Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session P39: Focus Session: Superconductivity-Theory and Computation (Mainly First Principles) |
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Sponsoring Units: DCOMP DCMP Chair: Ole Andersen, Max Planck Institute Room: Baltimore Convention Center 342 |
Wednesday, March 15, 2006 11:15AM - 11:51AM |
P39.00001: Effect of Spin Fluctuations on Electron-Phonon Superconductivity Invited Speaker: Most of our intuition regarding conventional superconductivity is derived from the McMillan formula. What are often believed to be generic properties of the Eliashberg equations, in reality hold only in the regime where the McMillan formula is applicable. I will show how common beliefs, such as a monotonic relation between the reduced gap and the coupling constant, or an exponential behavior of the NMR relaxation with the gap as the activation parameter, fail for more complicated spectral functions. Most interestingly, I will demonstrate that our conventional wisdom totally fails us in the rapidly developing field of superconductors near a magnetic instability. In particular, I will derive (and test against numerical solutions of the Eliashberg equations) an analogue of the McMillan formula, fully accounting for the pair-breaking effect of spin fluctuations, and will show that these {\it increase} the phonon isotope effect, sometimes by as much as a factor of two. This is counterintuitive and opposite to the effect of the high-energy Coulomb interactions (the so-called Coulomb pseudopotential). I will also discuss the possibility of observing this effect in specific materials, such as MgCNi$_3$. This work has been done in collaboration with Oleg Dolgov (MPI Stuttgart) and Alexander Golubov (U. Twente). [Preview Abstract] |
Wednesday, March 15, 2006 11:51AM - 12:03PM |
P39.00002: Properties from spin-phonon coupling in high-T$_C$ superconductors: HgBa$_2$CuO$_4$ and La$_{(2-x)}$Sr$_x$CuO$_4$ Thomas Jarlborg The mechanism of spin-phonon coupling (SPC) in high-T$_C$ copper oxides is explored from band calculations on LSCO and HBCO systems. The LMTO band calculations, based on the local density approximation, are made for cells containing frozen phonon displacements and/or spin waves within the CuO plane. The virtual crystal approximation is used for studies of hole doped systems. The main result is that phonons are favorable for spin waves and vice-versa, and that pseudogaps appear naturally in the band structures of striped materials with strong SPC. The wave length of the spin-phonon modulation is related to doping, and the mutual enhancement of SPC is strongest when the non-doped system is close to an anti-ferro magnetic ground state. The calculated band results are used for modelling of different properties, such as isotope effects, phonon softening, shear dependences and T-variations. The results are discussed and compared with experiment. It is speculated that perpendicular SPC, with different behavior along x- and y-directions, can produce double gap structures. A moderate correction to LDA, which stabilizes the AFM state for the undoped material, will enhance the coupling constant for spin fluctuations $\lambda_{sf}$ for doped cases. These results suggest that properties of high-T$_C$ superconductors should depend both on phonons and magnetic fluctuations. [Preview Abstract] |
Wednesday, March 15, 2006 12:03PM - 12:15PM |
P39.00003: Superconductivity of Li, Al and K under pressure Gianni Profeta C Franchini$^{\S}$, N. N. Lathiotakis$^{\dag}$, A. Floris$^{\dag\S}$, A. Sanna$^{\S}$, M. A. L. Marques$^{\dag}$, M.~L{\"u}ders$^{\ddag}$, S. Massidda$^{\S}$, E. K. U. Gross$^{\dag}$, A. Continenza$^{*}$.\\ $^{*}$ CASTI - INFM and Dip. Fis., Univ. di L'Aquila, I-67010 Coppito (L'Aquila) Italy; $^{\S}$ SLACS INFM and Dip. Fis., Univ. di Cagliari, I-09042 Monserrato (Ca), Italy; $^{\dag}$ Institut f{\"u}r Theoretische Physik, Freie Universit{\"a}t Berlin, Arnimallee 14, D-14195 Berlin, Germany; $^{\ddag}$ Daresbury Lab., Warrington WA4 4AD, United Kingdom.\\ Extreme pressure strongly affects the superconducting properties of ``simple'' metals, like Li, K and Al. Using the new ab-initio method of density functional theory of the superconducting state, we report investigations on the superconducting properties of dense Li, K and Al. Our results show an unprecedented agreement with experiments, assess the predictive power of the method over a wide range of densities and electron-phonon couplings, and provide predictions for K, where no experiments exist so far. For fcc K we predict a superconducting phase transition at 18 GPa, with a maximum critical temperature of about 2 K at 23 GPa, the pressure where the crossover between the fcc and the K $III$ structure experimentally occurs. We studied the effect of pressure on the electronic and vibrational properties of alkali, showing a progressive phonon softening near the K point of the Brillouin zone and a concomitant enhancement of the electron-phonon coupling constant $\lambda$. [Preview Abstract] |
Wednesday, March 15, 2006 12:15PM - 12:27PM |
P39.00004: Superconductivity of Alkali Metals under High Pressure Lei Shi, Dimitrios Papaconstantopoulos We calculated the superconductivity properties of alkali metals under high pressure using the results of band theory and the rigid-muffin theory of Gaspari and Gyorffy. Our results suggest that at high pressures Lithium, Potassium, Rubidium and Cesium would be superconductors with transition temperatures approaching 10-20 K. Our calculations also show that Sodium would not be a superconductor under high pressure even if compressed to less than half of its equilibrium volume. We found that the compression of the lattice strengthens the electron-phonon coupling through a delicately balanced increase of both the electronic and phononic components of this coupling. This increase of the electron-phonon coupling in Li is due to an enhancement of the s-p channel of the interaction, while in the heavier elements the p-d channel is the dominant component. [Preview Abstract] |
Wednesday, March 15, 2006 12:27PM - 12:39PM |
P39.00005: Superconductivity and Lattice Instability in Compressed Lithium from Fermi Surface Hot Spots Deepa Kasinathan, Jan Kunes, Richard Scalletar, Warren Pickett, Amy Lazicki, Choong-Shik Yoo, Helge Rosner Lithium, a simple metal not superconducting above 5mK at ambient pressure, becomes a 20 K superconductor at 50 GPa. This high T$_c$ is shown to arise from critical (formally divergent) electron-phonon coupling to the transverse phonon branch along intersections of Kohn anomaly surfaces with the Fermi surface. First principles linear response calculations of the phonon spectrum and spectral function $\alpha^2 F(\omega)$ reveal (harmonic) instability already at 25 GPa. Our results imply that the fcc phase is anharmonically stabilized in the 25-38 GPa range. [Preview Abstract] |
Wednesday, March 15, 2006 12:39PM - 12:51PM |
P39.00006: DFT Study of the Single-Band Layered TMO LiNbO$_2$ Erik Ylvisaker, Warren Pickett We establish using first principles methods that LiNbO$_2$ is a realization of a triangular lattice ``single band'' system. The bandwidth (less than 2 eV) suggests the interesting possibility of correlation effects that should be kept in mind. We present a tight-binding model for the valence band of LiNbO$_2$, composed primarily of Nb d$_{z^2}$ states, finding that intralayer second neighbor hopping $t_2 ~ 100$ meV is dominant over the significantly smaller first neighbor interactions $t_1 ~ 70$ meV. The nearest neighbor coupling is strongly modified by oxygen displacements, and the electron-phonon coupling may provide the coupling mechanism for superconductivity in Li-deficient samples ($T_c \approx 5$K). We will present the Nb-centered Wannier function, which provides insight into this unusual electronic structure. Calculations of the Born effective charges for the metal ions are also found to have anisotropy that reflects the layered nature of the electronic bonding. Their deviation from formal charge values indicates important covalent character, which is also evident in the Wannier function. [Preview Abstract] |
Wednesday, March 15, 2006 12:51PM - 1:03PM |
P39.00007: Electron-phonon Interaction in Graphite-Intercalation Compounds Lilia Boeri, Matteo Giantomassi, Giovanni B. Bachelet, Ole Krogh Andersen After the discovery of superconductivity with a Tc of 11.5 K in Ca-intercalated graphite (CaC6 )[1], the interest in graphite-intercalation compounds has been revived. Different pairing mechanisms, based on excitons or electron-phonon interactions, have been put forward [2]. In this contribution we first analyze, using the NMTO[3] method, the electronic structure of CaC6. We then propose a simple model, based on pure graphite, to explain superconductivity in this class of compounds. Implications on the design of new materials with similar superconducting properties are also discussed. \newline [1] T.E. Weller {\em et al.}, Nature Physics {\bf 1}, 39 (2005). [2] G. Csany {\em et al.}, Nature Physics {\bf 1}, 42 (2005); I. I. Mazin, cond-mat/0504127; M. Calandra and F. Mauri, . [3] O.K. Andersen and T. Saha-Dasgupta, Phys. Rev. B 62, R16219 and O.K. Andersen, T. Saha-Dasgupta and S. Ezhov, Bull. Mat. Sci. 26, 19 (2003). [Preview Abstract] |
Wednesday, March 15, 2006 1:03PM - 1:15PM |
P39.00008: Gap anisotropy in density functional theory of the superconducting state A. Floris, A. Continenza, C. Franchini, E.K.U. Gross, N.N. Lathiotakis, M. L{\"u}ders, M. Marques, S. Massidda, G. Profeta, A. Sanna The discovery of superconductivity in MgB$_{2}$ ($T_{c}=39.5$K), with the clear presence of two gaps, has renewed the interest not only in electron-phonon mediated superconductivity, but also on the problem of anisotropic superconductivity. Here we use the recently introduced density functional theory of the superconducting state, that allows calculations of material-specific properties without the use of any adjustable parameters. The method, extended to ${\bf k},{\bf k'}$ resolved matrix elements of phonon-mediated and coulomb interactions, allows for a fully ${\bf k}$-resolved gap structure. Within this approach, we obtain the critical temperature and the two gaps of MgB$_{2}$ in good agreement with experiment. We will report on the existence of two different gaps also in Pb, and show that this is related to the different strength of the electron-phonon coupling associated with the two bands crossing the Fermi level. The calculated anisotropy is in good agreement with experiment. The same aproach is used for Nb3Sn, where recent experiments (Guritanu et al., Phys. Rev. B 70, 184526 (2004)) point to a possible two-gap behaviour. Our calculations show how our formalism is able to capture, in absence of any ad-hoc model, the features of multi-gap superconductors. [Preview Abstract] |
Wednesday, March 15, 2006 1:15PM - 1:27PM |
P39.00009: Band structure trend in cuprates and correlation with T$c\ max$ Ove Jepsen, Eva Pavarini, Wenhui Xie, Ole Krogh Andersen, Indra Dasgupta, Tanusri Saha-Dasgupta Parameters in model Hamiltonians are derived from LDA band structures for cuprate high T$_c$ superconductors. The materials and structural dependences of these are discussed. The most essential material dependent parameter is the range of the intralayer hopping. Furthermore, the range of this hopping correlates with T$_c$, i.e. materials with larger hopping ranges have higher maximum T$_c$'s. [Preview Abstract] |
Wednesday, March 15, 2006 1:27PM - 1:39PM |
P39.00010: First-Principles Construction of the Zhang-Rice singlet: Role of the apical oxygen in the mobility of the doped hole. Wei Ku, Wei-Guo Yin The Zhang-Rice singlet (ZRS) has been well accepted as the most relevant low-energy states in high $T_c$ cuprates. Based on a novel Wannier state analysis [1] of the LDA+$U$ electronic structure, a realistic ZRS is constructed from properly orthogonalized local Cu $d_{x^2-y^2}$ and symmetric combination of O-p states ($p^{(s)}$), leading to a realistic derivation of low-energy effective t-t'- t''-J Hamiltonian. Interestingly, symmetrized apical oxygen $p_z$ orbital with the Cu $d_{z^2}$ symmetry is found to be close to the ZRS in energy (~0.7 eV) and thus significantly facilitates the hopping to the second and third nearest neighbors. [1] W.-G. Yin, D. Volja, and W. Ku, cond-mat/0509075. [Preview Abstract] |
Wednesday, March 15, 2006 1:39PM - 1:51PM |
P39.00011: Band structure and Fermi surface of Ba$_{2}$Ca$_{3}$Cu$_{4}$O$_{8}$F$_{2}$ Wenhui Xie, Ove Jepsen, Ole K. Andersen, Zhi-Xun Shen Recently Y. Chen \textit{et} \textit{al}. have measured the Fermi Surface (FS) of the fluorinated four CuO$_{2}$ layer superconductor, Ba$_{2}$Ca$_{3}$Cu$_{4}$O$_{8}$F$_{2}$, using angular resolved photoemission spectroscopy (ARPES). Surprisingly, they found only two large ($\pi $,$\pi $) centered hole FS sheets, while four would have been expected. In order to investigate the reason for this we have performed first-principles electronic band structure calculations for this compound. As expected four antibonding copper-oxygen bands cross the Fermi level of which the two have dominantly orbital character on the two inner-layers and the other two have most orbital character on the outer-layers. The splitting between these bands is, however,much smaller than the splitting between the two measured FS sheets. The fluorine were claimed to replace all apical oxygens, however, by partly substituting apical oxygen as well as oxygen in the four CuO$_2$ layers by fluorine, good agrement with the experimental FS could be obtained. [Preview Abstract] |
Wednesday, March 15, 2006 1:51PM - 2:03PM |
P39.00012: The role of the Fermi surface sampling in first-principles calculations of electron-phonon coupling Feliciano Giustino, Marvin L. Cohen, Steven G. Louie A quantitative understanding of the electron-phonon interaction is crucial to the understanding of conventional and possibly high-T$_{\rm c}$ superconductivity, as well as to the study of transport and spectroscopic (such as optical and photoemission) properties of bulk and nanoscale systems. Despite the enormous interest in calculating electron-phonon interaction from first principles, present methods carry severe practical limitations. We present here a comparative study of several existing methods for computing this quantity. We show that, independent of the approximation adopted, a common computational bottleneck is that the Fermi surface must be sampled with extremely high accuracy, leading to prohibitively expensive calculations for complex systems. We also reformulate the problem of evaluating phonon linewidths in terms of self-consistent linear response theory, and demonstrate our approach through application to magnesium diboride. [Preview Abstract] |
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