Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session Y8: Superconductivity: Theory |
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Sponsoring Units: DCMP Chair: Alexei Koshelev, Argonne National Laboratory Room: Colorado Convention Center Korbel 1C |
Friday, March 9, 2007 11:15AM - 11:27AM |
Y8.00001: Impurity scattering interference in high-T$_{c}$ superconductors Chung-Pin Chou, Ting Kuo Lee, Noboru Fukushima Recent STM measurements have observed many inhomogeneous patterns of the local density of state (LDOS) on the surface of high-$T_{c}$ cuprates. In particular, for Bi2212 crystals, well defined interference patterns in the momentum space has been seen at low bias voltage. And recently, for the underdoped sample, it has been observed that the spatially ``checkerboard'' LDOS modulations appear at higher energies. By using a simple impurity scattering potential with BCS Hamiltonian, we describe all LDOS features in Bi2212 materials in terms of quasiparticle scattering interference. We are able to obtain all these features seen by STM experiments in both real- and momentum- space. Our results also show that the observed ``checkerboard'' patterns are dispersionless. Additionally, by using variational Monte Carlo method, we show that the impurity scattering are greatly suppressed due to the presence of strong correlations. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y8.00002: Identifying Collective Modes in $d_{x^2-y^2}$-wave Superconductors via Impurities Roy Nyberg, Dirk Morr, Enrico Rossi We demonstrate that magnetic impurities can be employed to identify the nature of collective modes in the cuprate superconductors. Specifically, we show that a magnetic impurity in an external magnetic field pins an antiferromagnetic collective mode, thus inducing to a local spin-density wave (SDW), i.e., a magnetic droplet. Using a scattering ${\hat T}$-matrix formalism, we find that the presence of such a droplet significantly changes the local electronic structure of the $d_{x^2-y^2}$-wave superconductor. In particular, it suppresses the local density of states (LDOS) inside the droplet on the energy scale of the superconducting gap without inducing an impurity states inside the gap. Moreover, the spin-resolved LDOS exhibits characteristic differences on the two sublattices of the antifferomagnetic droplet. This effect, together with the spatial dependence of the LDOS provides inside into the characteristic momentum of the mode as well as its correlation length. Since these features are absent for other collective modes such as phonons or charge-density waves, our study provides future experiments with the possibility to identify the nature of collective modes. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y8.00003: Identifying Collective Modes in $d_{x^2-y^2}$-wave superconductors via Impurities: II Dirk Morr, Roy Nyberg, Enrico Rossi In the preceeding talk, we demonstrated that magnetic impurities can be employed to identify the nature of collective modes in the cuprate superconductors. In particular we showed that a magnetic impurity in an external magnetic field pins an antiferromagnetic spin mode and induces to a local magnetic droplet. This droplet in turn changes the local electronic structure of the $d_{x^2-y^2}$-wave superconductor. Using a non self-consistent ${\hat T}$-matrix formalism, we identified several characteristic features in the local density of states (LDOS) that arise from the presence of the magnetic droplet. The question naturally arises whether the suppression of the superconducting order parameter (SCOP) in the droplet will alter our conclusions. To investigate this question, we employed a Bogoliubov de Gennes formalism that allows us to self-consistently compute the spatial form of the SCOP. Our results are two-fold. First, we find that the SCOP is significantly changed from its bulk value only in the center of the droplet, and that it recovers the bulk value within a few lattice spacings from the center of the droplet. Second, the suppression of the SCOP only leads to small quantitative changes in the LDOS. Hence our conclusions obtained within the ${\hat T}$-matrix formalism remain unchanged. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y8.00004: Dispersing and non-dispersing peaks and two-energy scales in AC-Arpes in underdoped cuprates Belen Valenzuela, Elena Bascones Recent experiments have shown that the structure in Autocorrelation (AC) Arpes maps compare well with the one observed in Fourier Transform Scanning Tunneling Spectroscopy (FT-STS). In particular dispersing peaks (consistent with the octet model due to scattering induced interference) are observed at low energies in the superconducting state and non-dipersing ones are seen in the pseudogap state and at higher energies in the superconducting state of underdoped cuprates. We have computed the AC-Arpes using the Yang, Rice and Zhang (YRZ) model for the pseudogap. This model assumes that pseudogap and superconductivity compete below a critical doping and has been succesfully used (cond-mat/0611154) to explain the two energy scales found in Raman and ARPES experiments below Tc. We will show that the computed AC-ARPES compares well with the experimental results. The pseudogap is characterized by non-dispersing peaks. In underdoped superconducting cuprates with pseudogap scale larger than the superconducting order parameter, dispersing peaks, associated to the superconducting order parameter appear at low energies and non-dispersing ones, related to the pseudogap, at higher energies. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y8.00005: From the BCS equations to the Anisotropic Superconductivity equations. Jose Samuel Millan, Luis Antonio Perez, Chumin Wang Since the discovery of cuprate superconductors, many new correlated electronic models have been proposed in order to understand their substantially different features, such as high transition temperature (Tc) at an optimal doping, quasi two-dimensional behavior, d-symmetry superconducting order parameter, less influence of the isotope effect, and a power-law behavior of the superconducting specific heat. Recently, we have studied a two-dimensional generalized Hubbard model, in which a second-neighbor correlated hopping is included in addition to the on-site and nearest-neighbor repulsions [1]. This model has the advantage to be able to give some insights on all these new features within the BCS formalism. In this work, we report a unified description of s-, p-, and d-wave superconductivities, in which the experimental power-law behavior of anisotropic superconducting specific heat can be nicely reproduced [2]. [1] J.S. Mill\'{a}n, L.A. P\'{e}rez, and C. Wang, Phys. Lett. A \textbf{335}, 505 (2005). [2] J.S. Mill\'{a}n, L.A. P\'{e}rez, and C. Wang, Proceedings of AIP \textbf{850}, 563 (2006). [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y8.00006: ABSTRACT HAS BEEN MOVED TO C1 |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y8.00007: Tuning effective interactions in high-$T_c$ cuprates via apical oxygen atoms: New realization from the first-principles Wannier function approach Weiguo Yin, Wei Ku Based on a novel first-principles Wannier function approach, the low-energy effective Hamiltonian for high-$T_c$ cuprates is derived. The apical oxygen atoms are found to significantly modify the mobility and distribution of the Zhang-Rice singlets in the CuO$_2$ plane, by tuning the effective hopping parameters $t^\prime$ and $t^{\prime\prime}$, and local chemical potential, $\mu_{\mathrm{eff}}$. Most remarkably, $V_{\mathrm{eff}}$, an additional effective repulsion (de- pairing) between neighboring doped holes, is found to be significantly tuned by a ``vacuum fluctuation'' mechanism inherited from the correlated multiband nature of the cuprates. Our results identify the apical oxygen states as the main material dependence of these systems and provide a microscopic avenue to the understanding of recent spectroscopic imaging STM data [K. McElroy \textsl{et al.}, Science \textbf{309}, 1048 (2005)]. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y8.00008: Realistic Model of Cuprate High-Energy Pseudogaps J.C. Phillips Cuprates become metallic only when doped, much like semiconductor impurity bands. The unique properties of the cuprates are the result of self-organization of the dopants to form off-lattice filamentary networks (``pearls on strings''). The internal structure of these glassy networks is optimized by maximizing their dielectric screening of internal ionic fields. At low energies ARPES peaks in energy and momentum distributions give similar quasiparticle dispersions, but Lanzara has identified a spectral domain between 0.3 eV and 0.8 eV where the two distributions yield orthogonal dispersion relations. I explain this quasiparticle bifurcation with my model, which also explains: why the cuprate phase diagram exhibits an intermediate phase (IP), and only the IP is superconductive; chemical trends in Tcmax (R), where R is the average number of Pauling resonating bonds; the unique architectonic properties of the CuO2 planes; the nature of the glassy Davis 3 nm nanodomains, and the glassy Davis dopant sites; the two Ando lines in the planar resistivity occurring at the pseudogap transition temperature T* and at optimal doping; the Shen Fermi arcs that evolve with doping, whose angular strength ratio has a step-function at optimal doping; a similar step-function jump in the relaxation of spectral holes at 1.5 eV; all the Lanzara angular isotopic trends observed across the phase diagram by ARPES, and the diamagnetic anomalies associated with the pseudogap, with onset temperatures To as large as 2Tcmax. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y8.00009: High-T$_{c}$ superconductivity originates in BaO or similar planes, not in cuprate-planes. John D. Dow CuO$_{2}$ planes are not needed for high-T$_{c}$ superconductivity, as demonstrated by Sr$_{2}$YRuO$_{6}$ and Ba$_{2}$YRuO$_{6}$, weakly doped on Ru sites with Cu but having \textit{no cuprate-planes}. These materials have onsets of superconductivity at 49K and 93K, respectively. We have shown that the related Cu-Ru materials Gd$_{2-z}$Ce$_{z}$Sr$_{2}$Cu$_{2}$RuO$_{10}$ and GdSr$_{2}$Cu$_{2}$RuO$_{8}$ do not superconduct in their cuprate planes, which are magnetic, but in their SrO layers (with onset T$_{c }\approx $ 45K) [1]. The claims that the cuprate-planes superconduct are based on a one-point unconfirmed jump in the Bell Labs data that was supposedly confirmed by Jorgensen [2], although \textit{Jorgensen's data actually} \textit{contradict the Bell datum}. In all the materials we have studied, and even in YBa$_{2}$Cu$_{3}$O$_{7}$, the superconductivity occurs in the bulk in layers that do not contain Cu, namely \textit{in BaO layers of} YBa$_{2}$Cu$_{3}$O$_{7}$, which have $s$-wave character, not $d$-wave character [3]. \newline [1] J. D. Dow \textit{et al.,} J. Vac. Sci. Technol. \textbf{B 24}, 1977 (2006). \newline [2] J. D. Jorgensen, Phys. Today, 34 (\textbf{June}, 1991). \newline [3] D. R. Harshman \textit{et al}., Phys. Rev. \textbf{B 69}, 174505 (2004). [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y8.00010: Ba$_2$YRuO$_6$: High-T$_c$ superconductivity without CuO$_2$ planes. Hermann Azemtsa Donfack Doped Ba$_2$YRuO$_6$ begins superconducting at 93~K, although it has no cuprate-planes. It does have Cu as a dopant, but superconducts with so little Cu (1\%) [S. M. Rao et al., J. Crystal Growth {\bf 235,} 271 (2002)] that it is clearly not a cuprate-plane superconductor. This means that CuO$_2$ planes are not needed for high-T$_c$ superconductivity. It also means that all theories of high-T$_c$ superconductivity based on cuprate-planes superconducting are incorrect, or else that there are at least two theories of high-T$_c$ superconductivity, not just one: one for cuprate-plane materials, and one for ruthenates. (It is our opinion that there is just one theory of high-T$_c$ superconductivity, and that it involves superconductivity in the BaO or similar layers.) In doped Sr$_2$YRuO$_6$, a sister compound of Ba$_2$YRuO$_6$ that begins superconducting at 49~K, the superconductivity is clearly in the (SrO)$_2$ layers, not in the Cu-doped YRuO$_4$ layers, which contain magnetic fields of order 3~kG in zero applied field, and so are unlikely layers to superconduct [J. D. Dow and D. R. Harshman, J. Low Temp. Phys. {\bf 131,} 483 (2003)]. Unlike doped Ba$_2 $YRuO$_6$, doped Ba$_2$GdRuO$_6$ does not superconduct, because Cooper pairs in the BaO layer are disrupted by the magnetic ion Gd. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y8.00011: Nodeless Ground State Symmetry of YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ Dale R. Harshman, Anthony T. Fiory, John D. Dow Muon spin rotation measurements were conducted on a single-crystal of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (T$_{c}$ = 91.3 K) as a function of temperature and magnetic field [1]. By correctly accounting for the temperature-activated fluxon depinning and disorder known to exist in these and other high-quality YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ crystals grown today, the underlying ground state symmetry was found to be nodeless, consistent with s-wave (or extended s-wave) pairing. This result is in agreement with earlier data acquired on samples with strongly pinned vortices. The best fit was obtained assuming the two-fluid model. Analysis of the data assuming a d-wave gap function proved to be extremely unsatisfactory. In fact, the probability that the d-wave model tested gives a better fit than the two-fluid model is less than 4 $\times $ 10$^{-6}$. By ignoring the effects of fluxon depinning, others have erroneous claimed evidence of d-wave pairing. This result effectively negates that possibility. \begin{enumerate} \item D. R. Harshman et el., Phys. Rev. B \textbf{69}, 174505 (2004). \end{enumerate} [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y8.00012: Superconductivity in the ordered limit Vladimir Cvetkovic A novel mechanism for superconductivity is proposed based on the duality in the quantum elasticity. Using a charge crystal as a starting point, these superconductors can be viewed as liquid crystal phases of charge in sense that the broken translational symmetries are restored by the Bose condensation of dislocation defects. Although the crystalline correlations are lost at long distance, the order (and the shear rigidity of the solid) persists at scales large comparable to the lattice spacing. This leads to a host of unconventional properties predicted for this `ordered' superconductor: Meissner effect with oscillating currents, overscreening of Coulomb force, long-range topological order, and the presence of a new excitation in the dynamical electric response. The origin of this excitation lies in the short range shear rigidity, i.e., transient order of a solid. Therefore, an experiment designed to measure the presence of the predicted excitations in the cuprate superconductors could be used to unambiguously (dis)prove the existence of fluctuating stripes. [Preview Abstract] |
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