+alnn, wherecandaare some constants,nis the size of the pool, andLis the cumulative loss incurred by the best expert in the pool. We find the set of those pairs (c, a) for which this is true.
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