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  • 1.
    Jacobs, Thorsten
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Katterwe, Sven Olof
    Stockholm University, Faculty of Science, Department of Physics.
    Motzkau, Holger
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Maljuk, A.
    Helm, T.
    Putzke, C.
    Kampert, E.
    Kartsovnik, M. V.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Electron-tunneling measurements of low-T-c single-layer Bi2+xSr2-yCuO6+delta: Evidence for a scaling disparity between superconducting and pseudogap states2012In: Physical Review B, ISSN 2469-9950, Vol. 86, no 21, article id 214506Article in journal (Refereed)
    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.

  • 2.
    Jacobs, Thorsten
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Katterwe, Sven-Olof
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Superconducting correlations above T-c in the pseudogap state of Bi2Sr2CaCu2O8+delta cuprates revealed by angular-dependent magnetotunneling2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 22, article id 220501Article in journal (Refereed)
    Abstract [en]

    We present an angular-dependent magnetotunneling technique, which facilitates unambiguous separation of superconducting (supporting circulating screening currents) and nonsuperconducting (not supporting screening currents) contributions to the pseudogap phenomenon in layered Bi2Sr2CaCu2O8+delta cuprates. Our data indicate persistence of superconducting correlations at temperatures up to 1.5T(c) in a form of both phase and amplitude fluctuations of the superconducting order parameter. However, despite a profound fluctuations region, only a small fraction of the pseudogap spectrum is caused by superconducting correlations, while the dominating part comes from a competing nonsuperconducting order, which does not support circulating orbital currents.

  • 3.
    Katterwe, Sven Olof
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Jacobs, Thorsten
    Stockholm University, Faculty of Science, Department of Physics.
    Maljuk, A.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Low anisotropy of the upper critical field in a strongly anisotropic layered cuprate Bi2.15Sr1.9CuO6+δ: Evidence for a paramagnetically limited superconductivity2014In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 89, no 21, article id 214516Article in journal (Refereed)
    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.

  • 4.
    Katterwe, Sven Olof
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Temperature dependences of geometrical and velocity-matching resonances in Bi(2)Sr(2)CaCu(2)O(8+x) intrinsic Josephson junctions2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 21, p. 214519-Article in journal (Refereed)
    Abstract [en]

    We study temperature dependence of geometrical (Fiske) and velocity-matching (Eck) resonances in the flux-flow state of small Bi(2)Sr(2)CaCu(2)O(8+x) mesa structures. It is shown that the quality factor of resonances is high at low T, but rapidly decreases with increasing temperature. We also study T dependencies of resonant voltages and the speed of electromagnetic waves (the Swihart velocity). Surprisingly it is observed that the Swihart velocity exhibits a flat T dependence at low T, following T dependence of the c-axis critical current, rather than the expected linear T dependence of the London penetration depth. Our data indicate that self-heating is detrimental for operation of mesas as coherent THz oscillators because it limits the emission power via suppression of the quality factor. On the other hand, significant temperature dependence of the Swihart velocity allows broad-range tunability of the output frequency.

  • 5.
    Katterwe, Sven-Olof
    Stockholm University, Faculty of Science, Department of Physics.
    Confinement and resonant phenomena in small Bi2Sr2CaCu2O8 mesa structures2010Licentiate thesis, monograph (Other academic)
    Abstract [en]

    In this thesis, intrinsic Josephson junctions, naturally formed in the stronglyanisotropic high-temperature superconductor Bi2Sr2CaCu2O8, are studied experimentally.Small mesa structures are fabricated on the surface of singlecrystals using micro- and nano-fabrication tools, focused ion beam is used toreduce the mesa-area to ≈ 1 × 1 μm2.

    Intrinsic tunneling spectroscopy is performed to study the properties ofcharge transport between adjacent copper-oxide layers. For this purpose, currentvoltage characteristics along the crystallographic c-axis are measured and temperature and bias-dependence of the c-axis quasiparticle resistance is studied. A change in the interlayer transport-mechanism is observed at the criticaltemperature Tc: from thermal activation like above Tc to multiparticle transport below Tc.

    In magnetic fields B parallel to the copper-oxide planes, Josephson-vortices (fluxons) penetrate the intrinsic Josephson junctions in the form of a vortexlattice.Information about the static lattice structure can be obtained fromthe magnetic field dependence of the critical current Ic. In this thesis, Ic(B) is measured for mesas with different sizes in magnetic fields up to 4 T and at atemperature of 1.6 K. Modulations similar to the Fraunhofer diffraction patternare found. For long mesas, the period of oscillations is equal to half a fluxquantum per junction, but it changes to one flux quantum per junction if the size of the mesa is reduced, or if the magnetic field is increased. This indicatesa transition from a triangular to a rectangular fluxon-lattice by geometrical confinement in small mesas.

    Flux-flow characteristics and resonance phenomena in mesa structures of Bi2Sr2CaCu2O8+x and Bi2−yPbySr2CaCu2O8+x are finally studied. From measurementsof the flux-flow voltage in magnetic fields up to 17 T, the maximumvelocity of fluxons is calculated to cmax = 4.3 × 105 m/s. Phonon resonances are observed in the current-voltage characteristics. Their voltage positions are shown to be temperature-independent. The analysis of phonon-resonances in magnetic fields reveals a shift in the phonon frequencies with increasing fields. Evidence for an interaction between flux-flow and phonon resonances is found. In addition, Fiske steps (cavity-resonances) are observed in the current-voltagecharacteristics.

    It is argued that the employed technique for miniaturization of mesas andthe obtained results can be useful for better understanding of fundamentalproperties of high-temperature superconductors and for realization of a coherentflux-flow oscillator in the important THz frequency range.

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  • 6.
    Katterwe, Sven-Olof
    Stockholm University, Faculty of Science, Department of Physics.
    Properties of small Bi2Sr2CaCu2O8 intrinsic Josephson junctions: confinement, flux-flow and resonant phenomena2011Doctoral 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.

    Download full text (pdf)
    fulltext
  • 7.
    Katterwe, Sven-Olof
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir
    Stockholm University, Faculty of Science, Department of Physics.
    Stabilization of the in-phase fluxon state by geometrical confinement in small Bi2Sr2CaCu2O8+x mesa structures2009In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 80, no 2, p. 020502(R)-Article in journal (Refereed)
    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.

  • 8.
    Katterwe, Sven-Olof
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Temperature dependence of geometrical and velocity matching resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctionsManuscript (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.

  • 9.
    Katterwe, Sven-Olof
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Motzkau, Holger
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Coherent generation of phonon-polaritons in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions2011In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 83, no 10, article id 100510Article in journal (Refereed)
    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.

  • 10.
    Katterwe, Sven-Olof
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Doping-induced change in the interlayer yransport mechanism of Bi2Sr2CaCu2O8+δ near the superconducting transition temperature2008In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 101, no 8, p. 087003-Article in journal (Refereed)
    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.

  • 11.
    Katterwe, Sven-Olof
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Interaction between flux-flow and phonon resonances in small Bi-2212 mesa structures2010In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 234, no 4, p. 042016-Article in journal (Refereed)
    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.

  • 12.
    Katterwe, Sven-Olof
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Motzkau, Holger
    Stockholm University, Faculty of Science, Department of Physics.
    Kulakov, A. B
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Superluminal geometrical resonances observed in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 2, article id 024517Article in journal (Refereed)
    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.

  • 13.
    Krasnov, Vladimir M.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Katterwe, Sven-Olof
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Signatures of the electronic nature of pairing in high-T-c superconductors obtained by non-equilibrium boson spectroscopy2013In: Nature Communications, E-ISSN 2041-1723, Vol. 4, article id 2970Article in journal (Refereed)
    Abstract [en]

    Understanding the pairing mechanism that gives rise to high-temperature superconductivity is one of the longest-standing problems of condensed-matter physics. Almost three decades after its discovery, even the question of whether or not phonons are involved remains a point of contention to some. Here we describe a technique for determining the spectra of bosons generated during the formation of Cooper pairs on recombination of hot electrons as they tunnel between the layers of a cuprate superconductor. The results obtained indicate that the bosons that mediate pairing decay over micrometre-scale distances and picosecond timescales, implying that they propagate at a speed of around 106 m s−1. This value is more than two orders of magnitude greater than the phonon propagation speed but close to Fermi velocity for electrons, suggesting that the pairing mechanism is mediated by unconventional repulsive electron–electron, rather than attractive electron–phonon, interactions. 

  • 14.
    Krasnov, Vladimir M.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Motzkau, Holger
    Stockholm University, Faculty of Science, Department of Physics.
    Golod, Taras
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Katterwe, Sven-Olof
    Stockholm University, Faculty of Science, Department of Physics.
    Kulakov, A. B.
    Comparative analysis of tunneling magnetoresistance in low-Tc Nb/Al-AlOx/Nb and high-Tc Bi2−yPbySr2CaCu2O8+δ intrinsic Josephson junctions2011In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 84, no 5, article id 054516Article in journal (Refereed)
    Abstract [en]

    We perform a detailed comparison of magnetotunneling in conventional low-Tc Nb/Al-AlOx/Nb junctions with that in slightly overdoped Bi2−yPbySr2CaCu2O8+δ [Bi(Pb)-2212] intrinsic Josephson junctions and with microscopic calculations. It is found that both types of junctions behave in a qualitatively similar way. Both magnetic field and temperature suppress superconductivity in the state-conserving manner. This leads to the characteristic sign change of tunneling magnetoresistance from the negative at the subgap to the positive at the sum-gap bias. We derived theoretically and verified experimentally scaling laws of magnetotunneling characteristics and employ them for accurate extraction of the upper critical field Hc2. For Nb an extended region of surface superconductivity at Hc2<H<Hc3 is observed. The parameters of Bi(Pb)-2212 were obtained from self-consistent analysis of magnetotunneling data at different levels of bias, dissipation powers, and for different mesa sizes, which precludes the influence of self-heating. It is found that Hc2(0) for Bi(Pb)-2212 is ≃T→Tc T and decreases significantly at T→Tc. The amplitude of subgap magnetoresistance is suppressed exponentially at T>Tc/2, but remains negative, although very small, above Tc. This may indicate the existence of an extended fluctuation region, which, however, does not destroy the general second-order type of the phase transition at Tc.

  • 15.
    Motzkau, Holger
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Jacobs, Thorsten
    Stockholm University, Faculty of Science, Department of Physics.
    Katterwe, Sven-Olof
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures2012In: Physical Review B, ISSN 2469-9950, Vol. 85, no 14, article id 144519Article in journal (Refereed)
    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.

  • 16.
    Motzkau, Holger
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Katterwe, Sven-Olof
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Size-dependent transformation from triangular to rectangular fluxon lattice in Bi-2212 mesa structures2012In: 26th International Conference on Low Temperature Physics (LT26), Institute of Physics (IOP), 2012, article id 052022Conference paper (Refereed)
    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.

  • 17.
    Motzkau, Holger
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Katterwe, Sven-Olof
    Stockholm University, Faculty of Science, Department of Physics.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Strong polaritonic interaction between flux-flow and phonon resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions: Angular dependence and the alignment procedure2013In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 491, p. 51-55Article in journal (Refereed)
    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.

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