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 unexplored. 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 thirdorder magnetic response, which is believed to be dominated by SF above T_{c}. Here, we present a phenomenological theoretical model for the nonlinear susceptibility of layered superconductors. Our model, based on the LawrenceDoniach functional, predicts powerlaw 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 Sr_{2}RuO_{4} [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

Tuesday, March 3, 2020 11:51AM  12:03PM 
G49.00004: PhotoManipulation of Superconductivity in an Extended Hubbard Model WeiChih Chen, Yao Wang, ChengChien Chen Photoinduced 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 16site square cluster. Our groundstate phases at various carrier doping match previous studies based on meanfield and functional renormalization group methods. On top of the equilibrium ground states, we then investigate the nonequilibrium 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 thinfilm 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 solidstate systems, these decoherence mechanisms affects all experiments involving disordered superconductors, and even more so for devices with smaller crosssections 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 dWave Superconducting Heterostructures Corey Ostrove, Linda E Reichl We present analytical expressions for the noise spectrum of a chiral dwave/normal/chiral dwave Josephson junction. The scattering matrix is derived from the Bogoliubov deGennes 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 nonchiral dwave order parameter. Recent experimental work has produced a number of candidate materials hypothesized to have chiral dwave 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 Febased Superconductors: Elastic Tunneling Contribution John Zasadzinski, Beverly Lowell, Noah Samuelson The abovegap, SIN tunneling spectral dip (bosonic mode) feature in the bilayer cuprate Bi_{2}Sr_{2}CaCu_{2}O_{8+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 selfenergy. The observation of similar bosonic mode features in the tunneling spectra of Febased 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 singlelayer cuprates. It is demonstrated that SIS break junctions and intrinsic caxis 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 midinfrared photoexcitation Angela Montanaro, Francesca Giusti, Daniele Fausti Superconducting fluctuations in optimallydoped 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 midinfrared 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 optimallydoped YBi2212 by a 3pulse 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 midinfrared 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 midinfrared 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

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 lockin 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 lockin 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 inphase 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 fluxdensity distributions must be powerful for the understandings of the dynamics of vortices. However, observations of the timeevolution of the fluxdensity 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 magnetooptical microscopy. The fluxdensity 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 fieldcooled state, the remanent state, the zerofieldcooled (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 rfsusceptibility of ferromagnetic superconductor EuFe_{2}(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 (T_{C}=24K, T_{Curie}=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 12fold 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 typeII superconductor, consisting of superimposed six and 12fold 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 12fold anisotropy amplitudes is varied. The transition causes the VL to split into domains that have rotated clockwise or counterclockwise, with grain boundaries that are "decorated" by dislocations consisting of five and sevenfold coordinated vortices. We also find intradomain dislocations and defects, and characterize them in terms of their energy cost. 
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