Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H11: Focus Session: MgB2-like: Computational Design of Novel Superconductors |
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Sponsoring Units: DMP Chair: Igor Mazin, Naval Research Laboratory Room: Morial Convention Center RO9 |
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H11.00001: Quasi-two-dimensional electronic states in a dense three-dimensional Li-Be alloy Richard G. Hennig, Ji Feng, Roald Hoffmann, N.W. Ashcroft High pressure can affect electronic structure and crystal packing, and in some cases even induce compound formation between elements that do not bond under ambient conditions. Our computational study for the Li-Be system shows that the reactivity of Li and Be is fundamentally altered by pressure. These two lightest of all metallic elements are immiscible at ambient conditions. Using structure search methods we discover four stoichiometric Li$_x$Be$_{1-x}$ compounds that are stable over a range of pressures. The electronic density of states of one of them displays a remarkable step-like feature and plateau at the bottom of the valence band, which is typical of a quasi-2D electronic structure and rather unexpected in a 3D crystal environment. We attribute this feature to large size differences between the ionic cores of Li and Be. Under increased pressure, the Li cores start to overlap and thereby expel valence electrons into quasi-two-dimensional layers characterized by delocalized free-particle-like states in the vicinity of Be ions. These alloys are also interesting from the perspective of superconductivity. The T$_c$ in the LiBe alloy is expected to be substantially higher than that of the component elements due to the increased density of states at the Fermi energy compared particularly to elemental Be. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H11.00002: First-principles search for potential high temperature superconductors in the Mg-B-A (A=alkaline metal) system with high boron content Roman Chepulskyy, Igor Mazin, Stefano Curtarolo Possible superconductivity at 50K was recently reported [1] in the Mg-B-A (A=Cs, Rb, Ba) system. Although attempts to reproduce this finding have been unsuccessful so far [2], if a stable or metastable phase could be found by a first principle search similar to that in Ref. 3, this would have lent credibility to the experimental finding [1] and outline possible further directions. The results of Ref. 1 suggest that (a) the superconducting phase is not similar to MgB$_2$ (B site has cubic or similarly high symmetry) and (b) boron content is higher than in MgB$_2$. We report first-principles study of the thermodynamics of alkali and alkaline earth doping in the boron- rich part of the Mg-B phase diagram (MgB$_n$ with $n>2$), searching for a phase that could explain the results of Ref. [1]. [1] A.V. Palnichenko {\it et al.}, JETP Letters \textbf{86}, 272 (2007). [2] R K Singh {\it et al} http://arxiv.org/abs/0709.4001v1. [3] S. Curtarolo {\it et al}, Calphad \textbf{29}, 163 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H11.00003: Engineering superconductors with ab initio methods: ternary metal borides Aleksey Kolmogorov, Matteo Calandra, Stefano Curtarolo We have performed a targeted search for novel superconducting ternary borides starting from the recently identified class of binary metal sandwich (MS) structures [1]. Our {\it ab initio} calculations suggest that a theoretically-devised MS lithium monoboride gains in stability when alloyed with electron-rich metals [2,3]. In an effort to pre-select compounds with the strongest electron-phonon coupling we evaluate the softening of the in-plane boron phonon mode in a large class of metal borides. Our results reveal interesting general trends for the frequency of the in-plane boron phonons as a function of the boron-boron bond length and the valence of the metal [4]. Research supported by ONR and NSF. [1] PRB 73, 180501(R) (2006) [2] PRB 74, 224507 (2006). [3] PRB 75, 144506 (2007) [4] A.N. Kolmogorov, M. Calandra, S. Curtarolo, submitted to PRB. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H11.00004: Electron-Phonon interaction in hexagonal layered compounds Lilia Boeri, Matteo Giantomassi, Giovanni B. Bachelet, Ole Krogh Andersen The discovery of superconductivity in MgB$_2$ has initiated a thorough search for new electron-phonon ($e-ph$) superconductors, particulary among hexagonal layered compounds. In this talk I will describe, using {\em ab-initio} methods, the factors that determine the electron-phonon properties of two classes of recently discovered superconductors, namely alkali-earth intercalated graphites (highest T$_c$ 15.1 K for CaC$_6$)[1] and metal-intercalated ternary compounds MaAlSi. [2] \newline References: \newline [1] J. S. Kim, L. Boeri, R. K. Kremer, and F. S. Razavi Phys. Rev. B 74, 214513 (2006), Phys. Rev. Lett. 96, 217002 (2006), and Phys. Rev. Lett. 027001 (2007). \newline L. Boeri, G.B. Bachelet, M. Giantomassi, O.K. Andersen, Phys. Rev. B 76, 064510 (2007). \newline [2] M. Giantomassi, L. Boeri, and G. B. Bachelet, Phys. Rev. B 72, 224512 (2005) [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H11.00005: Structure and Superconductivity of Calcium under Pressure Z. P. Yin, F. Gygi, W. E. Pickett The structure, phonon spectrum and electron phonon coupling of Ca under pressure is studied by first principle calculations. Experimentally, Ca at room temperature is simple cubic (SC) at pressure between 30 GPa and 109 GPa and goes to unknown structure above 109GPa. Its superconducting T$_c$ increases significantly in the SC phase, increasing to 23 K at 109 GPa (25 K at 161 GPa). Linear response calculations reveal that SC Ca is horribly unstable in the corresponding pressure range (at T=0). Ab initio molecule dynamics calculations on a 4x4x4 supercell find the SC phase is distorted into a four-atom bcc structure that is dynamically stable in the 40-110 GPa range. At even higher pressure this bcc structure becomes dynamically unstable (imaginary frequencies). T. Ishikawa {\it et al.} (private communication) proposed an orthorhombic structure around 120 GPa. We confirmed this structure with minor differences of internal parameters. The theoretical x-ray diffraction (XRD) pattern of this structure has very good match with the experimental XRD pattern of the Ca V phase. Our linear response calculations further confirm that it is dynamically stable, and indications of strong electron-phonon coupling in this phase will be presented. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H11.00006: Constraints on $T_c$ for superconductivity in heavily boron-doped diamond Jonathan E. Moussa, Marvin L. Cohen Calculations of electron-phonon coupling are performed for boron-doped diamond structures without electronically compensating defects over a wide range of boron concentration. The effects of boron substitutional disorder are incorporated through the use of randomly generated supercells, leading to a disorder-broadened distribution of results. After averaging over disorder, this study predicts a maximum bulk $T_c$ near 55 K for boron concentrations between $20\% - 30\%$, assuming the validity of the simple structural model used and a Coulomb pseudopotential of $\mu^* = 0.12$. Considering only the largest electron-phonon coupling values of the distribution, superconductivity may still percolate through the material at higher temperatures, up to 80 K, through the regions of large coupling. A synthesis path is proposed to experimentally access this class of materials. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:48AM |
H11.00007: Superconductivity in high-pressure solids Invited Speaker: The structural principle behind the unusual features in the high-pressure phases of simple alkali elements is reviewed. It is shown that there exists a pressure regime in which the elemental solids are likely to adopt a layer structure. There are two novel characteristics associated with this structure type. The system tends to be at the proximity of phonon and electronic instabilities. The combined effect is a significant enhancement of electron-phonon coupling, resulting in a superconducting state. We demonstrate this empirical observation with selected examples including a recently predicted novel structure of high pressure SnH$_4$ which shows very high superconducting critical temperature. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H11.00008: Layered Structures Favor Superconductivity in Compressed Solid SiH$_{4}$ Hai-Qing Lin, Xiao-Jia Chen, Jiang-Long Wang, Viktor V. Struzhkin, Ho-kwang Mao The electronic and lattice-dynamics properties of compressed solid SiH$_{4}$ have been calculated over the pressure regime up to 300 GPa with density functional theory. We find that the structures having layered network with eight-fold SiH$_{8}$ coordination favor the metallization and superconductivity. The layered $Cmca$ SiH$_{4}$ is predicted to have a superconducting transition temperature of 75 K at 70 GPa thus opening new possibilities for exploring high temperature superconductivity in the hydrogen-rich system. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H11.00009: First-Principles Study of Superconductivity in boron-doped SiC Jesse Noffsinger, Feliciano Giustino, Steven Louie, Marvin Cohen The discovery of superconductivity in materials such as intercalated graphite, alkali-doped fullerenes, and boron-doped diamond has drawn significant interest to carbon-based superconductors. Recent experiments indicate that boron-doped cubic SiC may superconduct above 1 K [1]. We investigate the superconductivity in cubic SiC using a first-principles approach. We describe the electronic structure within density functional theory and the lattice dynamics within density functional perturbation theory. The electron-phonon interaction matrix elements are calculated via a recently developed method based on Wannier functions [2]. The boron doping is accounted for by a virtual crystal approximation. In addition to the coupling of Fermi surface electronic states to optical phonon modes, there appears to be a non-negligible contribution to the electron-phonon coupling arising from acoustic phonons. Superconductivity is discussed by analyzing the similarities and the differences with respect to the closely related boron-doped diamond. [1] Z-A. Ren et. al, \textit{private communication}. [2] F. Giustino et. al, Phys. Rev. B 76, 165108 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H11.00010: First principles study of Al and C-doped MgB$_2$: evolution of two gaps and critical temperature Omar De la Pe\~na-Seaman, Romeo de Coss, Rolf Heid, Klaus-Peter Bohnen We have studied the electron-phonon and superconducting properties of the Mg$_{1-x}$Al$_x$B$_2$ and MgB$_{2(1-x)}$C$_{2x}$ alloys within the framework of density functional perturbation theory, using a mixed-basis pseudopotential method and the virtual crystal approximation (VCA) for modeling the alloys. For both systems, the Eliashberg spectral function ($\alpha^{2}F(\omega)$) and the electron-phonon coupling parameter ($\lambda$) have been calculated in the two band model ($\sigma$,$\pi$) for several concentrations until $x(\mathrm{Al})=0.55$ and $x(\mathrm{C})=0.175$. Using the calculated $\alpha^{2}_{ij}F(\omega)$ and a diagonal expression for the Coulomb pseudopotential matrix, $\mu^{*}$, we solved numerically the Eliashberg gap equations in the two band model without interband scattering. We reproduce the experimental decreasing behavior of $\Delta_{\sigma}(x)$, $\Delta_{\pi}(x)$, and $T_{c}(x)$ for both alloy systems. The role of the interband scattering in the observed behavior of the superconducting gaps and $T_c$ in the Al- and C-MgB$_2$ alloys is discussed. This research was supported by Consejo Nacional de Ciencia y Tecnolog\'ia (Conacyt) under Grant No. 43830-F. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H11.00011: Effects of Quenched Random Gap Inhomogeneities on the Specific Heat of a Model High-$T_c$ Superconductor David Stroud, Daniel Valdez-Balderas In many cuprate superconductors, scanning tunneling microscopy experiments show that the energy gap has substantial quenched random spatial variations. We have calculated how such gap variations affect the specific heat $C_V$ in a model for the most anisotropic of these materials. The model is based on a Ginzburg-Landau free energy functional in which position- dependent coefficients are used to model quenched inhomogeneity. Using Monte Carlo simulations, we evaluate $C_V$ for different disorder strengths. Near optimal doping, we find that quenched gap disorder substantially broadens the specific heat anomaly near the phase ordering transition $T_c$, compared to that due to thermal fluctuations alone. But for strongly underdoped samples, in which $T_c$ is greatly separated from the pseudogap temperature $T_{c0}$, disorder only slightly increases the broadening beyond the already substantial amount due to thermal fluctuations. We compare these results to recent experiments. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H11.00012: The temperature dependent gap edge in strong-coupling superconductors David G. Walmsley, XueHeng Zheng Using the theory of Eliashberg and Nambu for strong coupling superconductors we have calculated the gap function for a model superconductor and a selection of real superconductors including the elements Al, Sn, Tl, Nb, In, Pb, and Hg and one alloy, Bi$_2 $Tl. We have determined the temperature dependent gap edge in each and found that in materials with weak electron-phonon coupling ($\lambda<0.8$)it is single-valued but in materials with intermediate coupling ($0.8\leq\lambda\leq1.2$) the gap edge is double-valued whereas in materials with strong coupling ($\lambda>1.20$) not only is the gap edge double-valued but it also departs significantly from the BCS form and develops a shoulder-like structure which may in some cases denote a gap edge exceeding the $T=0$ value. These computational results support the insights obtained by Leavens in an analytic consideration of the general problem. Both the shoulder and double value arise from a common origin seated in the form of the gap function in strongly coupled materials at finite temperatures. From the calculated gap function we can determine the densities of states in the materials and the form of the tunneling current-voltage characteristics for junctions with these materials as electrodes. By way of illustration results are shown for the contrasting cases of Sn ($\lambda=0.74$) and Hg ($\lambda=1.63$). The reported results are distinct in several ways from BCS predictions. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H11.00013: A DFT study of the $UCoGe$ magnetic superconductor Pablo de la Mora, Oracio Navarro Recently Huy \textit{et al.} (PRL 99, 067006) found that $UCoGe$ is a superconductor coexisting with magnetism. Electronic structure calculations were performed on this compound using the WIEN2k package, results show that its magnetism is anisotropic, with the easy magnetic axis in the $c$-direction. The magnetic moment of the U atom is quite small, but there is a large moment in the Co atom which is in disagreement the experimental result of Huy \textit{et al.}. These results contrast with the isostructural magnetic superconductor $URhGe$, in this latter compound the U-atom magnetic moment is relatively large; while for the Rh atom it is small. The main contribution at $E_F$ is due to U-5f and Co-3d, Ge has little contribution. There are many similarities with the $MgB_2$ superconductor, there are many bands at $E_F$ and $UCoGe$ has a distorted $MgB_2$ crystalline structure, but the superconductivity mechanism seems to be of different origin. [Preview Abstract] |
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