Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K39: Superconductivity: Neutrons and Spin Resonance |
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Sponsoring Units: DCMP Chair: Takeshi Egami, University of Tennessee Room: 386 |
Wednesday, March 15, 2017 8:00AM - 8:12AM |
K39.00001: Crystal Field Excitations Across High Tc Phase Diagram in $La_{1.6-x}Nd_{0.4}Sr_{x}CuO_{4}$ Qianli Ma, Dalini Maharaj, Connor Buhariwalla, Alexander Kolesnikov, Matthew Stone, Bruce Gaulin The family of high Tc superconductors(SC) $La_{1.6-x}Nd_{0.4}Sr_{x}CuO_{4}$ $($Nd-LSCO$)$ has been studied as it displays a complex picture of the canonical hole-doped high Tc phase diagram. It displays static charge and spin stripe order over a range of Sr doping, which are optimized around x=0.125. Nd-LSCO evolves from an AFM insulating phase at x=0, to a region $($0.05$<$x$<$0.17$)$ where static charge and spin stripe order co-exist with superconductivity at low temperatures, to an optimally-doped SC(x=0.19) with Tc=20K, and to what is believed to be a conventional non-superconducting Fermi liquid for $x>0.25$. Here I present time-of-flight inelastic neutron scattering data of the Nd$^{3+}$ crystalline electric field (CEF) levels on polycrystalline samples of Nd-LSCO over a range of compositions up to x=0.4. The experiments were performed on the SEQUOIA chopper spectrometer at the Spallation Neutron Source, and observe the evolution of the splitting of the J=9/2 multiplet appropriate to Nd$^{3+}$ as a function of Sr concentrations (x=0.04,0.12,0.2,0.24 and 0.4) and (4K, 35K and 200K) temperature. We observe sharp CEF transitions near 21 meV and 27 meV at low x, evolve to a single transition near 24meV near x=0.1, in agreement with the onset of a superconducting ground state. [Preview Abstract] |
Wednesday, March 15, 2017 8:12AM - 8:24AM |
K39.00002: Neutron Scattering Study of Low Energy Magnetic Excitation in superconducting Te-vapor annealed under-doped FeTeSe zhijun xu, J. A. Shneeloch, Guangyong Xu, Ming Yi, Yang Zhao, D. M. Pajerowski, Masaaki Matsuda, Genda Gu, J. M. Tranquada, R.J. Birgeneau, J. W. Lynn To study the interplay between magnetism and superconductivity, we have performed neutron scattering measurements on a group of Te vapor annealed single crystal FeTe1-xSex (Tc\textasciitilde 11-13K) samples. The Te vapor annealed process is found to reduce/remove the excess Fe in the as-grown sample and make the under-doped originally non-superconducting sample become good superconducting sample. Our neutron scattering studies have mapped the magnetic excitation in the (HK0) plane in these compounds. A spin resonance can be observed at incommensurate wave-vectors away from (0.5,0.5). The change of low energy magnetic excitations with temperature suggests a possible phase with SC and NSC coexist on a microscopic level. [Preview Abstract] |
Wednesday, March 15, 2017 8:24AM - 8:36AM |
K39.00003: Helical magnetic ground states in the vicinity of the superconducting state of MnP Sachith Dissanayake, M. Matsuda, F. Ye, S. Chi, J.-G. Cheng, J. Ma, H. D. Zhou, J.-Q. Yan, K. Matsubayashi, T. Okada, J. Gouchi, Y. Uwatoko MnP, the first Mn-based superconductor under pressure, exhibits superconductivity near the critical pressure of 7.5 GPa. It shows a ferromagnetic order followed by a helical order (helical-c) with the spins lying in the ab plane and the helical rotation propagating along c axis at ambient pressure. We performed high pressure single crystal neutron diffraction study up to 7 GPa, which is in the vicinity of the superconducting phase. Our results indicate that the magnetic phase in the vicinity of the superconducting phase has a helical-b structure with the spins lying in the ac plane and the magnetic propagation vector along b axis. Polarized single crystal neutron diffraction measurements at 1.8 GPa confirmed the helicity in the ac plane. With increasing pressure, incommensurability $\delta $ increases and the magnetic moment decreases. Furthermore, small lattice anomalies were observed at different magnetic transitions, indicating that magnetoelastic coupling works to stabilize the magnetic order. [Preview Abstract] |
Wednesday, March 15, 2017 8:36AM - 8:48AM |
K39.00004: Magnetic Excitations in Superconducting La2-xCa1+xCu2O6+d John Schneeloch, Ruidan Zhong, Zhijun Xu, Alexander Kolesnikov, Matthew Stone, Guangyong Xu, Genda Gu, John Tranquada We report inelastic neutron scattering experiments on superconducting and non-superconducting La2-xCa1+xCu2O6+d single crystals, with superconductivity induced by high-pressure oxygen annealing. The magnetic excitations in the non-superconducting composition are similar to those in other weakly hole-doped cuprates, being commensurate and having a similar intensity temperature-dependence. In the superconducting samples, though the excitations appear commensurate, the intensity temperature-dependence is much different and there is broadening and a decrease in the steepness of the dispersion. For the elastic magnetic scattering, changes are also seen, with 3-dimensional antiferromagnetic order in the non-superconducting sample being replaced by 2-dimensional magnetic correlations in the superconducting samples. We will discuss these results and how they fit into the trends seen for other hole-doped cuprates. [Preview Abstract] |
Wednesday, March 15, 2017 8:48AM - 9:00AM |
K39.00005: Neutron Diffraction Studies of the Quasi-1D Superconductor K$_2$Cr$_3$As$_3$ Keith Taddei, Qiang Zheng, Athena Sefat, Clarina dela Cruz The recently discovered $A_2$Cr$_3$As$_3$ (with $A$ = K, Rb or Cs) superconductors (with $T_c \sim$ 7K and hexagonal space group symmetry $P$\={6}$m2$) offer an exciting new system to study unconventional superconductivity (UNSC). Much like their predecessors \textemdash\ the cuprates and the iron-based superconductors (FBS) \textemdash\ these intercalated CrAs compounds couple superconductivity with reduced dimensionality, an expanded phase space accessible through changing elements in a ‘charge reservoir’ portion of the structure and the occurrence of magnetic elements which contribute strongly to the Fermi surface. While the latter two of these features strongly suggest the role of magnetism in UNSC and give a tuning parameter with which to study it, the former not only gives rise to interesting phenomena but also, importantly, allows greater accessibility of the material to theoretical and computational treatments. The $A_2$Cr$_3$As$_3$ system, therefore, both opens new avenues for the study of UNCS and displays early indications of novel and exotic quantum phenomenon related to their low dimensionality. In this talk, we present preliminary results of neutron scattering experiments exploring the magnetism and structure of polycrystalline K$_2$Cr$_3$As$_3$. [Preview Abstract] |
Wednesday, March 15, 2017 9:00AM - 9:12AM |
K39.00006: Investigation of fluctuating intra-unit cell magnetic order in cuprates by $\mu $SR Jeff Sonier, A. Pal, K. Akintola, A. Fang, M. Potma, M. Ishikado, H. Eisaki, W.N. Hardy, D.A. Bonn, R. Liang A mysterious intra-unit cell (IUC) magnetic order has been observed by spin polarized neutron scattering in the pseudogap phase of four different cuprate families. However, the origin of the IUC magnetic order is unclear, in particular because it has not been detected by the local magnetic probe methods nuclear magnetic resonance (NMR) and muon spin relaxation ($\mu $SR). One possible explanation is that the IUC magnetic order fluctuates at a rate slow enough to appear static on the time scale of neutron scattering, but too fast to cause relaxation of the $\mu $SR or NMR signal. I will discuss recent $\mu $SR measurements on Bi$_{\mathrm{2+x}}$Sr$_{\mathrm{2-x}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+\delta }}$ and YBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{y}}$ that explore this possibility. The new results place narrow limits on the fluctuation rate of the unidentified IUC magnetic order, and raise new questions concerning the magnetism detected by $\mu $SR in high-$T_{\mathrm{c}}$ cuprates. [Preview Abstract] |
Wednesday, March 15, 2017 9:12AM - 9:24AM |
K39.00007: Discovery of slowly fluctuating magnetic fields by MuSR in cuprates Lei Shu, J. Zhang, Z.F. Ding, C. Tan, K. Huang, D. E. MacLaughlin, C. M. Varma, A. D. Hillier, P. Biswas, O. O. Bernal, P.-C. Ho, H. Xiang, X. Yao The origin of a mysterious pseudogap region in high-$T_c$ superconductors is a challenging issue. The proposed time-reversal breaking order through loop-currents is consistent with five different classes of symmetry-sensitive experiments: polarized neutron scattering, Kerr effect, bireferingence, dichroic ARPES, and second harmonic generation. However, local probes such as $\mu$SR and NMR do not see the magnetic fields expected for such an order. Such local probes have much longer time scales than the others. It has been suggested that the local magnetic fields may be motionally narrowed by fluctuations among the different possible directions of the loop-current order. We have performed zero field and longitudinal field $\mu$SR measurements on YBa$_2$Cu$_3$O$_y$ ($T_c$ = 73K, 80K, 88K, and 91K) single crystals. A magnetic field with rms width of about 20 Gauss fluctuating at about 10$^8$ Hz has been discovered, consistently at similar temperatures ($T^{\ast}$) as the onset of order in the other experiments. Critical slowing down of magnetic fluctuation near $T^{\ast}$ has also been found. [Preview Abstract] |
Wednesday, March 15, 2017 9:24AM - 9:36AM |
K39.00008: Influence of apical oxygen on the extent of in-plane exchange interaction in cuprate superconductors yingying peng, Giacomo Ghiringhelli In high T$_{\mathrm{c}}$ superconductors the magnetic and electronic properties are determined by the probability that valence electrons virtually jump from site to site in the CuO$_{\mathrm{2}}$ planes, a mechanism opposed by on-site Coulomb repulsion and favored by hopping integrals. The spatial extent of the latter is related to transport properties, including superconductivity, and to the dispersion relation of spin excitations (magnons). Here, measuring by resonant inelastic x-ray scattering over a significant portion of the reciprocal space and with unprecedented accuracy, we compare the magnetic spectra for three antiferromagnetic parent compounds (single-layer Bi$_{\mathrm{2}}$Sr$_{\mathrm{0.99}}$La$_{\mathrm{1.1}}$CuO$_{\mathrm{6+\delta }}$, double-layer Nd$_{\mathrm{1.2}}$Ba$_{\mathrm{1.8}}$Cu$_{\mathrm{3}}$O$_{\mathrm{6}}$ and infinite-layer CaCuO$_{\mathrm{2}})$ differing by the number of apical atoms. We observe that the absence of apical oxygens increases the in-plane hopping range and, in CaCuO$_{\mathrm{2}}$, it leads to a genuine 3D exchange-bond network. These results establish a corresponding relation between the exchange interactions and the crystal structure, and provide fresh insight into the materials dependence of the superconducting transition temperature. [Preview Abstract] |
Wednesday, March 15, 2017 9:36AM - 9:48AM |
K39.00009: $^{19}$F-NMR in LaO$_{0.5}$F$_{0.5}$BiS$_2$: Anomalous Lineshape Behaviour Below 10 Kelvin Shrishti Yadav, Oscar Bernal, Duygu Yazici, Kevin Huang, Lei Shu, M.B. Maple A recently discovered layered superconductor, LaO$_{0.5}$F$_{0.5}$BiS$_2$ is of current interest for its surprising properties. $T_c$ is in the vecinity of 3~K for room-pressure-grown samples, but it reaches 10~K in samples grown under pressure. NMR is a microscopic tool that can probe the static and fluctuating local fields at nuclear sites. In our studies of $^{19}$F-NMR in a room-pressure-grown sample, we have found anomalous features in the spectral lineshape and curious behaviour of the spin-lattice relaxation rate as functions of temperature and magnetic field. We report our measurements and discuss some of our results in light of the potential properties predicted by others for the parent as well as the F-doped compounds. [Preview Abstract] |
Wednesday, March 15, 2017 9:48AM - 10:00AM |
K39.00010: Fluctuating spin stripes in the normal state of high-Tc cuprate superconductors Edwin Huang, Christian Mendl, Shenxiu Liu, Steve Johnston, Hong-Chen Jiang, Brian Moritz, Thomas Devereaux Recent experiments have established charge stripes as universal in underdoped cuprate superconductors. In contrast, spin stripes, which are intimately tied to charge stripes in many doped Mott insulators, are seemingly absent in the majority of cuprates, at least in the static long-ranged form. Whether spin stripes exist in a subtle fluctuating form in these cuprates is controversial, with proponents suggesting a link between fluctuating stripes and the mechanism of high-$T_c$ superconductivity. Here, we use numerically exact determinant quantum Monte Carlo calculations to demonstrate dynamical spin stripe correlations in the three-band Hubbard model, which represents the local electronic structure of a Cu-O plane in a cuprate superconductor. Unlike existing ground state studies, our simulations are conducted at temperatures approaching the pseudogap regime, indicated by a decrease in the Knight shift upon cooling, and hence reflect properties of the normal, non-superconducting state. Calculations of the dynamic spin susceptibility strongly support the interpretation of a variety of experimental observations in terms of the physics of fluctuating stripes, including the ubiquitous hourglass magnetic dispersion and the Yamada plot of incommensurability vs. doping. [Preview Abstract] |
Wednesday, March 15, 2017 10:00AM - 10:12AM |
K39.00011: Spin Waves in the Normal State of the Two-Dimensional Hubbard Model James LeBlanc, Xi Chen, Ryan Levy, Andrey Antipov, Andrew Millis, Emanuel Gull Dynamical mean field theory and dual fermi methods are used to compute the dynamical spin response of the 2D Hubbard model for a variety of interaction strengths both at and away from half-filling. We compute generalized two-particle correlated vertex functions within dynamical mean-field theory and then extend beyond the dynamical mean field approximation using the technique of dual-fermions. A direct comparison to spin-wave theory and to inelastic neutron scattering data for the insulating parent cuprate La$_2$CuO$_4$ reveals excellent agreement and demonstrates the power of the method. The evolution of the spin response with electron and hole doping is presented and discussed in terms of the physics of the cuprates. [Preview Abstract] |
Wednesday, March 15, 2017 10:12AM - 10:24AM |
K39.00012: Knight shifts, nuclear spin-relaxation rates, and spin echo decay times in the pseudogap regime of the cuprates: Simulation and relation to experiment Xi Chen, James LeBlanc, Emanuel Gull We study the temperature and doping evolution of the NMR Knight shift, spin relaxation rate, and spin echo decay time in the pseudogap regime of the two-dimensional Hubbard model for parameters believed to be relevant to cuprate superconductors using cluster dynamical mean field theory. We recover the suppression of the Knight shift seen in experiment upon entering the pseudogap regime and find agreement between single and two-particle measures of the pseudogap onset temperature $T^*$. The simulated spin-echo decay time shows a linear in $T$ behavior at high $T$ which flattens off as $T$ is lowered, and increases as doping is increased. The relaxation rate shows a marked increase as $T$ is lowered but no indication of a pseudogap on the Cu site, and a clear downturn on the O site, consistent with experimental results on single layer materials but different from double layer materials. The consistency of the simulated susceptibilities with experiment, along with similar agreement on the single-particle level and the absence of long-range order and symmetry breaking suggests that the pseudogap is well described by strong short-range correlation effects and that long-range order and multi-orbital effects are not required. [Preview Abstract] |
Wednesday, March 15, 2017 10:24AM - 10:36AM |
K39.00013: Dynamical Nuclear Magnetic Resonance Imaging of Micron-scale Liquids Aimee Sixta, Alexandra Choate, Jake Maeker, Sophia Bogat, Daniel Tennant, Shirin Mozaffari, John Markert We report our efforts in the development of Nuclear Magnetic Resonance Force Microscopy (NMRFM) for dynamical imaging of liquid media at the micron scale. Our probe contains microfluidic samples sealed in thin-walled (µm) quartz tubes, with a micro-oscillator sensor nearby in vacuum to maintain its high mechanical resonance quality factor. Using ~10 µm spherical permalloy magnets at the oscillator tips, a 3D $T_1$-resolved image of spin density can be obtained by reconstruction from our magnetostatics-modelled resonance slices; as part of this effort, we are exploring single-shot $T_1$ measurements for faster dynamical imaging. We aim to further enhance imaging by using a 2$\omega$ technique to eliminate artifact signals during the cyclic inversion of nuclear spins. The ultimate intent of these efforts is to perform magnetic resonance imaging of individual biological cells. [Preview Abstract] |
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