Ultrathin Topological Insulator Research Articles

Preservation of Topological Surface States in Millimeter-Scale Transferred Membranes.

Ultrathin topological insulator membranes are building blocks of exotic quantum matter. However, traditional epitaxy of these materials does not facilitate stacking in arbitrary orders, while mechanical exfoliation from bulk crystals is also challenging due to the non-negligible interlayer coupling therein. Here we liberate millimeter-scale films of the topological insulator Bi2Se3, grown by molecular beam epitaxy, down to 3 quintuple layers. We characterize the preservation of the topological surface states and quantum well states in transferred Bi2Se3 films using angle-resolved photoemission spectroscopy. Leveraging the photon-energy-dependent surface sensitivity, the photoemission spectra taken with 6 and 21.2 eV photons reveal a transfer-induced migration of the topological surface states from the top to the inner layers. By establishing clear electronic structures of the transferred films and unveiling the wave function relocation of the topological surface states, our work lays the physics foundation crucial for the future fabrication of artificially stacked topological materials with single-layer precision.

Charge qubits based on ultra-thin topological insulator films

Charge qubits based on ultra-thin topological insulator films

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.

Surface coupling in Bi2Se3 ultrathin films by screened Coulomb interaction

Single-particle band theory has been very successful in describing the band structure of topological insulators. However, with decreasing thickness of topological insulator thin films, single-particle band theory is insufficient to explain their band structures and transport properties due to the existence of top and bottom surface-state coupling. Here, we reconstruct this coupling with an equivalently screened Coulomb interaction in Bi2Se3 ultrathin films. The thickness-dependent position of the Dirac point and the magnitude of the mass gap are discussed in terms of the Hartree approximation and the self-consistent gap equation. We find that for thicknesses below 6 quintuple layers, the magnitude of the mass gap is in good agreement with the experimental results. Our work provides a more accurate means of describing and predicting the behaviour of quasi-particles in ultrathin topological insulator films and stacked topological systems.

Robustness of negative low-field magnetoconductance in ultrathin topological insulator films

In the ultrathin films of three-dimensional topological insulators (TIs), modification of the Berry phase caused by surface hybridization was predicted to allow for a crossover between weak antilocalization (WAL) and weak localization (WL). Here we report on a systematic study of the magnetotransport properties of ${({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{2}{\mathrm{Te}}_{3}$ (BST) thin films with well-controlled thickness, doping level, and chemical potential. We found that the sign of magnetoconductance in perpendicular magnetic field remains negative in all circumstances, suggesting absence of global WL in the ultrathin BST films. This is attributed to long-range disorder, as well as conductance corrections by quantum interference and electron-electron interaction. Our work shows that the electron transport in ultrathin TIs is distinctively different from the WAL regime in the systems with symplectic symmetry, and the WL or weak insulator regime associated with the orthogonal symmetry class.

Synthesis of Ultrathin Topological Insulator β-Ag2 Te and Ag2 Te/WSe2 -Based High-Performance Photodetector.

β-Ag2 Te has attracted considerable attention in the application of electronics and optoelectronics due to its narrow bandgap, high mobility, and topological insulator properties. However, it remains a significant challenge to synthesize 2D Ag2 Te because of the non-layered structure of Ag2 Te. Herein, the synthesis of large-size, ultrathin single crystal topological insulator 2D Ag2 Te via the van der Waals epitaxial method for the first time is reported. The 2D Ag2 Te crystal exhibits p-type conduction behavior with high carrier mobility of 3336 cm2 V-1 s-1 at room temperature. Taking advantage of the high mobility and perfect electron structure of Ag2 Te, the Ag2 Te/WSe2 heterojunctions are fabricated via mechanical stacking and show an ultrahigh rectification ratio of 2×105 . Ag2 Te/WSe2 photodetector also exhibits self-driven properties with a fast response speed (40 µs/60 µs) in the near-infrared region. High responsivity (219mA W-1 ) and light ON/OFF ratio of 6×105 are obtained under the photovoltaic mode. The overall performance of the Ag2 Te/WSe2 photodetector is significantly competitive among all reported 2D photodetectors. These results indicate that 2D Ag2 Te is a promising candidate for future electronic and optoelectronic applications.

Optical conductivity of ultrathin Floquet topological insulators

In this paper, we theoretically investigate the topological aspects of a 3D ultrathin topological insulator in the presence of an irradiated off-resonant circularly-polarized light field. Based on the Floquet formalism we treat the light field as an external perturbation which introduces an effective energy term in the system Hamiltonian and hence, gives mass to the massless Dirac fermions at the surfaces of the Floquet topological insulators (FTIs). By controlling the strength of the off-resonant light field we manipulate the interplay between the hybridization energies of the top and bottom surface states (SSs), and the effective energies induced by the irradiated light field to open and close band gaps of the SSs Dirac electrons. This gives rise to topologically nontrivial and trivial phases. We calculate the longitudinal and transverse conductivities of the FTI thin film and show that these strongly depend on the strength of the irradiated light field. The behavior of conductivities signatures TQPTs between distinct topological insulating phases.

Optical and magnetic response of ultrathin film topological insulators in tilted magnetic field

Faraday’s rotation and magnetic susceptibility of an ultrathin film of topological insulator, placed in tilted magnetic field, are studied. A Hall effect analogue is studied in terms of Faraday’s rotation. It is an isotropic response and tunable with magnetic field. Susceptibility has three contributions; spin, orbital and spin–orbital. All three contributions are calculated for magnetic field tilted at an angle θ with respect to the perpendicular axis on the plane. Spin susceptibility for low temperature varies directly with Fermi energy, orbital part show diamagnetic divergence and spin orbital contribution arises as a step function at Fermi energy.

Visualizing the in-Gap States in Domain Boundaries of Ultra-Thin Topological Insulator FilmsSupported by the National Natural Science Foundation of China (Grant Nos. 61804056 and 92065102), and the Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University.

Ultra-thin topological insulators provide a platform for realizing many exotic phenomena such as the quantum spin Hall effect, and quantum anomalous Hall effect. These effects or states are characterized by quantized transport behavior of edge states. Experimentally, although these states have been realized in various systems, the temperature for the edge states to be the dominating channel in transport is extremely low, contrary to the fact that the bulk gap is usually in the order of a few tens of milli-electron volts. There must be other in-gap conduction channels that do not freeze out until a much lower temperature. Here we grow ultra-thin topological insulator Bi2Te3 and Sb2Te3 films by molecular beam epitaxy and investigate the structures of domain boundaries in these films. By scanning tunneling microscopy and spectroscopy we find that the domain boundaries with large rotation angles have pronounced in-gap bound states, through which one-dimensional conduction channels are suggested to form, as visualized by spatially resolved spectroscopy. Our work indicates the critical role played by domain boundaries in degrading the transport properties.

Enhanced Spin-to-Charge Conversion Efficiency in Ultrathin Bi2Se3 Observed by Spintronic Terahertz Spectroscopy.

Owing to their remarkable spin-charge conversion (SCC) efficiency, topological insulators (TIs) are the most attractive candidates for spin-orbit torque generators. The simple method of enhancing SCC efficiency is to reduce the thickness of TI films to minimize the trivial bulk contribution. However, when the thickness reaches the ultrathin regime, the SCC efficiency decreases owing to intersurface hybridization. To overcome these contrary effects, we induced dehybridization of the ultrathin TI film by breaking the inversion symmetry between surfaces. For the TI film grown on an oxygen-deficient transition-metal oxide, the unbonded transition-metal d-orbitals affected only the bottom surface, resulting in asymmetric surface band structures. Spintronic terahertz emission spectroscopy, an emerging tool for investigating the SCC characteristics, revealed that the resulting SCC efficiency in symmetry-broken ultrathin Bi2Se3 was enhanced by up to ∼2.4 times.

Highly tunable magnetic coupling in ultrathin topological insulator films due to impurity resonances

We theoretically investigate the exchange interaction between magnetic impurities in ultrathin Bi$_2$Se$_3$ topological insulator films by taking into account the low-energy states produced by the impurities. We find that the locally induced impurity resonances strongly influence the exchange interaction between magnetic moments. In particular, we find a non-collinear alignment being more favorable than the collinear ferromagnetic alignment preferred when impurity states are ignored and only the pristine topological insulator band structure is considered. Moreover, we show that by applying of an electric field perpendicular to the ultrathin film, the exchange interaction can be drastically enhanced. This opens for the possibility of highly tunable magnetism by electric field.

Spin polarization in nanojunctions of quantum anomalous Hall insulators

We theoretically investigate spin polarized conductance through nanojunctions made by magnetic domains in a thin films of three dimensional ferromagnetic topological insulator (TI) as a quantum anomalous Hall insulator. Writing of magnetic domains is now feasible in recent experiments. Ferromagnetism induced in such materials can control topological phase of the band structure and as consequence, configuration of the states in each band. By means of band analysis, it is demonstrated that in topologically non-trivial bands with non-zero Chern number, there exists a change in spin dominant states among Brillouin zone while spin polarization of trivial bands shows dominant of one spin type. Indeed, there is an interplay between topological invariant of the band spectrum and contribution of the spin states in conductance. We show that in-plane magnetization or structure inversion asymmetry result in a k-dependent parity configuration of surface states in ultrathin film topological insulator. In the conduction and valence bands, a nearly perfect spin polarized conductance is found in a nanojunction composed of anti-parallel magnetic domains which is mediated by a layer under application of in-plane magnetization or perpendicular applied bias. This spin polarized region is extendable with the magnetization. To the sake of completeness, proposed polarization in conductance was also supported by interpretation based on spin and parity local density of states along the interfaces of magnetic domain walls. Analysis of the spin-parity states sheds light to the quantum transport through magnetically doped TI thin-film’s nanojunctions with different mass terms.

Dynamical density response and collective modes of topological-insulator ultrathin films

We analytically calculate the intra- and inter-surface dynamical density-density linear responses of ultra-thin topological insulator films with finite tunneling between their top and bottom surfaces in both metallic and insulating regimes. Employing the random phase approximation we investigate the dispersions of in-phase and out-of-phase collective density modes of this system in the metallic regime. We find that in contrast to the bilayers of the conventional two-dimensional electron gas, where finite tunneling gaps out the out-of-phase mode, in topological insulator thin films, this mode remains linear at long wavelengths. Depending on different system parameters, the velocity of out-of-phase mode can be tuned to be larger or substantially smaller than the Fermi velocity of electrons on the isolated surfaces of the topological insulator. Finite tunneling generally reduces the energy of collective modes, making them more confined in space.

A generic method to control hysteresis and memory effect in Van der Waals hybrids

The diverse properties of two-dimensional materials have been utilized in a variety of architecture to fabricate high quality electronic circuit elements. Here we demonstrate a generic method to control hysteresis and stable memory effect in Van der Waals hybrids with a floating gate as the base layer. The floating gate can be charged with a global back gate-voltage, which it can retain in a stable manner. Such devices can provide a very high, leakage-free effective gate-voltage on the field-effect transistors due to effective capacitance amplification, which also leads to reduced input power requirements on electronic devices. The capacitance amplification factor of ∼10 can be further enhanced by increasing the area of the floating gate. We have exploited this method to achieve highly durable memory action multiple genre of ultra-thin 2D channels, including graphene, MoS2, and topological insulators at room temperature.

Revealing surface-state transport in ultrathin topological crystalline insulator SnTe films

SnTe is a topological crystalline insulator that exhibits crystal symmetry protected topological surface states (SS), which are useful for the development of novel devices, such as low-dissipation transistors. However, major obstacles remain to probe the SS and realize the application of SnTe and other topological insulators. Due to unintentional doping by Sn vacancies, bulk conduction may overwhelm the transport through SS in SnTe. Synthesis of SnTe films thin enough to suppress bulk conduction has proven difficult due to the formation of discontinuous domain structures. By introducing a novel deposition method that builds upon molecular beam epitaxy, we achieve ultrathin continuous films of single-orientation SnTe (001) on SrTiO3 (STO) (001) substrates. We separate the carrier concentrations in the bulk and in the SS and discover that conduction through the SS dominates (a majority of hole carriers occupy the SS) in films thinner than 40 unit cells, with a large temperature independent hole density of SS nS = 5 × 1014 cm−2. Unlike the depletion of SS carriers observed at the vacuum/SnTe interface that inhibits topological behavior, we show that SS carriers are buried and protected from depletion at the SnTe/STO interface, which is enabled by the relatively large bandgap of STO and its favorable band alignment with SnTe. This work provides an important pathway for probing and realizing SS transport in SnTe and other TIs even when bulk conduction coexists.

Closing the Surface Bandgap in Thin Bi2Se3/Graphene Heterostructures.

Topological insulator (TI), a band insulator with topologically protected edge states, is one of the most interesting materials in the field of condensed matter. Bismuth selenide (Bi2Se3) is the most spotlighted three-dimensional TI material; it has a Dirac cone at each top and bottom surface and a relatively wide bandgap. For application, suppression of the bulk effect is crucial, but in ultrathin TI materials, with thicknesses less than 3 QL, the finite size effect works on the linear dispersion of the surface states, so that the surface band has a finite bandgap because of the hybridization between the top and bottom surface states and Rashba splitting, resulting from the structure inversion asymmetry. Here, we studied the gapless top surface Dirac state of strained 3 QL Bi2Se3/graphene heterostructures. A strain caused by the graphene layer reduces the bandgap of surface states, and the band bending resulting from the charge transfer at the Bi2Se3-graphene interface induces localization of surface states to each top and bottom layer to suppress the overlap of the two surface states. In addition, we verified the independent transport channel of the top surface Dirac state in Bi2Se3/graphene heterostructures by measuring the magneto-conductance. Our findings suggest that the strain and the proximity effect in TI/non-TI heterostructures may be feasible ways to engineer the topological surface states beyond the physical and topological thickness limit.

Signatures of long-range-correlated disorder in the magnetotransport of ultrathin topological insulators

In an ultrathin topological insulator (TI) film, a hybridization gap opens in the TI surface states, and the system is expected to become either a trivial insulator or a quantum spin Hall insulator when the chemical potential is within the hybridization gap. Here we show, however, that these insulating states are destroyed by the presence of a large and long-range-correlated disorder potential, which converts the expected insulator into a metal. We perform transport measurements in ultrathin, dual-gated topological insulator films as a function of temperature, gate voltage, and magnetic field, and we observe a metallic-like, non-quantized conductivity, which exhibits a weak antilocalization-like cusp at the low magnetic field and gives way to a nonsaturating linear magnetoresistance at large field. We explain these results by considering the disordered network of electron- and hole-type puddles induced by charged impurities. We argue theoretically that such disorder can produce an insulator-to-metal transition as a function of increasing disorder strength, and we derive a condition on the band gap and the impurity concentration necessary to observe the insulating state. We also explain the linear magnetoresistance in terms of strong spatial fluctuations of the local conductivity, using both numerical simulations and a theoretical scaling argument.

Angular-momentum nanometrology in an ultrathin plasmonic topological insulator film

Complementary metal–oxide–semiconductor (CMOS) technology has provided a highly sensitive detection platform for high-resolution optical imaging, sensing and metrology. Although the detection of optical beams carrying angular momentum have been explored with nanophotonic methods, the metrology of optical angular momentum has been limited to bulk optics. We demonstrate angular-momentum nanometrology through the spatial displacement engineering of plasmonic angular momentum modes in a CMOS-compatible plasmonic topological insulator material. The generation and propagation of surface plasmon polaritons on the surface of an ultrathin topological insulator Sb2Te3 film with a thickness of 100 nm is confirmed, exhibiting plasmonic figures of merit superior to noble metal plasmonics in the ultraviolet-visible frequency range. Angular-momentum nanometrology with a low crosstalk of less than −20 dB is achieved. This compact high-precision angular-momentum nanometrology opens an unprecedented opportunity for on-chip manipulation of optical angular momentum for high-capacity information processing, ultrasensitive molecular sensing, and ultracompact multi-functional optoelectronic devices.

Enhancement of crossed Andreev reflection in a normal-superconductor-normal junction made of thin topological insulator

We theoretically investigate the subgapped transport phenomena through a normal-superconductor-normal (NSN) junction made up of ultra thin topological insulator with proximity induced superconductivity. The dimensional crossover from three dimensional ($3$D) topological insulator (TI) to thin two-dimensional ($2$D) TI introduces a new degree of freedom, the so-called hybridization or coupling between the two surface states. We explore the role of hybridization in transport properties of the NSN junction, especially how it affects the crossed Andreev reflection (CAR). We observe that a rib-like pattern appears in CAR probability profile while examined as a function of angle of incidence and length of the superconductor. Depending on the incoming and reflection or transmission channel, CAR probability can be maneuvered to be higher than $97\%$ under suitable coupling between the two TI surface states along with appropriate gate voltage and doping concentration in the normal region. Coupling between the two surfaces also induces an additional oscillation envelope in the behavior of the angle averaged conductance, with the variation of the length of the superconductor. The behavior of co-tunneling (CT) probability is also very sensitive to the coupling and other parameters. Finally, we also explore the shot noise cross correlation and show that the behavior of the same can be monotonic or non-monotonic depending on the doping concentration in the normal region. Under suitable circumstances, shot noise cross correlation can change sign from positive to negative or vice versa depending on the relative strength of CT and CAR.

Magneto-optical properties of topological insulator thin films with broken inversion symmetry

We determine the optical response of ultrathin film topological insulators in the presence of a quantizing external magnetic field taking into account both hybridization between surface states, broken inversion symmetry and explicit time reversal symmetry breaking by the magnetic field. We find that breaking of inversion symmetry in the system, which can be due to interaction with a substrate or electrical gating, results in Landau level crossings and opening of additional optical transition channels that were previously forbidden. We show that by tuning the hybridization and symmetry breaking parameters, a transition from the normal to a topological insulator phase occurs with measureable signatures in both static (dc) and dynamic (optical) conductivity. Moreover, we find that these signatures in the optical Hall conductivity remain robust against a significant range of disorder strength.