Enrico Giannini

Gate-tuned normal and superconducting transport at the surface of a topological insulator

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 Bi2Se3 single crystals, equipped with a gate electrode. In the presence of perpendicular magnetic field B, sweeping the gate voltage enables us to observe the filling 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 filling. Our results demonstrate how gated nano-electronic devices give control over normal and superconducting transport of Dirac fermions at an individual surface of a three-dimensional topological insulators.

Indirect-to-Direct Band Gap Crossover in Few-Layer MoTe2

We study the evolution of the band gap structure in few-layer MoTe2 crystals, by means of low-temperature microreflectance (MR) and temperature-dependent photoluminescence (PL) measurements. The analysis of the measurements indicate that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions associated with recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe2 behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer, decreases slightly for trilayer, and it is significantly lower in the tetralayer. The analysis of this behavior and of all our experimental observations is fully consistent with mono and bilayer MoTe2 being direct band gap semiconductors with tetralayer MoTe2 being an indirect gap semiconductor and with trilayers having nearly identical direct and indirect gaps. This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides for which monolayers are found to be direct band gap semiconductors, and thicker layers have indirect band gaps that are significantly smaller (by hundreds of meV) than the direct gap. We discuss the relevance of our findings for experiments of fundamental interest and possible future device applications.

Effect of Fe excess on structural, magnetic and superconducting properties of single-crystalline Fe1+xTe1−ySey

Abstract Single crystals of Fe1+xTe1−ySey have been grown with a controlled Fe excess and Se doping, and the crystal structure has been refined for various compositions. The systematic investigation of magnetic and superconducting properties as a function of the structural parameters shows how the material can be driven into various ground states, depending on doping and the structural modifications. Our results prove that the occupation of the additional Fe site, Fe2, enhances the spin localization. By reducing the excess Fe, the antiferromagnetic ordering is weakened, and the superconducting ground state is favored. We have found that both Fe excess and Se doping in synergy determine the properties of the material and an improved 3-dimensional phase diagram is proposed.

Surface transport and band gap structure of exfoliated 2H-MoTe2 crystals

Semiconducting transition metal dichalcogenides (TMDs) have emerged as materials that can be used to realize two-dimensional (2D) crystals possessing rather unique transport and optical properties. Most research has so far focused on sulfur and selenium compounds, while tellurium-based materials have attracted little attention so far. As a first step in the investigation of Te-based semiconducting TMDs in this context, we have studied MoTe2 crystals with thicknesses above 4 nm, focusing on surface transport and a quantitative determination of the gap structure. Using ionic-liquid gated transistors, we show that ambipolar transport at the surface of the material is reproducibly achieved, with hole and electron mobility values between 10 and 30 cm2 V−1s−1 at room temperature. The gap structure is determined through three different techniques: ionic-liquid gated transistors and scanning tunneling spectroscopy, which allow the measurement of the indirect gap (Eind), and optical transmission spectroscopy on crystals of different thickness, which enables the determination of both the direct (Edir) and the indirect gap. We find that at room temperature Eind = 0.88 eV and Edir = 1.02 eV. Our results suggest that thin MoTe2 layers may exhibit a transition to a direct gap before mono-layer thickness. They should also drastically extend the range of direct gaps accessible in 2D semiconducting TMDs.

Observation of large topologically trivial Fermi arcs in the candidate type-II Weyl semimetal WT e 2

The authors present here several advances towards a comprehensive understanding of WTe

Direct observation of the Bi,Pb(2223) phase formation inside Ag-sheathed tapes and quantitative secondary phase analysis by means of in situ high-temperature neutron diffraction

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Direct role of structural dynamics in electron-lattice coupling of superconducting cuprates

. Using microfocus laser ARPES, they resolve for the first time the distinct electronic structure of both inequivalent top and bottom (001) surfaces of WTe

Fermi Arcs and Their Topological Character in the Candidate Type-II Weyl Semimetal MoTe 2

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The influence of thermal precompression on the mechanical behaviour of Ag-sheathed (Bi,Pb)2223 tapes with different matrices

. Moreover, they demonstrate for the first time the presence of large surface state Fermi arcs on both surfaces. Using surface electronic structure calculations, they further demonstrate that these Fermi arcs are topologically trivial and that their existence is independent of the presence of type-II Weyl points in the bulk band structure. Contrary to common belief, the observation of surface state Fermi arcs is thus not suitable to robustly identify a type-II Weyl semimetal. Finally, the authors use the observation of Fermi arcs and distinct top and bottom surfaces to clarify the controversial bulk electronic structure of WTe

Charge carrier interaction with a purely electronic collective mode: plasmarons and the infrared response of elemental bismuth.

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