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Unraveling the cuprate superconductor phase diagram: Intrinsic tunneling spectroscopy and electrical doping
Stockholm University, Faculty of Science, Department of Physics. (Experimental Condensed Matter Physics)ORCID iD: 0000-0002-7553-2816
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

High-temperature superconductors belong to the group of strongly correlated materials. In these compounds, complex repulsive electron interactions and a large number of degrees of freedom lead to a rich variety of states of matter. Exotic phases like the pseudogap, charge-, spin- and pair-density waves, but also the remarkable phenomenon of superconductivity emerge, depending on doping level and temperature. However, up to now it is unclear what exactly causes these states, to what extent they are coexisting or competing, and where their borders in the phase diagram lie. A better understanding could help in finding the mechanism behind high-temperature superconductivity, but would also provide a better insight into the puzzling behavior of strongly correlated materials.

This thesis tries to resolve some of these questions with focus on the underdoped pseudogap regime. Mesa structures of bismuth-based cuprate superconductors were studied using intrinsic tunneling, which allows spectroscopic characterizations of electronic density of states inside the material. A micro/nano fabrication method was developed to further reduce mesa areas into the sub square-micrometer range, in order to minimize the effect of crystal defects and measurement artifacts caused by heating induced by the measurement current.

The comparison of energy scales in Bi-2201 and Bi-2212 cuprates shows that the pseudogap phenomenon is not connected to superconductivity, but possibly represents a competing spin-singlet order that is universal to all cuprates. The analysis of the upper critical field in Bi-2201 reveals a low anisotropy, which gives evidence of paramagnetically limited superconductivity. Furthermore, a new electrical doping method is demonstrated, which enables the reversible tuning the doping level of Bi-2212 and study a broad doping range upon a single sample. Using this method, two distinct critical points were observed under the superconducting dome in the phase diagram: one at the overdoped side, associated with the onset of the pseudogap and a metal to insulator transition, and one at optimal doping, associated with an enhanced "dressed" electron energy. Finally, a novel angular-dependent magnetotunneling technique is introduced, which allows for the separation of the superconducting and non-superconducting contributions to the pseudogap phenomenon. The method reveals that after an abrupt decay of the energy gap for TTc, weak superconducting correlations persist up to several tens of degrees above Tc.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2016. , 77 p.
Keyword [en]
Superconductivity, cuprates, intrinsic tunneling spectroscopy, Josephson junctions, mesa structures, micro/nanoscale fabrication, electrical doping, pseudogap, Bi-2212, Bi-2201
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-129270ISBN: 978-91-7649-434-9 (print)OAI: oai:DiVA.org:su-129270DiVA: diva2:920919
Public defence
2016-06-13, sal FB54, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2016-05-19 Created: 2016-04-19 Last updated: 2016-06-15Bibliographically 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
Show others...
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. Low anisotropy of the upper critical field in a strongly anisotropic layered cuprate Bi2.15Sr1.9CuO6+delta: Evidence for a paramagnetically limited superconductivity
Open this publication in new window or tab >>Low anisotropy of the upper critical field in a strongly anisotropic layered cuprate Bi2.15Sr1.9CuO6+delta: Evidence for a paramagnetically limited superconductivity
2014 (English)In: Physical Review B, ISSN 2469-9950, Vol. 89, no 21, 214516Article in journal (Refereed) Published
Abstract [en]

We study angular-dependent magnetoresistance in a low-T-c layered cuprate Bi2.15Sr1.9CuO6+delta. The low Tc similar to 4 K allows complete suppression of superconductivity by modest magnetic fields and facilitates accurate analysis of the upper critical field H-c2. We observe a universal exponential decay of fluctuation conductivity in a broad range of temperatures above Tc and propose a method for extraction of H-c2(T) from the scaling analysis of the fluctuation conductivity at T > T c. Our main result is observation of a surprisingly low H-c2 anisotropy similar to 2, which is much smaller than the effective mass anisotropy of the material similar to 300. We show that the anisotropy is decreasing with increasing field and saturates at a small value when the field reaches the paramagnetic limit. We argue that the dramatic discrepancy of high-field and low-field anisotropies is clear evidence for paramagnetically limited superconductivity.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-106332 (URN)10.1103/PhysRevB.89.214516 (DOI)000337943500005 ()
Note

AuthorCount:4;

Available from: 2014-08-07 Created: 2014-08-04 Last updated: 2016-05-11Bibliographically approved
3. Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures
Open this publication in new window or tab >>Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures
Show others...
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
4. Sequence of Quantum Phase Transitions in Bi2Sr2CaCu2O8+delta Cuprates Revealed by In Situ Electrical Doping of One and the Same Sample
Open this publication in new window or tab >>Sequence of Quantum Phase Transitions in Bi2Sr2CaCu2O8+delta Cuprates Revealed by In Situ Electrical Doping of One and the Same Sample
Show others...
2016 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 116, no 6, 067001Article in journal (Refereed) Published
Abstract [en]

Our recently discovered electrical doping technique allows a broad-range variation of carrier concentration without changing the chemical composition. We show that it is possible to induce superconductivity in a nondoped insulating sample and to tune it reversibly all the way to an overdoped metallic state. This way, we can investigate the whole doping diagram of one and the same sample. Our study reveals two distinct critical points. The one at the overdoped side is associated with the onset of the pseudogap and with the metal-to-insulator transition in the c-axis transport. The other at optimal doping is associated with the appearance of a dressed electron energy. Our study confirms the existence of multiple phase transitions under the superconducting dome in cuprates.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-127346 (URN)10.1103/PhysRevLett.116.067001 (DOI)000369737100007 ()
Available from: 2016-03-11 Created: 2016-03-02 Last updated: 2017-11-30Bibliographically approved

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