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-Sn
摘要： Gray tin, also known as α-Sn, can be turned into a three-dimensional topological insulator (3D-TI) by strain and finite-size effects. Such room-temperature 3D-TI is peculiarly interesting for spintronics due to the spin-momentum locking along the Dirac cone (linear dispersion) of the surface states. Angle-resolved photoemission spectroscopy (ARPES) has been used to investigate the dispersion close to the Fermi level in thin (001)-oriented epitaxially strained films of α-Sn for different film thicknesses as well as for different capping layers (Al, AlOx, and MgO). Indeed a proper capping layer is necessary to be able to use α-Sn surface states for spintronic applications. In contrast with free surfaces or surfaces coated with Ag, coating the α-Sn surface with Al or AlOx leads to a drop in the Fermi level below the Dirac point, and an important consequence for electronic transport is the presence of bulk states at the Fermi level. α-Sn films coated by AlOx are studied by electrical magnetotransport: Despite magnetotransport properties of the bulk electronic states of the Γ8 band playing an important role as suggested by ab initio calculations, there is clear evidence of surface states revealed by Shubnikov–de Haas oscillations corresponding to the ARPES observation.
关键词： topological insulator,spin-momentum locking,α-Sn,Shubnikov–de Haas oscillations,Dirac cone,ARPES
更新于2025-09-23 15:21:01
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Theoretical Analysis of a Non-Quantized Square-Root Topological Insulator using Photonic Aharonov-Bohm Cages
摘要： Topological insulators have to date seen a variety of manifestations. All available realizations of topological insulators, however, share a common feature: their spectral bands are attributed with a nonlocal topological index that is quantized. In this work, we report a new type of insulator exhibiting spectral bands with nonquantized indices, yet robust boundary states. We provide a theoretical analysis based on the quantization of the indices in the corresponding system where the square of the Hamiltonian is taken and exemplify the general paradigm using photonic Aharonov-Bohm cages. Taking the square-root of the Klein-Gordon Hamiltonian has opened up an entirely new realm of physics: the resulting Dirac operator provided a relativistic quantum description of massive spin-1/2 fermionic particles, thus disclosing the fine-structure spectra of atoms or the anomalous Zeeman effect. Along a similar line, we consider a three-band model that can have nonquantized topological indices, but with a spectral symmetry that gives rise to quantized topological invariants when the square of the Hamiltonian is taken. This relation generates the topological characterization of the square-root model and allows us to establish a topological bulk-boundary correspondence with states that can be used as in-gap, protected and controllable qubits. Notably, whereas taking the square of the Dirac operator leads to the topologically trivial Klein-Gordon model, in our case, the squared model maps to an effective Su-Schrieffer-Heeger (SSH) model in its nontrivial phase. Optical settings prove to be ideally suited for realizing various topological phenomena. In this vein, we employ photonic waveguide lattices with effective negative hopping amplitudes to realize a 3-band quasi-1D chain made of photonic Aharonov-Bohm cages that exemplifies our general description. In summary, we predict and demonstrate the physics of a square-root topological insulator, using a photonic platform. Specifically, we show that the AB cages have in-gap states, above bands possessing nonquantized Zak’s phases. Furthermore, we find that these states are robust, both in energy and localization, against symmetry-preserving perturbation and disorder. We show that this robustness stems from the corresponding system where the square of the Hamiltonian is taken and where the bands exhibit quantized topological indices with associated in-gap boundary states.
关键词： nonquantized indices,photonic Aharonov-Bohm cages,square-root topological insulator,robust boundary states,Topological insulators
更新于2025-09-23 15:19:57
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Valley polarization reversal and spin ferromagnetism and antiferromagnetism in quantum dots of the topological insulator monolayer bismuthene on SiC
摘要： The valley and spin polarizations associated with electronic transport in quantum dots of the large-gap topological insulator (TI) monolayer bismuthene on SiC are investigated in the linear response regime using a minimal tight-binding model that accurately describes the low-energy electronic band structure of this TI. It is found that for zigzag edges the electronic edge states are strongly valley polarized if the Fermi energy lies in the bulk energy band gap. We predict the edge-state valley polarizations to switch between valleys K and K (cid:2) as the Fermi energy varies from the top of the valence band to the bottom of the conduction band or if the direction of electric current through the dot is reversed. If the electrostatic potential in the dot is nonuniform, we predict that the valley polarization of an electron can reverse as it travels through the dot. The valley polarization reversal is due to the zigzag edge-state dispersion crossing the center of the Brillouin zone that separates valleys K and K (cid:2) and is therefore predicted to be a general phenomenon. Although the spin polarization within the edge states is ferromagnetic, as expected for spin Hall devices, our calculations reveal the out-of-plane component of the spin polarization of the bulk valence band scattering states to be antiferromagnetic, and the direction of the out-of-plane component of the Neel vector to depend on whether the electronic accumulation belongs primarily to valley K or K (cid:2).
关键词： quantum dots,ferromagnetism,bismuthene,topological insulator,spin polarization,antiferromagnetism,SiC,tight-binding model,valley polarization
更新于2025-09-23 15:19:57
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Nonlayered tellurene as an elemental 2D topological Insulator: experimental evidence from scanning tunneling spectroscopy
摘要： We report formation of nonlayered tellurene monolayer in its alpha-phase through an anisotropic ultrasonication method. The nonlayered tellurene has so far been predicted to exhibit a topologically insulating state of matter in two-dimensional (2D) form with an insulating interior and metallic edge-states propagating along the perimeter of the 2D objects. In this work, we report a direct evidence of elemental topological insulator behavior in the material through a localized mode of measurement, that is, scanning tunneling spectroscopic studies. We moreover deliberate on the length-scale the time-reversal symmetry-protected edge-states extend towards the interior. The metallic edge, which has been found to span over a 3 nm region, opens and widens monotonically into gapped states. The appearance of elemental 2D topological insulator phase has been explained in terms of built-in strains in the systems as viewed through a shift in Raman modes.
关键词： Scanning Tunneling Spectroscopy,Elemental 2D Topological Insulator,Nonlayered Tellurene
更新于2025-09-19 17:15:36
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Preparation and magnetic properties for FeSe <sub/><i>x</i> </sub> /Bi <sub/>2</sub> Se <sub/>3</sub> bilayer films on silicon substrates by RF magnetron sputtering
摘要： FeSex/Bi2Se3 bilayer thin films were grown by RF magnetron sputtering on silicon substrates with different thicknesses of Bi2Se3 and the structural, morphological and magnetic properties were investigated. FeSex/Bi2Se3 bilayer films had Bi2Se3 crystallites oriented with c-axis perpendicular to the film plane, and exhibited weak ferromagnetism at low temperature due to the ferromagnetic FeSe2. The thickness of Bi2Se3 layer affected both crystalline structure of Fe–Se layer and the magnetic property.
关键词： magnetic phase,RF magnetron sputtering,paramagnetism,Topological insulator,bilayer films
更新于2025-09-19 17:15:36
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Molecular beam epitaxy of superconducting PdTe2 films on topological insulator Bi2Te3
摘要： Majorana fermions have been observed in topological insulator/s-wave superconductor heterostructures. To manipulate Majorana fermions, superconducting materials should be deposited on the surfaces of topological insulators. In this study, high-quality superconducting PdTe2 films are deposited on the topological insulator Bi2Te3 surface using molecular beam epitaxy. The surface topography and electronic properties of PdTe2/Bi2Te3 heterostructures are investigated via in situ scanning tunneling microscopy/spectroscopy. Under Te-rich conditions, the Pd atoms presumably form PdTe2 film on Bi2Te3 surface rather than diffuse into Bi2Te3. The superconductivity of the PdTe2/Bi2Te3 heterostructure is detected at a transition temperature of ~1.4 K using the two-coil mutual inductance technique. This study proposes a method for fabricating superconducting materials on topological insulator surfaces at low doping levels, paving ways for designing nanodevices that can manipulate Majorana fermions.
关键词： superconductor,heterostructure,molecular beam epitaxy,topological insulator
更新于2025-09-19 17:15:36
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Experimental Demonstration of a Non-Quantized Square-Root Topological Insulator using Photonic Aharonov-Bohm Cages
摘要： In many applications, topological effects can be characterized by a quantized topological invariant [1]. In this work, we investigate a new type of topological insulator that stands apart from this formalism, since it exhibits spectral bands with non-quantized topological properties. As it turns out, this system can nevertheless be reconciled with the existing paradigm, since a quantization can be established by squaring the Hamiltonian Fig. 1(b). Arrays of evanescently coupled waveguides have proven to be a versatile platform for probing topological phenomena [2]. Along these lines, we employ femtosecond laser-written [3] photonic Aharonov-Bohm cages to experimentally verify these claims, making use of this versatile platform to implement the desired Hamiltonian and to probe its topological features by observing propagation dynamics. The specific structure is sketched in Fig. 1(a), where the lattice sites are coupled with the strength t and every plaquette contains a flux of φ. We fabricate samples according to the sketch in Fig. 1(c), where the negative hopping in each plaquette is realized by means of a detuned ancillary waveguide. The associated flux of φ = π gives rise to compact states that form flat bands [5, 6]. After verifying the flux by demonstrating the existence of these localized modes Fig. 1(d), we show the existence of a topological boundary mode that is linked to the topological quantity of the squared Hamiltonian. In conclusion, we theoretically predicted the existence of a topological insulator with non-quantized winding number, and experimentally verified its characteristics in photonic lattices. Since these findings are not limited to a specific experimental platform, they may pave the road for further experiments and new insights in complex topological systems across numerous fields of physics.
关键词： photonic Aharonov-Bohm cages,femtosecond laser-written waveguides,topological insulator,non-quantized topological properties
更新于2025-09-16 10:30:52
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Growth Habits of Bismuth Selenide (Bi2Se3) layers and nanowires over Stranski–Krastanov Indium Arsenide Quantum Dots
摘要： Bismuth selenide layers and nanowires have been grown by molecular beam epitaxy on self-assembled Stranski–Krastanov InAs quantum dots of different sizes and densities on GaAs substrates. The size and density of the InAs quantum dots were modified by changes in the growth rate and composition. The structure and growth habits of the Bi2Se3 layers were studied by high-resolution x-ray diffraction, scanning probe microscopy, energy-dispersive x-ray spectroscopy and high-resolution electron microscopy. The epitaxial growth of continuous layers of (0001) Bi2Se3 was observed over flat InAs surfaces. In contrast, the presence of InAs quantum dots induced the growth of 100 nm-long and 20 nm-wide Bi2Se3 nanowires primarily oriented along [01-1] and [0-1-1] directions. The nanowires coalesced into full layers when the growth proceeded further. Better understanding and control of the Bi2Se3 growth habits over these surfaces should lead to novel nanostructures with enhanced physical properties.
关键词： InAs quantum dots,Bi2Se3,nanowires,topological insulator,Bismuth selenide,molecular beam epitaxy
更新于2025-09-12 10:27:22
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A Bi <sub/>2</sub> Te <sub/>3</sub> Topological Insulator as a New and Outstanding Counter Electrode Material for High-Efficiency and Endurable Flexible Perovskite Solar Cells
摘要： Inverted flexible perovskite solar cells (PSCs) typically employ expensive metals as the counter electrodes, which are brittle and corrodible by perovskite, leading to a sharp performance drop under continuous bending, air exposure, thermal stress or light illumination and eventually retards the commercialization. Herein, a low-cost Bi2Te3 counter electrode was employed by using a simple thermal evaporation process. The resultant device achieved an excellent power conversion efficiency of 18.16%, which was among the highest reported efficiencies, much higher than the reference Ag PSC (15.90%). The improvement should be attributed to the intrinsic suppressed electron backscattering in Bi2Te3 topological insulator. Simultaneously, the Bi2Te3 device obtained a significantly improved mechanical flexibility and long-term operational stability. The present strategy will help to open up a new avenue for future commercialization of flexible photovoltaic applications.
关键词： Topological insulator,Bi2Te3,Flexible perovskite solar cells,Counter electrode,CH3NH3PbI3
更新于2025-09-11 14:15:04
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Inverse Spin Hall Effect in Electron Beam Evaporated Topological Insulator Bi <sub/>2</sub> Se <sub/>3</sub> Thin Film
摘要： Spintronics exploiting pure spin current in ferromagnetic (FM)/heavy metals (HM) is a subject of intense research. Topological insulators having spin momentum locked surface states exhibit high spin–orbit coupling and thus possess a huge potential to replace the HM like Pt, Ta, W, etc. In this context, the spin pumping phenomenon in Bi2Se3/CoFeB bilayers has been investigated. Bi2Se3 thin films are fabricated by electron beam evaporation method on Si (100) substrate. In order to confirm the topological nature of Bi2Se3, low temperature magnetotransport measurement on a 30 nm thick Bi2Se3 film which shows 10% magnetoresistance (MR) at 1.5 K has been performed. A linear increase in MR with applied magnetic field indicates the presence of spin momentum-locked surface states. A voltage has been measured at room temperature to quantify the spin pumping which is generated via inverse spin Hall effect (ISHE). For the separation of spin rectification effects mainly produced by the FM CoFeB layer, in plane angular dependence of the dc voltage with respect to applied magnetic field has been measured. Our analysis reveals that spin pumping induced ISHE is the dominant contribution in the measured voltage.
关键词： spin pumping,inverse spin Hall effect,topological insulator/ferromagnetic interface,magnetoresistance,ferromagnetic resonance
更新于2025-09-10 09:29:36