Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L32: Focus Session: Superconductivity: Superconductivity (Mostly Electron-Phonon) |
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Sponsoring Units: DCOMP DCMP Chair: A. A. Golubov, University of Tweute, Holland Room: LACC 507 |
Tuesday, March 22, 2005 2:30PM - 3:06PM |
L32.00001: Using first-principles calculations for understanding MgB$_{2}$ Invited Speaker: Electronic structure methods have been the key to understand the peculiar properties of the multi-band superconductor MgB$_2$. After briefly reviewing the current state of our understanding of the material we will discuss some of the more recent observations on Al or C doped MgB$_2$.\\ We will demonstrate, based on first-principles band structure calculations and Eliashberg theory, that the experimentally observed decrease of the critical temperature $T_c$ of Al and C doped MgB$_2$ samples can be understood mainly in terms of a band filling effect due to the electron doping by Al and C. A simple scaling of the electron-phonon coupling constant $\lambda$ by the variation of the density of states (DOS) as function of electron doping is sufficient to capture qualitatively the observed behavior. Using the virtual crystal approximation to account for the electron doping we calculate the change of the DOS due to doping. The expected hardening of the $E_{2g}$ phonon frequency at $\Gamma$ is also well reproduced by our calculations using this approximation. These two contributions together, the change of DOS and phonon frequency, reproduce the observed dependence of $T_c$ on doping concentration astonishingly well not using any free parameter to fit the data.\\ Further, we also explain the long standing open question of the experimental observation of a nearly constant $\pi$ gap as function of doping by a compensation of the effect of band filling and interband scattering. Both effects together generate a nearly constant $\pi$ gap and shift the merging point of both gaps to higher doping concentrations, resolving the discrepancy between experiment and theoretical predictions based on interband scattering only.\\ The author acknowledges many fruitful collaborations and discussions on this exciting topic with O.K. Andersen, I.I. Mazin, A.A. Golubov, O.V. Dolgov, O. Jepsen and R.K. Kremer over the past years. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:18PM |
L32.00002: Zero-point band gap renormalization and superconductivity in diamond Manuel Cardona During the past decade the zero-point renormalizations of band gaps have been determined both experimentally and theoretically for many semiconductors. Tetrahedral materials containing elements of the second row of the periodic table C,N,O) have been shown to have fundamental gap renormalizations considerably larger (up to one order of magnitude) than those containing only other elements. Diamond, for instance, has a renormalization of $\sim $ 500meV as compared with 60 meV for silicon and germanium. This effect has been attributed to the lack of d-electrons in the core of the carbon atoms [1]. Superconductivity, with $T_{c}$ up to $\sim $9K, has been recently discovered in heavily boron doped diamond [2,3]. It will be conjectured that this phenomenon is related to the large coupling between the optical phonons and the holes at the top of the valence band, which is also related to the large band gap renormalization$. $An estimate of $T_{c}^{ }$based on available values of the hole-phonon deformation potential $d_{o }$and of the hole effective masses will be shown to explain the experimental values found for diamond. The corresponding $T_{c }$for Si and Ge should be either very small or nonexistent. \begin{enumerate} \item M.Cardona Solid State Commun. \textbf{133}, 3 (2004) \item E.A. Ekimov et al., Nature \textbf{428}, 542 (2004) \item Y.Takano et al., Appl. Phys. Lett. \textbf{85}, 2851 (2004) \end{enumerate} [Preview Abstract] |
Tuesday, March 22, 2005 3:18PM - 3:30PM |
L32.00003: Origin of Superconductivity in Boron-doped Diamond Kwan-Woo Lee, Warren E. Pickett Superconductivity of heavily boron-doped diamond having B concentration of 5$\times 10^{21}$ cm$^{-3}$, reported at T$_c$=4 K by Ekimov {\it et} {\it al}.(April, 2004), is investigated exploiting its electronic and vibrational analogies to MgB$_2$. The deformation potential of the hole states arising from the C-C bond stretch mode is 60\% larger than the corresponding quantity in MgB$_2$ that drives its high T$_c$, leading to very large electron-phonon matrix elements. The calculated coupling strength $\lambda \approx$ 0.5 leads to T$_c$ in the 5-10 K range and makes phonon coupling the likely mechanism. Higher doping should increase T$_c$ somewhat, but effects of three dimensionality primarily on the density of states keep doped diamond from having a T$_c$ closer to that of MgB$_2$. [Preview Abstract] |
Tuesday, March 22, 2005 3:30PM - 3:42PM |
L32.00004: Electron-phonon coupling and superconductivity in MgB$_2$ under hydrostatic pressure. Ramiro Quijano, Filiberto Ortiz, Romeo de Coss, Aaron Aguayo We have studied the dynamics and coupling of the $E_{2g}$ phonon mode with the $\sigma$-band in MgB$_2$ under pressure using the Frozen Phonon Approximation. The results were obtained by means of first-principles total-energy calculations using the full potential Linearized Augmented Plane Wave (LAPW) method and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We present results for the evolution of the anharmonicity and phonon frequency of the $E_ {2g}$ mode, the electron-phonon coupling constant, and $T_c$ as a function of hydrostatic pressure in the range 0-40 GPa. We find that the phonon frequency increases monotonically with pressure, but the the anharmonicity, the electron-phonon coupling and $T_c$ decreases with pressure. We have obtained a very good agreement between the calculated $T_c(P)$ and the experimental data available in the literature, in particular with the experimental data corresponding to monocystalline samples. This work was supported by Consejo Nacional de Ciencia y Tecnolog{\'\i}a (CONACYT, M{\'e}xico) under Grant No. 43830-F. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 3:54PM |
L32.00005: Searching for higher superconducting transition temperature in strained MgB2 using first principles calculations Jin-Cheng Zheng, Yimei Zhu Since the discovery of the amazing high superconducting transition temperature (T$_{c})$ in non-oxide MgB$_{2}$, great efforts have been made on the search for higher T$_{c}$ in MgB$_{2}$ and related materials by either chemical substitutions or by applying pressures to modify the lattice of MgB$_{2}$. Little success has been achieved so far due to the fact that the T$_{c}$ is always suppressed using either method. To tailor the T$_{c}$ in MgB$_{2}$, the full atomic-level understanding of underlying mechanism of chemical and lattice effects is required. According to the McMillan-Allen-Dynes analysis, T$_{c}$ in MgB$_{2}$ is controlled by the collective contributions from phonon frequency, electron-phonon coupling and Coulomb repulsion. Consideration of one single parameter alone cannot guarantee the improvement of T$_{c}$ because of the strong coupling and competing of other parameters. Here we present a detailed first-principles density functional analysis of the effects of lattice strains to superconducting properties of MgB$_{2.}$ On the basis of our results for structural, electronic, vibrational and superconducting properties of strained MgB$_{2}$, we show how higher superconducting transition temperature might be achieved in strained MgB$_{2}$ superconductor. [Preview Abstract] |
Tuesday, March 22, 2005 3:54PM - 4:06PM |
L32.00006: Electronic structure of scandium-doped MgB$_2$ Omar de la Pe{\~n}a, Romeo de Coss, Stefano Agrestini, Antonio Bianconi Recently has been reported the synthesis of a new superconducting alloy based on MgB$_2$, where Mg is partially substituted with Sc. In order to analyze the effect of Sc doping on the structural and superconducting properties of Mg$_ {1-x}$Sc$_x$B$_2$, we have performed a detailed study of the electronic structure for this new diboride. The calculations have been done using the first-principles LAPW method, within the supercell approach for modeling the doping. In this work we report results for the electronic band structure, Fermi surface, and density of states. The effect of the Sc-$d$ orbitals on the structural and electronic properties of Mg$_{1-x}$Sc$_x$B$_2$ is analyzed. Increasing the Sc concentration ($x$) the $\sigma$-band is gradually filled, because Sc have one valence electron more than Mg. Interestingly, the analysis of the band structure shows that even for ScB$_2$ the top of the $\sigma$-band remain above the Fermi level, nevertheless the $\sigma$-band presents high dispersion and has an important contribution of $d$ states. In this way, in addition to the band filling effect, Sc doping gradually reduces the two-dimensional character of the $\sigma$- band in Mg$_{1-x}$Sc$_x$B$_2$ as a result of increasing the $sp$(B)-$d$(Sc) hybridization. This research was partially supported by Consejo Nacional de Ciencia y Tecnolog{\'\i}a (CONACYT, M{\'e}xico) under Grant. No. 43830-F [Preview Abstract] |
Tuesday, March 22, 2005 4:06PM - 4:18PM |
L32.00007: Nonlocal screening, electron-phonon coupling, and phonon renormalization in metals Peihong Zhang, Steven G. Louie, Marvin L. Cohen A new method for calculating the phonon self-energy in metals arising from the coupling between phonons and electrons near the Fermi surface is developed. The essence of this scheme is the separation of the inter-and intra-band parts of the electron polarizability. The intra-band contribution provides extra screening and is closely related to the electron-phonon coupling and phonon softening in metals. Applications of this scheme to phonons in MgB$_2$ give excellent results when compared with experiments and previous theoretical work. In addition, both electron and hole dopings are found to reduce the renormalization effect of the $E_{2g}$ phonon mode. This indicates weakened electron-phonon couplings in the doped systems. This is consistent with the experimental observations that the superconducting transition temperature of MgB$_2$ drops upon substituting Mg with either Al or Li. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L32.00008: Phonon renormalization and anharmonicity in Al-doped MgB$_2$ Filiberto Ortiz, Ramiro Quijano, Omar de la Pe{\~n}a, Romeo de Coss, Aar{\'o}n Aguayo We have studied the evolution of the $E_{2g}$ phonon mode dynamics in Mg$_{1-x}$Al$_x$B$_2$ as a function of doping using the Frozen Phonon Approximation (FPA). The doping was modeled in the ab-initio Virtual Crystal Approximation (VCA). The results were obtained by means of first-principles total-energy calculations using the full potential Linearized Augmented Plane Wave (LAPW) method and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We present results for the evolution of the phonon frequency and anharmonicity of the $E_{2g}$ mode as a function of Al concentration ($x$). From a comparison of the experimental data with the calculated $E_{2g}$ phonon frequency we show that the VCA-FPA reproduces the observed phonon renormalization in the whole range of Al concentrations. More interestingly, we find that the anharmonicity gradually decreases with Al doping and vanishes for $x$(Al)$>$0.5, that behaviour correlates with the evolution of the measured Raman linewidth in Al-doped MgB$_2$. The significance of these results are discussed in the light of the experimentally observed loss of superconductivity in Mg$_{1- x}$Al$_x$B$_2$.This work was supported by Consejo Nacional de Ciencia y Tecnolog{\'\i}a (CONACYT, M\'exico) under Grant. No. 43830-F. [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L32.00009: Electrons and phonons in YbC$_6$ Igor Mazin Electronic structure and selected zone center phonons, as well as the electron-phonon coupling are calculated for a novel intercalated graphite supercoductor, YbC$_6$, using LDA+U method (fully localized version). The only stable solution either in LDA or in LDA+U is with zero spin and orbital polarization and the 4f band fully occupied. We show that Yb d states are present at the Fermi level and assess a hypothesis that superconductivity may arise from Yb phonons. [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L32.00010: Electronic structure properties and superconductivity of the $\beta$-pyrochlore Os oxides, $A$Os$_2$O$_6$ ($A$=alkali metal) R. Saniz, A. J. Freeman The recently discovered\footnote{T. Muramatsu {\it et al.} J. Phys. Soc. Jpn. {\bf 73}, 10 (2004).} family of superconducting $\beta$-pyrochlores {\it A}Os$_2$O$_6$ ($A$=alkali metal) represents a particularly interesting example of the interplay between superconductivity and orbital and crystal structure degrees of freedom. Indeed, the pyrochlore lattice formed by the Os-O staggered chains appears to lead to very high Sommerfeld coefficients, increasing of $T_c$ under positive pressure, and other intriguing properties. We present results of a first-principles study of the electronic structure and superconducting properties of these materials ($A$=Na, K, Rb, and Cs) using the highly precise full-potential linearized augmented plane wave (FLAPW) method.\footnote{Wimmer, Krakauer, Weinert, Freeman, Phys. Rev. B {\bf 24}, 864 (1981).} We show that the observed increase of $T_c$ with decreasing mass of $A$ as well as under positive hydrostatic pressure can both be well understood within a conventional phonon-mediated pairing picture. Furthermore, the density of states at $E_F$ depends critically on spin-orbit coupling, due to a van Hove singularity near $E_F$, with a direct effect on $T_c$; the Fermi surface shows strong nesting, which is reflected in the dynamic susceptibility and thus indicates that spin fluctuations may play an important role in these materials. [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:06PM |
L32.00011: Anomalous Dynamics of K Ion in $\beta$-Pyrochlore Jan Kunes, Tae Seong Jeong, Warren E. Pickett Among the recently discovered $\beta$-pyrochlore superconductors AOs$_2$O$_6$ (A=K, Rb, and Cs) KOs$_2$O$_6$ exhibits several anomalous features (electrical resistivity, 1/T$_1$ NMR decay). We have studied electronic structure of AOs$_2$O$_6$ using the density functional theory based methods. We have found a moderate Stoner enhancement of the Pauli susceptibility of 2.15 and a sizable thermal mass enhancement of 2.9 in KOs$_2$O$_6$. While the electronic structure of the three systems is very similar the structural stability of the alkali atom site is rather different. In particular the K ion potential well has a flat bottom, beyond small anharmonic corrections, allowing for large excursions from the symmetric position. We suggest that this feature is behind the striking differences in behavior of otherwise similar AOs$_2$O$_6$ compounds. [Preview Abstract] |
Tuesday, March 22, 2005 5:06PM - 5:18PM |
L32.00012: Interface Electronic Structure and Possible Superconductivity in CuCl/Si(111) S.H. Rhim, R. Saniz, A.J. Freeman, J. Yu To investigate a possible interfacial superconductivity \footnote{B.L. Mattes, Physica C {\bf 162},554 (1989); B.L. Mattes and C. L. Foiles, Physica 135B, 139 (1985)} in CuCl/Si(111), we carried out electronic structure calculations using the highly precise FLAPW \footnote{Wimmer, Krakauer, Weinert, and Freeman, Phys.Rev.B, {\bf 24}, 864 (1981)} method. As a result of charge transfer between CuCl and Si layers, two-dimensional (2D) metallic states are found to be formed at the interface. From the geometry relaxation, it is shown that the ionic bonding of CuCl is weakened and there is mixed metallic and covalent bonding at the interface. The 2D conduction bands at the interface, sandwiched by the highly polarizable dielectric layers, resemble the 2D Cu-O dp$\sigma$ bands of the Cu-oxide superconductors, which are considered to be responsible for high $T_c$ superconductivity. To obtain $T_c$ of the CuCl/Si interface based on the conventional electron-phonon(e-p) interactions, we calculated the e-p coupling constant, $\lambda$, within the rigid ion approximation \footnote{G. D. Gaspari and B. L. Gyorffy, Phys. Rev. Lett. {\bf 28} 801 (1972)}. The results indicate that a strong e-p coupling is present at the interface layers but is not enough to explain the previously reported high transition temperature$^{2}$. [Preview Abstract] |
Tuesday, March 22, 2005 5:18PM - 5:30PM |
L32.00013: Non-empirical Calculations of the upper-critical field $H_{c2}$ for Nb, NbSe$_{2}$, and MgB$_{2}$ Takafumi Kita, Masao Arai Detailed Fermi-surface structures are essential to describe the upper critical field $H_{c2}$ in type-II superconductors, as first noticed by Hohenberg and Werthamer [Phys.\ Rev.\ {\bf 153},\ 493 (1967)] and shown explicitly by Butler for high- purity cubic Niobium [Phys.\ Rev.\ Lett.\ {\bf 44},\ 1516 (1980)]. However, most of $H_{c2}$ calculations performed so far have used simplified model Fermi surfaces and/or phenomenological fitting parameters. Due to this lack of {\em ab}-{\em initio}-type calculations, our understanding on $H_{c2}$ remains at a rather unsatisfactory level. With these observations, we have derived an $H_{c2}$ equation for classic type-II superconductors which is applicable to systems with anisotropic Fermi surfaces and/or energy gaps under arbitrary field directions. Based on the formalism, we have calculated $H_{c2}$ curves for clean type-II superconductors Nb, NbSe$_{2}$, and MgB$_{2}$ using Fermi surfaces from {\em ab initio} electronic structure calculations. The results for Nb and NbSe$_2$ excellently reproduce both temperature and directional dependences of measured $H_{c2}$ curves, including marked upward curvature of NbSe$_{2}$ near $T_{c}$. As for MgB$_2$, a good fit is obtained for a $\pi$/$\sigma$ gap ratio of $\sim\! 0.3$. Our results indicate essential importance of Fermi surface anisotropy for describing $H_{c2}$. [Preview Abstract] |
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