Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E30: Superconductivity: Disorder, Impurities, and Shape Effects |
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Sponsoring Units: DCMP Chair: Liuvan Zhao, University of Michigan Room: LACC 406B |
Tuesday, March 6, 2018 8:00AM - 8:12AM |
E30.00001: Magnetic Impurity Effects on Cooper Pair Localization Xue Zhang, James Joy, Wu Chunshu, Jimmy Xu, James Valles The Cooper pair insulator(CPI) state has been observed and probed by various experiments in the past decade. Nevertheless, the microscopic mechanisms driving Cooper pair localization and giving rise to its signature activated transport and giant magnetoresistance peak remain unelucidated. In this talk, I will show our recent studies using magnetic impurity doping, which weakens Cooper pairing, to probe the microscopic properties of quasi-two dimensional nano-patterned films to gain insight into the microscopic properties of the CPI state. The films are patterned with a hole array that allows the application of magnetic frustration to the CPI state. With increasing doping concentration in situ, the low temperature resistance increases and peaks much like the giant magneto-resistance(MR) peak. In addition, the evolution of the transport activation energy with doping shows an unexpected magnetic frustration dependence. I will describe how a theoretical model based on a competition between a localizing charging energy renormalized by quasiparticle tunneling and a delocalizing Josephson energy can account for these results. |
Tuesday, March 6, 2018 8:12AM - 8:24AM |
E30.00002: Monte Carlo Simulations of a Disordered Superconductor-Metal Quantum Phase Transition Ahmed Ibrahim, Thomas Vojta We investigate the quantum phase transitions of a disordered nanowire from superconducting to metallic behavior by employing extensive Monte Carlo simulations. To this end, we map the quantum action onto a (1+1)-dimensional classical XY model with long-range interactions in imaginary time. We show that the critical behavior belongs to the random transverse-field Ising universality class as predicted by a strong disorder renormalization group calculation [1]. We also study transport properties of the system such as the dynamic conductivity. |
Tuesday, March 6, 2018 8:24AM - 8:36AM |
E30.00003: Evidence for enhanced phase fluctuations in nanostructured niobium thin films Ting-Hui Chen, Hsiang-Hsi Kung, Chang-Ran Wang, Chia-Tso Hsieh, Wei-Li Lee In a superconducting nanostructure, phase fluctuations are prominent and give rise to finite resistance below superconducting transition temperature Tc. By using a monolayer polymer/nanosphere hybrid we developed previously, we fabricated a large array of interconnected niobium (Nb) honeycomb lattices with the thinnest interconnected linewidth d ranging from 36 nm to 89 nm. The honeycomb cells form a highly ordered triangular lattice with more than 108 unit cells extending over few mm2 area, which enables the detailed transport study at nanometer scales. We found Tc gradually drops with decreasing d due to the phase-slip effect, while the critical field at lower temperature tends to follow that of a continuous Nb thin film. One likely scenario is to consider a model system of numerous superconducting islands interconnected by short phase-slip junctions, where the phase coherence is dictated by the phase slippage in the nanoconstriction. (PHYSICAL REVIEW B 96, 020506(R) (2017)) |
Tuesday, March 6, 2018 8:36AM - 8:48AM |
E30.00004: Quantum metal phase in ultrathin niobium films Marta Cieplak, Iryna Zaytseva, Aleksander Abaloszew We study the magnetic field induced transition from superconducting to “dirty” metallic phase in niobium films, of the thickness d varying from 1.1 nm to 20 nm, sandwiched between Si buffer layers. As d is reduced, the structure of the Nb films changes from polycrystalline to amorphous, and the Hall coefficient evolves from positive to negative [1]. In the presence of the magnetic field a crossing point of the resistance isotherms appears, usually associated with the superconductor-insulator transition. In the thinnest films at mK temperatures the resistance saturates at the level smaller than the normal state resistance, signaling the likely presence of quantum metal phase. The power-law scaling of the resistance with the magnetic field is consistent with the Bose-metal model [2]. The evolution of the scaling with d indicates a direct relation of the scaling exponent to the activation energy for vortex pinning, extracted from the Arrhenius law at higher temperatures. We are grateful to Leyi Y. Zhu and Chia-Ling Chien (Johns Hopkins University) for growing the films. |
Tuesday, March 6, 2018 8:48AM - 9:00AM |
E30.00005: Impurity Effects on Superconducting Critical Temperature for Nano-structured Superconductor Masaki Umeda, Masaru Kato Superconductors have their own superconducting critical temperature Tc. However, for nano-structured superconductors, their sizes and shapes change Tc, [1]. Nishizaki showed that high pressure torsion (HPT) make many fine grain in bulk of Nb, and make Tc higher experimentally [2]. However he also found that HPT makes Tc lower for bulk of V, and discussesed that impurities in the bulk of V affects the Tc [3]. |
Tuesday, March 6, 2018 9:00AM - 9:12AM |
E30.00006: Superconductivity of UHV Grown Pb Nanocrystals at the Anderson Limit Tianzhen Zhang, Sergio vlaic, Stephane Pons, Alexandre Assouline, Alexandre Zimmers, Christophe David, Guillemin Rodary, Jean-Christophe Girard, Dimitri Roditchev, Herve Aubin What is the size limit for the existence of superconductivity in isolated superconducting nanocrystals? In 1959, P.W. Anderson predicted that superconductivity should disappear in nanocrystals where the mean level spacing becomes larger than the superconducting gap energy. Despite many attempts, this Anderson limit has never been demonstrated experimentally. |
Tuesday, March 6, 2018 9:12AM - 9:24AM |
E30.00007: Revisiting Disorder in High-Temperature Superconductors Miguel Antonio Sulangi, Milan Allan, Jan Zaanen It is known that the crystal structures of copper-oxide superconductors are rather disordered. We attempt to understand various disorder-related electronic properties in these materials using exact real-space numerical methods. We calculate the quasiparticle density of states (DOS) using various models of disorder and find that a finite DOS arises at the Fermi energy from smooth disorder due to off-plane dopants, and show that a residual linear-T contribution to the specific heat naturally results at realistic doping levels. We also find that smooth disorder has a more muted effect on the localization of quasiparticles than pointlike disorder. We model quasiparticle scattering interference as measured in scanning tunneling spectroscopy (STS) and find that the peaks seen in experiment are far better defined than according to our simulations, indicating that a more microscopic modeling of the tunneling process is needed to fully reproduce experimental results. We also study the effects on the STS spectra coming from the interplay between disorder and electronic self-energies parametrizing the effects of interactions. |
Tuesday, March 6, 2018 9:24AM - 9:36AM |
E30.00008: Disorder Effects on Superconductor-Graphene-Superconductor Arrays Rita Garrido Menacho, Vincent HUMBERT, Nadya Mason Graphene coupled to a superconducting array of islands undergoes a continuous Quantum Phase Transition (QPT) from a superconducting state to an insulating or metallic state as a function of gate voltage or applied magnetic field. To study the effects of disorder on this phase transition and the related ground states, we performed transport measurements on graphene proximity-coupled to an array of Sn superconducting islands, where we added point disorder (random displacements) to each island site. We studied the Superconductor-to-Insulator Transition (SIT) as a function of applied magnetic field for devices having varying amounts of point disorder. For low disorder, a clear critical crossing point is observed from magnetoresistance measurements, and the extracted critical exponents were consistent with a continuous 2D SIT. In contrast, high disorder devices showed a disrupted crossing point and disrupted scaling. These signatures suggest unusual behavior as a function of disorder near the Quantum Critical Point (QCP) for devices having resistances less than the quantum of resistance (RQ). |
Tuesday, March 6, 2018 9:36AM - 9:48AM |
E30.00009: Evolution of specific heat with controlled disorder in filled skutterudite superconductor LaRu4As12 Yuta Mizukami, Ohei Tanaka, Marcin Konczykowski, Satoshi Kurata, Yuji Matsuda, Zygmunt Henkie, Tomasz Cichorek, Takasada Shibauchi In filled skutterudite compounds, the lanthanoid atoms are surrounded by cages of pnictogen atoms, often exhibiting characteristic motion in the anharmonic potential known as rattling. It is proposed that the rattling motion is related to the exotic heavy-fermion state or unconventional superconductivity in some filled skutterudite compounds containing 4f electrons[1,2]. However, it remains unclear how the rattling motion affects the normal-state and superconducting properties when the system does not possess 4f electrons. The filled skutterudite LaRu4As12 shows multigap superconductivity at Tc = 10.5 K and has relatively large Sommerfeld coefficient γ ~ 60 mJ/molK2 in spite of the absence of 4f electrons. Here, we introduce defects of the guest atoms and cage atoms in LaRu4As12 single crystals using electron irradiation which allows us to investigate the impact of atomic defects in a controllable manner. We report on the evolution of specific heat with increasing atomic defects and discuss the superconducting gap structure and normal-state electronic properties. |
Tuesday, March 6, 2018 9:48AM - 10:00AM |
E30.00010: Evolution of Collective (Higgs and phase) Modes in Disordered s-wave Superconductors. ABHISEK SAMANTA, Amulya Ratnakar, Nandini Trivedi, Rajdeep Sensarma The destruction of superconductivity by strong disorder and the resultant superconductor-insulator phase transition (SIT) is an active area of research in condensed matter physics. We use functional integral techniques to study the attractive Hubbard model on a square lattice, in the presence of random on-site potential. Expanding the theory in quantum fluctuations around the spatially inhomogeneous saddle point given by BdG mean field theory, we study the evolution of the amplitude (Higgs) and phase fluctuations with disorder. We find a phase transition where the Higgs gap vanishes and low energy phase fluctuations develop large spectral weight. Near the transition, the system breaks up into patches of superconducting islands that host localized low energy Higgs modes, separated from regions with low energy incoherent phase fluctuations. We discuss the implications of our results for optical conductivity and Josephson spectroscopy disordered superconductors. |
Tuesday, March 6, 2018 10:00AM - 10:12AM |
E30.00011: Disorder effects in unconventional multiband superconductor junctions Maxim Dzero, Alex Levchenko We study the transport properties of superconducting junctions consisting of unconventional multinand superconductors in the presence of nonmagnetic disorder. We employ the quasiclassical approximation to derive a set of Eilenberger equations for the quasiclassical Green's functions. By implementing the generalized boundary conditions for the quasiclassical functions, we solve the Eilenberger equations numerically and compute the tunneling conductance as well as the spatial dependence of superconducing pairing amplitude. The application of our results to the analysis of current experimental data will be discussed. |
Tuesday, March 6, 2018 10:12AM - 10:24AM |
E30.00012: Theory of the supercyclotron resonance and Hall response in flux flow metals Sean Hartnoll, Luca Delacretaz, Blaise Gouteraux Anomalous metallic phases have now been observed in a large variety of 2D materials near zero temperature -- and often at high field -- challenging theoretical dogmas. Recent measurements of the optical conductivity (showing the absence of a cyclotron mode) and the dc Hall resitivity (tending towards zero at zero temperature) have highlighted the unconventional properties of these metallic phases. |
Tuesday, March 6, 2018 10:24AM - 10:36AM |
E30.00013: Superconductiviy in Boron-doped Amorphous Carbon Yuki Sakai, James Chelikowsky, Marvin Cohen We examine the effect of boron doping in superconducting forms of amorphous carbon. By judiciously optimizing boron substitutional sites in simulated amorphous carbon we predict a superconducting transition temperature near 37 K at 14 % boron concentration. Our findings have direct implications for understanding the recently discovered high Tc superconductivity in Q-carbon. |
Tuesday, March 6, 2018 10:36AM - 10:48AM |
E30.00014: Radiation enhanced antiferromagnetic exchange between spins in a superconducting host Kamphol Akkaravarawong, Jukka Vayrynen, Jay Sau, Leonid Glazman, Norman Yao A magnetic impurity on a conventional superconductor can host a localized bound state whose energy lies inside the superconducting gap. If the distance between two such impurities is smaller than the coherence length, the presence of these so-called Yu-Shiba-Rusinov (YSR) bound states can induce an antiferromagnetic exchange interaction between the impurities, falling off as 1/r2. The small magnitude of the YSR interaction makes its obesrvation extremely challenging. We demonstrate that both the strength of the YSR interaction, as well as its spatial dependence, can be controlled via radiation assisted virtual occupation of bound states. Finally, we propose an experimental scheme for observing such radiation enhanced YSR interactions in a circuit-QED system. |
Tuesday, March 6, 2018 10:48AM - 11:00AM |
E30.00015: Influence of the metal-insulator transition on normal state electronic transport of HTS Michael Osofsky, Joseph Prestigiacomo The nature of the normal state properties of HTS is still not well established. It is generally accepted that, as doping increases, the HTS evolve from an anti-ferromagnetic insulator to a “strange metal” with a pseudo-gap to a conventional Fermi liquid. The “strange metal” phase is the subject of a great deal of research. In this presentation we will demonstrate that the normal state electronic transport properties of the HTS are consistent with materials near the disorder driven metal-insulator transition |
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