Session A23: Proximity Effect and Josephson Junctions

Vertical Transport in Ferroelectric/Superconductor Heterostructures

We study electric field-effects in superconducting films by measuring vertical transport in ferroelectric/superconductor heterostructures. These are based on ultrathin (4 to 8 nm thick) BiFeO3-Mn grown on YBa2Cu3O7 by pulsed laser deposition. Nanoscale contacts are defined on the BiFeO3 via a series of nanofabrication steps which include e-beam lithography, metal deposition (Nb or Co capped with Pt) and lift-off. Conductive-tip atomic force microscopy and piezoresponse force microscopy are used to characterize the transport across the ferroelectric barrier as a function of its polarization (up/down). The observed electro-resistance, measured at various temperatures, allows studying the different electric-field screening in the normal and superconducting states.   

Two-dimensional simulations of the superconducting proximity in superconductor-semiconductor junctions

We simulate the consequences of the superconducting proximity effect on the DC current response of a semiconductor-superconductor proximity device within the quasiclassical formalism in the diffusively disordered limit. The device is modeled on in-situ fabricated NS junctions of superconducting Nb films on metallic doped InAs films, with electrical terminals placed in an N-S-N T-junction configuration. Due to the non-collinear configuration of this three terminal device, a theoretical model based on coupled two dimensional spectral and distributional Usadel equations was constructed and numerically solved using Finite-Elements methods. In the regime of high junction conductance, our numerical results demonstrate strong temperature and spatial dependencies of the proximity induced modifications to spectral and transport properties. Such characteristics deviate strongly from usual tunnel junction behavior and aspects of this have been observed in prior experiments[arXiv:1402.6055].   

STM/STS on proximity-coupled superconducting graphene

Graphene in good electrical contact with a superconductor has been observed to have an enhanced proximity effect. Application of a magnetic field is expected to generate an Abrikosov lattice of superconducting vortices, each containing Andreev bound states in its core. With our versatile, homebuilt, low temperature scanning tunneling force microscope (STM/SFM), we investigate the electronic properties of graphene on superconducting NbSe$_2$ in a magnetic field and search for signatures of these vortex core states.   

Closing the gap in the Andreev spectrum in a three-terminal superconducting junction

Quasiclassical circuit theory [1] is used to investigate transport in a mesoscopic junction with three superconducting terminals. Our study reveals the closing of the gap in the Andreev spectrum for a wide range of phase-biases and transparencies, in agreement with previous work [2]. In this regime a superconducting current flows in the junction, while the proximity mini-gap is closed. The corresponding parameter region is studied systematically, both analytically in the low transparency limit and numerically. We provide a microscopic explanation for the closing of the gap in terms of multiple pair processes that correlate the superconducting currents flowing between different pairs of terminals [3]. We show that multi-terminal superconducting junctions provide unique opportunities for applications in quantum devices based on Josephson and/or Majorana physics. \\[4pt] [1] Yu. V. Nazarov, Superlattices Microstruct. 25, 1221-1231 (1999). \\[0pt] [2] B. van Heck, S. Mi, and A. R. Akhmerov, Phys. Rev. B 90, 155450 (2014).\\[0pt] [3] A. Freyn, B. Dou\c{c}ot, D. Feinberg and R. M\'{e}lin, Phys. Rev. Lett. 106, 257005 (2011).   

Proximity Effects in Superconductor--Graphene Junctions

Superconducting proximity effects are of particular interest in graphene: because of its band structure, an unconventional (specular) Andreev reflection is expected [1]. In this context, high-Tc superconductor-graphene junctions are especially attractive. In these, the size of the superconducting energy-gap may exceed the graphene doping inhomogeneities around the Dirac point, which should favor the observation of the specular Andreev reflection. Yet, the fabrication of high-Tc superconductor-graphene junctions is challenging: the usual growth and lithography processes in both materials are incompatible. We report here on a fabrication method that allow us to fabricate planar cuprate superconductor-graphene junctions, which we characterize via conductance spectroscopy. We analyze the features in the conductance spectra as a function of graphene doping, and discuss them in the framework of the Andreev reflection. [1] C. W. J. Beenakker. Phys. Rev. Lett. 97, 067007 (2006)   

AC measurements of the superconducting proximity effect in metal nanowires

We quantitatively probe the admittance of diffusive superconductor/normal-metal/superconductor (SNS) weak links. Measurements are performed from 0.4 GHz to 12.8 GHz which includes regimes above and below the predicted characteristic energy scale for the superconducting proximity effect. Each device consists of a flux-biased SNS SQUID chain where N is a gold-palladium nanowire. The chain has strong capacitive coupling to a multimode microwave resonator. Measurements of the resonance frequency and quality factor for each resonator mode reveal the dissipative and reactive parts of the admittance of the SNS SQUID chain. The measurement results are valuable because they provide a direct test for theories of non-equilibrium superconductivity in SNS weak links [1,2] and because AC measurements have only recently been reported in the literature [3]. The data presented is also crucial for understanding losses in microwave circuits that employ SNS weak links, including nanobolometers [4]. \textit{References:} [1] P.~Virtanen et al.~Phys.~Rev.~B \textbf{83}, 144514 (2011), [2] F.~Kos et al.~Phys.~Rev.~B \textbf{87}, 174521 (2013), [3] B.~Dassonneville et al.~Phys.~Rev.~Lett.~\textbf{110}, 217001 (2013).~[4] J.~Govenius et al. Phys. Rev. B \textbf{90}, 064505 (2014)   

Cascading Proximity Effect and Singlet-Triplet Mixing in Rotating Magnetization Junctions

The proximity of a superconductor to an inhomogeneous magnetic material induces singlet and triplet pair correlations in the hybrid structure. The amount of each component and their presence deep in the magnetic material strongly depends on the magnetic inhomogeneity. We present a comparative study of pair correlations in a diffusive magnetic Josephson junction involving a multilayer with misaligned magnetization, a cosine-type helical structure and a more flexible and realistic domain wall of an exchange spring. Using the cascading effect [1] we demonstrate that the three systems induce qualitatively different mixtures of correlations. In particular, we show that misaligned and continuously rotating magnetizations do not display the same physical state. Analyzing the Gor'kov functions we find that so-called short range singlet correlations can be found deep in the magnetic material and compete with triplet correlations giving rise to the so-called singlet-triplet $0-\pi$ transition [2].\\[.2cm] $[1]$ T.E.~Baker, A.~Richie-Halford, O.E.~Icreverzi, and A.~Bill, Europhys.~Lett.~{\bf 107}, 17001 (2014).\\ $[2]$ T.E.~Baker, A.~Richie-Halford, and A.~Bill, New J.~Phys.~{\bf 16}, 093048 (2014). See also the video abstract!   

Spin-transfer torque effect in nanopillar superconducting-magnetic hybrid Josephson junctions

We have developed single nanopillar Josephson junctions with pseudo-spin-valve barriers with a feature size 50 nm or larger. We observed changes in Josephson critical current depending on the magnetization state of the barrier (parallel or anti-parallel) through the superconductor-ferromagnet proximity effect. The magnetization states of the pseudo-spin-valve barriers could also be switched with applied bias currents which is consistent with the spin-transfer torque effect in room-temperature spin valve devices. Our results demonstrate devices that combine superconducting and spintronic functions promising for a nanoscale cryogenic memory technology.   

Josephson Interference due to Orbital States in Nanowire Proximity Effect Junctions

The Josephson effect in a nanowire-based superconductor-normal-superconductor ($SNS$) junction is studied theoretically and experimentally, focusing on the effects of nanoscale confinement on the current-phase relationship of the junction. An axial external magnetic field is applied. The theory of a previously unstudied type of Josephson interference is described, based on the coupling between the axial flux and $N$-section Andreev quasiparticles (continuum states or bound states) occupying subbands of non-zero orbital angular momentum. The Bogoliubov-de Gennes equations are solved while considering the transverse subbands in the $N$-section, yielding energy-versus-phase curves that are shifted in phase in the presence of the flux. A similar phase shift is observed in the continuum current of the junction. An intuitive, semi-classical version of the theory is presented. The critical current $I_c$ of the junction is numerically calculated, and shown to oscillate versus the axial flux. Experimental observations of the oscillations of $I_c$ in an Nb-InAs nanowire-Nb junction are reported. It is shown that the observed oscillations can be described by the semi-classical picture. The scope and applicability of the theory to experimental devices is discussed.