Terahertz‐induced effects on excitons in magnetic field

The Feranchuk-Komarov operator method is developed by combining with the Levi-Civita transformation in order to construct analytical solutions of the Schrödinger equation for a two-dimensional exciton in a uniform magnetic field of arbitrary strength. As a result, analytical expressions for the energy of the ground and excited states are obtained with a very high precision of up to four decimal places. Especially, the precision is uniformly stable for the whole range of the magnetic field. This advantage appears due to the consideration of the asymptotic behaviour of the wave-functions in strong magnetic field. The results could be used for various physical analyses and the method used here could also be applied to other atomic systems.

Nanocomposites and magnetic field effects on nanostructures have received great attention in recent years. A large amount of research work was focused on developing the proper theoretical framework for describing many physical effects appearing in structures on nanoscale level. Great step in this direction was successful application of nonlocal continuum field theory of Eringen. In the present paper, the free transverse vibration analysis is carried out for the system composed of multiple single walled carbon nanotubes (MSWCNT) embedded in a polymer matrix and under the influence of an axial magnetic field. Equivalent nonlocal model of MSWCNT is adopted as viscoelastically coupled multi-nanobeam system (MNBS) under the influence of longitudinal magnetic field. Governing equations of motion are derived using the Newton second low and nonlocal Rayleigh beam theory, which take into account small-scale effects, the effect of nanobeam angular acceleration, internal damping and Maxwell relation. Explicit expressions for complex natural frequency are derived based on the method of separation of variables and trigonometric method for the “Clamped-Chain” system. In addition, an analytical method is proposed in order to obtain asymptotic damped natural frequency and the critical damping ratio, which are independent of boundary conditions and a number of nanobeams in MNBS. The validity of obtained results is confirmed by comparing the results obtained for complex frequencies via trigonometric method with the results obtained by using numerical methods. The influence of the longitudinal magnetic field on the free vibration response of viscoelastically coupled MNBS is discussed in detail. In addition, numerical results are presented to point out the effects of the nonlocal parameter, internal damping, and parameters of viscoelastic medium on complex natural frequencies of the system. The results demonstrate the efficiency of the suggested methodology to find the closed form solutions for the free vibration response of multiple nanostructure systems under the influence of magnetic field.

In van der Waals heterostructures consisting of stacked ${\mathrm{MoSe}}_{2}$ and ${\mathrm{WSe}}_{2}$ monolayers, optically bright interlayer excitons can be observed when the constituent layers are crystallographically aligned. The symmetry of the monolayers allows for two different types of alignment, in which the momentum-direct interlayer transitions are either valley conserving ($R$-type alignment) or change the valley index ($H$-type antialignment). Here, we study the valley polarization dynamics of interlayer excitons in magnetic fields up to 30 T by time-resolved photoluminescence. For all interlayer exciton types, we find a finite initial photoluminescence circular degree of polarization after unpolarized excitation in applied magnetic fields. For interlayer excitons in $H$-type heterostructures, we observe a systematic increase of the photoluminescence circular degree of polarization with time in applied magnetic fields, which saturates at values close to unity for the largest fields. By contrast, for interlayer excitons in $R$-type heterostructures, the photoluminescence circular degree of polarization shows a decrease and a zero crossing before saturating with opposite polarization. This unintuitive behavior can be explained by a model considering the different interlayer exciton states in $H$- and $R$-type heterostructures and their selection rules coupling photoluminescence helicity and valley polarization.

You surely know that the transition between the liquid and solid states of materials can be controlled by changing the temperature. But did you know that this transition can also be achieved using magnetic or electric fields? Some substances, called electrorheological fluids, can have their physical state changed by an electric field. Other substances, called magnetorheological fluids, can change states under the influence of a magnetic field. Under the influence of magnetic or electric fields, the particles in these fluids line up a certain way, forming structures that turn the fluids from a flowing liquid to a state similar to a solid. Electrorheological and magnetorheological fluids have many possible uses, such as controlling vibration in buildings or in certain kinds of robots.

The influence of a constant magnetic field on the radiation of a planar undulator is studied analytically with the use of generalized Bessel functions. The behavior of on-axis radiation intensity in the undulator and the influence of a magnetic field on off-axis radiation are investigated. The critical magnitude of the constant component of the magnetic field, which has a significant effect on the motion of electrons in the undulator, is calculated analytically. The examples of several planar undulators are discussed, and the influence of the Earth’s magnetic field on their spectrum is evaluated.

The exciton fine structure in self-assembled coupled quantum dots with barriers of varying widths is studied in detail. For narrow barriers we find doublet splittings of the molecule ground state exciton in magnetic field, while for wide barriers in some cases a multiplet of emission lines is observed. Pronounced anticrossings occur in the field dispersion of such a multiplet with details depending on the particular molecule geometry. Strong variations of the fine structure including avoided crossings are observed also for the excited states that arise from the coupling-induced splitting of the quantum dot $s$-shell excitons. Values for the exciton diamagnetic shifts and spin splittings as functions of barrier width are given.

Heat transfer coefficient is a key parameter for efficiency evaluation of heat exchangers. Good stability and high heat transfer coefficient are essential for the application of nanofluids in heat exchangers and solar systems. In this work, nanofluids with good stability were prepared, and the influence of vertical magnetic field on flow and heat exchange of magnetic nanofluids under laminar and turbulent conditions was mainly studied. The flow and heat transfer rules of Fe3O4 nanofluids with or without magnetic field conditions, magnetic field strength, magnetic field distribution, the nanoparticle concentration and nanofluids temperature were systematically studied by setting up an experimental platform. The results show that the intensity and distribution of magnetic field had a significant influence on the heat transfer of magnetic nanofluids, whether in laminar or turbulent flow. When the magnetic field strength is 800G and 1000G, the convective heat transfer coefficient increases by an average of 23.89% and 26.12%. However, the influence of magnetic field on its flow characteristics is not obvious, and the effect on resistance coefficient increases by only 2.01%. In addition, the characteristics of magnetic nanofluids also have a certain influence on its flow and heat transfer. When the temperature of magnetic nanofluids is increased, the convective heat transfer coefficient will increase. When the concentration of magnetic nanofluids is increased, the pressure drop will also increase, but it has little effect on the drag coefficient.

In this study, two dimensional numerical simulations are performed to investigate the influence of the magnetic field on the nanofluid flow inside a sinusoidal channel. This work reveals the influence of variable magnetic field in the heat transfer of heat exchanger while the mixture is in a single phase. In this heat exchanger, the inner tube is sinusoidal and the outer tube is considered smooth. The magnetic field is applied orthogonal to the axis of the sinusoidal tube. In our study, the ferrofluid (water with 4 vol% nanoparticles (Fe3O4)) flows in a channel with sinusoidal bottom. The finite volume method with the SIMPLEC algorithm is used for handling the pressure-velocity coupling. The numerical results present validated data with experimentally measured data and show good agreement with measurement. The influence of different parameters, like the intensity of magnetic field and Reynolds number, on the heat transfer is investigated. According to the obtained results, the sinusoidal formation of the internal tube significantly increases the Nusselt number inside the channel. Our findings show that the magnetic field increases the probability of eddy formation inside the cavities and consequently enhances the heat transfer (more than 200%) in the vicinity of the magnetic field at low Reynolds number (Re=50). In addition, the variation of the skin friction shows that the magnetic field increases the skin friction (more than 600%) inside the sinusoidal channel.

Electrodeposition under the influence of magnetic field has sparked the interest of researchers for decades. The beneficial effects of homogenous magnetic fields on electrodeposition enable increased deposition rates as well as smoother deposits.[1] These effects have been intensively studied and attributed to magnetohydrodynamic forces. Conversely, several questions remain unanswered with respect to the beneficial influence of heterogeneous magnetic fields.[2] There, local convection allows structuring of deposit morphologies; however, it has not yet been reported if magnetic gradients can be used to induce local compositional changes.[3] Here, we investigate the opportunity to induce both, topographic and chemical structuring of electrodeposits by magnetic gradients.For this purpose, potentiostatic deposition of CuNi films is carried out on an Au coated glass slide using a sulphate-based aqueous electrolyte. An iron wire template is magnetised by a permanent magnet and placed below the working electrode to apply well defined, high magnetic gradients during electrodeposition. The obtained Cu-Ni deposits are analysed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The results of these measurements reveal that the deposited film topography follows the distribution of superimposed magnet field gradients and that a local compositional change of the deposits is possible. Thus, our results open up a new path to control the local morphology of complex deposits structures and other properties of films by magnetic gradient assisted electrodeposition. References (1) Uhlemann, M.; Tschulik, K.; Gebert, A.; Mutschke, G.; Fröhlich, J.; Bund, A.; Yang, X.;Eckert, K. Eur. Phys. J. Spec. Top. 2013, 220, 287–302.(2) Mutschke, G.; Tschulik, K.; Weier, T.; Uhlemann, M.; Bund, A.; Fröhlich, J.Electrochimica Acta 2010, 55, 9060–9066.(3) Karnbach, F.; Uhlemann, M.; Gebert, A.; Eckert, J.; Tschulik, K. Electrochimica Acta2014, 123, 477–484

Based on the replica exchange algorithm of the Monte Carlo method, the influence of an external magnetic field on phase transitions and thermodynamic properties of the two-dimensional antiferromagnetic Potts model with the number of spin states q = 4 on a hexagonal lattice was studied. The studies were conducted in the range of external magnetic field values of 0.0 ≤ h ≤ 10.0. The magnetic field is measured in relative units of exchange interaction between nearest neighbors |J1|. The magnetic structures of the ground state were obtained, and the nature of phase transitions in the considered field range was analyzed. It has been established that in the intervals 0.0 ≤ h ≤ 3.0 and 6.0 ≤ h ≤ 6.5 a first-order phase transition is observed. It is shown that at the external magnetic field values h = 3.5 and 5.5, the system is frustrated, and in the magnetic field intervals 4.0 ≤ h ≤ 5.0 and 7.0 ≤ h ≤ 8.5, the system approaches the frustration regime. It was found that strong magnetic fields (h ≥ 9.0) suppress the phase transition in the system.

Influence of magnetic field on photoinduced electron transfer in non-Markovian solvents

Introduction. When heating with high-frequency currents (HFCs) at high speeds, more significant strengthening effects can be observed compared to using machine generators. Therefore, hardening at high frequencies is more efficient. However, the increase in the generator frequency results in a decrease in the depth of penetration of eddy currents and an increased unevenness of heating across the cross-section. The application of a constant external magnetic field during HFC hardening can increase the depth of eddy current penetration and create more uniform heating. Unfortunately, there is not enough information available on the effect of the external magnetic field on HFC heating processes and phase transformations in steel. Currently, there are no quantitative estimates for the impact of an external magnetic field on changes in the kinetics of electric heating and the penetration depth of eddy currents. In connection with the above, the aim of this paper is to investigate changes in the kinetics of high-frequency heating of iron-carbon alloys when an external constant magnetic field is applied and, and, based on this, to consider the potential for technological applications. Materials and Methods. Theoretical assessment of the influence of an external magnetic field on the change in the kinetics of electric heating and the penetration depth of eddy currents is based on the general theory of induction heating kinetics. An experimental study of the influence of a magnetic field on the kinetics of high-frequency current heating was performed on samples of 45 steel, pearlitic gray (SCh30), and ferritic malleable cast iron (KCh30-6). The temperature distribution over the cross-section of ferromagnetic materials during induction heating with an external magnetic field has been studied using special samples of iron, 45 steel, and SCh30 gray pearlitic cast iron. Electric tempering processes have been investigated on samples of U8A steel using a vacuum tube generator (heating temperature — 450℃, heating rate — 750℃/s). Changes in austenite grain size after high-speed heating with external magnetization have been examined on samples of reduced-hardenability 55PP steel. To study the processes of thermal treatment in a magnetic field during experiments involving heating samples using high-frequency currents, a specially designed electromagnet was created to apply an external constant magnetic field. Results. Theoretical curves were constructed for heating conditions with and without an external constant magnetic field. Experimental data on the effect of an external constant magnetic field on induction heating in the surface layer of various materials were summarized in kinetic diagrams. Evidence that the observed changes were due to increased depth of penetration of eddy currents came from experiments on cylindrical samples of 45 steel with different wall thicknesses. Kinetic curves were provided for estimating the temperature field (at 6 points at different depths) during high-frequency current heating with and without external magnetization. The paper presents experimental data on the micro-hardness distribution across the cross-section of a U8 steel sample after quenching, quenching and electric tempering, quenching and electric tempering with external magnetization, and quenching and bulk tempering. It also includes the results of the study of the austenite grain size of 55PP steel after high-speed heating with external magnetization and conventional (slow) deep heating. Discussion. The application of a high-intensity external constant magnetic field during the first quasi-stationary process resulted in a decrease in the rate of induction heating of the ferromagnetic material and an increase in the depth of its uniform heating. However, above the Curie point, the effect of the magnetic field was negligible due to the low magnetic susceptibility of the material, and the heating rate remained unchanged as if there was no field present. In addition, due to the insignificant difference in the values of magnetic permeability below and above the Curie point during heating in the field, the thermal curve did not exhibit the characteristic inflection typical of kinetic curves observed during the transition of the surface layer to a paramagnetic state. Experiments with electric tempering have demonstrated that by applying an external field, it was possible to temper a material to the desired depth and it could be done on a single high-frequency current setup. The size of the austenite grains after high-speed heating with magnetization was reduced compared to conventional deep heating of steel with low hardenability, eliminating the issue of induction heating for low-hardenability steel. Conclusion. The results of the study demonstrated that the use of an external magnetic field enabled the achievement of strengthening effects during heating at higher frequencies, thereby eliminating the drawbacks of such heating methods.

In this paper, the influence of an external magnetic field on the vibration responses and damping ratios of a vascular beam filled with three different magnetic nanofluids are investigated. First, the equation of motion of a fluid-filled beam is derived based on the published works and some amendments are made to simultaneously include the damping ensuing from the surrounding air and also the effects of viscosity of the fluid inside the vascular channel. Then, the effects of an external magnetic field on the viscosity of the fluid is studied and incorporated into the equation of motion of the beam. Finally, based on three magnetic nanofluid samples, simulations are conducted to represent the changes in frequency response functions and damping ratios of the vascular channel beam. The maximum increase observed in the viscosity of the magnetic nanofluid samples under the influence of magnetic field with the intensity of 1 Tesla was 16%, while the reduction in the vibration amplitude found 12.7% and 33.8% for the first and second vibration mode under the influence of the maximum magnetic field intensity of 0.3 Tesla. Moreover, the maximum decrement of the damping ratio was 1.1% under the same magnetic field. However, it seems that the higher values for vibration reduction can be achieved by using the magnetic nanofluid with higher magnetic particle concentrations and viscosities.

In this study, we investigated the effect of a heat source on blood flow through a gradient-tapered vessel under the influence of a gradient magnetic field by reformulating the problem using a mathematical model representing the blood momentum and energy equations. The partial differential equations were dimensionlessly scaled using a scaling parameter and further reduced to a system of ordinary differential equations. A coupled system of regular equations was solved using the series method to obtain analytical solutions for temperature and blood velocity profiles. Numerical simulations were performed using Wolfram Mathematica version 12 and varied parameters relevant to the investigation. The results showed that the relevant parameters—magnetic field, radiation parameter, Grashof number, and tilt angle contributed to liquid temperature and blood flow velocity, respectively. The novelty of this study is the fact that heat can be introduced from a heat source for the purpose of helping blood circulation, and magnetic fields tend to accelerate blood flow velocity.

Results of experimental studies concerning the influence of permanent magnetic field on wear of lubricated sliding contact operating at short stroke and high frequency are presented. It was found that horizontal magnetic is affecting performance of the contact. The jagged delamination regions produced in the presence of a magnetic field can be regarded as easily undergoing oxidisation because oxygen is readily adsorbed there. Magnetic field is likely to intensify the abrasive action by wear particles and mitigate wear of the plate specimen. It is postulated that all these effects are caused by the influence of magnetic field on the electrical charge of the worn surface created on Si3N4 ball.