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Intrinsic tunneling spectroscopy of low-Tc cuprate superconductors: Pseudogap, fluctuations and electrical doping
Stockholm University, Faculty of Science, Department of Physics. (Experimental Condensed Matter Physics)
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, intrinsic tunneling and magnetic transport measurements of the cuprate superconductor Bi2+xSr2-yCuO6+δ (Bi-2201) are presented. The compound is characterized by a single-CuO2 plane, has a very low critical temperature Tc ~ 4K and a relatively low upper critical field Hc2 ~ 10T. Therefore, superconductivity can be suppressed even at low temperatures, which opens the possibility to study normal-state properties with a relatively low influence of thermal fluctuations. Understanding the mechanism of Tc suppression in this compound might help understanding the mechanism of high Tc in other cuprates. The measurements where performed on small mesa structures with areas less than a square micrometer, which allow for measurements with nearly no artifacts caused by heating or crystal defects.

This series of experiments demonstrate that all superconducting characteristics are reduced inproportion to Tc , but the corresponding c-axis pseudogap characteristics remains similar to that in high-Tc Bi-2212 and Bi-2223 compounds with 20-30 times larger Tc. This large disparity reveals that the pseudogap is not connected to superconductivity, instead it possibly represents a different spin-singlet order that is universal to all cuprates. Furthermore, the experiments show that the thermal fluctuation region is not extraordinary large in this compound and is therefore not a cause of the low Tc, which leads to the assumption that this is caused by a weaker coupling, resulting in a smaller Cooper pair energy.

In addition, the change of mesa properties by electrical doping is studied on this compound, as well as on Bi-2212. The superconducting and normal state properties can be altered in a similar way than by conventional doping, without the use of different samples. This makes doping dependent studies much simpler and helps to understand the puzzling behavior of existing states within these compounds.

Place, publisher, year, edition, pages
Stockholm University, 2014. , 53 p.
Keyword [en]
Bi-2201, Bi-2212, low-Tc, cuprates, intrinsic tunneling, transport measurements, pseudogap, fluctuations, electrical doping
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-108236OAI: oai:DiVA.org:su-108236DiVA: diva2:755983
Presentation
2014-10-15, A2:1041, Albanova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2014-11-04 Created: 2014-10-15 Last updated: 2014-11-04Bibliographically approved
List of papers
1. Electron-tunneling measurements of low-T-c single-layer Bi2+xSr2-yCuO6+delta: Evidence for a scaling disparity between superconducting and pseudogap states
Open this publication in new window or tab >>Electron-tunneling measurements of low-T-c single-layer Bi2+xSr2-yCuO6+delta: Evidence for a scaling disparity between superconducting and pseudogap states
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2012 (English)In: Physical Review B, ISSN 2469-9950, Vol. 86, no 21, 214506Article in journal (Refereed) Published
Abstract [en]

We experimentally study intrinsic tunneling and high magnetic field (up to 65 T) transport characteristics of the single-layer cuprate Bi2+xSr2-yCuO6+delta, with a very low superconducting critical temperature T-c less than or similar to 4 K. It is observed that the superconducting gap, the collective bosonic mode energy, the upper critical field, and the fluctuation temperature range are scaling down with T-c, while the corresponding pseudogap characteristics remain the same as in high-T-c cuprates with 20 to 30 times higher T-c. The observed disparity of the superconducting and pseudogap scales clearly reveals their different origins.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-86498 (URN)10.1103/PhysRevB.86.214506 (DOI)000312290600002 ()
Note

AuthorCount:10;

Available from: 2013-01-14 Created: 2013-01-14 Last updated: 2016-05-11Bibliographically approved
2. Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures
Open this publication in new window or tab >>Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures
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2012 (English)In: Physical Review B, ISSN 2469-9950, Vol. 85, no 14, 144519Article in journal (Refereed) Published
Abstract [en]

Application of a significantly large bias voltage to small Bi2Sr2CaCu2O8+x mesa structures leads to persistent doping of the mesas. Here, we employ this effect for analysis of the doping dependence of the electronic spectra of Bi-2212 single crystals by means of intrinsic tunneling spectroscopy. We are able to controllably and reversibly change the doping state of the same single crystal from underdoped to overdoped state, without changing its chemical composition. It is observed that such physical doping is affecting superconductivity in Bi-2212 similar to chemical doping by oxygen impurities: with overdoping, the critical temperature and the superconducting gap decrease; with underdoping, the c-axis critical current rapidly decreases due to progressively more incoherent interlayer tunneling and the pseudogap rapidly increases, indicative for the presence of the critical doping point. We distinguish two main mechanisms of persistent electric doping: (i) even-in-voltage contribution, attributed to a charge transfer effect, and (ii) odd-in-voltage contribution, attributed to reordering of oxygen vacancies.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-80276 (URN)10.1103/PhysRevB.85.144519 (DOI)000302958900003 ()
Note

AuthorCount:5;

Available from: 2012-09-27 Created: 2012-09-17 Last updated: 2016-05-11Bibliographically approved

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