Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U49: Superconductivity: Response to Electromagnetic Fields (Optical and Raman Spectroscopy, Surface Impedance, Etc.) |
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Sponsoring Units: DCMP Chair: Richard Klemm, Univ of Central Florida Room: Mile High Ballroom 1B |
Thursday, March 5, 2020 2:30PM - 2:42PM |
U49.00001: Effects of Forward Scattering from Dopant Impurities in Overdoped Cuprates Peter Hirschfeld, Vivek Mishra, Nicholas Lee-Hone, Ulas Ozdemir, David Broun Many properties of overdoped cuprate superconductors can be explained by simple BCS and Landau Fermi liquid theory, supplemented by an appropriate description of disorder from dopant impurities. In addition to simply providing weaker scattering, out-of-plane dopants give rise to extended impurity potentials that scatter preferentially in the forward direction. Here we investigate the effects of such forward scattering on the optical conductivity and the penetration depth, and make specific quantitative comparisons with both LSCO and Tl-2201. We discuss previous work on forward scattering in ARPES in the same context. |
Thursday, March 5, 2020 2:42PM - 2:54PM |
U49.00002: Gauge-invariant microscopic kinetic theory of superconductivity in response to electromagnetic fields I: Application to current excitation Fei Yang, Ming-Wei Wu Within a gauge-invariant microscopic kinetic theory, we study the electromagnetic response in the superconducting states. Both superfluid and normal-fluid dynamics are involved. We predict that the normal fluid is present only when the excited superconducting velocity is larger than a threshold. Interestingly, with the normal fluid, we find that there exists friction between the normal-fluid and superfluid currents. Due to this friction, part of the superfluid becomes viscous. Therefore a three-fluid model, normal fluid and nonviscous and viscous superfluids, is proposed to capture both the magnetic and optical responses. Moreover, an exotic phase in which both the resistivity and superconducting gap are finite is predicted. |
Thursday, March 5, 2020 2:54PM - 3:06PM |
U49.00003: Phase-sensitive analysis of Higgs oscillations in quenched superconductors with time- and angle-resolved photo emission spectroscopy Lukas Schwarz, Dirk Manske Higgs oscillations in nonequilibrium superconductors provide a unique tool to obtain information about the underlying order parameter. Several quantities like the absolute value, existence of multiple gaps and the symmetry of the order parameter can be encoded in the Higgs oscillation frequency. Most works so far concentrate on experiments, where momentum averaged quantities like the optical conductivity or third-harmonic effects in the transmitted light field are investigated, which does not allow to access all possible information contained in the Higgs oscillations. Here, we study the time-resolved spectral function measured in angle-resolved photo emission spectroscopy after quenching the system using a general approach. We analyze the induced oscillations all over momentum space to study the creation process of collective Higgs oscillations and we extract phase information of the order parameter from the oscillations of the spectral function. |
Thursday, March 5, 2020 3:06PM - 3:18PM |
U49.00004: Driving the Higgs oscillation in cuprates: unveiling collective modes or interactions coupled to Cooper pairs Hao Chu, Min-Jae Kim, Kota Katsumi, Sergey Kovalev, Robert Dawson, Lukas Schwarz, Naotaka Yoshikawa, Gideok Kim, Daniel Putzky, Jan-Christoph Deinert, Nilesh Awari, Min Chen, Georg Christiani, Gennady Logvenov, Yann Gallais, Alexander Boris, Bernhard Keimer, Andreas P Schnyder, Dirk Manske, Michael Gensch, Zhe Wang, Ryo Shimano, Stefan Kaiser The Higgs mode of a superconductor is a collective excitation of the amplitude of the superconducting order parameter. It can be coherently driven at a frequency 2ω by a multicycle terahertz pulse of frequency ω. This then leads to third harmonic generation (THG) as a result of sum frequency generation between the oscillating condensate and the terahertz driving pulse. We applied such experimental scheme to different families of cuprate high-Tc superconductors. By phase-resolving the THG response with respect to the terahertz drive, we uncover the universal anti-resonance of the driven Higgs oscillation in all samples. This may be understood in terms of an additional collective mode coupled to the Higgs mode. Careful analysis of the anti-resonance feature as a function of hole doping with the help of a coupled oscillators model suggests that the coupled mode could be the magnetic resonant mode. Our findings may shed light on the pairing mechanism in d-wave superconductors. |
Thursday, March 5, 2020 3:18PM - 3:30PM |
U49.00005: Spectroscopic and optical response of odd-frequency Berezinskii superconductors Pavlo Sukhachov, Alexander Balatsky By using a simple model with a parabolic dispersion law and a non-magnetic disorder, the spectral function, the electron density of states, and the optical conductivity are calculated in clean and disordered Berezinskii superconductors for a few gap ansatzes. The strong dependence of the spectral function and the electron DOS on the frequency profile of the gap is rather unusual and could explain why it is experimentally hard to identify the Berezinskii state. It is found also that the OF pairing allows for peaks in the real part of the optical conductivity, whose form and position depend on the specific frequency profiles of the gap. For the frequencies that correspond to these peaks, sharp cusps appear in the imaginary part. The form of these cusps is nontrivial for the OF pairing. In particular, the imaginary part of the optical conductivity can become negative allowing for the optical transparency windows. |
Thursday, March 5, 2020 3:30PM - 3:42PM |
U49.00006: Control of Supercurrents and Collective Modes in Superconductors by Terahertz Lightwave Propagation Martin Mootz, Ilias Perakis, Jigang Wang Terahertz (THz) lightwave driving of supercurrents as a dynamical symmetry-breaking principle in superconductors allows for control of collective modes and excitation of non-equilibrium phases such as gapless superconductivity. Here we demonstrate that THz light-induced nonlinear response together with THz lightwave propagation can induce an unipolar ac electric field pulse inside the superconductor for sufficiently weak current relaxation. The resulting dynamical inversion symmetry breaking in form of the induced Cooper-pair condensate flow manifests itself by high harmonic generation at equilibrium-symmetry forbidden frequencies and allows for control of collective modes by THz-pulse shaping. To systematically describe the dynamic interplay of THz lightwave acceleration of Cooper-pair condensate and THz lightwave propagation inside the superconductor, we solve full nonlinear gauge-invariant Maxwell-Bloch equations beyond the Anderson pseudo-spin precession model that we derived by using a gauge-invariant non-equilibrium density matrix approach. |
Thursday, March 5, 2020 3:42PM - 3:54PM |
U49.00007: Giant Microwave Absorption in d-Wave Superconductors Michael Smith, Anton Andreev, Boris Spivak We discuss a new mechanism of microwave absorption in d-wave superconductors which arises in the presence of a dc supercurrent. This mechanism produces a contribution to the ac conductivity that is proportional to the inelastic quasiparticle relaxation time and determines the supercurrent dependence of the conductivity. This new contribution may also be much larger than the conventional contribution because, in superconductors, the inelastic relaxation time typically exceeds the elastic one by several orders of magnitude. We show that the calculation of the aforementioned contribution to the conductivity can be reduced to the calculation of the single particle density of states in the presence of a dc supercurrent. Our results may enable determination of the inelastic relaxation time in superconductors from microwave absorption measurements. |
Thursday, March 5, 2020 3:54PM - 4:06PM |
U49.00008: High-Frequency Nonlinear Response of Superconducting Cavity-Grade Surfaces Bakhrom Oripov, Steven Anlage Superconducting Radio Frequency (SRF) cavities are used to accelerate charged particles to high energies, but their performance is limited by surface defects which lead to cavity breakdown at high accelerating gradients. A novel near-field magnetic microwave microscope that can study these defects was successfully built using a magnetic writer from a conventional magnetic recording hard-disk drive. We study the 3rd harmonic response-V_{3f} as a function of rf field amplitude (H_{rf}) and temperature (T). In our experiments on bulk and thin-film Nb surfaces, we observed 2 different classes of nonlinearity, which we call Low-field and Periodic. The low-field response is the intrinsic response of the sample due to the dynamics of vortex semiloops created by the magnetic writer (arXiv:1909.02714). In the periodic case there exists a relatively strong onset nonlinear response signal with periodic dips at H_{rf} = H_1(T), H_2(T), H_3(T)… This response can be linked to the Josephson effect at or near the surface and is in good agreement with the nonlinear response expected from rf-current-biased RCSJ model (PRApplied 11, 064030 (2019)). Efforts to image third harmonic response across Nb surfaces will be presented. |
Thursday, March 5, 2020 4:06PM - 4:18PM |
U49.00009: 2D THz spectroscopy and anomalous non-linear response of La2−xSrxCuO4 Dipanjan Chaudhuri, Fahad Mahmood, David Barbalas, Xi He, Ivan Bozovic, Peter Armitage La2−xSrxCuO4 (LSCO) is a prototypical example of hole-doped single layer cuprate superconductor. Here we investigate the low frequency broadband non-linear optical response of LSCO across a wide range of doping as a function of temperature. In addition to the large optical non-linearities that is expected in the superconducting state, we find anomalous non-linear response even in the normal state, well above the superconducting critical temperature. This is in sharp contrast to the clean BCS type superconductors where the strong non-linear response at low temperatures promptly disappear at the critical temperature. Furthermore, we perform 2D THz spectroscopy on these samples to identify the different underlying non-linear processes that give rise to the overall response. We compare our results to existing theories on the superconductivity in this class of materials. |
Thursday, March 5, 2020 4:18PM - 4:30PM |
U49.00010: Terahertz conductivity of superconducting Beta-tungsten thin films Peter Armitage, Prashant Chauhan, Ramesh Budhani Ultrathin films of β-tungsten host superconductivity with strong spin-orbit coupling (SOC). This non-equilibrium phase of tungsten has attracted considerable attention in recent years for spintronic applications due to its giant spin Hall effect and the potential promise of exotic superconductivity. After more than 60 years of its discovery, the superconductivity in this material is still not well understood. We investigate the superconducting order in β-Tunsgten using high precision time-domain THz transmission spectroscopy. Superconductivity is exhibited below 4.2 K for 70 nm film and the transition temperature reduces with increasing thickness. We study the frequency response of the complex conductivity of a thin film with a Tc of 4.2 K. In the superconducting state down to 1.6 K, the superfluid spectral weight developed is much less than the expected spectral weight for a BCS-type superconductor. This anomalous spectral weight is a possible indication of unconventional superconductivity in this system. The magnetic field response of the optical conductivity that can give additional information about the superconducting order parameter of this system, is currently under investigation. |
Thursday, March 5, 2020 4:30PM - 4:42PM |
U49.00011: Thickness dependence of superfluid density scaling in DyBa2Cu3O7-x thin films Robert Dawson, Daniel Putzky, Georg Christiani, Gennady Logvenov, Bernhard Keimer, Alexander Boris Understanding the nature of the pseudogap phase and the relationship between charge order and superconductivity (SC) in the cuprates remains an outstanding challenge [1]. Recently, 2D SC has gained much interest due to its discovery in a variety of unconventional materials, and advancements in MBE growth of thin films have shown that 2D SC can also exist in the cuprates [2]. One interesting approach, therefore, is to investigate the evolution of cuprate physics in the ultra-thin film limit where only a few superconducting CuO2 planes are present. Here, we report a study of the temperature dependence of the superfluid density as a function of thickness as well as measurements of the terahertz and infrared optical conductivity in a series of DyBa2Cu3O7-x SC thin films. We find that in the ultra-thin film limit the temperature scaling rapidly approaches that expected for dirty s-wave SC, while with decreasing oxygen content the scaling shows the opposite trend. These results suggest an increasing isotropic order parameter component below a threshold thickness due to distortions of the Fermi surface in the 2D limit. |
Thursday, March 5, 2020 4:42PM - 4:54PM |
U49.00012: Non-linear Vortex Dynamics and Dissipation at High-amplitude Microwave Drive Mattia Checchin, Anna Grassellino, Alexander Romanenko In this talk, the description of the vortex dynamics and dissipation at arbitrary value of microwave field amplitude is discussed. |
Thursday, March 5, 2020 4:54PM - 5:06PM |
U49.00013: Optical properties of severely overdoped La1.75Sr0.25CuO4 Christopher Homes, Genda Gu, P. M. Lozano, Qiang Li The temperature dependence of the optical properties of the severely overdoped cuprate La1.75Sr0.25CuO4 have been determined above and below the critical temperature Tc ~16 K over a wide frequency range for light polarized in the a-b plane. The real part of the optical conductivity can be reproduced reasonably well at low frequency using a simple Drude model, yielding an estimate for the plasma frequency wpD ~ 9300 cm-1. Below Tc the superconducting plasma frequency wps ~ 2700 cm-1 is determined from the Ferrell-Glover-Tinkham sum rule, indicating that less than 8% of the free carriers collapse into the superconducting condensate, in agreement with a recent study [1]. However, there is unusual structure below ~0.1 eV that suggests that the free-carrier scattering rate is strongly renormalized with frequency. In addition, the nature of the magnetic susceptibility through the superconducting transition suggests that this compound may not be homogeneous. |
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U49.00014: Doping-dependent phonon anomaly and charge order in Hg-family of high-Tc cuprates Lichen Wang, Biqiong Yu, Ran Jing, Xiangpeng Luo, Junbang Zeng, Jiarui Li, Izabela Bialo, Martin Bluschke, Yang Tang, Jacob Freyermuth, Guichuan Yu, Ronny Sutarto, Feizhou He, Eugen Weschke, Wojciech Tabis, Martin Greven, Yuan Li Charge order is ubiquitously found in high-Tc cuprate superconductors, yet their microscopic origin has been elusive. The Hg-family of cuprates are known for their high Tc and relatively simple structure, and are therefore promising systems for avoiding material-specific complexity in experimental studies. Using resonant X-ray diffraction and Raman spectroscopy to study high-quality single crystals [1,2] of the first two members of the family, Hg1201 and Hg1212, we establish universal existence of charge order in these compounds . The most robust charge order is found in underdoped samples near a hole-carrier concentration of 0.09. Interestingly, we observe concomitant anomalies in the lattice dynamics that involve the motion of atoms in and/or adjacent to the CuO2 layers, around the same doping in both materials. As these anomalies are already present at room temperature, before the formation of charge order, we attribute them to a doping-dependent instability of the CuO2 layers that drives the formation of charge order as an emergent phenomenon [3]. |
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U49.00015: Gauge-invariant microscopic kinetic theory of superconductivity in response to electromagnetic fields II: Application to collective modes Fei Yang, Ming-Wei Wu We show that the gauge-invariant kinetic equation of superconductivity provides an efficient approach to study the electromagnetic response of the gapless Nambu-Goldstone and gapful Higgs modes on an equal footing. We prove that the Fock energy in the kinetic equation is equivalent to the generalized Ward's identity. Hence, the gauge invariance directly leads to the charge conservation. Both linear and second-order responses are investigated. The linear response of the Higgs mode vanishes in the long-wave limit. The linear response of the Nambu-Goldstone mode is in consistency with the previous works. Whereas the second-order response exhibits interesting physics. On one hand, a finite second-order response of the Higgs mode is obtained in the long-wave limit. We reveal that this response, which has been experimentally observed, is attributed solely to the drive effect rather than the widely considered Anderson-pump effect. On the other hand, the second-order response of the Nambu-Goldstone mode, free from the influence of the long-range Coulomb interaction and hence the Anderson-Higgs mechanism, is predicted. We find that both Anderson-pump and drive effects play important role in this response. A tentative scheme to detect this second-order response is proposed. |
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