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In alphabetical order by speaker last name.

Local Characterization of Superconductivity in Iron Pnictides

Ophir Auslaender, Technion

We use magnetic force microscopy (MFM) to characterize superconductivity across the superconducting dome in P-Ba122, an isovalently doped pnictide that exhibits a unique peak in the penetration depth at optimal doping, as shown previously only in sample-wide measurements. Our local measurements show the peak at optimal doping and a dependence on Tc that is similar on both sides of optimal doping. Near the underdoped edge of the superconducting dome the penetration depth increases sharply, suggesting that superconductivity competes with another phase. In underdoped P-Ba122 we find that superconductivity is enhanced along stripes parallel to twin boundaries. These stripes of enhanced diamagnetic response repel vortices when we cool in finite magnetic field and act as barriers when we drag vortices with the magnetic MFM tip. The stripes disappear when we warm up the sample towards Tc.

Visualizing Majorana bound states using the generalized Majorana polarization

Cristina Bena, CEA Saclay

We study the solutions of generic Hamiltonians exhibiting particle-hole mixing. We show that there exists a universal quantity that can describe locally the Majorana nature of a given state. We name this pseudo-spin like two-component quantity 'Majorana polarization'. We apply this to open two-dimensional Kitaev systems, to one-dimensional topological wires, as well as to open two-dimensional square lattices with general anisotropic Rashba coupling, in the presence of an applied Zeeman field and in the proximity of a superconductor. We show that the Majorana polarization characterization is a necessary and sufficient criterion to test whether a state is a Majorana or not, and we use it to numerically determine the topological phase diagram of various topological systems.

4π­-periodic Josephson Supercurrent in Topological Josephson Junctions

Erwann Bocquillon, Universität Würzburg

In the surface states of a three­-dimensional topological insulator (3D TI), transport is mediated by Dirac­like fermions which exhibit a helical spin polarization. When a 3D TI is coupled to superconductors in a Josephson junction, topologically protected gapless Andreev bound states are predicted. Their energy varies 4π­ periodically with the superconducting phase difference, resulting in a fractional 4π­ periodic Josephson current. The non­ambiguous observation of such gapless states is regarded as an important experimental signature of the unconventional superconductivity in topological insulators, but no robust evidence has been reported yet.

In topological Josephson junctions based on the 3D TI HgTe, we observe a suppression of the first Shapiro step at low frequencies. We attribute it to the existence of a 4π­periodic component in the supercurrent, as an experimental signature of fractional Josephson effect. Recent results obtained on 2D TI also support this interpretation. Our experimental observations thus provide robust indications of the presence of 4π­periodic zero­energy Andreev bound states.

Quantum Phase Transitions in Proximity Superconducting Graphene

Vincent Bouchiat, Institut Néel/CNRS

When subjected to an external field, quantum phases such as a disordered superconductor, terminates into either metallic or insulating states. This remarkable phase transition in the vicinity of a quantum critical point has been intriguing both theoretically and experimentally and is still subject of an active debate in condensed matter physics. Here, we focus on a specific type of  quantum phase transition : the superconductor-to-metal transition in 2D, which  is believed to be driven by quantum phase fluctuations.  The main interesting feature of this transition is that it can be precisely controlled by the conductance of a 2D metallic film, which can be tuned in case of a semiconducting system.

Graphene turns out to offer a highly suitable platform for addressing this study: its surface-exposed and  chemically inert 2D electron/hole gas is open to the intimate coupling of superconductors, giving rise to long range superconducting proximity effect. Meanwhile, gate-tunability of the electron mean free path opens the possibility to study the stability of the superconducting phase against quantum fluctuations. In this work, graphene monolayers were surface-conjugated to arrays of superconducting disk-shaped islands, whose inter-island distances were patterned to be in the quasi-ballistic limit of the underlying 2D electron gas. Arrays can be made on a large range of geometry and density, up to the highly diluted limit with less than 5% surface coverage and few micrometers in between islands. In the lower temperature limit (<100 mK) , despite of the long distance in between islands, a supercurrent was observed among the whole graphene sheet. Interestingly, the superconducting state vanishes exponentially in gate voltage and rests in a metallic state. This peculiar behaviour provide evidence for our recently developed theory, and may provide a hint to the understanding of long-standing issue of “zero-temperature” bosonic metallic state observed earlier in a many systems.

Non-Abelian gauge theory description of (dynamical) spin-orbit coupling effects in Fermi systems

Cosimo Gorini, Universität Regensburg

Spin-orbit coupling heavily influences the dynamics of charge carriers in a solid, where its strength can be enhanced by orders of magnitude as compared to the vacuum.  Remarkable consequences are phenomena such as the spin Hall and Edelstein effects, where spin currents and polarizations are generated by purely electrical means.  The intricacies of such rich spin-charge coupled dynamics can be described within a non-Abelian gauge theory approach, based on Keldysh non-equilibrium formalism.  Thanks to a symmetric treatment of spin and charge degrees of freedom, and the removal of ambiguities related to spin non-conservation in the presence of spin-orbit coupling, a physically transparent picture is achieved.  Furthermore, the non-Abelian language, by virtue of its universal character, has the advantage of treating on the same footing standard spin-orbit interaction in solid state systems and exotic forms of (pseudo) spin-orbit coupling which arise, or can be engineered, in different contexts.

Attraction by Repulsion: Paring Electrons using Electrons

Shahal Ilani, Weizmann Institute of Science

One of the fundamental properties of electrons is their mutual Columbic repulsion. If electrons are placed in a solid, however, this basic property may change. A famous example is that of superconductors, where coupling to lattice vibrations makes electrons attractive and leads to the formation of bound pairs. But what if all the degrees of freedom in the solid are electronic? Is it possible to make electrons attract each other only by their repulsion to other electrons? Such an ‘excitonic’ mechanism for attraction was proposed fifty years ago by William Little, with the hope that it could lead to better and more exotic superconductivity. Yet, despite many efforts to synthesize materials that possess this unique property, to date there is still no evidence for electronic-based attraction. In this talk I will present our recent experiments that observe this unusual electronic attraction using a different, bottom-up approach. Our experiments are based on a new generation of quantum devices made from pristine carbon nanotubes, combined with precision cryogenic manipulation. Using this setup we can now assemble the fundamental building block of the excitonic attraction and demonstrate that two electrons that naturally repel each other can be made attractive using an independent electronic system as the binding glue. I will discuss the lessons learned from these experiments on what is achievable with plain electrostatics, and on the possibility to use the observed mechanism for creating exotic states of matter.

Signatures of Majorana Zero Modes in Current Correlations, Interference and Charge Sensors

Yuval Oreg, Weizmann Institute of Science

After the first experiments on the observation of Majorana Bound States at zero energy were published it became clear that there might be other scenarios exhibiting zero-bias-conductance-peak which do not have necessarily a Majorana bound state. It is crucial to find different probes that will give additional evidences for the existence of a Majorana bound state. In this talk I will discuss three such probes: (i) We study current correlations in a T-junction composed of a grounded topological superconductor and of two normal-metal leads. We show that the existence of an isolated Majorana zero mode in the junction dictates a universal behavior for the cross correlation of the currents through the two normal-metal leads of the junction. This behavior is robust in the presence of disorder and multiple transverse channels, and persists at finite temperatures. In contrast, an accidental low-energy Andreev bound state gives rise to non-universal behavior of the cross correlation. (ii) The visibility of an interference experiment through a Majorana bound state always increases with the temperature (iii) While the tunneling DOS of coupled Majoranas is concentrated at the edges of the wire their charge is uniformly distributed.

Sequential tunnelling in InAs nanowires

Alexander Palevski (Tel Aviv)

We have studied the electronic transport in InAs one dimensional quantum wires. We have observed that in sufficiently disordered wires the electronic transport is dominated by so called Coulomb blockade regime, where the conductivity can be well described by the tunneling through the dot embedded between the one dimensional Luttinger liquids. The conductance peaks reduce their values at low temperature in contrast with the Coulomb blockade peaks reported so far in all the experiments in other material systems. This phenomenon was predicted theoretically (A. Furusaki, Phys. Rev. B 57, 7141 (1998)) for the materials with the Luttinger interaction parameter. By fitting the data to the theory, we found that in InAs wires the value of.In my talk, I will give few possible explanations why the effective Luttinger parameter  in the InAs wires is smaller than the one observed in other 1D systems, like Carbon nanotubes, or GaAs quantum wires reported so far.

Current at a distance and resonant transparency in Weyl semi-metals

Ady Stern, Weizmann Institute of Science

Surface Fermi arcs are the most prominent manifestation of the topological nature of Weyl semimetals. In the presence of a static magnetic field oriented perpendicular to the sample surface, their existence leads to unique inter-surface cyclotron orbits. We propose two experiments which directly probe the Fermi arcs: a magnetic field dependent non-local DC voltage and sharp resonances in the transmission of electromagnetic waves at frequencies controlled by the field. We show that these experiments do not rely on quantum mechanical phase coherence, which renders them far more robust and experimentally accessible than quantum effects. We also comment on the applicability of these ideas to Dirac semimetals.

Strong interface induced spin-orbit coupling in graphene-on-WS₂ heterostructure

Zhe Wang, Université de Genève

Disorder-free graphene is the first predicted topological insulator, whose characteristics have not been observed experimentally because the strength of intrinsic spin-orbit interaction (SOI) in graphene is too weak. Here we explore this issue by exploiting interfacial interactions in graphene-on-WS₂ heterostructure, whose basic transport characteristics confirm the high device quality. Robust weak anti-localization effect is observed at all accessible carrier density range down to 250 mK, which is the first time in graphene and constitutes unambiguous evidence of strong SOI induced in graphene. The extracted spin-relaxation time is 2-3 order shorter than that in graphene on SiO₂ or hexagonal boron nitride (hBN) substrates, and is comparable to the intervalley scattering time. The experimental findings are consistent with first-principle electronic structure calculations, which shows interfacial interactions with WS₂ substrate indeed induce strong SOI in graphene. Furthermore, the same analysis also suggests opening of a gap due to SOI which can become a two-dimensional topological insulator. Our work therefore clearly demonstrates strong SOI induced in high-quality graphene using interfacial interactions with WS₂, and opens a possible new route to access topological states of matter in graphene-based systems.