Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W1: Neutron Scattering in High Tc Cuprates |
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Sponsoring Units: DCMP Chair: Eugene Demler, Harvard University Room: LACC 152 |
Thursday, March 24, 2005 2:30PM - 3:06PM |
W1.00001: The structure of the high-energy spin excitations in YBa$_2$Cu$_3$O$_{6+x}$ Invited Speaker: The most obvious feature in the magnetic excitations of high-$T_{c}$ superconductors is the so-called `resonance-mode'. This mode is strongly coupled to the superconductivity, however, it has not been found in the La$_{2-x}$(Ba,Sr)$_{x}$CuO$_{4}$ family and is not universally present in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$. Here we use inelastic neutron scattering to characterize other excitations at higher energies which may be relevant to the superconductive pairing in YBa$_{2}$Cu$_{3}$O$_{6.6}$. We observe a square-shaped continuum of excitations in reciprocal space [1]. These excitations have energies greater than the superconducting pairing energy, are present at $T_{c}$, and have spectral weight far exceeding that of the `resonance'. The discovery of similar excitations in La$_{2-x}$Ba$_{x}$CuO$_{4}$\ [2] suggests that they are a general property of the copper oxides, and a candidate for mediating the electron pairing. Our data show that the high-energy magnetic excitations in the high-temperature superconductor YBa$_{2}$Cu$_{3}$O$_{6.6}$ consists of a continuum of scattering bounded by a square and peaked at wavevector positions \textbf{Q}$_{\varepsilon }$ =(1/2$\pm \varepsilon $,1/2$\pm \varepsilon )$ and (1/2$\pm \varepsilon $,1/2$\mp \varepsilon )$. A similar structure is observed in the high-energy magnetic excitations of the magnetically ordered but weakly superconducting compound La$_{1.85}$Ba$_{0.125}$CuO$_{4}$\ [2]. This suggests there is universality, both in the low-energy and the high-energy spin dynamics between two very different classes of high-$T_{c}$ superconductor. \\ \ [1] S.M. Hayden, H.A. Mook, P.C. Dai, T.G. Perring, and F. Dogan, Nature \textbf{429}, 531-534 (2004) \\ \ [2] J.M. Tranquada, H. Woo, T.G. Perring, H. Goka, G.D. Gu , G. Xu, M. Fujita, K.Yamada K, Nature \textbf{429,} 534-538 (2004). [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:42PM |
W1.00002: Quantum magnetic excitations from stripes in copper-oxide superconductors Invited Speaker: Recent inelastic neutron scattering studies show that the magnetic excitation spectra of two well-studied families of cuprate superconductors are much more similar than previously believed. In particular, I will present results we have obtained on La$_{2-x}$Ba$_{x}$CuO$_{4}$ (LBCO) with $x $= 0.125 [1,2]. Using very large single crystals grown at Brookhaven, we were able to measure the magnetic excitations up to 200 meV using the MAPS time-of-flight spectrometer at the ISIS spallation source. While the lowest energy excitations are split incommensurately, these disperse inwards towards the antiferromagnetic wave vector with increasing energy, merging at $\sim $50 meV. At higher energies the excitations disperse outwards again. There is a significant enhancement of the \textbf{Q}-integrated magnetic scattering near $\sim $50 meV compared to lower energies, suggestive of quantum correlations and distinct from spin-wave predictions. Many features of the spectrum are quite similar to those found in YBa$_{2}$Cu$_{3}$O$_{6.6}$ [3]. One can qualitatively characterize the results with a universal excitation spectrum, together with a material-dependent spin gap in the superconducting state. It is important to note that the LBCO sample exhibits static stripe order [2], as this has significant implications for the origin of the magnetic excitations in superconducting cuprates. \begin{enumerate} \item J. M. Tranquada, H. Woo, T. G. Perring, H. Goka, G. D. Gu, G. Xu, M. Fujita, and K. Yamada, Nature \textbf{429}, 534 (2004). \item M. Fujita, H. Goka, K. Yamada, J. M. Tranquada, and L.-P. Regnault, Phys. Rev. B \textbf{70}, 104517 (2004). \item S. M. Hayden, H. A. Mook, P. C. Dai, T. G. Perring, and F. Dogan, Nature \textbf{429}, 531 (2004). \end{enumerate} [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 4:18PM |
W1.00003: Fermi Surface Approach to Inelastic Neutron Scattering Data in Cuprates Invited Speaker: A large body of calculations has been done to address inelastic neutron scattering data on cuprate superconductors based on linear response theory with a d-wave energy gap. I will review these calculations, emphasizing unique aspects that they can explain in regards to the data. This includes the magnetic resonance, with its unusual ``reversed magnon'' dispersion [1], and a novel new resonance mode which occurs at higher energies and at smaller momenta, which can account for the unusual ``45 degree rotation'' of the 2D momentum pattern seen in recent neutron data [2]. Finally, I will discuss limitations of these calculations in regards to certain experimental findings.\hfil\break [1] M. R. Norman, Phys. Rev. B 61, 14751 (2000) and 63, 092509 (2001).\hfil\break [2] I. Eremin, D. K. Morr, A. V. Chubukov, K. Bennemann, M. R. Norman, cond-mat/0409599 [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:54PM |
W1.00004: Near the superconducting edge: the central mode and spin confinement in cuprates Invited Speaker: Marginally above the critical doping for superconductivity, we find a dramatic softening of the spin fluctuations and a central mode spectrum similar to that above certain structural and magnetic phase transitions. We demonstrate in YBCO$_{6+x }$for x= 6.354, where T$_{c}$=18K is five times less than optimal, that the spins fluctuate an order of magnitude more slowly than a typical resonance energy$^{1, 2}$, with a relaxation rate of only $\sim $3 meV. The central mode width is less than 0.1 meV but is finite and decreases below T$_{c}$. Its strength grows on cooling but there is no antiferromagnetic (AF) transition since the correlations indicate that spin clusters are confined to 8 cells in the plane and one cell between planes. Surprisingly, polarized neutrons show that the spins fluctuate equally in all directions with triplet symmetry, with none of the anisotropy of the ordered AF Mott insulator, x=6.15. Although the doping of 6{\%} is close to critical for superconductivity, the system properties indicate it is relatively far from the AF critical point. Topological structures such as the Nagaoka polaron that destroy AF correlations over many sites per doped hole may be needed to account for a spin response that is both short range and unpolarized. The spin resonance behaves with reduced doping as a damped soft mode that drives a central mode and destroys the superconducting phase. 1. C. Stock, W.J.L. Buyers, et al., Phys. Rev. B 69, 014502 (2004) 2. C. Stock, W.J.L. Buyers, et al., condmat/0408071, Phys. Rev. B accepted (2005). [Preview Abstract] |
Thursday, March 24, 2005 4:54PM - 5:30PM |
W1.00005: The bosonic peak in Bi-2212: Is it caused by phonons or the 41 meV magnetic resonance? Invited Speaker: Tom Timusk The infrared spectra of the high temperature superconductors are dominated by two features, a broad continuous background absorption extending to $\approx$ 1 eV and a sharp onset at $\hbar\omega \approx 8 k_BT_c$. Dispersion curves of the free carriers in the same materials measured by angle resolved photo emission (ARPES) display strong curvature (termed the ``kink'') in the same spectral region suggesting that the IR absorption features are spectroscopic signatures of the bosonic excitations that scatter the carriers. The feature at $8 k_BT$ has been attributed to the neutron scattering resonance, seen at $(\pi,\pi)$ momentum transfer. We test this notion by tracking the IR mode as a function of doping level in Bi$_2$Sr$_2$CaCu$_2$O$_8$ and compare our data with ARPES dispersion. We also show new data on the temperature dependence of the mode strength in YBa$_2$Cu$ _3$O$_x$ for the $x=6.5$, the Ortho II, material where neutron data of the resonance is available for samples from the same source. The mode, as seen in the IR, weakens in a linear fashion as the temperature increases to completely vanish at $\approx 200$ K in parallel with the neutron scattering mode. A phonon model for the bosonic mode would predict a temperature independent strength. Another interesting feature of the mode its frequency which scales with the superconducting transition temperature over a wide range of materials and doping levels in the optimally and over doped regions with an intercept at zero frequency. Model calculations for a $d$-wave superconductor are in accord with this behavior provided both the mode frequency and the gap scale with $T_c$, ruling out the phonon origin for the mode which is expected to have a doping and material independent frequency. [Preview Abstract] |
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