Direction Of External Field Research Articles

Excitation of transverse and longitudinal thermomagnetic waves in anisotropic conducting media in the presence of a temperature gradient T   without an external magnetic fieldH

In anisotropic conducting media, the excited thermomagnetic waves at different orientations of the magnetic field and temperature gradient significantly depend on the direction of the anisotropic medium. Theoretical calculations of the electrical conductivity tensor ik  depending on the frequency of the thermomagnetic wave are of scientific interest. In this theoretical work, the frequencies of the thermomagnetic wave at k T   || and at k T ⊥   were found, and it was proven that at longitudinal (k T   || ) and at transverse (k T ⊥   ) the direction of the frequency and growth rate of these waves depends differently on the external magnetic field. The work theoretically studies the conditions for the excitation of thermomagnetic waves. It is indicated that the directions of external fields play a significant role in the appearance of growing waves in the sample. It is shown that, depending on the value of the electrical conductivity tensor ik  , thermomagnetic waves are excited in the longitudinal (i.e. k T   || ) and transverse (i.e.k T ⊥   ) directions. The frequencies of these thermomagnetic waves in both the longitudinal and transverse directions have been calculated. The growth rates of these waves are determined by the values of the inverse electrical conductivity tensor ik  . It has been proven that the excited wave is mainly of a thermomagnetic nature. In theory, the dispersion equation obtained is of algebraically high powers relative to the oscillation frequency. The dispersion equation in both cases (longitudinalk T   || and transverse k T ⊥   ) contains terms in which there are thermomagnetic frequencies in a low degree of frequency. It has been proven that if the value of the electrical conductivity tensor ik  is the same, then the propagation frequencies of thermomagnetic waves are different. The theory is constructed without an external magnetic field 0 0 =Н . In the presence of an external magnetic field, the conditions for the excitation of thermomagnetic waves, and of course the conditions for their growth, will change significantly.

Electron-related properties in a GaAs/GaAlAs Ultra-thin Core/Shell Film through external field direction for energy and photonic devices

In this paper, we investigate electron-related properties in GaAs/GaAlAs Ultra-thin Core/Shell Film Quantum dot (UTFQD), examining the effects of external electric fields, impurity position, and aluminum fraction. Using the finite element method (FEM) within the effective mass approximation, we numerically solve the Schrödinger equation to study electron probability density, polarizability, and diamagnetic susceptibility. Our results reveal that electric fields can shift electron probability density due to the Stark effect. Without an electric field, the density centers near the shell's core. However, with electric fields, the density shifts opposite to the field's direction. Moreover, polarizability and diamagnetic susceptibility are influenced by impurity position and aluminum fraction. Polarizability increases with electric field intensity, indicating a transition from geometrical confinement to the Stark effect. Diamagnetic susceptibility rises with more aluminum, indicating stronger electron confinement. These findings suggest that adjusting electric fields, impurity position, and aluminum fraction in GaAs/GaAlAs core/shell QDs can control electron behavior, providing flexibility for photonic devices. This study offers a foundation for optimizing quantum dot-based components in photonics.

Influence of external electric field on molecular microscopic descriptors of non-polar insulating gases

Molecular microscopic descriptors without external electric field are commonly used for the prediction of insulating strength in gaseous medium. However, insulating gases are subjected to external electric field in electrical equipment, and the molecular microscopic descriptors are affected. Therefore, it is necessary to investigate the influence of external electric field on molecular microscopic descriptors. The Gaussian 16 software is employed to calculate the global parameter with the M06-2X functional and the 6–311++G (d, p) basis set. The wavefunction analysis software Multiwfn is used to obtain electrostatic potential parameter. In this paper, three directions of external electric field with magnitude ranging from 0 to 0.03 a.u. have been set to 10 non-polar insulating gas molecules. According to the type of molecular point group, the effect of direction and magnitude of external electric field on the molecular microscopic descriptors has been analyzed. The results indicate that the molecular microscopic descriptors are affected by the direction and magnitude of external electric field. The change rate of microscopic descriptors increases with the increase of external electric field magnitude. And the influence of the direction of external electric field is associated with the molecular symmetry. In conclusion, the effect of direction and magnitude of external electric field is needed to be considered when analyzing the effect of external electric field on molecular microscopic descriptors.

Several nonlinear waves in spin quantum electron-positron plasmas

By using the reductive perturbation method, we obtained a nonlinear Schrödinger equation considering spin properties for a magnetized electron-positron plasmas. Several nonlinear wave were studied. The results indicate that various types of nonlinear waves exist in a magnetized electron-positron plasmas and they are spatially localized in both parallel and vertical directions to the external magnetized field direction. Additionally, the dependence of the wave amplitudes, wave width in both direction, group velocity of the envelop wave and the phase velocity of the background waves for these kinds of the nonlinear waves on the system parameters are all given in the present paper.

Polarization properties of E-FISH signals and optimization of simultaneous measurement of electric field vectors

Electric field measurements based on the electric field induced second harmonic (E-FISH) method have been employed in a wide range of studies. Most studies typically measure two components of the electric field separately. Although there have been proposals for the simultaneous measurement of electric field vectors, the imbalance of the two corresponding E-FISH signals has limited its application. Furthermore, the relationship between the polarization of the E-FISH signal and the direction of external electric field remains unclear. In this paper, the general expressions for the polarization and power of both components of E-FISH signals are derived, assuming arbitrary probe beam polarization and external electric field direction. The theoretical results indicate that the polarization of E-FISH signals varies along the interaction length. The final signal’s polarization is elliptically polarized for arbitrary electric field distribution and is correlated with the polarization of the probe beam, which deviates from what is commonly assumed to be consistent with the external electric field. If the polarization of the probe beam is not parallel to the axes, the power of each signal component is determined by both components of the external electric field, which lays the foundation for the simultaneous measurement of electric field vectors. This theoretical prediction is subsequently validated by experimental results. Finally, the power maps suggest that the optimal polarization angle of the probe beam is 45° or 135° to achieve a balanced signal power when measuring an unknown electric field vector. Both components of the electric field can be simultaneously obtained according to the theoretical relationship.

Hysteresis in ferromagnetic materials and its impact on electromagnetic devices

Hysteresis is a well-known phenomenon that occurs in ferromagnetic materials when they are exposed to external magnetic fields. It results from the inherent properties of magnetic domains, which tend to align with an external field direction. However, when the field is removed, the domains do not return to their initial state but retain a residual magnetization, which leads to the formation of a hysteresis loop. This behavior can have a significant impact on the performance of electromagnetic devices, which rely on magnetic fields for their operation. Hysteresis can cause energy losses, reduced efficiency, and other undesired effects in devices such as transformers, inductors, and electric motors

Structural Transformation of Relaxor PbMg1/3Nb2/3O3 under Electric Field Switching

One from the most important features of relaxors is the possibility of inducing a stable ferroelectric phase in them by applying an electric field. This induced phase transition has been multiply investigated; in particular, the structural changes during the transition and its kinetics were traced. However, the processes occurring under electric field switching remain little-studied, and the structural transformation during switching generally has not been investigated at all. To fill in this gap, the temporal evolution of the pattern of Bragg and diffuse X-ray scattering in classical relaxor PbMg1/3Nb2/3O3 at T = 175 K during multiple switching of the direction of a dc external field E = ± 6.25 kV/cm has been traced. It is shown that the glasslike state decays and a mixed ferro-glass phase is formed after switched on a field. Switching the field sign leads to a rise of dipole-glass correlations; however, no signs of occurrence of an inhomogeneous state with limited regions of dipole-glass and ferroelectric phases were found. Switching the field to the initial direction causes rapid formation of the ferroelectric phase, without any rise in the dipole-glass correlations. A repeated switching leads to a decrease in the Bragg scattering intensity, related to the long-range order. This effect is presumably due to the occurrence of random weakly correlated ionic displacements, violating the long-range order.

Numerical Study of Free-Surface Electro-Hydrodynamic Wave Turbulence

Direct numerical simulation of threedimensional chaotic motion of a dielectric liquid with a free surface under the action of external horizontal electric field is carried out. The numerical model takes into account the effects of surface tension, viscosity, and external isotropic random forcing acting at large scales. A transition from dispersive capillary wave turbulence to quasi-isotropic nondispersive EHD surface turbulence with increase of the external electric field strength is numerically observed for the first time. At the regime of developed EHD wave turbulence, the total electrical energy is found to be much greater than the energy of capillary waves, i.e., electrohydrodynamic effects play a dominant role. At the same time, anisotropic effects are detected that lead to the generation of capillary wave packets traveling perpendicular to the external field direction. Despite the revealed anisotropy, the calculated spectrum of EHD wave turbulence is in very good agreement with the analytical spectrum obtained on the basis of dimensional analysis of weak turbulence spectra.

Switching of magnon parametric oscillation by magnetic field direction

Parametric oscillation occurs when the resonance frequency of an oscillator is periodically modulated. Owing to time-reversal symmetry breaking in magnets, nonreciprocal magnons can be parametrically excited when spatial-inversion symmetry breaking is provided. This means that magnons with opposite propagation directions have different amplitudes. Here we demonstrate switching on and off the magnon parametric oscillation by reversing the external field direction applied to a Y3Fe5O12 micro-structured film. The result originates from the nonreciprocity of surface mode magnons, leading to field-direction dependence of the magnon accumulation under nonuniform microwave pumping. Our numerical calculation well reproduces the experimental result.

Instability in multi-valley semiconductors in external electric and magnetic fields

It is theoretically proved that the excited wave in two-valley semiconductors is growing. It is indicated that the directions of external fields play an essential role for the appearance of growing waves in the sample. It is shown that oscillations can occur at certain values of the sample dimensions L ,L ,L x y z Analytical formulas for the frequency of the growing waves are obtained. The interval of variation of the external electric field in a strong magnetic field μH>>c has been determined. The paper takes into account that the time of transition from the lower valley to the upper valley differs from the time of transition from the upper valley to the lower valley. It means τ12≠τ21 In the sample, the total concentration is constant, and therefore, n0=na+nb=const. The changes in the corresponding concentrations are equal to each other and have na'=-nb'. It is taken into account that at critical values of the electric and magnetic fields and the corresponding concentrations they change as a monochromatic wave. And the change in these quantities differs little from their equilibrium value. For simplicity of mathematical calculations, the external electric and magnetic fields are directed in the same way, i.e. in x direction. Since the current oscillations in one direction (for example, along x) are studied in the experiment, the following equalities were taken into account jy'=0, jz'=0. In the vicinity of the critical field at the beginning of the current oscillation in the sample, the current oscillation frequency is ϖ=ϖ0+iϖ1, ϖ1<<ϖ0. In addition, the magnitude of the critical electric field, at which the current fluctuation changes depending on the magnetic field as follows .

Crystal structure and hyperfine fields in Mn1-xCoFexGe magnetocaloric alloys

The phase analysis, site occupancy, and hyperfine fields of Mn1-xCoFexGe alloys (x = 0.01, 0.02, 0.05, 0.10) after annealing procedure are presented. Mössbauer spectroscopy was performed with and without an external magnetic field of B = 1.3 T parallel to the beam direction at room temperature. RT X-ray diffraction shows that Mn1-xCoFexGe compound is two phases of Ni2In type hexagonal structure and TiNiSi type orthorhombic structure. Samples with × = 0.01, 0.02 and 0.05 exhibit (pre)dominant contribution of the orthorhombic structure while for × = 0.10 the amount of hexagonal phase increases significantly. In the series of alloys, the lattice parameters and the unit cell volume remain weakly sensitive to Fe concentration at both phases. The measured Mössbauer spectra are superpositions of two doublets which come from the Fe atoms located in hexagonal structure and two broad sextets of Fe orthorhombic structure origin. The values of isomer shift and quadrupole splitting do not change with increasing iron content. Using information about the composition of local environment and measured values of isomer shift, we identify the origin of individual subspectra, and site occupation of Fe atoms in the alloy. The Mössbauer measurements carried out in external magnetic field shows that induced hyperfine magnetic field in the hexagonal structure is much higher for Fe at Mn sites than the observed for Fe at Co sites. Analysis of sextets shows that magnetic moments of Fe tend to arrange along the external field direction with almost the same values of about 0.87 µB.

High order harmonic spectra of CO under external electrostatic field

In this work, we use Lewenstein’s theory to calculate the high order harmonic spectra of CO molecule in a linearly polarized laser field combined with an external electrostatic field. The results show that the characteristics of the high order harmonic spectra of CO molecule depend strongly on the direction of the external static electric field relative to the orientation of CO. Especially, when the direction of the external static electric field points from O to C, the plateau of the harmonic spectrum becomes wider and the cutoff frequency reaches a larger value than the scenario without external static electric field. When the direction of the external static electric field points from C to O, the harmonic spectrum shows a double-plateau structure. Using Lewenstein theory, these phenomena can be understood from the viewpoint that the harmonic generation comes from a coherent superposition of the contributions of two kinds of channels characterized by C-end ionization and O-end ionization. The C(O)-end ionization channel means that the electron is ionized from C(O) end, then accelerated by the driving electric field, finally recombines with its parent molecular ion at either C or O end, emitting the harmonics. For the same harmonic order, the contribution of the C-end ionization channel is greater than that of the the O-end ionization channel. The two kinds of channels emit harmonics in adjacent half period of laser, where the external static electric field causes the cutoff frequency of the harmonic spectrum to increase and decrease in the adjacent half period of the laser field. Especially, when the direction of external static electric field is from the C to O, the cutoff frequency of the harmonic spectrum of the C-end ionization channel decreases, resulting in a higher first plateau in the spectrum. While, the increase of cut-off frequency of the O-end ionization channel will result in a lower second plateau. When the direction of external static electric field is reversed, the cutoff frequency of the harmonics of the C-end channel increases, leading the plateau of CO harmonic spectrum to become wider than that without the external static electric field. The cut-off frequency of the O-end ionization channel decreases. Because the contribution of the O-end ionization channel can be ignored compared with that of C-end ionization channel, the C-end channel dominates the contribution to harmonic generation and hence there is only one plateau in the harmonic spectrum. This work provides a clear physical picture for the formation of a double-plateau structure of CO harmonic spectra under the action of an external static electric field.

Atomic Force Manipulation of Single Magnetic Nanoparticles for Spin-Based Electronics.

Magnetic nanoparticles (MNPs) are instrumental for fabrication of tailored nanomagnetic structures, especially where top-down lithographic patterning is not feasible. Here, we demonstrate precise and controllable manipulation of individual magnetite MNPs using the tip of an atomic force microscope. We verify our approach by placing a single MNP with a diameter of 50 nm on top of a 100 nm Hall bar fabricated in a quasi-two-dimensional electron gas (q2DEG) at the oxide interface between LaAlO3 and SrTiO3 (LAO/STO). A hysteresis loop due to the magnetic hysteresis properties of the magnetite MNPs was observed in the Hall resistance. Further, the effective coercivity of the Hall resistance hysteresis loop could be changed upon field cooling at different angles of the cooling field with respect to the measuring field. The effect is associated with the alignment of the MNP magnetic moment along the easy axis closest to the external field direction across the Verwey transition in magnetite. Our results can facilitate experimental realization of magnetic proximity devices using single MNPs and two-dimensional materials for spin-based nanoelectronics.

Thermodynamic parameters of atomically thin superconductors derived from the upper critical field

The amplitude of the ground-state superconducting energy gap Δ(0) and relative jump in the electronic specific heat at the transition temperature are the primary fundamental parameters of any superconductor. Several well-established techniques are available for measuring these values in bulk samples. However, a limited number of techniques can be used to measure these parameters in atomically thin superconductors. Here, we propose a new approach for extracting Δ(0) and in atomically thin superconductors by utilizing the upper critical field data from perpendicular, B c2,Δ(T) (when a magnetic field is applied perpendicular to the film surface) and parallel, B c2,‖(T) (when a magnetic field is applied in the direction parallel to the film surface), external field directions. The deduced parameters for few-layer-thick Al, Sn, NbSe2, MoS2, magic angle twisted trilayer graphene, and WTe2 are well-matched values expected for strongly and moderately strongly coupled electron-phonon-mediated superconductors. In many reports, the enhancement of B c2,‖(0) above the Pauli–Clogston–Chandrasekhar limiting field (i.e. magnetic field required to break the Cooper pair) in atomically thin superconductors has been explained based on the assumption of exotic pairing mechanisms, for instance, Ising-type pairing. Here, we explain the observed B c2,‖(0) enhancement based on the geometrical enhancement factor that originates from the sample geometry. This approach does not assume the existence of novel exotic pairing mechanisms in atomically thin superconductors.

Na uptake at TiO2 anatase surfaces under electric field control: A first-principles study

Na-ion batteries (NIBs) are promising devices for large-scale energy-storage facilities. Nanostructured TiO2 is an efficient NIB negative electrode, showing good cycling performance and rate capability, but its activity depends on the crystalline facets exposed by anatase nanoparticles. Hence, we propose here a DFT+U study of Na+ adsorption and insertion at (101), (100) and (001)-TiO2 surfaces under the influence of external electric fields, which are simulated by adding a sawtooth-like electrostatic potential to the bare ionic potential. We find that field polarization affects Na+ uptake as well as titania electronic features, promoting redox processes within Ti sublattice, as in battery charge/discharge cycling. Our results highlight the high-energy (001) surface to be the most active, for both directions of external fields, proving its activity to be exerted reversibly. Besides further insights, these outcomes pave the route for further exploration and design of electrode materials by simulation of battery in operando conditions.Graphical

Anisotropic effect of dipolar interaction in ordered ensembles of nanoparticles

We implement extensive computer simulations to investigate the hysteresis characteristics in the ordered arrays (lx × ly) of magnetic nanoparticles as a function of aspect ratio Ar = ly/lx, dipolar interaction strength hd, and external magnetic field directions. We have considered the aligned anisotropy case, α is the orientational angle. It provides an elegant en route to unearth the explicit role of anisotropy and dipolar interaction on the hysteresis response in such a versatile system. The superparamagnetic character is dominant with weak dipolar interaction (hd ≤ 0.2), resulting in the minimal hysteresis loop area. Remarkably, the double-loop hysteresis emerges even with moderate interaction strength (hd ≈ 0.4), reminiscent of antiferromagnetic coupling. These features are strongly dependent on α and Ar. Interestingly, the hysteresis loop area increases with hd, provided Ar is enormous, and the external magnetic field is along the y-direction. The coercive field μoHc, remanent magnetization Mr, and the heat dissipation EH also depend strongly on these parameters. Irrespective of the external field direction and weak dipolar interaction (hd ≤ 0.4), there is an increase in μoHc with hd for a fixed α and Ar ≤ 4.0. The dipolar interaction also elevates Mr as long as Ar is huge and the field is along the y-direction. EH is minimal for negligible and weak dipolar interaction, irrespective of Ar, α, and the field directions. Notably, the magnetic interaction enhances EH if Ar is enormous and the magnetic field is along the long axis of the system. These results are beneficial in various applications of interest such as digital data storage and spintronics. (a) Schematic of the two-dimensional array of nanoparticles. The variation of heat dissipation EH as a function of dipolar interaction strength and anisotropy axis orientation is shown in (b)-(d). The magnetic field is applied along the x-direction in (b). In contrast, the magnetic field is applied along y-direction in (c) and (d).

Ramping Loss Analysis of No-Insulation HTS Coil Under External Field Using an Improved Equivalent Circuit Model

This article is to study the ramping loss of no-insulation (NI) high temperature superconducting (HTS) racetrack coil under different external magnetic fields during the charging process. An improved equivalent circuit electromagnetic coupling model for the HTS racetrack coil is introduced with good computational speed and accuracy. The simulation results show that the turn-to-turn loss and the change of the lowest critical current position are obviously influenced by the external field that have not been mentioned from our knowledge base. During the rising time of the power supply current, there is little difference in current distribution under different external fields. But after that, steady state of charging process is deeply influenced by the external field. A larger external field will lead to a lower charging current and the external bypass phenomenon. Another result is that the relationship between direction of external field and orientation of coils will influence the lowest critical current amount and position. The relationship between turn-to-turn resistivity of co-winding materials and ramping loss is also discussed in the paper. The distributed unevenness and value of the turn-to-turn loss change with the increase of turn-to-turn resistivity.

A femtosecond magnetic circular dichroism spectrometer.

We describe the development of a broadband magneto-optical spectrometer with femtosecond temporal resolution. The absorption spectrometer is based on a white-light supercontinuum (∼320 to 750 nm) using shot-to-shot temporal and spectral referencing at 1 kHz. Static and transient absorption spectra using circularly polarized light are collected in a magnetic field. The difference spectra with respect to the external field direction give the static and transient magneto-optical Faraday rotation (magnetic optical rotary dispersion) and ellipticity (magnetic circular dichroism) spectra. An achromatic quarter-wave plate is used, and the impact of the deviation from ideal retardance on the spectra is discussed. Results from solution-based and thin-film samples are used to demonstrate the performance and wide applicability of the instrument. The sensitivities for the static and time-resolved data were found to be 5 and 0.4 mdeg, respectively. The method presents a simple way to measure magneto-optical spectra using a transient absorption spectrometer and an electromagnet.

Measurement of the thulium ion spin Hamiltonian in an yttrium gallium garnet host crystal

We characterize the magnetic properties for thulium ion energy levels in the Y$_3$Ga$_5$O$_{12}$ (Tm:YGG) lattice with the goal to improve decoherence and reduce line-width broadening caused by local host spins and crystal imperfections. More precisely, we measure hyperfine tensors for the lowest level of the, $^3$H$_6$, and excited, $^3$H$_4$, states using a combination of spectral hole burning, absorption spectroscopy, and optically detected nuclear magnetic resonance. By rotating the sample through a series of angles with an applied external magnetic field, we measure and analyze the orientation dependence of the Tm$^{3+}$ ion's spin-Hamiltonian. Using this spin-Hamiltonian, we propose a set of orientations to improve material properties that are important for light-matter interaction and quantum information applications. Our results yield several important external field directions: some to extend optical coherence times, another to improve spin inhomogeneous broadening, and yet another that maximizes mixing of the spin states for specific sets of ions, which allows improving optical pumping and creation of lambda systems in this material.

Magnetic relaxation time for an ensemble of nanoparticles with randomly aligned easy axes: A simple expression

A critical parameter in characterizing the properties of single-domain nanoparticles is their magnetic relaxation time. It must be known, for example, to estimate the anisotropy from magnetization versus temperature measurements. The time it takes for the magnetization to relax also determines the behavior of particles in various oscillating applied fields, which is critically important for their application in magnetic particle imaging and hyperthermia treatment. However, an analytic expression for this relaxation time has been generally missing. Brown's [Phys. Rev. 130, 1677 (1963)] famous result is only valid for the easy anisotropy axes of each particle in the ensemble aligned along the external field direction and overestimates the relaxation time. Despite this overestimation, this expression is most commonly used to extract magnetic nanoparticle parameters such as anisotropy energy from magnetometry data. Here, we use Brown's formalism to derive a different, simple, approximate relaxation time expression that is valid for randomly aligned easy axes. Using parameters appropriate for magnetite, we compare our results to other results in the literature and with stochastic Landau-Lifshitz-Gilbert simulations to show that our result is more accurate across a range of applied field strengths and temperatures. We note that several analytic expressions for the relaxation time do a reasonable job, as long as one uses a full calculation for the attempt time, rather than the commonly used estimate ${\ensuremath{\tau}}_{0}\ensuremath{\sim}1$ ns.