Time-dependent Magnetic Field Research Articles

Nonparametric distributions of tensor-valued Lorentzian diffusion spectra for model-free data inversion in multidimensional diffusion MRI.

Magnetic resonance imaging (MRI) is the method of choice for noninvasive studies of micrometer-scale structures in biological tissues via their effects on the time- and frequency-dependent (restricted) and anisotropic self-diffusion of water. While new designs of time-dependent magnetic field gradient waveforms have enabled disambiguation between different aspects of translational motion that are convolved in traditional MRI methods relying on single pairs of field gradient pulses, data analysis for complex heterogeneous materials remains a challenge. Here, we propose and demonstrate nonparametric distributions of tensor-valued Lorentzian diffusion spectra, or "D(ω) distributions," as a general representation with sufficient flexibility to describe the MRI signal response from a wide range of model systems and biological tissues investigated with modulated gradient waveforms separating and correlating the effects of restricted and anisotropic diffusion.

Magnetic field expulsion in optically driven YBa2Cu3O6.48.

Coherent optical driving in quantum solids is emerging as a research frontier, with many reports of interesting non-equilibrium quantum phases1-4 and transient photo-induced functional phenomena such as ferroelectricity5,6, magnetism7-10 and superconductivity11-14. In high-temperature cupratesuperconductors, coherent driving of certain phonon modes has resulted in a transient state with superconducting-like optical properties, observed far above their transition temperatureTc and throughout the pseudogap phase15-18. However, questions remain on the microscopic nature of this transient state and how to distinguish it from a non-superconducting state with enhanced carrier mobility. For example, it is not known whether cuprates driven in this fashion exhibit Meissner diamagnetism. Here we examine the time-dependent magnetic field surrounding an optically driven YBa2Cu3O6.48 crystal by measuring Faraday rotation in a magneto-optic material placed in the vicinity of the sample. For a constant applied magnetic field and under the same driving conditions that result in superconducting-like optical properties15-18, a transient diamagnetic response was observed. This response is comparable in size with that expected in an equilibrium type II superconductor of similar shape and size with a volume susceptibility χv of order -0.3. This value is incompatible with a photo-induced increase in mobility without superconductivity. Rather, it underscores the notion of a pseudogap phase in which incipient superconducting correlations are enhanced or synchronized by the drive.

Spin torque-driven electron paramagnetic resonance of a single spin in a pentacene molecule.

Control over quantum systems is typically achieved by time-dependent electric or magnetic fields. Alternatively, electronic spins can be controlled by spin-polarized currents. Here, we demonstrate coherent driving of a single spin by a radiofrequency spin-polarized current injected from the tip of a scanning tunneling microscope into an organic molecule. With the excitation of electron paramagnetic resonance, we established dynamic control of single spins by spin torque using a local electric current. In addition, our work highlights the dissipative action of the spin-transfer torque, in contrast to the nondissipative action of the magnetic field, which allows for the manipulation of individual spins based on controlled decoherence.

Scaling and universality at ramped quench dynamical quantum phase transitions

The nonequilibrium dynamics of a periodically driven extended XY model, in the presence of linear time dependent magnetic field, is investigated using the notion of dynamical quantum phase transitions (DQPTs). Along the similar lines to the equilibrium phase transition, the main purpose of this work is to search fundamental concepts such as scaling and universality at the ramped quench DQPTs. We have shown that the critical points of the model, where the gap closing occurs, can be moved by tuning the driven frequency and consequently the presence of or absence of DQPTs can be flexibly controlled by adjusting the driven frequency. We have uncovered that, for a ramp across the single quantum critical point, the critical mode at which DQPTs occur is classified into three regions: the Kibble–Zurek (KZ) region, where the critical mode scales linearly with the square root of the sweep velocity, the pre-saturated (PS) region, and the saturated (S) region where the critical mode makes a plateau versus the sweep velocity. While for a ramp that crosses two critical points, the critical modes disclose just the KZ and PS regions. On the basis of numerical simulations, we find that the dynamical free energy scales linearly with time, as approaches to DQPT time, with the exponent ν=1±0.01 for all sweep velocities and driven frequencies.

Peculiarities of the Interstellar Dust Distribution in the Heliosphere Induced by the Time-Dependent Magnetic Field

Interstellar dust enters the heliosphere due to the relative motion of the Sun and the Local Interstellar Cloud, which contains the Sun. The dynamics of interstellar dust particles is governed mainly by the electromagnetic force. The direction of this force depends on the polarity of the heliospheric magnetic field. In turn, polarity is a function of position and time and depends on the orientation of the solar magnetic dipole axis relative to the solar rotation axis. Previously it was shown that for the case when the magnetic dipole axis coincides with the solar rotation axis, the electromagnetic force acting on dust particles is directed towards the solar equatorial plane in both the northern and southern solar hemispheres. As a result, under the influence of such a force, the distribution of interstellar dust becomes highly inhomogeneous and, in particular, thin regions of increased number density (caustics) are formed. The goal of this work is to study the nature of caustics for a more realistic time-dependent model, when it is assumed that the magnetic dipole axis rotates relative to the solar rotation axis with a period of 22 years in accordance with the 22-year solar cycle. In addition, the magnetic dipole axis rotates due to the rotation of the Sun with a period of 25 days. To calculate the dust number density, the Lagrangian Osiptsov method is used. The shape and evolution of the resulting caustics are examined and the physical mechanisms of their origin are discussed. It is shown that, when taking into account time-dependent effects, caustics appear only in certain phases of the 22-year solar cycle, and then disappear.

Dynamic magnetic and thermodynamic behaviors of edge-modified graphene-like nanoparticles in a time-dependent magnetic field

Dynamic magnetic and thermodynamic behaviors of edge-modified graphene-like nanoparticles in a time-dependent magnetic field

Exploration of dynamic phase transition of 3D Ising model with a new long-range interaction by using the Monte Carlo method

In this paper, we present a novel interpretation of the long-range interaction for exploring the magnetic characteristics of the 3D Ising model. To describe the long-range interaction, we utilize spin chains, which enable us to explore the dynamic magnetic behaviors of this model. The research investigates the impacts of the amplitude of the time-dependent magnetic field h0 and period τ on the dynamic phase transition by obtaining phase diagrams and midplane cross-sections of the system. Critical exponents are also discussed, which are consistent with other theoretical results. The dynamic hysteresis behaviors are then examined through various physical parameters. As the temperature increases, the area and shape of the dynamic hysteresis loop change. Our theoretical conclusions are consistent with those of other theoretical and experimental studies.

Time dependent magnetic field effects on the MHD flow and heat transfer in a rectangular duct

AbstractWe consider the effect of time dependent magnetic field on the natural convection magnetohydrodynamic flow in a rectangular duct. The flow is two‐dimensional, unsteady, laminar and the fluid is electrically conducting and incompressible. The heat transfer is taken into account using the Boussinesq approximation and the buoyancy force is included in the momentum equations for the thermal coupling. The flow is considered at low magnetic Reynolds number setting and hence, the induced magnetic field is neglected. The proposed model in which the governing equations are given in terms of stream function, vorticity and temperature, is approximated by using a Chebyshev spectral collocation method for the spatial discretisation coupled with a backward difference scheme that is unconditionally stable for the temporal integration. The flow and heat transfer characteristic are analysed for several definitions of applied time varying magnetic field. Increase of the Hartmann number and the time variation of the applied magnetic field, changes the flow behaviour, especially at transient time levels and at the steady‐state.

Micromagnetic investigation of dynamic hysteresis and dynamic phase transition properties in Co, Fe and Ni nanodisks

We have investigated the dynamical hysteresis behaviors of Co,Fe, and Ni nanodisks under the effect of the time-dependent magnetic field by solving the Landau-Lifshitz-Gilbert equation with the OOMMF software. A nanodisk with a diameter of 360nm constructed in the xy plane is exposed to a sinusoidal x-directed magnetic field with varying frequency and amplitude values. All nanodisks exhibit intricate frequency-dependent variations in the hysteresis loop area, remanent magnetization, and coercive field. Our simulation results emphasize that while the dynamical order occurs exclusively in the x direction for Fe nanodisks, dynamical order is observed in the y and z directions for Ni nanodisks within a specific frequency range.

Role of Time-Varying Magnetic Field on QGP Equation of State

The phase diagram of quantum chromodynamics (QCD) and its associated thermodynamic properties of quark-gluon plasma (QGP) are studied in the presence of time-dependent magnetic field. The study plays a pivotal role in the field of cosmology, astrophysics, and heavy-ion collisions. In order to explore the structure of quark-gluon plasma to deal with the dynamics of quarks and gluons, we investigate the equation of state (EoS) not only in the environment of static magnetic field but also in the presence of time-varying magnetic fields. So, for determining the equation of state of QGP at nonzero magnetic fields, we revisited our earlier model where the effect of time-varying magnetic field was not taken into consideration. Using the phenomenological model, some appealing features are noticed depending upon the three different scales: effective mass of quark, temperature, and time-independent and time-dependent magnetic fields. Earlier the effective mass of quark was incorporated in our calculations, and in the current work, it is modified for static and time-varying magnetic fields. Thermodynamic observables including pressure, energy density, and entropy are calculated for a wide range of temperature- and time-dependent as well as time-independent magnetic fields. Finally, we claim that the EoS are highly affected in the presence of a magnetic field. Our results are notable compared to other approaches and found to be advantageous for the measurement of QGP equation of state. These crucial findings with and without time-varying magnetic field could have phenomenological implications in various sectors of high-energy physics.

Polarization effects in higher-order guiding-centre Lagrangian dynamics

The extended guiding-centre Lagrangian equations of motion are derived by the Lie-transform perturbation method under the assumption of time-dependent and inhomogeneous electric and magnetic fields that satisfy the standard guiding-centre space–time orderings. Polarization effects are introduced into the Lagrangian dynamics by the inclusion of the polarization drift velocity in the guiding-centre velocity and the appearance of finite-Larmor-radius corrections in the guiding-centre Hamiltonian and guiding-centre Poisson bracket.

Faraday kinks connecting parametric waves in magnetic wires

Kinks are domain walls connecting symmetric equilibria and emerge in several branches of science. Here, we report topological kinks connecting Faraday-type waves in a magnetic wire subject to dissipation and a parametric injection of energy. We name these structures Faraday kinks. The wire magnetization is excited by a time-dependent magnetic field and evolves according to the one-dimensional Landau–Lifshitz–Gilbert equation. In the case of high magnetic anisotropy and low energy injection and dissipation, this model is equivalent to a perturbative sine-Gordon equation, which exhibits 2π kinks that connect uniform states. We show that kinks connecting Faraday-type waves also exist in the damped and parametrically driven sine-Gordon equation, corresponding to the localized structures observed in the magnetic system. The solutions are robust; indeed, the bifurcation diagram reveals that kinks are stable, independently if the Faraday patterns are standing waves or have a dynamic amplitude or phase. Analysis of the nearly integrable limit of the sine-Gordon equation, as well as its description in terms of a fast and a slow variable, i.e., the Kapitza limit, provide a useful interpretation of the kink as a non-parametric emitter that barely alters the fast standing waves. The existence of topological kinks connecting Faraday-type waves in the parametrically driven and damped Landau–Lifshitz–Gilbert and sine-Gordon equations, which model magnetic media, forced pendulum chains, and Josephson junctions, among other systems, suggest the universality of this self-organized structure.

Collective excitations of a hot QCD medium in a time dependent background magnetic field

Collective excitations of a hot QCD medium in a time dependent background magnetic field

Variational formulation of higher-order guiding-center Vlasov–Maxwell theory

Extended guiding-center Vlasov–Maxwell equations are derived under the assumption of time-dependent and inhomogeneous electric and magnetic fields that obey the standard guiding-center space-timescale orderings. The guiding-center Vlasov–Maxwell equations are derived up to second order, which contains dipole and quadrupole contributions to the guiding-center polarization and magnetization that include finite-Larmor-radius corrections. Exact energy-momentum conservation laws are derived from the variational formulation of these higher-order guiding-center Vlasov–Maxwell equations.

Dynamic hysteretic properties and specific loss power of magnetic nanoparticles in a viscous medium at different thermal baths

A system of magnetite single-domain magnetic nanoparticles in an aqueous colloidal suspension at different temperatures is simulated. In this framework, we study the response of the magnetization of the system to the presence of a time-dependent magnetic field at certain frequency. To do that, a Hamiltonian that includes the Zeeman interaction and the uniaxial magneto-crystalline anisotropy energy is considered. The dynamics of the system is driven according to the solution of the stochastic Landau–Lifshitz–Gilbert differential equation, in combination with a torque term acting on the nanoparticle in order to consider the Brown rotation influenced by the viscosity of the solvent. Thus, both Néel and Brown rotation mechanisms are contemplated. Our results allow us to conclude that Brownian relaxation mediates the alignment of the anisotropy axes with the external field. Thus, depending on the solid or liquid state of the solvent, it is possible to determine the conditions under which a reinforcement of the magnetic anisotropy can take place in order to increase the remanence and squareness of the hysteresis loops.

Exploring the equilibrium and dynamic phase transition properties of the Ising ferromagnet on a decorated triangular lattice.

We study the equilibrium and dynamic phase transition properties of a two-dimensional Ising model on a decorated triangular lattice under the influence of a time-dependent magnetic field composed of a periodic square wave part plus a time-independent bias term. Using Monte Carlo simulations with a standard Metropolis algorithm, we determine the equilibrium critical behavior in zero field. At a fixed temperature corresponding to the multidroplet regime, we locate the relaxation time and the dynamic critical half period at which a dynamic phase transition takes place between ferromagnetic and paramagnetic states. Benefiting from finite-size scaling theory, we estimate the dynamic critical exponent ratios for the dynamic order parameter and its scaled variance, respectively. The response function of the average energy is found to follow a logarithmic scaling as a function of lattice size. At the critical half period and in the vicinity of a small bias field regime, the average of the dynamic order parameter obeys a scaling relation with a dynamic scaling exponent which is very close to the equilibrium critical isotherm value. Finally, in the slow critical dynamics regime, investigation of metamagnetic fluctuations in the presence of bias field reveals a symmetric double-peak behavior for the scaled variance contours of the dynamic order parameter and average energy. Our results strongly resemble those previously reported for kinetic Ising models.

Constraints on axion-like dark matter from a SERF comagnetometer

Ultralight axion-like particles are well-motivated relics that might compose the cosmological dark matter and source anomalous time-dependent magnetic fields. We report on terrestrial bounds from the Noble And Alkali Spin Detectors for Ultralight Coherent darK matter (NASDUCK) collaboration on the coupling of axion-like particles to neutrons and protons. The detector uses nuclei of noble-gas and alkali-metal atoms and operates in the Spin-Exchange Relaxation-Free (SERF) regime, achieving high sensitivity to axion-like dark matter fields. Conducting a month-long search, we cover the mass range of 1.4 × 10−12 eV/c2 to 2 × 10−10 eV/c2 and provide limits which supersede robust astrophysical bounds, and improve upon previous terrestrial constraints by over two orders of magnitude for many masses within this range for protons, and up to two orders of magnitude for neutrons. These are the sole reliable terrestrial bounds reported on the coupling of protons with axion-like dark matter, covering an unexplored terrain in its parameter space.

Singlet-triplet dephasing in radical pairs in avian cryptochromes leads to time-dependent magnetic field effects.

Cryptochrome 4a (Cry4a) has been proposed as the sensor at the heart of the magnetic compass in migratory songbirds. Blue-light excitation of this protein produces magnetically sensitive flavin-tryptophan radical pairs whose properties suggest that Cry4a could indeed be suitable as a magnetoreceptor. Here, we use cavity ring-down spectroscopy to measure magnetic field effects on the kinetics of these radical pairs in modified Cry4a proteins from the migratory European robin and from nonmigratory pigeon and chicken. B1/2, a parameter that characterizes the magnetic field-dependence of the reactions, was found to be larger than expected on the basis of hyperfine interactions and to increase with the delay between pump and probe laser pulses. Semiclassical spin dynamics simulations show that this behavior is consistent with a singlet-triplet dephasing (STD) relaxation mechanism. Analysis of the experimental data gives dephasing rate constants, rSTD, in the range 3-6 × 107s-1. A simple "toy" model due to Maeda, Miura, and Arai [Mol. Phys. 104, 1779-1788 (2006)] is used to shed light on the origin of the time-dependence and the nature of the STD mechanism. Under the conditions of the experiments, STD results in an exponential approach to spin equilibrium at a rate considerably slower than rSTD. We attribute the loss of singlet-triplet coherence to electron hopping between the second and third tryptophans of the electron transfer chain and comment on whether this process could explain differences in the magnetic sensitivity of robin, chicken, and pigeon Cry4a's.

Real-time time-dependent self-consistent field methods with dynamic magnetic fields.

The first finite basis set implementation of the real-time time-dependent self-consistent field method in a dynamic (time-dependent) magnetic field using London atomic orbitals (LAOs) is presented. The accuracy of the finite basis approach using LAOs is benchmarked against numerical results from the literature for the hydrogen atom and H2 in the presence of rapidly oscillating magnetic fields. This comparison is used to inform the choice of appropriate basis sets for studies under such conditions. Remarkably, relatively modest compact LAO basis sets are sufficient to obtain accurate results. Analysis of electron dynamics in the hydrogen atom shows that LAO calculations correctly capture the time evolution of orbital occupations. The Fourier transformation of the autocorrelation function yields a power spectrum exhibiting harmonics associated with coherent emission, which closely matches the literature and further confirms the accuracy of this approach. The dynamical response of the electron density in H2 for a magnetic field parallel to the internuclear axis shows similar behavior to benchmark studies. The flexibility of this implementation is then demonstrated by considering how the dynamical response changes as a function of the orientation of the molecule relative to the applied field. At non-parallel orientations, the symmetry of the system is lowered and numerical benchmark data, which exploit cylindrical symmetry, are no-longer readily available. The present study demonstrates the utility of LAO-based calculations for extreme dynamic magnetic fields, providing a stress-test on the choice of basis. Future applications of this approach for less extreme dynamic magnetic fields are briefly discussed.

Dynamic hysteresis curves in a mixed spin ternary alloy system under the time-dependent magnetic field

The dynamic hysteresis features in mixed spins- ternary alloy system with formula was examined through Glauber-type stochastic dynamic and mean-field theory. The system includes two interpenetrating lattices, one has spin- components, while the other includes a random distribution of spin- and spin- compounds. The effect of Hamiltonian parameters on the dynamic hysteresis behavior was investigated in detail and observed that the physical characteristics have a significant impact on the shape and quantity of hysteresis loops. We compared our findings with other theoretical and experimental investigations; a high level of agreement is obtained.