Influence of topological edge states on the properties ofAl/Bi2Se3/Alhybrid Josephson devices (original) (raw)
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Signatures of Majorana Fermions in Hybrid Superconductor-Topological Insulator Josephson Junctions
Bulletin of the American Physical Society, 2018
The ability to measure and manipulate complex particles in the solid state is a cornerstone of modern condensed-matter physics. Typical excitations of a sea of electrons, called quasiparticles, have properties similar to those of free electrons. However, in recent years exotic excitations with very different properties have been created in designer quantum materials, including Dirac fermions in graphene 1 and fractionally-charged quasiparticles in fractional quantum Hall systems 2. Here we report signatures of a new quasiparticle-the Majorana fermion-in Josephson junctions consisting of two superconducting leads coupled through a three-dimensional topological insulator 3. We observe two striking departures from the common transport properties of Josephson junctions: a characteristic energy that scales inversely with the width of the junction, and a low characteristic magnetic field for suppressing supercurrent. To explain these effects, we propose a phenomenological model in which a one-dimensional wire of Majorana fermions is present along the width of the junction, similar to a theoretical prediction by Fu and Kane 4. These results present an opening into the investigation of Majorana fermions in the solid state and their exotic properties, including non-Abelian statistics 5 , a suggested basis for fault-tolerant quantum computation 6 .
Progress in Superconductor-Semiconductor Topological Josephson Junctions
2024
Majorana bound states (MBSs) are quasiparticles that are their own antiparticles. They are predicted to emerge as zero-energy modes localized at the boundary of a topological superconductor. No intrinsic topological superconductor is known to date. However, by interfacing conventional superconductors and semiconductors with strong spin-orbit coupling, it is possible to create a system hosting topological states. Hence, epitaxial superconductors and semiconductors have emerged as an attractive material system with atomically sharp interfaces and broad flexibility in device fabrications incorporating Josephson junctions. We discuss the basics of topological superconductivity and provide insight into how to go beyond current state-of-the-art experiments. We argue that the ultimate success in realizing MBS physics requires the observation of non-Abelian braiding and fusion experiments.
Josephson supercurrent through a topological insulator surface state
Nature Materials, 2012
Topological insulators [1-11] are characterized by an insulating bulk with a finite band gap and conducting edge or surface states, where charge carriers are protected against backscattering. These states give rise to the quantum spin Hall effect [2] without an external magnetic field, where electrons with opposite spins have opposite momentum at a given edge. The surface energy spectrum of a threedimensional topological insulator [3, 5] is made up by an odd number of Dirac cones with the spin locked to the momentum. The long-sought yet elusive Majorana fermion [12] is predicted to arise from a combination of a superconductor and a topological insulator [13-15]. An essential step in the hunt for this emergent particle is the unequivocal observation of supercurrent in a topological phase. Here, we present the first measurement of a Josephson supercurrent through a topological insulator. Direct evidence for Josephson supercurrents in superconductor (Nb)-topological insulator (Bi 2 Te 3)-superconductor e-beam fabricated junctions is provided 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 length 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 [16].
Enhanced topological superconductivity in spatially modulated planar Josephson junctions
2021
We propose a semiconductor-superconductor hybrid device for realizing topological superconductivity and Majorana zero modes consisting of a planar Josephson junction structure with periodically modulated junction width. By performing a numerical analysis of the effective model describing the low-energy physics of the hybrid structure, we demonstrate that the modulation of the junction width results in a substantial enhancement of the topological gap and, consequently, of the robustness of the topological superconducting phase and associated Majorana zero modes. This enhancement is due to the formation of minibands with strongly renormalized effective parameters, including stronger spin-orbit coupling, generated by the effective periodic potential induced by the modulated structure. In addition to a larger topological gap, the proposed device supports a topological superconducting phase that covers a significant fraction of the parameter space, including the low Zeeman field regime, in the absence of a superconducting phase difference across the junction. Furthermore, the optimal regime for operating the device can be conveniently accessed by tuning the potential in the junction region using, for example, a top gate.
Journal of Applied Physics, 2020
We have used Bi2Se3 nanoribbons, grown by catalyst-free physical vapor deposition to fabricate high quality Josephson junctions with Al superconducting electrodes. In our devices, we observe a pronounced reduction of the Josephson critical current density Jc by reducing the width of the junction, which in our case corresponds to the width of the nanoribbon. Because the topological surface states extend over the entire circumference of the nanoribbon, the superconducting transport associated with them is carried by modes on both the top and bottom surfaces of the nanoribbon. We show that the Jc reduction as a function of the nanoribbon width can be accounted for by assuming that only the modes traveling on the top surface contribute to the Josephson transport as we derive by geometrical consideration. This finding is of great relevance for topological quantum circuitry schemes since it indicates that the Josephson current is mainly carried by the topological surface states.
Physical Review B, 2016
Topological insulators (TIs) hold great promise for topological quantum computation in solidstate systems. Recently, several groups reported experimental data suggesting that signatures of Majorana modes have been observed in topological insulator Josephson junctions (TIJJs). A prerequisite for the exploration of Majorana physics is to obtain a good understanding of the properties of low-energy Andreev bound states (ABS) in a material with topologically non-trivial band structure. Here, we present experimental data and a theoretical analysis demonstrating that the band structure inversion close to the surface of a TI has observable consequences for supercurrent transport in TIJJs prepared on surface-doped Bi 2 Se 3 thin films. Electrostatic carrier depletion of the film surface leads to an abrupt drop in the critical current of such devices. The effect can be understood as a relocation of low-energy ABS from a region deeper in the bulk of the material to the more strongly disordered surface which is driven by the topology of the effective band structure in the presence of surface dopants.
Superconducting Quantum Interference in Edge State Josephson Junctions
Physical Review Letters, 2020
We study superconducting quantum interference in a Josephson junction linked via edge states in two-dimensional (2D) insulators. We consider two scenarios in which the 2D insulator is either a topological or a trivial insulator supporting one-dimensional (1D) helical or nonhelical edge states, respectively. In equilibrium, we find that the qualitative dependence of critical supercurrent on the flux through the junction is insensitive to the helical nature of the mediating states and can, therefore, not be used to verify the topological features of the underlying insulator. However, upon applying a finite voltage bias smaller than the superconducting gap to a relatively long junction, the finite-frequency interference pattern in the non-equilibrium transport current is qualitatively different for helical edge states as compared to nonhelical ones.
Josephson current mediated by ballistic topological states in Bi2Te2.3Se0.7 single nanocrystals
Communications Materials
Superconducting proximity devices using low-dimensional semiconducting elements enable a ballistic regime in the proximity transport. The use of topological insulators in such devices is considered promising owing to the peculiar transport properties these materials offer, as well the hope of inducing topological superconductivity and Majorana phenomena via proximity effects. Here we demonstrate the fabrication and superconducting properties of proximity Josephson devices integrating nanocrystals single of Bi2Te2.3Se0.7 with a thickness of a few unit cells. Single junctions display typical characteristics of planar Josephson devices; junctions integrating two nanocrystals behave as nanodimensional superconducting quantum interference devices. A peculiar temperature and magnetic field evolution of the Josephson current along with the observed excess current effect point towards the ballistic proximity regime of topological channels. This suggests the proposed devices are promising fo...
Gate-tuned normal and superconducting transport at the surface of a topological insulator
Nature Communications, 2011
Three-dimensional topological insulators are characterized by the presence of a bandgap in their bulk and gapless Dirac fermions at their surfaces. New physical phenomena originating from the presence of the Dirac fermions are predicted to occur, and to be experimentally accessible via transport measurements in suitably designed electronic devices. Here we study transport through superconducting junctions fabricated on thin Bi 2 Se 3 single crystals, equipped with a gate electrode. In the presence of perpendicular magnetic fi eld B, sweeping the gate voltage enables us to observe the fi lling of the Dirac fermion Landau levels, whose character evolves continuously from electron-to hole-like. When B = 0, a supercurrent appears, whose magnitude can be gate tuned, and is minimum at the charge neutrality point determined from the Landau level fi lling. Our results demonstrate how gated nano-electronic devices give control over normal and superconducting transport of Dirac fermions at an individual surface of a threedimensional topological insulators.