Films Of Topological Insulator Bi2Se3 Research Articles

Transition from weak antilocalization to linear magnetoresistance by tuning structure geometry and chemical potential in nanostructured Bi2Se3 films

We report the electrical and magnetic transport behavior in vertical Cu-doped Bi2Se3 nanoplate films prepared by the chemical vapor deposition method. In vertical Cu-doped Bi2Se3 nanoplate films, the topological surface states are tuned by both the large surface-to-bulk ratio and the Cu doping. Due to their high specific surface area, the magnetoresistance of the vertical undoped Bi2Se3 nanoplate film exhibits a weak antilocalization effect, and it indicates that the topological surface state properties are greatly enhanced. In vertical Cu-doped Bi2Se3 nanoplate films, the electron doping is inhibited, and the carrier type is changed from n-type to p-type. The observed linear magnetoresistance is attributed to have a quantum origin from the topological surface states. When the Cu concentration reaches 1.73 at.% in vertical Bi2Se3 nanoplate film, the linear magnetoresistance can be maintained up to 100 K. Meanwhile, these vertical nanoplate films exhibit the 3D magnetotransport property. Thus, using the same material system with a broad range of carrier density and type, our work shows the transition from a weak-antilocalization to linear magnetoresistance in nanostructured topological insulator Bi2Se3 films with an unusual morphology where the nanoplates are vertically aligned to increase the surface area.

Coherent surface-to-bulk vibrational coupling in the 2D topologically trivial insulator Bi2Se3 monitored by ultrafast transient absorption spectroscopy

Ultrafast carrier relaxation in the 2D topological insulator (TI) Bi2Se3 [gapped Dirac surface states (SS)] and how it inherits ultrafast relaxation in the 3D TI Bi2Se3 (gapless Dirac SS) remains a challenge for developing new optoelectronic devices based on these materials. Here ultrashort (~ 100 fs) pumping pulses of ~ 340 nm wavelength (~ 3.65 eV photon energy) were applied to study ultrafast electron relaxation in the 2D TI Bi2Se3 films with a thickness of 2 and 5 quintuple layers (~ 2 and ~ 5 nm, respectively) using transient absorption (TA) spectroscopy in the ultraviolet–visible spectral region (1.65–3.9 eV). The negative and positive contributions of TA spectra were attributed to absorption bleaching that mostly occurs in the bulk states and to the inverse bremsstrahlung type free carrier absorption in the gapped Dirac SS, respectively. Owing to this direct and selective access to the bulk and surface carrier dynamics, we were able to monitor coherent longitudinal optical (LO) phonon oscillations, which were successively launched in the bulk and surface states by the front of the relaxing electron population within the LO-phonon cascade emission. We have also recognized the coherent surface-to-bulk vibrational coupling that appears through the phase-dependent amplitude variations of coherent LO-phonon oscillations. This unique behavior manifests itself predominantly for the topologically trivial insulator phase of the 2D TI Bi2Se3 (2 nm thick film) in the photon energy range (~ 2.0–2.25 eV) where efficient energy exchange between the bulk and surface states occurs. We also found that the coherent surface-to-bulk vibrational coupling significantly weakens with increasing both the Bi2Se3 film thickness and pumping power.

Size Effect in the Electrical Conductivity of Thin Films of Topological Insulator Bi2Se3

The electrical resistivity of thin films of topological insulator (TI) Bi2Se3 10 to 75 nm thick is measured in the temperature range of 4.2 to 300 K. A size effect is observed in the electrical conductivity of the Bi2Se3 films; i.e., there is a linear dependence of a film’s conductivity on its inverse thickness. It is assumed that a similar effect can be observed in other TIs and in systems with nonuniform distributions of current over the cross section of a sample.

Topological insulator Bi2Se3 films on rare earth iron garnets and their high-quality interfaces

The integration of quantum materials like topological insulators (TIs) with magnetic insulators (MIs) has important technological implications for spintronics and quantum computing. Here, we report excellent crystallinity of c-axis oriented epitaxial TI films of Bi2Se3 grown on MI films, a rare earth iron garnet (ReIG), such as thulium iron garnet (Tm3Fe5O12, TmIG), by molecular beam epitaxy using Se-buffered low-temperature growth technique. Strained-TmIG films with robust perpendicular magnetic anisotropy were deposited by off-axis sputtering. We demonstrated a streaky reflection high-energy electron diffraction pattern starting from the very first quintuple layer of Bi2Se3, indicating the high-quality interface between TmIG and Bi2Se3, a prerequisite for studying interfacial exchange coupling effects. The strong interfacial exchange interaction was manifested by the observation of an anomalous Hall effect in the Bi2Se3/TmIG bilayer and a shift of the ferromagnetic resonance field of TmIG induced by Bi2Se3. We have reproducibly grown high-quality Bi2Se3/ReIG and interfaces using this TI growth method, which may be applied to grow other types of van der Waals hetero-structures.

Enhanced coherence and decoupled surface states in topological insulators through structural disorder

To harness the true potential of topological insulators as quantum materials for information processing, it is imperative to maximise topological surface state conduction while simultaneously improving their quantum coherence. However, these goals have turned out to be contradictory. Surface dominated transport in topological insulators has been achieved primarily through compensation doping of bulk carriers which introduces tremendous electronic disorder and drastically deteriorates electronic coherence. In this work, we use structural disorder instead of electronic disorder to manipulate the electrical properties of thin films of topological insulator Bi2Se3. We achieve decoupled surface state transport in our samples and observe significantly suppressed carrier dephasing rates in the coupled surface state regime. As the film thickness is decreased, the dephasing rate evolves from a linear to a super-linear temperature dependence. While the former is consistent with Nyquist electron-electron interactions, the latter leads to significantly enhanced coherence at low temperatures and is indicative of energy exchange due to frictional drag between the two surface states. Our work opens up the way to harness topological surface states, without being afflicted by the deleterious effects of compensation doping.

From linear magnetoresistance to parabolic magnetoresistance in Cu and Cr-doped topological insulator Bi2Se3 films

We report on the magnetotransport properties of Cu- and Cr-doped topological insulator Bi2Se3 films grown on silicon (100) substrate by chemical vapor deposition. Upon doping of non-magnetic Cu, Bi2Se3 film's carrier density is reduced and the magnetoresistance changes from a strong positive parabolic magnetoresistance to a linear magnetoresistance. This linear magnetoresistance is similar to the linear magnetoresistance in other gapless topological states of matter and has a strong temperature dependence. In contrast, the magnetoresistance of Cr-doped Bi2Se3 films remains to be parabolic and becomes temperature independent upon increased Cr-doping. These results suggest that the absence of linear magnetoresistance in Cr-doped Bi2Se3 is related to the magnetic dopants that induce time reversal symmetry breaking and surface states gap opening.

Properties of topological insulator Bi2Se3 films prepared by thermal evaporation growth on different substrates

Topological insulator Bi2Se3 thin films were grown by the thermal evaporation deposition on different substrates and their phase structures and magneto-transport properties were discussed. The films growth is along the c-axis, and their surfaces exhibited terrace-like quintuple layers. Resistivity upturn as well as weak anti-localization was observed only in the film with Si and LAO substrates. The linear magneto-resistance (LMR) under high field was found, which was associated with the gapless topological surface states and the quantum origin. The results indicate that the films are highly uniform.

Strain state, film and surface morphology of epitaxial topological insulator Bi2Se3 films on Si(111)

Epitaxial Bi2Se3 films were grown by molecular beam epitaxy on Si(111)-Bi(3×3)R30° at temperatures between 200 and 250°C. The surface and bulk morphology was characterized by high resolution low energy electron diffraction, X-ray diffraction, and atomic force microscopy for various film thicknesses between 6 and 90nm. The films are atomically smooth without small angle mosaics or small angle rotational domains. The precise determination of lattice parameter reveals that films grown at higher temperature exhibit a smaller value for the vertical lattice parameter. The presence of random stacking faults in the film is reflected by a parabolic increase of the width of the diffraction peaks in X-ray diffraction.

Observation of quantum-tunnelling-modulated spin texture in ultrathin topological insulator Bi2Se3 films.

Understanding the spin-texture behaviour of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nanodevices. Here, by using spin-resolved photoemission spectroscopy with p-polarized light in topological insulator Bi2Se3 thin films, we report tunnelling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition. We report a systematic binding energy- and wavevector-dependent spin polarization for the topological surface electrons in the ultrathin gapped-Dirac-cone limit. The polarization decreases significantly with enhanced tunnelling realized systematically in thin insulating films, whereas magnitude of the polarization saturates to the bulk limit faster at larger wavevectors in thicker metallic films. We present a theoretical model that captures this delicate relationship between quantum tunnelling and Fermi surface spin polarization. Our high-resolution spin-based spectroscopic results suggest that the polarization current can be tuned to zero in thin insulating films forming the basis for a future spin-switch nanodevice.

Fully gapped topological surface states in Bi2Se3 films induced by a d-wave high-temperature superconductor

Topological insulators are a new class of material1,2, that exhibit robust gapless surface states protected by time-reversal symmetry3,4. The interplay of such symmetry-protected topological surface states and symmetry-broken states (for example, superconductivity) provides a platform for exploring new quantum phenomena and functionalities, such as one-dimensional chiral or helical gapless Majorana fermions5, and Majorana zero modes6 that may find application in fault-tolerant quantum computation7,8. Inducing superconductivity on the topological surface states is a prerequisite for their experimental realization1,2. Here, by growing high-quality topological insulator Bi2Se3 films on a d-wave superconductor Bi2Sr2CaCu2O8+δ using molecular beam epitaxy, we are able to induce high-temperature superconductivity on the surface states of Bi2Se3 films with a large pairing gap up to 15 meV. Interestingly, distinct from the d-wave pairing of Bi2Sr2CaCu2O8+δ, the proximity-induced gap on the surface states is nearly isotropic and consistent with predominant s-wave pairing as revealed by angle-resolved photoemission spectroscopy. Our work could provide a critical step towards the realization of the long sought Majorana zero modes.

Achieving Surface Quantum Oscillations in Topological Insulator Thin Films of Bi2Se3

High-quality thin films of topological insulator Bi2Se3 grown on sapphire substrates present a long phase coherence length and allow direct observations of surface quantum oscillations. The key to achieving high mobility of surface electrons is to raise the main deposition temperature to 300−320 °C, which necessitates a two-step deposition procedure with the initial epilayer deposited at 110−130 °C.

High-field linear magneto-resistance in topological insulator Bi2Se3 thin films

Linear magneto-resistance is observed in high magnetic field in topological insulator Bi2Se3 films. As revealed by tilted magnetic field measurement, this linear magneto-resistance is associated with the gapless topological surface states and of quantum origin. In the ultra-thin limit, the inter-surface tunneling induced surface state gap opening quenches the linear magneto-resistance. Instead, weak negative magneto-resistance is observed in high magnetic fields in ultra-thin films.

Growth characteristics of topological insulator Bi2Se3 films on different substrates

Molecular -beam epitaxy of topological insulator Bi2Se3 on different substrates is carried out, including on clean and hydrogen-terminated Si(111), bismuth (Bi) induced Si(111)–(3×3) and –(3×3) surfaces, GaN, and several selenide compounds. Epifilms surface and structural properties are characterized and the effects of surface structure and lattice misfit are indicated. It is found that substrates that are inert to chemical reaction with Bi and/or selenium (Se) and have small lattice misfits with Bi2Se3 appear advantageous to single-crystalline Bi2Se3 film growth. By using vicinal substrates, rotation or twin domains are effectively suppressed.

Topological insulator Bi2Se3 thin films grown on double-layer graphene by molecular beam epitaxy

Atomically flat thin films of topological insulator Bi2Se3 have been grown on double-layer graphene formed on 6H–SiC(0001) substrate by molecular beam epitaxy. By a combined study of reflection high energy electron diffraction and scanning tunneling microscopy, we identified the Se-rich condition and temperature criterion for layer-by-layer growth of epitaxial Bi2Se3 films. The as-grown films without doping exhibit a low defect density of 1.0±0.2×1011/cm2, and become a bulk insulator at a thickness of ten quintuple layers, as revealed by in situ angle resolved photoemission spectroscopy measurement.

Quintuple-layer epitaxy of thin films of topological insulator Bi2Se3

Atomically smooth, single crystalline Bi2Se3 thin films were prepared on Si(111) by molecular beam epitaxy. Scanning tunneling microscopy, low-energy electron diffraction, x-ray photoelectron emission spectroscopy, and Raman spectroscopy were used to characterize the stoichiometry and crystallinity of the film. The films grow in a self-organized quintuple layer by quintuple-layer mode, and atomically smooth films can be obtained, with controllable thickness down to one quintuple layer (∼1 nm).