Magnetic Field Dependent Energy Research Articles

Toward the Origin of Magnetic Field-Dependent Storage Properties: A Case Study on the Supercapacitive Performance of FeCo2O4 Nanofibers.

Despite the many reports in the literature on the magnetic field-dependent energy storage properties of metal oxides, the origin of magnetic field-dependent supercapacitive properties is still not clear. This is because electrode's properties such as physical (electrical and magnetic properties), structural and microstructural (surface area, pore size, and their distribution), and electrolyte's properties (ionic diffusion, ionic conductivity, cation size, etc.) are very crucial for investigating the effect of a magnetic field on the energy storage properties of metal oxides. In this article, the effect of a magnetic field on some of the abovementioned properties and thereby on the supercapacitive properties of FeCo2O4 (FCO) nanofibers is thoroughly investigated. The local magnetic environment of the magnetized electrode (magnetic gradient force, susceptibility, etc.) is proposed to be crucial for tuning the storage properties of the electrode material. Magnetic field-mediated resistive properties of the electrode material and thereby the induced magnetic gradient force at the electrode surface seem to be helpful in lowering the Nernst layer thickness and improving the electrode/electrolyte interface for a smoother ionic exchange resulting in 56% increment in the capacitance values of FCO nanofibers. A series of electrochemical experiments (cyclic voltammetry and galvanostatic charge-discharge) and magnetic property evaluation of bare and cycled electrodes are carried out, and the proposed mechanism/hypothesis is validated by studying the ex situ magnetic properties and the results are discussed in detail.

Structural, multiferroic, and magnetoelectric properties of (1 − x)Bi0.85La0.15FeO3–xBaTiO3 composite ceramics

Multiferroic magnetoelectric composites are beneficial in device fabrication because of their tunable ferroelectricity and magnetism. The partial substitution in bismuth ferrite (BiFeO3) is one of the best possible ways for synthesizing pure phase BiFeO3-based materials. A comprehensive study of (1 − x)Bi0.85La0.15FeO3–xBaTiO3 (x = 0–0.3) was done by coalescing ferroelectric, dielectric, magnetic, and magnetoelectric properties with structural and microstructural characterizations to explore the effect of BaTiO3 (BT) into Bi0.85La0.15FeO3 (BLFO) and forming composite ceramics. The X-ray diffraction study reveals the phase purity in BLFO and a structural transformation from rhombohedral to cubic phase with increasing content of BT. The Raman spectroscopy and scanning electron micrographs confirm the co-existence of composite formation in BLFO–xBT. The Raman modes shift towards lower wavenumber with increasing BT concentration suggests lattice compression. The room temperature M–H hysteresis curve shows the existence of weak ferromagnetism in BLFO–BT composites and superparamagnetism in BLFO–10BT ceramic. The curve fitting of M–H curve for BLFO–10BT showed the existence of superparamagnetic particles. The ferroelectric hysteresis P–E loop measurements produced unsaturated oval-shaped loops with high leakage and displayed a lossy dielectric nature. The effect of magnetic field on polarization versus electric field curve reveals the interfacial interaction due to the origin of magnetoelectric interaction in BLFO–BT composite ceramics. All the samples display peak broadening in temperature–permittivity plot and confirm relaxor behavior. The superparamagnetic behavior and magnetic field-dependent energy storage capacity of BLFO–10BT composite ceramic make this material a potential candidate for magnetoelectric devices.

Chiral phase transition and quantum revivals in graphene

We explain the dynamics of charge carriers in graphene using a two dimensional Dirac oscillator in the presence of an external magnetic field. The energy dispersion relation with linear behavior corresponds to monolayer graphene in a relativistic regime, whereas parabolic behavior appears in the case of bilayer graphene in a non-relativistic regime. We show that in the bilayer graphene model, a magnetic field-dependent energy gap exists, whereas a changing external magnetic field leads to a chiral phase transition. Our model explains the phenomenon of collapse and revivals and chiral phase transition in the presence of a magnetic field in monolayer and bilayer graphene. The displayed collapses and revivals occur due to Zitterbewegung and classical cyclotron motion.

Energy spectra of ABC-stacked trilayer graphene in magnetic and electric fields

We have developed the generalized tight-binding model to understand how the electronic structures of ABC-stacked trilayer graphene can be modulated by external fields. A band-like Hamiltonian matrix is used to obtain the electronic properties efficiently. A uniform perpendicular magnetic field gives rise to three groups of Landau levels that can be distinguished from one another by the subenvelope functions of the distinct sublattices. Intergroup Landau-level anticrossings occurring between any two groups and intragroup anticrossings coming from the second group are revealed in the magnetic field-dependent energy spectrum. Such anticrossings are induced by specific interlayer atomic interactions. In the presence of an electric field, each Landau-level group will split into two subgroups, which are characterized by the two different localization centers. The anticrossings in the second group can be suppressed or produced by varying the electric field strength, the reason being the significant modification of the band structures. Furthermore, the intragroup anticrossings arise in two split subgroups of the first group.

Magnetoexcitons in nanostructures exhibiting cylindrical symmetry

The problem of an exciton in the cylindrical nanostructure exposed to an external static magnetic field is investigated. The theoretical model assumes anisotropic masses which are different inside and outside the nanostructure. The confinement potential has finite value at the boundaries and magnetic field is parallel to the axis of the cylinder. The screened Coulomb interaction between an electron and a hole is assumed. The consistent mathematical procedure is developed to calculate the magnetoexciton eigenfunctions and eigenenergies. Our method applies to the systems exhibiting cylindrical symmetry where, due to confinement effects accompanied by the e-h Coulomb interaction, the separation of relative- and center-of-mass motion is not possible. Numerical calculations have been performed for the quantum disk, the cylinder and the quantum rod. The magnetic field dependent energy spectrum and corresponding wave functions, expressed in terms of known one-particle electron and hole eigenfunctions, are calculated. Additionally, we point out the different role of Coulomb interaction in every case.

Proposal for a magnetic field induced graphene dot

Quantum dots induced by a strong magnetic field applied to a single layer of graphene in the perpendicular direction are investigated. The dot is defined by a model potential which consists of a well of depth ΔV relative to a flat asymptotic part and quantum states formed from the zeroth Landau level are considered. The energy of the dot states cannot be lower than −ΔV relative to the asymptotic potential. Consequently, when ΔV is chosen to be about half of the gap between the zeroth and first Landau levels, the dot states are isolated energetically in the gap between Landau level 0 and Landau level −1. This is confirmed with numerical calculations of the magnetic field dependent energy spectrum and the quantum states. Remarkably, an antidot formed by reversing the sign of ΔV also confines electrons but in the energy region between Landau level 0 and Landau level +1. This unusual behaviour gives an unambiguous signal of the novel physics of graphene quantum dots.

A Mean Field Theory for the Quantum Hall Liquid. II: The Vortex Solution

In the Fractional Quantum Hall state, we introduce a bi-local mean field and get vortex mean field solutions. Rotational invariance is imposed and the solution is constructed by means of numerical self-consistent method. It is shown that vortex has a fractional charge, a fractional angular momentum and a magnetic field dependent energy. In $\nu=1/3$ state, we get finite energy gap at $B=10,15,20[T]$. We find that the gap vanishes at $B=5.5[T]$ and becomes negative below it. The uniform mean field becomes unstable toward vortex pair production below $B=5.5[T]$.