Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session P8: Superconductivity: Proximity Effects and SN Junctions I |
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Sponsoring Units: DCMP DMP Room: 304 |
Wednesday, March 16, 2016 2:30PM - 2:42PM |
P8.00001: Anisotropic Suppression of Superconductivity on Pb(111)/Mn$_{5}$Ge$_{3}$/Ge(111) Howon Kim, Yuki Nagai, Takeo Kato, Yukio Hasegawa When a superconductor comes in a good contact to non-superconducting materials, the superconducting pair correlation and its breaking penetrate each other from the interface. In particular, superconductor/ferromagnet(S/F) interface has been of great interests because of emergence of exotic superconducting (inverse) proximity effect on both sides. In spite of extensive efforts for the S/F interfaces, the experimental approach was limited because of difficulties in fabricating the high quality interface and probing it in the nanometer scales. Here, we studied superconductivity of Pb(111) layers which are formed on the ferromagnetic Mn$_{5}$Ge$_{3}$ island structures grown on a Ge(111) substrate at T\textasciitilde 0.5 K. From our spatially-resolved local tunneling spectra over the top surface of Pb(111) on the ferromagnetic islands, we found that the superconducting property above the magnetic island was strongly suppressed and that the suppression was laterally spread from the strongly suppressed area in an anisotropic manner. By considering the scattering and propagating behaviors of the broken cooper pairs at the fermi surface of Pb(111), we calculate the local density of states at the top of the Pb(111) layer above the Mn$_{5}$Ge$_{3}$ island, and found that the anisotropic suppression of superconductivity mainly due to the anisotropic shape of the fermi surface of Pb. Possible origins of anisotropic suppression of superconductivity will be discussed. [Preview Abstract] |
Wednesday, March 16, 2016 2:42PM - 2:54PM |
P8.00002: Spin-triplet superconducting proximity effect in SrRuO$_{\mathrm{3}}$/Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ hybrids Muhammad Shahbaz Anwar, Seungran Lee, R Ishiguro, Y Sugimoto, Y Tano, S. J. Kang, Y.J. Shin, Shingo Yonezawa, H Takayanagi, Tae Won Noh, Yoshiteru Maeno Spin-\textit{triplet} superconducting correlations can be induced into a ferromagnet (FM) out of a spin-\textit{singlet} superconductor (SSC) via magnetic inhomogeneity at the SSC/FM interface. In this case, however, the proximity effect is not readily controllable because spins are quenched. In contrast, superconducting spintronics can be realized by using spin-\textit{triplet} superconductors (TSCs) and FM hybrids. Theoretically, it has been predicted that spin-triplet proximity effect can be controlled by the relative orientations between the magnetization in the FM and the Cooper pair spin in TSC. We fabricate Au(600-nm)/SrRuO$_{\mathrm{3}}$(15-nm) /Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ junctions by growing epitaxial SrRuO$_{\mathrm{3}}$ FM thin films on Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ TSC single crystals. Differential conductance vs voltage shows the conductance enhancements with superconducting transitions at three different characteristic voltages. These three features can naturally be interpreted as originating from the SC gaps in bulk Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ as well as at two distinct interfaces (Au/SrRuO$_{\mathrm{3}}$ and SrRuO$_{\mathrm{3}}$/Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}})$. The effect of applied magnetic field reveals that the proximity effect is robust against the loss of magnetic inhomogeneity. [Preview Abstract] |
Wednesday, March 16, 2016 2:54PM - 3:06PM |
P8.00003: Waiting times of entangled electrons in normal-superconducting junctions Denis Chevallier, Mathias Albert, Pierre Devillard We consider a normal-superconducting junction in order to investigate the effect of new physical ingredients on waiting times. First, we study the interplay between Andreev and specular scattering at the interface on the distribution of waiting times of electrons or holes separately. In that case the distribution is not altered dramatically compared to the case of a single quantum channel with a quantum point contact since the interface acts as an Andreev mirror for holes. We then consider a fully entangled state originating from spliting of Cooper pairs at the interface and demonstrate a significant enhancement of the probability to detect two consecutive electrons in a short time interval. Finally, we discuss the electronic waiting time distribution in the more realistic situation of partial entanglement. [Preview Abstract] |
Wednesday, March 16, 2016 3:06PM - 3:18PM |
P8.00004: Measurements of the superconducting proximity effect in Pd/Al NS bilayers at GHz frequencies. Ani Nersisyan, Riccardo Manenti, Michael Peterer, Einar Magnusson, Giovanna Tancredi, Andrew Patterson, Peter Leek The superconducting proximity effect, well known since the 1960s, describes superconductivity in the case of a superconductor contacted to a normal metal, and is typically studied experimentally using transport techniques such as tunneling spectroscopy [1-4]. Here we will present studies of the superconducting proximity effect in thin film palladium/aluminum NS bilayers using microwave frequency lumped element LC resonators. Measurements of the resonance frequency and quality factor as a function of temperature and film thickness reveal properties of the NS bilayers such as the critical temperature and penetration depth. Our results should be useful for understanding losses in superconducting quantum circuits that incorporate thin normal layers, and, in the particular case of Pd, should aid in design of hybrid superconducting quantum devices incorporating carbon nanotubes with high contact transparency [5]. [1] C. J. Adkins and B. W. Kingston, Phys. Rev 177, 777 (1969). [2] J. R. Toplicar and D. K. Finnemore, Phys. Rev. B 16, 2072 (1977). [3] A. Kastalsky, et. al., Phys. Rev. Lett. 64, 958 (1990). [4] S. Gu\'{e}ron, et. al., Phys. Rev. Lett. 77, 3025 (1996). [5] Y. Chai, et. al., IEEE Transactions on Electron Devices , 59, 1 (2012). [Preview Abstract] |
Wednesday, March 16, 2016 3:18PM - 3:30PM |
P8.00005: Electrostatic effects in semiconductor-superconductor heterostructures. Piyapong Sitthison, Tudor Stanescu We study the effects of an interface-induced bias on the ~charge distribution, ~proximity-induced superconducting gap, and spin-orbit coupling strength in semiconductor-superconductor hybrid structures. The effective bias potential is generated by the work function difference across the interface. ~We show that the size of the induced superconducting gap (relative to the bulk gap) depends on the geometry of the structure and, in addition, is controlled by two effective parameters: the work function difference and the effective semiconductor-superconductor coupling.~The interface-induced bias also breaks inversion symmetry, which leads to a nonzero ~value of the Rashba spin orbit coupling. We systematically study the dependence of the induced gap and Rashba coupling strength on the geometry of the structure (e.g., the diameter of the wire) and the relevant effective parameters (i.e. work function difference and semiconductor-superconductor coupling strength). [Preview Abstract] |
Wednesday, March 16, 2016 3:30PM - 3:42PM |
P8.00006: Differential Conductance in Semiconductor-Superconductor Hybrid Structures John Stenger, Tudor Stanescu We construct a theory for calculating the differential conductance in semiconductor-superconductor hybrid structures that accounts for both the current carried by quasiparticles in the bulk superconductor and the contributions due to proximity effects induced in the semiconductor. Starting with a Blonder-Tinkham-Klapwijk (BTK) type approach, we show that the superconductor degrees of freedom can be conveniently integrated out and replaced by an interface `Green function' determined by the properties of the superconductor and the original outgoing-wave boundary conditions corresponding to quasiparticle propagation. We find that the features present in the differential conductance are associated with both semiconductor and bulk superconductor spectral features, with a relative strength that depends on the parameters of the structure. We systematically investigate the dependence of the differential conductance on the parameters of the system, including coupling strength, semiconductor band occupancy, and barrier transparency, and correlate our findings with recent experimental measurements on proximity-coupled semiconductor wires. [Preview Abstract] |
Wednesday, March 16, 2016 3:42PM - 3:54PM |
P8.00007: Super-Hard induced gap in InSb nanowires Jun Chen, Peng Yu, Moïra Hocevar, Sébastien Plissard, Diana Car, Erik Bakkers, Sergey Frolov In recent years, Majorana bound states were observed experimentally in InSb nanowire-superconductor hybrid devices, which manifested themselves as a zero-bias conductance peak (ZBP). However, there was still significant conductance inside the superconducting gap, which would smear sub-gap features. Moreover, fermionic states inside the gap would also break topological protection. Therefore, a hard gap is required in search of more deterministic signatures of Majorana bound states, and building up Majorana qubits. We report the observation of a hard induced gap in an InSb Josephson junction with an optimized superconducting contact recipe. The gap is resolved in magnetic field up to 2 Tesla, and demonstrates a peculiar kinked field dependence. In addition, we observed rich sub-gap features: Andreev levels appeared close to pinch off regime, while multiple Andreev reflection appeared in open regime. [Preview Abstract] |
Wednesday, March 16, 2016 3:54PM - 4:06PM |
P8.00008: Proximity semiconducting nanowire junctions from Josephson to quantum dot regimes Kaveh Gharavi, Gregory Holloway, Jonathan Baugh Experimental low-temperature transport results are presented on proximity-effect Josephson junctions made from low bandgap III-V semiconductor nanowires contacted with Nb. Two regimes are explored in terms of the Nb/nanowire interface transparency $t$. (i) High $t$ allows a supercurrent to flow across the junction with magnitude $I_c$, which can be modulated using the voltage $V_g$ on a global back gate or a local gate. Relatively high values are obtained for the figure-of-merit parameter $I_c R_N/(e \Delta) \sim 0.5$, and $t \sim 0.75$, where $R_N$ is the normal state resistance and $\Delta$ the superconducting gap of the Nb leads. With the application of an axial magnetic field, $I_c$ decays but exhibits oscillations before being fully suppressed. The period and amplitude of the oscillations depend on $V_g$. Possible explanations for this behaviour are presented, including Josephson interference of the orbital subbands in the nanowire. (ii) Lower transparency correlates with a spontaneous quantum dot (QD) formed in the nanowire channel. Pairs of Andreev Bound States (ABS) appear at energies $|E| < \Delta$, with one pair unexpectedly pinned at $E=0$ for a wide range of $V_g$. A description of the QD-ABS system beyond the Anderson model is presented to explain the latter results. [Preview Abstract] |
Wednesday, March 16, 2016 4:06PM - 4:18PM |
P8.00009: Metal organic chemical vapor deposition of core-shell InAs-Al nanowires for proximity-induced superconductivity T. R. Hartke, J. Stehlik, J. R. Petta The zero-bias conductance peaks observed in proximitized InSb nanowires have been interpreted as evidence of Majorana fermions.\footnote{V. Mourik \textit{et al.}, Science \textbf{336}, 1003 (2012).} However, these observations are complicated by the presence of a non-zero conductance throughout the gap, which has been termed a ``soft-gap.'' The characteristics of the gap can be improved by using MBE to epitaxially grow a superconducting aluminum shell around an InAs core.\footnote{W. Chang \textit{et al.}, Nature Nanotech. \textbf{10}, 232 (2015).} Here we use metal organic chemical vapor deposition (MOCVD) to grow high quality InAs nanowires on predefined Au catalyst sites. An aluminum shell is deposited immediately after the InAs growth is terminated. The resulting core-shell nanowires are structurally and electrically characterized. [Preview Abstract] |
Wednesday, March 16, 2016 4:18PM - 4:30PM |
P8.00010: Superconducting gap closing and Zero-bias peak in InSb nanowire Peng Yu, Jun Chen, Moïra Hocevar, Sébastien Plissard, Diana Car, Erik Bakkers, Sergey Frolov In a 1D superconductor-nanowire-normal contact system, Majorana bound states are expected to appear after topological phase transition. Although there are many experiments reported possible zero-bias conductance peak from Majorana bound states, mapping out of the topological phase diagram is still missing.In our InSb nanowire hybrid devices, we observed possible superconducting gap closing and re-opening with magnetic field. These gap closings appear near conductance resonances which show some feature of 1D subband edges. Interestingly, zero-bias conductance peak appears inside the split regime of crossings at finite magnetic field. The magnetic field onset of the zero-bias peak can be tuned by gates underneath the superconductor, which may result from the changing of chemical potential. [Preview Abstract] |
Wednesday, March 16, 2016 4:30PM - 4:42PM |
P8.00011: Symmetry breaking in SNS junctions: edge transport and field asymmetries Henri Suominen, Fabrizio Nichele, Morten Kjaergaard, Asbjorn Rasmussen, Jeroen Danon, Karsten Flensberg, Leonid Levitov, Javad Shabani, Chris Palmstrom, Charles Marcus We study magnetic diffraction patterns in a tunable superconductor-semiconductor-superconductor junction. By utilizing epitaxial growth of aluminum on InAs/InGaAs we obtain transparent junctions which display a conventional Fraunhofer pattern of the critical current as a function of applied perpendicular magnetic field, $B_\perp$. By studying the angular dependence of the critical current with applied magnetic fields in the plane of the junction we find a striking anisotropy. We attribute this effect to dephasing of Andreev states in the bulk of the junction, leading to SQUID like behavior when the magnetic field is applied parallel to current flow. Furthermore, in the presence of both in-plane and perpendicular fields, asymmetries in $\pm B_\perp$ are observed. We suggest possible origins and discuss the role of spin-orbit and Zeeman physics together with a background disorder potential breaking spatial symmetries of the junction. [Preview Abstract] |
Wednesday, March 16, 2016 4:42PM - 4:54PM |
P8.00012: Proximity induced Shiba states in an organic radical molecular junction Joshua Island, Rocco Gaudenzi, Enrique Burzuri, Herre van der Zant Superconductors containing magnetic impurities lead to interesting phenomena derived from the interaction between Cooper pairing and Kondo screening. Here, we present measurements on proximity induced superconducting break-junctions hosting a magnetic impurity in the form of a neutral and stable, all organic radical molecule. Transport measurements reveal sub-gap excitations which are characteristic of a spin-induced, Yu-Shiba-Rusniov (Shiba) bound state due to the interaction of the radical's unpaired spin with a strongly coupled, proximity-induced superconductor. We show that by applying an external magnetic field to suppress the proximity induced superconductivity, a zero bias peak emerges signaling Kondo screening of the radical's unpaired spin coupled to normal leads. Our results show that Shiba states are a robust feature of the interaction between a magnetic impurity and a proximity induced superconducting junction. [Preview Abstract] |
Wednesday, March 16, 2016 4:54PM - 5:06PM |
P8.00013: Observation of conductance doubling in an Andreev quantum point contact M Kjaergaard, F Nichele, H Suominen, M Nowak, M Wimmer, A Akhmerov, J Folk, K Flensberg, J Shabani, C Palmstrom, C Marcus One route to study the non-Abelian nature of excitations in topological superconductors is to realise gateable two dimensional (2D) semiconducting systems, with spin-orbit coupling in proximity to an s-wave superconductor. Previous work on coupling 2D electron gases (2DEG) with superconductors has been hindered by a non-ideal interface and unstable gateability. We report measurements on a gateable 2DEG coupled to superconductors through a pristine interface, and use aluminum grown in situ epitaxially on an InGaAs/InAs electron gas. We demonstrate quantization in units of $4e^2/h$ in a quantum point contact (QPC) in such hybrid systems. Operating the QPC as a tunnel probe, we observe a hard superconducting gap, overcoming the soft-gap problem in 2D superconductor/semiconductor systems. Our work paves way for a new and highly scalable system in which to pursue topological quantum information processing. [Preview Abstract] |
Wednesday, March 16, 2016 5:06PM - 5:18PM |
P8.00014: Origin of Mesoscopic Superconductivity at Cd$_3$As$_2$ Point-Contacts Leena Aggarwal, Abhishek Gourav Sinha, Gohil S. Thakur, Zeba Haque, Ashok K. Ganguli, Goutam Sheet I will present our point-contact spectroscopy results on the nature and origin of superconductivity that is observed at the mesoscopic interfaces between the conventional metals and the 3-D Dirac semimetal Cd$_3$As$_2$. From our experiments with metallic tips of varying mechanical properties we show that the local superconducting phase does not emerge due to pressure. We show that quantum fluctuations may play a significant role in the emergence of such novel superconducting phase. [Preview Abstract] |
Wednesday, March 16, 2016 5:18PM - 5:30PM |
P8.00015: Two-dimensional epitaxial superconductor-semiconductor heterostructures: A platform for topological superconducting networks J. Shabani, M. Kjaergaard, H. J. Suominen, F. Nichele, Y. Kim, K. Pakrouski, S. Kraemer, T. Stankevic, P. Krogstrup, R. Feidenhans’l, R. M. Lutchyn, C. Nayak, M. Troyer, C. M. Marcus, C. J. Palmstrom Theory suggests that the interface between a one-dimensional semiconductor (Sm) with strong spin-orbit coupling and a superconductor (S) hosts Majorana modes with nontrivial topological properties. A key challenge in fabrication of such hybrid devices is forming highly transparent contacts between Sm and S. Recently, it has been shown that a near perfect interface and a highly transparent contact can be achieved using epitaxial growth of aluminum on InAs nanowires [1, 2]. In this work, we present the first two-dimensional epitaxial superconductor-semiconductor material system that can serve as a platform for topological superconductivity, and the search for quasiparticles such as Majorana zero modes that are predicted to obey non-abelian statistics. We show that our material system, Al-InAs, satisfies all the requirements necessary to reach into the topological superconducting regime by individual characterization of the semiconductor two dimensional electron system, superconductivity of Al and performance of S-Sm-S junctions [3]. [Preview Abstract] |
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