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Properties of small Bi2Sr2CaCu2O8 intrinsic Josephson junctions: confinement, flux-flow and resonant phenomena
Stockholm University, Faculty of Science, Department of Physics. (Experimental Condensed Matter Physics)
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, intrinsic Josephson junctions, naturally formed in the strongly anisotropic high-temperature superconductor Bi2Sr2CaCu2O8 (Bi-2212), are studied experimentally. For this purpose, small mesa structures are fabricated on the surface of single crystals using micro- and nano-fabrication tools, focused ion beam is used to reduce the area of the mesa-structures down to ≈ 1 × 1 μm2.

The properties of charge transport across copper-oxide layers inside the mesas are studied by intrinsic tunneling spectroscopy. Temperature, bias and magnetic field dependences of current-voltage characteristics are examined.

In the main part of the thesis, the behavior of intrinsic Josephson junctions in magnetic fields B parallel to the copper-oxide planes is studied. Parallel magnetic fields penetrate the junctions in the form of Josephson vortices (fluxons). At high magnetic fields, fluxons are arranged in a regular lattice and are accelerated by a sufficient high transport current. As the fluxon lattice is moving through the mesa, it emits electromagnetic waves in the important THz frequency range. Properties of Bi-2212 mesas in this flux-flow regime are studied in this thesis.

The following new observations were made during the course of this work: a crossover from thermal activation above Tc to quantum tunneling below Tc is seen in the interlayer transport-mechanism, the Fraunhofer pattern of Ic(B) is observed clearly in Bi-2212, superluminal electromagnetic cavity resonances and phonon-polaritons are observed in Bi-2212.

It is argued that the employed technique for miniaturization of mesas and the obtained results can be useful for a better understanding of fundamental properties of high-temperature superconductors and for the realizations of coherent flux-flow oscillators and coherent phonon-polariton generators in the important THz frequency range.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2011. , 105 p.
Keyword [en]
high-temperature superconductivity, intrinsic Josephson junctions, tunneling, fluxons, flux-flow oscillator, THz-emission, cavity resonances, polaritons, micro/nano-fabrication
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-62583ISBN: 978-91-7447-358-2 (print)OAI: oai:DiVA.org:su-62583DiVA: diva2:443129
Public defence
2011-10-27, FB41, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 6: Manuscript. Available from: 2011-10-05 Created: 2011-09-23 Last updated: 2011-10-07Bibliographically approved
List of papers
1. Doping-induced change in the interlayer yransport mechanism of Bi2Sr2CaCu2O8+δ near the superconducting transition temperature
Open this publication in new window or tab >>Doping-induced change in the interlayer yransport mechanism of Bi2Sr2CaCu2O8+δ near the superconducting transition temperature
2008 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 101, no 8, 087003- p.Article in journal (Refereed) Published
Abstract [en]

We perform a detailed study of temperature, bias, and doping dependence of interlayer transport in the layered high temperature superconductor Bi2Sr2CaCu2O8+delta. We observe that the shape of interlayer characteristics in underdoped crystals exhibits a remarkable crossover at the superconducting transition temperature: from thermal activation-type above Tc to almost T-independent quantum tunneling-type below Tc. Our data provide insight into the nature of interlayer transport and indicate that its mechanism changes with doping: from the conventional single quasiparticle tunneling in overdoped to a progressively increasing Cooper pair contribution in underdoped crystals.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-14611 (URN)10.1103/PhysRevLett.101.087003 (DOI)000258643600060 ()18764651 (PubMedID)
Available from: 2008-10-13 Created: 2008-10-13 Last updated: 2017-12-13Bibliographically approved
2. Stabilization of the in-phase fluxon state by geometrical confinement in small Bi2Sr2CaCu2O8+x mesa structures
Open this publication in new window or tab >>Stabilization of the in-phase fluxon state by geometrical confinement in small Bi2Sr2CaCu2O8+x mesa structures
2009 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 80, no 2, 020502(R)- p.Article in journal (Refereed) Published
Abstract [en]

Thein-phase (rectangular) fluxon lattice is required for achieving coherent THzemission from stacked Josephson junctions. Unfortunately, it is usually unstabledue to mutual repulsion of fluxons in neighbor junctions, whichfavors the out-of-phase (triangular) lattice. Here we experimentally study magneticfield modulation of the critical current in small Bi-2212 mesastructures with different sizes. Clear Fraunhofer-like modulation is observed whenthe field is aligned parallel to CuO planes. For longmesas the periodicity of modulation is equal to half theflux quantum per intrinsic Josephson junction, corresponding to the triangularfluxon lattice. However, the periodicity is changed to one fluxquantum, characteristic for the rectangular fluxon lattice, both by decreasingthe length of the mesas and by increasing magnetic field.Thus, we demonstrate that the stationary in-phase fluxon state canbe effectively stabilized by geometrical confinement in small Bi-2212 mesastructures.

Keyword
bismuth compounds, calcium compounds, critical currents, flux-line lattice, high-temperature superconductors, Josephson effect, strontium compounds, terahertz waves
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-32819 (URN)10.1103/PhysRevB.80.020502 (DOI)000268617500014 ()
Available from: 2009-12-16 Created: 2009-12-16 Last updated: 2017-12-12Bibliographically approved
3. Interaction between flux-flow and phonon resonances in small Bi-2212 mesa structures
Open this publication in new window or tab >>Interaction between flux-flow and phonon resonances in small Bi-2212 mesa structures
2010 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 234, no 4, 042016- p.Article in journal (Refereed) Published
Abstract [en]

Resonant phenomena are important for the use of intrinsic Josephson junction as THz-oscillators, due to the decreased linewidth of emitted radiation when biasing the junctions near a resonance. We perform a detailed study of flux-flow characteristics and phonon resonances in small Bi(Pb)2Sr2CaCu2O8+x mesa structures. Magnetic field dependence of flux-flow characteristics up to 17 T and temperature and magnetic field dependence of phonon resonances at temperatures from 2 K to 80 K and in fields up to 15 T are analyzed. A shift of the phonon resonances in the presence of external magnetic fields and an interaction between flux-flow and phonon resonances are observed.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-48328 (URN)10.1088/1742-6596/234/4/042016 (DOI)
Available from: 2010-12-07 Created: 2010-12-07 Last updated: 2017-12-11Bibliographically approved
4. 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
5. 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
6. Temperature dependence of geometrical and velocity matching resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions
Open this publication in new window or tab >>Temperature dependence of geometrical and velocity matching resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We study temperature dependence of geometrical (Fiske) and velocity-matching (Eck) resonances in the flux flow state of small Bi2Sr2CaCu2O8+x mesa structures. It is shown that the quality factor of resonances is high at low T, but rapidly decreases with increasing temperature already at T > 10 K. This indicates that self-heatingis strongly detrimental for operation of mesas as coherent THz oscillators and ultimately limits the emission power via suppression of the quality factor. We also study T-dependence of the resonant voltage and the speed of electromagnetic waves (Swihart velocity). Surprisingly it is observed that the Swihart velocity exhibits very weak T-dependence at low T, following T−dependence of the Josephson plasma frequency, rather than the expected linear T-dependence of the London penetration depth.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-62580 (URN)
Available from: 2011-09-23 Created: 2011-09-23 Last updated: 2011-09-23Bibliographically approved

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