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  • 51.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    A distributed active patch antenna model of a Josephson oscillator2023Ingår i: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 14, s. 151-164Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Optimization of Josephson oscillators requires a quantitative understanding of their microwave properties. A Josephson junction has a geometry similar to a microstrip patch antenna. However, it is biased by a dc current distributed over the whole area of the junction. The oscillating electric field is generated internally via the ac-Josephson effect. In this work, I present a distributed, active patch antenna model of a Josephson oscillator. It takes into account the internal Josephson electrodynamics and allows for the determination of the effective input resistance, which couples the Josephson current to cavity modes in the transmission line formed by the junction. The model provides full characterization of Josephson oscillators and explains the origin of the low radiative power efficiency. Finally, I discuss the design of an optimized Josephson patch oscillator capable of reaching high efficiency and radiation power for emission into free space.

  • 52.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Coherent flux-flow emission from stacked Josephson junctions: Nonlocal radiative boundary conditions and the role of geometrical resonances2010Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, nr 13, s. 134524-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    I derive simple nonlocal dynamic boundary conditions, suitable for modeling of radiation emission from stacked Josephson junctions in an arbitrary dynamic state. Coherent flux-flow emission from intrinsic Josephson junctions in high-Tc superconductors is analyzed. It is shown that due to the lack of Lorentz contraction of fluxons in stacked junctions, high-quality geometrical resonances are prerequisite for high power flux-flow emission from the stack. This leads to a dual role of the radiative impedance: on one hand, small impedance increases the efficiency of emission from the stack, on the other hand, enhanced radiative losses reduce the quality factor of geometrical resonances, which may decrease the total emission power. Optimal conditions are achieved when radiative losses are comparable to resistive losses inside the stack.

  • 53.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Comment on “Essence of intrinsic tunneling: Distinguishing intrinsic features from artifacts”2007Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 75, nr 14, artikel-id 146501Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In a recent paper, Zavaritsky [Phys. Rev. B 72, 094503 (2005)] has argued that interlayer (c axis) current-voltage characteristics of high-temperature superconductors (HTSC) are Ohmic and that a simple self-heating model based on this assumption “provides a qualitative and quantitative description of key finding of intrinsic tunneling spectroscopy.” In this Comment, I demonstrate that the genuine interlayer current-voltage characteristics are strongly non-Ohmic. Therefore, the self-heating model, advocated by Zavaritsky, can hardly provide correct explanation of nonlinearities observed in intrinsic tunneling spectra of HTSC.

  • 54.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    How to Write a Contemporary Scientific Article?2022Ingår i: Education Research International, ISSN 2090-4002, E-ISSN 2090-4010, Vol. 2022, artikel-id 5156888Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Today, scientists are drowned in information and have no time to read all publications, even in a specific area. Information is sifted and only a small fraction of articles is read. Under these circumstances, scientific articles have to be properly adjusted to pass through the superficial sifting. Here, I present instructions for PhD students with almost serious advice on how to write (and how not to write) a contemporary scientific article. I argue that it should “tell a story” and should answer on the three main questions: Why, What, and So what?

  • 55.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Moscow Institute of Physics and Technology, State University, Russia.
    Josephson junctions in a local inhomogeneous magnetic field2020Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 101, nr 14, artikel-id 144507Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A Josephson junction can be subjected to a local, strongly inhomogeneous magnetic field in various experimental situations. Here this problem is analyzed analytically and numerically. A modified sine-Gordon type equation in the presence of time-dependent local field is derived and solved numerically in static and dynamic cases. Two specific examples of local fields are considered: induced either by an Abrikosov vortex, or by a tip of a magnetic force microscope (MFM). It is demonstrated that a time-dependent local field can induce a dynamic flux-flow state in the junction with shuttling or unidirectional ratchetlike Josephson vortex motion. This provides a mechanism for detection and manipulation of Josephson vortices by an oscillating MFM tip. In a static case, the local field leads to a distortion of the critical current versus magnetic field, Ic(H), pattern. The distortion is sensitive to both the shape and the amplitude of the local field. Therefore, the Ic(H) pattern carries information about the local field distribution within the junction. This opens the possibility for employing a single Josephson junction as a scanning probe sensor with spatial resolution not limited by its geometrical size, thus obviating a known problem of a trade-off between the field sensitivity and the spatial resolution of a sensor.

  • 56.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Observation of a subharmonic gap singularity in interlayer tunneling characteristics of Bi2Sr2CaCu2O8+δ2016Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 93, nr 6, artikel-id 064518Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A subharmonic structure in Josephson junctions appears due to Andreev reflections within the junction. Here we report on experimental observation of a subharmonic half-gap singularity in interlayer tunneling characteristics of a layered high temperature superconductor Bi2Sr2CaCu2O8+delta. The singularity is most pronounced in optimally doped crystals and vanishes with decreasing doping. It indicates the existence of nonvanishing electronic density of states and certain metallic properties in the intermediate BiO layers, which grows stronger with increasing doping. This provides an additional coherent interlayer transport channel and can explain a gradual transition from an incoherent quasi-two-dimensional c-axis transport in underdoped to a coherent metallic transport in overdoped cuprates. Furthermore, due to a very small subgap current, the singularity allows unambiguous extraction of the superconducting gap, without distortion by self-heating.

  • 57.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Quantum Cascade Phenomenon in Bi2Sr2CaCu2O8+δ Single Crystals2006Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 97, artikel-id 257003Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study interlayer transport in Bi2Sr2CaCu2O8+δ cuprates, which represent stacks of atomic scale intrinsic Josephson junctions. A series of resonant dips in conductance is observed at condition when bremsstrahlung and recombination bands in nonequilibrium spectrum of Josephson junctions overlap. The phenomenon is explained in terms of self-detection of a new type of collective strongly nonequilibrium state in natural atomic superlattices, bearing certain resemblance with operation of a quantum cascade laser. Conclusions are supported by in situ generation-detection experiments and by numerical simulations.

  • 58.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Radiative annihilation of a soliton and an antisoliton in the coupled sine-Gordon equation2012Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, nr 13, s. 134525-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the sine-Gordon equation solitons and antisolitons in the absence of perturbations do not annihilate. Here, I present numerical analysis of soliton-antisoliton collisions in the coupled sine-Gordon equation. It is shown that in such a system, soliton-antisoliton pairs (breathers) do annihilate even in the absence of perturbations. The annihilation occurs via a logarithmic-in-time decay of a breather caused by emission of plasma waves in every period of breather oscillations. This also leads to a significant coupling between breathers and propagating waves, which may lead to self-oscillations at the geometrical resonance conditions in a dc-driven system. The phenomenon may be useful for achieving superradiant emission from coupled oscillators.

  • 59.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Superconducting condensate residing on small Fermi pockets in underdoped cuprates2015Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, nr 22, artikel-id 224508Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    How does Fermi surface develop upon doping of cuprates, does it consist of large barrels or small pockets, which of them is responsible for superconductivity, and what is a role of the pseudogap? Those are actively debated questions, important for understanding of high temperature superconductivity. We study doping dependence of intrinsic tunneling in Bi2Sr2Ca1-x YxCu2O8+delta cuprates, which allows independent analysis of Cooper pair and quasiparticle transport. We observe that with decreasing doping the supercurrent is rapidly decreasing at a rate much faster than the doping level, but the quasiparticle resistance at a sufficiently high bias remains almost doping independent. This remarkable discrepancy indicates that Cooper pairs and quasiparticles are originating from different parts of the Brillouin zone: Cooper pairs are residing on small pockets, which are progressively shrinking with decreasing doping, but the majority of the quasiparticle current is integrated over large barrels, which are only weakly doping dependent. The expanding pseudogap areas along the barrels do not contribute to pair current. This provides direct evidence for nonsuperconducting origin of the pseudogap. We present numerical calculations, taking into account Fermi surface topology, that support our conclusions.

  • 60.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Temperature dependence of the bulk energy gap in underdoped BiSrCaCuO: Evidence for the mean-field superconducting transition2009Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, s. 214510-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Understanding of the puzzling phenomenon of high-temperature superconductivity requires reliable spectroscopic information about temperature dependence of the bulk electronic density of states. Here I present a detailed analysis of the T evolution of bulk electronic spectra in Bi2Sr2CaCu2O8+δ obtained by intrinsic tunneling spectroscopy on small mesa structures. Unambiguous spectroscopic information is obtained by obviation of self-heating problem and by improving the spectroscopic resolution. The obtained data allow accurate determination of the superconducting transition temperature and indicate that (i) the superconducting transition maintains the mean-field character down to moderate underdoping and is associated with a rapid opening of the superconducting gap, which is well described by the conventional BCS T dependence. (ii) The mean-field critical temperature reaches maximum at the optimal doping and decreases with underdoping. Such behavior is inconsistent with theories assuming intimate connection between superconducting and antiferromagnetic spin gaps and supports proposals associating high-temperature superconductivity with the presence of competing ground states and a quantum critical point near optimal doping.

  • 61.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Terahertz electromagnetic radiation from intrinsic Josephson junctions at zero magnetic field via breather-type self-oscillations2011Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, nr 17, s. 174517-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    I propose a new mechanism of intense high-frequency electromagnetic wave generation by spatially uniform stacked Josephson junctions at zero magnetic field. The ac-Josephson effect converts the dc-bias voltage into ac supercurrent; however, in the absence of spatial variation of the Josephson phase difference it does not provide dc-to-ac power conversion, needed for emission of electromagnetic waves. Here I demonstrate that at geometrical resonance conditions, spatial homogeneity of the phase can be spontaneously broken by the appearance of breathers (bound fluxon-antifluxon pairs), facilitating effective dc-to-ac power conversion. This leads to self-oscillations at cavity-mode frequencies, accompanied by the emission of radiation. The proposed mechanism explains all major features of recently observed THz radiation from large-area Bi(2)Sr(2)CaCu(2)O(8+x) mesa structures.

  • 62.
    Krasnov, Vladimir M.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Golod, Taras
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Bauch and P. Delsing, T
    Anticorrelation between temperature and fluctuations of the switching current in moderately damped Josephson junctions2007Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, nr 22, s. 224517-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Presented are the results of calculations suggesting that the quasi-one-dimensional organic superconductors (TMTSF)2X (where TMTSF represents tetramethyltetraselenafulvalene and X is PF6 AsF6, ClO4, etc.) may show a substantial increase in their superconducting and spin-density-wave ordering temperatures when the Fermi level is raised through application of an electrostatic gating voltage. A rich behavior is observed, strongly dependent on the form of the superconducting order parameter, as the Fermi level approaches the Van Hove singularity at ka=0. Included are predictions for the behavior of these materials under zero and moderate applied pressure. It is found that TSDW as high as 50 K and superconducting Tc as high as 20 K may be achieved at optimal gate voltages of approximately 100 mV.

  • 63.
    Krasnov, Vladimir M.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Katterwe, Sven-Olof
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Rydh, Andreas
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Signatures of the electronic nature of pairing in high-T-c superconductors obtained by non-equilibrium boson spectroscopy2013Ingår i: Nature Communications, E-ISSN 2041-1723, Vol. 4, artikel-id 2970Artikel i tidskrift (Refereegranskat)
    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. 

  • 64.
    Krasnov, Vladimir M.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Motzkau, Holger
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Golod, Taras
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Rydh, Andreas
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Katterwe, Sven-Olof
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Kulakov, A. B.
    Comparative analysis of tunneling magnetoresistance in low-Tc Nb/Al-AlOx/Nb and high-Tc Bi2−yPbySr2CaCu2O8+δ intrinsic Josephson junctions2011Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 84, nr 5, artikel-id 054516Artikel i tidskrift (Refereegranskat)
    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.

  • 65.
    Motzkau, Holger
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Jacobs, Thorsten
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Katterwe, Sven-Olof
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Rydh, Andreas
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures2012Ingår i: Physical Review B, ISSN 2469-9950, Vol. 85, nr 14, artikel-id 144519Artikel i tidskrift (Refereegranskat)
    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.

  • 66.
    Motzkau, Holger
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Katterwe, Sven-Olof
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Rydh, Andreas
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Size-dependent transformation from triangular to rectangular fluxon lattice in Bi-2212 mesa structures2012Ingår i: 26th International Conference on Low Temperature Physics (LT26), Institute of Physics (IOP), 2012, artikel-id 052022Konferensbidrag (Refereegranskat)
    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.

  • 67.
    Motzkau, Holger
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Katterwe, Sven-Olof
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Rydh, Andreas
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Strong polaritonic interaction between flux-flow and phonon resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions: Angular dependence and the alignment procedure2013Ingår i: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 491, s. 51-55Artikel i tidskrift (Refereegranskat)
    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.

  • 68.
    Rydh, Andreas
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Golod, Taras
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Field- and current controlled switching between vortex states in a mesoscopic superconductor2009Ingår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 153, nr 1, s. 012027-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the controllable manipulation of vortices in a mesoscopic, superconducting "island" of Nb, using an integrated Josephson junction as a field-sensitive vortex detector. The island, divided by a single Josephson junction and suspended by Nb microbridges, was fabricated from a Nb/P11-xNix/Nb tri-layer using a focused ion beam. We find that the system at select magnetic fields behaves as a vortex memory cell, where current pulses can be used to switch the vortex configuration between metastable states of distinctly different junction critical currents. Non-destructive read-out of a state is then easily done with an intermediate current. Furthermore, we show that the Josephson junction displays a strong magnetoresistive effect at current bias well above the junction critical current but below the onset of flux flow. This enables the junction to be used as a quantitative probe of magnetic field with better than single flux quantum resolution.

  • 69.
    Tagliati, Stella
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Rydh, Andreas
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Differential membrane-based nanocalorimeter for high-resolution measurements of low-temperature specific heat2012Ingår i: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 83, nr 5, s. 055107-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A differential, membrane-based nanocalorimeter for general specific heat studies of very small samples, ranging from 0.5 mg to sub-mu g in mass, is described. The calorimeter operates over the temperature range from above room temperature down to 0.5 K. It consists of a pair of cells, each of which is a stack of heaters and thermometer in the center of a silicon nitride membrane, in total giving a background heat capacity less than 100 nJ/K at 300 K, decreasing to 10 pJ/K at 1 K. The device has several distinctive features: (i) The resistive thermometer, made of a Ge1-xAux alloy, displays a high dimensionless sensitivity |dlnR/dlnT| greater than or similar to 1 over the entire temperature range. (ii) The sample is placed in direct contact with the thermometer, which is allowed to self-heat. The thermometer can thus be operated at high dc current to increase the resolution. (iii) Data are acquired with a set of eight synchronized lock-in amplifiers measuring dc, 1st and 2nd harmonic signals of heaters and thermometer. This gives high resolution and allows continuous output adjustments without additional noise. (iv) Absolute accuracy is achieved via a variable-frequency-fixed-phase technique in which the measurement frequency is automatically adjusted during the measurements to account for the temperature variation of the sample heat capacity and the device thermal conductance. The performance of the calorimeter is illustrated by studying the heat capacity of a small Au sample and the specific heat of a 2.6 mu g piece of superconducting Pb in various magnetic fields.

  • 70.
    Zeinali, Arash
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Golod, Taras
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Surface superconductivity as the primary cause of broadening of superconducting transition in Nb films at high magnetic fields2016Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, nr 21, artikel-id 214506Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the origin of broadening of superconducting transition in sputtered Nb films at high magnetic fields. From simultaneous tunneling and transport measurements we conclude that the upper critical field H-c2 always corresponds to the bottom of transition R similar to 0, while the top R similar to R-n occurs close to the critical field for destruction of surface superconductivity H-c3 similar or equal to 1.7H(c2). The two-dimensional nature of superconductivity at H > H-c2 is confirmed by cusplike angular dependence of magnetoresistance. Our data indicates that surface superconductivity is remarkably robust even in disordered polycrystalline films and, surprisingly, even in perpendicular magnetic fields. We conclude that surface superconductivity, rather than flux-flow phenomenon, inhomogeneity, or superconducting fluctuations, is the primary cause of broadening of superconducting transition in magnetic field.

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