Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G49: Superconductivity: Fluctuation, Noise, Nonequilibrium, and Localization Effects |
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Sponsoring Units: DCMP Chair: James Eckstein Room: Mile High Ballroom 1B |
Tuesday, March 3, 2020 11:15AM - 11:27AM |
G49.00001: Quantum Synchronization and Superconductivity Jonathan Curtis, Victor Galitski Synchronization is a phenomenon whereby weak interactions between oscillators enables a macroscopic phase coherence to develop. While this phenomenon is well known in the context of classical dynamical systems, its extension to quantum oscillators has been relatively un-explored. In this talk we will consider a model where the quantum oscillators are realized by a local superconducting order parameter. We will then study under what circumstances interactions lead to synchronization of the phase, enhancing the phase coherence and superconductivity of the sample. |
Tuesday, March 3, 2020 11:27AM - 11:39AM |
G49.00002: Theory of the nonlinear susceptibility of layered superconductors: application to strontium ruthenate Fei Chen, Damjan Pelc, Martin Greven, Rafael Fernandes Probing superconducting fluctuations (SF) in unconventional superconductors is essential to elucidate their nature. Much work has been done to elucidate the contributions from SF to the conductivity and the linear susceptibility, disentangling them from normal state contributions can be challenging. An alternative that has been recently explored is to measure the nonlinear susceptibility via third-order magnetic response, which is believed to be dominated by SF above Tc. Here, we present a phenomenological theoretical model for the nonlinear susceptibility of layered superconductors. Our model, based on the Lawrence-Doniach functional, predicts power-law behaviors in distinct temperature and magnetic field regimes. Comparison with data from conventional BCS superconductors reveals an excellent agreement between theory and experiment. The same does not happen, however, for oxide superconductors, such as the strontium ruthenate Sr2RuO4 [D Pelc et al., Nat. Commun. 10, 2729 (2019)]. We show that inclusion of disorder can capture some of the experimental features. Finally, we discuss the limitations and possible extensions of our model. |
Tuesday, March 3, 2020 11:39AM - 11:51AM |
G49.00003: Current induced Non equilibrium states In NbTi superconducting Bridge Khalil Harrabi, Jean Paul Maneval
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Tuesday, March 3, 2020 11:51AM - 12:03PM |
G49.00004: Photo-Manipulation of Superconductivity in an Extended Hubbard Model Wei-Chih Chen, Yao Wang, Cheng-Chien Chen Photo-induced phase transition is a promising approach to engineering material properties and has been demonstrated experimentally in several strongly correlated systems. Here, we numerically study intertwined orders involving charge, spin, and superconductivity in an extended Hubbard model by exact diagonalization on a 16-site square cluster. Our ground-state phases at various carrier doping match previous studies based on mean-field and functional renormalization group methods. On top of the equilibrium ground states, we then investigate the non-equilibrium dynamics of various orders during and after a realistic pump pulse. We found that these intertwined orders and superconducting pairing symmetries can be selectively manipulated through light polarization, frequency, and amplitude. This work shows that using ultrafast light to control superconductivity of desired pairing symmetry is possible. |
Tuesday, March 3, 2020 12:03PM - 12:15PM |
G49.00005: Charged fluctuators as a limit to the microscopic and macroscopic coherence of superconductors Nicolas Bourlet, Hélène le Sueur, Artis Svilans, Anil MURANI, Laurent Berge, Louis Dumoulin, Philippe Joyez By analyzing experiments on thin-film resonators of NbSi and TiN, we shed light on a novel decoherence mechanism at work in disordered superconductors. This decoherence is caused by charged Two Level Systems (TLS) which couple to the conduction electrons in the BCS ground state, inducing fluctuations of the kinetic inductance. Standard theories of mesoscopic disordered conductors are used to describe this effect, linking electronic (microscopic) decoherence and electromagnetic (macroscopic) decoherence in superconductors. This model is compared to the so called Generalised Tunelling Model (GTM), used to describe the impact of fluctuating TLS on the dielectric properties of the resonators. Given the omnipresence of charged TLS in solid-state systems, these decoherence mechanisms affects all experiments involving disordered superconductors, and even more so for devices with smaller cross-sections through the new mechanism presented here. In particular, we show it easily explains the poor coherence observed in quantum phase slip experiments and may contribute to lowering the quality factors in disordered superconductor resonators. |
Tuesday, March 3, 2020 12:15PM - 12:27PM |
G49.00006: Analytical Noise Spectra for Chiral d-Wave Superconducting Heterostructures Corey Ostrove, Linda E Reichl We present analytical expressions for the noise spectrum of a chiral dwave/normal/chiral d-wave Josephson junction. The scattering matrix is derived from the Bogoliubov de-Gennes equations without use of the Andreev approximation. We use the derived scattering coefficients to calculate the currentcurrent correlation function directly and from this the noise spectrum (and in particular the shot noise contribution) is generated. The effects of the chiral nature of the order parameter in the scattering and noise properties of the structures are investigated and are compared to those of those of the non-chiral d-wave order parameter. Recent experimental work has produced a number of candidate materials hypothesized to have chiral d-wave order parameters, which makes theoretical characterization of these structures of particular importance. |
Tuesday, March 3, 2020 12:27PM - 12:39PM |
G49.00007: Bosonic Mode in the Tunneling Spectra of Cuprate and Fe-based Superconductors: Elastic Tunneling Contribution John Zasadzinski, Beverly Lowell, Noah Samuelson The above-gap, SIN tunneling spectral dip (bosonic mode) feature in the bilayer cuprate Bi2Sr2CaCu2O8+δ (Bi2212) is highly reproducible and its doping dependence links it to the resonance spin excitation. SIS junctions on Bi2212 reveal a strongly enhanced dip strength and symmetric, decreasing conductance backgrounds, indicating the elastic tunneling origin of the bosonic mode and the connection to the pairing self-energy. The observation of similar bosonic mode features in the tunneling spectra of Fe-based superconductors and other cuprate superconductors is discussed. Scanning tunneling spectroscopy (STS) on FeSe reveals a similar background shape to Bi2212 indicating the symmetric bosonic mode features also are dominated by elastic tunneling. Disorder effects which show up as a depairing rate, Γ, in the fitting of the density of states (DOS) become increasingly important for lower Tc single-layer cuprates. It is demonstrated that SIS break junctions and intrinsic c-axis junctions allow the observation of a bosonic mode in strongly disordered Bi2201 with Tc values ≤ 5K. The scaling of Ω with Tc over nearly two decades, and over two different classes of superconductors suggests that the bosonic mode is a universal signature of unconventional superconductivity. |
Tuesday, March 3, 2020 12:39PM - 12:51PM |
G49.00008: Dynamics of phase coherence in superconducting cuprates upon mid-infrared photoexcitation Angela Montanaro, Francesca Giusti, Daniele Fausti Superconducting fluctuations in optimally-doped cuprates are known to survive well above the critical temperature, making these systems a perfect playground to investigate the possibility of transiently controlling superconductivity through ultrashort light pulses. While it has been widely shown that high photon energy electromagnetic fields melt the superconducting phase [1], there are evidences that mid-infrared photoexcitation can trigger the onset of superconductivity in regions of the phase diagram in which the system is not superconducting at the equilibrium [2,3]. We performed measurements on optimally-doped Y-Bi2212 by a 3-pulse technique which allows to disentangle these two effects. The approach is based on selectively destroying the superconducting state using a visible pump, and then further exciting the sample by means of a mid-infrared source. By probing the system through a broadband supercontinuum, we reveal the details of the transient dynamics of the condensate phase coherence solely driven by mid-infrared pulses with photon energy close to the superconducting gap. |
Tuesday, March 3, 2020 12:51PM - 1:03PM |
G49.00009: Josephson phenomena in Van der Waals heterostructures Liam Farrar, Geetha Balakrishnan, Simon J Bending
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Tuesday, March 3, 2020 1:03PM - 1:15PM |
G49.00010: Vortex effects as an indication of a transition to the FFLO state Calvin Bales, Raju Ghimire, Vesna F Mitrovic, John A Schlueter, Akiko Kobayashi, Charles C Agosta The FFLO state is an exotic superconducting state that allows materials to remain in a superconducting state at higher fields than the paramagnetic limit where a magnetic field would normally break the singlet state of the Cooper pairs. The angular dependence of the rf penetration depth of the sample relative to the field is sensitive to distinct data signatures such as vortex lock-in where the Josephson vortices are very lightly pinned and increase the penetration depth. This presentation will discuss the process by which we were able to analyze these vortex data signatures as an indication of vortex lock-in and the transition to the FFLO state. The analysis of angular and field sweeps as well as the transformation from angular sweeps to field sweeps and vice versa make evident the validity of the features we are seeing as well as the connections between the features and vortex motion. This presentation will focus on the results of measurements of λ-(BETS)2GaCl4 and κ-(ET)2Cu(NCS)4 superconductors. |
Tuesday, March 3, 2020 1:15PM - 1:27PM |
G49.00011: Imaging dissipative current in superconducting Niobium film using scanning SQUID susceptometry Eli Mueller, John Robert Kirtley, Ruby Shi, Huiyuan Man, Kathryn Ann Moler The quasiparticle density of states is a strong diagnostic of the superconducting gap and may be observed as dissipation in two coil mutual inductance experiments. Mutual inductance experiments have typically been limited to millimeter spatial resolution and therefore measure nonzero impedance resulting from motion of vortices. Susceptibility measurements with greater spatial resolution may be able to detect the additional dissipation resulting from scattering of excited quasiparticles. Scanning Superconducting QUantum Interference Device (SQUID) susceptibility measurements offer micron scale spatial resolution with the in-phase component measuring superfluid density and the quadrature component measuring dissipation. In this talk, we report on micron scale scanning SQUID susceptometry measurements on a Niobium film in an effort to observe the frequency and temperature dependence of quasiparticle scattering. Successfully observing the temperature dependence of quasiparticle excitations in Niobium would be a proof of concept to measure dissipation in unconventional superconductors with micron scale spatial resolution. |
Tuesday, March 3, 2020 1:27PM - 1:39PM |
G49.00012: Dynamic Evolutions of Flux Distributions in a Superconductor by a Pulsed Current Hodaka Kurokawa, Yuto Kinoshita, Fuyuki Nabeshima, Masashi Tokunaga, Atsutaka Maeda The dynamics of vortices in a superconductor has long been investigated because of its practical importance for the application of the superconducting magnets. The direct imaging of flux-density distributions must be powerful for the understandings of the dynamics of vortices. However, observations of the time-evolution of the flux-density distributions have been scarcely reported. From this viewpoint, it is still unclear how the static state of vortices collapses into the flow state. Hence, we observed the changes of flux–density distributions in a superconductor by a pulsed current with the magneto-optical microscopy. The flux-density distributions in a NbN film were measured up to 10000 frames per second. We investigated for different initial flux distributions in a comparative manner; the field-cooled state, the remanent state, the zero-field-cooled (ZFC) state. In the remanent state and the ZFC state, local reconfiguration of vortices occurred even below the critical current, which was qualitatively explained within the critical state model. However, some deviations between experiments and the theory were observed in the ZFC state, indicating that the current flows in the sample even far below the critical current. |
Tuesday, March 3, 2020 1:39PM - 1:51PM |
G49.00013: Effect of electron irradiation on rf-susceptibility of ferromagnetic superconductor EuFe2(As,P)2 Sunil Ghimire, Kyuil Cho, Makariy A Tanatar, Tsuyoshi Tamegai, Ruslan Prozorov High resolution AC magnetic susceptibility was measured in single crystals of ferromagnetic superconductor (TC=24K, TCurie=18K), |
Tuesday, March 3, 2020 1:51PM - 2:03PM |
G49.00014: Rotational transition, domain formation, dislocations and defects in vortex systems with combined six- and 12-fold anisotropic interactions Maciej W Olszewski, Morten Eskildsen, Charles Reichhardt, Cynthia Reichhardt We introduce a new model for a pairwise repulsive interaction potential of vortices in a type-II superconductor, consisting of superimposed six- and 12-fold anisotropies. Using numerical simulations we study how the vortex lattice configuration varies as the magnitudes of the two anisotropic interaction terms change. A triangular lattice appears for all values, and rotates through 30° as the ratio of the six- and 12-fold anisotropy amplitudes is varied. The transition causes the VL to split into domains that have rotated clockwise or counter-clockwise, with grain boundaries that are "decorated" by dislocations consisting of five- and seven-fold coordinated vortices. We also find intra-domain dislocations and defects, and characterize them in terms of their energy cost. |
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