Magnetic Barrier Research Articles

Self-similar transmission patterns induced by magnetic field effects in graphene

In this work we study the propagation of Dirac electrons through Cantor-like structures in graphene. In concrete, we are considering structures with magnetic and electrostatic barriers arrange in Cantor-like fashion. The Dirac-like equation and the transfer matrix approach have been used to obtain the transmission properties. We found self-similar patterns in the transmission probability or transmittance once the magnetic field is incorporated. Moreover, these patterns can be connected with other ones at different scales through well-defined scaling rules. In particular, we have found two scaling rules that become a useful tool to describe the self-similarity of our system. The first expression is related to the generation and the second one to the length of the Cantor-like structure. As far as we know it is the first time that a special self-similar structure in conjunction with magnetic field effects give rise to self-similar transmission patterns. It is also important to remark that according to our knowledge it is fundamental to break some symmetry of graphene in order to obtain self-similar transmission properties. In fact, in our case the time-reversal symmetry is broken by the magnetic field effects.

Coupling Influences SMM Properties for Pure 4 f Systems.

Increasing both the energy barrier for magnetization reversal and the coercive field of the hysteresis loop are significant challenges in the field of single-molecule magnets (SMMs). Coordination geometries of lanthanide ions and magnetic interactions between lanthanide ions are both important for guiding the magnetic behavior of SMMs. We report a high energy barrier of 657 K (457 cm-1 ) in a diamagnetic-ion-diluted lanthanide chain compound with a constrained bisphenoid symmetry (D2d ); this energy barrier is substantially higher than the barrier of 567 K (394 cm-1 ) of the non-diluted chain compound with intrachain ferromagnetic interactions. Although intrachain magnetic interaction lowers the energy barrier for magnetization reversal, it can greatly enhance the coercive fields and zero-field remanence of the hysteresis loops, which is crucial for the rational design of high-performance SMMs. Factors related to the coordination sphere of the lanthanide center, which govern the high magnetic relaxation barriers through the second excited Kramer's doublets and the magnetic interactions that affect the hysteresis loops, were revealed through ab initio calculations.

Terahertz response in the quantum-Hall-effect regime of a quantum-well-based charge-sensitive infrared phototransistor

The characteristics of a charge-sensitive infrared phototransistor (CSIP) based on a GaAs/AlGaAs multiple quantum-well (QW) structure are studied under a magnetic field. In the CSIP, the upper QWs serve as a floating gate that is charged by photoexcitation. The photoinduced charges are detected using the resistance of the lowest QW conducting channel. The conducting channel exhibits the integer quantum Hall effect (QHE) in a perpendicular high magnetic field, yielding the magnetic field dependence of the terahertz (THz) response ΔR. We found two different features of ΔR. One is that ΔR switches sign across the QHE plateau, which is explained simply by an increased electron density in the conducting channel. The other feature is observed as an enhanced positive ΔR when a potential barrier is formed in the conducting channel. The latter mechanism can be interpreted as the promotion of edge/bulk scattering due to photoinduced charges. These findings suggest ways to enhance the THz response by using magnetic fields and potential barriers.

Conductance of graphene based normal-superconductor junction with double magnetic barriers

We study conductance of a graphene based normal metal-superconductor junction with two magnetic barriers. The magnetic barriers are induced via two applied magnetic fields with the same magnitudes and opposite directions accompanied by an applied electrostatic potential. We solve Dirac-Bogoliubov-De-Gennes (DBdG) equation to calculate conductance of the junction. We find that applying the magnetic field leads to suppression of the Andreev reflection and conductance for all energies. On the other hand, we observe a crossover from oscillatory to tunneling behavior of the conductance as a function of the applied potential by increasing the magnetic field.

Detection of fractional solitons in quantum spin Hall systems

We propose two experimental setups that allow for the implementation and the detection of fractional solitons of the Goldstone-Wilczek type. The first setup is based on two magnetic barriers at the edge of a quantum spin Hall system for generating the fractional soliton. If then a quantum point contact is created with the other edge, the linear conductance shows evidence of the fractional soliton. The second setup consists of a single magnetic barrier covering both edges and implementing a long quantum point contact. In this case, the fractional soliton can unambiguously be detected as a dip in the conductance without the need to control the magnetization of the barrier.

Revival of cloaking effect in a driven bilayer graphene vector barrier

Transmission profiles in bilayer graphene are studied theoretically through a rectangular vector potential (magnetic) barrier with and without the presence of an oscillatory potential. Unlike the electrostatic barrier, the Fano resonances (FR) are noted in the transmission spectra both for normal and glancing incidences due to non-conservation of chirality for a static vector barrier. The results for normal incidence indicate that the cloaking effect is a manifestation of the chirality conservation in charge transport through bilayer graphene scalar barriers. It is also noted that the aforesaid FR for a static vector barrier might disappear (photon induced electronic cloaking effect) due to the predominant photon exchange processes in presence of an external oscillating potential. The study of Fano resonances in transmission spectrum is in high demand in respect of localization of charge carriers in graphene nano structures for its potential applications in digital device fabrications.

Magnetized advective accretion flows: formation of magnetic barriers in magnetically arrested discs

We discuss the importance of large scale strong magnetic field in the removal of angular momentum outward, as well as the possible origin of different kinds of magnetic barrier in advective, geometrically thick, sub-Keplerian accretion flows around black holes. The origin of this large scale strong magnetic field near the event horizon is due to the advection of the magnetic flux by the accreting gas from the environment, say, the interstellar medium or a companion star, because of flux freezing. In this simplest vertically averaged, 1.5-dimensional disc model, we choose the maximum upper limit of the magnetic field, which the disc around a black hole can sustain. In this so called magnetically arrested disc (MAD) model, the accreting gas either decelerates or faces the magnetic barrier near the event horizon by the accumulated magnetic field depending on the geometry. The magnetic barrier may knock the matter to infinity. We suggest that these types of flow are the building block to produce jets and outflows in the accreting system. We also find that in some cases, when matter is trying to go back to infinity after knocking the barrier, matter is prevented being escaped by the cumulative action of strong gravity and the magnetic tension, hence by another barrier. In this way, magnetic field can lock the matter in between these two barriers and it might be a possible explanation for the formation of episodic jet.

Effect of a gap opening on the conductance of graphene with magnetic barrier structures

In the present study Klein tunneling in a single-layer gapped graphene was investigated by transfer matrix method under normal magnetic field for one and two magnetic barriers. Calculations show that electron transmission through a magnetic barrier is deflected to positive angles and reduces as the magnitude of magnetic field and especially the energy gap increases. This reduction is even more significant in larger fields so that after reaching a specific value of energy gap, an effective confinement for fermions and suppression of Klein tunneling is reached particularly in normal incidence and the conductance becomes zero. Unlike one barrier, the process of tunneling through two magnetic barriers induces symmetric transmission probability versus the incident angle; even, for lower energy gaps, electron transmission probability increases which in turn reduces total conductance via proper changes in the value of the magnetic field and energy gap. In general, it is concluded that confining electrons in asymmetric transmission through one barrier is conducted better than two barriers.

Spin-dependent electronic transport characteristics in Fe4N/BiFeO3/Fe4N perpendicular magnetic tunnel junctions

Perpendicular magnetic tunnel junctions (MTJs) have attracted increasing attention owing to the low energy consumption and wide application prospects. Herewith, against Julliere's formula, an inverse tunnel magnetoresistance (TMR) appears in tetragonal Fe4N/BiFeO3/Fe4N perpendicular MTJs, which is attributed to the binding between the interface resonant tunneling state and central (bordered) hot spots. Especially, antiferromagnetic BiFeO3 shows an extra spin-polarized resonant state in the barrier, which provides a magnetic-barrier factor to affect the tunneling transport in MTJs. Meanwhile, due to the spin-polarized transport in Fe4N/BiFeO3/Fe4N MTJs, the sign of TMR can be tuned by the applied bias. The tunable TMR and resonant magnetic barrier effect pave the way for clarifying the tunneling transport in other junctions and spintronic devices.

The electronic states and magnetization of coupled AlGaAs/GaAs quantum dots in magnetic fields

We present a theoretical study of electronic states and magnetization of two interacting electrons confined in coupled quantum dots (CQDs) presented in a magnetic field. We obtain the eigenenergies of the CQD by solving the relative two-dimensional (2D) Hamiltonian using the combined variational–exact diagonalization method. The dependence of magnetization on temperature, magnetic field strength, confining frequency and barrier height has been investigated. We have shown the singlet–triplet transitions in the ground state of the CQD spectra and the corresponding jumps in the magnetization curves. The comparisons show that our results are in very good agreement with the reported works.

Lateral shift in a spin-orbit-coupling modulated magnetic-barrier nanostructure

Spin-dependent Goos–Hänchen（GH）effect of the electron in a magnetic barrier nanostructure modulated by the spin-orbit coupling (SOC) is investigated. Two kinds of intrinsic SOCs (Rashba and Dresselhaus types) are taken into account, and spin-dependent lateral shifts are obtained by the transfer matrix method and the stationary phase approximation. The spin-dependent lateral shift is found to be related closely to the SOC. Both magnitude and sign of the spin-polarised lateral shift can be controlled by properly adjusting the strength of Rashba or Dresselhaus SOC. These interesting features can provide an alternative approach to manipulate spin-polarised electrons in the semiconductor, and such a nanostructure can serve as a controllable spatial spin splitter for spintronics applications.

Is a strong axial crystal-field the only essential condition for a large magnetic anisotropy barrier? The case of non-Kramers Ho(iii) versus Tb(iii ).

A monometallic non-Kramers Ho(iii) complex with a strong uniaxial ligand field displays a record high energy barrier of 355 K with hysteresis opening up to 4 K at a sweep rate of 0.027 T s-1, while the isomorphous, more anisotropic non-Kramers Tb(iii) complex displays strong quantum tunnelling between the ground states. Ab initio calculations reveal that the presence of strong axial ligands and moderately weaker equatorial ligands is sufficient to quench the QTM between the ground pseudo doublets (mJ = ±8) in the case of Ho(iii) complex, where the relaxation is found to proceed via the first excited state. However, the equatorial field disrupts the stabilization of ground pseudo doublets in Tb(iii), inducing QTM and inhibiting SIM behaviour. Further insights into the decisive role played by the secondary coordination sphere and hydrogen-bonding in the SIM characteristics of these two non-Kramers ions were also gained. This combined experimental and computational approach highlights that although strong axiality holds the key for designing high temperature SIMs based on oblate non-Kramers ions, the strength of the equatorial ligand field also cannot be ignored.

Propagation of Plasma Bunches through a Transverse Magnetic Barrier

The injection of a plasma bunch into a multipolar trap can be applied to fill the trap with a plasma. The injection of the bunch into a tokamak-like trap can be considered an additional means for controlling the processes of plasma heating and fuel delivery to the central zone of a thermonuclear reactor. In both cases, the bunch is injected normally to the magnetic field of the trap. It has been shown theoretically, experimentally, and by numerical simulation that the depth of plasma bunch penetration into the magnetic field varies in direct proportion to the bunch energy and in inverse proportion to the magnetic pressure and the cross-sectional area of the plasma bunch. The data of this work allow researchers to estimate the values of plasma bunch parameters at which the bunch will be trapped. As a result, the process of plasma bunch trapping has been optimized.

Electronic transport in armchair graphene nanoribbon under double magnetic barrier modulation

We present a theoretical investigation of the transport properties and the magnetoresistance effect in armchair graphene nanoribbons (AGNRs) under modulation by two magnetic barriers. The energy levels are found to be degenerate for a metallic AGNR but are not degenerate for a semiconducting AGNR. However, the conductance characteristics show quantized plateaus in both the metallic and semiconducting cases. When the magnetization directions of the barriers change from parallel to antiparallel, the conductance plateau in the metallic AGNR shows a degenerate feature due to matching between the transport modes in different regions. As the barrier height increases, the conductance shows more oscillatory behavior with sharp peaks and troughs. Specifically, the initial position of nonzero conductance for the metallic AGNR system moves towards a higher energy regime, which indicates that an energy gap has been opened. In addition, the magnetoresistance ratio also shows plateau structures in certain specific energy regions. These results may be useful in the design of electron devices based on AGNR nanostructures.

Spin-polarized charge transport in HgTe/CdTe quantum well topological insulator under a ferromagnetic metal strip

Electron tunneling through a single magnetic barrier in a HgTe topological insulator has been theoretically investigated. We find that the perpendicular magnetic field would not lead to spin-flip of the edge states due to the conservation of the angular moment. By tuning the magnetic field and the Fermi energy, the edge channels can be transited from switch-on states to switch-off states and the current from unpolarized states can be filtered to fully spin polarized states. These features offer us an efficient way to control charge/spin transport in a HgTe/CdTe quantum well, and pave a way to construct the nanoelectronic devices utilizing the topological edge states.

One-way transmission of electrons on the topological insulator surface modulated by magnetic potential

We investigate the transport properties of Dirac fermions on the surface of a three-dimensional topological insulator (TI) with magnetic modulation potentials. By using the transfer-matrix method, the transmission coefficients are obtained as a function of incident energy and incident angle. It is shown that the forward and backward propagating carriers possess different transmission coefficients at some incident energies when the charge carriers incident obliquely, which originates from the break of time reversal symmetry. Particularly, the magnetic barrier introduces asymmetric scattering; thus, the scattered angles are different for the forward and backward propagating carriers. As a consequence, the transmission in one direction is permitted while it is blocked in its reversal direction. Therefore, the unidirectional transmission of electrons is achieved on the surface of TI. Furthermore, unidirectional transmission is demonstrated by the electronic charge distributions in the system. The investigations may have potential applications in the design of TI-based one-way quantum devices.

Giant coercivity and high magnetic blocking temperatures for N23\u2212 radical-bridged dilanthanide complexes upon ligand dissociation

Increasing the operating temperatures of single-molecule magnets—molecules that can retain magnetic polarization in the absence of an applied field—has potential implications toward information storage and computing, and may also inform the development of new bulk magnets. Progress toward these goals relies upon the development of synthetic chemistry enabling enhancement of the thermal barrier to reversal of the magnetic moment, while suppressing alternative relaxation processes. Herein, we show that pairing the axial magnetic anisotropy enforced by tetramethylcyclopentadienyl (CpMe4H) capping ligands with strong magnetic exchange coupling provided by an N23− radical bridging ligand results in a series of dilanthanide complexes exhibiting exceptionally large magnetic hysteresis loops that persist to high temperatures. Significantly, reducing the coordination number of the metal centers appears to increase axial magnetic anisotropy, giving rise to larger magnetic relaxation barriers and 100-s magnetic blocking temperatures of up to 20 K, as observed for the complex [K(crypt-222)][(CpMe4H2Tb)2(μ−{rm{N}}_2^ cdot)].

Temperature Induced Spin Polarization in Magnetically Stripped Ferromagnetic Film: The Coexisting Phases and the Two Regime Behavior

The paper deals with the study of coexisting majority and minority components in thin ferromagnetic film which is magnetically stripped by applying external magnetic field in selected regions. There occurs continuous transition from ferromagnetic barriers to a coexisting phase separated region through the formation of a depletion layer. Formation of magnetic barriers, channels and step like structure have been explored. Temperature induced two regime behavior is observed – first one dominated by the ferroic nature of the film and the second one by the paramagnetic nature of film. Two sizes of lattice in 3 dimensions of 100 × 100 × 100 and 200 × 100 × 100 are used for Monte Carlo simulation using Ising model with Metropolis algorithm. The spin polarization, average magnetization and spin correlation functions are plotted to reveal the underlying physics. The micrographs of the magnetic barrier and phase separated regions are also shown.

Spin-valley-dependent transport and giant tunneling magnetoresistance in silicene with periodic electromagnetic modulations**Project supported by the National Natural Science Foundation of China (Grant Nos. 11547249, 51501102, and 11647157) and the Science Foundation for Excellent Youth Doctors of Three Gorges University, China (Grant No. KJ2014B076).

The transport property of electrons tunneling through arrays of magnetic and electric barriers is studied in silicene. In the tunneling transmission spectrum, the spin-valley-dependent filtered states can be achieved in an incident energy range which can be controlled by the electric gate voltage. For the parallel magnetization configuration, the transmission is asymmetric with respect to the incident angle θ, and electrons with a very large negative incident angle can always transmit in propagating modes for one of the spin-valley filtered states under a certain electromagnetic condition. But for the antiparallel configuration, the transmission is symmetric about θ and there is no such transmission channel. The difference of the transmission between the two configurations leads to a giant tunneling magnetoresistance (TMR) effect. The TMR can reach to 100% in a certain Fermi energy interval around the electrostatic potential. This energy interval can be adjusted significantly by the magnetic field and/or electric gate voltage. The results obtained may be useful for future valleytronic and spintronic applications, as well as magnetoresistance device based on silicene.

Energy relaxation in a quantum dot at the edge of a two-dimensional topological insulator

The calculation of energy relaxation rates due to transitions involving phonons is performed for quantum dots of varying size which are formed by magnetic barriers at the edge of a two-dimensional topological insulator based on a HgTe/CdTe quantum well. Relaxation both into the discrete and continuous spectrum of edge states is considered, as well as into the continuous spectrum of the bulk material. The obtained results demonstrate the existence of a region of system parameters which provide relatively slow energy relaxation, indicating that the considered objects are promising, e.g., for the design of novel types of solid state qubits.