The Superconductivity of the Topologically Protected Surface States of Bi${}_2$Se${}_3$: Experiment (original) (raw)
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Physical Review B, 2016
In this paper we present scanning tunneling microscopy of a large Bi2Se3 crystal with superconducting PbBi islands deposited on the surface. Local density of states measurements are consistent with induced superconductivity in the topological surface state with a coherence length of order 540 nm. At energies above the gap the density of states exhibits oscillations due to scattering caused by a nonuniform order parameter. Strikingly, the spectra taken on islands also display similar oscillations along with traces of the Dirac cone, suggesting an inverse topological proximity effect.
OSS 2018 Oxide Superconducting Spintronic Workshop
2018
Traditionally, the symmetries that protect superconductivity are time-reversal and parity. Here, we examine the minimal symmetries protecting superconductivity in two dimensions and find that timereversal symmetry and inversion symmetry are not required, and having a combination of either symmetry with a mirror operation on the basal plane is sufficient. We classify superconducting states stabilized by these two symmetries, when time-reversal and inversion symmetries are not present, and provide realistic minimal models as examples. Interestingly, several experimentally realized systems, such as transition metal dichalcogenides and the two-dimensional Rashba system fall into this scenario, when subject to an applied magnetic field.
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Artificially engineered topological superconductivity has emerged as a viable route to create Majorana modes. In this context, proximity-induced superconductivity in materials with a sizable spin−orbit coupling has been intensively investigated in recent years. Although there is convincing evidence that superconductivity may indeed be induced, it has been difficult to elucidate its topological nature. Here, we engineer an artificial topological superconductor by progressively introducing superconductivity (Nb), strong spin−orbital coupling (Pt), and topological states (Bi 2 Te 3). Through spectroscopic imaging of superconducting vortices within the bare s-wave superconducting Nb and within proximitized Pt and Bi 2 Te 3 layers, we detect the emergence of a zerobias peak that is directly linked to the presence of topological surface states. Our results are rationalized in terms of competing energy trends which are found to impose an upper limit to the size of the minigap separating Majorana and trivial modes, its size being ultimately linked to fundamental materials properties.
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Scientific Reports
interest in topological states of matter burgeoned over a decade ago with the theoretical prediction and experimental detection of topological insulators, especially in bulk three-dimensional insulators that can be tuned out of it by doping. their superconducting counterpart, the fully-gapped threedimensional time-reversal-invariant topological superconductors, have evaded discovery in bulk intrinsic superconductors so far. the recently discovered topological metal β-pdBi 2 is a unique candidate for tunable bulk topological superconductivity because of its intrinsic superconductivity and spinorbit-coupling. in this work, we provide experimental transport signatures consistent with fullygapped 3D time-reversal-invariant topological superconductivity in K-doped β-pdBi 2. in particular, we find signatures of odd-parity bulk superconductivity via upper-critical field and magnetization measurements-odd-parity pairing can be argued, given the band structure of β-pdBi 2 , to result in 3D topological superconductivity. in addition, Andreev spectroscopy reveals surface states protected by time-reversal symmetry which might be possible evidence of Majorana surface states (Majorana cone). Moreover, we find that the undoped bulk system is a trivial superconductor. Thus, we discover β-pdBi 2 as a unique bulk material that, on doping, can potentially undergo an unprecedented topological quantum phase transition in the superconducting state. According to the traditional Landau-Ginzburg paradigm, states of matter are defined by the symmetries broken in thermal equilibrium that are preserved by the underlying Hamiltonian, and phase transitions acquire universal features that only depend on the symmetries involved and the spatial dimension. However, this definition proves inadequate for topological phases, in which the ground state wavefunction of the bulk system is characterized by a global, topological quantum number which distinguishes it from a conventional phase with the same symmetries 1-3. Naturally, critical points separating these phases fall outside the traditional paradigm as well. The most striking consequence of the non-trivial bulk topology is the presence of robust surface states where the bulk terminates. One of the most celebrated topological phases in condensed matter systems is the time-reversal symmetric strong topological insulator (TI) in three dimensions, which is characterized by a Z 2 topological invariant ν = odd/even 4,5. The surface manifestation of the bulk topology in this phase is the presence of an odd number of pseudo-relativistic, helical surface states (Dirac Cone) that are robust against non-magnetic perturbations. Numerous materials have been predicted to be in this phase, and many of them have been experimentally confirmed. Additionally, several TIs can be tuned into trivial insulators with doping, thus allowing experimental access to the quantum critical point separating them. A close cousin of the topological insulator is the time-reversal symmetric topological superconductor (TSC) in 3D (Class 3D III) 2. Here, the superconducting gap plays the role of the insulating gap of the insulator, the topological invariant is ν ∈ = … Z 0, 1, 2 , and the surface hosts ν helical Majorana fermions (Majorana cone) instead of Dirac fermions (Dirac Cone). The sufficient conditions for 3D time-reversal invariant (TRI) topological superconductivity are: One, the normal state Fermi surfaces enclose an odd number of time-reversal invariant momenta, two, the bulk superconductivity is fully gapped, and three, odd-parity 6,7. Once these conditions are met, the surface states are spanned by robust, helical Majorana surface states. The 2D Majorana surface states also referred to as the Majorana cone (can be regarded as the superconducting analog of the 2D Dirac cone)-and is distinct from the Majorana Zero Mode (MZM). The transport signatures of 2D Majorana surface states are also
Superconducting doped topological materials
Physica C: Superconductivity and its Applications, 2015
Recently, the search for Majorana fermions (MFs) has become one of the most important and exciting issues in condensed matter physics since such an exotic quasiparticle is expected to potentially give rise to unprecedented quantum phenomena whose functional properties will be used to develop future quantum technology. Theoretically, the MFs may reside in various types of topological superconductor materials that is characterized by the topologically protected gapless surface state which are essentially an Andreev bound state. Superconducting doped topological insulators and topological crystalline insulators are promising candidates to harbor the MFs. In this review, we discuss recent progress and understanding on the search of MFs based on timereversal-invariant superconducting topological materials to deepen our understanding and have a better outlook on both the search for and realization of MFs in these systems. We also discuss some advantages of these bulk systems to realize MFs including remarkable superconducting robustness against nonmagnetic impurities.
Possible transport evidence of topological superconductivity in
Nature - Sci. Reports, 2019
Interest in topological states of matter burgeoned over a decade ago with the theoretical prediction and experimental detection of topological insulators, especially in bulk three-dimensional insulators that can be tuned out of it by doping. Their superconducting counterpart, the fully-gapped three-dimensional time-reversal-invariant topological superconductors, have evaded discovery in bulk intrinsic superconductors so far. The recently discovered topological metal β-PdBi2 is a unique candidate for tunable bulk topological superconductivity because of its intrinsic superconductivity and spin-orbit-coupling. In this work, we provide experimental transport signatures consistent with fully-gapped 3D time-reversal-invariant topological superconductivity in K-doped β-PdBi2. In particular, we find signatures of odd-parity bulk superconductivity via upper-critical field and magnetization measurements— odd-parity pairing can be argued, given the band structure of β-PdBi2, to result in 3D topological superconductivity. In addition, Andreev spectroscopy reveals surface states protected by time-reversal symmetry which might be possible evidence of Majorana surface states (Majorana cone). Moreover, we find that the undoped bulk system is a trivial superconductor. Thus, we discover β-PdBi2 as a unique bulk material that, on doping, can potentially undergo an unprecedented topological quantum phase transition in the superconducting state.
Spin-orbital ground states of superconducting doped topological insulators: A Majorana platform
Physical Review B, 2011
The Bi2Se3 class of topological insulators has recently been shown to undergo a superconducting transition upon hole or electron doping (Cux-Bi2Se3 with TC =3.8 o K and Pdx-Bi2Te3 with TC =5 o K), raising the possibilities that these are the first known "topological superconductors" or realizes a superconducting state that can be potentially used as Majorana platforms (L. A. Wray et.al., Nature Phys. 6, 855-859 (2010)). We use angle resolved photoemission spectroscopy to examine the full details of the spin-orbital groundstates of these materials including Bi2Te3, observing that the spin-momentum locked topological surface states remain well defined and non-degenerate with respect to bulk electronic states at the Fermi level in the optimally doped superconductor and obtaining their experimental Fermi energies. The implications of this unconventional surface (that undergoes superconducting at lower temperatures) topology are discussed, and we also explore the possibility of realizing the same topology in superconducting variants of Bi2Te3 (with TC ∼ 5 o K). Characteristics of the experimentally measured three dimensional bulk states are examined in detail for these materials with respect to the superconducting state and topological properties, showing that a single Majorana fermion zero mode is expected to be bound at each superconducting vortex on the surface. Systematic measurements also reveal intriguing renormalization and charge correlation instabilities of the surface-localized electronic modes.
Pressure-induced superconductivity in topological parent compound Bi 2 Te 3
Proceedings of the National Academy of Sciences, 2010
We report a successful observation of pressure-induced superconductivity in a topological compound Bi 2 Te 3 with T c of ∼3 K between 3 to 6 GPa. The combined high-pressure structure investigations with synchrotron radiation indicated that the superconductivity occurred at the ambient phase without crystal structure phase transition. The Hall effects measurements indicated the hole-type carrier in the pressure-induced superconducting Bi 2 Te 3 single crystal. Consequently, the first-principles calculations based on the structural data obtained by the Rietveld refinement of X-ray diffraction patterns at high pressure showed that the electronic structure under pressure remained topologically nontrivial. The results suggested that topological superconductivity can be realized in Bi 2 Te 3 due to the proximity effect between superconducting bulk states and Dirac-type surface states. We also discuss the possibility that the bulk state could be a topological superconductor.
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Targeted covalent modification is assuming consolidated importance in drug discovery. In this context, the electrophilic tuning of redox-dependent cell signaling is attracting major interest, as it opens prospect for treating numerous pathologic conditions. Herein, we discuss the rationale and the issues of electrophile-based approaches, focusing on the transcriptional Nrf2-Keap1 pathway as a test case. We also highlight relevant medicinal chemistry strategies researchers have devised to meet the ambitious goal, dwelling on the investigational and therapeutic potential of modulating redox-signaling networks through regulatory cysteine switches.
The Journal of Physical Chemistry Letters, 2021
We report on structural and electronic properties of superconducting nano-hybrids made of Pb grown in the ultrahigh vacuum on the atomically clean surface of single crystals of topological Bi 2 Te 3. In-situ Scanning Tunneling Microscopy and Spectroscopy demonstrated that the resulting network is composed of Pbnanoislands dispersed on the surface and linked together by an amorphous atomic layer of Pb, which wets Bi 2 Te 3. As a result, the superconducting state of the system is characterized by a thickness-dependent superconducting gap of Pb-islands and by a very unusual positionindependent proximity gap between them. Furthermore, the data analysis and DFT calcu