Topological Insulator Nanowires Research Articles

Selective-Area Epitaxy of Bulk-Insulating (BixSb1-x)2Te3 Films and Nanowires by Molecular Beam Epitaxy.

Selective-area epitaxy (SAE) is a useful technique to grow epitaxial films with a desired shape on a prepatterned substrate. Although SAE of patterned topological-insulator (TI) thin films has been performed in the past, there has been no report of SAE-grown TI structures that are bulk-insulating. Here we report the successful growth of Hall-bars and nanowires of bulk-insulating TIs using the SAE technique. Their transport properties show that the quality of the selectively grown structures is comparable to that of bulk-insulating TI films grown on pristine substrates. In SAE-grown TI nanowires, we were able to observe Aharonov-Bohm-like magnetoresistance oscillations that are characteristic of the quantum-confined topological surface states. The availability of bulk-insulating TI nanostructures via the SAE technique opens the possibility to fabricate intricate topological devices in a scalable manner.

Effect of bending deformation on suspended topological insulator nanowires: Towards a topological insulator based NEM switch

Nanodevices consisting of the suspended and supported parts of topological insulator Bi2Se3 nanowires were fabricated and measured at low and room temperatures. Probing of topological surface states, accompanied by the electrostatic field effect used to dynamically manipulate bending deformation, was carried out to monitor the external strain introduced into the suspended and supported parts within the same Bi2Se3 nanowire. Depending on the device geometry, pure elastic and elastoplastic types of concave deformation, as well as convex buckling deformation, were realized in the suspended parts of the nanowires. For various types of observed deformations, different magnitudes of increase in the Source-Drain resistance of the deformed part compared to the relaxed part of the same devices were determined. All suspended devices exhibit external strain-sensitive Shubnikov-de Haas oscillation frequencies representing the carriers of top and bottom surface states and bulk, whereas, in the case of supported devices, the bottom surface states are masked by a trivial 2DEG. The obtained results may be useful for strain engineering of TI materials, as well as for applications in NEMS and other areas related to suspended nanostructures.

Electrostatic environment and Majorana bound states in full-shell topological insulator nanowires

Electrostatic environment and Majorana bound states in full-shell topological insulator nanowires

Preparation of Bi2Se3 topological insulator nanowires via topochemical transformation and their potential in anisotropic optical and optoelectronic applications

Herein, a facile, catalyst-free, and dry topochemical transformation strategy for transforming the Bi2S3 nanowires to the Bi2Se3 nanowires, which has been realized through an atmospheric pressure high-temperature selenization treatment, has been developed. The complete topochemical transformation has been verified by combining x-ray diffraction, Raman spectroscopy, energy dispersive spectrometer, x-ray photoelectron spectroscopy, transmission electron microscopy, and selected area electron diffraction measurements. Systematical optical characterizations, including polarization-resolved optical microscopy images and polarization-dependent Raman spectra, have revealed the strong anisotropy of the Bi2Se3 nanowires. Furthermore, finite-different time-domain simulations have consolidated that the Bi2Se3 nanowires possess highly anisotropic absorption cross sections across the ultraviolet to far infrared spectral range, laying a solid foundation for the realization of ultra-broadband polarized optoelectronic applications. On the whole, this pioneering study depicts a unique avenue for topological material design accompanied with the integration of additional functionalities beyond the intrinsic counterparts, opening up an attractive research field for polarized photonics and optoelectronics.

Electromagnetic response of the surface states of a topological insulator nanowire embedded within a resonator

Exploring the interplay between topological phases and photons opens new avenues for investigating novel quantum states. Here we show that superconducting resonators can serve as sensitive probes for properties of topological insulator nanowires (TINWs) embedded within them. By combining a static, controllable magnetic flux threading the TINW with an additional oscillating electromagnetic field applied perpendicularly, we show that orbital resonances can be generated and are reflected in periodic changes of the Q-factor of the resonator as a function of the flux. This response probes the confinement of the two-dimensional Dirac orbitals on the surface of the TINW, revealing their density of states and specific transition rules, as well as their dependence on the applied flux. Our approach represents a promising cross-disciplinary strategy for probing topological solid state materials using state-of-the-art photonic cavities, which would avoid the need for attaching contacts, thereby enabling access to electronic properties closer to the pristine topological states.

Realization of superconducting transmon qubits based on topological insulator nanowires

Topological-material-based Josephson junctions have the potential to be used to host Majorana zero modes and to construct topological qubits. For operating the topological qubits at an appropriate timescale to avoid decoherence and quasiparticle poisoning, one would eventually go to the time domain and embed the topological qubits into quantum electrodynamic circuits. Here, we constructed a topological-insulator-nanowire-based transmon qubit and demonstrated its strong coupling to a coplanar waveguide resonator. The flux-tunable spectrum and Rabi oscillations with a qubit lifetime T 1 of ∼ 0.5 μ s were observed. Such a hybrid platform, containing topological materials and quantum electrodynamic circuits, can further be used to study the physical properties such as Majorana zero modes in topological quantum circuits.

Top-Down Fabrication of Bulk-Insulating Topological Insulator Nanowires for Quantum Devices

In a nanowire (NW) of a three-dimensional topological insulator (TI), the quantum confinement of topological surface states leads to a peculiar sub-band structure that is useful for generating Majorana bound states. Top-down fabrication of TINWs from a high-quality thin film would be a scalable technology with great design flexibility, but there has been no report on top-down-fabricated TINWs where the chemical potential can be tuned to the charge neutrality point (CNP). Here we present a top-down fabrication process for bulk-insulating TINWs etched from high-quality (Bi1-xSbx)2Te3 thin films without degradation. We show that the chemical potential can be gate-tuned to the CNP, and the resistance of the NW presents characteristic oscillations as functions of the gate voltage and the parallel magnetic field, manifesting the TI-sub-band physics. We further demonstrate the superconducting proximity effect in these TINWs, preparing the groundwork for future devices to investigate Majorana bound states.

Confined vs. extended Dirac surface states in topological crystalline insulator nanowires

Confining two dimensional Dirac fermions on the surface of topological insulators has remained an outstanding conceptual challenge. Here we show that Dirac fermion confinement is achievable in topological crystalline insulators (TCI), which host multiple surface Dirac cones depending on the surface termination and the symmetries it preserves. This confinement is most dramatically reflected in the flux dependence of these Dirac states in the nanowire geometry, where different facets connect to form a closed surface. Using SnTe as a case study, we show how wires with all four facets of the \langle 100 \rangle‹100› type display novel Aharonov-Bohm oscillations, while nanowires with the four facets of the \langle 110 \rangle‹110› type such oscillations are absent due to strong confinement of the Dirac states to each facet separately. Our results place TCI nanowires as a versatile platform for confining and manipulating Dirac surface states.

Anomalous critical supercurrent and half-integer Shapiro steps based on Josephson junction of topological insulator nanowires

Topological insulator-based Josephson junction, as a candidate device for searching for Majorana zero energy modes, has attracted much attention. One of the key issues along this direction is to fabricate Josephson junctions with high-quality interfaces, hoping to searching for 4π-period current-phase relation in topologically non-trivial Josephson junction. In this work, the Josephson junctions based on three-dimensional topological insulator nanowires Bi<sub>2</sub>Te<sub>3</sub> and Bi<sub>2</sub>(Se<sub><i>x</i></sub>Te<sub>1–<i>x</i></sub>)<sub>3</sub> are fabricated to study their superconducting proximity effects, multiple Andreev reflections, and current-phase relations. A number of interesting phenomena are observed, including the anomalous enhancement in junctions’ critical supercurrent with magnetic field, and the appearance of half-integer Shapiro steps in the ac Josephson effect. And, we discuss the possible origins of the observed anomalous behaviors in general, and their relation with the ferromagnetic layer of TiTe alloy formed at the interface between the topological insulator nanowires and the Ti buffer layer of the metallic electrodes, in particular. We provide the experimental evidence for the formation of a ferromagnetic TiTe alloy layer at the interface of our device. And, we believe that the formation of such a layer in our Josephson device breaks the time reversal symmetry, leading to the observed anomalous enhancement of the critical supercurrent with magnetic field, as well as the appearance of half-integer Shapiro steps. Our results suggest that to study the topologically non-trivial behaviors such as 4π-period current-phase relation, one still needs to improve the interface quality of the superconductor-normal metal-superconductor type of Josephson junction devices.

Quantum interference probed by the thermovoltage in Sb-doped Bi2Se3 nanowires

The magnetic-flux-dependent dispersions of sub-bands in topologically protected surface states of a topological insulator nanowire manifest as Aharonov-Bohm oscillations (ABOs) observed in conductance measurements, reflecting the Berry's phase of π because of the spin-helical surface states. Here, we used thermoelectric measurements to probe a variation in the density of states at the Fermi level of the surface state of a topological insulator nanowire (Sb-doped Bi2Se3) under external magnetic fields and an applied gate voltage. The ABOs observed in the magnetothermovoltage showed 180° out-of-phase oscillations depending on the gate voltage values, which can be used to tune the Fermi wave number and the density of states at the Fermi level. The temperature dependence of the ABO amplitudes showed that the phase coherence was kept to T= 15K. We suggest that thermoelectric measurements could be applied for investigating the electronic structure at the Fermi level in various quantum materials.

Superconducting diode effect due to magnetochiral anisotropy in topological insulators and Rashba nanowires

The critical current of a superconductor can depend on the direction of current flow due to magnetochiral anisotropy when both inversion and time-reversal symmetry are broken, an effect known as the superconducting (SC) diode effect. Here, we consider one-dimensional (1D) systems in which superconductivity is induced via the proximity effect. In both topological insulator and Rashba nanowires, the SC diode effect due to a magnetic field applied along the spin-polarization axis and perpendicular to the nanowire provides a measure of inversion symmetry breaking in the presence of a superconductor. Furthermore, a strong dependence of the SC diode effect on an additional component of magnetic field applied parallel to the nanowire as well as on the position of the chemical potential can be used to detect that a device is in the region of parameter space where the phase transition to topological superconductivity is expected to arise.

Electronic confinement of surface states in a topological insulator nanowire

We analyze the confinement of electronic surface states in a model of a topological insulator nanowire. Spin-momentum locking in the surface states reduces unwanted backscattering in the presence of nonmagnetic disorder and is known to counteract localization for certain values of magnetic flux threading the wire. We show that intentional backscattering can be induced for a range of conditions in the presence of a nanowire constriction. We propose a geometry for a nanowire that involves two constrictions and show that these regions form effective barriers that allow for the formation of a quantum dot. We analyze the zero-temperature noninteracting electronic transport through the device using the Landauer-B\"uttiker approach and show how externally applied magnetic flux parallel to the nanowire and electrostatic gates can be used to control the spectrum of the quantum dot and the electronic transport through the surface states of the model device.

Giant magnetochiral anisotropy from quantum-confined surface states of topological insulator nanowires

Wireless technology relies on the conversion of alternating electromagnetic fields into direct currents, a process known as rectification. Although rectifiers are normally based on semiconductor diodes, quantum mechanical non-reciprocal transport effects that enable a highly controllable rectification were recently discovered1–9. One such effect is magnetochiral anisotropy (MCA)6–9, in which the resistance of a material or a device depends on both the direction of the current flow and an applied magnetic field. However, the size of rectification possible due to MCA is usually extremely small because MCA relies on inversion symmetry breaking that leads to the manifestation of spin–orbit coupling, which is a relativistic effect6–8. In typical materials, the rectification coefficient γ due to MCA is usually ∣γ∣ ≲ 1 A−1 T−1 (refs. 8–12) and the maximum values reported so far are ∣γ∣ ≈ 100 A−1 T−1 in carbon nanotubes13 and ZrTe5 (ref. 14). Here, to overcome this limitation, we artificially break the inversion symmetry via an applied gate voltage in thin topological insulator (TI) nanowire heterostructures and theoretically predict that such a symmetry breaking can lead to a giant MCA effect. Our prediction is confirmed via experiments on thin bulk-insulating (Bi1−xSbx)2Te3 (BST) TI nanowires, in which we observe an MCA consistent with theory and ∣γ∣ ≈ 100,000 A−1 T−1, a very large MCA rectification coefficient in a normal conductor.

Metallization and proximity superconductivity in topological insulator nanowires

A heterostructure consisting of a topological insulator (TI) nanowire brought into proximity with a superconducting layer provides a promising route to achieve topological superconductivity and associated Majorana bound states (MBSs). Here, we study effects caused by such a coupling between a thin layer of an $s$-wave superconductor and a TI nanowire. We show that there is a distinct phenomenology arising from the metallization of states in the TI nanowire by the superconductor. In the strong coupling limit, required to induce a large superconducting pairing potential, we find that metallization results in a shift of the TI nanowire subbands ($\sim 20$ meV) as well as it leads to a small reduction in the size of the subband gap opened by a magnetic field applied parallel to the nanowire axis. Surprisingly, we find that metallization effects in TI nanowires can also be beneficial. Most notably, coupling to the superconductor induces a potential in the portion of the TI nanowire close to the interface with the superconductor, this breaks inversion symmetry and at finite momentum lifts the spin degeneracy of states within a subband. As such coupling to a superconductor can create or enhance the subband splitting that is key to achieving topological superconductivity. This is in stark contrast to semiconductors, where it has been shown that metallization effects always reduce the equivalent subband-splitting caused by spin-orbit coupling. We also find that in certain geometries metallization effects can reduce the critical magnetic required to enter the topological phase. We conclude that, unlike in semiconductors, the metallization effects that occur in TI nanowires can be relatively easily mitigated, for instance by modifying the geometry of the attached superconductor or by compensation of the TI material.

4π -periodic supercurrent tuned by an axial magnetic flux in topological insulator nanowires

Topological insulator (TI) nanowires in proximity to conventional superconductors have been proposed as a tunable platform to realize topological superconductivity and Majorana zero modes. The tuning is done using an axial magnetic flux $\ensuremath{\phi}$ which allows transforming the system from trivial at $\ensuremath{\phi}=0$ to topologically nontrivial when half a magnetic flux quantum ${\ensuremath{\phi}}_{0}/2$ threads the cross-section of the wire. Here, we explore the expected topological transition in TI-wire-based Josephson junctions as a function of magnetic flux by probing the $4\ensuremath{\pi}$-periodic fraction of the supercurrent, which is considered an indicator of topological superconductivity. Our data suggest that this $4\ensuremath{\pi}$-periodic supercurrent is at lower magnetic field largely of trivial origin but that, at magnetic fields above $\ensuremath{\sim}{\ensuremath{\phi}}_{0}/4$, topological $4\ensuremath{\pi}$-periodic supercurrents take over.

Disorder effects in topological insulator nanowires

Three-dimensional topological insulator (TI) nanowires with quantized surface subband spectra are studied as a main component of Majorana bound states (MBS) devices. However, such wires are known to have large concentration $N\ensuremath{\sim}{10}^{19}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ of Coulomb impurities. It is believed that a MBS device can function only if the amplitude of long-range fluctuations of the random Coulomb potential $\mathrm{\ensuremath{\Gamma}}$ is smaller than the subband gap $\mathrm{\ensuremath{\Delta}}$. Here we calculate $\mathrm{\ensuremath{\Gamma}}$ for recently experimentally studied large-dielectric-constant ${({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{2}{\mathrm{Te}}_{3}$ wires in a small-dielectric-constant environment (no superconductor). We show that provided by such a dielectric-constant contrast, the confinement of electric field of impurities within the wire allows more distant impurities to contribute into $\mathrm{\ensuremath{\Gamma}}$, leading to $\mathrm{\ensuremath{\Gamma}}\ensuremath{\sim}3\mathrm{\ensuremath{\Delta}}$. We also calculate a TI wire resistance as a function of the Fermi level and carrier concentration due to scattering on Coulomb and neutral impurities and do not find observable discrete subband-spectrum-related oscillations at $N\ensuremath{\gtrsim}{10}^{18}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.

Crossed Andreev reflection in topological insulator nanowire T junctions

We numerically study crossed Andreev reflection (CAR) in a topological insulator nanowire T junction where one lead is proximitized by a superconductor. We perform realistic simulations based on the three-dimensional (3D) Bernevig-Hughes-Zhang model and compare the results with those from an effective two-dimensional (2D) surface model, whose computational cost is much lower. Both approaches show that CAR should be clearly observable in a wide parameter range, including perfect CAR in a somewhat more restricted range. Furthermore, it can be controlled by a magnetic field and is robust to disorder. Our effective 2D implementation allows us to model systems of micron size, typical of experimental setups but computationally too heavy for 3D models.

Fabry–Pérot interferometry with gate-tunable 3D topological insulator nanowires

Three-dimensional topological insulator (3D TI) nanowires display remarkable magnetotransport properties that can be attributed to their spin-momentum-locked surface states such as quasiballistic transport and Aharonov–Bohm oscillations. Here, we focus on the transport properties of a 3D TI nanowire with a gated section that forms an electronic Fabry–Pérot (FP) interferometer that can be tuned to act as a surface-state filter or energy barrier. By tuning the carrier density and length of the gated section of the wire, the interference pattern can be controlled and the nanowire can become fully transparent for certain topological surface-state input modes while completely filtering out others. We also consider the interplay of FP interference with an external magnetic field, with which Klein tunneling can be induced, and transverse asymmetry of the gated section, e.g. due to a top-gated structure, which displays an interesting analogy with Rashba nanowires. Due to its rich conductance phenomenology, we propose a 3D TI nanowire with gated section as an ideal setup for a detailed transport-based characterization of 3D TI nanowire surface states near the Dirac point, which could be useful towards realizing 3D TI nanowire-based topological superconductivity and Majorana bound states.

Quantum confinement of the Dirac surface states in topological-insulator nanowires

The non-trivial topology of three-dimensional topological insulators dictates the appearance of gapless Dirac surface states. Intriguingly, when made into a nanowire, quantum confinement leads to a peculiar gapped Dirac sub-band structure. This gap is useful for, e.g., future Majorana qubits based on TIs. Furthermore, these sub-bands can be manipulated by a magnetic flux and are an ideal platform for generating stable Majorana zero modes, playing a key role in topological quantum computing. However, direct evidence for the Dirac sub-bands in TI nanowires has not been reported so far. Here, using devices fabricated from thin bulk-insulating (Bi1−xSbx)2Te3 nanowires we show that non-equidistant resistance peaks, observed upon gate-tuning the chemical potential across the Dirac point, are the unique signatures of the quantized sub-bands. These TI nanowires open the way to address the topological mesoscopic physics, and eventually the Majorana physics when proximitized by an s-wave superconductor.

Accessing topological surface states and negative MR in sculpted nanowires of Bi2Te3 at ultra-low temperature

Milling of 2D flakes is a simple method to fabricate nanomaterial of any desired shape and size. Inherently milling process can introduce the impurity or disorder which might show exotic quantum transport phenomenon when studied at the low temperature. Here we report temperature dependent weak antilocalization (WAL) effects in the sculpted nanowires of topological insulator in the presence of perpendicular magnetic field. The quadratic and linear magnetoconductivity (MC) curves at low temperature (>2 K) indicate the bulk contribution in the transport. A cusp feature in magnetoconductivity curves (positive magnetoresistance) at ultra low (<1 K) temperature and at magnetic field (<1 T) represent the WAL indicating the transport through surface states. The MC curves are discussed by using the 2D Hikami–Larkin–Nagaoka theory. The cross-over/interplay nature of positive and negative magnetoresistance observed in the MR curve at ultra-low temperature. Our results indicate that transport through topological surface states (TSS) in sculpted nanowires of Bi2Te3 can be achieved at mK range and linear MR observed at ∼2 K could be the coexistence of electron transport through TSS and contribution from the bulk band.