Amplitude Of Driving Field Research Articles

Field-dependent vortex pinning strength in a periodic array of antidots

We explore the dynamic response of vortex lines in a Pb thin film with a periodic array of antidots by means of ac-susceptibility measurements. For low drive field amplitudes, within the Campbell regime, vortex motion is of intra-valley type and the penetration depth is related to the curvature of the pinning potential well, $\alpha$. For dc-fields below the first matching field $H_1$, $\alpha$ reaches its highest value associated with a Mott Insulator-like phase where vortex lines are strongly localized at the pinning sites. For $H_1<H_{dc}<H_2$, the response is mainly due to the interstitial vortices and $\alpha$ drops to smaller values as expected for this metallic-like regime. Strikingly, for $H_2<H_{dc}<H_3$, we observe that $\alpha$ reduces further down. However, for $H_3<H_{dc}<H_4$, a reentrance in the pinning strength is observed, due to a specific configuration of the flux line lattice which strongly restricts the mobility of vortices. We present a possible explanation for the measured $\alpha(H_{dc})$ dependence based on the different flux line lattice configurations.

Spatial control of a classical electron state in a Rydberg atom by adiabatic synchronization.

An adiabatic synchronization approach is used to control orbital eccentricity and inclination of a highly excited electron in a hydrogen atom. The approach is based on persisting nonlinear phase locking (autoresonance) between spatially uniform, chirped frequency oscillating electric field, and the classical Keplerian motion of the electron in the atom. Efficient control in three dimensions is achieved by slow passage through and capture into different resonances. Scenarios guaranteeing the capture and continuing synchronization in the system are outlined, all requiring the driving field amplitude to exceed a threshold. The threshold scales as A(3/4), where A is the sweep rate of the driving frequency at resonance. The adiabatic synchronization allows one to accelerate the electron gradually by using dipolar fields, until approaching the stochastic ionization limit.

Noise-Induced Resonances in Superparamagnetic Particles

Noise-Induced Resonances in Superparamagnetic Particles

Contributions to the nonlinear dielectric and piezoelectric response of ferroelectric thin films and ceramics

The dielectric and piezoelectric response of ferroelectric thin films and ceramics was investigated as a function of the driving field amplitude. Detailed analysis of the amplitude and phase angle of the third harmonic of the dielectric displacement and piezoelectrically induced charge shows that the usual phenomenological approach is inadequate to describe experimentally observed field dependence of the dielectric and piezoelectric response. Qualitatively better agreement with experimental data is obtained using Rayleigh relations, suggesting that the observed field dependence of the polarization and piezoelectric charge may be associated with domain-wall displacement in a medium with a random distribution of pinning centers. The results indicate that the mechanism which controls the electromechanical response is very complex, and that it depends on history of samples, preparation conditions, material microstructure and on driving field amplitude.

Atomic cooperation and nonlinear dynamics in a mesoscopic maser

We consider a mesoscopic maser, in which a beam of excited two-level Rydberg atoms interacts with a single radiation mode during propagation through a resonant cavity. The number of atoms in the cavity ranges from a few tens to a few hundreds. Steady-state and nonlinear dynamics are investigated both with and without a coherent driving field. At steady state, atomic cooperation can induce multistability and temporal instabilities. A novel type of bistability is predicted for the amplitude of the coherently driven cavity field as a function of atomic density. The system dynamics exhibits a variety of regimes, including chaos. We have explored them by varying the experimentally accessible parameters: atomic density, cavity linewidth and driving field amplitude.

30 Hz–10 kHz hysteresis loop tracer with field up to 0.1 T for high-Tc superconductors

A hysteresis loop tracer to measure the ac magnetization of high-Tc superconductors in the frequency range of 30 Hz–10 kHz is described, with driving field amplitudes up to 0.1 T. Due to the high frequency of the magnetic field, the hysteresis loops could be displayed on an oscilloscope and their details could be observed. The demagnetization factor, the stability, and the frequency characteristics of the tracer were tested. The ac magnetization of granular high-Tc superconductors was studied as a function of field amplitude and frequency.

Period doubling and strange attractors in quantum wells.

A realistic model of a doped quantum well heterostructure exhibits a subharmonic signal in the presence of intense far-infrared radiation. Nonlinearity enters the model through the effective potential due to the density of electrons in the well. Dephasing and energy relaxation are introduced through a simple density matrix approach. The resultant dynamics show the period-doubling routes to chaos, in the form of strange attractors. All material parameters used are well within the present capabilities of present quantum well technology, and the driving field amplitudes and frequencies are easily obtainable with a free electron laser.

A new method for fluxgate magnetometers using the coupling property of odd and even harmonics

A new method suitable for the single core fluxgate magnetometer has been proposed. The method makes use of the coupling property of odd and even harmonics generated by the self-product of the induced voltage during the magnetization of ferromagnetic material by an external field superposed on the AC driving magnetic field. The power difference between the two half-periods of magnetization was expressed by the coupling of odd and even harmonics, which is proportional to the external field under the condition that Hext<<HAC and HAC is constant, where Hext and HAC are the external magnetic field and AC driving field amplitude respectively.

Effect of Driving Field and Temperature on the Response Behavior of Ferroelectric Actuator and Sensor Materials

Three commonly used ferroelectric actuator and sensor materials, namely polyvinylidene fluoride copolymers, lead zirconate titanate piezoceramics (PZT), and lead magnesium niobate-lead titanate relaxors (PMN-PT) at low PT content, are characterized with respect to temperature and driving field amplitude. It is shown that changes in the response and loss with the driving field amplitude are mainly caused by irreversible process and hysteresis effect in the materials, which are different from a nonlinear process. To analyze the driving field amplitude dependence of the response of a piezoelectric material, a representation is proposed which divides the material response behavior into three regions with increased driving field amplitude: 1) a linear response region, 2) a region with increased response sensitivity, nonlinearity and hysteresis, and 3) a saturation and depoling region. Based on this, the operating field limit is clearly defined for different materials and different applications. For ferroelectric relaxor materials, since the magnitude of the field induced piezoelectric coefficients depends on both the dielectric constant and the polarization level, a material with a smaller difference between the temperatures of depolarization and dielectric constant maximum will yield higher piezoelectric constants. In addition, the relaxor materials exhibit large nonlinear effects which may be useful in smart materials and structures.

High-frequency flux dynamics in single-crystal Nd1.85Ce0.15CuO4.

Flux dynamics in a ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ single crystal is investigated in the 20 kHz--3 MHz frequency range by using a mutual-inductance technique. We show that the superconducting ac response is due to thermally activated flux motion; from the dc field, ac amplitude, and frequency dependencies of this ac response, we extract the effective energy barriers U(T,B,J)=${\mathit{U}}_{0}$(1-T/${\mathit{T}}_{\mathit{c}}$)${\mathit{B}}^{\mathrm{\ensuremath{-}}0.66}$ln(${\mathit{J}}_{\mathit{c}}$/J). Significantly, these energies account for the time-dependent response of these superconductors over ten decades of time. As a consequence of them, the ac response is found to be strongly nonlinear, i.e., the ac susceptibility depends on the driving-field amplitude for ac fields as small as ${10}^{\mathrm{\ensuremath{-}}6}$ T superposed to dc fields up to 2 T. The ac transition is well described by a critical-state model with a frequency-dependent effective critical current ${\mathit{J}}_{\mathit{c}}$(T,B,f) stemming from the thermally activated flux motion. Our analysis allows us to obtain the temperature and frequency dependencies of ${\mathit{J}}_{\mathit{c}}$(T,B,f). ${\mathit{J}}_{\mathit{c}}$(f) follows a power law which is shown to be in agreement with the observed logarithmic U(J) dependence. Finally, isothermal cuts carried out for different driving-field amplitudes are used to explore the onset and limits of the nonlinear response.

Microwave ionization of Rb Rydberg atoms: Frequency dependence.

We report on an experimental study of the frequency dependence of the microwave ``ionization'' probability for laser-excited Rb(np${)}^{2}$${\mathit{P}}_{3/2}$(\ensuremath{\Vert}${\mathit{M}}_{\mathit{J}}$\ensuremath{\Vert}=1/2) Rydberg atoms. Most data are for n=84. The microwave field was produced by a low-noise frequency synthesizer, and the laser-excited atoms subsequently driven by microwaves were kept in an environment cooled to 77 K in order to minimize the perturbing effects of the thermal, blackbody-radiation field. The driving frequency covered 4--14-MHz intervals around three frequencies between 8.87 and 16.14 GHz. Data are shown for 1- and 5-\ensuremath{\mu}s interaction times, which correspond to the driving frequency being resolved by the atoms to about 1 and 0.2 MHz, respectively. The turnon and turnoff parts of the microwave pulse shape were slow, lasting about 6 ns (circa 70 field oscillations). For the narrow frequency intervals used, the microwave field amplitude was calibrated in situ with use of two-photon Rabi-nutation measurements, which also directly demonstrated the spatial uniformity of the driving field amplitude and its coherence over at least a 1-\ensuremath{\mu}s time scale. Within the frequency intervals studied experimentally, we have observed no sharp, frequency-dependent structure outside of that expected from experimental counting statistics. We give physical arguments based on experimental data and previous experimental and theoretical work on microwave ``ionization'' that show that the present experiments have been in the range of the scaled frequency above 1. Thus, these experimental data examine in a continuous fashion the frequency dependence of microwave ``ionization'' in the high-scaled-frequency regime. Finally, we discuss our results in the light of published theoretical discussions of what might be expected to be the frequency dependence of the microwave ``ionization'' probability for various quantal models for the one-dimensional hydrogen atom in this high-frequency regime.

Applications of amorphous wire

In-water-quenched amorphous magnetic metal wire has been available for a decade. In this time, the unique properties of the wire have created many new uses, especially in the field of electronic devices and sensors. Because of the near-ideal mechanical properties of amorphous metal, the wire can be die drawn to various wire diameters which can be much smaller than is possible with polycrystalline magnetic materials. Diameters of any value from the as-cast value of about 135 μm down to 10 μm are available. Also the wire can be used in applications where mechanical strength as well as unique magnetic characteristics are needed. Since magnetic metals are typically magnetostrictive, changing the chemical composition of the amorphous wire can be used to change the magnetostriction which, in combination with residual or applied stress, provides yet another opportunity to tailor the magnetic character to the device needs. Flux reversal lies at the heart of all electronic devices since the ultimate output is always a voltage to interact with the other electronic elements. Wire can have a longitudinal flux change or a circumferential flux change or a combination of two. The most curious, although not necessarily the most useful, is the re-entrant longitudinal reversal found in magnetostrictive as-cast and die-drawn tension-annealed wire. Re-entrant reversal takes the same time and hence generates the same voltage essentially independent of the drive field amplitude or frequency within the range of electromechanical devices. It is useful for tachometers where the frequency changes or security sensors where the drive field changes over a wide range. Since the reversal mechanism involves a magnetic domain wall propagating along the wire, distance sensors are also possible. By twisting the magnetostrictive wire, the longitudinal flux can be coupled to the circumferential flux which links the wire itself. This Matteucci effect allows the sensor to be driven or the output voltage observed between each end of the wire, providing a great convenience and flexibility for some types of devices. The ability to drive the reversal using a current through the wire is also popular with non-magnetostrictive wires. Here, the sharp and predictable demagnetizing effects resulting from the wire geometry make the wire attractive for electronic device applications such as the data tablet, miniature chokes and transformers.

Non-Bloch decay of transient nutations inS=1/2 systems: An experimental investigation

The decay of transient nutations has been experimentally investigated in S=1/2 spin systems at microwave frequency: E' centers in silica and [${\mathrm{AlO}}_{4}$${]}^{0}$ centers in quartz have been studied. We have found that the damping is well described by a single exponential decay function, as expected from a ${\mathit{T}}_{1}$-${\mathit{T}}_{2}$ model (Bloch model). However, the agreement is only qualitative. In fact the measured decay rate \ensuremath{\Gamma} is faster than expected and depends on the driving-field amplitude: it tends to the Bloch value ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{B}}$=1/2${\mathit{T}}_{2}$ in the low-power limit and becomes faster and faster on increasing the input power. In all the cases examined the power dependence of the decay rate is fit well by a simple linear dependence of \ensuremath{\Gamma} on the induced Rabi frequency \ensuremath{\chi}. The observed power dependence of \ensuremath{\Gamma} cannot be ascribed to the inhomogeneity of \ensuremath{\chi} over the sample volume nor to the radiation damping, since both effects are negligible in our experiments. Other mechanisms, which can, in principle, yield a \ensuremath{\chi} dependence of \ensuremath{\Gamma}, e.g., the direct interaction of the driving field with structural two-level systems or the spreading of the spin-field coupling constant, are not compatible with the experimental conditions. So, our results suggest that the homogeneous dephasing time of each isochromat contains an intrinsic term and a \ensuremath{\chi}-dependent one. The latter may originate in a field-induced enhancement of the hyperfine or dipolar interaction; however, neither of these mechanisms completely fits the experimental features. The relationship with the decay properties of other coherent regimes is also discussed.

Vortex response to AC-fields in Bi2Sr2CaCu2O8+ delta

Results on the driving-field amplitude dependence of the AC-susceptibility of a Bi2Sr2CaCu2O8+ delta -single crystal measured in an applied DC magnetic field are presented. The data obtained in DC fields higher than 2.5T strongly support 2D plastic creep as determining the diamagnetic transition. At lower fields the susceptibility behaviour is more complex, although at all fields flux motion seems to be diffusive.

Dislocation-mediated flux creep in Bi2Sr2CaCu2O8+ delta.

We have investigated the possible role of two-dimensional vortex-lattice defects in thermally assisted flux flow for the very anisotropic high-temperature superconductor ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (Bi-Sr-Ca-Cu-O). At low current densities, this mechanism, which we shall refer to as plastic flux creep, is expected to prevail over creep of elastically correlated flux bundles, or elastic creep. It is assumed that vortices are pinned by oxygen vacancies, for which the elementary interaction is obtained. Measurements of the ac susceptibility in an applied dc field, ${\mathit{B}}_{\mathit{c}1}$${\mathrm{\ensuremath{\mu}}}_{0}$H0.2${\mathit{B}}_{\mathit{c}2}$, carried out using a sufficiently low driving-field amplitude, were performed on a Bi-Sr-Ca-Cu-O single-crystalline sample. It is shown that under the above assumptions the experimental data may be consistently interpreted. The field dependence of the activation barrier is explained. Furthermore, the irreversibility line is reproduced, as well as the shape of the ac-susceptibility transition. Deviations from the plastic-flux-creep model can be qualitatively understood using a criterion describing the crossover to elastic flux creep.

Attractors and chaos in the laser with injected signal

We revisit a homogeneously broadened ring laser that is driven by an external coherent signal. We identify new basins of attraction and calculate the dimensionality of the chaotic regions by using both the Kol’mogorov and Lyapunov formalisms. As a function of the driving-field amplitude, we see the dimensionality of the chaotic regions globally decrease in fractal dimension and locally decrease linearly immediately before the emergence of a periodic attractor. New evidence shows that this complex system is governed by only a few attractors; these attractors can coexist, govern independently, or compete for dominance in the chaotic regions. Arguments reveal that, at least for this system, the ideas of distinct chaotic attractors and windows of periodicity within chaos are misnomers and are misleading.

Classical dynamics and resonance structures in laser-induced dissociation of a Morse oscillator.

We study laser-induced dissociation of a Morse oscillator by an ir field as a prototype of multiphoton excitation and dissociation of molecular systems. The driving field is modeled both by a sinusoidal interaction term and by a sequence of \ensuremath{\delta} impulses. We have found that the latter case yields results which closely resemble those of the former and considerably facilitates the computation and analysis. Primary and a sequence of secondary resonances are found in our classical dynamical study and are analyzed by using Chirikov's nonlinear resonance theory. We have also calculated the dissociation rate as a function of time, dissociation fractions, and half-lives, all for ensembles of initial states uniformly distributed over regions in the phase space. The half-life as a function of the driving field amplitude follows a scaling law with a critical exponent in close agreement with that predicted for the standard map. This fact and the time-dependent dissociation rate reflect the effects on dynamic flows of the partial barriers formed by cantori.

Chaotic Response of Driven Charge Density Wave Systems

Abstract We investigate the response parameters of the charge density wave condensates of (TaSe4)2I and NbSe3 for ac, dc, or ac+dc combined electric drive fields. Chaotic conductivity obtains for well-defined ranges of temperature, driving field amplitude, and ac drive frequency. We discuss these results in terms of simple equations of motion for CDW transport.

Theory of collective resonance fluorescence in strong driving fields

An S-conserving theory of collective resonance fluorescence by $N$ two-level atoms driven by a uniform strong field is presented. The detuning between field and atoms can be varied. A canonical transformation to dress the atoms by the driving field and Langevin equations for dressed operators are used to investigate the behavior of the system. For sufficiently large detuning the dressed atoms are shown to reach a steady state after a transient super-radiant-like decay in times short, compared with the long decay time which is attained on resonance and which is the same as the single-atom spontaneous-decay time. In the transient regime the parameters of the quasi-super-radiant decay are shown to vary with detuning and with the driving-field amplitude. Both the amplitude spectrum and the integrated intensity of the fluorescence field, emitted off resonance by the dressed atoms during the transient, are shown to bear pulselike traces of the quasi-super-radiant decay. The elastic components of the off-resonance amplitude spectrum at the end of the super-radiant-like transient are broadened with respect to resonance, and the integrated intensity exhibits a sudden transient decrease in an appropriate range of detuning, which has been given the name superdarkening and which disappears on resonance. After the steady-state regime is attained, the resonance intensity spectrum is shown to coincide with that obtained by other authors, while off resonance the inelastic lines are broadened in a super-radiant-like fashion.

Temperature dependence of the internal bias in TGS-Alanine crystals near Tc

Abstract The temperature, driving field amplitude and frequency dependence of hysteresis loops of triglycine sulfate doped with L-alanine has been examined in the neighborhood of the Curie temperature, Tc. The magnitude of the internal bias associated with L-alanine content shows a remarkable temperature dependence near Tc, decreasing by more than two orders of magnitude within ±1°C of Tc. A semiquantitative interpretation of this behavior in terms of the interaction between orientable (glycine) dipoles and fixed (L-alanine) dipoles is given