Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X38: Superconductivity; Fluctuations and Non-equilibrium Effects |
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Sponsoring Units: DCMP Chair: Thomas Devereaux, Stanford Unviersity Room: 385 |
Friday, March 17, 2017 8:00AM - 8:12AM |
X38.00001: Fluctuations of superconductivity near the quantum critical point of overdoped La$_{\mathrm{2-x}}$Sr$_{\mathrm{x}}$CuO$_{\mathrm{4}}$ Fahad Mahmood, Ivan Bozovic, N. Peter Armitage It was widely believed that the overdoped regime of high-temperature cuprate superconductors can be described by the conventional physics of BCS superconductors. However, recent measurements by Bozovic et. al. (Nature 536, 309-311), conclude that even in this regime the superfluid density is a smooth function of the transition temperature (T$_{\mathrm{c}})$ rather than being independent of it, in contrast with BCS predictions. It is important to understand the mechanism by which normal state charge carriers condense to form the superconducting state. Moreover, one expects that for critically overdoped La$_{\mathrm{2-x}}$Sr$_{\mathrm{x}}$CuO$_{\mathrm{4}}$, superconducting fluctuations will be very significant due to the low superfluid densities. Here, we combine kHz range mutual inductance measurements and time-domain THz spectroscopy (TDTS) to probe the fluctuations of superconductivity in overdoped La$_{\mathrm{2-x}}$Sr$_{\mathrm{x}}$CuO$_{\mathrm{4}}$ films grown by atomic-layer- by-layer molecular beam epitaxy (ALL-MBE). The fluctuations are described by quantifying and directly comparing both the low (kHz) and high (THz) frequency phase stiffness. [Preview Abstract] |
Friday, March 17, 2017 8:12AM - 8:24AM |
X38.00002: Signatures of pair-density wave order via measurement of the current-phase relation in La$_{2-x}$Ba$_x$CuO$_4$ Josephson junctions David Hamilton, Adam Weis, Genda Gu, Dale Van Harlingen La$_{2-x}$Ba$_x$CuO$_4$ (LBCO) exhibits a sharp drop in the transition temperature near $x=1/8$ doping. In this regime, charge, spin and superconducting orders are intertwined and superconductivity is believed to exist in a pair-density wave (PDW) state, an ordered stripe phase characterized by sign changes in the superconducting order parameter between adjacent stripes. We present direct measurements of the current-phase relation (CPR) of Josephson junctions patterned onto crystals of LBCO at $x=1/8$ and $x=0.155$ (optimal doping) using a phase-sensitive Josephson interferometry technique. In contrast to the approximately sinusoidal CPR observed at optimal doping, we find the proportion of higher harmonics in the CPR increases at $x=1/8$ doping, consistent with the formation of a PDW state. In parallel, we are carrying out measurements of the resistance noise in thin films of LBCO of various doping levels to identify features that signify the onset of charge order and changes in the dynamics of charge stripes. [Preview Abstract] |
Friday, March 17, 2017 8:24AM - 8:36AM |
X38.00003: Search for flux noise sources in superconducting material using polarized neutron reflectivity Timothy Charlton, Michael O’Keeffe, Alexander Melville, Andrew J. Kerman, Jonilyn Yoder, William Oliver Magnetic flux noise is a dominant source of decoherence in superconducting qubits. Flux noise is believed to originate from unpaired spins at the surfaces and interfaces of superconducting thin films. Possible origins of these fluctuating spins include dangling bonds, interface states, and adsorbed molecules. To our knowledge, there has been no direct observation of magnetic induction at interfaces of these devices. Polarized neutron reflectometry is uniquely suited to provide depth-resolved spin information in layered structures. In this presentation we will report the magnetic induction as a function of depth from the surface of superconducting thin films used in qubit devices, near the superconducting transition temperature. [Preview Abstract] |
Friday, March 17, 2017 8:36AM - 8:48AM |
X38.00004: Identification of the microscopic origin of unpaired surface spins on superconducting devices using XMCD simulations Zhe Wang, Hui Wang, Clare Yu, Ruqian Wu Unpaired surface spins reside on superconducting quantum interference devices (SQUIDs) are significant sources of the 1/f flux noise, which limits the coherence time of superconducting devices. Recently, the x-ray magnetic circular dichroism (XMCD) technique has been successfully used to identify the species of these unpaired spins, where adsorbed molecular O2 was demonstrated to be the dominant contributor. Here, we present systematic density functional theory (DFT) investigations on the XMCD spectra of adsorbed O2 and other likely candidates include dangling bonds, surface vacancies. Pronounced peaks can be found in the x-ray absorption and XMCD spectra due to the transition from the 1s core to 2$\pi $* orbitals; and the peak intensity decreases when the bond direction of O2 molecule deviates away from the incident x-rays. In contrast, the XMCD signals from the lattice are either zero or in much higher energy position, indicating that the XMCD technique can be used in detecting the origin of unpaired spins on qubits. [Preview Abstract] |
Friday, March 17, 2017 8:48AM - 9:00AM |
X38.00005: Pair symmetry conversion in driven multiband superconductors Christopher Triola, Alexander Balatsky |
Friday, March 17, 2017 9:00AM - 9:12AM |
X38.00006: Abstract Withdrawn Recent advances, both theoretical and experimental, have made it possible to observe hydrodynamic flow in electron systems such as graphene and extract hydrodynamic transport coefficients such as the shear viscosity. Following the same logic, I will show how certain flows in superconductors could show signatures of fluctuating superconductivity. |
Friday, March 17, 2017 9:12AM - 9:24AM |
X38.00007: Twisting Anderson pseudospins with light: Quench dynamics in THz-pumped BCS superconductors Yang-Zhi Chou, Yunxiang Liao, Matthew Foster We study the preparation and the detection of coherent far-from-equilibrium BCS superconductor dynamics in THz pump-probe experiments. In a recent experiment [Matsunaga et al, PRL. 111, 057002 (2013)], an intense monocycle THz pulse with center frequency $\omega \simeq \Delta$ was injected into a superconductor with BCS gap $\Delta$; the post-pump evolution was detected via the optical conductivity. It was argued that nonlinear coupling of the pump to the Anderson pseudospins of the superconductor induces coherent dynamics of the Higgs mode $\Delta(t)$. We validate this picture in a 2D BCS model with a combination of exact numerics and the Lax reduction, and we compute the dynamical phase diagram. The main effect of the pump is to scramble the orientations of Anderson pseudospins along the Fermi surface by twisting them in the xy-plane. We show that more intense pulses can induce a far-from-equilibrium gapless phase (phase I), originally predicted in the context of interaction quenches. We show that the THz pump can reach phase I at much lower energy densities than an interaction quench, and we demonstrate that Lax reduction provides a quantitative tool for computing coherent BCS dynamics. We also compute the optical conductivity for the states discussed here. [Preview Abstract] |
Friday, March 17, 2017 9:24AM - 9:36AM |
X38.00008: Time resolved optical conductivity in interacting fermionic systems after a quantum quench Yonah Lemonik, Aditi Mitra We study dynamics following a quantum quench from an initial normal metal phase to the superconducting critical point. We identify a scaling regime where universal behavior appears in an intermediate time regime before thermalization sets in. The scaling is characterized in terms of a new critical exponent that controls the anomalous dimension of fields at short times after the quench. Motivated by experiments in pump-probe spectroscopy of correlated materials, we make predictions for how the optical conductivity evolves in time. Our results show that transient superconducting fluctuations and nonequilibrium critical exponents can be measured directly by experimental probes. [Preview Abstract] |
Friday, March 17, 2017 9:36AM - 9:48AM |
X38.00009: The dynamics of Anderson localization in a quasi 1D system: role of symmetry and topology Eslam Khalaf, Pavel Ostrovsky Since its discovery 60 years ago, Anderson localization has been an active area of research. Despite this, little is known about the dynamical correlations in Anderson localized systems beyond the strictly 1D case, where effects of symmetry and topology cannot be taken into account. Quasi-1D systems, on the other hand, provide realistic models for Anderson localization, where symmetry and topology play a non-trivial role. The main technical difficulty in computing dynamical correlations in these systems lies in the complicated form of the transfer matrix Hamiltonian derived from the sigma model at finite frequency. We show that the zero mode of this Hamiltonian has a surprisingly simple form, leading to a remarkable identity that enables the computation of any dynamical local correlation function in a quasi-1D system in any of the Wigner-Dyson classes. The result is used to compute the quantum return probability in these classes including the effects of topology for classes A, where an arbitrary number of channels can be topologically protected and AII, where a single channel can be topologically protected. Physical realizations of such systems include the interface between two quantum Hall systems, a Weyl semimetal in magnetic field or the edge of a 2D topological insulator. [Preview Abstract] |
Friday, March 17, 2017 9:48AM - 10:00AM |
X38.00010: Controlling competing electronic orders \emph{via} non-equilibrium acoustic phonons Michael Schuett, Peter Orth, Alex Levchenko, Rafael Fernandes The interplay between multiple electronic orders is a hallmark of strongly correlated systems displaying unconventional superconductivity. While doping, pressure, and magnetic field are the standard knobs employed to assess these different phases, ultrafast pump-and-probe techniques opened a new window to probe these systems. Recent examples include the ultrafast excitation of coherent optical phonons coupling to electronic states in cuprates and iron pnictides. In this work, we demonstrate theoretically that non-equilibrium acoustic phonons provide a promising framework to manipulate competing electronic phases and favor unconventional superconductivity over other states. In particular, we show that electrons coupled to out-of-equilibrium anisotropic acoustic phonons enter a steady state in which the effective electronic temperature varies around the Fermi surface. Such a momentum-dependent temperature can then be used to selectively heat electronic states that contribute primarily to density-wave instabilities, reducing their competition with superconductivity. We illustrate this phenomenon by computing the microscopic steady-state phase diagram of the iron pnictides, showing that superconductivity is enhanced with respect to the competing antiferromagnetic phase. [Preview Abstract] |
Friday, March 17, 2017 10:00AM - 10:12AM |
X38.00011: Why superconducting vortices follow to moving hot sport? Andrei Sergeev, Reizer Michael Recent experiments reported in Nature Comm. 7, 12801, 2016 show that superconducting vortices follow to the moving hot sport created by a focused laser beam, i.e. vortices move from the cold area to the moving hot area. This behavior is opposite to the vortex motion observed in numerous measurements of the vortex Nernst effect, where vortices always move against the temperature gradient. Taking into account that superconducting magnetization currents do not transfer entropy, we analyze the balance of forces acting on a vortex in stationary and dynamic temperature gradients. We show that the dynamic measurements may be described by a single vortex approximation, while in stationary measurements interaction between vortices is critical. [Preview Abstract] |
Friday, March 17, 2017 10:12AM - 10:24AM |
X38.00012: Dynamic Transitions of Vortices into Phase Slips in Josephson Junctions Under DC and AC Currents Ahmad Sheikhzada, Alex Gurevich We present extensive numerical simulations of nonlinear dynamics of vortices driven by strong dc and ac currents in thin film Josephson junctions of finite length. We solved a nonlinear integro-differential equation which takes into account the nonlocal electrodynamics of films, vortex Cherenkov radiation and the essential effects of interaction of vortices with the edges of the junction. Our simulations have shown that in overdamped junctions vortices expand as they move faster and turn into phase slips as current increases. In underdamped junctions vortices entering from the edges become unstable due to Cherenkov radiation and produce a cascade of expanding vortex-antivortex pairs, which ultimately drives the whole junction into resistive phase slip state. Vortices driven by ac currents can exhibit a variety of complicated dynamic states which manifest themselves in jumps and regions with negative differential resistance on current-voltage characteristics. [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 10:36AM |
X38.00013: Electronic and energetics properties of oxygen defects in La2-xSrxCuO4 in relation to doping and strain Sohee Park, Changwon Park, Mina Yoon The level of oxygen defects in La2-xSrxCuO4 (LSCO), a high temperature superconductor, is known to drastically change LSCO’s structural and electronic properties. However, the atomistic understanding of the role of oxygen defects is far from being complete. Using first-principles calculations, we investigated the electronic and energetic properties of oxygen vacancies in LSCO in relation to external parameters such as degree of Sr doping amount and external strain. We find that the relative stabilities between the equatorial vacancy induced in the CuO2 layer and the apical vacancy in the LaO layer can be altered by strain. In addition, Sr doping plays a crucial role in their relative stabilities. Therefore, the complex interplay between those key parameters essentially determines the overall oxygen density. Our finding can be instrumental in the experimental development of LSCO with desired oxygen density. [Preview Abstract] |
Friday, March 17, 2017 10:36AM - 10:48AM |
X38.00014: Kohn anomalies and electron-phonon coupling in T-graphene M.E. Cifuentes-Quintal, R. de Coss T-graphene is a metastable phase of graphene which consist in tetragonal rings of carbon atoms with a metallic behaviour. The Fermi surface of T-graphene consist of a hole pocket at the $\Gamma$ point and a electron pocket at the M point, with large parallel portions which is expected to induce strong Kohn anomalies and a strong electron-phonon (e-ph) coupling. In this work, we present DFT first-principles calculations of the Kohn anomalies and electron-phonon coupling in T-graphene. Our calculations are based in the Plane-Waves and Pseudopotential method, with the GGA-PBE exchange-correlation functional. Dynamical matrices and e-ph coupling properties were computed with the linear response theory. We found a strong Kohn anomaly in two optical phonon branches at the M point, which consist in inter-band transitions from the two fermi surface pocket. The squared average of the electron-phonon coupling matrix elements at the Fermi surface of the Kohn anomalies are lower in comparison with the hexagonal ground-state graphene. The Eliashberg function, e-ph coupling constant, and the possibility of superconductivity are also analyzed. [Preview Abstract] |
Friday, March 17, 2017 10:48AM - 11:00AM |
X38.00015: A time-dependent mean field description of pump-probe ARPES Neil J. Robinson, Jonathan D. Rameau, Robert M. Konik We consider the problem of describing pump-probe ARPES experiments in the $t$-$J$ model. Specifically, to compute the time-resolved ARPES intensity we combine a time-dependent generalization of slave boson mean field theory with the Schwinger-Keldysh formalism for computing the two-time Green's function. The pump field is treated through a Peierls substitution, under the assumption that it is not strong enough to excite to the upper Hubbard band, whilst the probe field is taken to be weak and treated under the sudden approximation. We compare our results to pump-probe ARPES experiments on Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212). [Preview Abstract] |
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