Session A21: Superconductivity: Josephson, Tunneling and Proximity Effects

Equal-spin Andreev reflection from quasiparticle interference effects in high-T$_{C}$/half metallic ferromagnet junctions

We report evidence for long-range superconducting correlations in the half-metallic ferromagnet (F) La$_{0.7}$Ca$_{0.3}$MnO$_{3}$, obtained from conductance measurements along the c-axis in SFS and SF junctions (with S the high-T$_{C}$ superconductor YBa$_{2}$Cu$_{3}$O$_{7})$. Well below the superconducting T$_{C}$, we observed oscillations in the differential conductance as a function of the voltage bias, in which we identified two sets of resonances: i) quasiparticle interferences in the S (top) layer (Tomasch resonances) and ii) in the half-metallic F layer (McMillan-Rowell resonances). Both of them imply Andreev-like reflections at the SF interface and the coherent propagation of the resulting phase-conjugated quasiparticles through the entire S and F layers thickness (up to 30 nm for the F layer). Because conventional Andreev reflection and the coherent long-range propagation of the outcoming opposite-spin electron/hole pairs are heavily suppressed in strongly polarized ferromagnets, the observation of the above effect implies the occurrence of equal-spin Andreev reflection at the studied SF interfaces.   

Proximity Effect in Mesoscopic Superconductor-Normal-Superconductor Arrays

Systems of superconducting islands on normal metal films provide a tunable medium with which to study the superconducting proximity effect, phase transitions, and vortex dynamics. Such systems are predicted to exhibit 2D zero-temperature metallic states. Although there has been experimental evidence of such states, they cannot be explained by conventional transport theory. Here, we report transport measurements on triangular arrays of mesoscopic, proximity-coupled Nb islands placed on normal metal Au films. The arrays undergo a two-step transition to a superconducting state; we characterize the superconducting transitions in these systems as a function of island thickness and spacing. The temperature of the first step of the transition linearly decreases with increasing island spacing, and the spacing-dependence of the second step deviates from conventional theories. Moreover, the trends of both steps suggest that the system is approaching zero-temperature metallic states. Through a phenomenological model, we resolve these transitions as a consequence of intra- and inter-island coupling between superconducting phases of individual Nb grains.   

Coherent Two Photon Production in Superconductor-Semiconductor Heterostructures

Connecting a thin (direct band gap) p-n junction to a superconductor allows Cooper pairs to tunnel into the junction. This leads to an enhancement of the luminescence at the junction via Cooper pair based radiative recombination[1,2], an effect that has recently been observed experimentally[3]. Due to the proximity-induced Cooper pairs in the junction, anomalous photon production related to coherent two photon processes becomes allowed. Using a simple model for direct band gap luminescence we study a superconductor-p-n-superconductor heterostructure where the two photon state depends on the relative phase between the two superconductors. We investigate to what extend the production rate of entangled photons is controlled by the phase difference between the attached superconductors. \newline \newline [1] E. Hanamura, Phys. Stat. Sol. (b) 234, 166 (2002). \newline [2] Y. Asano, I. Suemune, H. Takayanagi, and E. Hanamura, Phys. Rev. Lett. 103, 187001 (2009). \newline [3] I. Suemune, T. Akazaki, K. Tanaka, M. Jo, K. Uesugi, M. Endo1, H. Kumano, E. Hanamura, H. Takayanagi, M. Yamanishi and H. Kan, Jpn. Journ. of Appl. Phys. 45, 9264 (2006).   

Field-Dependent Effects in Superconductors Coupled to Semiconducting Heterostructures

We report transport measurements between superconducting leads separated by a small InAs gap. The NbTi superconducting layer is grown in-situ on top of the InAs to allow good contact. The samples are then fabricated into Hall bars with narrow gaps between the superconducting leads. Differential resistance and IV characteristics are measured in two and four terminal setups at 300mK both on and off quantum Hall plateaus. Multiple Andreev reflection peaks are observed (up to n=4 in some cases) and their field dependence measured. As the field increases, the MAR peaks shift to lower voltages and follow a general scaling law which holds across devices with different gap lengths. One device shows a supercurrent at all fields and in this device, MAR peaks corresponding both to the superconducting gap as well as a proximity-induced normal gap are seen. Explanations for the field-dependence of the MAR peaks will be discussed.   

Transport Properties Characterized by Magnetic Domain-Wall Motion in Josephson Junction

Nano-scale magnetic materials for spintronics devices are extensively studied due to many advantages. Non-volatile memory using a magnetic domain wall (DW) is one example of such devices, and the oscillatory DW is examined toward several applications. Once such devices are realized in a microscopic circuit, one needs to measure the DW motion more precisely. In this talk, we discuss the transport properties characterized by a magnetic domain wall (DW) motion in a ferromagnetic Josephson junction, which is composed of a ferromagnetic wire with DW and two superconducting electrodes. Our previous theory [1] is developed to include the DW motion into the Josephson current. By supposing a simplified DW structure, we find that the current-voltage curve exhibits stepwise structures, when DW oscillates in the ferromagnetic wire. The mechanism behind this result is common to the electric field generated by the vortex motion in the superconductor. \\[4pt] [1] S. Hikino, M. Mori, S. Takahashi, and S. Maekawa, J. Phys. Soc. Jpn, {\bf 77}, 053707 (2008).   

Dramatic suppression of the Josephson effect in floating superconductors

A number of devices exploiting the Josephson effect consist of floating superconducting islands. A well-known example are SISIS junctions (where S and I stand for superconductor and insulating barrier, respectively) present in many applications such as SQUIDs. The Josephson effect relies on the macroscopic phase describing the wave function of the electrons in the superconducting electrodes. This phase is fixed by the electrochemical potential of the superconductor that is settled experimentally. An intriguing situation arises then when dealing with floating superconducting islands that have no well-defined reference potential. Here we present an experimental characterization of Al/AlOx/Al/AlOx/Al junctions. We focus, on the one hand, on genuine SISIS junctions in which the central aluminium island remains floating and, on the other, on virtually grounded samples in which the central aluminium island is referred to a fixed potential through a metallic contact. We observe that, under identical circumstances, floating junctions exhibit a dramatic suppression of their Josephson current compared to their grounded counterparts. Possible reasons such as the overheating of the central island or quasiparticle fluctuations are addressed.   

Escape and retrapping phenomena in HTS and LTS Josephson Junctions

We investigate escape and retrapping dynamics in Josephson junctions characterized by different levels of dissipation. Measurements are carried out both on high (HTS) and low (LTS) critical temperature superconductor Josephson systems, characterized by different types of barriers, i.e. grain boundary and standard insulating layers. Based on the damping level we observe various regimes ranging from macroscopic quantum tunneling, thermal activation and phase diffusion processes. Experimental data are compared with a numerical model allowing a precise determination of the damping parameter.   

Dominant second harmonic in the Josephson current--phase relation: Manifestation of the long-range spin-triplet proximity effect in SFF'S junctions

We present theoretical study of the Josephson effect and pairing correlations in planar SFF'S junctions that consist of conventional superconductors connected through two metallic monodomain ferromagnets. Both singlet and triplet pair amplitudes, the Josephson current-phase relations, and density of states for arbitrary orientation of magnetizations are calculated from the self-consistent solutions of Eilenberger equations in the clean limit and for moderate disorder in ferromagnets. We find that in highly asymmetric SFF'S junctions the long-range spin-triplet proximity effect manifests itself as a dominant second harmonic in the Josephson current-phase relation [1] and gives distinctive tunneling conductance spectra [2]. Unambiguous detection of the long-range spin-triplet proximity effect by tunneling spectroscopy and experimental realization of the Josephson junctions ground state degeneracy (like at 0-pi transtions) is accessible for small interface roughness and moderate disorder in ferromagnets at low temperatures. \\[4pt] [1] L. Trifunovic, Z. Popovic and Z. Radovic, Phys. Rev. B 84, 064511 (2011).\\[0pt] [2] M. Knezevic, L. Trifunovic, and Z. Radovic (to be published).   

Andreev bound states in proximitized InAs nanowires

We present measurements of individual Andreev bound states (ABS) in semiconducting InAs nanowires contacted by a superconductor. The two ends of a U-shaped Al lead are deposited on the nanowire to form low-resistance contacts, between which a normal lead is then deposited to form a high-resistance tunnel contact to the nanowire. A tunneling current through the nanowire is formed by grounding the Al leads and by applying a voltage bias to the tunnel contact. Measurements of the differential conductance of the device as a function of voltage bias, magnetic field, and backgate show resonances that are associated with the density of states in the proximitized nanowire. This sub-gap structure depends periodically on magnetic flux, with a period of \textit{$\phi $}$_{0}=h$/2$e$. At voltages and magnetic fields exceeding the gap and the critical field of the Al leads, this structure disappears completely. Further control is achieved via a global backgate, with the tunneling current completely switched off at sufficiently low gate voltages. We interpret this structure as ABS arising from the normal electronic properties of the nanowires.   

A Superconductor in the Presence of an Exchange Field and Spin-orbit Interaction

A superconductor (SC) in contact with a magnetic semiconductor experiences a large internal exchange field (IEF) that acts on the spins [1,2]. The quasi particle density of states (\textit{q-DOS}) in the SC splits by the Zeeman energy, 2$\mu $H$_{EX}$ where $\mu $ is the electron magnetic moment and H$_{EX}$ is the IEF [1,2]. Here we combine IEF and spin-orbit (S-O) interaction to investigate the properties of Al by superconductive (SIS) tunneling spectroscopy [1]. Thin film sandwich junction 4EuS/4.5Al/($d$-Au)/Al$_{2}$O$_{3}$/8Al (film thickness in nm and $d$ from 0 to 0.06nm) was studied: ferromagnetic EuS layer provides the IEF whereas the Au layer creates the S-O scattering in Al. Tunnel conductance measured at 0.4K showed a large Zeeman splitting of \textit{q-DOS} in Al film due to the IEF, even at H$_{appl}$ = 0. In zero H the large asymmetry in the conductance peaks and reduction in the Zeeman splitting were observed due to S-O scattering when compared to control junctions (no Au). The sharp features in the SIS tunnel conductance allowed these studies even at the lowest level of S-O scattering.\\[4pt] [1] R. Meservey and P. M. Tedrow, Phys. Rep. 238, 173 (1994).\\[0pt] [2] J. S. Moodera, T. S. Santos and T. Nagahama, J. Phys.: Condens. Matter 19, 165202 (2007)   

Quantum tunneling of Normal-Superconducting interfaces in a type-I superconductor

The magnetic irreversibility in the intermediate state in various disks of pure type-I superconducting lead (Pb) with structural defects of diverse nature is presented. The results are discussed in terms of the contributions of the shape-dependent geometrical barrier (which is the origin of the so-called topological hysteresis) and the energy barriers associated to stress defects that act as pinning centers on normal-superconductor interfaces (NSI). The effect of the defects is to enhance the capability of the system to trap magnetic flux along the descending branch of the hysteresis cycle driving the system in a set of metastable states that would originate the occurrence of time-dependent phenomena. Magnetic relaxation studies reveal that the dynamics of the intermediate state of a type-I superconductor with defects is ruled by nonthermal processes for low enough temperatures. It is attributed to quantum tunneling of NSI mediated by the formation/flattening of bumps at the defects of the sample. The average value of the tunneling barriers is estimated and the temperature of crossover from the thermal to the quantum regime is obtained from the Caldeira-Leggett theory. Comparison between theory and experiment points to tunneling of interface segments of a size comparable to the coherence length, by steps of the order of 1 nm. Finally, the effect of an applied magnetic field on the quantum dynamics of the system is also explored.   

Josephson superjunction: an SIS junction with a correlated insulator

We consider the properties of an SIS Josephson junction, in which the insulating region is itself a superconductor, but below its superconducting-insulating transition. The junction could, for example, be made of a superconducting film, in which the central region is much thinner than the two regions on either side. To treat quantum fluctuations in both the insulating and the superconducting region, we model the insulator as itself an SIS Josephson array below its S/I transition and the superconductor as an array above that transition. We calculate the coupling energy of the junction as a function of insulator thickness using two methods. The first is a mean-field approach, generalized to allow for spatial variation of the order parameter. The second approach is to use quantum Monte Carlo simulation, which we use to calculate the phase stiffness as a function of insulator thickness. In both cases, the properties of the junction can be tuned by varying the thickness of the insulating region, and the properties of both superconducting and insulating materials.   

Neural dynamics in superconducting networks

We discuss the use of Josephson junction networks as analog models for simulating neuron behaviors. A single unit called a ``Josephson Junction neuron'' composed of two Josephson junctions [1] displays behavior that shows characteristics of single neurons such as action potentials, thresholds and refractory periods. Synapses can be modeled as passive filters and can be used to connect neurons together. The sign of the bias current to the Josephson neuron can be used to determine if the neuron is excitatory or inhibitory. Due to the intrinsic speed of Josephson junctions and their scaling properties as analog models, a large network of Josephson neurons measured over typical lab times contains dynamics which would essentially be impossible to calculate on a computer We discuss the operating principle of the Josephson neuron, coupling Josephson neurons together to make large networks, and the Kuramoto-like synchronization of a system of disordered junctions.\\[4pt] [1] ``Josephson junction simulation of neurons,'' P. Crotty, D. Schult and K. Segall, Physical Review E 82, 011914 (2010).   

Half-metallic $d$-wave Josephson junctions

We examine the dc Josephson effect in a ballistic superconductor/half-metal/superconductor junction by means of the Bogoliubov--de Gennes equations. We study the role of spin-active interfaces and compare how different superconductor symmetries affect the Josephson effect. We analyze critical current as a function of junction width, spin-flip strength and direction, and temperature. We show that the temperature-dependence of the supercurrent in the $d_{xy}$-symmetry case differs qualitatively from the $s$- and $d_{x^2-y^2}$-symmetries. Finally, we have found a general analytical expression for the Andreev Bound State-energies which shows how we can either induce $0-\pi$-transitions, or continuously change the ground state phase of the junction by controlling the magnetic misalignment at the interfaces.   

A Microfabricated Phonon Spectrometer to Spectrally Resolve Phonon Transport

In this work we show how frequency-dependent phonon transport can be probed with non-equilibrium phonons. We demonstrate a method of generating and detecting non-equilibrium phonons in micron- and nanoscale structures [1]. Our scalable method of fabricating phonon generators and detectors involves formation of Al-AlxOy-Al superconducting tunnel junctions on the sidewalls of a silicon mesa. In the line-of-sight path in these mesas, we generate and detect non-equilibrium, ballistically propagating phonons with a frequency $\sim$ 100 GHz. Phonons are generated through the decay of quasiparticles injected into one superconducting film of the generator. This process excites phonons in a tunable, non-thermal spectral distribution. The phonons radiate into the mesa and are observed by the detector after passing through the mesa. By utilizing electron-beam lithography and plasma etching, we demonstrate the fabrication of high aspect ratio nanostructures along the ballistic path in order to observe surface scattering effects. This work is supported by DOE (DE-SC0001086).\\[4pt] [1] J. B. Hertzberg et al, Rev. Sci. Instrum. 82, 104905 (2011).