Proximity‐Effect‐Induced Superconductivity in Nb/Sb ₂ Te ₃ ‐Nanoribbon/Nb Junctions

Wang X., Cheng L., Zhu D., Wu Y., Chen M., Wang Y., Zhao D., Boothroyd C.B., Lam Y.M., Zhu J., Battiato M., Song J.C., Yang H., Chia E.E.

Strong spin-orbit coupling, resulting in the formation of spin-momentum-locked surface states, endows topological insulators with superior spin-to-charge conversion characteristics, though the dynamics that govern it have remained elusive. Here, we present an all-optical method that enables unprecedented tracking of the ultrafast dynamics of spin-to-charge conversion in a prototypical topological insulator Bi$_2$Se$_3$/ferromagnetic Co heterostructure, down to the sub-picosecond timescale. Compared to pure Bi$_2$Se$_3$ or Co, we observe a giant terahertz emission in the heterostructure than originates from spin-to-charge conversion, in which the topological surface states play a crucial role. We identify a 0.12-picosecond timescale that sets a technological speed limit of spin-to-charge conversion processes in topological insulators. In addition, we show that the spin-to-charge conversion efficiency is temperature independent in Bi$_2$Se$_3$ as expected from the nature of the surface states, paving the way for designing next-generation high-speed opto-spintronic devices based on topological insulators at room temperature.

Li C., de Boer J.C., de Ronde B., Ramankutty S.V., van Heumen E., Huang Y., de Visser A., Golubov A.A., Golden M.S., Brinkman A.

Although signatures of superconductivity in Dirac semimetals have been reported, for instance by applying pressure or using point contacts, our understanding of the topological aspects of Dirac semimetal superconductivity is still developing. Here, we utilize nanoscale phase-sensitive junction technology to induce superconductivity in the Dirac semimetal Bi1−xSbx. Our radiofrequency irradiation experiments then reveal a significant contribution of 4π-periodic Andreev bound states to the supercurrent in Nb–Bi0.97Sb0.03–Nb Josephson junctions. The conditions for a substantial 4π contribution to the supercurrent are favourable because of the Dirac cone’s very broad transmission resonances and a measurement frequency faster than the quasiparticle poisoning rate. In addition, we show that a magnetic field applied in the plane of the junction allows tuning of the Josephson junctions from 0 to π regimes. Our results open the technologically appealing avenue of employing the topological bulk properties of Dirac semimetals for topological superconductivity research and topological quantum computer development.Superconductivity is induced in an accidental Dirac semimetal via the proximity effect.

Ghatak S., Breunig O., Yang F., Wang Z., Taskin A.A., Ando Y.

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.

Moors K., Schüffelgen P., Rosenbach D., Schmitt T., Schäpers T., Schmidt T.L.

We study the magnetotransport properties of patterned 3D topological insulator nanostructures with several leads, such as kinks or Y-junctions, near the Dirac point with analytical as well as numerical techniques. The interplay of the nanostructure geometry, the external magnetic field and the spin-momentum locking of the topological surface states lead to a richer magnetoconductance phenomenology as compared to straight nanowires. Similar to straight wires, a quantized conductance with perfect transmission across the nanostructure can be realized across a kink when the input and output channels are pierced by a half-integer magnetic flux quantum. Unlike for straight wires, there is an additional requirement depending on the orientation of the external magnetic field. A right-angle kink shows a unique $\pi$-periodic magnetoconductance signature as a function of the in-plane angle of the magnetic field. For a Y-junction, the transmission can be perfectly steered to either of the two possible output legs by a proper alignment of the external magnetic field. These magnetotransport signatures offer new ways to explore topological surface states and could be relevant for quantum transport experiments on nanostructures which can be realized with existing fabrication methods.

Chen M., Chen X., Yang H., Du Z., Wen H.

Superconductivity with twofold symmetry is observed by STM in Bi2Te3/FeTe0.55Se0.45 heterostructures. Topological superconductors are an interesting and frontier topic in condensed matter physics. In the superconducting state, an order parameter will be established with the basic or subsidiary symmetry of the crystalline lattice. In doped Bi2Se3 or Bi2Te3 with a basic threefold symmetry, it was predicted, however, that bulk superconductivity with order parameters of twofold symmetry may exist because of the presence of odd parity. We report the proximity effect–induced superconductivity in the Bi2Te3 thin film on top of the iron-based superconductor FeTe0.55Se0.45. By using the quasiparticle interference technique, we demonstrate clear evidence of twofold symmetry of the superconducting gap. The gap minimum is along one of the main crystalline axes following the so-called Δ4y notation. This is also accompanied by the elongated vortex shape mapped out by the density of states within the superconducting gap. Our results provide an easily accessible platform for investigating possible topological superconductivity in Bi2Te3/FeTe0.55Se0.45 heterostructures.

Skryabina O.V., Egorov S.V., Goncharova A.S., Klimenko A.A., Kozlov S.N., Ryazanov V.V., Bakurskiy S.V., Kupriyanov M.Y., Golubov A.A., Napolskii K.S., Stolyarov V.S.

We report on a fabrication method and electron-transport measurements for submicron Josephson junctions formed by Cu nanowires coupling to superconducting planar Nb electrodes. The Cu nanowires with a resistivity of ρCu≃1 μΩ cm at low temperatures consisting of single-crystalline segments have been obtained by templated electrodeposition using anodic aluminum oxide as a porous matrix. The current-voltage characteristics of the devices have been studied as a function of temperature and magnetic field. For all junctions, the critical current monotonically decreases with a magnetic field. The measured temperature and magnetic field dependencies are consistent with the model for one-dimensional diffusive superconductor/normal metal/superconductor (SNS) Josephson junctions within the quasiclassical theory of superconductivity.

Huang G., Xu H.Q.

Signatures of Majorana fermion bound states in one-dimensional topological insulator (TI) nanowires with proximity effect induced superconductivity are studied. The phase diagram and energy spectra are calculated for single TI nanowires and it is shown that the nanowires can be in the topological invariant phases of winding numbers $W=0, \pm 1$, and $\pm 2$ corresponding to the cases with zero, one and two pairs of Majorana fermions in the single TI nanowires. It is also shown that the topological winding numbers, i.e., the numbers of pairs of Majorana fermions in the TI nanowires can be extracted from the transport measurements of a Josephson junction device made from two TI nanowires, while the sign in the winding numbers can be extracted using a superconducting quantum interference device (SQUID) setup.

Weyrich C., Merzenich T., Kampmeier J., Batov I.E., Mussler G., Schubert J., Grützmacher D., Schäpers T.

We report on the Shubnikov–de Haas oscillations in the longitudinal resistance of thin films of three-dimensional topological insulator Sb2Te3 grown by means of molecular beam epitaxy. The oscillations persist up to the temperatures of 30 K, and the measurements at various tilt angles reveal that they originate from a two-dimensional system. Using a top gate, we further study the change of oscillation amplitude and frequency, which in combination with the standard Hall measurements suggest the origin of oscillations to be at the interface between the film and the Si substrate.

Kim B., Kim H., Yang Y., Peng X., Yu D., Doh Y.

We report the fabrication of strongly coupled nanohybrid superconducting junctions using PbS semiconductor nanowires and Pb0.5In0.5 superconducting electrodes. The maximum supercurrent in the junction reaches up to ~15 \mu A at 0.3 K, which is the highest value ever observed in semiconductor-nanowire-based superconducting junctions. The observation of microwave-induced constant voltage steps confirms the existence of genuine Josephson coupling through the nanowire. Monotonic suppression of the critical current under an external magnetic field is also in good agreement with the narrow junction model. The temperature-dependent stochastic distribution of the switching current exhibits a crossover from phase diffusion to a thermal activation process as the temperature decreases. These strongly coupled nanohybrid superconducting junctions would be advantageous to the development of gate-tunable superconducting quantum information devices.

Arango Y.C., Huang L., Chen C., Avila J., Asensio M.C., Grützmacher D., Lüth H., Lu J.G., Schäpers T.

We report on low-temperature transport and electronic band structure of p-type Sb2Te3 nanowires, grown by chemical vapor deposition. Magnetoresistance measurements unravel quantum interference phenomena, which depend on the cross-sectional dimensions of the nanowires. The observation of periodic Aharonov-Bohm-type oscillations is attributed to transport in topologically protected surface states in the Sb2Te3 nanowires. The study of universal conductance fluctuations demonstrates coherent transport along the Aharonov-Bohm paths encircling the rectangular cross-section of the nanowires. We use nanoscale angle-resolved photoemission spectroscopy on single nanowires (nano-ARPES) to provide direct experimental evidence on the nontrivial topological character of those surface states. The compiled study of the bandstructure and the magnetotransport response unambiguosly points out the presence of topologically protected surface states in the nanowires and their substantial contribution to the quantum transport effects, as well as the hole doping and Fermi velocity among other key issues. The results are consistent with the theoretical description of quantum transport in intrinsically doped quasi-one-dimensional topological insulator nanowires.

Bocquillon E., Deacon R.S., Wiedenmann J., Leubner P., Klapwijk T.M., Brüne C., Ishibashi K., Buhmann H., Molenkamp L.W.

In recent years, Majorana physics has attracted considerable attention because of exotic new phenomena and its prospects for fault-tolerant topological quantum computation. To this end, one needs to engineer the interplay between superconductivity and electronic properties in a topological insulator, but experimental work remains scarce and ambiguous. Here, we report experimental evidence for topological superconductivity induced in a HgTe quantum well, a 2D topological insulator that exhibits the quantum spin Hall (QSH) effect. The a.c. Josephson effect demonstrates that the supercurrent has a 4π periodicity in the superconducting phase difference, as indicated by a doubling of the voltage step for multiple Shapiro steps. In addition, this response like that of a superconducting quantum interference device to a perpendicular magnetic field shows that the 4π-periodic supercurrent originates from states located on the edges of the junction. Both features appear strongest towards the QSH regime, and thus provide evidence for induced topological superconductivity in the QSH edge states.

Wiedenmann J., Bocquillon E., Deacon R.S., Hartinger S., Herrmann O., Klapwijk T.M., Maier L., Ames C., Brüne C., Gould C., Oiwa A., Ishibashi K., Tarucha S., Buhmann H., Molenkamp L.W.

The Josephson effect describes the generic appearance of a supercurrent in a weak link between two superconductors. Its exact physical nature deeply influences the properties of the supercurrent. In recent years, considerable efforts have focused on the coupling of superconductors to the surface states of a three-dimensional topological insulator. In such a material, an unconventional induced p-wave superconductivity should occur, with a doublet of topologically protected gapless Andreev bound states, whose energies vary 4π-periodically with the superconducting phase difference across the junction. In this article, we report the observation of an anomalous response to rf irradiation in a Josephson junction made of a HgTe weak link. The response is understood as due to a 4π-periodic contribution to the supercurrent, and its amplitude is compatible with the expected contribution of a gapless Andreev doublet. Our work opens the way to more elaborate experiments to investigate the induced superconductivity in a three-dimensional insulator.

Jauregui L.A., Pettes M.T., Rokhinson L.P., Shi L., Chen Y.P.

The spin-helical Dirac fermion topological surface states in a topological insulator nanowire or nanoribbon promise novel topological devices and exotic physics such as Majorana fermions. Here, we report local and non-local transport measurements in Bi2Te3 topological insulator nanoribbons that exhibit quasi-ballistic transport over ∼2 μm. The conductance versus axial magnetic flux Φ exhibits Aharonov-Bohm oscillations with maxima occurring alternately at half-integer or integer flux quanta (Φ0 = h/e, where h is Planck's constant and e is the electron charge) depending periodically on the gate-tuned Fermi wavevector (kF) with period 2π/C (where C is the nanoribbon circumference). The conductance versus gate voltage also exhibits kF-periodic oscillations, anti-correlated between Φ = 0 and Φ0/2. These oscillations enable us to probe the Bi2Te3 band structure, and are consistent with the circumferentially quantized topological surface states forming a series of one-dimensional subbands, which undergo periodic magnetic field-induced topological transitions with the disappearance/appearance of the gapless Dirac point with a one-dimensional spin helical mode.

Eschbach M., Młyńczak E., Kellner J., Kampmeier J., Lanius M., Neumann E., Weyrich C., Gehlmann M., Gospodarič P., Döring S., Mussler G., Demarina N., Luysberg M., Bihlmayer G., Schäpers T., et. al.

Three-dimensional (3D) topological insulators are a new state of quantum matter, which exhibits both a bulk band structure with an insulating energy gap as well as metallic spin-polarized Dirac fermion states when interfaced with a topologically trivial material. There have been various attempts to tune the Dirac point to a desired energetic position for exploring its unusual quantum properties. Here we show a direct experimental proof by angle-resolved photoemission of the realization of a vertical topological p–n junction made of a heterostructure of two different binary 3D TI materials Bi2Te3 and Sb2Te3 epitaxially grown on Si(111). We demonstrate that the chemical potential is tunable by about 200 meV when decreasing the upper Sb2Te3 layer thickness from 25 to 6 quintuple layers without applying any external bias. These results make it realistic to observe the topological exciton condensate and pave the way for exploring other exotic quantum phenomena in the near future. Topological insulators possess dispersionless electronic surface states with perpendicular spin-momentum locking which may be utilised in spintronic devices. Here, the authors demonstrate p–n junctions formed from two topological insulator thin films, tuning the junction type by film thickness.

Nechaev I.A., Aguilera I., De Renzi V., di Bona A., Lodi Rizzini A., Mio A.M., Nicotra G., Politano A., Scalese S., Aliev Z.S., Babanly M.B., Friedrich C., Blügel S., Chulkov E.V.

I. A. Nechaev,1,2 I. Aguilera,3 V. De Renzi,4,5 A. di Bona,5 A. Lodi Rizzini,4,5 A. M. Mio,6 G. Nicotra,6 A. Politano,7 S. Scalese,6 Z. S. Aliev,1,8,9 M. B. Babanly,8 C. Friedrich,3 S. Blügel,3 and E. V. Chulkov1,2,10,11,12 1Donostia International Physics Center (DIPC), 20018 San Sebastián/Donostia, Basque Country, Spain 2Tomsk State University, Laboratory for Nanostructured Surfaces and Coatings, 634050 Tomsk, Russia 3Peter Grünberg Institute and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA, D-52425 Jülich, Germany 4Dipartimento di Scienze Fisiche Informatiche e Matematiche, Università di Modena e Reggio Emilia, Via Campi, 213/a, 41125 Modena, Italy 5CNR-Nanoscience Institute, S3 Center, 41125 Modena, Italy 6CNR-IMM, Strada VIII, 5, 95121 Catania, Italy 7Università della Calabria, Dipartimento di Fisica, 87036 Rende (CS) Italy 8Institute of Catalysis and Inorganic Chemistry, ANAS, AZ1143 Baku, Azerbaijian 9Institute of Physics, ANAS, AZ1143 Baku, Azerbaijian 10Departamento de Fı́sica de Materiales UPV/EHU, Facultad de Ciencias Quı́micas, UPV/EHU, Apdo. 1072, 20080 San Sebastián/Donostia, Basque Country, Spain 11Centro de Fı́sica de Materiales CFM MPC, Centro Mixto CSIC-UPV/EHU, 20080 San Sebastián/Donostia, Basque Country, Spain 12Saint Petersburg State University, Saint Petersburg 198504, Russia (Received 26 March 2015; revised manuscript received 26 May 2015; published 11 June 2015)

Bakurskiy Sergey, Ruzhickiy Vsevolod, Neilo Alexey, Klenov Nikolay, Soloviev Igor, Elistratova Anna, Shishkin Andrey, Stolyarov Vasily, Kupriyanov Mikhail

We have studied the Thouless energy in Josephson superconductor – normal metal – superconductor (SN-N-NS) bridges analytically and numerically, considering the influence of the sub-electrode regions. We have discovered a significant suppression of the Thouless energy with increasing interfacial resistance, consistent with experimental results. The analysis of the temperature dependence of the critical current in Josephson junctions in comparison with the expressions for the Thouless energy may allow the determination of the interface parameters of S and N-layers.

Maggiora J., Wang X., Zheng R.

The interfaces between superconductors and other materials have long been established as being an important part in the exploration of new physics to aid in our understanding of superconductivity and open us up to new technological advancements. Herein this article we analyse the recent progress made in the understanding of superconductivity at the interfaces involving a wide range of functional materials, mostly looking at two-dimensional (2D) systems. We start off in the first half of this review by focusing on magnetic and superconductive hybrid heterostructures, as well as the resulting physical phenomena from these systems. The first is a section on vortex and anti-vortex phenomena; the second key area is ferromagnet–superconductor hybrid phenomena with particular interest of magnetic skyrmions, the third is the novel frontier based on 2D magnetic and superconductive interfaces particularly examining Ising superconductivity at these interfaces; the fourth is superconductivity at anti-ferromagnetic interfaces and finally half-metals at superconducting interfaces. The second half of this review focuses on superconductivity at insulating and other functional interfaces. Examining firstly, Mott insulator interfaces with wide ranging discussions about how such interfaces can enhance our understanding in high-temperature superconductive cuprates and other unconventional superconductor systems such as the nickelates; in the second section the interface of 2D and 3D ferroelectric materials with superconductors with a key emphasis on devices that have been developed to control the superconducting phase; Topological insulators at interfaces with superconductors is the third section; and lastly 2D twisted material interfaces are explored, including the newly discovered magic angle interfaces discovered with graphene and other van Der Waals materials. It is anticipated that this review will lead to further interest in such interfaces to improve our understanding and expose the exotic science behind these interfaces.

Ruzhickiy V., Bakurskiy S., Kupriyanov M., Klenov N., Soloviev I., Stolyarov V., Golubov A.

In this paper, we present a theoretical study of electronic transport in planar Josephson Superconductor–Normal Metal–Superconductor (SN-N-NS) bridges with arbitrary transparency of the SN interfaces. We formulate and solve the two-dimensional problem of finding the spatial distribution of the supercurrent in the SN electrodes. This allows us to determine the scale of the weak coupling region in the SN-N-NS bridges, i.e., to describe this structure as a serial connection between the Josephson contact and the linear inductance of the current-carrying electrodes. We show that the presence of a two-dimensional spatial current distribution in the SN electrodes leads to a modification of the current–phase relation and the critical current magnitude of the bridges. In particular, the critical current decreases as the overlap area of the SN parts of the electrodes decreases. We show that this is accompanied by a transformation of the SN-N-NS structure from an SNS-type weak link to a double-barrier SINIS contact. In addition, we find the range of interface transparency in order to optimise device performance. The features we have discovered should have a significant impact on the operation of small-scale superconducting electronic devices, and should be taken into account in their design.

Kudriashov A., Babich I., Hovhannisyan R.A., Shishkin A.G., Kozlov S.N., Fedorov A., Vyalikh D.V., Khestanova E., Kupriyanov M.Y., Stolyarov V.S.

AbstractTypically, topological superconductivity is reachable via proximity effect by a direct deposition of superconductor (S) on top of a topological insulator (TI) surface. Here, the double critical current in the Josephson junctions based on the topological insulator is observed in the fabricated planar Superconducting Quantum Interference Devicea. By measuring critical currents as a function of temperature and magnetic field, it is shown that the second critical current stems from the intrinsic superconductivity of the S–TI interface, which is supported by the modified Resistively Shunted Junction model and Transmission Electron Microscopy studies. This complex structure of the interface should be taken into account when the technological process involves Ar‐plasma cleaning.

Zhang J., Tse P., Jalil A., Kölzer J., Rosenbach D., Luysberg M., Panaitov G., Lüth H., Hu Z., Grützmacher D., Lu J.G., Schäpers T.

Despite the fact that GeTe is known to be a very interesting material for applications in thermoelectrics and for phase-change memories, the knowledge on its low-temperature transport properties is only limited. We report on phase-coherent phenomena in the magnetotransport of GeTe nanowires. From universal conductance fluctuations measured on GeTe nanowires with Au contacts, a phase-coherence length of about 280 nm at 0.5 K is determined. The distinct phase-coherence is confirmed by the observation of Aharonov–Bohm type oscillations for parallel magnetic fields. We interpret the occurrence of these magnetic flux-periodic oscillations by the formation of a tubular hole accumulation layer. For Nb/GeTe-nanowire/Nb Josephson junctions we obtained a critical current of 0.2 μA at 0.4 K. By applying a perpendicular magnetic field the critical current decreases monotonously with increasing field, whereas in a parallel field the critical current oscillates with a period of the magnetic flux quantum confirming the presence of a tubular hole channel. A deep understanding of low-temperature transport properties of GeTe material remains a challenge. Here, the authors investigate phase-coherent phenomena in GeTe nanowire structures where the occurrence of magnetic flux-periodic oscillations come from the formation of a tubular hole accumulation layer.

Tse P.L., Tian F., Mugica-Sanchez L., Rüger O., Undisz A., Möthrath G., Ronning C., Takahashi S., Lu J.G.

Scaling information bits to ever smaller dimensions is a dominant drive for information technology (IT). Nanostructured phase change material emerges as a key player in the current green-IT endeavor with low power consumption, functional modularity and promising scalability. In this work, we present the demonstration of microwave AC voltage induced phase change phenomenon at 3 GHz in single Sb$_2$Te$_3$ nanowires. The resistance change by a total of 6 - 7 orders of magnitude is evidenced by a transition from the crystalline metallic to the amorphous semiconducting phase, which is cross-examined by temperature dependent transport measurement and high-resolution electron microscopy analysis. This discovery could potentially tailor multi-state information bit encoding and discrimination along a single nanowire, rendering technology advancement for neuro-inspired computing devices.

Chen J., Tse P.L., Krivorotov I.N., Lu J.G.

Unique spin–momentum locking in topological surface states of Sb2Te3 nanowires exhibits an unusual symmetry in non-local voltage signal.