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The quantum-spin-Hall (QSH) phase and its helical edge spins of two-dimensional (2D) topological insulators (TIs) are attracting increasing attention. The helical edge spin currents are a favorite to applications in dissipationless spintronic devices with low error rates because the spin currents existing along sample 1D edges are topologically protected in time-reversal symmetry and are easily controlled by external bias voltages. Moreover, combining the 1D edge spin currents with ferromagnetic or superconductor electrodes yields various exciting phenomena and those applications to innovative quantum devices. Nevertheless, reports of 2D TIs and QSH phases (particularly at high temperatures) have been rare and mainly in semiconductor 2D quantum wells. Here, I introduce and discuss how 2D TI states and QSH phases can be created and applied to innovative quantum (spintronic) devices, particularly in atomically thin layers (such as graphene and transition metal dichalcogenide family, which is energetically studied and have demonstrated large topological bulk gaps recently). Research of 2D TIs on atom-thin (or few) layers and those application must open a door to next-generation quantum architectures (such as topological quantum computation utilizing the Majorana fermion). [ABSTRACT FROM AUTHOR]

A unique system of nanostructures, composed of nonlithographically fabricated arrays of nano-wires directly electrodeposited into porous anodic aluminum oxide templates, is presented. Results of X-ray diffraction, Raman and magnetic measurements on metal and semiconductor wires are shown. The I-V characteristics of two terminal devices constructed from the nano-arrays are seen to display room temperature periodic conductance oscillations and Coulomb-blockade like current staircases. Single-electron tunneling in the presence of interwire coupling in these arrays causes spontaneous electrostatic polarization of the wires.

Twistronics, a novel engineering approach involving the alignment of van der Waals (vdW) integrated two-dimensional materials at specific angles, has recently attracted significant attention. Novel nontrivial phenomena have been demonstrated in twisted vdW junctions (the so-called magic angle), such as unconventional superconductivity, topological phases, and magnetism. However, there have been only few reports on integrated vdW layers with large twist angles θ t , such as twisted interfacial Josephson junctions using high-temperature superconductors. Herein, vdW homojunctions of the thin-magnetic flakes, Fe 3 GeTe 2 (FGT), with large θ t ranging from 0° to 90°, without inserting any tunnel barriers are assembled. Nevertheless, these vdW homojunctions exhibit tunnel-magnetoresistance (TMR) like behavior (pseudo-TMR (PTMR) effect) with the ratios highly sensitive to the θ t values, revealing that the vdW gap at the junction interface between the twisted FGT layers behaves like a tunnel barrier and the θ t serves a control parameter for PTMR by drastically varying magnitudes of the lattice-mismatch and the subsequent appearance of antiferromagnetic (AFM) spin alignment. First-principles calculations considering vacuum gaps indicate strong dependence of TMR on the θ t driven by the sixfold screw rotational symmetry of bulk FGT. The present homojunctions hold promise as a platform for novel AFM spin-dependent phenomena and spintronic applications., (© 2025 Wiley‐VCH GmbH.)

SrTiO 3 (STO) substrate, a perovskite oxide material known for its high dielectric constant (ɛ), facilitates the observation of various (high-temperature) quantum phenomena. A quantum Hall topological insulating (QHTI) state, comprising two copies of QH states with antiparallel two ferromagnetic edge-spin overlap protected by the U(1) axial rotation symmetry of spin polarization, has recently been achieved in low magnetic field (B) even as high as ≈100 K in a monolayer graphene/thin hexagonal boron nitride (hBN) spacer placed on an STO substrate, thanks to the high ɛ of STO. Despite the use of the heavy STO substrate, however, proximity-induced quantum spin Hall (QSH) states in 2D TI phases, featuring a topologically protected helical edge spin phase within time-reversal-symmetry, is not confirmed. Here, with the use of a monolayer hBN spacer, it is revealed the coexistence of QSH (at B = 0T) and QHTI (at B ≠ 0) states in the same single graphene sample placed on an STO, with a crossover regime between the two at low B. It is also classified that the different symmetries of the two nontrivial helical edge spin phases in the two states lead to different interaction with electron-puddle quantum dots, caused by a local surface pocket of the STO, in the crossover regime, resulting in a spin dephasing only for the QHTI state. The results obtained using STO substrates open the doors to investigations of novel QH spin states with different symmetries and their correlations with quantum phenomena. This exploration holds value for potential applications in spintronic devices., (© 2024 Wiley‐VCH GmbH.)