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Register a new userQuasi-one-dimensional metallic conduction channels in exotic ferroelectric topological defects
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Ferroelectric topological objects (e.g. vortices, skyrmions) provide a fertile ground for exploring emerging physical properties that could potentially be utilized in future configurable nanoelectronic devices. Here, we demonstrate quasi-one-dimensional metallic high conduction channels along two types of exotic topological defects, i.e. the topological cores of (i) a quadrant vortex domain structure and (ii) a center domain (monopole-like) structure confined in high quality BiFeO3 nanoisland array, abbreviated as the vortex core and the center core. We unveil via phase-field simulations that the superfine (< 3 nm) metallic conduction channels along center cores arise from the screening charge carriers confined at the core whereas the high conductance of vortex cores results from a field-induced twisted state. These conducting channels can be repeatedly and reversibly created and deleted by manipulating the two topological states via an electric field, leading to an apparent electroresistance effect with an on/off ratio higher than 103. These results open up the possibility of utilizing these functional one-dimensional topological objects in high-density nanoelectronic devices such as ultrahigh density nonvolatile memory.
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We have investigated from first-principles an electronic structure and magnetism in MnB4 compound with experimentally observed orthorhombic C12/m1 structure. It is found that Mn tetra-borides (MnB4) is found to have metallic ground state with well defined local Mn magnetic moments. This conclusion was drawn from calculation within full potential Linear Augmented Plane Wave method and Korringa-Kohn-Rostocker method using Disordered Local Moment Approximation. We have shown using Lichtenstein Green function method that magnetic exchange interactions between Mn moments are strongly ferromagnetic along 1D-chains of Mn atoms and they are practically vanishing between the chains. The metallic state appears to exhibit a strongly one-dimensional character since the single metallic band show dispersion only in one reciprocal lattice dimension. Thus it appears that MnB4 may be a perfect one-dimensional one-band Hubbard model system. Although LSDA predicts the magnetism in this system it may well be superconductor at low temperatures due to the correlation effects beyond mean field like approach.
The major breakthroughs in the understanding of topological materials over the past decade were all triggered by the discovery of the Z$_2$ topological insulator (TI). In three dimensions (3D), the TI is classified as either strong or weak, and experimental confirmations of the strong topological insulator (STI) rapidly followed the theoretical predictions. In contrast, the weak topological insulator has so far eluded experimental verification, since the topological surface states emerge only on particular side surfaces which are typically undetectable in real 3D crystals. Here we provide experimental evidence for the WTI state in a bismuth iodide, $beta$-Bi4I4. Significantly, the crystal has naturally cleavable top and side planes both stacked via van-der-Waals forces, which have long been desirable for the experimental realization of the WTI state. As a definitive signature of it, we find quasi-1D Dirac TSS at the side-surface (100) while the top-surface (001) is topologically dark. Furthermore, a crystal transition from the $beta$- to $alpha$-phase drives a topological phase transition from a nontrivial WTI to the trivial insulator around room temperature. This topological phase, viewed as quantum spin Hall (QSH) insulators stacked three-dimensionally, and excellent functionality with on/off switching will lay a foundation for new technology benefiting from highly directional spin-currents with large density protected against backscattering.
Electrical conduction is studied along parabolically confined quasi-one dimensional channels, in the framework of a revised linear-response theory, for elastic scattering. For zero magnetic field an explicit multichannel expression for the conductance is obtained that agrees with those of the literature. A similar but new multichannel expression is obtained in the presence of a magnetic field B||z perpendicular to the channel along the x axis. An explicit connection is made between the characteristic time for the tunnel-scattering process and the transmission and reflection coefficients that appear in either expression. As expected, for uncoupled channels the finite field expression gives the complete (Landauer-type) conductance of N parallel channels, a result that has not yet been reported in the literature. In addition, it accounts explicitly for the Hall field and the confining potential and is valid, with slight modifications, for tilted magnetic fields in the (x,z) plane.
Quasi-one-dimensional (1D) materials provide a superior platform for characterizing and tuning topological phases for two reasons: i) existence for multiple cleavable surfaces that enables better experimental identification of topological classification, and ii) stronger response to perturbations such as strain for tuning topological phases compared to higher dimensional crystal structures. In this paper, we present experimental evidence for a room-temperature topological phase transition in the quasi-1D material Bi$_4$I$_4$, mediated via a first order structural transition between two distinct stacking orders of the weakly-coupled chains. Using high resolution angle-resolved photoemission spectroscopy on the two natural cleavable surfaces, we identify the high temperature $beta$ phase to be the first weak topological insulator with gapless Dirac cones on the (100) surface and no Dirac crossing on the (001) surface, while in the low temperature $alpha$ phase, the topological surface state on the (100) surface opens a gap, consistent with a recent theoretical prediction of a higher-order topological insulator beyond the scope of the established topological materials databases that hosts gapless hinge states. Our results not only identify a rare topological phase transition between first-order and second-order topological insulators but also establish a novel quasi-1D material platform for exploring unprecedented physics.
Recent progress in the field of topological states of matter(1,2) has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs)(3-6), followed by closely related ternary compounds(7-16) and predictions of several weak TIs(17-19). However, both the conceptual richness of Z$_2$ classification of TIs as well as their structural and compositional diversity are far from being fully exploited. Here, a new Z$_2$ topological insulator is theoretically predicted and experimentally confirmed in the $beta$-phase of quasi-one-dimensional bismuth iodide Bi$_4$I$_4$. The electronic structure of $beta$-Bi$_4$I$_4$, characterized by Z$_2$ invariants (1;110), is in proximity of both the weak TI phase (0;001) and the trivial insulator phase (0;000). Our angle-resolved photoemission spectroscopy measurements on the (001) surface reveal a highly anisotropic band-crossing feature located at the point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.
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