Strong Ac Research Articles

Effect of a dc magnetic field on the magnetization relaxation of uniaxial single-domain ferromagnetic particles driven by a strong ac magnetic field

Abstract The nonlinear ac stationary response of the magnetization of noninteracting uniaxial single-domain ferromagnetic particles acted on by superimposed dc and ac magnetic fields applied along the anisotropy axis is evaluated from the Fokker–Planck equation, expressed as an infinite hierarchy of recurrence equations for Fourier components of the relaxation functions governing longitudinal relaxation of the magnetization. The exact solution of this hierarchy comprises a matrix continued fraction, allowing one to evaluate the ac nonlinear response and reversal time of the magnetization. For weak ac fields, the results agree with perturbation theory. It is shown that the dc bias field changes substantially the magnetization dynamics leading to new nonlinear effects. In particular, it is demonstrated that for a nonzero bias field as the magnitude of the ac field increases the reversal time first increases and having attained its maximum at some critical value of the ac field, decreases exponentially.

Nanorod dynamics in ac electric fields

Metal and semiconductor nanorods polarized by an external electric field tend to alignparallel to this field. We derived the equation of motion for this alignment, taking intoaccount electrostatic forces, thermal fluctuations and viscous resistance of the liquid thenanorods are suspended in. It was solved for a strong ac electric field, as well as for thecombination of strong dc and weak ac fields. The results were used for calculations of thecapacity of the nanorod solution, its frequency dispersion and dependence on the fieldstrength. Modification of the nanorod absorption spectra under the influence of an electricfield was also considered. It was shown that metal nanorods in laser radiation, with thefrequency belonging to the interval between longitudinal and transverse plasmon modes,tend to align perpendicular, rather than parallel, to the optical electric field.

Spin-glass-like properties ofLa0.8Ca0.2MnO3nanoparticles ensembles

Magnetic properties of compacted La0.8Ca0.2MnO3 manganite nanoparticles with average particle size of 18 and 70 nm and Curie temperatures TC 231 K and TC 261 K, respectively, have been investigated. The relative volume of the ferromagnetic phase has been estimated to be 52% for ensembles of 18 nm particles and 92% for 70 nm particles. It was found that applied hydrostatic pressure enhances TC of La0.8Ca0.2MnO3 nanoparticles at a rate dTC /dP 1.8– 1.9 K / kbar, independently on the average particle size. Pronounced irreversibility of magnetization below Tirr 208 K and strong frequency dependent ac susceptibility below TC for smaller 18 nm particles have been observed. 18 nm particles have also shown aging and memory effects in zero-field-cooled ZFC and field-cooled magnetization. These features indicate the appearance of spin-glasslike state, partially reminiscent the behavior of La1�xCaxMnO3 crystals, doped below the percolation threshold xxC = 0.225. In contrast, ensembles of larger 70 nm particles have shown insignificant irreversibility of magnetization only and no frequency dependence of ac susceptibility, similarly to the behavior of La1�xCaxMnO3 crystals with xxC. The temperature of the ZFC magnetization maximum for 18 nm particles decreases with increasing magnetic field and forms a critical line with an exponent 1.89 0.56. The results suggest that superspin-glass features in ensembles of interacting 18 nm particles appear along with superferromagnetic-like features.

Quantum and classical resonant escapes of a strongly driven Josephson junction

The properties of phase escape in a dc superconducting quantum interference device (SQUID) at 25 mK, which is well below quantum-to-classical crossover temperature ${T}_{cr}$, in the presence of strong resonant ac driving have been investigated. The SQUID contains two $\text{Nb}/{\text{Al-AlO}}_{x}/\text{Nb}$ tunnel junctions with Josephson inductance much larger than the loop inductance so it can be viewed as a single junction having adjustable critical current. We find that with increasing microwave power $W$ and at certain frequencies $\ensuremath{\nu}$ and $\ensuremath{\nu}/2$, the single primary peak in the switching current distribution, which is the result of macroscopic quantum tunneling of the phase across the junction, first shifts toward lower bias current $I$ and then a resonant peak develops. These results are explained by quantum resonant phase escape involving single and two photons with microwave-suppressed potential barrier. As $W$ further increases, the primary peak gradually disappears and the resonant peak grows into a single one while shifting further to lower $I$. At certain $W$, a second resonant peak appears, which can locate at very low $I$ depending on the value of $\ensuremath{\nu}$. Analysis based on the classical equation of motion shows that such resonant peak can arise from the resonant escape of the phase particle with extremely large oscillation amplitude resulting from bifurcation of the nonlinear system. Our experimental result and theoretical analysis demonstrate that at $T⪡{T}_{cr}$, escape of the phase particle could be dominated by classical process, such as dynamical bifurcation of nonlinear systems under strong ac driving.

Effect of a dc bias field on the dynamic hysteresis of single-domain ferromagnetic particles

Dynamic magnetic hysteresis in uniaxial superparamagnetic nanoparticles in superimposed ac and dc magnetic fields of arbitrary amplitude is considered using Brown’s model of coherent rotation of the magnetization. The dependence of the area of the dynamic hysteresis loop on the temperature, frequency, and ac and dc bias fields is analyzed. In particular, the dynamic hysteresis loop of a single-domain ferromagnetic particle is substantially altered by applying a relatively weak dc field. Furthermore, it is shown that at intermediate to low ac field amplitudes, the dc bias field permits tuning of the magnetic power absorption of the particles, while for strong ac field amplitudes the effect becomes entirely analogous to that produced by the exchange biased anisotropy. Simple analytical formulas are provided in the linear response regime for the steady-state magnetization and loop area, exhibiting perfect agreement with the numerical solution of Brown’s Fokker–Planck equation. Comparison with previous results is also given.

Dissipative two-level system under strong ac driving: A combination of Floquet and Van Vleck perturbation theory

We study the dissipative dynamics of a two-level system (TLS) exposed to strong ac driving. By combing Floquet theory with Van Vleck perturbation theory in the TLS tunneling matrix element, we diagonalize the time-dependent Hamiltonian and provide corrections to the renormalized Rabi frequency of the TLS, which are valid for both a biased and unbiased TLS and go beyond the known high-frequency and rotating-wave results. In order to mimic environmental influences on the TLS, we couple the system weakly to a thermal bath and solve analytically the corresponding Floquet-Bloch-Redfield master equation. We give a closed expression for the relaxation and dephasing rates of the TLS and discuss their behavior under variation of the driving amplitude. Further, we examine the robustness of coherent destruction of tunneling (CDT) and driving-induced tunneling oscillations (DITO). We show that also for a moderate driving frequency an almost complete suppression of tunneling can be achieved for short times and demonstrate the sensitiveness of DITO to a change of the external parameters.

Photovoltaic Berry curvature in the honeycomb lattice

Photovoltaic Hall effect — the Hall effect induced by intense, circularly-polarized light in the absence of static magnetic fields — has been proposed in Phys. Rev. B 79, 081406R (2009) for graphene where a massless Dirac dispersion is realized. The photovoltaic Berry curvature (a nonequilibrium extension of the standard Berry curvature) is the key quantity to understand this effect, which appears in the Kubo formula extended to Hall transport in the presence of strong AC field backgrounds. Here we elaborate the properties of the photovoltaic curvature such as the frequency and field strength dependence in the honeycomb lattice.

Dynamic Characteristics Analysis of Jindo-Cheju HVDC System

The case that HVDC system is connected to the strong AC network has little the considering problems. However, because there are many problems(example for voltage stability, inertia and low frequency resonance) in case of connecting the weak AC system, a detailed attentions are needed. In particular, because HVDC system can control the flow of AC network freely, have to contain the range of stability and safe operation range. This paper deals with HVDC control algorithms, which are operated within 1 second on viewpoint of time domain, that is, transient stability. The target of HVDC introduced in this paper is Jindo-Cheju HVDC system, which will be operated at 2011. And the introduced algorithm can be actually implemented to Jindo-Cheju HVDC system.

Relaxation of the magnetization in uniaxial single-domain ferromagnetic particles driven by a strong ac magnetic field

The magnetization spectrum and reversal time of uniaxial single-domain ferromagnetic particles driven by an ac magnetic field of arbitrary strength applied along the easy axis are evaluated by solving the infinite hierarchy of recurrence equations for the Fourier components of the relaxation functions governing the frequency dependence of the magnetization. A simple analytic equation for the reversal time is also obtained via the mean first passage time method exhibiting pronounced nonlinear effects such as a rapid decrease in the reversal time as the strength of the ac field increases. For weak ac fields, the results agree with perturbation theory.

Semiclassical treatment of a Brownian ratchet using the quantum Smoluchowski equation

Quantum effects in the noninertial Brownian motion of a particle in a one-dimensional ratchet potential are treated in the high temperature and weak bath-particle coupling limit by solving a quantum Smoluchowski equation for the time evolution of the Wigner function in configuration space. In particular, an analytical expression for the stationary average drift velocity for constant driving forces is presented including quantum corrections to any order in Planck's constant. The corresponding frequency response is determined using continued fractions in both the linear approximation holding for small ac driving amplitude and in the nonlinear regime for arbitrary driving amplitude exhibiting pronounced ac induced frequency dependence of the dc component of the average drift velocity. Moreover, Shapiro steps are apparent in the dc characteristics for strong ac driving just as in the dc current-voltage characteristics of a point Josephson junction.

DNA condensation by field-induced non-equilibrium noise.

DNA molecules under good solvent conditions condense when subjected to strong AC electrical fields. It is shown that AC electrical fields couple charge fluctuations to molecular shape fluctuations, which produces a non-equilibrium noise source that can dominate over equilibrium thermal noise in the long-wavelength limit. The field-induced excess charge fluctuations amplify the Asakura-Oosawa fluctuation attraction force between neighboring chains, providing a mechanism for field-induced DNA condensation.

Quasi-energy spectra of a charged particle in planar honeycomb lattices

The low-energy spectrum of a particle in planar honeycomb lattices is conical, which leads to the unusual electronic properties of graphene. In this paper, we calculate the quasi-energy spectra of a charged particle in honeycomb lattices driven by a strong ac field, which is of fundamental importance for its time-dependent dynamics. We find that depending on the amplitude, direction and frequency of the external field, many interesting phenomena may occur, including band collapse, renormalization of the velocity of ‘light’, gap opening etc. Under suitable conditions, by increasing the magnitude of the ac field, a series of phase transitions from gapless phases to gapped phases appear alternately. At the same time, the Dirac points may disappear or change to a line. We suggest possible realization of the system in honeycomb optical lattices.

Bloch oscillation, dynamical localization, and optical probing of electron gases in quantum-dot superlattices in high electric fields

In this paper, we present numerical results for steady-state and time-dependent currents as well as for a long-time average current in strong nonlinear dc and ac electric fields for an electron gas in a one-dimensional (1D) quantum-dot superlattice. A microscopic model is employed for the scattering of electrons by phonons and static impurities by means of the Boltzmann equation method. The dc results are favorably compared with recent exact analytic results based on a relaxation-time model for electron-phonon scattering. Our results demonstrate the different roles played by elastic and inelastic scattering on the damped Bloch oscillations as well as the nonlinear steady-state current and their opposite roles on the damped dynamical localization. We also find a suppression of dynamical localization by strong Bloch oscillations and features in the Esaki-Tsu peaks in the presence of an ac electric field when electron scattering is included. On the basis of a nonequilibrium electron distribution obtained from the Boltzmann equation, a self-consistent-field approach is employed to establish a general formalism for the optical response of current-driven electrons in both the linear and nonlinear regimes to a 1D quantum-dot superlattice. The dc-field dependences of both the peak energy and peak strength in the absorption spectrum for a 1D quantum-dot superlattice are calculated, from which we find: (1) both the peak energy and its strength are significantly reduced with increasing dc electric field; and (2) the peak energy and peak strength are anomalously enhanced by raising the temperature for the nonlinear transport of electrons when a strong dc electric field is applied.

Floquet formulation for the investigation of multiphoton quantum interference in a superconducting qubit driven by a strong ac field

We present a Floquet treatment of multiphoton quantum interference in a strongly driven superconducting flux qubit. The periodically time-dependent Schr\"odinger equation can be reduced to an equivalent time-independent infinite-dimensional Floquet matrix eigenvalue problem. For resonant or nearly resonant multiphoton transitions, we extend the generalized Van Vleck (GVV) nearly degenerate high-order perturbation theory for the treatment of the Floquet Hamiltonian, allowing the reduction of the infinite-dimensional Floquet matrix to an $N\ifmmode\times\else\texttimes\fi{}N$ effective Hamiltonian, where $N$ is the number of eigenstates under consideration. The GVV approach allows accurate treatment of ac Stark shift, power broadening, time-dependent and time-averaged transition probability, etc., well beyond the rotating wave approximation. We extend the Floquet and GVV approaches for numerical and analytical studies of the multiphoton resonance processes and quantum interference phenomena for the superconducting flux qubit system $(N=2)$ driven by intense ac fields.

Photo-induced metallic liquid in a one-dimensional Mott insulator in AC fields

We theoretically study the properties of the photo-carriers created in a one-dimensional Mott insulator subject to strong AC electric fields. The time-dependent density matrix renormalization group method is used to calculate the nonlinear optical conductivity, for which a metallic, gapless mode is found to emerge. The mode has linear dispersion with a charge velocity that differs from the free electron velocity, which indicates that the photo-doped carriers behave collectively as in the Tomonaga-Luttinger liquid.

Carbon Nanotube Electrodes for Hot‐Wire Electrochemistry

The use and preparation of single-walled carbon nanotubes (SWCNTs) at thin metallic wire electrodes for hot-wire electrochemical studies is described. The nanotubes were deposited on metal substrates such as gold by electrophoresis from a dispersion containing sodium dodecyl sulphate as an anionic surfactant. The formation of a layer of pure SWCNTs is achieved by thermal treatment at 350 degrees C. When heated in situ by a strong ac current, the electrodes can be used for electrochemical studies of nanotubes at increased temperatures. The state and functionality of the electrodes were characterized by Raman spectroscopy, scanning electron microscopy, and cyclic voltammetry with both anionic and cationic redox systems (dopamine, ferrocene carboxylic acid). First time experiments at the heated SWCNT electrodes demonstrated an excellent suitability of these as-prepared electrodes for thermoelectrochemical studies.

Nonlinear noninertial response of a quantum Brownian particle in a tilted periodic potential to a strong ac force as applied to a point Josephson junction

The quantum Smoluchowski equation for the reduced Wigner function in configuration space pertaining to the quantum Brownian motion of a particle in a tilted cosine potential in the high dissipation or noninertial limit as applied to a model point Josephson junction namely, a resistively shunted junction in the presence of noise and an arbitrarily large microwave ac driving current is considered. The solution of the resulting recurrence relations for the Fourier amplitudes of the statistical moments describing the nonlinear dynamics of the junction ignoring the capacitance is obtained using the matrix continued fractions previously developed for the stationary ac field solution of the corresponding classical problem. Quantum effects in the nonlinear response of the junction to an ac microwave current of arbitrary amplitude nonlinear microwave impedance, frequency dependence of the dc current-voltage characteristic, etc. are estimated.

Photoinduced Tomonaga-Luttinger-like liquid in a Mott insulator

Photoinduced metallic states in a Mott insulator are studied for the half-filled one-dimensional Hubbard model with the time-dependent density-matrix renormalization group. An irradiation of strong ac fields is found to create, in the nonequilibrium steady state, a linear dispersion in the optical spectrum (current-current correlation) reminiscent of the Tomonaga-Luttinger liquid for the doped Mott insulator in equilibrium. The spin spectrum in nonequilibrium retains the des Cloizeaux--Pearson mode with the spin velocity differing from the charge velocity. The mechanism of the photocarrier doping, along with the renormalization in the charge velocity, is analyzed in terms of an effective Dirac model.

Correlated electron systems periodically driven out of equilibrium:Floquet+DMFTformalism

We propose to combine the Floquet formalism for systems in ac fields with the dynamical mean-field theory to study correlated electron systems periodically driven out of equilibrium by external fields such as intense laser light. This approach has a virtue that we can nonperturbatively include both the correlation effects and nonlinear effects due to the driving field, which is imperative in analyzing recent experiments for photoinduced phase transitions. In solving the problem, we exploit a general theorem that the Hamiltonian in a Floquet matrix form can be exactly diagonalized for single-band noninteracting systems. As a demonstration, we have applied the method to the Falicov-Kimball model in intense ac fields to calculate the spectral function. The result shows that photoinduced midgap states emerge from strong ac fields, triggering an insulator-metal transition.

Possible THz Bloch gain in dc–ac-driven superlattices

We study theoretically the feasibility of amplification and generation of THz radiation in dc–ac-driven semiconductor superlattices in the absence of electric domains. We find that if in addition to dc bias strong ac pump fields are applied, Bloch gain profile for a small THz signal can be achieved under conditions of positive static differential conductivity. We briefly review the case of THz pump and present preliminary results on the use of pump fields belonging to the microwave frequency band.