Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S31: Superconductivity: Films and interfaces |
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Sponsoring Units: DCOMP Chair: Lei Wang, Yale University Room: LACC 407 |
Thursday, March 8, 2018 11:15AM - 11:27AM |
S31.00001: Integration of superconducting NbN thin films with III-Nitride semiconductors John Wright, Rusen Yan, Guru Khalsa, Scott Katzer, Neeraj Nepal, Brian Downey, David Meyer, Huili Xing, Debdeep Jena The superconducting properties of metallic NbN have been known and studied for several decades, but it has been successfully integrated epitaxially with the Gallium Nitride family of wide-bandgap semiconductors only recently. In this work, we report the physical properties of epitaxial NbN integrated with SiC, AlN, and GaN grown by molecular beam epitaxy. We study the crystallinity, surface and interface qualities of the superconductor/semiconductor heterojunctions, and the superconducting properties (critical temperatures of ~16K, coherence lengths of ~10nm, and critical Meissner fields in excess of 35T) of epitaxial NbN. Progress towards the realization of all-epitaxial NbN/Semiconductor heterojunctions, superconductors integrated with nitride transistors, and epitaxial Josephson junctions, all towards establishing a fundamentally new platform for superconducting electronics will be presented. |
Thursday, March 8, 2018 11:27AM - 11:39AM |
S31.00002: Synthesis of high-oxidation Y-Ba-Cu-O phases in superoxygenated thin films Hao Zhang, Nicolas Gauquelin, Christopher McMahon, Chao Zhang, David Hawthorn, Gianluigi Botton, John Wei It is known that solid-state reaction in high-pressure oxygen can stabilize high-oxidation phases of Y-Ba-Cu-O superconductors in powder form. We extend this superoxygenation concept of synthesis to thin films which, due to their large surface-to-volume ratio, are more reactive thermodynamically. Epitaxial thin films of YBa2Cu3O7-δ grown by pulsed laser deposition are annealed at up to 700 atm O2 and 900 ○C, in conjunction with Cu enrichment by solid-state diffusion. The films show clear formation of Y2Ba4Cu7O15-δ and Y2Ba4Cu8O16 as well as regions of YBa2Cu5O9-δ and YBa2Cu6O10-δ phases, according to scanning transmission electron microscopy, x-ray diffraction and x-ray absorption spectroscopy. Similarly annealed YBa2Cu3O7-δ powders show no phase conversion. Our results demonstrate a novel route of synthesis towards discovering more complex phases of cuprates and other superconducting oxides [1]. |
Thursday, March 8, 2018 11:39AM - 11:51AM |
S31.00003: Magnetic properties of YBa2Cu3O7/La0.7Sr0.3MnO3 Multi-layer Interface by X-Ray Magnetic Circular Dichroism (XMCD) Measurement Marco Miguel Parel, Suling Cheng, H. Lin, Jauyn Grace Lin, Pu-Cheng Chien, Shih-Jye Sun, Hsiung Chou Interface effect of heterostructure due to spin, orbital and charge interaction provide abundant exotic phenomena. In this study, the magnetic properties of Cu and Mn at the interfaces of YBa2Cu3O7/La0.7Sr0.3MnO3 multi-layer system is carried out by an X-Ray Magnetic Circular Dichroism (XMCD) measurement on the L-edges at various magnetic field intensities and temperatures. The magnetic polarizations of both Cu and Mn are parallel to the external field. Results show that the total magnetic moments decrease with increasing magnetic field intensity. The orbital moments of Cu at interfaces are not quenched and have larger values than that of the spin moments. For Cu at 0.1 T, morb/mspin is ~2.3 with a total moment of ~1μB. This result indicates that there is a possible spin-split coupling for LSMO layer that affects a thin layer of one or two unit cells near the interface of YBCO, altering the environment of Cu ions which leads to an asymmetric distribution of crystal field. Moreover, it is observed that there is a sufficient amount of decrease in the Cu orbital moment from 0.3 T to 0.4 T, while the spin magnetic moment shows a continuously exponential decay. |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S31.00004: Deposition of NbN Superconducting Thin Films Using Ion Beam Assisted Sputtering Tomas Polakovic, Sergi Lendinez, John Pearson, Trupti Khaire, John Arrington, Kawtar Hafidi, Goran Karapetrov, Valentyn Novosad Transition metal nitrides are a large family of materials with wide range of electronic and mechanical properties, making them a popular choice for applications in protective coatings, quantum optics or superconducting electronic devices. Typical member of this family is NbN, which is a superconductor used predominantly in the field of single photon detectors. Using the common preparation method of reactive magnetron sputtering, temperatures of up to 650 oC are required to achieve high-Tc supercondicting crystal phase in this material, making it incompatible with many processes and materials. In this presentation, we report on a new method of deposition of NbN films, where we utilize ion beam assisted sputtering. Using this method, high-Tc NbN films deposited at room temperature can be achieved with added benefit of dramaticaly reduced sensitivity of Tc to N2 concentration. We also present results of systematic study of the materials electric, magnetic and structural properties as functions of the nitrogen ion concentration and energy and compare them to films deposited by reactive magnetron sputtering. |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S31.00005: Inhomogeneity of the gap from TiN films studied by low-temperature STM Wan-Ting Liao, Peng Xu, Kevin Osborn, Robert Butera, Frederick Wellstood, Christopher Lobb, Michael Dreyer Titanium nitride films with high kinetic inductance show great promise for use in superconducting qubits and resonators. The magnitude of the kinetic inductance and the loss in the films depend crucially on the preparation conditions and resulting film morphology. Here we report STM measurements performed at 400 mK on a 20 nm-thick high kinetic inductance film and a 40 nm thick oxygen treated film. We have found that in a 40 nm thick film the superconducting gap can be observed after gentle sputter cleaning of the surface. However, we find that the gap can vary from grain to grain by more than 20% and there is significant broadening of the gap in all regions. We will present spatially resolved gap maps on both films, examine the correlation between the surface topography and gap, and discuss some of the implications of these results for superconducting qubits. |
Thursday, March 8, 2018 12:15PM - 12:27PM |
S31.00006: Al/Nb bilayer superconducting density of states measurements at 4 K Zac Barcikowski, Joshua Pomeroy, Michael Stewart Tunneling spectroscopy is performed using tunnel junctions with a thin film Al/Nb hybrid superconducting bilayer. With this hybrid Al/Nb system, we take advantage of the relatively high superconducting critical temperature of Nb (9.2 K in bulk) and the low spin-orbit coupling of Al (b ) for Meservey-Tedrow type measurements near liquid helium temperatures (4 K). While altering the thickness of the bilayer elements, we measure the bilayer superconducting transition temperature and tunneling spectroscopy density of states to explore the interplay of the superconducting proximity effect and spin orbit coupling between the Al and Nb. |
Thursday, March 8, 2018 12:27PM - 12:39PM |
S31.00007: Stoichiometry effect on the superconductivity of YB6 thin films Xiaohang Zhang, Seunghun Lee, Drew Stasak, H M Iftekhar Jaim, Joshua Higgins, Richard Greene, Ichiro Takeuchi Rare earth borides exhibit a rich range of physical properties including superconductivity, metal-metal transitions, mixed-valence chemistry, etc. Rare earth borides display complex phase diagrams with various chemical compounds, and the stoichiometric effect in these materials is considered to play a critical role in altering their physical properties. Yttrium hexaboride (YB6) is a known superconducting material (Tc ~ 7 K), but its superconducting properties have been reported to be strongly dependent on the composition. To systematically address its stoichiometric effect on the physical properties of YB6, we fabricated natural compositional spread films of YB6 by sputtering. Here we show the change in the structural and superconducting characteristics of YB6 thin films depending on the B/Y ratio. It is found that B deficiency improves the superconducting transition temperature (Tc), but the superconducting coherence length is maximum near stoichiometric YB6. YB6 thin films with robust superconductivity and its extremely small lattice misfit with SmB6 provide a platform to explore exotic superconductivity with topological insulator SmB6 by fabricating isostructural SmB6/YB6 bilayers. |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S31.00008: Low Energy Electrodynamics of a Superconducting Epitaxial Bismuth/Nickel Bilayer Prashant Chauhan, Fahad Mahmood, Di Yue, Peng-Chao Xu, Xiaofeng Jin, Peter Armitage Epitaxial Bi film grown on thin Ni film has strong spin-orbit coupling and no inversion symmetry. Recent results show that this Bi/Ni bilayer has superconductivity that breaks time-reversal symmetry (TRS). It is believed to be a candidate to realize a 2D topological superconductor. We investigate superconductivity in this interesting and unusual 2D bilayer system using time domain THz transmission spectroscopy (TDTS). Superconductivity is exhibited below 4.1K, which is surprising as it is known that in isolation both ferromagnetic Ni and rhombohedral Bi films are not superconducting. We study the frequency response of the complex conductivity of the bilayer. A superconducting gap structure with an energy scale close to the weak coupling s-wave BCS value is clearly seen. High-precision THz polarimetry provides additional information about time-reversal symmetry breaking in this compound. |
Thursday, March 8, 2018 12:51PM - 1:03PM |
S31.00009: Magneto –THz spectroscopy in spinel superconductor LiTi2O4 thin films Yue Huang, Wei Zhang, Wei Hu, Jie Yuan, Kui Jin, Daniel Mittleman We investigate a spinel superconductor, LiTi2O4 using continues wave(CW) terahertz spectroscopy under magnetic fields. By measuring both the magnitude and phase of the transmission, the inversion to complicated conductivity can be done directly and without Kramers-Kronig transformation. LiTi2O4 (LTO)is an oxide superconductor with a spinel crystal structure. Although the phase transition temperature is 10K, it was estimated that the spin fluctuations played an important role in the microscopic mechanism for superconductivity. We observed a clear superconducting gap opening at about 1.9meV and a coherent peak. The superconducting state was strongly suppressed by magnetic field up to 9T. It was the first time that the superconducting gap was observed in the compounds LiTi2O4 with CW terahertz spectroscopy. |
Thursday, March 8, 2018 1:03PM - 1:15PM |
S31.00010: Selective Equal Spin Andreev Reflection at Vortex Core Center in Magnetic Semiconductor-Superconductor Heterostructure Chuang Li, Lun-Hui Hu, Yi Zhou, Fu-Chun Zhang Sau, Lutchyn, Tewari and Das Sarma (SLTD) proposed a heterostructure consisting of a semiconducting thin film sandwiched between an s-wave superconductor and a magnetic insulator and showed possible Majorana zero mode. Here we study spin polarization of the vortex core states and spin selective Andreev reflection at the vortex center of the SLTD model. In the topological phase, the differential conductance at the vortex center contributed from the Andreev reflection, is spin selective and has a quantized value (dI/dV)Atopo =2e^2/h at zero bias. In the topological trivial phase, (dI/dV)Atrivial at the lowest quasiparticle energy of the vortex core is spin selective due to the spin-orbit coupling. Unlike in the topological phase, (dI/dV)Atrivial is suppressed in the Giaever limit and vanishes exactly at zero bias due to the quantum destruction interference. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S31.00011: Link between the superconducting dome and spin-orbit interaction in the (111) LaAlO3/SrTiO3 interface Eran Maniv, Prasanna Rout, Yoram Dagan We measure the gate voltage (Vg) dependence of the superconducting properties and the spin-orbit interaction in (111) oriented LaAlO3/SrTiO3 interface. Superconductivity is observed in a dome-shaped region in the carrier density-temperature phase diagram with the maxima of superconducting transition temperature Tc and the upper critical fields lying at the same Vg. The spin-orbit interaction determined from the superconducting parameters and confirmed by weak-antilocalisation measurements follows the same gate voltage dependence as Tc. The correlation between superconductivity and spin-orbit interaction as well as the enhancement of parallel upper critical field, well beyond the Chandrasekhar-Clogston limit suggest that superconductivity and the spin-orbit interaction are linked in a non-trivial fashion. We propose possible scenarios to explain this unconventional behavior [1]. |
Thursday, March 8, 2018 1:27PM - 1:39PM |
S31.00012: Suppression of superconductivity across the Lifshitz transition in bulk SrTiO3 and LaAlO3/SrTiO3 interfaces Thais Victa Trevisan, Michael Schütt, Rafael Fernandes The nature of the superconducting (SC) state of doped SrTiO3 (STO) remains a subject of intense research. It is known experimentally that the SC transition temperature Tc displays a double-dome shape as function of the carrier density n. Quantum oscillations experiments reveal that multiple bands become occupied as the concentration n changes, i.e. STO is a multi-band superconductor. Surprisingly, around the critical concentration where the second band starts to be occupied, signaling a Lifshitz transition, a suppression of Tc is observed. The same phenomenon is also observed in LaAlO3/SrTiO3 interfaces. Here we show that such a suppression of Tc across a Lifshitz transition can be explained due to the combination of disorder and repulsive inter-band interactions, since in this case the pair-breaking caused by inter-band impurity scattering can overcome the increase of Tc favored by the enhancement of the density of states. We also show that this process is accompanied by a change of symmetry of the SC state from s+- to s++, indicating that STO is an unconventional multi-band superconductor. |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S31.00013: The c-axis characterizations of Bi2Sr2CaCu2O8+δ films grown by liquid-phase epitaxy for THz applications Yilmaz Simsek, Vitalii Vlasko-Vlasov, Manabu Tsujimoto, Timothy Benseman, Alexei Koshelev, ibrahim kesgin, Yang Hao, Helmut claus, John Pearson, Wai-Kwong Kwok, Ulrich Welp Intrinsic Josephson junctions (IJJ) in the high-Tc superconducting Bi2Sr2CaCu2O8+δ (Bi-2212) exhibit the fundamental phenomena associated with the Josephson effect opening an avenue towards compact THz chip system with integrated Josephson plasma wave devices for THz applications. To realize the THz-on-a-chip concept, we have grown high-quality and thick epitaxial Bi-2212 films by liquid-phase epitaxy (LPE). The structural and stoichiometric characterizations of our best films by XRD and EDS confirm single-phase, highly c-axis aligned crystalline Bi-2212 films. Sharp superconducting transitions in transport measurements on annealed films occur at approximately 85K. In the c-axis transport measurements, we have observed strongly hysteretic I-V characteristics with relatively high Jc of 600 A/cm2 which is a clear signature of underdamped IJJs and a prerequisite for THz generation. We will discuss LPE growth conditions allowing to improve the size of single phase Bi-2212 grains and their structural and superconducting properties for leading to the THz-on-a-chip systems. |
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