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Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physics.
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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.

Place, publisher, year, edition, pages
2012. Vol. 85, no 14, 144519
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-80276DOI: 10.1103/PhysRevB.85.144519ISI: 000302958900003OAI: oai:DiVA.org:su-80276DiVA: diva2:557144
Note

AuthorCount:5;

Available from: 2012-09-27 Created: 2012-09-17 Last updated: 2016-05-11Bibliographically approved
In thesis
1. Intrinsic tunneling spectroscopic investigations of Bi2Sr2CaCu2O8+x superconductors: The fluxon lattice, resonant phenomena,flux-flow and current doping
Open this publication in new window or tab >>Intrinsic tunneling spectroscopic investigations of Bi2Sr2CaCu2O8+x superconductors: The fluxon lattice, resonant phenomena,flux-flow and current doping
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, the tunneling between individual atomic layers in structures of Bi2Sr2CaCu2O8+x based high-temperature superconductors are experimentally studied employing the intrinsic Josephson effect. A special attention is paid on the fabrication of small mesa structures using micro and nanofabrication techniques.

Different resonant phenomena are studied in the dynamic fluxon state at high magnetic fields, including Eck-resonances and Fiske steps.

The periodic Fraunhofer-like modulation of the critical current of the junctions as a function of in-plane magnetic field is investigated. A transition from a modulation with a half flux quantum to a flux quantum periodicity is demonstrated with increasing field and decreasing junction length. It is interpreted in terms of the static fluxon lattice of stacked, strongly cou- pled intrinsic Josephson junctions and compared with theoretical predictions. A fluxon phase diagram is constructed. It is experimentally found that a stable rectangular fluxon lattice can be obtained at a flux density larger than 1.3 fluxons per Josephson penetration length and junction, Ф0Js. Numerical simulations have been carried out to complement the experimental data.

The resistive switching of mesas at high bias is studied. It is attributed to a persistent electri- cal doping of the crystal. Superconducting properties such as the critical current and tempera- ture and the tunneling spectra are analyzed at different doping states of the same sample. The dynamics of the doping is studied, and attributed to two mechanisms; a charge-transfer effect and oxygen reordering.

Place, publisher, year, edition, pages
Department of Physics, Stockholm University, 2013. 61 p.
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:su:diva-89385 (URN)
Presentation
2013-05-15, FA32, Fysikum, Albanova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

At the time of the defence the following paper was unpublihed and had a status as follows:

Paper 4: in Press

Available from: 2013-05-14 Created: 2013-04-23 Last updated: 2013-05-14Bibliographically approved
2. Intrinsic tunneling spectroscopy of low-Tc cuprate superconductors: Pseudogap, fluctuations and electrical doping
Open this publication in new window or tab >>Intrinsic tunneling spectroscopy of low-Tc cuprate superconductors: Pseudogap, fluctuations and electrical doping
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
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:nbn:se:su:diva-108236 (URN)
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
3. High-frequency phenomena in small Bi2Sr2CaCu2O8+x intrinsic Josephson junctions
Open this publication in new window or tab >>High-frequency phenomena in small Bi2Sr2CaCu2O8+x intrinsic Josephson junctions
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, the tunneling between individual atomic layers in structures of Bi2Sr2CaCu2O8+x based high-temperature superconductors are experimentally studied employing the intrinsic Josephson effect. A special attention is paid to the fabrication of small mesa structures using micro and nanofabrication techniques.

In the first part of the thesis, the periodic Fraunhofer-like modulation of the critical current of the junctions as a function of in-plane magnetic field is investigated. A transition from a modulation with a half flux quantum to a flux quantum periodicity is demonstrated with increasing field and decreasing junction length. It is interpreted in terms of the transformation of the static fluxon lattice of stacked, strongly coupled intrinsic Josephson junctions and compared with theoretical predictions. A fluxon phase diagram is constructed.Numerical simulations have been carried out to complement the experimental data.

In the second part of the thesis, different resonant phenomena are studied in the dynamic flux-flow state at high magnetic fields, including Eck-resonances and Fiske steps. Different resonant modes and their velocities, including superluminal modes, are identified.

In the third part, different experiments attempting to detect radiation from small mesa structures using different setups based on hot-electron bolometer mixers and calorimeters are described. No distinct radiation with emission powers higher than about 500pW could be detected. Furthermore, the interaction with external GHz-radiation is studied. Resonances attributed to an induced flux-flow are observed, and the reflectivity of the sample can be tuned by switching mesas between the superconducting and quasiparticle state.

In the last part, the resistive switching of mesas at high bias is studied. It is attributed to a persistent electrical doping of the crystal. Superconducting properties such as the critical current and temperature and the tunneling spectra are analyzed at different doping states of the same sample. The dynamics of the doping is studied, and attributed to two mechanisms; a charge-transfer effect and oxygen reordering

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2015
Keyword
high-temperature superconductivity, Bi-2212, cuprates, intrinsic Josephson junctions, intrinsic tunneling, fluxons, flux-flow oscillator, THz-emission, cavity resonances, polaritons, electrical doping, micro/nano-fabrication
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-115582 (URN)978-91-7649-157-7 (ISBN)
Public defence
2015-05-20, sal FD41, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2015-04-28 Created: 2015-03-26 Last updated: 2015-05-05Bibliographically approved
4. Unraveling the cuprate superconductor phase diagram: Intrinsic tunneling spectroscopy and electrical doping
Open this publication in new window or tab >>Unraveling the cuprate superconductor phase diagram: Intrinsic tunneling spectroscopy and electrical doping
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
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:nbn:se:su:diva-129270 (URN)978-91-7649-434-9 (ISBN)
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

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