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Register a new userConfiguration-Sensitive Transport on Domain Walls of a Magnetic Topological Insulator
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We study the transport on the domain wall (DW) in a magnetic topological insulator. The low-energy behaviors of the magnetic topological insulator are dominated by the chiral edge states (CESs). Here, we find that the spectrum and transport of the CESs at the DW are strongly dependent on the DW configuration. For a Bloch wall, two co-propagating CESs at the DW are doubly degenerate and the incoming electron is totally reflected. However, for a N{e}el wall, the two CESs are split and the transmission is determined by the interference between the CESs. Moreover, the effective Hamiltonian for the CESs indicates that the component of magnetization perpendicular to the wall leads to the distinct transport behavior. These findings may pave a way to realize the low-power-dissipation spintronics devices based on magnetic DWs.
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The electronic orders in magnetic and dielectric materials form the domains with different signs of order parameters. The control of configuration and motion of the domain walls (DWs) enables gigantic, nonvolatile responses against minute external fields, forming the bases of contemporary electronics. As an extension of the DW function concept, we realize the one-dimensional quantized conduction on the magnetic DWs of a topological insulator (TI). The DW of a magnetic TI is predicted to host the chiral edge state (CES) of dissipation-less nature when each magnetic domain is in the quantum anomalous Hall state. We design and fabricate the magnetic domains in a magnetic TI film with the tip of the magnetic force microscope, and clearly prove the existence of the chiral one-dimensional edge conduction along the prescribed DWs. The proof-of-concept devices based on the reconfigurable CES and Landauer-Buttiker formalism are exemplified for multiple-domain configurations with the well-defined DW channels.
We present a theoretical investigation of electron states hosted by magnetic domain walls on the 3D topological insulator surface. The consideration includes the domain walls with distinct vectorial and spatial textures. The study is carried out on the basis of the Hamiltonian for quasi-relativistic fermions by using a continual approach and tight-binding calculations. We derive the spectral characteristics and spatial localization of the the one-dimensional low-energy states appearing at the domain walls. The antiphase domain walls are shown to generate the topologically protected chiral states with linear dispersion, the group velocity and spin-polarization direction of which depend on an easy axis orientation. In the case of an easy plane anisotropy, we predict a realization of a dispersionless state, flat band in the energy spectrum, that is spin-polarized along the surface normal. Modification of the surface states in the multi-domain case, which is approximated by a periodic set of domain walls, is described as well. We find that the magnetic domain walls with complex internal texture, such as Neel-like or Bloch-like walls, also host the topological states, although their spectrum and spin structure can be changed compared with the sharp wall case.
We study transport across a time-dependent magnetic barrier present on the surface of a three-dimensional topological insulator. We show that such a barrier can be implemented for Dirac electrons on the surface of a three-dimensional topological insulator by a combination of a proximate magnetic material and linearly polarized external radiation. We find that the conductance of the system can be tuned by varying the frequency and amplitude of the radiation and the energy of an electron incident on the barrier providing us optical control on the conductance of such junctions. We first study a $delta$-function barrier which shows a number of interesting features such as sharp peaks and dips in the transmission at certain angles of incidence. Approximate methods for studying the limits of small and large frequencies are presented. We then study a barrier with a finite width. This gives rise to some new features which are not present for a $delta$-function barrier, such as resonances in the conductance at certain values of the system parameters. We present a perturbation theory for studying the limit of large driving amplitude and use this to understand the resonances. Finally, we use a semiclassical approach to study transmission across a time-dependent barrier and show how this can qualitatively explain some of the results found in the earlier analysis. We discuss experiments which can test our theory.
Cylindrical nanowires made of soft magnetic materials, in contrast to thin strips, may host domain walls of two distinct topologies. Unexpectedly, we evidence experimentally the dynamic transformation of topology upon wall motion above a field threshold. Micromagnetic simulations highlight the underlying precessional dynamics for one way of the transformation, involving the nucleation of a Bloch-point singularity, however, fail to reproduce the reverse process. This rare discrepancy between micromagnetic simulations and experiments raises fascinating questions in material and computer science.
We study transport across either a potential or a magnetic barrier which is placed on the top surface of a three-dimensional thin topological insulator (TI). For such thin TIs, the top and bottom surfaces interact via a coupling $lambda$ which influences the transport properties of junctions constructed out of them. We find that for junctions hosting a potential barrier, the differential conductance oscillates with the barrier strength. The period of these oscillations doubles as the coupling $lambda$ changes from small values to a value close to the energy of the incident electrons. In contrast, for junctions with a magnetic barrier, the conductance approaches a non-zero constant as the barrier strength is increased. This feature is in contrast to the case of transport across a single TI surface where the conductance approaches zero as the strength of a magnetic barrier is increased. We also study the spin currents for these two kinds of barriers; in both cases, the spin current is found to have opposite signs on the top and bottom surfaces. Thus this system can be used to split applied charge currents to spin currents with opposite spin orientations which can be collected by applying opposite spin-polarized leads to the two surfaces. We show that several of these features of transport across finite width barriers can be understood analytically by studying the $delta$-function barrier limit. We discuss experiments which may test our theory.
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