Topological Insulator Bi Research Articles

Surface potential dependence of the topological insulator Bi[formula omitted]Se[formula omitted] studied by scanning photoemission and Kelvin probe microscopy

The atomic composition and electronic structure of topological insulators (TI) Bi2Se3 grown by the two-temperature Physical Vapor Deposition on n-type Si patterned substrates, have been followed in details during slight variations of the growth temperatures and values of thickness close to that of the topological transition (about 6 nm), by means of submicron lateral probe photoemission with enhanced surface sensitivity (less than 1 nm). Raman and atomic force microscopy together with Kelvin probe microscopy provide a coherent picture where the fine tuning of Se doping and of the growth temperatures constrain the on–set of the gapless state at the exact middle point of the gap.

Study of dielectric polarization and electrical transport in Bi1·2Sb0·8Te0·4Se2.6 nanofilms

Studying the dielectric response of topological insulators (TIs) can unveil their unique physical mechanisms such as charge transport and spin-orbit coupling effects. However, due to the manifestation of material's topological nature and band structure primarily in nanofilm, such thickness poses challenges for dielectric testing. To date, research on TI dielectric aspects remains relatively unexplored. Therefore, this paper successfully synthesizes nanofilm of quaternary topological insulator Bi1·2Sb0·8Te0·4Se2.6 (BSTS) using laser molecular beam epitaxy (LMBE) technique. Utilizing a wide-frequency dielectric spectrometer and a comprehensive physical properties measurement system (PPMS), we measured and thoroughly analyzed the dielectric polarization and charge transport characteristics of BSTS. We observed various polarization responses in the frequency range of 101–103 Hz, with the dipole orientation gradually failing to keep pace with the frequency increase in the range of 103–105 Hz, and the relaxation polarization unable to establish itself in the range of 105–107 Hz, with polarization primarily contributed by displacement polarization. Subsequently, we further analyzed the dependence of BSTS dielectric polarization response on temperature and film thickness, which will help reveal the influence of external factors on TI dielectric response, providing crucial insights for controlling TI materials' dielectric response. This not only deepens our understanding of the fundamental physical properties of this novel material but also offers important scientific basis and technological support for its applications in quantum computing, photonics, spintronics, and other fields.

Spin-Orbit Torques and Magnetization Switching in (Bi,Sb)2Te3/Fe3GeTe2 Heterostructures Grown by Molecular Beam Epitaxy.

Topological insulators (TIs) hold promise for manipulating the magnetization of a ferromagnet (FM) through the spin-orbit torque (SOT) mechanism. However, integrating TIs with conventional FMs often leads to significant device-to-device variations and a broad distribution of SOT magnitudes. In this work, we present a scalable approach to grow a full van der Waals FM/TI heterostructure by molecular beam epitaxy, combining the charge-compensated TI (Bi,Sb)2Te3 with 2D FM Fe3GeTe2 (FGT). Harmonic magnetotransport measurements reveal that the SOT efficiency exhibits a non-monotonic temperature dependence and experiences a substantial enhancement with a reduction of the FGT thickness to 2 monolayers. Our study further demonstrates that the magnetization of ultrathin FGT films can be switched with a current density of Jc ∼ 1010 A/m2, with minimal device-to-device variations compared to previous investigations involving traditional FMs.

Time-Resolved Second-Harmonic Generation in Topological Insulator Bi2Te3: Competing Contributions from Dirac Surface States, Surface Photovoltage, and Band Bending

Time-Resolved Second-Harmonic Generation in Topological Insulator Bi₂Te₃: Competing Contributions from Dirac Surface States, Surface Photovoltage, and Band Bending

Utilizing Topological Insulator Two-Dimensional Bismuth for Ultrasensitive Acoustic Detection.

Topological insulators (TIs) are characterized by a full insulating gap in the bulk and gapless edge or surface states, which have attracted tremendous attention. 2D Bi (110), as a typical TI, is of particular interest due to its low symmetry structure and topologically protected and spin-momentum-locked Dirac surface states. However, the material's potential applications are hindered by difficulties in fabrication, due to its strong semi-metallic bonding and poor stability. In this study, a novel electrochemical intercalation method for the fabrication of ultrathin Bi (110) nanosheets with the highest yield ever reported is presented. These nanosheets are stabilized through cathodic exfoliation in a reductive environment and further modification with polymer ionic liquids. The versatility of these nanosheets is demonstrated by fabricating flexible acoustic sensors with ultrahigh sensitivity. These sensors can even detect sounds as quiet as 45dB. Furthermore, these sensors are utilized for acoustic-to-electric energy conversion and information transfer. This work offers a promising approach for scalable fabrication and preservation of ultrathin 2D TI Bi (110) nanosheets and paves the way for their integration into smart devices.

Exchange-Driven Intermixing of Bulk and Topological Surface States by Chiral Excitons in Bi2Se3

Topological surface states (TSS) in the prototypical topological insulator (TI) Bi_{2}Se_{3} are frequently characterized using optical probes, but electron-hole interactions and their effect on surface localization and optical response of the TSS remain unexplored. Here, we use ab initio calculations to understand excitonic effects in the bulk and surface of Bi_{2}Se_{3}. We identify multiple series of chiral excitons that exhibit both bulk and TSS character, due to exchange-driven mixing. Our results address fundamental questions about the degree to which electron-hole interactions can relax the topological protection of surface states and dipole selection rules for circularly polarized light in TIs by elucidating the complex intermixture of bulk and surface states excited in optical measurements and their coupling to light.

Twist-angle dependent proximity induced spin-orbit coupling in graphene/topological insulator heterostructures

The proximity-induced spin-orbit coupling (SOC) in heterostructures of twisted graphene and topological insulators (TIs) Bi$_2$Se$_3$ and Bi$_2$Te$_3$ is investigated from first principles. To build commensurate supercells, we strain graphene and correct thus resulting band offsets by applying a transverse electric field. We then fit the low-energy electronic spectrum to an effective Hamiltonian that comprises orbital and spin-orbit terms. For twist angles 0$^\circ\leq\Theta \lessapprox 20^\circ$, we find the dominant spin-orbit couplings to be of the valley-Zeeman and Rashba types, both a few meV strong. We also observe a sign change in the induced valley-Zeeman SOC at $\Theta\approx 10^\circ$. Additionally, the in-plane spin structure resulting from the Rashba SOC acquires a non-zero radial component, except at $0^\circ$ or $30^\circ$. At $30^\circ$ the graphene Dirac cone interacts directly with the TI surface state. We therefore explore this twist angle in more detail, studying the effects of gating, TI thicknesses, and lateral shifts on the SOC parameters. We find, in agreement with previous results, the emergence of the proximitized Kane-Mele SOC, with a change in sign possible by electrically tuning the Dirac cone within the TI bulk band gap.

Topological Insulator Bi2 Se3 -Assisted Heterostructure for Ultrafast Charging Sodium-Ion Batteries.

The development of fast charging materials offers a viable solution for large-scale and sustainable energy storage needs. However, it remains a critical challenge to improve the electrical and ionic conductivity for better performance. Topological insulator (TI), a topological quantum material that has attracted worldwide attention, hosts unusual metallic surface states and consequent high carrier mobility. Nevertheless, its potential in promising high-rate charging capability has not been fully realized and explored. Herein, a novel Bi2 Se3 -ZnSe heterostructure as excellent fast charging material for Na+ storage is reported. Ultrathin Bi2 Se3 nanoplates with rich TI metallic surfaces are introduced as an electronic platform inside the material, which greatly reduces the charge transfer resistance and improves the overall electrical conductivity. Meanwhile, the abundant crystalline interfaces between these two selenides promote Na+ migration and provide additional active sites as well. As expected, the composite delivers the excellent high-rate performance of 360.5 mAh g-1 at 20 A g-1 and maintains its electrochemical stability of 318.4 mAh g-1 after 3000 long cycles, which is the record high for all reported selenide-based anodes. This work is anticipated to provide alternative strategies for further exploration of topological insulators and advanced heterostructures.

Field-induced reentrant insulator state of a gap-closed topological insulator ( Bi1−xSbx ) in quantum-limit states

In the extreme quantum limit states under high magnetic fields, enhanced electronic correlation effects can stabilize anomalous quantum states. Using band-tuning with a magnetic field, we realized a spin-polarized quantum limit state in the field-induced semimetallic phase of a topological insulator Bi_{1-x}Sb_x. Further increase in the field injects more electrons and holes to this state and results in an unexpected reentrant insulator state in this topological semimetallic state. A single-particle picture cannot explain this reentrant insulator state, reminiscent of phase transitions due to many-body effects. Estimates of the binding energy and spacing of electron-hole pairs and the thermal de Broglie wavelength indicate that Bi_{1-x}Sb_x may host the excitonic insulator phase in this extreme environment.

Evolution of Dopant-Concentration-Induced Magnetic Exchange Interaction in Topological Insulator Thin Films

To date, the quantum anomalous Hall effect has been realized in chromium (Cr)- and/or vanadium(V)-doped topological insulator (Bi,Sb)2Te3 thin films. In this work, we use molecular beam epitaxy to synthesize both V- and Cr-doped Bi2Te3 thin films with controlled dopant concentration. By performing magneto-transport measurements, we find that both systems show an unusual yet similar ferromagnetic response with respect to magnetic dopant concentration; specifically the Curie temperature does not increase monotonically but shows a local maximum at a critical dopant concentration. We attribute this unusual ferromagnetic response observed in Cr/V-doped Bi2Te3 thin films to the dopant-concentration-induced magnetic exchange interaction, which displays evolution from van Vleck-type ferromagnetism in a nontrivial magnetic topological insulator to Ruderman-Kittel-Kasuya-Yosida (RKKY)-type ferromagnetism in a trivial diluted magnetic semiconductor. Our work provides insights into the ferromagnetic properties of magnetically doped topological insulator thin films and facilitates the pursuit of high-temperature quantum anomalous Hall effect.

Dirac Bands in the Topological Insulator Bi2Se3 Mapped by Time‐Resolved Momentum Microscopy

AbstractThe energy dispersion of the unoccupied Dirac bands of the topological insulator Bi2Se3 has been studied up to large parallel momenta and intermediate state energies using a setup for laser‐based time‐resolved momentum microscopy with 6 eV probe‐photons. A strongly momentum‐dependent evolution of the topologically protected Dirac states into a conduction band resonance is observed, highlighting the anisotropy dictated by the symmetry of the surface. The results are in remarkable agreement with the theoretical surface spectrum obtained from a GW‐corrected tight‐binding model, suggesting the validity of the approach in the prediction of the quasiparticle excitation spectrum of large systems with non‐trivial topology. After photoexcitation with 0.97 eV photons, assigned to a bulk valence band‐conduction band transition, the out‐of‐equilibrium population of the surface state evolves on a multi‐picosecond time scale, in agreement with a simple thermodynamical model with a fixed number of particles, suggesting a significant decoupling between bulk and surface states.

Plasmonic hot carrier injection from single gold nanoparticles into topological insulator Bi2Se3 nanoribbons.

Plasmonic gold nanoparticles injecting hot carriers into the topological insulator (TI) interface of Bi2Se3 nanoribbons are studied by resonant Raman spectroscopy. We resolve the impact of individual gold particles with sizes ranging from 140 nm down to less than 40 nm on the topological surface states of the nanoribbons. In resonance at 1.96 eV (633 nm), we find distinct phonon renormalization in the Eg2- and A1g2-modes that can be associated with plasmonic hot carrier injection. The phonon modes are strongly enhanced by a factor of 350 when tuning the excitation wavelengths into interband transition and in resonance with the surface plasmon of gold nanoparticles. At 633 nm wavelength, a plasmonic enhancement factor of 18 is observed indicating a contribution of hot carriers injected from the gold nanoparticles into the TI interface. Raman studies as a function of gold nanoparticle size reveal the strongest hot carrier injection for particles with size of 108 nm in agreement with the resonance energy of its surface plasmon. Hot carrier injection opens the opportunity to locally control the electronic properties of the TI by metal nanoparticles attached to the surface of nanoribbons.

Pressure-Induced Electronic Topological Transition and Superconductivity in Topological Insulator Bi2Te2.1Se0.9.

One approach to discovering topological superconductors is establishing superconductivity based on well-identified topological insulators. However, the coexistence of superconductivity and a topological state is always arcane. In this paper, we report how pressure tunes the crystal structure, electronic structure, and superconductivity in topological insulator Bi2Te2.1Se0.9. At ∼2.5 GPa, the abnormal changes in c/a and the full width at half-maximum of the A1g1 mode indicate the occurrence of an electronic topological transition. The pressure-induced superconductivity in Bi2Te2.1Se0.9 pinned with an electronic topological transition presents at 2.4 GPa, which is far below the structural phase transition pressure of 8.4 GPa. These results suggest that the appearance of an electronic topological transition is closely correlated with superconductivity in the initial phase, where the topological surface state persists. Our work clarifies the complex electronic structure of Bi2Te2.1Se0.9 and sheds light on the mechanism for superconductivity in topological insulators.

Emergent topological superconductivity in Bi-intercalated van der Waals layered SiTe2

Van der Waals (vdW) layered materials have attracted extensive attention and become a platform for investigating various exotic physical properties of interest. Considering experimentally synthesized vdW layered ${\mathrm{SiTe}}_{2}$ and monolayer Bi, here we propose a feasible way to obtain topological superconductivity by intercalating monolayer Bi into the bulk ${\mathrm{SiTe}}_{2}$. Using Allen-Dynes-modified McMillan equation based on the first principles calculations, we predict the bulk ${\mathrm{SiTe}}_{2}$ to be a superconductor with superconducting transition temperature ${\mathrm{T}}_{C}$ of about 2.9 K, which could be effectively modulated by hydrostatic and uniaxial pressures. Intercalating a two-dimensional topological insulator Bi into the bulk ${\mathrm{SiTe}}_{2}$, we find that the system exhibits both superconductivity with ${\mathrm{T}}_{C}$ of about 1.7 K and topological surface states near the Fermi energy. In addition, the ${\mathrm{T}}_{C}$ of ${\mathrm{SiTe}}_{2}$ monolayer remains almost intact when exfoliated from its bulk. Our work paves a new way to realize topological superconductivity by intercalating topological matter into vdW layered materials.

Fascinating Electrical Transport Behavior of Topological Insulator Bi2 Te3 Nanorods: Toward Electrically Responsive Smart Materials.

Electrical conductivity and dielectric parameters are general inherent features of materials. Controlling these characteristics through applied bias will add a new dimension to regulate the dynamic response of smart materials. Here, a fascinating electrical transport behavior is observed in topological insulator (TI) Bi2 Te3 nanorods, which will play a vital role in intelligent materials or devices as a unit for information reception, processing or feedback. The Bi2 Te3 nanorod aggregates exhibit a monotonic resistance response to voltage, with observed four-fold change of electrical conductivity in a small range electric field of 1Vmm-1 . The dielectric constant and dielectric loss of Bi2 Te3 nanorod composites also show strong dependences on bias voltage due to the unique electrical transport characteristics. The unique voltage-controlled electrical responses are attributed to the change of Fermi levels within the band structure of disordered TInanorods, which are non-parallel to the applied electric field. The excellent controllable inherent characteristics through electric field endows Bi2 Te3 nanomaterials bright prospects for applications in smart devices and resistive random access memories.

Strong Anharmonicity‐Induced Low Thermal Conductivity and High n‐type Mobility in the Topological Insulator Bi1.1Sb0.9Te2S

AbstractIntrinsically low lattice thermal conductivity (κlat) while maintaining the high carrier mobility (μ) is of the utmost importance for thermoelectrics. Topological insulators (TI) can possess high μ due to the metallic surface states. TIs with heavy constituents and layered structure can give rise to high anharmonicity and are expected to show low κlat. Here, we demonstrate that Bi1.1Sb0.9Te2S (BSTS), which is a 3D bulk TI, exhibits ultra‐low κlat of 0.46 Wm−1 K−1 along with high μ of ≈401 cm2 V−1 s−1. Sound velocity measurements and theoretical calculations suggest that chemical bonding hierarchy and high anharmonicity play a crucial role behind such ultra‐low κlat. BSTS possesses low energy optical phonons which strongly couple with the heat carrying acoustic phonons leading to ultra‐low κlat. Further, Cl has been doped at the S site of BSTS which increases the electron concentration and reduces the κlat resulting in a promising n‐type thermoelectric figure of merit (zT) of ≈0.6 at 573 K.

Strong Anharmonicity-Induced Low Thermal Conductivity and High n-type Mobility in the Topological Insulator Bi1.1 Sb0.9 Te2 S.

Intrinsically low lattice thermal conductivity (κlat ) while maintaining the high carrier mobility (μ) is of the utmost importance for thermoelectrics. Topological insulators (TI) can possess high μ due to the metallic surface states. TIs with heavy constituents and layered structure can give rise to high anharmonicity and are expected to show low κlat . Here, we demonstrate that Bi1.1 Sb0.9 Te2 S (BSTS), which is a 3D bulk TI, exhibits ultra-low κlat of 0.46 Wm-1 K-1 along with high μ of ≈401 cm2 V-1 s-1 . Sound velocity measurements and theoretical calculations suggest that chemical bonding hierarchy and high anharmonicity play a crucial role behind such ultra-low κlat . BSTS possesses low energy optical phonons which strongly couple with the heat carrying acoustic phonons leading to ultra-low κlat . Further, Cl has been doped at the S site of BSTS which increases the electron concentration and reduces the κlat resulting in a promising n-type thermoelectric figure of merit (zT) of ≈0.6 at 573 K.

Sensitivity of superconducting states to the impurity location in layered materials

The family of multi-layered superconductors derived from the doped topological insulator Bi$_2$Se$_3$ has been found to be unusually robust against non-magnetic disorder. Recent experimental studies have highlighted the fact that the location of impurities could play a critical role for this puzzling robustness. Here we investigate the effects of four different types of impurities, on-site, interstitial, intercalated and polar, on the superconducting critical temperature. We find that different components of the scattering potential are active depending on the impurity configuration and choice of orbitals for the effective low-energy description of the normal state. For the specific case of Bi$_2$Se$_3$-based superconductors, we find that only the symmetric share of impurity configurations contribute to scattering, such that polar impurities are completely inactive. We also find that a more dominant mass-imbalance term in the normal-state Hamiltonian can make the superconducting state more robust to intercalated impurities, in contrast to the case of on-site or interstitial impurities.

Superconducting properties of Nb, NbN, and MoN thin films grown on topological insulator Bi1.95Sb0.05Se3: a comparative study

The superconducting properties of DC magnetron sputtered Nb, NbN, and MoN films deposited on Sb-doped bismuth selenide (BSS) topological insulator films have been studied. In this two-fold study, firstly, thick superconducting films of ∼100 nm have been studied to find the modifications in the superconducting properties of the films caused by the surface morphology of pulsed laser deposition-grown BSS substrates. A drastic suppression in the critical current density has been observed in the bilayers, which has been ascribed to the strong influence of the substrate on the morphology of the superconducting film and the consequent enhancement in the density of superconducting weak links. The temperature dependence of the critical magnetic field has been carefully analyzed within the purview of the Werthamer–Hohenberg–Helfand theory, taking into account the effects of spin paramagnetism and spin–orbit scattering. In the second part of the study, ultra-thin bilayers of NbN-BSS, with thickness of the order of the coherence length (∼5 nm), have been grown to study the superconducting properties within the proximity regime. A ∼3.5 K suppression in T c is seen in the NbN(5 nm)-BSS bilayer, due to a combination of interface roughness and superconducting proximity, while a suppression of 0.6 K has been observed in the BSS(6 nm)-NbN(5 nm) bilayer, which is surmised to be solely due to the superconducting proximity effect.

Electron–phonon coupling in copper intercalated Bi_{2}Se_{3}

We report charge and heat transport studies in copper-intercalated topological insulator Bi_2Se_3 hybrid devices. Measured conductivity shows impact of quantum corrections, electron–electron and electron–phonon interactions. Our shot noise measurements reveal that heat flux displays a crossover between T^2 and T^4 with the increase of temperature. The results might be explained by a model of inelastic electron scattering on disorder, increasing the role of transverse acoustic phonons in the electron–phonon coupling process.