We consider an overdamped bistable oscillator subject to the action of a biharmonic force with very different frequencies, and study the response of the system when the parameters of the high-frequency force are varied. A resonantlike behavior is obtained when the amplitude or the frequency of this force is modified in an experiment performed by means of an analog circuit. This behavior, confirmed by numerical simulations, is explained on the basis of a theoretical approach.

We show that topological transitions in electronic spin transport are feasible by a controlled manipulation of spin-guiding fields. The transitions are determined by the topology of the fields texture through an effective Berry phase (related to the winding parity of spin modes around poles in the Bloch sphere), irrespective of the actual complexity of the nonadiabatic spin dynamics. This manifests as a distinct dislocation of the interference pattern in the quantum conductance of mesoscopic loops. The phenomenon is robust against disorder, and can be experimentally exploited to determine the magnitude of inner spin-orbit fields.PACS numbers: 71.70. Ej, 75.76.+j, In the early 1980s Berry showed that quantum states in a cyclic motion may acquire a phase component of geometric nature [1]. This opened a door to a class of topological quantum phenomena in optical and material systems [2]. With the development of quantum electronics in semiconducting nanostructures, a possibility emerged to manipulate electronic quantum states via the control of spin geometric phases driven by magnetic field textures [3]. After several experimental attempts An early proposal for the topological manipulation of electron spins by Lyanda-Geller involved the abrupt switching of Berry phases in spin interferometers [12]. These are conducting rings of mesoscopic size subject to Rashba spin-orbit (SO) coupling, where a radial magnetic texture B SO steers the electronic spin (Fig. 1a). For relatively large field strengths (or, alternatively, slow orbital motion) the electronic spins follow the local field direction adiabatically during transport, acquiring a Berry phase factor π of geometric origin (equal to half the solid angle subtended by the spins in a roundtrip) leading to destructive interference effects. By introducing an additional in-plane uniform field B, it was assumed that the spin geometric phase undergoes a sharp transition at the critical point beyond which the corresponding solid angle vanishes together with the Berry phase, and interference turns constructive. The transition should manifest as a step-like characteristic in the ring's conductance as a function of the coupling fields (so far unreported). However, this reasoning appears to be oversimplified: the adiabatic condition can not be satisfied in the vicinity of the transition point, since the local steering field vanishes and reverses direction abruptly at the rim of the ring. Moreover, typical experimental conditions correspond to moderate field strengths, resulting in nonadiabatic effects in analogy to the case of spin transport in helical magnetic fields [13]. Hence, a more sophisticated approach is required. This includes identifying the role played by nonadiabatic Aharonov-Anandan (AA) geometric phases [14].Here, we report transport simulations showing that a topological phase transition is possible in loop-shaped spin interferometers away from the adiabatic limit. The transition is determined by the topology of the field texture through an effective Berry phase ...

We report on the effect of vibrational resonance in a spatially extended system of coupled noisy oscillators under the action of two periodic forces, a low-frequency one (signal) and a high-frequency one (carrier). Vibrational resonance manifests itself in the fact that for optimally selected values of high-frequency force amplitude, the response of the system to a low-frequency signal is optimal. This phenomenon is a synthesis of two effects, a noise-induced phase transition leading to bistability, and a conventional vibrational resonance, resulting in the optimization of signal processing. Numerical simulations, which demonstrate this effect for an extended system, can be understood by means of a zero-dimensional "effective" model. The behavior of this "effective" model is also confirmed by an experimental realization of an electronic circuit.

A unifying principle explaining the numerical bounds of quantum correlations remains elusive, despite the efforts devoted to identifying it. Here, we show that these bounds are indeed not exclusive to quantum theory: for any abstract correlation scenario with compatible measurements, models based on classical waves produce probability distributions indistinguishable from those of quantum theory and, therefore, share the same bounds. We demonstrate this finding by implementing classical microwaves that propagate along meter-size transmission-line circuits and reproduce the probabilities of three emblematic quantum experiments. Our results show that the "quantum" bounds would also occur in a classical universe without quanta. The implications of this observation are discussed.

The effect of a high-frequency signal on the FitzHugh-Nagumo excitable model is analyzed. We show that the firing rate is diminished as the ratio of the high-frequency amplitude to its frequency is increased. Moreover, it is demonstrated that the excitable character of the system, and consequently the firing activity, is suppressed for ratios above a given threshold value. In addition, we show that the vibrational resonance phenomenon turns up for sufficiently large noise strength values. DOI: 10.1103/PhysRevE.73.061102 PACS number͑s͒: 05.40.Ϫa, 02.50.Ϫr, 87.19.La, 05.90.ϩm Nonlinear noisy systems have been studied with ever growing interest due to the applicability to the modeling of a great variety of phenomena of relevance in physics, chemistry, and the life sciences ͓1,2͔. Perhaps, the simplest nonlinear noisy system studied in the literature is the bistable system, which has been used successfully to illustrate the phenomenon of stochastic resonance. In addition, the spiking activity of a neuron has been theoretically studied using nonlinear excitable noisy models ͓3͔, among them, the FitzHugh-Nagumo ͑FHN͒ system ͓4͔ being one of the most utilized due to its simplicity. Besides, networks of FHN units have been considered as simplified models useful in the description of both the cardiac tissue ͓5-8͔ and reactiondiffusion chemical systems ͓9,8͔.Recently, it has been shown, both theoretically and experimentally, that the addition of a high-frequency ͑HF͒ signal results in an improvement of the stochastic resonance in a bistable optical system ͓10͔. Nevertheless, many experimental studies have suggested that HF ͑nonionizing͒ fields may damage several structural and functional properties of neuronal membranes in single cells, as well as cause a number of negative physiological effects on typically excitable media such as cardiac tissue ͓11͔. It has also been found that certain HF signals are able to suppress the steady directed motion in a ratchet model ͓12͔. Thus, HF signals seem to play a twofold role, positive or negative, depending on the nonlinear system under consideration.In this paper we study the influence of a HF field on a rather simple excitable model, such as the archetypal FHN system. This system is governed by the following equations ͓13͔:In the context of neurophysiology, v͑t͒ is called the voltage variable, and w͑t͒ the recovery variable. Here S͑t͒ is an external forcing of period T, ͑t͒ a noisy term, and ⌫͑t͒ is an added HF signal which will be specified later on. The "neuronal" noise ͑t͒ is assumed to be an unbiased Gaussian white noise with the autocorrelation function ͗͑t͒͑s͒͘ =2D␦͑t − s͒. The values of the model parameters define the dynamical regimes of the system, and are discussed below. Let us focus our attention on the deterministic FHN model ͑D =0͒ in the absence of a HF signal ͓⌫͑t͒ =0͔. In the case of a time-independent external signal S͑t͒ = S 0 , the excitable character of the FHN model relies on the existence of a threshold value of this signal, S H , at which a Hopf bifurca...

We identify a series of topological transitions occurring in electronic spin transport when manipulating spin-guiding fields controlled by the geometric shape of mesoscopic interferometers.They manifest as distinct inversions of the interference pattern in quantum conductance experiments. We establish that Rashba square loops develop weak-(anti)localization transitions (absent in other geometries as Rashba ring loops) as an in-plane Zeeman field is applied. These transitions, boosted by non-adiabatic spin scattering, prove to have a topological interpretation in terms of winding numbers characterizing the structure of spin modes in the Bloch sphere.

We study the dynamics of a localized spin-1/2 driven by a time-periodic magnetic field that undergoes a topological transition. Despite the strongly non-adiabatic effects dominating the spin dynamics, we find that the field's topology appears clearly imprinted in the Floquet spin states through an effective Berry phase emerging in the quasienergy. This has remarkable consequences on the spin resonance condition suggesting a whole new class of experiments to spot topological transitions in the dynamics of spins and other two-level systems, from nuclear magnetic resonance to strongly-driven superconducting qubits.

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