Direct mapping of the spin-filtered surface bands of a three-dimensional quantum spin Hall insulator

Spin-polarized band structure of the three-dimensional quantum spin Hall insulator $\rm Bi_{1-x}Sb_{x}$ (x=0.12-0.13) was fully elucidated by spin-polarized angle-resolved photoemission spectroscopy using a high-yield spin polarimeter equipped with a high-resolution electron spectrometer. Between the two time-reversal-invariant points, $\bar{\varGamma}$ and $\bar{M}$, of the (111) surface Brillouin zone, a spin-up band ($\Sigma_3$ band) was found to cross the Fermi energy only once, providing unambiguous evidence for the strong topological insulator phase. The observed spin-polarized band dispersions determine the "mirror chirality" to be -1, which agrees with the theoretical prediction based on first-principles calculations

Anomalous Fraunhofer Patterns in Gated Josephson Junctions Based on the Bulk-Insulating Topological Insulator BiSbTeSe2

One-dimensional Majorana modes are predicated to form in Josephson junctions based on three-dimensional topological insulators (TIs). While observations of supercurrents in Josephson junctions made on bulk-insulating TI samples have been reported recently, the Fraunhofer patters observed in such Tl-based Josephson junctions, which sometimes present anomalous features, are still not well-understood. Here, we report our study of highly gate-tunable Tl-based Josephson junctions made of one of the most bulk-insulating TI materials, BiSbTeSe2, and Al. The Fermi level can be tuned by gating across the Dirac point, and the high transparency of the Al-BiSbTeSe2 interface is evinced by a high characteristic voltage and multiple Andreev reflections, with peak indices reaching 12 Anomalous Fraunhofer patterns with missing lobes were observed in the entire range of gate voltage. We found that, by employing an advanced fitting procedure to use the maximum entropy method in a Monte Carlo algorithm, the anomalous Fraunhofer patterns are explained as a result of inhomogeneous supercurrent distributions on the TI surface in the junction. Besides establishing a highly promising fabrication technology, this work clarifies one of the important open issues regarding Tl-based Josephson junctions

Anomalous Fraunhofer Patterns in Gated Josephson Junctions Based on the Bulk-Insulating Topological Insulator BiSbTeSe₂

One-dimensional Majorana modes are predicated to form in Josephson junctions based on three-dimensional topological insulators (TIs). While observations of supercurrents in Josephson junctions made on bulk-insulating TI samples have been reported recently, the Fraunhofer patters observed in such TI-based Josephson junctions, which sometimes present anomalous features, are still not well-understood. Here, we report our study of highly gate-tunable TI-based Josephson junctions made of one of the most bulk-insulating TI materials, BiSbTeSe₂, and Al. The Fermi level can be tuned by gating across the Dirac point, and the high transparency of the Al–BiSbTeSe₂ interface is evinced by a high characteristic voltage and multiple Andreev reflections, with peak indices reaching 12. Anomalous Fraunhofer patterns with missing lobes were observed in the entire range of gate voltage. We found that, by employing an advanced fitting procedure to use the maximum entropy method in a Monte Carlo algorithm, the anomalous Fraunhofer patterns are explained as a result of inhomogeneous supercurrent distributions on the TI surface in the junction. Besides establishing a highly promising fabrication technology, this work clarifies one of the important open issues regarding TI-based Josephson junctions

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

Spin waves above and below the Verwey transition in TbBaFe2O5

TbBaFe2O5+delta (TBFO) is a mixed valence compound with an antiferromagnetic order (T-N = 450 K) that changes along with the crystal structure and conductivity upon cooling below a Verwey-like transition (T-v = 280 K). This type of double-cell-layered perovskite is useful to study putative charge ordering in fractional valence systems because the crystal structure develops unique sites for the different valence states. While the evolution of the static magnetic structure in the different charge ordered states of TBFO is known, in this study we investigate the effect of charge ordering on the magnetic interactions. Dispersive magnetic excitations measured using single crystal inelastic neutron scattering experiments at T = 4 K T-v are modeled with linear-spin-wave theory to extract anisotropy and superexchange parameters. Below T-v, we observe a sizable magnon gap due to an easy-axis magnetic anisotropy that closes above T v due to the evolution of easy-plane anisotropy. The magnetic interactions are found to be highly three-dimensional and change across T-v. Above T-v, where the conductivity is greater, a damping term is required to model the spin waves that suggests a coupling to valence fluctuations

Spin waves above and below the Verwey transition in TbBaFe2O5

TbBaFe2O5+δ (TBFO) is a mixed valence compound with an antiferromagnetic order (TN=450K) that changes along with the crystal structure and conductivity upon cooling below a Verwey-like transition (TV=280K). This type of double-cell-layered perovskite is useful to study putative charge ordering in fractional valence systems because the crystal structure develops unique sites for the different valence states. While the evolution of the static magnetic structure in the different charge ordered states of TBFO is known, in this study we investigate the effect of charge ordering on the magnetic interactions. Dispersive magnetic excitations measured using single crystal inelastic neutron scattering experiments at T=4KTV are modeled with linear-spin-wave theory to extract anisotropy and superexchange parameters. Below TV, we observe a sizable magnon gap due to an easy-axis magnetic anisotropy that closes above TV due to the evolution of easy-plane anisotropy. The magnetic interactions are found to be highly three-dimensional and change across TV. Above TV, where the conductivity is greater, a damping term is required to model the spin waves that suggests a coupling to valence fluctuations

Topological transition in Bi1-xSbx studied as a function of Sb doping

International audienceSpin- and angle-resolved photoemission spectroscopy measurements were performed on Bi1-xSbx samples at x = 0.04, 0.07, and 0.21 to study the change of the surface band structure from nontopological to topological. Energy shift of the T and L-s bulk bands with Sb concentration is quantitatively evaluated. An edge state becomes topologically nontrivial at x = 0.04. An additional trivial edge state appears at the L band gap that forms at x > 0.04 and apparently hybridize with the nontrivial edge state. A scenario for the topological transition mechanism is presented. Related issues of self-energy and temperature dependence of the surface state are also considered