Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E38: Superconductivity: Proximity Effect |
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Sponsoring Units: DCMP Chair: Joseph Prestigiacomo, Naval Research Laboratory Room: 385 |
Tuesday, March 14, 2017 8:00AM - 8:12AM |
E38.00001: Numerical simulation of the superconducting proximity effect in 3D including magnetic and spin-orbit interactions: vortex excitations and anomalous supercurrent flow Morten Amundsen, Jacob Linder In numerical studies of superconducting hybrid structures, one often resorts to effective one dimensional models. This introduces a layer of abstraction between experiments and theory where many of the geometrical effects are neglected. In addition, inherently higher dimensional phenomena such as charge/spin Hall effects and topological excitations like vortices and skyrmions may not be described by this approach. The main obstacle when including higher dimensional effects is the large amount of computer resources required and the difficulty in describing experimentally relevant geometries with the commonly used finite difference methods. Here, the Usadel equation is solved in two and three dimensions by means of the finite element method, thereby simulating the superconducting proximity effect in the diffusive limit. Both normal metals and ferromagnets with spin-orbit interactions will be considered. Through relevant examples, it will be demonstrated the ease with which the method handles realistic systems exhibiting higher dimensional effects, such as non-trivial geometries and complicated magnetization textures, and that it predicts new physical phenomena related to vortex physics and supercurrent flow which cannot be captured in 1D models. [Preview Abstract] |
Tuesday, March 14, 2017 8:12AM - 8:24AM |
E38.00002: Gapped triplet $p$-wave superconductivity in strong spin-orbit-coupled semiconductor quantum wells in proximity to $s$-wave superconductor Ming-Wei Wu, Tao Yu We show that the gapped triplet superconductivity can be realized in strong spin-orbit-coupled (100) quantum wells in proximity to $s$-wave superconductor. It is revealed that with the singlet order parameter induced from the superconducting proximity effect, in quantum wells, not only can the triplet pairings arise due to the spin-orbit coupling, but also the triplet order parameter can be induced due to the repulsive effective electron-electron interaction. Specifically, we derive the effective Bogoliubov-de Gennes equation, in which the self-energies due to the effective electron-electron interactions contribute to the singlet and triplet order parameters. It is further shown that for the singlet order parameter, it is efficiently suppressed due to this self-energy renormalization; whereas for the triplet order parameter, it is the $p$-wave ($p_x\pm ip_y$) one with the ${\bf d}$-vector parallel to the effective magnetic field due to the spin-orbit coupling. Finally, we perform the numerical calculation in InSb (100) quantum wells. It shows that with proper electron density, the minimum of the renormalized singlet and the maximum of the induced triplet order parameters are comparable, and hence can be experimentally distinguished. [Preview Abstract] |
Tuesday, March 14, 2017 8:24AM - 8:36AM |
E38.00003: Non-Magnetic Suppression of Superconductivity in Heteroepitaxial Perovskite/YBCO/Perovskite Thin Films C. Zhang, H. Zhang, J. Y.T. Wei To distinguish between the effects of strain and magnetism on superconductivity in epitaxial thin-film heterostructures comprising YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) and half-metallic manganites, we study perovskite/YBCO/perovskite trilayer films, using either ferromagnetic La$_{2/3}$Ca$_{1/3}$MnO$_3$ (LCMO) or paramagnetic LaNiO$_3$ (LNO) as the buffer and capping layers. For comparison with trilayers that are lattice-symmetry matched, orthorhombic PrBa$_2$Cu$_3$O$_{7-\delta}$ (PBCO) was also used instead of the perovskites. LCMO/YBCO/LCMO and LNO/YBCO/LNO trilayers show similar reduction in superconducting transition temperature ($T_c$) vs. decreasing YBCO thickness, from 50 and 12.5 nm, but PBCO/YBCO/PBCO trilayers show no such $T_c$ reduction. The tetragonal La$_2-x$Sr$_x$CuO$_4$ (LSCO) is also used in place of the orthorhombic YBCO, to further elucidate the effect of lattice-symmetry mismatch. Our results suggest that heteroepitaxial strain plays a stronger role than the proximity to ferromagnetism, for the suppression of superconductivity observed in manganite/YBCO heterostructures. [Preview Abstract] |
Tuesday, March 14, 2017 8:36AM - 8:48AM |
E38.00004: Superconducting contacts for 2D materials Bosong Sun, Tauno Palomaki, Yongchao Tang, Zaiyao Fei, Paul Nguyen, Wenjin Zhao, Guoxing Miao, Xiaodong Xu, David Cobden The incorporation of superconducting materials into 2D heterostructure devices is important for many purposes. For example, such devices might enable gating or other new ways of controlling the superconductivity, or proximitizing of 2D materials such as topologically nontrivial monolayers of WTe$_{2}$ or ZrTe$_{5}$ with helical edges that could harbor Majorana zero modes or other unusual excitations. Furthermore, the electronic properties of these structures could be amenable to surface spectroscopy techniques such as STS. The challenge is that both the 2D materials and most superconductors, including layered superconductors such as NbSe$_{2}$ easily oxidize, and hence creating the necessary high quality interface and maintaining it requires carefully avoiding oxidation and final encapsulation, for example with h-BN. We will report our results on a variety of approaches to incorporating superconducting contacts in encapsulated structures, including using graphene-protected exfoliated NbSe$_{2}$ and sputtered superconducting films. [Preview Abstract] |
Tuesday, March 14, 2017 8:48AM - 9:00AM |
E38.00005: Measurement of proximity induced superconductivity in MoTe2 Wudi Wang, Minhao Liu, Quinn Gibson, R. J. Cava, N.P. Ong MoTe2 is predicted to have type-II Weyl nodes and many of its novel transport properties have been predicted and studied. Here we reported an experiment on the superconductivity in MoTe2 induced by proximity effect. We fabricated a SQUIPT-like device on mechanical exfoliated MoTe2 micro flakes via nanofabrication. The device contains an Aluminum tunneling probe with AlOx barrier and Al contact. We measured tunneling current from probe to the sample. By fitting the differential conductance (dI/dV), we obtained the superconducting gaps in MoTe2. The dependence of gap in MoTe2 on temperature and magnetic field was measured. We also measured the current-phase relation in Al-MoTe2-Al Josephson junctions with an inductance based measurement technique. [Preview Abstract] |
Tuesday, March 14, 2017 9:00AM - 9:12AM |
E38.00006: Bi-2212/TaS$_2$ Van der Waals Junctions: Interplay of proximity induced high-$T_c$ Superconductivity and CDW order Ang Li, Xiaochen Zhu, Gregory Stewart, Arthur Hebard We present an experimental observation of high-$T_c$ superconducting proximity effect by using four-terminal current-voltage measurements to study the Van der Waals interface between freshly exfoliated flakes of the high-$T_{c}$ superconductor, Bi-2212, and the CDW-dominated TMD layered material, 1T-TaS$_{2}$. For highly transparent barriers, there is a pronounced Andreev reflection feature providing evidence for proximity-induced high-$T_{c}$ superconductivity in 1T-TaS$_{2}$ with a surprisingly large energy gap ($\sim 20$meV) equal to half that of intrinsic Bi-2212 ($\sim 40$meV). Our systematic study using conductance spectroscopy $dI/dV$ of junctions with different transparencies also reveals the presence of two separate boson modes, each associated with a ``dip-hump" structure. We infer that the proximity-induced high-$T_c$ superconductivity in the TaS$_2$ is driven by coupling to the metastable metallic phase coexisting within the Mott-CCDW (commensurate CDW) phase. [Preview Abstract] |
Tuesday, March 14, 2017 9:12AM - 9:24AM |
E38.00007: Superconductivity in Bi2Se3 coupled with Nb islands Yang Bai, Can Zhang, Brian Mulcahy, James Eckstein Topological Materials contain bulk bands as well as surface states that have a spin-momentum locked band structure in the vicinity of the bulk band gap. The surface states hybridize when the layers are thin. Superconducting correlation can be introduced via proximity coupling to a normal superconductor. The superconductivity spreads in both the planar and vertical directions as the temperature is reduced. A lot of work has been done on the proximity effect between superconductors and normal metals. We performed a transport experiment to measure the proximity effect in devices consisting of an array of many niobium islands on a Bi2Se3 film. We did some preliminary analysis to extract parameters describing the Berezinskii Kosterlitz Thouless (BKT) phase stiffening transition in the resistance-temperature dependence. [Preview Abstract] |
Tuesday, March 14, 2017 9:24AM - 9:36AM |
E38.00008: Superconductivity Proximity Effect on Bulk and Surface States of Bi2Se3 Revealed by Conductance Peak and Plateau James Jun He, Hui Li, Tong Zhou, Huanwen Wang, Huachen Zhang, Hong-Chao Liu, Ya Yi, Changming Wu, Kam Tuen Law, Hongtao He, Jiangnong Wang We measured the transport properties of Bi2Se3/NbSe2 junctions. The resistance drops twice around temperatures of 7K and 2K respectively, indicating proximity-induced superconductivity in Bi2Se3. Further conductance measurements show plateaus in the superconducting gap of NbSe2, and a peak near zero bias. By numerical simulation with a tight-binding model, we found that the plateau is due to a quite transparent interface between Bi2Se3 and NbSe2 and thus the surface states of the Bi2Se3 couple with the superconductor strongly. And the zero-bias peak is from the bulk states of the conduction band with the chemical potential close to the band bottom. [Preview Abstract] |
Tuesday, March 14, 2017 9:36AM - 9:48AM |
E38.00009: Coherent quantum transport in hybrid Nb-InGaAs-Nb Josephson junctions Kaveh Delfanazari, R. Puddy, P. Ma, M. Cao, T. Yi, Y. Gul, I. Farrer, D. Ritchie, H. Joyce, M. Kelly, C. Smith Because of the recently reported detection of Majorana fermions states at the superconductor-semiconductor (S-Sm) interface in InAs nanowire devices, the study of hybrid structures has received renewed interest. In this paper we present experimental results on proximity induced superconductivity in a high-mobility two-dimensional electron gas in InGaAs heterostructures. Eight symmetric S-Sm-S Josephson junctions were fabricated on a single InGaAs chip and each junction was measured individually using a lock-in measurement technique. The superconducting electrodes were made of Niobium (Nb). The measurements were carried out in a dilution fridge with a base temperature of 40 mK, and the quantum transport of junctions were measured below 800 mK. Owing to Andreev reflections at the S-Sm interfaces, the differential resistance (\textit{dV/dI}) versus $V$ curve shows the well-known subharmonic energy gap structure (SGS) at $V=$2$\Delta_{\mathrm{Nb}}$/\textit{ne}. The SGS features suppressed significantly with increasing temperature and magnetic field, leading to a shift of the SGSs toward zero bias. Our result paves the way for development of highly transparent hybrid S-Sm-S junctions and coherent circuits for quantum devices capable of performing quantum logic and processing functions. [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:00AM |
E38.00010: Atomic Scale Proximity Effect at a Molecular Superconductor-Metal Boundary Kyaw Zin Latt, Sajida Khan, Anh Ngo, Hao Chang, Abdou Hassanien, Larry Curtiss, Saw-Wai Hla We report a new organization of nanoscale molecular clusters that exhibits superconducting properties on Ag(111) surface below 8K substrate temperature. The clusters here are composed of a few molecular chains formed by BETS (donors) and GaCl4 (acceptor). In STM images, these molecular clusters appear as ordered parallel chains resembling the ‘rafts’. Using scanning tunneling microscope (STM) manipulation, small molecular clusters are repositioned on the surface at desired locations. From the tip height signals, the dynamics of molecular clusters during their movements across the surface has been unveiled. Repeated manipulation experiments reveal that the rafts move only along [211] surface directions with single atomic site hops. Then by means of tunneling spectroscopy, interaction of two dimensional surface state electrons with these two dimensional clusters are investigated at the molecule-metal boundary regions. The results provide how electrons interact with the superconducting clusters in addition to the electronic and mechanical properties of these superconducting rafts. [Preview Abstract] |
Tuesday, March 14, 2017 10:00AM - 10:12AM |
E38.00011: Transport in Ferromagnet/Superconductor Spin Valves Evan Moen, Oriol T. Valls We consider transport in realistic Ferromagnet-Superconductor layered nanostructures with a spin-valve structure: $F_1/N/F_2/S$ where $F_1$ and $F_2$ denote the ferromagnets, $S$ the superconductor, and $N$ a normal metal spacer usually inserted in these spin-valve devices. These systems exhibit physically intricate proximity effects, including singlet to triplet conversion, which have a large impact on the transport properties and the spin-valve effect. We use a fully self consistent method that ensures that all conservation laws are satisfied. We obtain results for the conductance, $G$, of the devices as a function of voltage, for all values of the angle $\phi$ between the magnetizations of the $F_1$ and $F_2$ layers. These calculations are performed for experimentally relevant ranges of layer thicknesses, material properties, and interfacial scattering. We consider also spin currents and spin transfer torques in these structures. [Preview Abstract] |
Tuesday, March 14, 2017 10:12AM - 10:24AM |
E38.00012: Thin film textured erbium for superconducting spintronics Nathan Satchell, Joshaniel Cooper, Christy Kinane, Timothy Charlton, James Witt, Gavin Burnell, Machiel Flokstra, Peter Curran, Sean Langridge Bulk erbium has an exceptionally rich magnetic phase diagram. It exhibits antiferromagnetic, spiral and conical phases, which could be technologically important if these phases can be reproduced in the thin film regime. In previous works it was found that with reduced dimensionality many of the bulk phases are suppressed (even in films as thick as 2000 nm). We have studied strongly textured, DC magnetron sputtered films of Er in a thickness range 6 - 200 nm. Using neutron diffraction and polarised reflectometry, the nature of the spin spiral and conical phases were directly probed. On the thickest samples it was found, using neutron diffraction, that all bulk magnetic phases were retained for the first time in a thin film. On reduction to the thinnest film of 6 nm we used polarised neutron reflectometry to extract a depth dependent magnetisation profile. Even at this thickness there is evidence that the spiral magnetisation is retained. Finally, we combine Er with superconducting Nb and find a proximity effect which is both large in magnitude and tuneable with a field history. [Preview Abstract] |
Tuesday, March 14, 2017 10:24AM - 10:36AM |
E38.00013: Phonon engineering in proximity enhanced superconductor heterostructures Yongchao Tang, Sangil Kwon, Hamid Mohebbi, David Cory, Guo-Xing Miao The phonon density of states of superconducting film is an important factor in determining film behavior. We observe that the local quantized phonon spectra of Al cladding layers in proximity-enhanced Al/Nb/Al heterostructures has an effect on the superconducting resonator quality factors. We examine a model system of a proximity-enhanced three-layered Al/Nb/Al heterostructures, and consider the size effect of the cladding Al layers. The two Al layers are thin enough that the size effect of thin films becomes observable. Instead of a monotonic increase of quality factors with decreasing temperatures, it is observed that the quality factor reaches a maximum at 1.2K in the 5/50/5 nm Al/Nb/Al microstrip resonators. A quality factor as high as 20,000 is obtained at 15 mK under an in-plane 0.35T magnetic field for the 5/50/5 nm Al/Nb/Al resonators, which is higher than the corresponding results from a pure 50 nm Nb film (15,000). [Preview Abstract] |
Tuesday, March 14, 2017 10:36AM - 10:48AM |
E38.00014: Gapped superconductivity with all symmetries in InSb (110) quantum wells in proximity to Fulde-Ferrell-Larkin-Ovchinnikov superconductor Fei Yang, Ming wei Wu We show that all the singlet even-frequency (SE), singlet odd-frequency (SO), triplet even-frequency (TE) and triplet odd-frequency (TO) pairings and the corresponding order parameters (gaps) can be realized in InSb (110) spin-orbit-coupled quantum well in proximity to Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductor. It is revealed that with the proximity-induced SE order parameter, SO (TE) pairing is induced due to translational (spin-rotational) asymmetry by the center-of-mass momentum of Cooper pair in FFLO phase (spin-orbit coupling). With translational and spin-rotational asymmetries, the TO one is also realized. Then, we show that the corresponding order parameters can be obtained from the self-energy of the electron-electron Coulomb interaction with the dynamic screening. The SE and TE order parameters exhibit the conventional $s$-wave and $p$-wave characters in the momentum space, respectively. Whereas SO and TO ones show the $p$-wave and $d$-wave characters, respectively. Moreover, the p-wave character of the SO order parameter exhibits anisotropy with respect to the direction of the center-of-mass momentum. While for the TO one, we find that $d_{x^2}$- and $d_{xy}$-wave characters can be obtained with respect to the direction of the center-of-mass momentum. [Preview Abstract] |
Tuesday, March 14, 2017 10:48AM - 11:00AM |
E38.00015: Theory of surface Andreev bound states and tunneling spectroscopy in three-dimensional chiral superconductors Shun Tamura, Shingo Kobayashi, Lu Bo, Yukio Tanaka We study the surface Andreev bound states (SABSs) and quasiparticle tunneling spectroscopy of three-dimensional (3D) chiral superconductor by changing the surface (interface) misorientation angle of chiral superconductors. We obtain analytical formula of the energy dispersion of SABS for general pair potential when an original 4$\times$4 BdG Hamiltonian can be reduced to be two 2$\times$2 blocks. The resulting SABS for 3D chiral superconductors with pair potential given by $k_z(k_x+ik_y)^\nu$ ($\nu = 1, 2$) has a complicated energy dispersion due to the coexistence of both point and line nodes. We focus on the tunneling spectroscopy of this pairing in the presence of applied magnetic field which induces Doppler shift of quasiparticle spectra. By contrast to previous known Doppler effect in unconventional superconductors, zero bias conductance dip can change into zero bias conductance peak by external magnetic field. We also study SABSs and tunneling spectroscopy for possible pairing symmetries of UPt$_3$. For this purpose, we extend a standard formula of tunneling conductance of unconventional superconductor junctions. The tunneling spectroscopy in the presence of magnetic field can serve as a guide to determine the pairing symmetry of this material. [Preview Abstract] |
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