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Register a new userImproving the Josephson energy in High-T$_c$ superconducting junctions for ultra-fast electronics
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We report the electrical transport in vertical Josephson tunnel junctions (area 400 $mu m$$^2$) using GdBa$_2$Cu$_3$O$_7$$_{-delta}$ electrodes and SrTiO$_3$ as an insulating barrier (with thicknesses between 1 nm and 4 nm). The results show Josephson coupling for junctions with SrTiO$_3$ barriers of 1 nm and 2 nm. The latter displays a Josephson of 8.9 mV at 12 K. This value is larger than the usually observed in planar arrays of junctions. Our results are promising for the development of superconducting electronic devices in the terahertz regime.
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We study the electrical transport of vertically-stacked Josephson tunnel junctions using GdBa$_2$Cu$_3$O$_{7-d}$ electrodes and a BaTiO$_3$ barrier with thicknesses between 1 nm and 3 nm. The junctions with an area of 20 mm x 20 mm were fabricated combining optical lithography and ion etching using GdBa$_2$Cu$_3$O$_{7-d}$ (16 nm) / BaTiO$_3$ (1 - 3 nm) / GdBa$_2$Cu$_3$O$_{7-d}$ (16 nm) trilayers growth by sputtering on (100) SrTiO$_3$. Current-voltage measurements at low temperatures show a Josephson coupling for junctions with BaTiO$_3$ barriers of 1 nm and 2 nm. Reducing the barrier thickness bellow a critical thickness seems to suppress the ferroelectric nature of the BaTiO$_3$. The Josephson coupling temperature is strongly reduced for increasing barrier thicknesses, which may be related to the suppression of the superconducting critical temperature in the bottom GdBa$_2$Cu$_3$O$_{7-d}$ due to stress. The Josephson energies at 12 K are of $approx$ 1.5 mV and $approx$ 7.5 mV for BaTiO$_3$ barriers of 1 nm and 2 nm. Fraunhofer patterns are consistent with fluctuations in the critical current due to structural inhomogeneities in the barriers. Our results are promising for the development of Josephson junctions using high-T$_c$ electrodes with energy gaps much higher than those usually present in conventional low-temperature superconductors.
Conventional Josephson metal-insulator-metal devices are inherently underdamped and exhibit hysteretic current-voltage response due to a very high subgap resistance compared to that in the normal state. At the same time, overdamped junctions with single-valued characteristics are needed for most superconducting digital applications. The usual way to overcome the hysteretic behavior is to place an external low-resistance normal-metal shunt in parallel with each junction. Unfortunately, such solution results in a considerable complication of the circuitry design and introduces parasitic inductance through the junction. This paper provides a concise overview of some generic approaches that have been proposed in order to realize internal shunting in Josephson heterostructures with a barrier that itself contains the desired resistive component. The main attention is paid to self-shunted devices with local weak-link transmission probabilities so strongly disordered in the interface plane that transmission probabilities are tiny for the main part of the transition region between two superconducting electrodes, while a small part of the interface is well transparent. We consider the possibility of realizing a universal bimodal distribution function and emphasize advantages of such junctions that can be considered as a new class of self-shunted Josephson devices promising for practical applications in superconducting electronics operating at 4.2 K.
We have investigated the temperature and magnetic field dependence of the Hall coefficient of two well-characterized superconducting MgB$_2$ films (T$_{c0}$=38.0 K) in both the normal and superconducting states. Our results show that the normal-state Hall coefficient R$_H$ is positive and increases with decreasing temperature, independent of the applied magnetic field. Below T$_c$(H), R$_H$ decreases rapidly with temperature and changes sign before it reaches zero. The position and magnitude at which R$_H$ shows a minimum depends on the applied field. Quantitative analysis of our data indicates that the Hall response of MgB$_2$ behaves very similarly to that of high-T$_c$ cuprates: R$_H$ $propto$ T and cot$theta_H$ $propto$ T$^2$ in the normal state, and a sign reversal of R$_H$ in the mixed state. This suggests that the B-B layers in MgB$_2$, like the Cu-O planes in high-T$_c$ cuprates, play an important role in the electrical transport properties.
We have studied low-frequency resistance fluctuations in shadow-evaporated Al/AlOx/Al tunnel junctions. Between 300 K and 5 K the spectral density follows a 1/f-law. Below 5 K, individual defects distort the 1/f-shape of the spectrum. The spectral density decreases linearly with temperature between 150 K and 1 K and saturates below 0.8 K. At 4.2 K, the spectral density is about two orders of magnitude lower than expected from a recent survey [D. J. Van Harlingen et al., Phys. Rev. B 70, 064510 (2004)]. Due to the saturation below 0.8 K the estimated qubit dephasing times at 100 mK are only about two times longer than calculated by Van Harlingen et al.
We study the spectrum of Andreev bound states and Josephson currents across a junction of $N$ superconducting wires which may have $s$- or $p$-wave pairing symmetries and develop a scattering matrix based formalism which allows us to address transport across such junctions. For $N ge 3$, it is well known that Berry curvature terms contribute to the Josephson currents; we chart out situations where such terms can have relatively large effects. For a system of three $s$- or three $p$-wave superconductors, we provide analytic expressions for the Andreev bound state energies and study the Josephson currents in response to a constant voltage applied across one of the wires; we find that the integrated transconductance at zero temperature is quantized to integer multiples of $4e^2/h$, where $e$ is the electron charge and $h = 2pi hbar$ is Plancks constant. For a sinusoidal current with frequency $omega$ applied across one of the wires in the junction, we find that Shapiro plateaus appear in the time-averaged voltage $langle V_1 rangle$ across that wire for any rational fractional multiple (in contrast to only integer multiples in junctions of two wires) of $2e langle V_1 rangle/(hbar omega)$. We also use our formalism to study junctions of two $p$- and one $s$-wave wires. We find that the corresponding Andreev bound state energies depend on the spin of the Bogoliubov quasiparticles; this produces a net magnetic moment in such junctions. The time variation of these magnetic moments may be controlled by an external applied voltage across the junction. We discuss experiments which may test our theory.
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