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The stochastic resonance phenomenon has been studied experimentally and theoretically for a state-of-art metal-oxide memristive device based on yttria-stabilized zirconium dioxide and tantalum pentoxide, which exhibits bipolar filamentary resistive switching of anionic type. The effect of white Gaussian noise superimposed on the sub-threshold sinusoidal driving signal is analyzed through the time series statistics of the resistive switching parameters, the spectral response to a periodic perturbation and the signal-to-noise ratio at the output of the nonlinear system. The stabilized resistive switching and the increased memristance response are revealed in the observed regularities at an optimal noise intensity corresponding to the stochastic resonance phenomenon and interpreted using a stochastic memristor model taking into account an external noise source added to the control voltage. The obtained results clearly show that noise and fluctuations can play a constructive role in nonlinear memristive systems far from equilibrium. more...

This paper is motivated by modeling the cycle-to-cycle variability associated with the resistive switching operation behind memristors. As the data are by nature curves, functional principal component analysis is a suitable candidate to explain the main modes of variability. Taking into account this data-driven motivation, in this paper we propose two new forecasting approaches based on studying the sequential cross-dependence between and within a multivariate functional time series in terms of vector autoregressive modeling of the most explicative functional principal component scores. The main difference between the two methods lies in whether a univariate or multivariate PCA is performed so that we have a different set of principal component scores for each functional time series or the same one for all of them. Finally, the sample performance of the proposed methodologies is illustrated by an application on a bivariate functional time series of reset-set curves. more...

Swirling spin textures, including topologically non-trivial states, such as skyrmions, chiral domain walls, and magnetic vortices, have garnered significant attention within the scientific community due to their appeal from both fundamental and applied points of view. However, their creation, controlled manipulation, and stability are typically constrained to certain systems with specific crystallographic symmetries, bulk, or interface interactions, and/or a precise stacking sequence of materials. Here, we make use of the stray field of YBa2Cu3O7-{\delta} superconducting microstructures in ferromagnet/superconductor hybrids to imprint magnetic radial vortices in permalloy at temperatures below the superconducting transition temperature (TC), a method easily extended to other ferromagnets with in-plane magnetic anisotropy. We examine the size dependence and temperature stability of the imprinted magnetic configurations. We show that above TC, magnetic domains retain memory of the imprinted spin texture. Micromagnetic modelling coupled with a SC field model reveals that the stabilization mechanism leading to this memory effect is mediated by microstructural defects. Superconducting control of swirling spin textures below and above the superconducting transition temperature holds promising prospects for shaping spintronics based on magnetic textures. more...

Achieving high velocities of magnetic domain walls is a crucial factor for their use as information carriers in modern nanoelectronic applications. In nanomagnetism and spintronics, these velocities are often limited either by internal domain wall instabilities, known as the Walker breakdown phenomenon, or by spin wave emission, known as the magnonic regime. In the rigid domain wall model, the maximum magnon velocity acts as an effective "speed of light", providing a relativistic analogy for the domain wall speed limitation. Cylindrical magnetic nanowires are an example of systems with the absence of the Walker breakdown phenomenon. Here we demonstrate that in cylindrical nanowires with high magnetization such as Iron, also the magnonic limit could be outstandingly surpassed. Our numerical modelling shows the Bloch point domain wall velocities as high as 14 km/s, well above the magnonic limit estimated in the interval 1.7-2.0 km/s. The key ingredient is the conical shape of the domain wall which elongates and breaks during the dynamics, leading to domain wall acceleration due to the jet propulsion effect. This effect will be very important for three-dimensional spintronic networks based on cylindrical magnetic nanowires., Comment: 19 pages, 5 figures more...

The future developments of three-dimensional magnetic nanotechnology require the control of domain wall dynamics by means of current pulses. While this has been extensively studied in planar magnetic strips (planar nanowires), few reports exist in cylindrical geometry, where Bloch point domain walls are expected to have intriguing properties. Here we report this investigation in cylindrical magnetic Ni nanowires with geometrical notches. Experimental work based on synchrotron X-ray magnetic circular dichroism (XMCD) combined with photoemission electron microscopy (PEEM) indicates that large current densities induce domain wall nucleation while smaller currents move domain walls preferably against the current direction. In the region where no pinning centers are present we found domain wall velocity of about 1 km/s. The domain wall motion along current was also detected in the vicinity of the notch region. Pinning of domain walls has been observed not only at geometrical constrictions but also outside of them. Thermal modelling indicates that large current densities temporarily raise the temperature in the nanowire above the Curie temperature leading to nucleation of domain walls during the system cooling. Micromagnetic modelling with spin-torque effect shows that for intermediate current densities Bloch point domain walls with chirality parallel to the Oersted field propagate antiparallel to the current direction. In other cases, domain walls can be bounced from the notches and/or get pinned outside their positions. We thus find that current is not only responsible for the domain wall propagation but is also a source of pinning due to the Oersted field action. more...

Magnetic cylindrical nanowires are very fascinating objects where the curved geometry allows many novel magnetic effects and a variety of non-trivial magnetic structures. Micromagnetic modelling plays an important role in revealing the magnetization distribution in magnetic nanowires, often not accessible by imaging methods with sufficient details. Here we review the magnetic properties of the shape anisotropy-dominated nanowires and the nanowires with competing shape and magnetocrystalline anisotropies, as revealed by micromagnetic modelling. We discuss the variety of magnetic walls and magnetic domains reported by micromagnetic simulations in cylindrical nanowires. The most known domain walls types are the transverse and vortex (Bloch point) domain walls and the transition between them is materials and nanowire diameter dependent. Importantly, the field or current-driven domain walls in cylindrical nanowires can achieve very high velocities. In recent simulations of nanowires with larger diameter the skyrmion tubes are also reported. In nanowires with large saturation magnetization the core of these tubes may form a helicoidal ('corkscrew') structure. The topology of the skyrmion tubes play an important role in the pinning mechanism, discussed here on the example of FeCo modulated nanowires. Other discussed examples include the influence of antinotches ('bamboo' nanowires) on the remanent magnetization configurations for hcp Co and FeCo nanowires and Co-Ni multisegmented nanowires., Comment: 24 pages, 17 figures, 1 table more...

Domain wall propagation in modulated-diameter cylindrical nanowires is a key phenomenon to be studied with a view to designing three-dimensional magnetic memory devices. This paper presents a theoretical study of transverse domain wall behavior under the influence of a magnetic field within a cylindrical nanowire with diameter modulations. In particular, domain wall pinning close to the diameter modulation was quantified, both numerically, using finite element micromagnetic simulations, and analytically. Qualitative analytical model for gently sloping modulations resulted in a simple scaling law which may be useful to guide nanowire design when analyzing experiments. It shows that the domain wall depinning field value is proportional to the modulation slope., Comment: 9 figures. Regular manuscript more...

Magnetic nanowires (NW) are promising candidates for heat generation under AC-field application due to their large shape anisotropy. They may be used for catalysis, hyperthermia or water purification treatments. In the present work we theoretically evaluate the heat dissipated by a single magnetic nanowire, originated from the domain wall dynamics under the action of an AC-field. We compare the Permalloy NWs (which demagnetize via the transverse wall propagation) with the Co fcc NWs whose reversal mode is via a vortex domain wall. The average hysteresis loop areas -which are proportional to the Specific Absorption Rate (SAR)- as a function of the field frequency have a pronounced maximum in the range 200MHz-1GHz. This maximum frequency is smaller in Permalloy than in Co and depends on the nanowire length. A simple model related to the nucleation and propagation time and domain wall velocity (higher for the vortex than for the transverse domain wall) is proposed to explain the non-monotonic SAR dependence on the frequency., Comment: 12 pages, 5 figures more...

We present experimental evidence of metastable magnetic domains in cylindrical CoNi nanowires. Transverse (perpendicular) domains have been previously observed only in flat nanostrips. Here they are found together with more conventional vortex domains along the same cylindrical nanowire, denoting a bistable system with similar free energies. The surface and the inner magnetization distribution in both types of domains are analyzed by Photo-Emission Electron Microscopy (PEEM) with X-ray magnetic circular dichroism (XMCD) contrast, and hysteresis loop in individual nanowires are measured by Magneto-optical Kerr effect. These experimental data are understood and compared with complementary micromagnetic simulations. The transverse domain states are potentially more important for sensing and recording applications than commonly observed vortex domains since they create larger surface stray fields. more...

We study the Zitterbewegung in monolayer silicene under a perpendicular magnetic field. Using an effective Hamiltonian, we have investigated the autocorrelation function and the density currents in this material. Moreover, we have analyzed other types of periodicities of the system (classical and revival times). Finally, the above results are compared with their counterparts in two other monolayer materials subject to a magnetic field: graphene and MoS$_2$ more...

We have studied the mobility of the folding catalyst, protein disulphide-isomerase (PDI), by molecular dynamics and by a rapid approach based on flexibility. We analysed our simulations using measures of backbone movement, relative positions and orientations of domains, and distances between functional sites. Despite their different assumptions, the two methods are surprisingly consistent. Both methods agree that motion of domains is dominated by hinge and rotation motion of the a and a' domains relative to the central b-b' domain core. We identify the a' domain as showing the greatest intra-domain mobility. The flexibility method, which requires 10^4-fold less computer power, predicts additional large-scale features of inter-domain motion that have been observed experimentally. We conclude that the methods offer complementary insight into the motion of this large protein and provide detailed structural models that characterise its functionally-significant conformational changes., Comment: 20 pages text, 5 figures, and full supplemental information more...

Protein function frequently involves conformational changes with large amplitude on timescales which are difficult and computationally expensive to access using molecular dynamics. In this paper, we report on the combination of three computationally inexpensive simulation methods-normal mode analysis using the elastic network model, rigidity analysis using the pebble game algorithm, and geometric simulation of protein motion-to explore conformational change along normal mode eigenvectors. Using a combination of ELNEMO and FIRST/FRODA software, large-amplitude motions in proteins with hundreds or thousands of residues can be rapidly explored within minutes using desktop computing resources. We apply the method to a representative set of six proteins covering a range of sizes and structural characteristics and show that the method identifies specific types of motion in each case and determines their amplitude limits., Comment: 34 pages, 22 Figures, Phys. Biol. 9 (2012) more...

We present a rigidity analysis on a large number of X-ray crystal structures of the enzyme HIV-1 protease using the 'pebble game' algorithm of the software FIRST. We find that although the rigidity profile remains similar across a comprehensive set of high resolution structures, the profile changes significantly in the presence of an inhibitor. Our study shows that the action of the inhibitors is to restrict the flexibility of the beta-hairpin flaps which allow access to the active site. The results are discussed in the context of full molecular dynamics simulations as well as data from NMR experiments., Comment: 4 pages, 3 figures. Conference proceedings for CMMP conference 2010 which was held at the University of Warwick more...

Rigidity analysis using the "pebble game" can usefully be applied to protein crystal structures to obtain information on protein folding, assembly and the structure-function relationship. However, previous work using this technique has not made clear how sensitive rigidity analysis is to small structural variations. We present a comparative study in which rigidity analysis is applied to multiple structures, derived from different organisms and different conditions of crystallisation, for each of several different proteins. We find that rigidity analysis is best used as a comparative tool to highlight the effects of structural variation. Our use of multiple protein structures brings out a previously unnoticed peculiarity in the rigidity of trypsin., Comment: 10 Figures, 3 Tables, 16 pages, submitted to physicalbiology more...

Quantum noise in a model of singly resonant frequency doubling including phase mismatch and driving in the harmonic mode is analyzed. The general formulae about the fixed points and their stability as well as the squeezing spectra calculated linearizing around such points are given. The use of a nonlinear normalization allows to disentangle in the spectra the dynamic response of the system from the contributions of the various noisy inputs. A general ``reference'' model for one-mode systems is developed in which the dynamic aspects of the problem are not contaminated by static contributions from the noisy inputs. The physical insight gained permits the elaboration of general criteria to optimize the noise suppression performance. With respect to the squeezing in the fundamental mode the optimum working point is located near the first turning point of the dispersive bistability induced by cascading of the second order nonlinear response. The nonlinearities induced by conventional crystals appear enough to reach it being the squeezing ultimately limited by the escape efficiency of the cavity. In the case of the harmonic mode both, finite phase mismatch and/or harmonic mode driving allow for an optimum dynamic response of the system something not possible in the standard phase matched Second Harmonic Generation. The squeezing is then limited by the losses in the harmonic mode, allowing for very high degrees of squeezing because of the non-resonant nature of the mode. This opens the possibility of very high performances using artificial materials with resonantly enhanced nonlinearities. It is also shown how it is possible to substantially increase the noise reduction and at the same time to more than double the output power for parameters corresponding to reported experiments., Comment: 18 pages, 11 figures more...

We study the quantum noise in the harmonic mode of a singly resonant frequency doubler simultaneously driven in both modes. This simple extension of the frequency doubler greatly improves its performance as a bright squeezed light source. Specifically, for parameters corresponding to reported experiments, 80 % of noise suppression is easily achieved, the phase of the corresponding squeezed quadrature can be freely and easily chosen, and the output power is nearly doubled., Comment: 3 two column pages, two figures more...

The squeezing properties of a cavity Second Harmonic Generation (SHG) system with an added Kerr effect-like nonlinearity are studied as a function of the intra-cavity photon number. The competition between the second and the third order non-linearities shifts the Hopf bifurcation of the standard SHG towards higher intra-cavity energies eventually completely stabilizing the system. Remarkably, the noise suppression is at the same time strongly enhanced, so that almost perfect squeezing is obtained for arbitrarily large intra-cavity photon numbers. Possible experimental implementations are finally discussed., Comment: 4 pages, RevTex, 3 EPS figures, uses psfig.sty, psfig.tex and epsf.sty. Avoid line breaks in the psfig commands more...