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Intrinsic tunneling spectroscopic investigations of Bi2Sr2CaCu2O8+x superconductors: The fluxon lattice, resonant phenomena,flux-flow and current doping
Stockholm University, Faculty of Science, Department of Physics. (Experimental Condensed Matter Physics)
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: urn:nbn:se:su:diva-89385OAI: oai:DiVA.org:su-89385DiVA: diva2:617598
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
List of papers
1. Size-dependent transformation from triangular to rectangular fluxon lattice in Bi-2212 mesa structures
Open this publication in new window or tab >>Size-dependent transformation from triangular to rectangular fluxon lattice in Bi-2212 mesa structures
2012 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 400, 052022Article in journal (Refereed) Published
Abstract [en]

We present a systematic study of the field and size dependencies of the static fluxon lattice configuration in Bi-2212 intrinsic Josephson junctions and investigate conditions needed for the formation of a rectangular fluxon lattice required for a high power flux-flow oscillator. We fabricate junctions of different sizes from Bi2Sr2CaCu2O8+x and Bi1.75Pb0.25Sr2CaCu2O8+xsingle crystals using the mesa technique and study the Fraunhofer-like modulation of the critical current with magnetic field. The modulation can be divided into three regions depending on the formed fluxon lattice. At low field, no periodic modulation and no ordered fluxon lattice is found. At intermediate fields, modulation with half-flux quantum periodicity due to a triangular lattice is seen. At high fields, the rectangular lattice gives integer flux quantum periodicity. We present these fields in dependence on the sample size and conclude that the transitions between the regions depend only on λJ(Jc) and occur at about 0.4 and 1.3 fluxons per λJ, respectively. These numbers are universal for the measured samples and are consistent with performed numerical simulations.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-89383 (URN)10.1088/1742-6596/400/5/052022 (DOI)000314977100461 ()
Conference
26th International Conference on Low Temperature Physics (LT26), Beijing, Peoples Republic of China, Aug 10-17, 2011
Available from: 2013-04-23 Created: 2013-04-23 Last updated: 2017-12-06Bibliographically approved
2. Superluminal geometrical resonances observed in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions
Open this publication in new window or tab >>Superluminal geometrical resonances observed in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions
Show others...
2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 2, 024517Article in journal (Refereed) Published
Abstract [en]

We study Fiske steps in small Bi2Sr2CaCu2O8+x mesa structures, containing only a few stacked intrinsic Josephson junctions. Careful alignment of magnetic field prevents penetration of Abrikosov vortices and facilitates observation of a large variety of high-quality geometrical resonances, including superluminal with velocities larger than the slowest velocity of electromagnetic waves. A small number of junctions limits the number of resonant modes and allows accurate identification of modes and velocities. It is shown that superluminal geometrical resonances can be excited by subluminal fluxon motion and that flux flow itself becomes superluminal at high magnetic fields. We argue that observation of high-quality superluminal geometrical resonances is crucial for realization of the coherent flux-flow oscillator in the terahertz frequency range.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-48322 (URN)10.1103/PhysRevB.82.024517 (DOI)000280207900007 ()
Available from: 2010-12-07 Created: 2010-12-07 Last updated: 2017-12-11Bibliographically approved
3. Coherent generation of phonon-polaritons in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions
Open this publication in new window or tab >>Coherent generation of phonon-polaritons in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions
2011 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 83, no 10, 100510Article in journal (Refereed) Published
Abstract [en]

The Bi2Sr2CaCu2O8+x high-temperature superconductor represents a natural layered metamaterial composed of metallic CuO bilayers sandwiched between ionic BiO planes. Each pair of CuO bilayers forms an atomic-scale Josephson junction. Here we employ the intrinsic Josephson effect for in situ generation and self-detection of electromagnetic waves in Bi2Sr2CaCu2O8+x single crystals. We observe that electromagnetic waves form polaritons with several transverse optical phonons. This indicates the presence of unscreened polar response in cuprates, which may lead to unusually strong electron-phonon interaction. Our technique can provide intense local sources of coherent, monochromatic phonon-polaritons with kW/cm2 power densities.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-62392 (URN)10.1103/PhysRevB.83.100510 (DOI)000288510800001 ()
Available from: 2011-09-16 Created: 2011-09-16 Last updated: 2017-12-08Bibliographically approved
4. Strong polaritonic interaction between flux-flow and phonon resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions: Angular dependence and the alignment procedure
Open this publication in new window or tab >>Strong polaritonic interaction between flux-flow and phonon resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions: Angular dependence and the alignment procedure
2013 (English)In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 491, 51-55 p.Article in journal (Refereed) Published
Abstract [en]

Bi2Sr2CaCu2O8+x single crystals represent natural stacks of atomic scale intrinsic Josephson junctions, formed between metallic CuO2–Ca–CuO2 and ionic insulating SrO–2BiO–SrO layers. Electrostriction effect in the insulating layers leads to excitation of c-axis phonons by the ac-Josephson effect. Here we study experimentally the interplay between and velocity matching (Eck) electromagnetic resonances in the flux-flow state of small mesa structures with c-axis optical phonons. A very strong interaction is reported, which leads to formation of phonon-polaritons with infrared and Raman-active transverse optical phonons. A special focus in this work is made on analysis of the angular dependence of the resonances. We describe an accurate sample alignment procedure that prevents intrusion of Abrikosov vortices in fields up to 17 T, which is essential for achieving high-quality resonances at record high frequencies up to 13 THz.

Keyword
Bi-based cuprates, Intrinsic Josephson junctions
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-89384 (URN)10.1016/j.physc.2012.11.001 (DOI)000320597700013 ()
Available from: 2013-04-23 Created: 2013-04-23 Last updated: 2017-12-06Bibliographically approved
5. 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

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