Magnetic Length Research Articles

Вертикальная проводимость сверхрешетки в продольном сильном магнитном поле при внутризонных и межзонных переходах

The influence of interband and intraband transitions on oscillations of the vertical conductivity of superlattices in a strong magnetic field is considered. It was found that for the charged impurity scattering intraband transitions dominate in magnetic fields up to 2 T, and with a further increase in the magnetic field, interband transitions control. The conductivity oscillations are determined by a ratio of the magnetic length to the superlattice period and the effective mass anisotropy.

Chiral magnetic effect and Maxwell–Chern–Simons electrodynamics in Weyl semimetals

The Weyl semimetal, due to a non-zero energy difference in the pair of Weyl nodes, shows chiral magnetic effect (CME). This leads to a flow of dissipationless electric current along an applied magnetic field. Such a chiral magnetic effect in Weyl semimetals has been studied using the laws of classical electrodynamics. It has been shown that the CME in such a semimetal changes the properties namely, frequency-dependent skin depth, capacitive transport, plasma frequency, etc., in an unconventional way as compared to the conventional metals. In the low-frequency regime, the properties are controlled by a natural length scale due to CME called the chiral magnetic length. Furthermore, unlike the conventional metals, the plasma frequency in this case is shown to be strongly magnetic field-dependent. Since the plasma frequency lies below the optical frequency, the Weyl semimetals will look transparent. Such new and novel observations might help in exploiting these class of materials in potential applications which would completely change the future technology.

Magnetic measurement of nanocrystalline alloy based on a single sheet tester

Generally, for the measurement of magnetic characteristics of nanocrystalline alloy, the magnetic circuit length (lm) is assumed as a constant, and magnetic measurement under square waveform is rarely focused. This paper examines that magnetic characteristics of nanocrystalline alloy are measured under sinusoidal waveforms and square waveforms based on a single sheet tester. The factors related to lm are analysed from the tested data under sinusoidal excitations, and a novel method to determine lm is presented, which is then extended to the case under square waveforms. By combining with a power electronic inverter, the influence of power electronic topology on measurements is analysed, and the oscillation mechanism of excitation current of single sheet tester is revealed. As a result, the limitation of the H-coil method for acquiring the magnetic field intensity H under square waveforms is pointed out, and improved magnetic circuit method is introduced. It is shown from experimental results that the proposed measurement system is useful for acquiring magnetisation characteristics of the nanocrystalline alloy.

Electric‐Potential‐Induced Complete Control of Magnetization in MnZnSb Metallic Ferromagnets

AbstractMagnetoelectric coupling refers to electric‐field control of magnetism, which may offer low‐power memory and beyond‐CMOS electronics. However, contenders of magnetic phase change materials, such as the dilute magnetic semiconductors (DMS), show weak ferromagnetism whereas the ultra‐small screening lengths of robust metallic magnets in artificially stacked thin film heterostructures reduce the extent of controllable magnetization to sub‐atomic distances, thereby diminishing the sheer magnitude of the tunable magnetization. In contrast, an electrochemical control of magnetization has recently been proposed where reversible electrochemistry/ion‐exchange is used to control magnetism in bulk ferromagnets. However, so far, ionic control of magnetism is limited to spinel ferrites and highly correlated oxide systems. Here, it is reported that the ionic control of magnetism can be extended to metallic ferromagnets; complete and reversible switching of ferromagnetism is demonstrated in bulk MnZnSb intermetallic compounds at room temperature. An electrochemically controlled reversible tuning of magnetization across the magnetic phase transition temperature is demonstrated. The observed phenomenon can be explained by the distortion of the crystal lattice, upon Li‐ion insertion into the MnZnSb interstitial sites, accompanied by a change in the magnetic moment of the manganese ions; acting together, both these effects lead to the collapse of the ferromagnetic order in MnZnSb.

Dirac fermions in graphene using the position-dependent translation operator formalism

Within the position-dependent translation operator formalism for quantum systems, we obtain analytical expressions for the eigenstates and the Landau level spectrum of Dirac fermions in graphene in the presence of a perpendicularly applied magnetic field and, as a consequence of the formalism, with a generalized form of the momentum operator. Moreover, we explore the behavior of wave packet dynamics in such a system by considering different initial pseudospin polarizations and metric parameters. Our findings show that the Landau levels, the wave packet trajectories, and velocities are significantly affected by the choice of the metric in the non-Euclidean space of the deformed momentum operator, exhibiting a tunable energy level spacing. In the dynamics analysis, one observes an enhancement of the oscillation amplitude of the average positions for all investigated pseudospin polarizations due to the nonsymmetric evolution of the wave packet induced by the different metrics in the system. The present formalism is shown to be a theoretical platform to describe the effects of two scenarios due to (i) a lattice deformation in graphene, giving rise to a natural Fermi velocity renormalization, or even (ii) a nonuniform mass term, induced by a specific substrate, that varies on a length scale much greater than the magnetic field length.

Precise control of Jeff=12 magnetic properties in Sr2IrO4 epitaxial thin films by variation of strain and thin film thickness

We report on a comprehensive investigation of the effects of strain and film thickness on the structural and magnetic properties of epitaxial thin films of the prototypal $J_\mathrm{eff}=1/2$ compound Sr$_2$IrO$_4$ by advanced X-ray scattering. We find that the Sr$_2$IrO$_4$ thin films can be grown fully strained up to a thickness of 108 nm. By using X-ray resonant scattering, we show that the out-of-plane magnetic correlation length is strongly dependent on the thin film thickness, but independent of the strain state of the thin films. This can be used as a finely tuned dial to adjust the out-of-plane magnetic correlation length and transform the magnetic anisotropy from two-dimensional (2D) to three-dimensional (3D) behavior by incrementing film thickness. These results provide a clearer picture for the systematic control of the magnetic degrees of freedom in epitaxial thin films of Sr$_2$IrO$_4$ and bring to light the potential for a rich playground to explore the physics of $5d$-transition metal compounds.

Electron Spectrum and Interband Magneto-Absorption of Light by Two-Dimensional Systems with Antidots

The degeneracy of Landau levels in perforated two-dimensional electronic structures is lifted in a perpendicular magnetic field. The cylindrical symmetry of the problem with a circular antidot holds and the magnetic quantum number m = 0, ±1, ±2, … is a good quantum number. The splitting of Landau levels for the antidot smaller than the magnetic length, as well as the frequencies and intensities of interband transition lines for normal semiconductors and transition metal dichalcogenide monolayers, has been analytically found.

Accounting for Edge-Plasma Parameter Dynamics at JET Divertor Sputtering Estimation during Edge-Localized Modes

The sputtering of tungsten divertor tiles at the JET ITER-like wall (ILW) during edge-localized modes (ELMs) is one of the main sources of tungsten impurity in the core plasma. The analytical approach for interpretation of the Langmuir probe (LP) measurements of ion flux at the divertor tile is elaborated to model the ELM ion parallel transport and to estimate the flux of sputtered tungsten atoms from the divertor under inter-ELM and intra-ELM conditions. Accounting for the pedestal temperature and density decrease at the pedestal crash during ELM allows obtaining the profiles of the particle and heat fluxes at the divertor tile corresponding to the measurements at the physically correct magnetic connection length. Estimates of the tungsten sputtered flux under intra-ELM and inter-ELM conditions show that first type ELMs contribute significantly (~85%) to the overall tungsten sputtering, which is in a good agreement with the divertor optical emission spectroscopy of the 400.9 nm atomic neutral tungsten line in JET discharges.

Large-scale simulations of particle-hole-symmetric Pfaffian trial wave functions

We introduce a family of paired-composite-fermion trial wave functions for any odd Cooper-pair angular momentum. These wave functions are parameter-free and can be efficiently projected into the lowest Landau level. We use large-scale Monte Carlo simulations to study three cases: Firstly, the Moore-Read phase, which serves us as a benchmark. Secondly, we explore the pairing associated with the anti-Pfaffian and the particle-hole-symmetric Pfaffian. Specifically, we assess whether their trial states feature exponentially decaying correlations and thus represent gapped phases of matter. For Moore-Read and anti-Pfaffian we find decay lengths of $\xi_\text{Moore-Read}=1.30(5)$ and $\xi_\text{anti-Pfaffian}=1.38(14)$, in units of the magnetic length. By contrast, for the case of PH-Pfaffian, we find no evidence of a finite length scale for up to $56$ particles.

Nonequilibrium phase transition in an Ising model without detailed balance.

We study a two-dimensional ferromagnetic Ising model in which spins are updated using modified versions of the Metropolis and Glauber algorithms. These update rules do not obey the detailed balance condition. The steady-state behavior of the model is studied using molecular field theory and Monte Carlo simulations. This model is found to exhibit a nonequilibrium phase transition from a "paramagnetic" state with zero magnetization to a "ferromagnetic" state with nonzero magnetization as the variable that plays the role of temperature in the spin updates is decreased. From detailed Monte Carlo simulations using the modified Metropolis algorithm, we demonstrate explicitly the nonequilibrium nature of the transition and show that it cannot be described as an equilibrium transition with an effective temperature different from the temperature used in the spin updates. The critical exponents that characterize the singular behavior near this continuous phase transition are calculated from finite size scaling of specific heat, magnetization, susceptibility, and correlation length. We find that the values of these exponents are the same (within error bars) as those of the equilibrium Ising model in two dimensions.

Novel design of hydrodynamic–magnetic compound support for cylinder block–valve plate in axial piston pump

An innovative hydrodynamic–magnetic compound support (HMCS) is proposed for the cylinder block–valve plate pair of the axial piston pump. Compared with the traditional design method, the magnetic structure provides a fixed magnetic torque to balance the residual torque, which causes eccentric wear on the valve plate and keeps the compression coefficient constant. First, the structure and principle of the HMCS are introduced, the magnetic model is established together with relative hydraulic models, and the influence of the permanent magnet length on the magnetic force is obtained. By studying the magnetic moment, the optimal structure of the magnetic pole is determined. Next, the 3D finite element method is used to simulate the magnetic forces under different pole gaps. After that, a reasonable magnetic pole gap and flux density distribution are determined. To obtain the influence mechanism of the HMCS on the cylinder block, the changing law of the track radii of hydraulic forces and average difference in the track radii are analyzed. Finally, the trend of the resultant torque of the cylinder block under multiple working conditions is analyzed. The results show that the HMCS can balance the residual torque acting on the valve plate, which is beneficial in preventing the occurrence of eccentric wear of the valve plate.

Global Energetics of Solar Flares. XII. Energy Scaling Laws

Abstract In this study we test 18 versions of five fundamental energy scaling laws that operate in large solar flares. We express scaling laws in terms of the magnetic potential field energy E p , the mean potential field strength B p , the free energy E free, the dissipated magnetic flare energy E diss, the magnetic length scale L, the thermal length scale L th, the mean helically twisted flux tube radius R, the sunspot radius r, the emission-measure-weighted flare temperature T e , the electron density n e , and the total emission measure EM, measured from a data set of 173 GOES M- and X-class flare events. The five categories of physical scaling laws include (i) a scaling law of the potential field energy, (ii) a scaling law for helical twisting, (iii) a scaling law for Petschek-type magnetic reconnection, (iv) the Rosner–Tucker–Vaiana scaling law, and (v) the Shibata–Yokoyama scaling law. We test the self-consistency of these theoretical scaling laws with observed parameters by requiring two criteria: a cross-correlation coefficient of CCC > 0.5 between the theoretically predicted scaling laws and observed values, and a linear regression fit with a slope of α ≈ 1 within one standard deviation σ. These two criteria enable us (i) to corroborate some existing (or modified) scaling laws, (ii) to reject other scaling laws that are not consistent with the observations, (iii) to probe the dimensionality of flare geometries, and (iv) to predict various energy parameters based on tested scaling laws.

Coexistence of type-I and type-II superconductivity signatures in ZrB12 probed by muon spin rotation measurements

Superconductors usually display either type-I or type-II superconductivity and the coexistence of these two types in the same material, for example at different temperatures is rare in nature. We the employed muon spin rotation (muSR) technique to unveil the superconducting phase diagram of the dodecaboride ZrB12 and obtained clear evidence of both type-I and type-II characteristics. Most importantly, we found a region showing unusual behavior where the usually mutually exclusive muSR signatures of type-I and type-II superconductivity coexist. We reproduced that behavior in theoretical modeling that required taking into account multiple bands and multiple coherence lengths, which suggests that material has one coherence length larger and another smaller than the magnetic field penetration length (the type-1.5 regime). At stronger fields, a footprint of the type-II mixed state showing square flux-line lattice was also obtained using neutron diffraction.

Weak antilocalization beyond the fully diffusive regime in Pb1−xSnxSe topological quantum wells

We report the measurements and analysis of weak antilocalization (WAL) in Pb1-xSnxSe topological quantum wells in a new regime where the elastic scattering length is larger than the magnetic length. We achieve this regime through the development of high-quality epitaxy and doping of topological crystalline insulator (TCI) quantum wells. We obtain elastic scattering lengths that exceeds 100nm and become comparable to the magnetic length. In this transport regime, the Hikami-Larkin-Nagaoka model is no longer valid. We employ the model of Wittmann and Schmid to extract the coherence time from the magnetoresistance. We find that despite our improved transport characteristics, the coherence time may be limited by scattering channels that are not strongly carrier dependent, such as electron-phonon or defect scattering.

Measurement of gradient magnetic field from a ferrofluid drop

We report that the response of the ferrofluid drop in the magnetic field can be described by the concept of a magnetic dipole. The gradient of the magnetic field in space is measured by the proposed model. Ferrofluid material not only exhibits the fluid property like surface tension and element transport but also can be manipulated by the magnetic field. A model is proposed that regards the bulk magnetization of the ferrofluid drop in the magnetic field as a magnetic dipole. The magnetic length coefficient denoting the ratio of the magnetic length of the dipole and the length of the major axis of the deformed drop is found scaling relation and is consistent with experimental results. A function is proposed that represents the responses of the deformation and traveling motion of the drop due to the presence of the magnetic field. The prolate-shaped traveling ferrofluid drops are demonstrated to measure the gradient of the magnetic field in space.

Effects of electromagnetic field and asymmetric Gaussian potential on low energy state energy of bound polaron in quantum well

In this article, effects of electromagnetic field and asymmetric Gaussian potential (AGP) on the bound polaron’s low energy state in quantum well are explored theoretically by the Lee-Low-Pines unitary transformation and Pekar type variational method. The variation of the ground state energy and the first excited state energy of the polaron with the Coulomb bound potential (CBP) strength at different electron-phonon coupling (EPC) constants, electric field (EF) strengths, heights and ranges of the AGP and magnetic field adjustment lengths are obtained. Our numerical results indicate that the polaron’s low energy state depends on the EPC constant, the EF strength, the AGP’s height and range and the CBP strength.

Heteromoiré Engineering on Magnetic Bloch Transport in Twisted Graphene Superlattices.

Localized electrons subject to applied magnetic fields can restart to propagate freely through the lattice in delocalized magnetic Bloch states (MBSs) when the lattice periodicity is commensurate with the magnetic length. Twisted graphene superlattices with moiré wavelength tunability enable experimental access to the unique delocalization in a controllable fashion. Here, we report the observation and characterization of high-temperature Brown-Zak (BZ) oscillations which come in two types, 1/B and B periodicity, originating from the generation of integer and fractional MBSs, in the twisted bilayer and trilayer graphene superlattices, respectively. Coexisting periodic-in-1/B oscillations assigned to different moiré wavelengths are dramatically observed in small-angle twisted bilayer graphene, which may arise from angle-disorder-induced in-plane heteromoiré superlattices. Moreover, the vertical stacking of heteromoiré supercells in double-twisted trilayer graphene results in a mega-sized superlattice. The exotic superlattice contributes to the periodic-in-B oscillation and dominates the magnetic Bloch transport.

Physical Assessment of the Magnetic Path Length of Energy Loss Testers

Magnetic energy losses of SiFe sheets are determined by the standardized methods of single sheet tester (SST) or Epstein tester (ET). According to IEC standards, the total power consumption is measured in Watt-metric ways, and the portion that concerns the homogeneously magnetized region(s) is estimated on a nominal value l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> of the so-called conventional effective magnetic path length (PL). A clear definition of PL is lacking-a suggestion being provided here. Standards express l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> through the dependence between the magnetic field strength H and the magnetization current. However, they apply the same value l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> also for the determination of losses P. From the viewpoint of physics, two PL quantities l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m,H</sub> and l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m,P</sub> would be preferable. The problematic is enhanced by the fact that the flux distribution within the magnetic circuit of a tester may change dynamically during the period of time, with induction changes B(t). This means that the physical PL L is a function of time, according to a PL function L[B(t)]. Finally, dynamic changes in flux distributions prove to be distinctly influenced by hysteretic mechanisms which means that L[B(t)] exhibits loop character. The resulting complexity indicates that the concept of PL is not promising, if high accuracy of testing is demanded. However, for purposes of material comparisons, optimum PL values LOPT may be suitable that are determined for individual material families. Analyses of instantaneous loss power values p(t) indicate that energy dissipation is weak during high induction. Thus, determinations of LOPT should be focussed on moderate induction. The ET is characterized by strong modifications of 3-D flux distributions which means that individual values LOPT would be needed. On the other hand, the SST shows a clear design. For moderate induction, some energy dissipation is expected to occur within the yoke region. This indicates that LOPT exceeds the conventional value of 450 mm, in weak ways.

Deconfined metal-insulator transitions in quantum Hall bilayers

We propose that quantum Hall bilayers in the presence of a periodic potential at the scale of the magnetic length can host examples of a Deconfined Metal-Insulator Transition (DMIT), where a Fermi liquid (FL) metal with a generic electronic Fermi surface evolves into a gapped insulator (or, an insulator with Goldstone modes) through a continuous quantum phase transition. The transition can be accessed by tuning a single parameter, and its universal critical properties can be understood using a controlled framework. At the transition, the two layers are effectively decoupled, where each layer undergoes a continuous transition from a FL to a generalized composite Fermi liquid (gCFL). The thermodynamic and transport properties of the gCFL are similar to the usual CFL, while its spectral properties are qualitatively different. The FL-gCFL quantum critical point hosts a sharply defined Fermi surface without long-lived electronic quasiparticles. Immediately across the transition, the two layers of gCFL are unstable to forming an insulating phase. We discuss the topological properties of the insulator and various observable signatures associated with the DMIT.

Effect of Gaussian impurity parameters on the valence and conduction subbands and thermodynamic quantities in a doped quantum wire

In this paper, the influence of impurity parameters on the energy-dispersion spectrum of electrons and holes in a doped quantum wire with Gaussian impurity is studied. The numerical calculations have shown that the energy eigenvalues enhance, if the confinement of carriers increases. Then, the energy eigenvalues rise when the magnetic field, decay length and strength of impurity increase. Also, the type of impurity affects the energy subband structures. So that, the presence of attractive impurity reduces the energy eigenvalues, as compared to repulsive impurity. Moreover, the role of impurity parameters and the external magnetic field as a control parameter on the thermal quantities has been demonstrated in details. The results show that enhancement of magnetic field, decay length and strength of repulsive impurity decreases (increases) the entropy (internal energy) of quantum wire.