Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session R09: Superconductivity: Proximity Effect and Josephson Junctions II |
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Sponsoring Units: DCMP Chair: Richard Klemm, University of Central Florida Room: BCEC 151A |
Thursday, March 7, 2019 8:00AM - 8:12AM |
R09.00001: Ground-state Phase Oscillations in Josephson Junctions Containing Iron Alexander Madden, Ben Byrd, Reza Loloee, Norman Owen Birge Ferromagnetic Josephson junctions with a controllable ground-state phase difference are now well-established as a potential system for cryogenic memory applications. Many recent demonstrations of this phase control have used nickel as a fixed layer, with the phase controlled by the relative orientation of a magnetically soft layer. While nickel is a hard ferromagnet that allows high transmission of supercurrent, its multidomain structure and stray field have been obstacles to implementation in devices. Iron is another hard ferromagnet that has been investigated in S/F/S Josephson junctions and shown to undergo 0 – π phase oscillations with thickness [1]. We present a further investigation of iron Josephson junctions and their potential for use as memory devices. SQUID magnetometer measurements show that iron grown on a copper buffer layer gives a narrower magnetic transition and thinner magnetic dead layer than if grown directly on niobium. We also present junction transport measurements showing the critical current and 0 – π phase oscillations for a range of iron thickness grown on a 2nm copper buffer. |
Thursday, March 7, 2019 8:12AM - 8:24AM |
R09.00002: Appearance of above the gap resonances in dirty Josephson junctions with epitaxial contact Shahrzad Zare, William Andrew Mayer, Foster H Sabatino, Cody Youmans, Kaushini Wickramasinghe, Joseph Yuan, Matthieu Dartiailh, Aaron Somoroff, Pouyan Ghaemi Mohammadi, Javad Shabani In Josephson junctions, when the separation of the superconducting contacts is made long enough, the system can be approximated with two superconducting-normal metal junctions connected with a normal link. In devices fabricated on InAs-Al materials, we observe oscillations of conductance with driving bias above the superconducting gap. The frequency of conductance oscillations as a function of the bias is determined by the geometric properties of the junction. We interpret these geometric oscillations (GO) in interference of quasi-particles with energies above the superconductivity gap. This can be compared with a simple quantum mechanical problem involving a particle in the box where the states with energy above the box wall solely gain a phase shift as they pass over the box potential. In the case of GO the variation of the superconducting gap size affect the phase of the quasi-particles as they pass through. We compare our data with our theoretical model and find agreement in dependence of the data to bias, magnetic field and temperature. |
Thursday, March 7, 2019 8:24AM - 8:36AM |
R09.00003: Pair tunneling in La2-xSrxCuO4 junctions above Tc Panpan Zhou, Liyang Chen, Yue Liu, Anthony Travis Bollinger, Xi He, Ilya Sochnikov, Ivan Bozovic, Douglas Natelson High temperature superconductivity remains challenging more than thirty years after its initial discovery. One of the most captivating ongoing debates is about the mechanism and the normal state above the critical temperature. Various experiments imply paired charge carriers persisting above the transition temperature Tc, but without direct signatures of pairing. Shot noise, the intrinsic current fluctuations that result from the discreteness of current-carrying excitations, probes the magnitude of the charge of carriers. We report measurements on the shot noise of the tunneling current in c-axis epitaxial structure of La2-xSrxCuO4/La2CuO4/La2-xSrxCuO4 for several doping levels. At temperatures far above Tc, the shot noise agrees quantitatively with independent tunneling of carriers with charge magnitude e. Approaching but still above Tc, the noise exceeds the expectations of single charge tunneling, indicating paring of carriers. Well below Tc the noise is greatly enhanced within the bias region of the superconducting gap, consistent with multiple Andreev reflection processes. |
Thursday, March 7, 2019 8:36AM - 8:48AM |
R09.00004: Infrared and THz Optical Nanoscopy of High-Tc Superconductor Devices Adrian Gozar, Qianbo Lu, Anthony Travis Bollinger, Ivan Bozovic We use cryogenic atomic force microscopy (AFM) combined with scanning near-field optical microscopy (SNOM) to study thin films and nano-constriction devices in La2-xSrxCuO4 high-Tc cuprate superconductors. Using light in the mid-infrared region we demonstrate that our customized AFM-SNOM setup can provide three dimensional dielectric characterization of devices fabricated by tightly focused Helium ion beams. Light demodulation up to the 4th harmonic of the AFM tapping frequency allows us to observe in the dielectric response irradiation induced amorphization effects which extend on length scales that are orders of magnitude larger than the size of the focused ion beam. We ascribe this widespread damage to a Helium depth distribution substantially modified by internal device interfaces. Low-temperature data in the THz range enables detection of superconducting fluctuations in a 13 nm thick La2-xSrxCuO4 film with a superconducting critical temperature Tc ~ 31.5 K. Our results show that AFM-SNOM is a powerful technique for probing and characterizing superconducting interfaces and devices with nanometer scale resolution. |
Thursday, March 7, 2019 8:48AM - 9:00AM |
R09.00005: Non-vanishing Fraunhofer Pattern in WTe2 Josephson Junction Yong-Bin Choi, Chui-Zhen Chen, Jinho Park, Gaurav Rana, Hu-Jong Lee, Mazhar Ali, Kam Tuen Law, Kin Chung Fong, Gil-Ho Lee WTe2 is predicted to host Weyl nodes at the contact of electron and hole pockets in a momentum space (Type-II Weyl semimetal) and exhibit novel transport properties. We fabricated proximity Josephson junctions based on mechanically exfoliated WTe2 layers of thickness around 10-20 nm and studied their Josephson effects under magnetic field. Josephson critical current modulation as a function of perpendicular magnetic field, Ic(B), (so-called Fraunhofer pattern) exhibited non-vanishing lobes up to ~300 Gauss when Josephson current direction is parallel to a-axis of WTe2 crystal. This indicates that Josephson current through a-axis of WTe2 layer has enhanced contribution from the edges of the flake. However, flowing Josephson current parallel to b-axis of WTe2 resulted in ordinary Fraunhofer pattern. We will discuss about observed anisotropic quantum transport of WTe2 Josephson junction in regard to its topological nature. |
Thursday, March 7, 2019 9:00AM - 9:12AM |
R09.00006: Josephsonic diagnostic of competing orders in quantum critical multiband superconductors Maxim Dzero, Alex Levchenko Motivated by the recent experimentally observed manifestations of the quantum critical point fluctuations in the thermodynamic properties of multiband superconductors, we will present the derivation of a general expression for the Josephson current of various junctions between two superconductors in the phase of superconductivity coexistence with the spin-density-wave. We demonstrate that the critical current peaks at the quantum critical point that separates pure and mixed superconducting phases. We argue that our results are generic and, in particular, can be adopted to explain the recent observations of nonmonotonic dependence of the supercurrent on external pressure in the quantum critical superconductor CeRhIn5. |
Thursday, March 7, 2019 9:12AM - 9:24AM |
R09.00007: Supercurrent in InSb Quantum Point Contacts Bomin Zhang, Hao Wu, Sasa Gazibegovic, Ghada Badawy, Roy Op het Veld, Erik P. A. M. Bakkers, Sergey M Frolov We study superconductivity induced by NbTiN in InSb nanowires. We observe supercurrents of the regime of a single occupied subband and study their magnetic field evolution. In the multimode regime where multiple one-dimensional subbands are occupied, supercurrent interference due to magnetic field has been reported. It is predicted that in the limit of a single occupied subband interference is suppressed and supercurrents can be observed to high magnetic fields, allowing the study of spin-orbit and Zeeman effects on the Josephson effect. |
Thursday, March 7, 2019 9:24AM - 9:36AM |
R09.00008: Enhanced Superconductivity and Infinite Electro-resistance in Proximity Exchange Coupled Superconductor Nano-bridges by Electric Field - towards First Generation of Triplet Paired Superconductor FETs Mirko Rocci, Gilvania Da Silva Vilela, Dhavala Suri, Jagadeesh Moodera Recent breakthrough experiments on BCS superconductor based nano-bridges showed electric field control of the critical current (Ic) [1]. Their fundamental interest and high potential for cryogenic-nanoelectronics with this new type of all-metallic superconducting field-effect transistors (SuFETs) make them highly attractive. Here, we present results of gating experiments on superconducting thin NbN film (Tc ~12 K) based nano-SuFETs which show a tunable infinite electro-resistance (ER). Interestingly, we observe an enhancement in the Ic in stark contrast to the earlier work where a suppression of Ic was seen [1]. The observed ER is a consequence of a monotonic enhancement of the Ic (~25%) on applying electrostatic field through back-gate voltage in the range of ±30 V. Importantly, similar results are obtained in proximity coupled NbN/ferromagnetic-insulator nano-bridges. The ability to tune the superconducting properties of NbN layer with magnetic and electric fields will be discussed. This could lead to a novel triplet superconductor with exchange field [2] based SuFETs. |
Thursday, March 7, 2019 9:36AM - 9:48AM |
R09.00009: Shapiro steps in InAs-based Josephson junctions with epitaxial Al contacts Matthieu Dartiailh, Joseph Yuan, William Andrew Mayer, Kaushini Wickramasinghe, Dillon Liu, Aditi Mitra, Javad Shabani Semiconductor-based Josephson junction provide a platform to study the proximity effect and can serve as a platform for the realization of in topological superconductivity. Recently our group has demonstrated the possibility to have a highly transparent contact between the superconductor and the semiconductor by combining InAs high mobility surface 2D-dimensional electron gases with epitaxially grown aluminium. |
Thursday, March 7, 2019 9:48AM - 10:00AM |
R09.00010: Ultrasensitive Calorimetry in an Al/InAs Josephson Junction Raj Katti, Olli Saira, Matthew Matheny, Michael Roukes Calorimetry allows for the detection of single-shot energy pulses over a wide bandwidth. Pushing the limits of energy resolution to the yoctojoule (1e-24 J) regime would allow detection of individual thermal phonons and monitoring of dynamics in open quantum systems. We present work toward development of a Josephson junction-based calorimeter fabricated from an Al/InAs superconductor-semiconductor heterostructure, a material system being developed for scalable topological quantum computation. Ongoing work seeks to demonstrate a device proof-of-principle while simultaneously characterizing fundamental thermal properties of this material system. |
Thursday, March 7, 2019 10:00AM - 10:12AM |
R09.00011: Unconventional Josephson effect in 2D Josephson junctions based on In0.75Ga0.25As quantum wells Kaveh Delfanazari, Pengcheng Ma, Ian Farrer, David A Ritchie, Hannah Joyce, Michael Joseph Kelly, Charles G Smith Here, we report on the discovery of an unconventional Josephson effect in hybrid In0.75Ga0.25As two-dimensional electron gas-Nb junctions that are designed and fabricated for scalable hybrid quantum circuit integration [1-3]. By sweeping the in-plane magnetic field, we observe a completely new type of oscillation mechanism in the differential conductance, that has not been identified before. We find that the differential conductance as a function of source-drain voltage shows two symmetric in-gap resonances, which are strongly temperature and magnetic field dependent. The resonances amplitudes enhance with increasing in-plane magnetic fields up to a critical field where they gradually suppress and disappear. The observed supercurrent also gradually increases with applied magnetic field up to the same critical field. We believe that these striking observed behaviours in our devices-that cannot be explained by the conventional Josephson effect phenomenon- may be related to the context of topological superconductivity in hybrid 2D systems. |
Thursday, March 7, 2019 10:12AM - 10:24AM |
R09.00012: Effect of Material Quality on Superconducting Proximity Effect in Al-InAs Heterostructures Kaushini Wickramasinghe, William Andrew Mayer, Joseph Yuan, Tri Nguyen, Matthieu Dartiailh, Kasra Sardashti, Vladimir Manucharyan, Javad Shabani Renewed theoretical and experimental interest in InAs heterostructure with two-dimensional electron systems (2DESs) confined to the surface is partly due to their potential applications in topological and superconducting quantum computation. Here we study Josephson junctions with epitaxial Al-InAs contacts and find large super currents and substantial product of ICRN in our high mobility wafers. We also compare the product of excess current and normal resistance, IexeRN of high mobility samples and low mobility samples. The excess current is negligible in the devices made of lower mobility Al-InAs samples whereas it is substantial and independent of gate voltages and junction length in high mobility sample. For the devices made with high mobility samples, we found that ICRN/Δ~2.2. Further we discuss how the above parameters are linked to the material quality of the samples. |
Thursday, March 7, 2019 10:24AM - 10:36AM |
R09.00013: Probing pair-breaking mechanisms in proximity-induced hybrid superconducting interfaces Karthik Raman Understanding depairing effects in a hybrid-superconducting interface utilizing high spin-orbit materials is an important study in proximity-induced superconductivity. Experimentally, proximity-induced superconductivity is found to suppress at much lower magnetic fields compared to the superconducting layer without a good understanding of its cause. Here, we provide a phenomenological tool to characterize different pair-breaking mechanisms, the ones that break or preserve time reversal symmetry, and show how they affect the differential tunneling conductance response. Importantly, we probe the properties of the SC layer at the hybrid interface and observe conductance peak pinning at zero bias in a larger field range with eventual signs of weak peak splitting. Further, the effect of varying the spin-orbit scattering and the Lande g-factor in tuning the conductance peaks show interesting trends. |
Thursday, March 7, 2019 10:36AM - 10:48AM |
R09.00014: Wavefunction of Andreev bound states with topological Weyl singularity in
multi-terminal Josephson junction Tomohiro Yokoyama, Shiro Kawabata We theoretically investigate a four-terminal Josephson junction. |
Thursday, March 7, 2019 10:48AM - 11:00AM |
R09.00015: Reduced Parameter Spread in Arrays of Planar MgB2 Josephson Junctions Made by Focused Helium Ion Beam Leila Kasaei, Mengjun Li, Thomas Melbourne, Leonard C Feldman, Torgny Gustafsson, Ke Chen, Xiaoxing Xi Series arrays of up to 100 planar Josephson junctions on MgB2 film with less than 3 % spread in critical current at 10 K were successfully developed and characterized. This technique uses a 30 keV focused helium ion beam with nominal beam diameter less than 0.5 nm and range of dose from 0.9 to 3 × 10^16 (Ions/cm^2)[1] to locally damage the 25 nm-thick MgB2 thin films grown by hybrid physical-chemical deposition (HPCVD) on SiC substrates to form the junction barriers. A typical single junction has an IcRn of 70 μV at 20 K and RSJ-like current-voltage characteristics. Under microwave radiation, flat Shapiro steps up to 150 μA width appear at voltages Vn = Nn(Φ0) f, where N is the number of junction in the array, n is an integer representing Shapiro step index, and f is the applied microwave frequency. The greatly reduced spread in critical current is a significant improvement over the previous studies. This technique may lead to applications including Josephson voltage standards and arbitrary function generators that can work at around 20 K. |
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