Niobium nitride Josephson tunnel junctions with magnesium oxide barriers

Shoji, Akira; Aoyagi, Masahiro; Kosaka, S.; Shinoki, Fujitoshi; Hayakawa, Hisao

doi:10.1063/1.95774

Cited by 132 publications

(26 citation statements)

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“…[9][10][11][12] Among them, only NbN/MgO/NbN junctions have exhibited good quality so far, because both NbN and MgO have the same crystal structure with a lattice mismatch of less than 5%. 8 However, the barrier height of MgO (2.8 eV) is so high that it is difficult to fabricate wellcontrolled high-J c junctions.…”

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confidence: 99%

High-quality epitaxial NbN/AlN/NbN tunnel junctions with a wide range of current density

Wang

Terai

Qiu

et al. 2013

Applied Physics Letters

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confidence: 99%

High-quality epitaxial NbN/AlN/NbN tunnel junctions with a wide range of current density

Wang

Terai

Qiu

et al. 2013

Applied Physics Letters

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“…We measured circular junctions of various diameters ranging from 2 to 10µm and consistently found a critical current density of 3A/cm 2 . Large values of the gap voltage (V g = 5.7mV ) and of the I co R N product, with R N the normal state resistance, of about 5mV have been measured along with a small subgap leakage voltage (V m = 23.5mV measured at 3mV) 20 . From the magnetic field dependence of the critical current we estimated a London penetration depth (λ L ) of about 190nm meanwhile the Josephson penetration depth was λ J = 150µm.…”

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confidence: 99%

Quantum crossover in moderately damped epitaxial NbN/MgO/NbN junctions with low critical current density

Longobardi¹,

Massarotti

Rotoli³

et al. 2011

Applied Physics Letters

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High quality epitaxial NbN/MgO/NbN Josephson junctions have been realized with MgO barriers up to a thickness of d=1 nm. The junction properties coherently scale with the size of barrier, and low critical current densities down to 3 A/cm 2 have been achieved for larger barriers. In this limit, junctions exhibit macroscopic quantum phenomena for temperatures lower than 90 mK. Measurements and junction parameters support the notion of a possible use of these devices for multiphoton quantum experiments, taking advantage of the fast non equilibrium electron-phonon relaxation times of NbN.PACS numbers: 74.50.+r, 85.25.CpAdvances in the quality of the traditional Josephson structures based on Nb and Al technology as well as the search for high quality junctions of different materials are a major contribution in superconducting electronics.In addition they may respond to a wider spectrum of qubits requirements and functionalities, and may contribute to investigate quantum regimes 1,2 . We inquire on the quantum behavior of low critical current density (J c ) NbN/MgO/NbN Josephson junctions (JJs). These kinds of JJs are usually aimed to high J c values of about 10kA/cm 2 and used in superconducting electronic circuits 3,4 . In this work we show that the NbN junction technology can be extended to low J c values of a) Electronic mail: llongobardi@ms.cc.sunysb.edu about 3A/cm 2 , which are two to four orders of magnitude lower than previously reported values 5 . NbN is a material of great interest for sensor applications 6-9 and it guarantees both fast non equilibrium electron-phonon relaxation times (τ ) and higher gap values, when compared with traditional junction technologies based on Nb, Al and Pb 10 .In this work, a canonical quantum behavior typical of a moderately damped junction is observed along with a crossover temperature between the thermal and quantum regime of about 90 mK. We found a phase diffusion regime, with a damping parameter Q ∼ 3 consistent with what observed in various Josephson systems 11 . The possibility to achieve very low critical current densities in moderately damped NbN junctions, combined with their short electron relaxation time τ < 10ps 12 , paves the way to ex-

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“…For NbN junctions such a thermal oxidation process is not yet established. Currently the best results are obtained by direct deposition of the barrier material 9 , and defects occur due to "missing atoms" since the barrier is comprised of merely 1 -2 atomic layers in high current density junctions 10 . In Fig.…”

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Observation of Andreev Reflection Enhanced Shot Noise

et al. 1997

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We have experimentally investigated the quasiparticle shot noise in NbN/MgO/NbN superconductor -insulator -superconductor tunnel junctions. The observed shot noise is significantly larger than theoretically expected. We attribute this to the occurrence of multiple Andreev reflection processes in pinholes present in the MgO barrier. This mechanism causes the current to flow in large charge quanta (Andreev clusters), with a voltage dependent average value of m ≈ 1 + 2∆ eV times the electron charge. Because of this charge enhancement effect, the shot noise is increased by the factor m.PACS numbers: 72.70.+m, 74.50.+r, 74.80.Fp A dc current I flowing through a vacuum tube or a tunnel junction generates shot noise, time dependent fluctuations of the current due to the discreteness of charge carriers. Shot noise studies provide information on the nature of conduction not obtainable by conductance studies, e.g. the electric charge of carriers or the degree of correlation in the conducting system. For an uncorrelated system in which the electrons do not interact, the passage of carriers can be described by a Poisson distribution. The spectral density of current fluctuations S I then equals full shot noise: S I = 2qI = P Poisson for zero frequency and temperature 1 . The charge quantum q is normally the electron charge e.In superconductor -normal metal (SN) systems Andreev reflection occurs, causing an effective charge to be transferred of 2e. Due to this doubling of the charge, the shot noise in such a system is predicted to have a maximum of twice the Poisson noise 2-4 . More recently, giant shot noise in the supercurrent is predicted in a singlechannel superconductor -normal metal -superconductor (SNS) point contact 5 which is attributed to transport of large charge quanta (q ≫ e) at finite voltages caused by Multiple Andreev Reflection (MAR)6 . Observation of enhanced charge quanta in SN or SNS structures requires a combination of conductance and shot noise measurements. Despite extensive theoretical work, experimental results in this field are rare. A recent experiment 7 is performed on a NbN/c/Nb structure in which c is assumed to be a Nb constriction with a length of 7 nm and a diameter of 15 nm. At 9.5 K the structure acts like an NS interface but doubled shot noise is not observed. The predicted giant supercurrent shot noise is not observed either (at 4.2 K).From an applied point of view, shot noise in superconducting structures is of interest since this noise forms a major limitation to the sensitivity of NbN/MgO/NbN Superconductor-Insulator-Superconductor (SIS) THz radiation detectors 8 . For these reasons, we have investigated quasiparticle current transport and shot noise in an SNS structure in which the quasiparticle current is carried by MAR. Anticipating the experimental shot noise results presented in this paper we demonstrate that in a system in which multiple Andreev reflections occurs the quasiparticle shot noise at V < 2∆/e has a maximum value given by S I = (1 +

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Niobium nitride Josephson tunnel junctions with magnesium oxide barriers

Cited by 132 publications

References 10 publications

High-quality epitaxial NbN/AlN/NbN tunnel junctions with a wide range of current density

High-quality epitaxial NbN/AlN/NbN tunnel junctions with a wide range of current density

Quantum crossover in moderately damped epitaxial NbN/MgO/NbN junctions with low critical current density

Observation of Andreev Reflection Enhanced Shot Noise

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