Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B30: Superconductor-Insulator Transition |
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Sponsoring Units: DCMP Chair: Lance Cooper, University of Illinois Urbana-Champaign Room: LACC 406B |
Monday, March 5, 2018 11:15AM - 11:27AM |
B30.00001: Sequential superconductor-Bose insulator-Fermi insulator transitions in two dimensional a-WSi Xiaofu Zhang, Andreas Schilling A zero-temperature magnetic-field-driven superconductor to insulator transition (SIT) in quasi-two-dimensional superconductors is expected to occur when the applied magnetic-field crosses a certain critical value. A fundamental question is whether this transition is due to the localization of Cooper pairs or due to the destruction of them. Here we address this question by studying the SIT in amorphous WSi. Transport measurements reveal the localization of Cooper pairs at a first critical field B_{C}^{1} (Bose-insulator), with a product of the correlation length and dynamical exponents zν~1.33 near the quantum critical point (QCP). Beyond B_{C}^{1} superconducting fluctuations still persist. Above a second critical field B_{C}^{2}>B_{C}^{1}, the Cooper pairs are destroyed and the film becomes a Fermi-insulator. The different phases all merge at a tricritical point with zν=0.67. Our results suggest sequential phase transitions (superconductor - Bose insulator - Fermi insulator) in the zero-temperature limit, which differs from the conventional scenario involving a single quantum critical point. |
Monday, March 5, 2018 11:27AM - 11:39AM |
B30.00002: Vanishing Hall Conductance in the Phase Glass Bose Metal at Zero Temperature Julian May-Mann, Philip Phillips Motivated in part by the numerical simulations which reveal that the energy to create a defect in a gauge or phase glass scales as L θ with θ < 0 for 2D, thereby implying a vanishing stiffness, we re-examine the relevance of these kinds of models to the Bose metal in light of the new experiments which reveal that the Hall conductance is zero in the metallic state that disrupts the transition from the superconductor to the insulator in 2D samples. Because of the particle-hole symmetry in the phase glass model, we find that bosonic excitations in a phase glass background generate no Hall conductance at the Gaussian level. Furthermore, this result persists to any order in perturbation theory in the interactions. We show that when particle-hole symmetry is broken, the Hall conductance turns on with the same power law as does the longitudinal conductance. This prediction can be verified experimentally by applying a ground plane to the 2D samples. |
Monday, March 5, 2018 11:39AM - 11:51AM |
B30.00003: Vortex and Charge BKT Transition in Disordered Superconducting Thin Films Sarath Sankar, Valerii Vinokour, Vikram Tripathi Disorder degrades superconductivity through its twofold effects of reducing the Cooper pair (CP) binding energy and enhancing phase fluctuations. In many systems, there is strong experimental evidence for CP existence above the critical temperature and the predominance of phase fluctuations in the superconductor to insulator transition. We study two outstanding experimental mysteries: 1. the giant positive magnetoresistance and 2. a putative finite temperature insulator transition. Physically, CPs fill the deep wells of the disorder potential and Josephson tunneling connects these islands. We study the evolution of the islands in a magnetic field and construct an effective Josephson junction model with field dependent parameters. We calculate the magnetoresistance and superfluid stiffness in three parameter regimes. Two of these regimes are governed by vortex BKT criticality with strong phase disorder qualitatively affecting critical behavior. Coming to the finite temperature insulator, we show that this transition can be understood as a charge BKT transition. Strong disorder is shown to result in the Vogel-Fulcher-Tammann critical behaviour which we attribute to the freezing phenomenon of charge dipole excitations. |
Monday, March 5, 2018 11:51AM - 12:03PM |
B30.00004: Microwave spectroscopy studies on superconductor-to-insulator transition and the intervening Bose metal phase in indium oxide thin films Youcheng Wang, Idan Tamir, Dan Shahar, Peter Armitage The self-dual criticality of magnetic field tuned superconductor-to-insulator transition (H-SIT) in homogeneously disordered InO_{x} thin films is a profound problem, and could be connected to other types of quantum phase transitions. Although it is generally accepted that Bosons are dominant on both sides of the transition in this material and fluctuation of the phase of the order parameter is important, the nature of the insulating phase and an intervening Bose metal phase usually observed in less disordered samples remain mysteries to be unveiled. Here we present measurements of the broadband microwave conductivity spectra of InO_{x} thin films of different disorder levels across their magnetic field tuned quantum phase transitions. |
Monday, March 5, 2018 12:03PM - 12:15PM |
B30.00005: Complete Characterization of Static and Dynamic Electromagnetic Response near 2D Percolative Superconductor-Insulator Transitions Yen Lee Loh, Pragalv Karki We present a comprehensive description of the superfluid stiffness, diamagnetic susceptibility, electric susceptibility, and dynamical conductivity of random inductor-capacitor networks on square lattices. The results are relevant to superconductor-insulator transitions in coarse-grained superconductor-insulator composites dominated by percolation physics. Results were obtained using Chebyshev equation-of-motion and bond-propagation algorithms customized for our models, and found to be in mutual agreement. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B30.00006: Imaging Quantum Fluctuations Near Criticality Hasan Khan, Yen Lee Loh, Anna Kerman, Aviad Frydman, Beena Kalisky, Nandini Trivedi A quantum phase transition (QPT) is a fascinating physical phenomenon in which a system exhibits a dramatic change of behavior as a function of a non-thermal tuning parameter at zero temperature. The quantum fluctuations associated with the quantum critical point (QCP) leave an imprint over an extended temperature range. Here we explore the nature of these fluctuations by performing quantum Monte Carlo calculations of the local diamagnetic susceptibility of a 2D Josephson junction array model in the vicinity of the QPT from a superconductor to an insulator. Our work is supported by scanning SQUID measurements of diamagnetic currents in NbTiN thin film superconductors. We find fluctuations of the diamagnetic response in both space and time, and identify the origin of these fluctuations as Cooper pair tunneling events. As we approach the QCP the lateral dimension of these fluctuations grows and they survive well below the transition temperature, thus demonstrating the contribution from quantum fluctuations in addition to thermal fluctuations. This research provides a unique tool for studying QPTs and has implications for current efforts in quantum information and communication. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B30.00007: Dissipative phase transition with quantum frustration Gianluca Rastelli, Dominik Maile, Sabine Andergassen, Wolfgang Belzig We study the quantum phase transition of the one dimensional phase model in presence of quantum frustration, provided by a dissipative interaction of the system with the environment through two non-commuting operators. Such a model can be realized in Josephson junction chains with shunt resistances and resistances between the chain and the ground. Using a self-consistent harmonic approximation, we determine the phase diagram at zero temperature which exhibits a quantum phase transition between a long-range ordered phase, corresponding to the superconducting state, and a disordered phase, corresponding to the insulating state with charge localization. We find that the critical line separating the two phases has a nonmonotonic behavior as a function of the dissipative coupling strength. Moreover, we analyse the purity of the system that quantifies the degree of correlation between the system and the environment, and the logarithmic negativity as entanglement measure that encodes the internal quantum correlations in the system. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B30.00008: The Role of Temperature and Magnetic Field in Destroying a Superconducting Phase in a-Ta Thin Films Junghyun Shin, Sun-gyu Park, Eunseong Kim We investigated superconductor insulator transition induced by controlling temperature and magnetic field in the superconducting Tantalum thin films. Quantum metallic phase intervening superconducting and insulating phase were reproduced by increasing magnetic field at low temperature limit, which was characterized by the finite saturation in the electrical resistance as well as non-linear I-V characteristics without hysteresis. [PRB.73.100505(2006)]. Besides, we observed similar non-linear I-V characteristic behaviors with different features near the transition temperature. To clarify the role of temperature and magnetic field, we conducted precise dV/dI measurements as varying temperature and magnetic field. We found the non-linear IV characteristic and destruction of superconductivity could be understood in the framework of vortex dynamics with unpinning by thermal excitation and addition by magnetic field. |
Monday, March 5, 2018 12:51PM - 1:03PM |
B30.00009: Magnetic Field Tuned Superconductor-Weak-Insulator Transition in Indium Oxide Thin Film Nicholas Lewellyn, Allen Goldman |
Monday, March 5, 2018 1:03PM - 1:15PM |
B30.00010: Abstract Withdrawn The magnetic-field tuned superconductor to insulator transition (H-SIT) is the paradigmatic quantum phase transition. However, the transition and the ground-state (T = 0) phases proximate to the transition have consistently exhibited features that are seemingly at odds with the generally accepted theoretical expectations. In particular, the emergence of an intervening anomalous metallic phase is now ubiquitous in experiments seeking to study H-SIT. Thus, to further elucidate the conditions for the emergence of different phases in 2D superconducting films, we introduced an additional tuning "knobs" that allow us to vary the properties of the films in different directions. In this paper we present studies of the superconductor-metal-insulator transitions (SIT) in amorphous Indium Oxide (InO_{x}) thin films fabricated by electron beam evaporation on SrTiO_{3}. Carrier density and amount of disorder were varied by low-temperature annealing and subsequent in-situ external electrostatic fields. Longitudinal and Hall resistance of the samples were used to distinguish the various observed phases. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B30.00011: Superconductor to insulator transition in (111) SrTiO_{3}/LaAlO_{3} interface Michael Mograbi, Eran Maniv, Prasanna Rout, David Graf, Yoram Dagan We study the superconductor-to-insulator phase diagram of the (111) interface between the two band-insulators: LaAlO_{3} and SrTiO_{3}. This interface has a two dimensional six-fold crystalline structure, as recently detected in the magneto-transport properties [1]. It has also been predicted to host exotic electronic orders and exhibits correlation between superconductivity and spin-orbit interaction [2]. We tune the carrier density deep into the insulating state and studied the magneto-transport properties. Our data are analyzed in the framework of superconducting fluctuations within the insulating state. |
Monday, March 5, 2018 1:27PM - 1:39PM |
B30.00012: One-Dimensional Nature of Pairing and Superconductivity at the LaAlO_{3}/SrTiO_{3} Interface Yun-Yi Pai, Hyungwoo Lee, Jung-Woo Lee, Anil Annadi, Guanglei Cheng, Shicheng Lu, Michelle Tomczyk, Mengchen Huang, Chang-Beom Eom, Patrick Irvin, Jeremy Levy We examine superconductivity in LaAlO_{3}/SrTiO_{3} channels in which the channel width transitions from the 1D to 2D regime. The superconducting critical current is independent of the channel width and increases approximately linearly with the number of parallel channels. Signatures of electron pairing outside of the superconducting phase are also independent of channel width. Collectively, these results indicate that electron pairing and superconductivity exist at the boundary of these channels and are absent within the interior region of the channels. The intrinsic 1D nature of superconductivity at the LaAlO_{3}/SrTiO_{3} interface imposes strong physical constraints on possible electron pairing mechanisms. |
Monday, March 5, 2018 1:39PM - 1:51PM |
B30.00013: The study of a Superconductor–Insulator quantum phase transition using Nernst effect Arnab Roy, Efrat Shimshoni, Aviad Frydman The superconductor-insulator transition (SIT) is a quantum phase transition that has proved to be an interesting subject of study owing to the two very dissimilar microscopic mechanisms by which it can be brought about. The fermionic mechanism is dominated by amplitude fluctuations of the superconducting order parameter Ψ, whereas in the bosonic case, it is the phase φ that fluctuates. These mechanisms are not easy to address in experiment in some cases, and the usual probes for the SIT are sometimes not sensitive to it. A special case of interest is the SIT of amorphous Indium Oxide, which has never shown full conformity with either mechanism. |
Monday, March 5, 2018 1:51PM - 2:03PM |
B30.00014: Observation of Finite-frequency Superconductivity in Insulating 1D Josephson Junction Chains Nicholas Grabon, Roman Kuzmin, Nitish Mehta, Raymond Mencia, Natalia Pankratova, Vladimir Manucharyan A 1D chain of Josephson junctions undergoes a BKT-type superconductor to insulator transition (SIT) driven by quantum fluctuations of the phase of the superconducting order parameter. Away from the transition a superconducting chain experiences two types of fluctuations. The first type corresponds to small vibrations of the phase and gives rise to propagating 1D plasmon waves. The second type corresponds to a slip of the phase by 2π. Theory predicts that phase slips induce the insulating state once the wave impedance of plasmons exceeds the value of R_{Q}/4 ~ 2 kOhm, where R_{Q} = 6.5 kOhm is the superconducting resistance quantum. We report microwave spectroscopy of nanofabricated 1D chains of Al/AlOx/Al tunnel junctions, containing 30,000+ junctions. Remarkably, we find that plasmons propagate along the entire chain even when the wave impedance exceeds 20 kOhm, suggesting that the insulating chain behaves as superconductor to finite-frequency currents. This new experimental insight can help to clarify the nature of the insulating phase and the bosonic SIT transition. The high wave impedance of plasmons translates to an effective fine structure constant near unity which opens up a path to studying extreme regimes of light-matter coupling. |
Monday, March 5, 2018 2:03PM - 2:15PM |
B30.00015: The Composite Fermi Sea of Aluminum Arsenide Zheng Zhu, Inti Sodemann, Donna Sheng, Liang Fu We study two-component electrons in the lowest Landau level at total filling factor 1/2 with anisotropic mass tensors whose principal axes are rotated by π / 2 as realized in Aluminum Arsenide (AlAs) quantum wells. Combining exact diagonalization and the density matrix renormalization group we demonstrate that the system undergoes a quantum phase transition from a gapless state in which both flavors are equally populated to another gapless state in which all the electrons spontaneously polarize into a single flavor as a function of mass anisotropy. We propose that this phase transition is a form of itinerant Stoner transition between a two-component and a single-component composite fermi sea states and describe a set of trial wavefunctions which successfully capture the quantum numbers and shell filling effects in finite size systems as well as providing a physical picture for the energetics of these states. Our estimates indicate that the composite fermi sea state in AlAs is indeed a fully polarized itinerant Stoner-type magnet. |
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