17 settembre 2012

Tra il semiconduttore e il superconduttore c'è di mezzo lo scotch. Uno studio pubblicato su Nature Communications l'11 Settembre 2012.

Un gruppo di ricerca internazionale coordinato da Ken Burch (fisico dell'Università di Toronto, Ken Burch, nella foto di Diana Tyszko) ha scoperto che i semi-conduttori noti con il nome di "isolanti topologici" (semiconduttori all'interno e conduttori all'esterno) possono manifestare superconduttività se trattati opportunamente. I superconduttori sono materiali che conducono l'elettricità senza perdite di energia. Il problema è che questo accade solo a temperature estremamente basse. Solo alcuni composti di ferro, rame e ossigenomanifestano  proprietà superconduttive a temperature più elevate. La scoperta è molto importante dato che fino a ora si riteneva che questi materiali non potessero essere integrati con i semiconduttori. 
Lo studio, pubblicato l'11 Settembre 2012 su Nature Communications, mostra che l'impiego di normalissimo scotch biadesivo per le giunzioni tra semiconduttori e superconduttori induce superconduttività in questi ultimi.
La ricerca è finanziata da: Ministero per l’Innovazione dell’Ontario, Natural Sciences and Engineering Research Council of Canada, Canadian Foundation for Innovation, National Science Foundation.
Andrea Mameli www.linguaggiomacchina.it 17 Settembre 2012
Proximity-induced high-temperature superconductivity in the topological insulators Bi2Se3 and Bi2Te3
Parisa Zareapour, Alex Hayat, Shu Yang F. Zhao, Michael Kreshchuk, Achint Jain, Daniel C. Kwok, Nara Lee, Sang-Wook Cheong, Zhijun Xu, Alina Yang, G.D. Gu, Shuang Jia, Robert J. Cava & Kenneth S. Burch
Nature Communications 3, 1056. Published 11 September 2012
Interest in the superconducting proximity effect has been reinvigorated recently by novel optoelectronic applications as well as by the possible emergence of the elusive Majorana fermion at the interface between topological insulators and superconductors. Here we produce high-temperature superconductivity in Bi2Se3 and Bi2Te3 via proximity to Bi2Sr2CaCu2O8+δ, to access higher temperature and energy scales for this phenomenon. This was achieved by a new mechanical bonding technique that we developed, enabling the fabrication of high-quality junctions between materials, unobtainable by conventional approaches. We observe proximity-induced superconductivity in Bi2Se3 and Bi2Te3 persisting up to at least 80 K—a temperature an order of magnitude higher than any previous observations. Moreover, the induced superconducting gap in our devices reaches values of 10 mV, significantly enhancing the relevant energy scales. Our results open new directions for fundamental studies in condensed matter physics and enable a wide range of applications in spintronics and quantum computing.

Figure 1: Device fabrication and measurement set-up.

Atomic force microscope (AFM) image of the crystal surface of Bi2Se3. The scale bar corresponds to 2 μm. (b) Larger area AFM scan of the Bi2Te3 sample, demonstrating the vertical inhomogeneity of the surface is limited to ±2 unit cells. The scale bar corresponds to 3 μm. Junction fabrication technique: (c) Bi-2212 (BSCCO) crystal is cleaved using scotch tape. (d) Bi2Se3 (or Bi2Te3) is sandwiched between glass slides with double-sided tapes and the top glass slide is lifted off, cleaving a flat surface. (e) The Bi2Se3 (or Bi2Te3) is transferred to a Cu sample holder, and the cleaved Bi-2212 crystal is applied to Bi2Se3 or Bi2Te3 using GE varnish at the corners. (f) Contacts are made with Ag epoxy or evaporated Au/Ti. (g) Experimental set-up: four-point DC current-voltage and AC differential conductance measurements performed down to 4.5 K, using a liquid He-flow cryostat with lock-in amplifiers. DC bias from the power supply is combined with the AC signal from the voltage lock-in amplifier in a transformer-based adder.

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