Session T31: Focus Session: Topological Insulators: Synthesis and Characterization - Spin Transport and Superconductivity

Towards spin injection from silicon into topological insulators

Attempts to uncover evidence of spin-momentum coupling in a topological insulator (TI) using transport measurements are hampered by many challenges. Most importantly, injection of a spin polarized current from a ferromagnet that is in contact or close proximity to a topological insulator can easily give rise to anisotropic magnetoresistance signals or planar Hall effect from stray fields, which have the same symmetry and hence are indistinguishable from any signal coming from the spin-momentum-locked surface states. Here we propose a scheme to remove this difficulty by injecting spin-polarized electrons from undoped silicon into the TI surface states. In addition to providing a long-distance transport region to separate the ferromagnetic spin source from the TI by several hundred microns or even millimeters, this approach will also allow spin precession measurements (necessary for unambiguous identification of spin signals) whereas direct injection does not. Detection is provided by differential measurement from two ballistic current contacts on the topological insulator. We will describe our progress in fabrication and measurement of devices with exfoliated crystals of TI Bi$_2$Se$_3$, including the determination of the silicon-Bi$_2$Se$_3$ Schottky barrier height of 0.34 eV   

Spin-momentum helical locking induced spin-valve effects in topological insulator/ferromagnet heterostructures

Topological insulator is composed of an insulating bulk state and an odd number of massless spin-helical Dirac cone formed two dimensional surface state. Here we report a novel spin-valve effect in Bi$_{1.5}$Sb$_{0.5}$Te$_{1.8}$Se$_{1.2}$/CoFe heterostructures. This effect indicates the spin-momentum helical locking on the Bi$_{1.5}$Sb$_{0.5}$Te$_{1.8}$Se$_{1.2}$ topological surface state. It is also indicated that the characteristics of helical surface can be preserved at topological insulator/ferromagnet interface although the ferromagnetism can break the time reversal symmetry and therefore generate an energy gap at the topological Dirac cone.   

Spin Transport Experiments in Topological Insulator Bi$_{2}$Se$_{3}$ Thin Films

Topological insulators are an unusual phase of quantum matter with an insulating bulk gap and gapless spin-momentum locked Dirac surface states. Such a spin-helical surface state provides rich opportunities for potential applications in spintronics. We performed spin valve experiments on exfoliated Bi$_{2}$Se$_{3}$ thin films ($\sim $ 10 nm thick) with ferromagnetic electrodes using a DC driving current in an in-plane magnetic field. We observed the two- terminal resistances are asymmetric between the large positive ($>$ 0.5 T) and negative ($<$-0.5 T) in-plane magnetic fields. The high and low resistance states can be reversed by changing the direction of the driving DC current. Furthermore, the measured resistance asymmetry decreases as temperature increases. One interpretation of our observation is related to the spin-momentum helical locking of topological surface states producing a spin-polarized surface current. We also performed the non-local spin valve measurements, and observed an asymmetry in the measured signal between opposite large magnetic fields.   

Josephson supercurrent through a topological insulator surface state

The long-sought yet elusive Majorana fermion is predicted to arise from a combination of a superconductor and a topological insulator. An essential step in the hunt for this emergent particle is the unequivocal observation of supercurrent in a topological phase. Here, we present direct evidence for a Josephson supercurrent in superconductor (Nb) - topological insulator (Bi$_{2}$Te$_{3})$ - superconductor e-beam fabricated junctions by the observation of clear Shapiro steps under microwave irradiation, and a Fraunhofer-type dependence of the critical current on magnetic field. The dependence of the critical current on temperature and electrode spacing shows that the junctions are in the ballistic limit. Shubnikov-de Haas oscillations in magnetic fields up to 30 T reveal a topologically non-trivial two-dimensional surface state. We argue that the ballistic Josephson current is hosted by this surface state despite the fact that the normal state transport is dominated by diffusive bulk conductivity. The lateral Nb-Bi$_{2}$Te$_{3}$-Nb junctions hence provide prospects for the realization of devices supporting Majorana fermions.   

Hybrid Superconducting Junctions of Bi$_{2}$Se$_{3}$ Topological Insulator Nanoribbons

Topological insulators are exotic materials with bulk band gap and metallic edge states which are protected on their own boundary topologically. Here, we report on the fabrication and measurement results of the superconducting proximity junctions of topological insulator nanoribbons of Bi$_{2}$Se$_{3}$. Single-crystalline Bi$_{2}$Se$_{3}$ nanoribbons are synthesized using the vapor-liquid-solid method, while the superconducting Al electrodes are formed on top of the nanowire. When a magnetic field ($H)$ is applied along the axial direction, the magneto-resistance data exhibit quasi-periodic oscillations with an average periodicity of $H^{\ast } \quad \sim $ 0.4 T, which is consistent with the Aharonov-Bohm oscillations. In the superconducting state, the supercurrent branch with a critical current of $I_{c} \quad \sim $ 90 nA is clearly observed in the current-voltage curve as a result of the superconducting proximity effect in Bi$_{2}$Se$_{3}$ nanoribbon. Quantized voltage steps of the Bi$_{2}$Se$_{3}$ nanoribbon Josephson junction under the microwave irradiation satisfy the ac Josephson relation.   

Signature of Majorana Fermions in Josephson Junctions of Bi2Se3

At the surface of a three-dimensional topological insulator lie Dirac fermions. Placing a superconductor in proximity to these surface fermions has been theoretically shown to produce Majorana fermions, an as-yet unobserved elementary particle. We report on the fabrication and low-temperature transport of a topological insulator in proximity to two superconductors -- a device forming a Josephson Junction with a topological insulator (Bi2Se3) as a weak link. Several departures from conventional Josephson Junctions are observed and evaluated in the context of the presence of a one-dimensional wire of Majorana fermions induced in the device.   

Superconductivity in the topological semimetal YPtBi

Superconductivity was recently discovered in the half Heusler compound YPtBi. Electrical resistivity and Hall data provide compelling evidence that supports the idea that band structure calculations are correct and that YPtBi is indeed a semimetal with nontrivial topology. The low-temperature superconductivity emerges from a remarkable normal state with an extremely low carrier density, no crystalline inversion symmetry, and strong band inversion. I will discuss the normal state properties of YPtBi and details of its superconducting state, and compare them to the characteristics of other potential topological superconductors. This research was performed at the University of Maryland, College Park in collaboration with Paul Syers, Kevin Kirshenbaum, Andrew P. Hope, and Johnpierre Paglione.   

Superconductivity in Bi2Te3 type three dimensional topological compounds induced via pressure

We report experimental updates on pressure induced superconductivity in Bi$_{2}$Te$_{3}$ type topological compounds. The topological nature of the superconductivity observed will be discussed in conjunction with on site high pressure structure investigations {\&} first principles calculations. A phase diagram of superconductivity as function of pressure will be provided.   

Proximity-Induced High-Temperature Superconductivity in a Topological Insulator

New topological phases of matter have been proposed to exist at the surface of some materials with spin-orbit coupling called topological insulators. Among the different exotic features of topological insulators, the interface between a topological insulator and a superconductor is of great interest. It is predicted that combining these two materials would lead to the emergence of Majorana fermion excitations which enable several applications in spintronics and quantum computing. Towards this goal, we have investigated Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$(Bi-2212)- Bi$_{2}$Se$_{3}$ interface junctions made by a new mechanical-bonding technique. Current vs. voltage and differential conductance measurements have been performed in various temperatures ranging from room temperature to 5K. Several anomalies were observed in the Andreev spectra including a zero bias conductance peak appearing below the critical temperature of the superconductor Bi-2212 (85K), a reduced gap in Bi-2212 as well as the intrinsic gap of Bi-2212. These features suggest the induction of high-temperature superconductivity in the Bi$_{2}$Se$_{3}$ due to proximity to Bi-2212.   

Point-contact Andreev Reflection Spectroscopy on Bi$_2$Se$_3$ Single Crystal

Point-contact spectroscopy measurement is carried out on Bi$_2$Se$_3$ single crystals via approaching a superconducting niobium tip (T$_c$=9.5 K) to the crystal surface. The tip-sample junction conductance is studied as a function of DC bias voltage, temperature, and magnetic field. The resulting spectra show a conductance dip at zero bias, indicative of ballistic transport. A homebuilt positioning stage enables us to approach the superconducting tip in nanometer accuracy, to precisely control the inter-facial barrier strength. We find that our experimental results cannot be simply described by the standard Blonder, Tinkham and Klapwijk (BTK) model even when the inelastic scattering at the interface is considered. An improved model taking into account the spin-orbit coupling effect is needed to fit our data.   

Crystal growth and physical property of Bi-Sb-Te-Se topological insulator and CuxBi2Se3 topological superconductor materials

The discovery of 3D topological insulator and topological superconductor materials opens up a new research field in the condensed matter physics. In order to exploit the novel surface properties of these topological insulators, it is crucial to achieve a bulk-insulating state in these topological insulator crystals. Unfortunately, all available topological insulator crystals are not bulk-insulating. We have grown a number of Bi-Se, Bi-Te, Sb-Te-Se, Bi-Sb-Se, Bi-Sb-Te-Se and Bi-Sb-Te-Se-S topological insulator single crystals by using 5N and 6N pure elements. We have measured the physical properties on these single crystals. We have studied the effect of growth condition and impurity on the bulk electrical conductivity of these single crystals. We try to answer two questions if it is possible to grow the bulk-insulating topological insulator single crystals and which maximum resistivity of these topological insulator single crystals we can grow. We have also grown a number of CuxBi2Se3 topological superconductor single crystals.   

Crystal structure and superconducting properties in Cu$_{x}$Bi$_{2}$Se$_{3}$

The recent discovery of the anomalous superconductivity in Cu$_{x}$Bi$_{2}$Se$_{3}$ (0.10$<$x$<$0.25) has attracted much attention because of the relation between superconducting state and topological surface state. In order to understand the role of Cu doping in superconducting Bi$_{2}$Se$_{3}$, we study the doping dependence of the magnetic properties and the characteristics of crystal structures. We made high quality of Cu$_{x}$Bi$_{2}$Se$_{3}$ single crystals with several doping level of x by using melt growth technique. We confirmed the superconducting transition from Cu$_{x}$Bi$_{2}$Se$_{3}$ for several doping levels and determined the phase diagram of a function of x. The lattice constant increases with increasing Cu doping level, however it saturated around x=0.25 which corresponds to the saturation of $T_{c}$ as well. We analyzed the crystal structure in detail in explaining the occurrence of superconductivity. Details of the structural and magnetization study will be discussed in the conference.   

Electrical Transport Properties of Bi$_{2}$Te$_{3}$ Nanotubes and Nanowires

Electrical transport properties of promising topological insulator, Bi$_{2}$Te$_{3}$ nanowires and nanotubes are measured at different temperatures and magnetic field and the results are compared with the Bi$_{2}$Se$_{3}$ thin films. The nanotube and nanowire samples are synthesized by galvanostatic electrodepositon and solution phase methods, with diameters of 70nm (ex) and 50nm (in) for nanotubes and 80$\sim $100nm for nanowires, respectively. The contact leads (Pt) are fabricated by using Focusing Ion Beam (FIB). The magnetoresistance of Bi$_{2}$Te$_{3}$ nanotubes shows linear dependence as a function of magnetic field, with a notable peak around zero field. This is different from Bi$_{2}$Se$_{3}$ thin films which show quadratic behavior, with a significant dip near zero field. The results will be discussed based on the possible weak localization effect in the nanotubes in comparison with the weak anti-localization effect in the films.