I urval från Stockholms universitets publikationsdatabas

• High-frequency phenomena in small Bi₂Sr₂CaCu₂O_8+x intrinsic Josephson junctions

  2015. Holger Motzkau, Vladimir M. Krasnov, Reinhold Kleiner.

  Avhandling (Dok)

  In this thesis, the tunneling between individual atomic layers in structures of Bi₂Sr₂CaCu₂O_8+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

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• Evidence for Nonlocal Electrodynamics in Planar Josephson Junctions

  2013. Andrey A. Boris (et al.). Physical Review Letters 111 (11)

  Artikel

  We study the temperature dependence of the critical current modulation I-c(H) for two types of planar Josephson junctions: a low-T-c Nb/CuNi/Nb and a high-T-c YBa2Cu3O7-delta bicrystal grain-boundary junction. At low T both junctions exhibit a conventional behavior, described by the local sine-Gordon equation. However, at elevated T the behavior becomes qualitatively different: the I-c(H) modulation field Delta H becomes almost T independent and neither Delta H nor the critical field for the penetration of Josephson vortices vanish at T-c. Such an unusual behavior is in good agreement with theoretical predictions for junctions with nonlocal electrodynamics. We extract absolute values of the London penetration depth lambda from our data and show that a crossover from local to nonlocal electrodynamics occurs with increasing T when lambda(T) becomes larger than the electrode thickness.

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• Intrinsic tunneling spectroscopic investigations of Bi₂Sr₂CaCu₂O_8+x superconductors

  2013. Holger Motzkau, Vladimir M. Krasnov, Paul Müller.

  Avhandling (Lic)

  In this thesis, the tunneling between individual atomic layers in structures of Bi₂Sr₂CaCu₂O_8+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, Ф₀/λ_Js. 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.

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• Strong polaritonic interaction between flux-flow and phonon resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions

  2013. Holger Motzkau (et al.). Physica. C, Superconductivity 491, 51-55

  Artikel

  Bi₂Sr₂CaCu₂O_8+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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• Electron-tunneling measurements of low-T-c single-layer Bi2+xSr2-yCuO6+delta

  2012. Thorsten Jacobs (et al.). Physical Review B 86 (21)

  Artikel

  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.

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• Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures

  2012. Holger Motzkau (et al.). Physical Review B 85 (14)

  Artikel

  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.

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• Photoconductivity effects in mixed-phase BSCCO whiskers

  2012. M. Truccato (et al.). Superconductors Science and Technology 25 (10)

  Artikel

  We report on combined photoconductivity and annealing experiments in whisker-like crystals of the Bi-Sr-Ca-Cu-O (BSCCO) high-T-c superconductor. Both single-phase Bi2Sr2CaCu2O8+delta (Bi-2212) samples and crystals of the mixed phases Bi2Sr2Ca2Cu3O10+x (Bi-2223)/Bi-2212 have been subjected to annealing treatments at 90 degrees C in air in a few hours steps, up to a maximum total annealing time of 47 h. At every step, samples have been characterized by means of electrical resistance versus temperature (R versus T) and resistance versus time at fixed temperature (R versus t) measurements, both in the dark and under illumination with a UV-Vis halogen arc lamp. A careful comparison of the results from the two techniques has shown that, while for single-phase samples no effect is recorded, for mixed-phase samples an enhancement in the conductivity that increases with increasing annealing time is induced by the light at the nominal temperature T = 100 K, i.e. at an intermediate temperature between the critical temperatures of the two phases. A simple pseudo-1D model based on the Kudinov's scheme (Kudinov et al, 1993 Phys. Rev. B 47 9017-28) has been developed to account for the observed effects, which is based on the existence of Bi-2223 filaments embedded in the Bi-2212 matrix and on the presence of electronically active defects at their interfaces. This model reproduces fairly well the photoconductive experimental results and shows that the length of the Bi-2223 filaments decreases and the number of defects increases with increasing annealing time.

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• Size-dependent transformation from triangular to rectangular fluxon lattice in Bi-2212 mesa structures

  2012. Holger Motzkau (et al.). Journal of Physics, Conference Series 400

  Artikel

  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 Bi₂Sr₂CaCu₂O_8+x and Bi_1.75Pb_0.25Sr₂CaCu₂O_8+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 λ_J(J_c) and occur at about 0.4 and 1.3 fluxons per λ_J, respectively. These numbers are universal for the measured samples and are consistent with performed numerical simulations.

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• Coherent generation of phonon-polaritons in Bi₂Sr₂CaCu₂O_8+x intrinsic Josephson junctions

  2011. Sven-Olof Katterwe (et al.). Physical Review B Condensed Matter 83 (10)

  Artikel

  The Bi₂Sr₂CaCu₂O_8+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 Bi₂Sr₂CaCu₂O_8+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.

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• Comparative analysis of tunneling magnetoresistance in low-T_c Nb/Al-AlO_x/Nb and high-T_c Bi_2−yPb_ySr₂CaCu₂O_8+δ intrinsic Josephson junctions

  2011. Vladimir M. Krasnov (et al.). Physical Review B Condensed Matter 84 (5)

  Artikel

  We perform a detailed comparison of magnetotunneling in conventional low-T_c Nb/Al-AlO_x/Nb junctions with that in slightly overdoped Bi_2−yPb_ySr₂CaCu₂O_8+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 H_c2. For Nb an extended region of surface superconductivity at H_c2<H<H_c3 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 H_c2(0) for Bi(Pb)-2212 is ≃T→T_c T and decreases significantly at T→T_c. The amplitude of subgap magnetoresistance is suppressed exponentially at T>T_c/2, but remains negative, although very small, above T_c. 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 T_c.

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• Superluminal geometrical resonances observed in Bi₂Sr₂CaCu₂O_8+x intrinsic Josephson junctions

  2010. Sven-Olof Katterwe (et al.). Physical Review B. Condensed Matter and Materials Physics 82 (2)

  Artikel

  We study Fiske steps in small Bi₂Sr₂CaCu₂O_8+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.

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