Topological Insulator Bi2Se3 Research Articles

Ultrafast bipolar conductivity driven by intense single-cycle terahertz pulses in a topological insulator Bi2Se3

We present the measurement of ultrafast terahertz (THz) conductivity of a 50 nm thick topological insulator Bi2Se3 at low temperature 5 K by using a THz pump and THz probe spectroscopy scheme. The THz conductivity driven by a single-cycle intense THz pump exhibits a bipolar lineshape and frequency-dependent relaxation dynamics due to the separated surface and bulk charge carriers with distinctly different scattering rates and relaxation times. We also demonstrate THz pump field-dependent excitation of the surface and bulk spectral weights, which suggests one of the optimal THz excitation conditions for generating the most charge carriers from the topology-protected surface state relative to the bulk. This also allows THz control of surface and bulk transport channels in a selective way.

Back Cover: Erasable Superconductivity in Topological Insulator Bi2Se3 Induced by Voltage Pulse (Adv. Quantum Technol. 9/2021)

Emergent superconductivity is observed in the local mesoscopic point-contacts on the topological insulator Bi2Se3 by applying a voltage pulse through the Ag contacts. The superconductivity can be erased with thermal cycles by warming up to high temperatures (300 K) and repeatedly induced by the voltage pulse at base temperature (1.9 K), suggesting a significance for designing new types of quantum devices. For further details, see article number 2100067 by Tian Le, Xiaozhi Wang, Xin Lu and co-workers.

Observation of antiferromagnetic ordering from muon spin resonance study and the Kondo effect in a Dy-doped Bi2Se3 topological insulator

The transport and magnetic properties as well as the microscopic electronic properties of the Dy-doped topological insulator Bi2Se3 (Bi1.9Dy0.1Se3) are examined in detail. It is demonstrated that Dy doping induces antiferromagnetic (AFM) ordering in Bi2Se3. It is also observed that Dy doping opens a surface band gap of ∼52 meV at 6.5 K. The transition from AFM to ferromagnetic occurs at a lower temperature, ∼5 K in a magnetic field of ∼3 T. Furthermore, Dy doping in Bi2Se3 leads to the Kondo effect and weak localization to weak anti-localization crossover. Muon spin resonance measurements indicate that the signal measured with magnetization reflects the magnetic properties of 2% of the sample. The experimental findings are supported by theoretical calculations based on density functional theory.

Growth of topological insulator Bi2Se3 particles on GaAs via droplet epitaxy

The discovery of topological insulators (TIs) and their unique electronic properties has motivated research into a variety of applications, including quantum computing. It has been proposed that TI surface states will be energetically discretized in a quantum dot nanoparticle. These discretized states could then be used as basis states for a qubit that is more resistant to decoherence. In this work, prototypical TI Bi2Se3 nanoparticles are grown on GaAs (001) using the droplet epitaxy technique, and we demonstrate the control of nanoparticle height, area, and density by changing the duration of bismuth deposition and substrate temperature. Within the growth window studied, nanoparticles ranged from 5 to 15 nm in height with an 8–18 nm equivalent circular radius, and the density could be relatively well controlled by changing the substrate temperature and bismuth deposition time.

1D topological systems for next-generation electronics

1D topological systems for next-generation electronics

Low-temperature ultrafast optical probing of topological bismuth selenide

We report the optical response and temperature-dependent excited-state carrier dynamics in the flake of Bi2Se3, which is cleaved from its single crystal. The optical properties are explored using the visible-near infrared (VIS-NIR), infrared (IR), and photoluminescence (PL) spectroscopies. The VIS-NIR spectra are then used to find the Ɛ1 and Ɛ2 using Kramer’s Koning relations, confirming OBT of 2 eV and 1.4 eV. The DFT (density functional theory) calculations are also carried out to confirm the experimental optical transitions (OBT). To probe these OBT in the bismuth selenide, we have studied the temperature, fluence, and excitation dependent ultrafast transient reflectance over a wide spectral range from 2.58–0.77 eV (VIS-NIR) with different excitation energies 3.02 eV, 2.61 eV, 1.9 eV, and 1.4 eV to explore several previously unseen transitions that do not appear in PL spectroscopy and room temperature carrier analysis. The temperature-dependent excited-state dynamics are investigated at 5–300 K. This study clearly indicates the existence of Moss-Burstein shift in the visible region and Pauli blocking effect in the NIR region in the Bi2Se3 topological insulator. Moreover, the low-temperature TRUS confirms the transition to the second surface state present in bismuth selenide.

Scalable fabrication of Bi2O2Se polycrystalline thin film for near-infrared optoelectronic devices applications* *Project supported by the Hunan Provincial Natural Science Foundation of China (Grant No. 2019JJ40032).

We present a controlled, stepwise formation of layered semiconductor Bi2O2Se thin films prepared via the vapour process by annealing topological insulator Bi2Se3 thin films in low oxygen atmosphere for different reactions. Photodetectors based on Bi2O2Se thin film show a responsivity of 1.7 × 104 A/W at a wavelength of 980 nm. Field-effect transistors based on Bi2O2Se thin film exhibit n-type behavior and present a high electron mobility of 17 cm2/V⋅s. In addition, the electrical properties of the devices after 4 months keeping in the air shows little change, implying outstanding air-stability of our Bi2O2Se thin films. From the obtained results, it is evident that low oxygen annealing is a surprisingly effective method to fabricate Bi2O2Se thin films for integrated optoelectronic applications.

Quantum coherence modulation in bismuth selenide topological insulator thin film by ion irradiation

The robust surface states of bismuth selenide (Bi2Se3) topological insulator (TI) that could potentially be harnessed for quantum computing and spintronics applications are often eclipsed by defect-induced bulk carriers. With the objective of suppressing the electronic transport in the bulk, we explore the effects of ion irradiation-driven isoelectronic doping in thin films of Bi2Se3 with 600 keV antimony (Sb+) ions at 45° oblique incidence. The distribution of Sb+ ions introduced into the system has been mapped using Rutherford backscattering spectroscopy. The heavy Sb+ ion irradiation has been observed to cause a change from parabolic magnetoresistance to linear magnetoresistance behavior in the irradiated films due to mobility fluctuations. Subsequent annealing in selenium environment has been found to reverse the damage produced by the irradiation, but with significant modifications in the quantum coherence signatures. Weak antilocalization behavior has been observed in the weak magnetic field limit and has been analyzed within the purview of the Hikami-Larkin-Nagaoka model. It has been found that the channel-separated Bi2Se3 film becomes completely channel-mixed due to the induced defects, yielding a system with a single coherent channel upon irradiation. In the as-deposited film, the temperature-dependent power law decay of the coherence length has been found to have two different exponents within the measured temperature range. On the other hand, the irradiated-annealed film has a single decay exponent which has been found to tend to − 0.5, indicating that electron-electron interaction is the only de-phasing mechanism involved. This work establishes the effectiveness of ion irradiation in tuning the quantum coherence phenomena in Bi-based TI systems.

Erasable Superconductivity in Topological Insulator Bi2Se3 Induced by Voltage Pulse

Abstract3D topological insulators attract much attention due to their topologically protected Dirac surface states. Doping into topological insulators or their proximity with normal superconductors can promote the realization of topological superconductivity and Majorana fermions with potential applications in quantum computation. Here, an emergent superconductivity is observed in local mesoscopic point‐contacts on the topological insulator Bi2Se3 by applying a voltage pulse through the contacts, evidenced by the Andreev reflection peak in the point‐contact spectra and a visible resistance drop in the four‐probe electrical resistance measurements. More intriguingly, the superconductivity can be erased with thermal cycles by warming up to high temperatures (300 K) and induced again by the voltage pulse at the base temperature (1.9 K), suggesting a significance for designing new types of quantum devices. Nematic behavior is also observed in the superconducting state, similar to the case of CuxBi2Se3 as topological superconductor candidates.

Dynamic Opening of a Gap in Dirac Surface States of the Thin-Film 3D Topological Insulator Bi2Se3 Driven by the Dynamic Rashba Effect

Optical control of Dirac surface states (SS) in topological insulators (TI) remains one of the most challenging problems governing their potential applications in novel electronic and spintronic devices. Here, using visible-range transient absorption spectroscopy exploiting ∼340 nm (∼3.65 eV) pumping, we provide evidence for dynamic opening of a gap in the Dirac SS of the thin-film 3D TI Bi2Se3, which has been induced by the dynamic Rashba effect occurring in the film bulk with increasing optical pumping power (photoexcited carrier density). The observed effect appears through the transient absorption band associated with inverse-bremsstrahlung-type free carrier absorption in the gapped Dirac SS. We have also recognized experimental signatures of the existence of the higher energy Dirac SS in the 3D TI Bi2Se3 (in addition to those known as SS1 and SS2) with energies of ∼2.7 and ∼3.9 eV (SS3 and SS4). It is evidenced that the dynamic gap opening has the same effect on the Dirac SS occurring at any energy.

Proximity-induced anisotropic magnetoresistance in magnetized topological insulators

Topological insulators (TIs) host spin-momentum locked surface states that are inherently susceptible to magnetic proximity modulations, making them promising for nano-electronic, spintronic, and quantum computing applications. While much effort has been devoted to studying (quantum) anomalous Hall effects in magnetic magnetically doped TIs, the inherent magnetoresistance (MR) properties in magnetic proximity-coupled surface states remain largely unexplored. Here, we directly exfoliate Bi2Se3 TI flakes onto a magnetic insulator, yttrium iron garnet, and measure the MR at various temperatures. We experimentally observe an anisotropic magnetoresistance that is consistent with a magnetized surface state. Our results indicate that the TI has magnetic anisotropy out of the sample plane, which opens an energy gap between the surface states. By applying a magnetic field along any in-plane orientation, the magnetization of the TI rotates toward the plane and the gap closes. Consequently, we observe a large (∼6.5%) MR signal that is attributed to an interplay between coherent rotation of magnetization within a topological insulator and abrupt switching of magnetization in the underlying magnetic insulator.

Spectroscopic ellipsometry as an in-situ monitoring tool for Bi2Se3 films grown by molecular beam epitaxy

In-operando spectroscopic ellipsometry (SE) was used to measure the optical response of Bi2Se3 films grown on sapphire substrates in a molecular beam epitaxy (MBE) reactor during cool-down from a growth temperature of 225 °C. A temperature dependent dielectric model was refined for the topological insulator Bi2Se3 by taking SE spectra at different temperatures and fitting the amplitude ratio and phase difference of orthogonally polarized light using Bi2Se3 films with varying thickness. In-operando SE demonstrated here enabled determining the dielectric function of substrate and growing film unobscured by surface or interface reactions. Its sensitivity to sample temperature and film thickness variations allows determining growth temperature, absolute film thickness, and growth rate in real time, rendering it a reliable and universal approach for a direct comparison of growth conditions between different growth campaigns, thus offering the potential to improve reproducibility of the growth conditions for Bi2Se3 based films and heterostructures.

Local insight to the structural phase transition sequence of Bi2Se3 under quasi-hydrostatic and nonhydrostatic pressure

High-pressure behaviors of Bi2Se3, as one of layered 3D topological insulators, has attracted tremendous interest recent years. However, the phase transition sequence of Bi2Se3 remain controversial. In this work, we explore the structural phase sequence of topological insulator Bi2Se3 using high-pressure x-ray absorption fine structure (XAFS) spectroscopy under quasi-hydrostatic and nonhydrostatic pressure up to 42 GPa. By examining the XAFS features, we find that the appearance of C2/c phase of Bi2Se3 is strongly dependent on pressure condition, C2/c phase of Bi2Se3 only exists under quasi-hydrostatic pressure condition. The phonon dispersion calculations also show that C2/c phase is dynamic unstable. Furthermore, we confirm that Bi2Se3 possesses I4/mmm phase rather than Im-3m and 9/10-fold C2/m phase at high pressure. Combining the experimental and theoretical results, we determine the structural phase transition sequence for Bi2Se3 of R-3m → C2/m → C2/c → I4/mmm phase under quasi-hydrostatic pressure condition and R-3m → C2/m → I4/mmm phase under nonhydrostatic pressure condition. The dynamic unstability and pressure condition sensitivity of C2/c phase may be account for the absence of C2/c phase in the phase transition sequence under nonhydrostatic pressure condition. Our findings obtain the high-pressure phase transition sequences of Bi2Se3 under hydrostatic and nonhydrostatic pressure condition, which can facilitate researchers to explore the novel properties in layered 3D topological insulators.

Topological insulator Bi2Se3 as a tunable crystal for terahertz frequency generation

Topological insulator Bi2Se3 as a tunable crystal for terahertz frequency generation

Clarifying Ultrafast Carrier Dynamics in Ultrathin Films of the Topological Insulator Bi2Se3 Using Transient Absorption Spectroscopy

Ultrafast carrier dynamics in the topological insulator Bi2Se3 have recently been intensively studied using a variety of techniques. However, we are not aware of any successful experiments exploiting transient absorption (TA) spectroscopy for these purposes. Here we demonstrate that if the ~730 nm wavelength pumping (~1.7 eV photon energy) is applied to ultrathin Bi2Se3 films, TA spectra cover the entire visible region, thus unambiguously pointing to two-photon excitation (~3.4 eV). The carrier relaxation dynamics is found to be governed by the polar optical phonon cascade emission occurring in both the bulk states and the Dirac surface states (SS), including SS-bulk-SS vertical electron transport and being also exclusively influenced by whether the Dirac point is presented between the Dirac cones of the higher energy (~1.5 eV) Dirac SS (known as SS2). We have recognized that SS2 act as a valve substantially slowing down the relaxation of electrons when the gap between Dirac cones exceeds the polar optical phonon and resonant defects energies. The resulting progressive accumulation of electrons in the gapped SS2 becomes detectable through the inverse bremsstrahlung type free carrier absorption.

Imaging current distribution in a topological insulator Bi2Se3 in the presence of competing surface and bulk contributions to conductivity

Two-dimensional (2D) topological surface states in a three-dimensional topological insulator (TI) should produce uniform 2D surface current distribution. However, our transport current imaging studies on Bi2Se3 thin film reveal non-uniform current sheet flow at 15 K with strong edge current flow. This is consistent with other imaging studies on thin films of Bi2Se3. In contrast to strong edge current flow in thin films, in single crystal of Bi2Se3 at 15 K our current imaging studies show the presence of 3.6 nm thick uniform 2D sheet current flow. Above 70 K, this uniform 2D sheet current sheet begins to disintegrate into a spatially non-uniform flow. The flow becomes patchy with regions having high and low current density. The area fraction of the patches with high current density rapidly decreases at temperatures above 70 K, with a temperature dependence of the form 1/left| {T - 70} right|^{0.35}. The temperature scale of 70 K coincides with the onset of bulk conductivity in the crystal due to electron doping by selenium vacancy clusters in Bi2Se3. Thus our results show a temperature dependent competition between surface and bulk conductivity produces a temperature dependent variation in uniformity of current flow in the topological insulator.

Unusual Quadrupole NMR of Topological Insulator Bi2Te3

Very recently, nuclear magnetic resonance (NMR) of 209Bi has been shown to provide quantitative access to the bulk band inversion in the topological insulator Bi2Se3 and detected a highly unusual f...

Impurity band assisted carrier relaxation in Cr doped topological insulator Bi2Se3

Topological insulators (TIs) with unique band structures have wide application prospects in the fields of ultrafast optical and spintronic devices. The dynamics of hot carriers plays a key role in these TI-based devices. In this work, using the time- and angle-resolved photoemission spectroscopy technique, the relaxation process of the hot carriers in Cr-doped Bi2Se3 has been systematically studied since the ferromagnetic TI is one of the key building blocks for next-generation spintronics. It is found that electronic temperature (Te) and chemical potential (μ) decrease faster with the increase in the Cr doping concentration. Similarly, the lifetime (τ) of the excited electrons also decreases with more Cr doped into Bi2Se3. The results suggest a mechanism of impurity band-assisted carrier relaxation, where the impurity band within the bulk bandgap introduced by Cr doping provides significant recombination channels for the excited electrons. This work directly illustrates the dynamic process of the photon-generated carriers in Cr-doped Bi2Se3, which is expected to promote the applications of (Bi1-xCrx)2Se3 in photoelectric devices.

Growth of Bi2Se3/graphene heterostructures with the room temperature high carrier mobility

Heterostructures of Bi2Se3 topological insulators were epitaxially grown on graphene by means of the physical vapor deposition at 500 °C. Micrometer-sized flakes with thickness 1 QL (quintuple layer ~ 1 nm) and films of millimeter-scale with thicknesses 2–6 QL had been grown on CVD graphene. The minimum thickness of large-scaled continuous Bi2Se3 films was found to be ~ 8 QL for the regime used. The heterostructures with a Bi2Se3 film thickness of > 10 QL had resistivity as low as 200–500 Ω/sq and a high room temperature carrier mobility ~ 1000–3400 cm2/Vs in the Bi2Se3/graphene interface channel. Moreover, the coexistence of a p-type graphene-related conductive channel, simultaneously with the n-type conductive surface channel of Bi2Se3, was observed. The improvement of the bottom Bi2Se3/graphene interface with the increase in the growth time clearly manifested itself in the increase of conductivity and carrier mobility in the grown layer. The grown Bi2Se3/G structures have lower resistivities and more than one order of magnitude higher carrier mobilities in comparison with the van der Waals Bi2Se3/graphene heterostructures created employing exfoliation of thin Bi2Se3 layers. The grown heterostructures demonstrated the properties that are perspective for new functional devices, for a variety of signal processing and logic applications.

Quantum frequency doubling in the topological insulator Bi2Se3

The nonlinear Hall effect due to Berry curvature dipole (BCD) induces frequency doubling, which was recently observed in time-reversal-invariant materials. Here we report novel electric frequency doubling in the absence of BCD on a surface of the topological insulator Bi2Se3 under zero magnetic field. We observe that the frequency-doubling voltage transverse to the applied ac current shows a threefold rotational symmetry, whereas it forbids BCD. One of the mechanisms compatible with the symmetry is skew scattering, arising from the inherent chirality of the topological surface state. We introduce the Berry curvature triple, a high-order moment of the Berry curvature, to explain skew scattering under the threefold rotational symmetry. Our work paves the way to obtain a giant second-order nonlinear electric effect in high mobility quantum materials, as the skew scattering surpasses other mechanisms in the clean limit.