Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Z2: Pseudogap in High Tc Cuprates |
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Sponsoring Units: DCMP Chair: Martin Greven, University of Minnesota Room: Ballroom A2 |
Friday, March 25, 2011 11:15AM - 11:51AM |
Z2.00001: Loop-Current Order in Several Families of Cuprates Invited Speaker: In high temperature copper oxides superconductors, a novel long range 3D magnetic order associated with the pseudogap phase has been identified in two different cuprate families - ${\rm YBa_{2}Cu_3CuO_{6+x}}$ (YBCO),\footnote{B. Fauqu\'e, Y. Sidis, V. Hinkov, S. Pailh\`es, C.T. Lin, X. Chaud, and P. Bourges, {\it Phys. Rev. Lett.} {\bf 96}, 197001 (2006).} ${\rm HgBa_2CuO_4}$ (Hg1201)\footnote{Y. Li, V. Bal\'edent, N. Barisi\'c, Y. Cho, B. Fauqu\'e, Y., Sidis, G. Yu,, X. Zhao, P. Bourges, and M. Greven, {\it Nature} {\bf 455}, 372 (2008).} - over a wide region of temperature and doping. That magnetic order, evidended using polarized neutron diffraction, respects the translation symmetry of the lattice and can be described as a Q=0 antiferromagnetism with active role of in-plane oxygens atoms. Such a magnetic order can be associated with orbital moments in the circulating currents phase proposed by C. Varma. Similar magnetic ordering is observed in the archetypal cuprate ${\rm La_{2-x}Sr_xCuO_4}$ (LSCO) system below 120 K for x=0.085.\footnote{V. Bal\'edent, B. Fauqu\'e, Y. Sidis, N. B. Christensen, S. Pailh\`es, K. Conder, E. Pomjakushina, J. Mesot, and P. Bourges {\it Phys. Rev. Lett.} {\bf 105}, 027004 (2010).} In contrast to the previous reports, the magnetic ordering in LSCO is {\it\bf only} short range with an in-plane correlation length of $\sim$ 10 \AA\ and is bidimensional (2D). Such a less pronounced order suggests an interaction with other electronic instabilities. In particular, LSCO also exhibits a strong tendency towards stripes ordering at the expense of the superconducting state. Additional polarized neutron diffraction measurements have been performed in YBCO.\footnote{V. Bal\'edent, D. Haug, Y. Sidis, V. Hinkov, C.T. Lin and P. Bourges, preprint} At lower doping (8.5 \%), the magnetic order is observed at lower temperature ($\sim$ 150 K) than the generally assumed value for the pseudogap. It tends to vanish for dopings where the nematic electronic liquid crystal phase sets up. Recently, two others cuprates families have been studied: ${\rm Bi_2Ca_2SrCu_2O_{8+\delta}}$ (Bi2212) and electron doped ${\rm Nd_{2-x}Ce_xCuO_4}$ (NCCO). In both families, a magnetic order related to the pseudogap phase has been also observed. The recent results will be discussed during the talk. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:27PM |
Z2.00002: Novel magnetic excitations in a model cuprate high-$T_{c}$ superconductor Invited Speaker: Magnetic fluctuations might be essential to the mechanism of high-temperature superconductivity in the cuprates. For a long time, such fluctuations have been theoretically regarded as arising from the antiferromagnetic correlations within the copper-oxygen layers, and experimental studies of magnetic excitation spectrum have mainly been carried out near the corresponding wave vector (1/2,~1/2). Following neutron diffraction experiments which demonstrated the universal existence of a ``$q$~=~0 antiferromagnetic order'' in the pseudogap phase of three different cuprates [1-3], our recent inelastic neutron scattering experiments on the model compound HgBa$_{2}$CuO$_{4+\delta }$ (Hg1201) revealed the existence of unusual magnetic excitations that weakly disperse throughout the entire Brillouin zone [4,5]. Like the $q$~=~0 antiferromagnetic order, the new excitations are observed in the pseudogap phase and therefore appear to be associated with the order. The excitations possess very large spectral weights at well-defined characteristic energies that are comparable to the resonance energy [6] and to those of electron-boson-coupling features observed in a wide range of cuprates, highlighting their possible influence on the electronic structure. These findings demonstrate that the pseudogap state is a distinct phase of matter rather than a mere crossover. They furthermore cast doubt on the presumed predominant importance of the wave vector (1/2,~1/2) in the magnetic excitation spectrum, and have the profound implication that a single-band description of the cuprates is insufficient. \\[4pt] [1] B. Fauque \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{96}, 197001 (2006). \\[0pt] [2] Yuan Li \textit{et al.}, \textit{Nature} \textbf{455}, 372 (2008). \\[0pt] [3] V. Baledent \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{105}, 027004 (2010). \\[0pt] [4] Yuan Li \textit{et al.}, \textit{Nature} \textbf{468}, 283 (2010). \\[0pt] [5] Yuan Li \textit{et al.}, unpublished. \\[0pt] [6] G. Yu \textit{et al.}, \textit{Phys. Rev. B} \textbf{81}, 064518 (2010). [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 1:03PM |
Z2.00003: Collective Modes in Cuprates and their coupling to Fermions Invited Speaker: The quantum-critical fluctuations of the loop current order observed universally in underdoped Cuprates have been derived and shown to be local in space and power law in time. The coupling of these fluctuations to fermions are shown to promote d-wave pairing as well as to give the Marginal Fermi liquid single particle spectra in the normal state [1]. Three collective fluctuations modes in the loop order modes are derived [2]. They are massive weakly dispersive magnetic modes. Two of these branches have been discovered. Experiments are suggested to discover the third branch. \\[4pt] [1] V. Aji, A. Shekhter and C.M. Varma, Phys. Rev. B. 81, 06451 (2010).\\[0pt] [2] Yan He and C.M. Varma, arXiv:1008.3182. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:39PM |
Z2.00004: Disentangling Cooper-pair formation above {$T_{c}$} from the pseudogap state in the cuprates Invited Speaker: The discovery of the pseudogap in the cuprates created significant excitement amongst physicists as it was believed to be a signature of pairing, in some cases well above room temperature. This was supported by a number of experiments detecting phase-fluctuating superconductivity above {$T_{c}$}. However, several recent experiments reported that the pseudogap and superconducting state are characterized by different energy scales, and likely compete with each other, leaving open the question of whether the pseudogap is caused by pair formation. To address this issue, we investigate the spectral weights, which are easier to quantify and in many cases interpret than the spectral feature, which is traditionally used. A key such measure is the density of states at the Fermi energy $D(E_{F})$. In conventional, clean superconductors this weight is zero below $T_{c}$, but can be finite if there are strong impurity scattering effects. In such cases $D(E_{F})$ reflects the pair breaking states. A separate scenario is a generic ``density wave state" in the absence of pairing, which leads to a decrease of the $D(E_{F})$ due to the opening of the density wave gap. In addition there is also the possibility of the coexistence of superconductivity and the density wave state - inhomogeneous superconductors such as the cuprates, where superconducting and non-superconducting patches coexist in the sample. One can then expect that the temperature dependence of $D(E_{F})$ can be used to distinguish between these scenarios and disentangle the electronic ground states of the cuprates. Since the spectral gap in the cuprates displays significant momentum dependence, in our study we use the intensity of the spectral function at $E_{F}$, $I(E_{F}, k)$, which when integrated over all momenta equals $D(E_{F})$. This allows us to isolate the behavior at a specific $k$-point and avoid smearing due to averaging. In this talk, we report the discovery of a spectroscopic signature of pair formation and demonstrate that in a region commonly referred to as the ``pseudogap", two distinct states coexist: one that persists to an intermediate temperature {$T_{pair}$} and a second that extends up to { $T^{*}$}. The first state is characterized by doping independent scaling behavior and is due to pairing above {$T_{c}$}, but significantly below {$T^{*}$}. The second state is the ``proper" pseudogap - characterized by the loss of low energy spectral weight, anomalies in transport properties and the absence of pair formation. {$T_{pair}$} has a universal value around 120-150K even for materials with very different {$T_{c}$} and it likely sets limit on the highest attainable {$T_{c}$} in the cuprates. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 2:15PM |
Z2.00005: Phenomenology of electronic nematic and smectic states in STM studies of high T$_c$ cuprates Invited Speaker: Electronic liquid crystals are phases in which electronic structure of a material breaks the spatial symmetries of its crystal lattice: electronic nematic only breaks the point group symmetry, while smectic (stripe) additionally breaks the translational symmetry. Here I define two independent order parameter fields for nematic and smectic that can be constructed from STM data. Using these order parameters we find long range intra-unit cell nematicity in the pseudogap states [1]. In contrast, we observe many topological defects that disorder the smectic fields. However, these defects reveal a remarkable coupling between smectic tendency and fluctuations in the nematic order. From these observations, we propose a Ginzburg-Landau free energy describing the quantum nematic/smectic coupling and demonstrate how it can explain the coexistence of these states and correctly predict their interplay [2]. In principle, this understanding may enable us to disentangle the complexities of the system specific cuprate phase diagrams.\\[4pt] [1] M. J. Lawler, K. Fujita, Jhinhwan Lee, A. R. Schmidt, Y. Kohsaka, Chung Koo Kim, H. Eisaki, S. Uchida, J. C. Davis, J. P. Sethna, Eun-Ah Kim, ``Intra-unit-cell electronic nematicity of the high Tc copper-oxide pseudogap states'', Nature {\bf 466}, 347 (2010).\\[0pt] [2] A. Mesaros, K. Fujita H. Eisaki, S. Uchida, J.C. Davis, S. Sachdev, J. Zaanen, M.J. Lawler, and Eun-Ah Kim, ``How topological defects couple the smectic and nematic electronic structure of the cuprate pseudogap states'', submitted (2010). [Preview Abstract] |
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