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Superradiant THz wave emission from arrays of Josephson junctions
Stockholm University, Faculty of Science, Department of Physics.ORCID iD: 0000-0003-4815-5856
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

High-power, continuous-wave, compact and tunable THz sources are needed for a large variety of applications. Development of power-efficient sources of electromagnetic radiation in the 0.1-10 THz range is a difficult technological problem, known as the “THz gap.” Josephson junctions allow creation of monochromatic THz sources with an inherently broad range of tunability. However, emission power from a single junction is too small. It can be amplified in a coherent superradiant manner by phase-locking of many junctions. In this case, the emission power should increase as a square of the number of phase-locked junctions.The aim of this thesis is to study a possibility of achieving coherent super-radiant emission with significant power and frequency tunability from Joseph-son junction arrays. Two types of devices are studied, based either on stacks (one-dimensional arrays) of intrinsic Josephson junctions naturally formed in single crystals of high-temperature cuprate superconductor Bi2Sr2CaCu2O8+x, or two-dimensional arrays of artificial low-temperature superconducting Nb/NbSi/Nb junctions. Micron-size junctions are fabricated using micro- and nanofabrication tools.The first chapter of this thesis describes the theory of Josephson junctions and how mutual coupling between Josephson junctions can lead to self-syn-chronization, facilitating the superradiant emission of electromagnetic radia-tion. The second chapter is focused on the technical aspects of this work, with detailed descriptions of sample fabrication and experimental techniques. The third chapter presents main results and discussion. It is demonstrated that de-vices based on high-Tc cuprates allow tunable emission in a very broad fre-quency range 1-11 THz. For low- Tc junction arrays synchronization of up to 9000 junctions is successfully achieved. It is argued that an unconventional traveling-waves mechanism facilitates the phase-locking of such huge arrays. The obtained results confirm a possibility of creation of high-power, continu-ous-wave, compact and tunable THz sources, based on arrays of Josephson junctions.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2020. , p. 67
Keywords [en]
Josephson junction, Superconductor, ThZ emission, high-Tc
National Category
Physical Sciences
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-181234ISBN: 978-91-7911-178-6 (print)ISBN: 978-91-7911-179-3 (electronic)OAI: oai:DiVA.org:su-181234DiVA, id: diva2:1427134
Public defence
2020-09-09, sal FR4, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2020-08-17 Created: 2020-04-28 Last updated: 2022-03-04Bibliographically approved
List of papers
1. Josephson emission with frequency span 1–11 THz from small Bi2Sr2CaCu2O8+δ mesa structures
Open this publication in new window or tab >>Josephson emission with frequency span 1–11 THz from small Bi2Sr2CaCu2O8+δ mesa structures
2017 (English)In: Nature Communications, E-ISSN 2041-1723, Vol. 8, article id 1742Article in journal (Refereed) Published
Abstract [en]

Mesa structures made of Bi2Sr2CaCu2O8+δ high-temperature superconductor represent stacks of atomic scale intrinsic Josephson junctions. They can be used for generation of high-frequency electromagnetic waves. Here we analyze Josephson emission from small-but-high mesas (with a small area, but containing many stacked junctions). We have found strong evidence for tunable terahertz emission with a good efficacy in a record high-frequency span 1–11 THz, approaching the theoretical upper limit for this superconductor. Emission maxima correspond to in-phase cavity modes in the mesas, indicating coherent superradiant nature of the emission. We conclude that terahertz emission requires a threshold number of junctions N ~ 100. The threshold behavior is not present in the classical description of stacked Josephson junctions and suggests importance of laser-like cascade amplification of the photon number in the cavity. 

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-149974 (URN)10.1038/s41467-017-01888-4 (DOI)000416229300028 ()29170380 (PubMedID)2-s2.0-85035045188 (Scopus ID)
Funder
Swedish Research Council, 621-2014-4314The Swedish Foundation for International Cooperation in Research and Higher Education (STINT), IG2013-5453
Available from: 2017-12-29 Created: 2017-12-29 Last updated: 2023-03-28Bibliographically approved
2. Synchronization of Large Josephson-Junction Arrays by Traveling Electromagnetic Waves
Open this publication in new window or tab >>Synchronization of Large Josephson-Junction Arrays by Traveling Electromagnetic Waves
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2018 (English)In: Physical Review Applied, E-ISSN 2331-7019, Vol. 9, no 5, article id 054032Article in journal (Refereed) Published
Abstract [en]

Mutual synchronization of many Josephson junctions is required for superradiant enhancement of the emission power. However, the larger the junction array is, the more difficult is the synchronization, especially when the array size becomes much larger than the emitted wavelength. Here, we study experimentally Josephson emission from such larger-than-the-wavelength Nb/NbSi/Nb junction arrays. For one of the arrays we observe a clear superradiant enhancement of emission above a threshold number of active junctions. The arrays exhibit strong geometrical resonances, seen as steps in current-voltage characteristics. However, radiation patterns of the arrays have forward-backward asymmetry, which is inconsistent with the solely geometrical resonance (standing-wave) mechanism of synchronization. We argue that the asymmetry provides evidence for an alternative mechanism of synchronization mediated by unidirectional traveling-wave propagation along the array (such as a surface plasmon). In this case, emission occurs predominantly in the direction of propagation of the traveling wave. Our conclusions are supported by numerical modeling of Josephson traveling-wave antenna. We argue that such a nonresonant mechanism of synchronization opens a possibility for phase locking of very large arrays of oscillators.

Keywords
Josephson junction arrays, coherent THz sources, Synchronization
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-157734 (URN)10.1103/PhysRevApplied.9.054032 (DOI)000433041700002 ()2-s2.0-85047735487 (Scopus ID)
Available from: 2018-08-03 Created: 2018-08-03 Last updated: 2022-10-26Bibliographically approved
3. Unusual two-dimensional behavior of iron-based superconductors with low anisotropy
Open this publication in new window or tab >>Unusual two-dimensional behavior of iron-based superconductors with low anisotropy
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 13, article id 134512Article in journal (Refereed) Published
Abstract [en]

We study angular-dependent magnetoresistance in iron-based superconductors Ba1−xNaxFe2As2 and FeTe1−xSex. Both superconductors have relatively small anisotropies γ∼2 and exhibit a three-dimensional (3D) behavior at low temperatures. However, we observe that they start to exhibit a profound two-dimensional behavior at elevated temperatures and in applied magnetic field parallel to the surface. We conclude that the unexpected two-dimensional (2D) behavior of the studied low-anisotropic superconductors is not related to layeredness of the materials, but is caused by appearance of surface superconductivity when magnetic field exceeds the upper critical field Hc2(T) for destruction of bulk superconductivity. We argue that the corresponding 3D-2D bulk-to-surface dimensional transition can be used for accurate determination of the upper critical field.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-149008 (URN)10.1103/PhysRevB.96.134512 (DOI)000413050000004 ()2-s2.0-85037116435 (Scopus ID)
Available from: 2017-11-20 Created: 2017-11-20 Last updated: 2022-10-20Bibliographically approved
4. Phase-Sensitive Evidence for the Sign-Reversal s± Symmetry of the Order Parameter in an Iron-Pnictide Superconductor Using Nb/Ba1−xNaxFe2As2 Josephson Junctions
Open this publication in new window or tab >>Phase-Sensitive Evidence for the Sign-Reversal s± Symmetry of the Order Parameter in an Iron-Pnictide Superconductor Using Nb/Ba1−xNaxFe2As2 Josephson Junctions
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2018 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 120, no 6, article id 067001Article in journal (Refereed) Published
Abstract [en]

Josephson current provides a phase-sensitive tool for probing the pairing symmetry. Here we present an experimental study of high-quality Josephson junctions between a conventional s-wave superconductor Nb and a multiband iron-pnictide Ba1−xNaxFe2As2. Junctions exhibit a large enough critical current density to preclude the d-wave symmetry of the order parameter in the pnictide. However, the IcRn product is very small ≃3μV, which is not consistent with the sign-preserving s++ symmetry either. We argue that the small IcRn value, along with its unusual temperature dependence, provides evidence for the sign-reversal s± symmetry of the order parameter in Ba1−xNaxFe2As2. We conclude that it is the phase sensitivity of our junctions that leads to an almost complete (below a subpercent) cancellation of supercurrents from sign-reversal bands in the pnictide.

Keywords
Unconventional superconductivity, Tunneling, Josephson effect
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-153780 (URN)10.1103/PhysRevLett.120.067001 (DOI)000424193100011 ()29481253 (PubMedID)2-s2.0-85041822231 (Scopus ID)
Available from: 2018-03-20 Created: 2018-03-20 Last updated: 2022-10-25Bibliographically approved
5. Evidence of synchronization of large Josephson-junction arrays by traveling electromagnetic waves
Open this publication in new window or tab >>Evidence of synchronization of large Josephson-junction arrays by traveling electromagnetic waves
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2018 (English)In: EPJ Web of Conferences, E-ISSN 2100-014X, Vol. 195, article id 02004Article in journal (Refereed) Published
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-181232 (URN)10.1051/epjconf/201819502004 (DOI)
Conference
3rd International Conference “Terahertz and Microwave Radiation: Generation, Detection and Applications” (TERA-2018)
Available from: 2020-04-28 Created: 2020-04-28 Last updated: 2023-07-06Bibliographically approved

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Borodianskyi, Ievgenii

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