Orientador: Iakov Veniaminovitch Kopelevitch / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-22T01:33:07Z (GMT). No. of bitstreams: 1 Baring_LuisAugustoGomes_D.pdf: 21081310 bytes, checksum: 275f0ba5ff80d6f9f19f53cf8316e1a6 (MD5) Previous issue date: 2012 / Resumo: No presente trabalho estudamos os semimetais bismuto Bi, antimônio Sb e Bi1-xSbx, materiais com propriedades topologicamente não triviais. Observamos a ocorrência de supercondutividade intrínseca em bismuto com TC »= 8:5K. Construímos, a partir dos dados de magnetização e resistência, o diagrama de fase do campo crítico H versus a temperatura T. Esse diagrama de fase, pode ser ajustado segundo modelos da literatura válidos para supercondutividade granular. Detectamos, no bismuto, o aumento da corrente Josephson e acoplamento intergranular no limite quântico devidos à quantização de Landau. Isso se manifesta como uma supercondutividade reentrante. Foi também encontrada transição tipo metal-isolante induzida por campo magnético em todos os materiais estudados. O diagrama de fase H versus T mostra uma extraordinária semelhança entre os três materiais. A amostra Bi1-xSbx, com x = 0:052, revelou a ocorrência de transição semimetal-isolante topológico já em campo magnético zero. Fizemos uma comparação com resultados anteriores da literatura, analisando a dependência da temperatura em que ocorre essa transição em relação à concentração de antimônio x e ao campo magnético B e demonstramos a similaridade entre eles. Observamos, também, supercondutividade nos semimetais bismuto, antimônio e no Bi1-xSbx, induzida por dopagem com os metais ouro e índio, e mostramos que a supercondutividade está associada à interface entre os metais e os semimetais. Finalmente, encontramos a indução de supercondutividade mediante a aplicação de campo magnético em bismuto, consistente com a ocorrência de férmions de Majorana na interface entre esse material e a tinta prata usada para os contatos. Tal observação pode ser devida, também, à ocorrência de um estado supercondutor fora do equilíbrio. / Abstract: In this work we studied the semimetals bismuth Bi, antimony Sb and Bi1-xSbx, all of them with non-trivial topologic properties. We observed an intrinsic superconductivity in bismuth, with TC »= 8:5 K. The phasediagram of the critical field H versus the temperature T, based upon the magnetization and resistance data, may be well fitted according to theoretical models valid for granular superconductivity. We also detected, in bismuth, the increase of the Josephson current and interganular coupling in the quantum limit due to Landau quantization. This manifests itself as a reentrant superconducting state. Our results revealed a metal-insulator transition triggered by magnetic field, for all the studied materials. The phase diagram H ¡T shows a striking similarity between them. The sample Bi1-xSbx with x = 0:052 demonstrated a semimetal-insulator transition even at zero field. We compared our results with previous results of other groups and analyzed the temperature dependence of the transition as a function of the antimony amount x and the magnetic field B and demonstrated their similarity. We also observed supeerconductivity in the semimetals bismuth, antimony and Bi1-xSbx, triggered by doping with the metals gold and indium, and showed that the superconductivity is associated to the interface between the metals and the semimetals. Finally, we found the superconductivity induced by the aplication of magnetic field in bismuth, consistent with the Majorana fermions present in the interface between this material and the silver paste contacts. This may also be related to a non-equilibrium superconduting state. / Doutorado / Física / Doutor em Ciências

Die vorliegende Arbeit beschäftigt sich mit der elektrischen Charakterisierung 10 nm dünner Schichten bestehend aus amorphen Ga-Nanoclustern eingebettet in Ga-dotiertes polykristallines Si. Die Herstellung der Schichten geschieht via Ionen-Implantation in Si-Wafer samt anschließender thermischer Ausheilung. Elektrische Transportmessungen in Magnetfeldern von bis zu 50 T zeigen, dass die Schichten durch Variation der Ausheilparameter zwei strukturelle Supraleiter-Isolator-Übergänge durchlaufen. TEM-gestützte Strukturanalysen decken auf, dass den Übergängen eine Gefügetransformation zugrunde liegt, die das Wechselspiel zwischen supraleitender Cluster-Kopplung und kapazitiver Ladungsenergie sowie dem Ausmaß von thermischen und Quantenfluktuationen beeinflusst. Im supraleitenden Regime (Tc = 7 K) wird ein doppelt reentrantes Phänomen beobachtet, bei dem Magnetfelder von mehreren Tesla in anisotroper Form die Supraleitung begünstigen. Eine qualitative Erklärung gelingt via selbstentwickeltem theoretischen Modell basierend auf Phaseslip-Ereignissen für Josephson-Kontakt-Netzwerke. Für Anwendungen im Bereich der Sensor-Technologie und Quanten-Logik werden die Schichten erfolgreich via Fotolithographie und FIB (focused ion beam) mikro- und nanostrukturiert. Dadurch gelingt die erstmalige Beobachtung des Little-Parks-Effektes in einer Nanostruktur aus amorphem Ga. / The following thesis is devoted to the electrical characterization of 10 nm thin layers consisting of amorphous Ga nanoclusters embedded in Ga-doped polycrystalline Si. The preparation of the layers is realized via ion implantation in Si wafers plus subsequent thermal annealing. Electrical-transport measurements in magnetic fields of up to 50 T show that the layers undergo two structural superconductor-insulator transitions upon variation of the annealing parameters. Structural analyzes based on TEM investigations reveal an underlying transformation of the size and distance of the clusters. This influences the interplay of the superconducting cluster coupling and capacitive charging energy as well as the extent of thermal and quantum fluctuations. In the superconducting regime (Tc = 7 K) a double-reentrant phenomenon is observed. Here, magnetic fields of several Tesla facilitate superconductivity in an anisotropic way. A qualitative explanation is given via a self-developed theoretical model based on phase-slip events for Josephson-junction arrays. With respect to applications regarding sensor technology and quantum logic circuits the layers are successfully micro- and nanostructured via photolithography and FIB. This allows for the first observation of the Little-Parks effect in a nanostructure of amorphous Ga.

Die vorliegende Arbeit beschäftigt sich mit der elektrischen Charakterisierung 10 nm dünner Schichten bestehend aus amorphen Ga-Nanoclustern eingebettet in Ga-dotiertes polykristallines Si. Die Herstellung der Schichten geschieht via Ionen-Implantation in Si-Wafer samt anschließender thermischer Ausheilung. Elektrische Transportmessungen in Magnetfeldern von bis zu 50 T zeigen, dass die Schichten durch Variation der Ausheilparameter zwei strukturelle Supraleiter-Isolator-Übergänge durchlaufen. TEM-gestützte Strukturanalysen decken auf, dass den Übergängen eine Gefügetransformation zugrunde liegt, die das Wechselspiel zwischen supraleitender Cluster-Kopplung und kapazitiver Ladungsenergie sowie dem Ausmaß von thermischen und Quantenfluktuationen beeinflusst. Im supraleitenden Regime (Tc = 7 K) wird ein doppelt reentrantes Phänomen beobachtet, bei dem Magnetfelder von mehreren Tesla in anisotroper Form die Supraleitung begünstigen. Eine qualitative Erklärung gelingt via selbstentwickeltem theoretischen Modell basierend auf Phaseslip-Ereignissen für Josephson-Kontakt-Netzwerke. Für Anwendungen im Bereich der Sensor-Technologie und Quanten-Logik werden die Schichten erfolgreich via Fotolithographie und FIB (focused ion beam) mikro- und nanostrukturiert. Dadurch gelingt die erstmalige Beobachtung des Little-Parks-Effektes in einer Nanostruktur aus amorphem Ga. / The following thesis is devoted to the electrical characterization of 10 nm thin layers consisting of amorphous Ga nanoclusters embedded in Ga-doped polycrystalline Si. The preparation of the layers is realized via ion implantation in Si wafers plus subsequent thermal annealing. Electrical-transport measurements in magnetic fields of up to 50 T show that the layers undergo two structural superconductor-insulator transitions upon variation of the annealing parameters. Structural analyzes based on TEM investigations reveal an underlying transformation of the size and distance of the clusters. This influences the interplay of the superconducting cluster coupling and capacitive charging energy as well as the extent of thermal and quantum fluctuations. In the superconducting regime (Tc = 7 K) a double-reentrant phenomenon is observed. Here, magnetic fields of several Tesla facilitate superconductivity in an anisotropic way. A qualitative explanation is given via a self-developed theoretical model based on phase-slip events for Josephson-junction arrays. With respect to applications regarding sensor technology and quantum logic circuits the layers are successfully micro- and nanostructured via photolithography and FIB. This allows for the first observation of the Little-Parks effect in a nanostructure of amorphous Ga.