Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session Y39: Superconductivity: Mostly Devices |
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Sponsoring Units: DCMP Chair: Lei Wang, University of South Carolina Room: 386 |
Friday, March 17, 2017 11:15AM - 11:27AM |
Y39.00001: Josephson nano-resonators. Thomas McDermott, Hai-Yao Deng, Andreas Isacsson, Eros Mariani We study theoretically a suspended nano-electromechanical resonator coupled to superconducting contacts. The coupling between Josephson dynamics and mechanical oscillations is a subject that has had a small amount of theoretical studies (all in the voltage bias regime of difficult experimental realisability), but no experimental realisations so far. Here we show that by coupling the currents in the device to an in plane magnetic field, mechanical oscillations can be both activated and detected in suspended Josephson junctions under experimentally relevant DC current bias conditions. We~show that the~activation of mechanical oscillations has a dramatic effect on the IV characteristics of the junction, due to energy being converted from the Josephson system to mechanical oscillations,~allowing a fully-DC measurement of both the resonant frequency and the mechanical quality factor of the suspended device. [Preview Abstract] |
Friday, March 17, 2017 11:27AM - 11:39AM |
Y39.00002: The fabrication and application of nanoSQUIDs Lei Chen, Hao Wang, Xiaoyu Liu, Long Wu, Zhen Wang NanoSQUIDs with nano-bridge junctions can be miniaturized into the nano-scale to measure a single Bohr magneton. Recently, the Pb-SOT nanoSQUID achieved the spin sensitivity comparable to the magnet-tipped ultra-soft cantilever in MRFM. Therefore, it is possible to use a nanoSQUID as a spin sensor for the on-chip nanoMRI integration. However£¬current the performance of Nb planar nanoSQUIDs are limited by the shallow flux modulation depth. Here, we are presenting the research progress made 3D nanoSQUID at SIMIT. The 3D Nb nanoSQUID shows a above 60{\%} flux modulation depth and a reversible current-voltage curve. The working field range and flux noise of the Nb nanoSQUID is 0.5 T and 0.34 $\mu \Phi $0$\surd $Hz. Also, we used the nanoSQUID to measure the Meissner effect of a single indium particle (of 47 $\mu $m in diameter) and niobium particle (of 25 $\mu $m in diameter). The nanoSQUID measurement allowed us to observed a sharp Meissner effect transition of the small supercoductors which were greatly broadened in the commercial MPMS. [Preview Abstract] |
Friday, March 17, 2017 11:39AM - 11:51AM |
Y39.00003: Josephson Junctions Carrying Spin-Triplet Supercurrent for Cryogenic Memory Joseph A. Glick, Samuel Edwards, Demet Korucu, Bethany M. Niedzielski, Reza Loloee, W. P. Pratt Jr., Norman O. Birge Recent experiments with Josephson junctions containing ferromagnetic materials show that they have potential for cryogenic random access memory with extremely low power consumption [1]. We discuss a new class of junctions containing three magnetic layers that carry spin-triplet supercurrent and whose phase state can potentially be toggled between values of 0 and $\pi$ [2]. We discuss experiments in which the central magnetic layer, which must have magnetization perpendicular to both the top and bottom magnetic layers, is a synthetic anti-ferromagnet (SAF) with perpendicular magneto-anisotropy (PMA). One main advantage of this configuration is that PMA SAFs can be easily patterned into nano-magnets with very low stray fields, which may lead to better controlled magnetic switching of the other magnetic layers. Additionally, the long-range coherence of spin-triplet supercurrent would make the junctions less sensitive to the precise thickness of the ferromagnetic layers. Besides this there are enticing opportunities to explore the physics of spin-polarized supercurrent and the role of spin-dependent scattering asymmetries in these systems. \newline [1] E. C. Gingrich, et al., Nature Phys. 12, 564–567 (2016) \newline [2] M. Houzet and A. I. Buzdin, Phys. Rev. B 76, 060504(R) (2007) [Preview Abstract] |
Friday, March 17, 2017 11:51AM - 12:03PM |
Y39.00004: Optimization of Phase-Controllable Ferromagnetic Josephson Junctions for Cryogenic Memory Applications Bethany M. Niedzielski, Joseph A. Glick, Joshua Willard, Reza Loloee, W. P. Pratt Jr, Norman O. Birge Josephson junctions containing ferromagnetic layers are currently of interest for use in cryogenic memory. Last year, we successfully demonstrated that the phase of such a junction could be switched between 0 and $\pi $ by changing the magnetization direction of the two magnetic layers in the junction from the antiparallel to parallel configuration [1]. This shows great promise for superconducting memory, but to be used in applications, these phase-controllable junctions must have adequate critical current, low switching fields, moderate initialization fields, and a high degree of reproducibility. To address these concerns, we have carried out experiments to optimize the performance of the base superconducting electrode, normal metal spacer layers, and magnetic layers. We will report the results of these improvements as applied to our devices. [1] E. C. Gingrich, B. M. Niedzielski, J. A. Glick, Y. Wang, D. L. Miller, R. Loloee. W. P. Pratt Jr., and N. O. Birge, Nature Phys. \textbf{12,} 564--567 (2016)\textbf{.} [Preview Abstract] |
Friday, March 17, 2017 12:03PM - 12:15PM |
Y39.00005: Modeling Bloch oscillations in ultra-small Josephson junctions Heli Vora, Richard Kautz, Sae Woo Nam, Jose Aumentado In a seminal paper, Likharev et al. [1] developed a theory for ultra-small Josephson junctions with Josephson coupling energy (E$_{j}$) less than the charging energy (E$_{c}$) and showed that such junctions demonstrate Bloch oscillations which could be used to make a fundamental current standard that is a dual of the Josephson volt standard. Here, based on the model of Geigenm\"{u}ller and Sch\"{o}n [2], we numerically calculate the current-voltage relationship of such an ultra-small junction which includes various error processes present in a nanoscale Josephson junction such as random quasiparticle tunneling events and Zener tunneling between bands. This model allows us to explore the parameter space to see the effect of each process on the width and height of the Bloch step and serves as a guide to determine whether it is possible to build a quantum current standard of a metrological precision using Bloch oscillations.\\ $[1]$ K. K. Likharev and A. B. Zorin. Journal of Low Temperature Physics 59, 347 (1985)\\ $[2]$ U. Geigenm\"{u}ller and G. Sch\"{o}n, Physica B 152, 186 (1988) [Preview Abstract] |
Friday, March 17, 2017 12:15PM - 12:27PM |
Y39.00006: Rectification effect in artificial-spin-ice/superconductor hybrids Yong-Lei Wang, Jing Xu, Xiaoyu Ma, Zhili Xiao, Boldizsar Janko, Wai-Kwong Kwok Artificial spin ice, arrays of nanoscale single-domain bar magnets, coupled with a superconducting film can induce novel behavior at the interface of the two materials. Recently, we demonstrated a new twist in the design of artificial spin ice structures that can produce a magnetic charge ice with reconfigurable magnetic charge ordering and symmetries [Science 352, 962 (2016)]. This globally reconfigurable and locally writable magnetic charge ice structure could provide a new setting for designing and controlling the properties of superconducting films and other two-dimensional materials. Here, we present a reconfigurable rectification effect in an artificial-spin-ice/superconductor hybrid sample. By controlling the magnetic charge symmetries with an in-plane external magnetic field, we can achieve vortex rectification effects with in-situ tunable amplitude and polarization. [Preview Abstract] |
Friday, March 17, 2017 12:27PM - 12:39PM |
Y39.00007: Unusual occurrence of robust superconductivity and its coexistence with ferromagnetism in Ni/Ga bilayers Jagadeesh Moodera, Jia Song, Don Heiman, Valeria Lauter, Chen Li, Peng Wei We demonstrate in Ni/Ga bilayer an unusual superconductivity (SC), with T$_{\mathrm{C}}$ as high as 7K, that co-exists with ferromagnetism (FM). Tunneling spectroscopy studies at LHe temperatures showed the presence of three SC energy gaps in the bilayers, attributable to surface, interface and bulk states. The bottom Ni layers, ranging from 0.8 to 6nm thick, confirmed to be ferromagnetic by the hysteretic magnetization results. Moreover, spin polarized tunneling studies revealed that the tunneling electrons coming from the Ni surface were spin polarized and simultaneously displayed SC gap, supporting the co-existence of SC and FM. A hysteretic structural phase transition observed near room temperature indicates a possible Ni-Ga compound formation. Interfacial characterization with synchrotron x-ray and polarized neutron reflectivity measurements are being carried out to reveal the interfacial FM, chemical and structural properties. The interplay of SC and FM with the presence of spin polarized carriers in such bilayer system could be a strong case for triplet pairing and thus represents a significant step towards superconducting spintronics. Supported by NSF, ONR and Scientific User Facilities Division at ORNL by BES of DOE. [Preview Abstract] |
Friday, March 17, 2017 12:39PM - 12:51PM |
Y39.00008: True Josephson supercurrent in a Scanning Tunneling Microscope (STM) with a niobium tip and sample Wan-Ting Liao, J.R. Anderson, C.J. Lobb, F. Wellstood, S.K. Dutta, Michael Dreyer We have measured I-V characteristics of Josephson junctions formed between a Nb tip and a Nb surface in a 50 mK scanning tunneling microscope (STM). Depending on the distance between the tip and sample, which sets the normal state tunneling resistance R$_n$, the I-V characteristics are either in the phase-diffusion, underdamped or overdamped regime. For R$_n$ = 500 k$\Omega$ the I-V curves show a quasiparticle current rise of 2 nA when biased at V = 2$\Delta$/e, but the junction is in the phase-diffusion regime and no supercurrent is visible at V = 0. For R$_n$ between 5 k$\Omega$ and 50 k$\Omega$, the IV curves show a hysteretic switching response, as expected for an underdamped junction, and a true Josephson supercurrent at V = 0. For example, at R$_n$ = 5.95 k$\Omega$, the junction shows a quasiparticle current of 200 nA and a suppressed but true switching critical current of 15 nA. For R$_n$ $<$ 1 k$\Omega$, the tunneling characteristics are those of an overdamped junction suggesting that the tip may be in physical contact with the surface, producing a weak link. In this regime at R$_n$ = 272 $\Omega$, the critical current is about 3 $\mu$A and is non-hysteretic. We extract and discuss key tunneling parameters (gap, capacitance and loss) in the different regimes. [Preview Abstract] |
Friday, March 17, 2017 12:51PM - 1:03PM |
Y39.00009: Transport spectroscopy on trapped superconducting nano-islands of Lead Anshu Sirohi, Preetha Saha, Sirshendu Gayen, Avtar Singh, Goutam Sheet Mesoscopic point contacts on elemental lead (Pb) show dramatically higher critical magnetic field (up to several Tesla) than bulk Pb (Hc = 800 Oe). Due to this, when a magnetic field slightly higher than 800 Oe is applied on a Pb-point contact, the bulk of Pb becomes non-superconducting and a small mesoscopic superconducting region is trapped under the point contact which loses superconductivity at a much higher magnetic field. From detailed magnetic field dependent transport spectroscopy (Andreev reflection) of such nano-scale islands of Pb, we have found signature of an unconventional component in the superconductivity of Pb which might emerge due to high spin-orbit coupling in Pb. [Preview Abstract] |
Friday, March 17, 2017 1:03PM - 1:15PM |
Y39.00010: Magnetoanisotropic Josephson effect in superconductor/ferromagnet/superconductor (S/F/S) junctions Andreas Costa, Petra Hoegl, Jaroslav Fabian Heterostructures combining two nominally antagonistic states---superconductivity and ferromagnetism---are promising systems for future spintronic devices. Perhaps most striking in S/F/S Josephson junctions is the existence of $\pi$-states, in which an additional $\pi$-shift to the superconducting phase difference reverses the Josephson current flow compared to the usual (0-) state. Due to structure inversion asymmetry, interfacial spin-orbit fields invariably emerge in heterojunctions. By performing numerical calculations on S/F/S model junctions in the presence of interfacial Rashba and Dresselhaus spin-orbit fields, we study the unique signatures of the interplay of ferromagnetism and the spin-orbit fields in the Josephson current flow.\footnote{arXiv:1608.01218} We find that the Rashba fields can not only significantly enhance the Josephson current, but even induce transitions from 0- to $\pi$-states. As a clear indication for the spin-orbit fields, we predict marked magnetoanisotropies in the Josephson current. These anisotropies are huge compared to tunneling anisotropic magnetoresistance in normal-state junctions, particularly close to 0-$\pi$ transitions. Finally, we show that 0-$\pi$ transitions can also be manipulated by solely rotating the magnetization in the F layer. [Preview Abstract] |
Friday, March 17, 2017 1:15PM - 1:27PM |
Y39.00011: Magneto-transport in superconductor - half-metal - superconductor junctions Adrian Balan, Xavier Palermo, Anke Sander, David Sanchez, Victor Ruoco, Jacobo Santamaria, Javier Villegas Proximity-induced superconducting correlations can survive in half-metallic (HM) ferromagnets over relatively long distances, despite the nearly 100{\%} spin polarization of their conduction electrons, owing to the generation of equal-spin triplet pairs. This effect allows for long-range, fully spin-polarized supercurrents, which hold much promise for superconducting spintronic applications. Here we experimentally investigate the conditions under which this type of long-range supercurrents are generated in the MH ferromagnet LaCaMnO3, in junctions in which this material is sandwiched in between different types of superconductors, and where the thickness of the HM spacer is more than one order of magnitude higher than the coherence length of the singlet pairs. We will describe the fabrication techniques for those junctions, and show transport experiments in which the signatures of a triplet supercurrents include a periodic modulation of the conductance as a function of the applied magnetic field. [Preview Abstract] |
Friday, March 17, 2017 1:27PM - 1:39PM |
Y39.00012: Anomalous Meissner Screening Probed using Low Energy Muon Spin Spectroscopy R. Stewart, M. G. Flokstra, S. L. Lee, N. Satchell, S. Langridge, G. Burnell, H. Luetkens, T. Prokscha, E. Morenzoni In mesoscopic systems the Meissner response of a superconducting film can be very different from bulk. For normal metal (N) superconducting (S) bilayers screening can be enhanced depending on material properties.\footnote{W. Belzig, \textbf{PRB} 53, 5727} It has been shown that if N is replaced by an inhomogeneous ferromagnet (F) an inverse Meissner response is possible\footnote{M. Alidoust, \textbf{PRB} 89, 054508}. Low Energy Muon Spin Spectroscopy has recently been used to observe this effect\footnote{A. Di Bernadino, \textbf{PRX} 5, 041021}. We present low energy muon spin spectroscopy measurements on bi and trilayer systems where an enhancement of 300 \% is observed in the Meissner response when compared with an S layer. Using the quasiclassical approach we compare these results with calculations. Whilst for the N/S system theory and experiment are consistent, a large discrepancy results when adding an F layer to form an N/S/F trilayer. Rather than finding a reduction in the Meissner response we observe a 500 \% increase in flux expulsion. Moreover, this F layer enhancement can be tuned by modifying the interface. These observations contradict current SF proximity theory and are valuable for understanding more complex S, F, N systems in which there is much current interest. [Preview Abstract] |
Friday, March 17, 2017 1:39PM - 1:51PM |
Y39.00013: Anomalous Meissner effect in superconductor-ferromagnet proximity systems. Machiel Flokstra, Rhea Stewart, Steve Lee, Gavin Burnell, Nathan Satchell, Sean Langridge, Hubertus Luetkens, Thomas Prokscha, Elvezio Morenzoni We previously reported on induced magnetism in a normal metal using a superconducting spin-valve structure [Nature Physics 12, 57-61 (2016)] where a superconductor (S$=$Nb) separates a normal metal (N$=$Au) from a Co-based ferromagnetic (F) double layer. When cooling to below the superconducting transition temperature we observe an induced moment in the Au, opposite to the applied field and strongest near the Au/Nb interface. This result is in contradiction with theory and the mechanism behind it is unclear. To identify the requirement for this effect we measured simpler structures and found that in a N/S/F trilayer the flux expulsion is greatly enhanced (\textgreater 500{\%}) compared to the Meissner flux expulsion of a N/S bilayer. While the spatial distribution of the expulsion and its dependence on applied field suggest an enhancement of the Meissner effect, measurements close to zero applied field suggest there is also an additional effect. Moreover, when modifying the S/F interface region by placing a thin (few nm) insulating oxide barrier between the S and F layer we significantly enhance the flux expulsion, while in contrast placing a thin Pt layer instead the flux expulsion is significantly decreased. We believe that these results show the origin of the observed induced magnetism. [Preview Abstract] |
Friday, March 17, 2017 1:51PM - 2:03PM |
Y39.00014: Vortex matching effect in artificial-spin-ice /superconductor hybrids Jing xu, Yonglei Wang, Xiaoyu Ma, Zhili Xiao, Boldizsar Janko, Wai-Kwong Kwok Superconducting (SC) and ferromagnetic (FM) hybrid systems provide an intriguing combination of two contrasting phenomena and their mutual interaction has been extensively studied to tailor their electromagnetic behavior. Here, we present a novel FM/SC hybrid structure consisting of a unique artificial spin structure deposited onto a superconducting MoGe film. The spin structure can produce magnetic charge ice structures mimicking those of a square artificial spin ice with the added value that the long-range ordering can be easily realized and controlled with the application of an in-plane magnetic field [Science 352, 962 (2016)]. The magnetic charge ice can affect the behavior of superconducting vortices present in the underlying MoGe film. The novelty of the magnetic spin ice structure and its impact on vortex matching effect and dynamics will be presented. [Preview Abstract] |
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