3D Topological Insulator Bi2Te3 Research Articles

Comparing spin injection in Fe75Co25/Bi2Te3 at GHz and optical excitations

Spin-to-charge conversion (S2CC) processes in thin-film heterostructures have attracted much attention in recent years. Here, we describe the S2CC in a 3D topological insulator Bi2Te3 interfaced with an epitaxial film of Fe75Co25. The quantification of spin-to-charge conversion is made with two complementary techniques: ferromagnetic resonance based inverse spin Hall effect (ISHE) at GHz frequencies and femtosecond light-pulse induced emission of terahertz (THz) radiation. The role of spin rectification due to extrinsic effects like anisotropic magnetoresistance (AMR) and planar Hall effects (PHE) is pronounced at the GHz timescale, whereas the THz measurements do not show any detectible signal, which could be attributed to AMR or PHE. This result may be due to (i) homodyne rectification at GHz, which is absent in THz measurements and (ii) laser-induced thermal spin current generation and magnetic dipole radiation in THz measurements, which is completely absent in GHz range. The converted charge current has been analyzed using the spin diffusion model for the ISHE. We note that regardless of the differences in timescales, the spin diffusion length in the two cases is comparable. Our results aid in understanding the role of spin pumping timescales in the generation of ISHE signals.

Giant photoinduced inverse spin Hall effect of the surface states in three dimensional topological insulators Bi2Te3 with different thickness.

The photoinduced inverse spin Hall effect (PISHE) has been studied in three dimensional (3D) topological insulator (TI) Bi2Te3 thin films with different thicknesses (3, 5, 12 and 20 quintuple layer (QL)). The sign of the PISHE current flips only once in the 3- and 20-QL Bi2Te3 films, but it flips three times in the 5-, 7- and 12-QL samples. The three-times sign flip is due to the superposition of the PISHE current of the top and bottom surface states in Bi2Te3 films. By analyzing the x-ray photoelectron spectroscopy (XPS) of the Bi2Te3 films, we find that the top surface of the 3- and 20-QL Bi2Te3 films are severely oxidized, leading to only one sign flip in the PISHE. The PISHE contributed by the top and bottom surface states in Bi2Te3 films have been successfully separated by fitting a theoretical model to the PISHE current. The impact of the bulk states on PISHE current has been determined. The PISHE current is also measured at different light powers, and all the measurement results are in good agreement with the theoretical model. In addition, it is found that the PISHE current in Bi2Te3 films grown on Si substrate is more than two orders larger than that grown on SrTiO3 substrates, which can be attributed to the larger absorption coefficient for Bi2Te3/Si samples. It is revealed that the PISHE current in 3D TI Bi2Te3 is as large as 140 nA/W in the 3-QL Bi2Te3 film grown on Si substrate, which is more than one order larger than that reported in GaAs/AlGaAs heterojunction (about 2 nA/W) and GaN/AlGaN heterojunction (about 1.7 nA/W). The giant PISHE current demonstrates that the TIs with strong SOC may have good application prospects in spintronic devices with high spin-to-charge conversion efficiency.

Tendency of Gap Opening in Semimetal 1T′‐MoTe2 with Proximity to a 3D Topological Insulator

AbstractMonolayer (ML) 1T′‐MoTe2 has attracted intensive interest as a fascinating quantum spin Hall (QSH) insulator. However, there are two critical aspects impeding its exploration and potential applications of QSH effects. One is its semimetallic feature with a negative band gap, leading to nontrivial edge channels annihilated by the bulk states. The other is its fabrication always accompanied by a mixed phase of 1T′ and 2H. Based on first‐principles calculations, it is shown that the large work‐function difference results in strong interlayer interactions and proximity effects in ML 1T′‐MoTe2 via interfacing a 3D topological insulator Bi2Te3, facilitating the realization of pure 1T′ phase and even the band gap opening. It is further verified that the epi‐grown ML 1T′‐MoTe2 on Bi2Te3 is nearly in single phase. Furthermore, the measurements of angle resolved photoemission spectroscopy and scanning tunneling spectroscopy confirm the obvious separated‐tendency of conduction and valence bands as well as the strong metallic edge states in ML 1T′‐MoTe2. The results also reveal the nontrivial band topology in ML 1T′‐MoTe2 is preserved in 1T′‐MoTe2/Bi2Te3 heterostructure. This work offers a promising candidate to realize QSH effects and provides guidance for controlling the nontrivial band gap opening by proximity effects in van der Waals engineering.

Magnetic field-dependent resistance crossover and anomalous magnetoresistance in topological insulator Bi2Te3

We report a metal–insulator like transition in single-crystalline 3D topological insulator Bi2Te3 at a temperature of 230 K in the presence of an external magnetic field applied normal to the surface. This transition becomes more prominent at larger magnetic field strength with the residual resistance value increasing linearly with the magnetic field. At low temperature, the magnetic field dependence of the magnetoresistance shows a transition from logarithmic to linear behavior and the onset magnetic field value for this transition decreases with increasing temperature. The logarithmic magnetoresistance indicates the weak anti-localization of the surface Dirac electrons while the high temperature behavior originates from the bulk carriers due to intrinsic impurities. At even higher temperatures beyond ∼230 K, a completely classical Lorentz model type quadratic behavior of the magnetoresistance is observed. We also show that the experimentally observed anomalies at ∼230 K in the magneto-transport properties do not originate from any stacking fault in Bi2Te3.

Observation of the Spin Seebeck Effect in Bi2Te3 Topological Insulator without an External Magnetic Field

The 3D topological insulator Bi2Te3 is a well‐known semiconductor material with a large Seebeck coefficient, and it is widely used in thermoelectric generators. The spin‐momentum‐locked surface states in topological insulator induce electronic spin polarization which may be used for spin generation and accumulation. Under a temperature gradient, the accumulation of surface electrons with opposite spin directions at the hot and cold sides of the sample will occur. Thus, the spin Seebeck effect (SSE) is detected through the inverse spin Hall effect in heavy metal Pt electrodes with a strong spin–orbit interaction. The SSE is observed in the topological insulator Bi2Te3 without application of an external magnetic field. The spin Seebeck coefficient in the Bi2Te3 topological insulator increases with increasing surface disorder, which is explained via the Dyakonov–Perel mechanism of the spin relaxation time.

Bulk defects and surface state dynamics in topological insulators: The effects of electron beam irradiation on the ultrafast relaxation of Dirac fermions in Bi2Te3

One of the most important challenges in the study of topological insulators is the realization of materials that are really insulating in the bulk, in order to emphasize quantum transport in the protected surface states. Irradiation with electron beams is a very promising approach toward this goal. By studying a series of samples of the prototype 3D topological insulator Bi2Te3, we show that while the topological properties of Dirac surface states are preserved after electron irradiation, their relaxation dynamics are very sensitive to the related modifications of the bulk properties. Using time- and angle-resolved photoelectron spectroscopy, we can reveal two distinct relaxation regimes after optical excitation for non-irradiated and irradiated samples. While the faster regime, corresponding to the first few picoseconds, presents a similar temporal evolution of the photoexcited population for all studied samples, the slower regime is strongly influenced by the controlled generation of defects in the bulk lattice. By adjusting the irradiation parameters in this class of materials, one can thus not only change the bulk transport properties but also tune the ultrafast response of the topological surface states.

Unexplored photoluminescence from bulk and mechanically exfoliated few layers of Bi2Te3

We report the exotic photoluminescence (PL) behaviour of 3D topological insulator Bi2Te3 single crystals grown by customized self-flux method and mechanically exfoliated few layers (18 ± 2 nm)/thin flakes obtained by standard scotch tape method from as grown Bi2Te3 crystals. The experimental PL studies on bulk single crystal and mechanically exfoliated few layers of Bi2Te3 evidenced a broad red emission in the visible region from 600–690 nm upon 375 nm excitation wavelength corresponding to optical band gap of 2 eV. These findings are in good agreement with our theoretical results obtained using the ab initio density functional theory framework. Interestingly, the observed optical band gap is several times larger than the known electronic band gap of ~0.15 eV. The experimentally observed 2 eV optical band gap in the visible region for bulk as well as for mechanically exfoliated few layers Bi2Te3 single crystals clearly rules out the quantum confinement effects in the investigated samples which are well known in the 2D systems like MoS2,WS2, WSe2, and MoSe2 for 1–3 layers.

Magnetotransport response in the 3D topological insulator Bi2Te3 with indium superconducting electrodes

This report studied the magneto-transport properties in the hybrid structure which combines thin flakes of 3D topological insulator (TI) Bi2Te3 with topside indium superconducting electrodes. The observed anomalous magnetoresistance suggests two critical transitions. The first transition, obtained approximately at T = 3.4 K, is attributed to superconductivity in the indium electrodes. While the second transition, observed at lower temperature, is attributed to the proximity effect generated from the interface between the TI and the superconductor.

First‐principle calculations of optical properties of monolayer arsenene and antimonene allotropes

Recently a stable monolayer of antimony in buckled honeycomb structure called antimonene was successfully grown on 3D topological insulator Bi2Te3 and Sb2Te3, which displays novel semiconducting properties. By first‐principle calculations, we systematically investigate the electronic and optical properties of α‐ and β‐allotropes of monolayer arsenene/antimonene. The obtained electronic structures reveal that the direct band gap of α‐arsenene/antimonene is much smaller than the indirect band gap of their β‐counterpart, respectively. Significant absorption is observed in α‐antimonene, which can be used as a broad saturable absorber. For β‐arsenene/antimonene, the reflectivity is low and the absorption is negligible in the visible region when the polarization along the out‐plane direction, indicating that β‐arsenene/antimonene are polarizationally transparent materials.image

Experimental and density functional study of Mn doped Bi2Te3 topological insulator

We present a nanoscale structural and density functional study of the Mn doped 3D topological insulator Bi2Te3. X-ray absorption near edge structure shows that Mn has valency of nominally 2+. Extended x-ray absorption fine structure spectroscopy in combination with electron energy loss spectroscopy (EELS) shows that Mn is a substitutional dopant of Bi and Te and also resides in the van der Waals gap between the quintuple layers of Bi2Te3. Combination of aberration-corrected scanning transmission electron microscopy and EELS shows that Mn substitution of Te occurs in film regions with increased Mn concentration. First-principles calculations show that the Mn dopants favor octahedral sites and are ferromagnetically coupled.

Superconducting-contact-induced resistance-anomalies in the 3D topological insulator Bi2Te3

This study examines the magnetotransport response observed in flakes of the 3D topological insulator (TI) Bi2Te3, including indium superconducting electrodes, and demonstrates two critical transitions in the magnetoresistive response with decreasing temperatures below T=3.4 K. The first transition is attributed to superconductivity in the indium electrodes, and the second transition, with a critical field exceeding the transition field of indium, is attributed to a proximity effect at the 2D planar interface of this hybrid TI/superconductor structure.