Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A39: Superconductivity: Optical Probes |
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Sponsoring Units: DCMP Chair: Ricardo Lobo, ESPCI Room: 386 |
Monday, March 13, 2017 8:00AM - 8:12AM |
A39.00001: Wide-band spectroscopic study of in-gap excitations in epitaxial DyBa$_2$Cu$_3$O$_7$ superconducting films R. D. Dawson, B. Reber, G. Christiani, G. Logvenov, B. Keimer, A. V. Boris Investigations of the electronic ground state properties of copper oxide high-temperature superconductors have revealed a complex array of competing forms of order across the phase diagram, suggesting the existence of dipole-active excitations at low energies and across a wide range of doping levels. \footnote{B. Keimer, et al. Nature 518 (2015) 179.} These excitations are expected to manifest even at frequencies below 2$\Delta$; however, due to strong optical response of the superconducting condensate below T$_c$, signatures of in-gap excitations can be masked and their direct measurement is challenging. Here, we have applied three complementary phase-sensitive techniques of submillimeter interferometry using backward wave oscillator sources, high-resolution time domain terahertz spectroscopy utilizing the asynchronous optical sampling technique at 1GHz repetition rate, and synchrotron-based far-infrared ellipsometry to sensitively probe the in-gap states of DyBa$_2$Cu$_3$O$_7$ epitaxial films grown by atomic-layer-by-layer oxide MBE. By combining the measured data we have obtained the continuous complex dielectric function in the spectral range of 0.5meV to 0.5eV and have observed its evolution as a function of temperature between 7K and 300K. [Preview Abstract] |
Monday, March 13, 2017 8:12AM - 8:24AM |
A39.00002: Optical spectroscopy of La$_{2-x}$Ba$_{x}$CuO$_{4}$ single crystals: influence of stripe order L-Y. Yan, D.B. Tanner, Genda Gu The $ab$-plane and $c$-axis reflectance spectra of ten La$_{2-x}$Ba$_{x}$CuO$_{4}$ single crystals, with $x$ ranging from undoped to optimally doped, have been measured over a wide frequency range and at temperatures from 10 to 300 K. The influence of stripe order around $x=0.125$ appears in the spectra below $T = 50$~K, observed both as a reduction in the free-carrier (normal state) and superfluid (superconducting state) density and by the appearance of a relatively narrow conductivity band near 25 meV. The superfluid density is estimated from the real part of the dielectric function and the $f$-sum rule. The $c$-axis spectra are those of an insulator or very bad metal, with very little doping or temperature dependence. The Josephson plasma edge is not observed in any of these spectra. [Preview Abstract] |
Monday, March 13, 2017 8:24AM - 8:36AM |
A39.00003: Infrared optical studies of symmetry breaking in cuprate superconductors over a wide doping range Alok Mukherjee, Jungryeol Seo, Mumtaz Murat Arik, John Cerne, Hao Zhang, Chao Zhang, John Y.T. Wei, N.P. Armitage, T. Kirzhner, G. Koren We explore broken symmetry states in high-temperature cuprate superconductors (HTS) using mid-infrared (MIR) polarization sensitive measurements. We measure the complex MIR Faraday angle at zero magnetic field, as a function of temperature (10 -300 K), energy (0.1- 0.4 eV) and sample orientation with respect to the incident linear polarization. The measurements are performed on a series of HTS thin films with various doping levels, grown epitaxially by pulsed laser-ablated deposition. The Faraday rotation signal is strongest in underdoped films, depending on both temperature and sample orientation, which suggests that there is linear symmetry breaking in the pseudogap region. [Preview Abstract] |
Monday, March 13, 2017 8:36AM - 8:48AM |
A39.00004: An inclusive view of the pseudogap in high-temperature superconductors James Storey A longstanding dispute in cuprate high-temperature superconductors concerns the origin of an energy gap, known as the pseudogap, which persists high above the superconducting transition temperature ($T_c$). Does it represent precursor superconductivity, or some other non-superconducting order? The answer is “yes” to both. By modelling a range of thermodynamic, transport and spectroscopic data I will demonstrate how these two scenarios explain different aspects of the unusual temperature and field dependences that have stubbornly defied explanation in terms of conventional mean-field theories. This implies that the phase diagram of the high-$T_c$ cuprates is actually a blend of the two leading proposals, and that the two opposing sides of this dispute are each partially correct. [Preview Abstract] |
Monday, March 13, 2017 8:48AM - 9:00AM |
A39.00005: Superconductor-insulator transition in a stripe-ordered cuprate John Tranquada, C. Homes, G.D. Gu, Q. Li, M. Huecker We reconsider the case of La$_{2-x}$Ba$_x$CuO$_4$ with $x=1/8$, where spin-stripe order and 2D superconducting correlations develop simultaneously at 40 K. The thermal evolution of the in-plane optical reflectivity suggests the development of a Josephson plasma resonance (JPR) between charge stripes, by analogy with the JPR seen in c-axis reflectivity in the superconducting state of Josephson-coupled CuO$_2$ planes. At low-temperature, when the superconductivity is suppressed by a magnetic field, the resistivity exhibits insulating character. We interpret this as suppression of the Josephson coupling between pair correlations in neighboring charge stripes, with single-particle transport suppressed by the surviving spin-stripe order. To obtain direct evidence that the high-field insulator involves hole pairs localized to 1D stripes will require further experiments. [Preview Abstract] |
Monday, March 13, 2017 9:00AM - 9:12AM |
A39.00006: An infrared view of superconductivity in the iron-based materials Zhen Xing, M. M. Qazilbash, Shanta Saha, J. Paglione Appropriate chemical doping in the 122 iron arsenides ($A$Fe$_{\mathrm{2}}$As$_{\mathrm{2}})$ usually leads to suppression of the antiferromagnetic order. However, bulk superconductivity does not always occur upon suppression of the antiferromagnetism. In this work we study why some iron-based materials are superconducting while others are not. We have performed cryogenic optical spectroscopy measurements on single crystals of superconducting BaFe$_{\mathrm{1.9}}$Pt$_{\mathrm{0.1}}$As$_{\mathrm{2}}$ and non-superconducting, rare-earth-doped CaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$. The \textit{ab}-plane dielectric functions have been obtained and compared to one another to gain insight into the conditions necessary for superconductivity in these materials. We also compare the dielectric functions of a number of superconducting and non-superconducting iron-based materials from the published literature. We shall discuss our findings in light of the electrodynamics formulation of the BCS theory of superconductivity. [Preview Abstract] |
Monday, March 13, 2017 9:12AM - 9:24AM |
A39.00007: Complex optical properties of BaFe$_2$As$_2$ C. C. Homes, Y. M. Dai, A. Akrap, S. L. Bud'ko, P. C. Canfield The complex optical properties of BaFe$_2$As$_2$, the parent material for numerous iron-based superconductors, have been examined over a wide frequency range above and below the structural and magnetic transition at $T_N \simeq 140$~K. The minimal description of this multiband material requires the use of the two-Drude model, revealing a strong, broad component, and a weaker, much narrower component. Above $T_N$ both components display a weak temperature dependence; however, at $T_N$ both scattering rates decrease dramatically. Below $T_N$ the broad component shows little temperature dependence, while for the narrow component $1/\tau \simeq 30 \rightarrow 3$~cm$^{-1}$ for $T\simeq T_N \rightarrow 5$~K. The transition results in a reconstruction of the Fermi surface, revealing Dirac-like cones in the electronic dispersion\footnote{Z. P. Yin {\it et al.}, Nat. Phys. {\bf 7}, 294 (2011)}. Dirac and Weyl materials display very small scattering rates at low temperature; we speculate that the collapse of $1/\tau$ in the coherent component is a consequence of these non-parabolic bands. [Preview Abstract] |
Monday, March 13, 2017 9:24AM - 9:36AM |
A39.00008: Magnesium diboride coated bulk niobium: a new approach to higher acceleration gradient Leonardo Civale, Teng Tan, M Wolak, Xiaoxing Xi, Tsuyoshi Tajima Bulk niobium Superconducting Radio-Frequency cavities are a leading accelerator technology. Their performance is limited by the cavity loss and maximum acceleration gradient, which are negatively affected by vortex penetration into the superconductor when the peak magnetic field at the cavity wall surface exceeds the vortex penetration field (H$_{vp}$). It has been proposed that coating the inner wall of an SRF cavity with superconducting thin films increases H$_{vp}$. In this work, we utilized Nb ellipsoids to simulate an inverse SRF cavity and investigate the effect of coating it with magnesium diboride layer on the vortex penetration field. A significant enhancement of H$_{vp}$ was observed. At 2.8 K, H$_{vp}$ increased from 2100 Oe for an uncoated Nb ellipsoid to 2700 Oe for a Nb ellipsoid coated with $\sim$ 200 nm thick MgB$_2$ thin film. This finding creates a new route towards achieving higher acceleration gradient in SRF cavity accelerator beyond the theoretical limit of bulk Nb. [Preview Abstract] |
Monday, March 13, 2017 9:36AM - 9:48AM |
A39.00009: Phonons and the isotope effect in pyrochlore superconductor Cd$_2$Re$_2$O$_7$ Maureen Reedyk, Mojtaba Hajialamdari, Reinhard Kremer, Fereidoon Razavi Cd$_2$Re$_2$O$_7$ is a pyrochlore superconductor with a transition temperature near 2 K. The results of Raman scattering and far-infrared reflectance measurements will be presented. The temperature dependence of phonons has been investigated above and below T$_C$ via IR spectroscopy and as a function of Oxygen ($^{16}$O and $^{18}$O) and Cadmium ($^{112}$Cd and $^{116}$Cd) isotope substitution in the normal state via Raman scattering. The shift in phonon frequency upon isotope substitution will be compared with measurements of the isotope effect on the superconducting transition temperature. The dominant presence of lattice vibrational modes in the optical spectra suggests that electron-phonon interaction plays an important role in the normal and superconducting state properties. [Preview Abstract] |
Monday, March 13, 2017 9:48AM - 10:00AM |
A39.00010: Raman scattering and the role of magnetic frustration in competing iron chalcogenide spin orders Harrison Ruiz, Yao Wang, Brian Moritz, Thomas Devereaux Understanding the competition between different magnetic orders in the iron chalcogenides is crucial to unraveling the nature of leading instabilities in Fe-based superconductors. Previous studies have suggested that the magnetic properties can be well captured by an extended $S=1$ Heisenberg model. Using exact diagonalization, we map a phase diagram reflecting dominant magnetic orders and correlate this information with the temperature dependence of calculated Raman and spin response functions in the vicinity of phase boundaries. This behavior is reminiscent of recent experimental measurents on these materials and provides new perspectives on the role of frustration in iron chalcogenides. [Preview Abstract] |
Monday, March 13, 2017 10:00AM - 10:12AM |
A39.00011: Imaging Gap Nodal Structure of Unconventional Superconductors through the Anisotropic Nonlinear Meissner Effect Seokjin Bae, Yuewen Tan, Alexander Zhuravel, Steven Anlage We present a new measurement method which can be used to image gap nodal structure of superconductors whose pairing symmetry is unknown. This method utilizes photoresponse from a microwave resonance of the superconducting sample perturbed by a scanned laser spot. For an epitaxial thin film or single crystal sample, the anisotropy of this photoresponse is directly related to that of the gap function via the non-linear Meissner coefficient, so the gap nodal directions can be inferred from the photoresponse image. Also, this method is able to simultaneously measure the change in penetration depth, whose low temperature behavior gives an important clue for gap structure. By combining results for gap symmetry from photoresponse, and low temperature behavior of penetration depth, the presented method can make a more conclusive judgement on gap nodal structure and hence pairing symmetry. The data taken from an example unconventional superconductors will be presented and discussed. [Preview Abstract] |
Monday, March 13, 2017 10:12AM - 10:24AM |
A39.00012: Ultrafast dynamics in the presence of antiferromagnetic correlations in electron-doped cuprate La$_{2-x}$Ce$_x$CuO$_{4\pm\delta}$ Inna Vishik, Fahad Mahmood, Zhanybek Alpichshev, J. S. Higgins, R. L. Greene, Nuh Gedik We used femtosecond optical pump-probe spectroscopy to study the photoinduced change in reflectivity of thin films of the electron-doped cuprate La$_{2-x}$Ce$_x$CuO$_4$ (LCCO) with dopings of x$=$0.08 (underdoped) and x$=$0.11 (optimally doped). Above T$_c$, we observe fluence-dependent relaxation rates which onset at a similar temperature that transport measurements first see signatures of antiferromagnetic correlations. Upon suppressing superconductivity with a magnetic field, it is found that the fluence and temperature dependence of relaxation rates is consistent with bimolecular recombination of electrons and holes across a gap (2$\Delta_{AF}$) originating from antiferromagnetic correlations which comprise the pseudogap in electron-doped cuprates. This can be used to learn about coupling between electrons and high-energy ($\omega>2\Delta_{AF}$) excitations in these compounds and set limits on the timescales on which antiferromagnetic correlations are static. [Preview Abstract] |
Monday, March 13, 2017 10:24AM - 10:36AM |
A39.00013: Optical Conductivity From Pair Density Waves Zhehao Dai, Patrick Lee We present a theory of optical conductivity in systems with finite-momentum Cooper pairs. In contrast to the BCS pairing where AC conductivity is purely imaginary in the clean limit, there is nonzero AC absorption across the superconducting gap for finite-momentum pairing if we break the Galilean symmetry explicitly in the electronic Hamiltonian. Vertex correction is crucial for maintaining the gauge invariance in the mean-field formalism and dramatically changes the optical conductivity in the direction of the pairing momentum. We carried out a self-consistent calculation and gave an explicit formula for optical conductivity in a simple case. This result applies to the Fulde-Ferrell-Larkin-Ovchinnikov state and candidates with pair density waves proposed for High-Tc cuprates. It may help detect PDW and determine the pairing gap as well as the direction of the pairing momentum in experiments. [Preview Abstract] |
Monday, March 13, 2017 10:36AM - 10:48AM |
A39.00014: Driven polarons and bipolarons Pavel Kornilovich Small lattice polarons and bipolarons driven by strong external electric fields are considered. The time-dependent Schroedinger equation is integrated directly in time domain. The field agitates the ions both directly and through modulation of electron density. It is found that when the field is in resonance with the ion frequency, the (bi)polaron is liberated from its self-induced trap and the tunneling frequency increases exponentially. This implies a dramatic decrease of the apparent (bi)polaron mass compared to the undriven case. This effect offers an explanation for dynamic stabilization of superconductivity at high temperatures recently observed in layered cuprates. [Preview Abstract] |
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