scholarly journals Nearly quantized conductance plateau of vortex zero mode in an iron-based superconductor

Science ◽  
2019 ◽  
pp. eaax0274 ◽  
Author(s):  
Shiyu Zhu ◽  
Lingyuan Kong ◽  
Lu Cao ◽  
Hui Chen ◽  
Michał Papaj ◽  
...  
Keyword(s):  
Bound States ◽  
Zero Mode ◽  
Finite Energy ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Tunneling Conductance ◽  
Zero Energy ◽  
Tunnel Coupling ◽  
Topological Quantum ◽  
The Continuum

Majorana zero modes (MZMs) are spatially-localized zero-energy fractional quasiparticles with non-Abelian braiding statistics that hold promise for topological quantum computing. Owing to the particle-antiparticle equivalence, MZMs exhibit quantized conductance at low temperature. By utilizing variable-tunnel-coupled scanning tunneling spectroscopy, we study tunneling conductance of vortex bound states on FeTe0.55Se0.45 superconductors. We report observations of conductance plateaus as a function of tunnel coupling for zero-energy vortex bound states with values close to or even reaching the 2e2/h quantum conductance (here e is the electron charge and h is Planck’s constant). In contrast, no plateaus were observed on either finite energy vortex bound states or in the continuum of electronic states outside the superconducting gap. This behavior of the zero-mode conductance supports the existence of MZMs in FeTe0.55Se0.45.

scholarly journals Zero-energy bound states in the high-temperature superconductors at the two-dimensional limit

2020 ◽  
Vol 6 (13) ◽  
pp. eaax7547 ◽  
Author(s):  
Chaofei Liu ◽  
Cheng Chen ◽  
Xiaoqiang Liu ◽  
Ziqiao Wang ◽  
Yi Liu ◽  
...  
Keyword(s):  
High Temperature ◽  
Bound States ◽  
High Temperature Superconductors ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Two Dimensional ◽  
Zero Energy ◽  
Topological Superconductors ◽  
Topological Quantum ◽  
Energy Bound

Majorana zero modes (MZMs) that obey the non-Abelian statistics have been intensively investigated for potential applications in topological quantum computing. The prevailing signals in tunneling experiments “fingerprinting” the existence of MZMs are the zero-energy bound states (ZEBSs). However, nearly all of the previously reported ZEBSs showing signatures of the MZMs are observed in difficult-to-fabricate heterostructures at very low temperatures and additionally require applied magnetic field. Here, by using in situ scanning tunneling spectroscopy, we detect the ZEBSs upon the interstitial Fe adatoms deposited on two different high-temperature superconducting one-unit-cell iron chalcogenides on SrTiO3(001). The spectroscopic results resemble the phenomenological characteristics of the MZMs inside the vortex cores of topological superconductors. Our experimental findings may extend the MZM explorations in connate topological superconductors toward an applicable temperature regime and down to the two-dimensional (2D) limit.

scholarly journals A new Majorana platform in an Fe-As bilayer superconductor

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Wenyao Liu ◽  
Lu Cao ◽  
Shiyu Zhu ◽  
Lingyuan Kong ◽  
Guangwei Wang ◽  
...  
Keyword(s):  
Bound States ◽  
Zero Mode ◽  
Surface States ◽  
Scanning Tunneling ◽  
Tunneling Microscopy ◽  
Iron Pnictide ◽  
Iron Chalcogenide ◽  
Zero Modes ◽  
Topological Quantum ◽  
Iron Chalcogenide Superconductors

Abstract Iron-chalcogenide superconductors have emerged as a promising Majorana platform for topological quantum computation. By combining topological band and superconductivity in a single material, they provide significant advantage to realize isolated Majorana zero modes. However, iron-chalcogenide superconductors, especially Fe(Te,Se), suffer from strong inhomogeneity which may hamper their practical application. In addition, some iron-pnictide superconductors have been demonstrated to have topological surface states, yet no Majorana zero mode has been observed inside their vortices, raising a question of universality about this new Majorana platform. In this work, through angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy measurement, we identify Dirac surface states and Majorana zero modes, respectively, for the first time in an iron-pnictide superconductor, CaKFe4As4. More strikingly, the multiple vortex bound states with integer-quantization sequences can be accurately reproduced by our model calculation, firmly establishing Majorana nature of the zero mode.

scholarly journals Evidence for Majorana bound states in an iron-based superconductor

Science ◽  
2018 ◽  
Vol 362 (6412) ◽  
pp. 333-335 ◽  
Author(s):  
Dongfei Wang ◽  
Lingyuan Kong ◽  
Peng Fan ◽  
Hui Chen ◽  
Shiyu Zhu ◽  
...  
Keyword(s):  
Bound States ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Vortex Center ◽  
Zero Energy ◽  
Zero Bias ◽  
Majorana Bound States ◽  
Topological Materials ◽  
Field Temperature ◽  
Iron Based

The search for Majorana bound states (MBSs) has been fueled by the prospect of using their non-Abelian statistics for robust quantum computation. Two-dimensional superconducting topological materials have been predicted to host MBSs as zero-energy modes in vortex cores. By using scanning tunneling spectroscopy on the superconducting Dirac surface state of the iron-based superconductor FeTe0.55Se0.45, we observed a sharp zero-bias peak inside a vortex core that does not split when moving away from the vortex center. The evolution of the peak under varying magnetic field, temperature, and tunneling barrier is consistent with the tunneling to a nearly pure MBS, separated from nontopological bound states. This observation offers a potential platform for realizing and manipulating MBSs at a relatively high temperature.

scholarly journals Controlling in-gap end states by linking nonmagnetic atoms and artificially-constructed spin chains on superconductors

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Lucas Schneider ◽  
Sascha Brinker ◽  
Manuel Steinbrecher ◽  
Jan Hermenau ◽  
Thore Posske ◽  
...  
Keyword(s):  
Bound States ◽  
Magnetic Order ◽  
Tight Binding ◽  
Spectral Weight ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Topological Quantum ◽  
Topological Quantum Computation ◽  
Gap States ◽  
Spatial Transition

Abstract Chains of magnetic atoms with either strong spin-orbit coupling or spiral magnetic order which are proximity-coupled to superconducting substrates can host topologically non-trivial Majorana bound states. The experimental signature of these states consists of spectral weight at the Fermi energy which is spatially localized near the ends of the chain. However, topologically trivial Yu-Shiba-Rusinov in-gap states localized near the ends of the chain can lead to similar spectra. Here, we explore a protocol to disentangle these contributions by artificially augmenting a candidate Majorana spin chain with orbitally-compatible nonmagnetic atoms. Combining scanning tunneling spectroscopy with ab-initio and tight-binding calculations, we realize a sharp spatial transition between the proximity-coupled spiral magnetic order and the non-magnetic superconducting wire termination, with persistent zero-energy spectral weight localized at either end of the magnetic spiral. Our findings open a new path towards the control of the spatial position of in-gap end states, trivial or Majorana, via different chain terminations, and the realization of designer Majorana chain networks for demonstrating topological quantum computation.

scholarly journals Majorana zero modes in impurity-assisted vortex of LiFeAs superconductor

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Lingyuan Kong ◽  
Lu Cao ◽  
Shiyu Zhu ◽  
Michał Papaj ◽  
Guangyang Dai ◽  
...  
Keyword(s):  
Quantum Computation ◽  
Zero Mode ◽  
Scanning Tunneling ◽  
Homogeneous Material ◽  
Dirac Fermions ◽  
Tunneling Microscopy ◽  
Zero Modes ◽  
Topological Quantum ◽  
One Step ◽  
Topological Quantum Computation

AbstractThe iron-based superconductor is emerging as a promising platform for Majorana zero mode, which can be used to implement topological quantum computation. One of the most significant advances of this platform is the appearance of large vortex level spacing that strongly protects Majorana zero mode from other low-lying quasiparticles. Despite the advantages in the context of physics research, the inhomogeneity of various aspects hampers the practical construction of topological qubits in the compounds studied so far. Here we show that the stoichiometric superconductor LiFeAs is a good candidate to overcome this obstacle. By using scanning tunneling microscopy, we discover that the Majorana zero modes, which are absent on the natural clean surface, can appear in vortices influenced by native impurities. Our detailed analysis reveals a new mechanism for the emergence of those Majorana zero modes, i.e. native tuning of bulk Dirac fermions. The discovery of Majorana zero modes in this homogeneous material, with a promise of tunability, offers an ideal material platform for manipulating and braiding Majorana zero modes, pushing one step forward towards topological quantum computation.

scholarly journals Scalable Majorana vortex modes in iron-based superconductors

2020 ◽  
Vol 6 (9) ◽  
pp. eaay0443 ◽  
Author(s):  
Ching-Kai Chiu ◽  
T. Machida ◽  
Yingyi Huang ◽  
T. Hanaguri ◽  
Fu-Chun Zhang
Keyword(s):  
Tight Binding ◽  
Three Dimensional ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Zero Energy ◽  
Binding Model ◽  
Zero Bias ◽  
Iron Based Superconductors ◽  
Iron Based ◽  
Important Indication

The iron-based superconductor FeTexSe1−x is one of the material candidates hosting Majorana vortex modes residing in the vortex cores. It has been observed by recent scanning tunneling spectroscopy measurement that the fraction of vortex cores having zero-bias peaks decreases with increasing magnetic field on the surface of FeTexSe1−x. The hybridization of two Majorana vortex modes cannot simply explain this phenomenon. We construct a three-dimensional tight-binding model simulating the physics of over a hundred Majorana vortex modes in FeTexSe1−x. Our simulation shows that the Majorana hybridization and disordered vortex distribution can explain the decreasing fraction of the zero-bias peaks observed in the experiment; the statistics of the energy peaks off zero energy in our Majorana simulation are in agreement with the experiment. These agreements lead to an important indication of scalable Majorana vortex modes in FeTexSe1−x. Thus, FeTexSe1−x can be one promising platform having scalable Majorana qubits for quantum computing.

scholarly journals Moiréless correlations in ABCA graphene

2021 ◽  
Vol 118 (4) ◽  
pp. e2017366118 ◽  
Author(s):  
Alexander Kerelsky ◽  
Carmen Rubio-Verdú ◽  
Lede Xian ◽  
Dante M. Kennes ◽  
Dorri Halbertal ◽  
...  
Keyword(s):  
Theoretical Calculations ◽  
Van Der Waals ◽  
Bilayer Graphene ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Van Hove Singularity ◽  
Natural Interfaces ◽  
Simple Material ◽  
Electronic Band ◽  
Topological Quantum

Atomically thin van der Waals materials stacked with an interlayer twist have proven to be an excellent platform toward achieving gate-tunable correlated phenomena linked to the formation of flat electronic bands. In this work we demonstrate the formation of emergent correlated phases in multilayer rhombohedral graphene––a simple material that also exhibits a flat electronic band edge but without the need of having a moiré superlattice induced by twisted van der Waals layers. We show that two layers of bilayer graphene that are twisted by an arbitrary tiny angle host large (micrometer-scale) regions of uniform rhombohedral four-layer (ABCA) graphene that can be independently studied. Scanning tunneling spectroscopy reveals that ABCA graphene hosts an unprecedentedly sharp van Hove singularity of 3–5-meV half-width. We demonstrate that when this van Hove singularity straddles the Fermi level, a correlated many-body gap emerges with peak-to-peak value of 9.5 meV at charge neutrality. Mean-field theoretical calculations for model with short-ranged interactions indicate that two primary candidates for the appearance of this broken symmetry state are a charge-transfer excitonic insulator and a ferrimagnet. Finally, we show that ABCA graphene hosts surface topological helical edge states at natural interfaces with ABAB graphene which can be turned on and off with gate voltage, implying that small-angle twisted double-bilayer graphene is an ideal programmable topological quantum material.

scholarly journals Recent progresses of quantum confinement in graphene quantum dots

2021 ◽  
Vol 17 (3) ◽  
Author(s):  
Si-Yu Li ◽  
Lin He
Keyword(s):  
Quantum Dots ◽  
Quantum Confinement ◽  
Bound States ◽  
Berry Phase ◽  
Graphene Quantum Dots ◽  
Bound State ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Dirac Fermions ◽  
Tunneling Microscopy

AbstractGraphene quantum dots (GQDs) not only have potential applications on spin qubit, but also serve as essential platforms to study the fundamental properties of Dirac fermions, such as Klein tunneling and Berry phase. By now, the study of quantum confinement in GQDs still attract much attention in condensed matter physics. In this article, we review the experimental progresses on quantum confinement in GQDs mainly by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Here, the GQDs are divided into Klein GQDs, bound-state GQDs and edge-terminated GQDs according to their different confinement strength. Based on the realization of quasi-bound states in Klein GQDs, external perpendicular magnetic field is utilized as a manipulation approach to trigger and control the novel properties by tuning Berry phase and electron-electron (e-e) interaction. The tip-induced edge-free GQDs can serve as an intuitive mean to explore the broken symmetry states at nanoscale and single-electron accuracy, which are expected to be used in studying physical properties of different two-dimensional materials. Moreover, high-spin magnetic ground states are successfully introduced in edge-terminated GQDs by designing and synthesizing triangulene zigzag nanographenes.

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