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 discovered1,2,3,4,5,6,7,8,9. One such effect is magnetochiral anisotropy (MCA)6,7,8,9, 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 effect6,7,8. In typical materials, the rectification coefficient γ due to MCA is usually ∣γ∣ ≲ 1 A−1 T−1 (refs. 8,9,10,11,12) 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

Current-induced breakdown of the quantum anomalous Hall effect

The quantum anomalous Hall effect (QAHE) realizes dissipationless longitudinal resistivity and quantized Hall resistance without the need of an external magnetic field. However, when reducing the device dimensions or increasing the current density, an abrupt breakdown of the dissipationless state occurs with a relatively small critical current, limiting the applications of the QAHE. We investigate the mechanism of this breakdown by studying multiterminal devices and identified that the electric field created between opposing chiral edge states lies at the origin. We propose that electric-field-driven percolation of two-dimensional charge puddles in the gapped surface states of compensated topological-insulator films is the most likely cause of the breakdown

Novel self-epitaxy for inducing superconductivity in the topological insulator (Bi1-xSbx)(2)Te-3

Using the superconducting proximity effect for engineering a topological superconducting state in a topological insulator (TI) is a promising route to realize Majorana fermions. However, epitaxial growth of a superconductor on the TI surface to achieve a good proximity effect has been a challenge. We discovered that simply depositing Pd on thin films of the TI material (Bi1-xSbx)(2)Te-3 leads to an epitaxial self-formation of PdTe2 superconductor having the superconducting transition temperature of similar to 1 K. This self-formed superconductor proximitizes the TI, which is confirmed by the appearance of a supercurrent in Josephson-junction devices made on (Bi1-xSbx)(2)Te-3. This self-epitaxy phenomenon can be conveniently used for fabricating TI-based superconducting nanodevices to address the superconducting proximity effect in TIs

Two-dimensional tellurium superstructures on Au(111) surfaces

Two-dimensional (2D) allotropes of tellurium (Te), recently coined as tellurene, are currently an emerging topic of materials research due to the theoretically predicted exotic properties of Te in its ultrathin form and at the single atomic layer limit. However, a prerequisite for the production of such new and single elemental 2D materials is the development of simple and robust fabrication methods. In the present work, we report three different 2D superstructures of Te on Au(111) surfaces by following an alternative experimental deposition approach. We have investigated the superstructures using low-temperature scanning tunneling microscopy and spectroscopy, Auger electron spectroscopy (AES), and field emission AES. Three superstructures (13 x 13, 8 x 4, and & RADIC;11 x & RADIC;11) of 2D Te are observed in our experiments, and the formation of these superstructures is accompanied by the lifting of the characteristic 23 x & RADIC;3 surface reconstruction of the Au(111) surface. Scanning tunneling spectroscopy reveals a strong dependence of the local electronic properties on the structural arrangement of the Te atoms on the Au(111) support, and we observe superstructure-dependent electronic resonances around the Fermi level and below the Au(111) conduction band. In addition to the appearance of the new electronic resonances, the emergence of band gaps with a p-type charge character has been evidenced for two out of three Te superstructures (13 x 13 and root 11 x root 11) on the Au(111) support. Published under an exclusive license by AIP Publishing

Proximity-induced superconductivity in (Bi1−xSbx)2Te3 topological-insulator nanowires

When a topological insulator is made into a nanowire, the interplay between topology and size quantization gives rise to peculiar one-dimensional states whose energy dispersion can be manipulated by external fields. In the presence of proximity-induced superconductivity, these 1D states offer a tunable platform for Majorana zero modes. While the existence of such peculiar 1D states has been experimentally confirmed, the realization of robust proximity-induced superconductivity in topological-insulator nanowires remains a challenge. Here, we report the realization of superconducting topological-insulator nanowires based on (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ (BST) thin films. When two rectangular pads of palladium are deposited on a BST thin film with a separation of 100--200 nm, the BST beneath the pads is converted into a superconductor, leaving a nanowire of BST in-between. We found that the interface is epitaxial and has a high electronic transparency, leading to a robust superconductivity induced in the BST nanowire. Due to its suitable geometry for gate-tuning, this platform is promising for future studies of Majorana zero modes.Comment: 33 pages total; 22 pages of main text with 4 figures, 11 pages of supplementary information with 8 figures. This version of the article has been accepted for publication, after peer review, but is not the Version of Record and does not reflect post-acceptance improvements, or any correction

Relaxation dynamics of the optically driven nonequilibrium states in the electron- and hole-doped topological-insulator materials (Bi1-xSbx)(2)Te-3

We report on time-resolved mid-infrared-pump terahertz-transmission-probe studies of the topological insulator materials (Bi1-xSbx)(2)Te-3, in which by varying x charge carriers are chemically tuned to be of n-type or p-type. Relaxation dynamics is found to be different in various aspects for transitions below or above the band gap, which are selectively excited by changing the pump-pulse energy. For the below-band-gap excitation, an exponential decay of the pump-probe signals is observed, which exhibits linear dependence on the pump-pulse fluence. In contrast, the relaxation dynamics for the above-band-gap excitation is characterized by a compressed exponential decay and nonlinear fluence dependence at high pump flunences, which reflects interaction of the excited nonequilibrium states

Lattice Location Studies of the Amphoteric Nature of Implanted Mg in GaN

Despite the renewed interest in ion implantation doping of GaN, efficient electrical activation remains a challenge. The lattice location of Mg-27 is investigated in GaN of different doping types as a function of implantation temperature and fluence at CERN's ISOLDE facility. The amphoteric nature of Mg is elucidated, i.e., the concurrent occupation of substitutional Ga and interstitial sites: following room temperature ultra-low fluence (approximate to 2 x 10(10) cm(-2)) implantation, the interstitial fraction of Mg is highest (20-24%) in GaN pre-doped with stable Mg during growth, and lowest (2-6%) in n-GaN:Si, while undoped GaN shows an intermediate interstitial fraction of 10-12%. Both for p- and n-GaN prolonged implantations cause interstitial Mg-27 to approach the levels found for undoped GaN. Implanting above 400 degrees C progressively converts interstitial Mg to substitutional Ga sites due to the onset of Mg interstitial migration (estimated activation energy 1.5-2.3 eV) and combination with Ga vacancies. In all sample types, implantations above a fluence of 10(14) cm(-2) result in >95% substitutional Mg. Ion implantation is hence a very efficient method to introduce Mg into substitutional Ga sites, i.e., challenges toward high electrical activation of implanted Mg are not related to lack of substitutional incorporation

Lattice location of Mg in GaN: a fresh look at doping limitations

Radioactive ^{27}Mg (t_{1/2}=9.5  min) was implanted into GaN of different doping types at CERN's ISOLDE facility and its lattice site determined via β^{-} emission channeling. Following implantations between room temperature and 800 °C, the majority of ^{27}Mg occupies the substitutional Ga sites; however, below 350 °C significant fractions were also found on interstitial positions ∼0.6  Å from ideal octahedral sites. The interstitial fraction of Mg was correlated with the GaN doping character, being highest (up to 31%) in samples doped p type with 2×10^{19}  cm^{-3} stable Mg during epilayer growth, and lowest in Si-doped n-GaN, thus giving direct evidence for the amphoteric character of Mg. Implanting above 350 °C converts interstitial ^{27}Mg to substitutional Ga sites, which allows estimating the activation energy for migration of interstitial Mg as between 1.3 and 2.0 eV.status: publishe