Session U36: Electron Phonon Superconductivity and Isotope Effect

Fermiology and Superconductivity of LaNiGa$_2$

LaNiGa$_2$ has been identified as a possible centrosymmetric triplet superconductor based on observations of time reversal symmetry breaking in $\mu$SR experiments. Here we report calculations of the Fermiology and related properties. In spite of the seemingly layered crystal structure, the Fermi surface has several large sheets and is only moderately anisotropic, so that the material is best described as a three dimensional metal. These include sections that are open in the in-plane direction as well as a section that approaches the zone center. The density of states is high and primarily derived from Ga $p$ states, which hybridize with Ni $d$ states. Comparing with experimental specific heat data, we infer a superconducting $\lambda \le$ 0.55, which implies that this is a weak to intermediate coupling material. Ni occurs in a nominal $d^{10}$ configuration in this material, which places the compound far from magnetism. The implication is that this is most likely a standard electron phonon mediated s-wave superconductor.   

Electron-phonon coupling in potassium-doped superconducting picene

We explore the properties of electron-phonon couplings in K$_3$Picene, in the framework of density functional theory (DFT). By exploiting the maximally localized Wannier function formalism, we identify the contribution of the intra- and intermolecular phonon vibrations and the role of local and non-local electronic states in determining the electron-phonon coupling. Despite the molecular nature of the crystal, we find that the purely molecular contributions account for only 20{\%} of the total electron-phonon interaction $\lambda $. In particular, the Holstein-like contribution to $\lambda $ are four times smaller than those computed for an isolated neutral molecule, as they are strongly screened by the metallic bands of the doped crystal. The major contribution (80{\%}) to $\lambda $ in K$_3$Picene comes from non-local couplings due to phonon modulated hoppings. We show that the crystal geometry together with the molecular picene structure leads to a strong 1D spatial anisotropy of the non-local couplings. Finally, we propose a lattice model of the electron-phonon couplings in K3Picene that gives 90{\%} of the $\lambda $ obtained in first principles calculations [1]. \\[4pt] [1] M. Casula, M. Calandra and F. Mauri, PRL 107, 137006 (2011), PRB 86, 075445 (2012)   

First-principles study of cobalt pnictide SrCo$_{2}$N$_{2}$

With the recent discovery of high temperature superconductivity in BaFe$_{2}$As$_{2}$, there has been renewed interest in other members of the AT$_{2}$X$_{2}$ family (A $=$ alkaline earth element or lanthanide, T $=$ transition metal, X $=$ an element of groups IIIB-VIB) and in particle isovalent members of the 122 family. In this work, we describe a hypothetical cobalt pnictide, SrCo2N2, using density functional theory (DFT) in the local density approximation (LDA) with a Hubbard U correction. In this work, we determine both the lattice and chemical stability of SrCo$_{2}$N$_{2}$ as well as explore how the substitutions affect the electronic and magnetic properties in comparison to BaFe$_{2}$As$_{2}$ and 122 rare-earth cobalt phosphides.   

Dynamical Jahn-Teller effect in Cs$_{3}$C$_{60}$ superconductors

The Cs$_{3}$C$_{60}$ shows a superconducting critical temperature of 38K which is one of the highest among phonon-mediated superconductors. Recent infrared spectroscopy data of insulating Cs$_{3}$C$_{60}$ apparently support the presence of Jahn-Teller dynamics in this fulleride [1]. To check this possibility, we have performed the DFT calculations of vibronic constants and multiplet splitting parameters, and have calculated from the first principles the spectrum of low-lying vibronic states on C$_{60}^{3-}$ sites by diagonalizing the full vibronic Hamiltonian in a large vibrational basis. The splitting of the t$_{\mathrm{1u}}^{3}$ shell into degenerate multiplets and their vibronic mixing has been fully taken into account, as well as the effect of the environment on the local vibrations. The results show that in the insulating phase an unhindered dynamical Jahn-Teller effect takes place at each C$_{60}$ site. Using Gutzwiller approach in combination with LDA band structure, we demonstrate that the Jahn-Teller instability also persists in the metallic phase for a wide range of values of intrasite repulsion energy ($U)$.\\[4pt] [1] G. Klupp, P. Matus, K. Kamaras, A.Y. Ganin, A. McLennan, M.J. Rosseinsky, Y. Takabayashi, M.T. McDonald, K. Prassides, \textit{Nat. Comm}. \textbf{2012}, \textit{3}, 912.   

Repulsive interaction helps superconductivity in fullerides

Alkali metal (A) doped fullerides (A$_{3}$C$_{60})$ show not only superconductivity (SC) with high transition temperature Tc up to about 40K, but also antiferromagnetism (AF) with A$=$Cs. In view of nearby presence of the AF state, the Coulomb repulsion should play a significant role in the SC state. However, various experimental evidences point to a fully symmetric s-wave SC state being realized. In the conventional theory, the s-wave state is unfavorable in the presence of Coulomb repulsion. Then the fundamental question remains why the Tc in fullerides is so high. As a step toward the complete understanding, we study a purely repulsive interaction model with the characteristic band structure derived by degenerate molecular orbitals in fullerides. We calculate SC coupling constants for various symmetries of SC pairs by using the second order perturbation theory We find that even with the repulsive interaction model, the s-wave pair can be formed. With the electron-phonon interaction combined, it is likely that the s-wave pair becomes the most stable. According to our result, we propose that the cooperation between Coulomb repulsion and electron-phonon interaction is responsible for the high Tc.   

Phonon Vibrations and Superconductivity of a Bi-based Superconductor

Elastic and Inelastic neutron scattering experiments have been carried out on polycrystalline samples of the newly discovered layered superconductor LaO0.5F0.5BiS2, and its nonsuperconducting parent compound LaOBiS2 to determine their crystal structures and lattice vibrational modes. The Bragg peaks from the superconducting sample shows large broadening in width in the powder diffraction pattern. For the lattice vibrations, significant difference was observed upon F doping. Using the density functional perturbation theory, we identified all phonon modes, and show the major change in the phonon spectrum comes mainly from the change in the Oxygen mode. Even though strong electron phonon coupling constant was estimated, no significant difference in the phonon spectrum from BiS2 superconducting layer was found above and below Tc.   

Intercalant dependent electronic structure studies on alkali metal intercalated graphite compounds

We present electronic structure study results on various alkali-metal intercalated graphite compounds (GIC) using angle-resolved photoemission spectroscopy(ARPES). There are two competing theories on where the superconductivity occurs in this material; intercalant metal or charge doped graphene layer. To elucidate this issue, we measured electron doping amount of each alkali-metal intercalated GICs. In addition, there are some mysterious problems that can't be explained with current theories.   

Strong Variation of Density of States and Anomalous Isotope Effect in Low and High Tc Superconductors

In this work, first-principles density functional theory (DFT) calculations of electronic structures are integrated into the fundamental formalism of many-body physics for superconductivity in Zr, Nb$_{3}$Sn, and YBa2Cu3O7. It is shown that the electronic structures of the transition metals and compounds such as Zr, Nb$_{3}$Sn, and YBa2Cu3O7 are very complex. The electron densities of states around their Fermi levels possess sharp variations that have a large contribution to the anomalous isotope effect in these superconductors. The work was funded in part by NSF LASIGMA Project (Award No. EPS-1003897, NSF92010-15-RII-SUBR), AFOSR (FA9550-09-1-0367), and NSF project CBET-0754821.   

Charge density wave transport in heterogeneously doped NbSe$_{3}$ single crystals by masked ion (B$^{+}$, Li$^{+})$ implantation

Charge-density wave is competing electron spectrum instability of superconductivity (SC). CDW transport vs. SC with boundary between CDWs and SC are well known examples of correlated transport of macroscopic numbers of electrons.CDW superconductors include layered dichalcogenides, NbSe$_{3}$. On selective area medium energy ion (B$^{+}$, Li$^{+})$ implantation was used to create irradiated/unmodified/irradiated CDW heterostructures with well-defined interfaces on single NbSe$_{3}$ crystal. The effects of impurities go beyond simply increasing CDW pinning (J.P.McCarten.et.al J.Phys.IV France 9,1999).The dV/dI vs. bias at several temperatures, and zero-bias resistance vs. temperature, where two voltage contacts straddle the interface (near the boundary between B$^{+}$/ Li$^{+}$-implanted and unimplanted regions) are well studied. Injected charge B$^{+}$/Li$^{+}$ is a non-isoelectronic impurity. The results of ongoing investigations of similar studies of boron- and lithium-doped NbSe$_{3}$ will be discussed.   

A Time-Domain Susceptibility Model for a BCS Superconductor in FDTD Calculations

We have developed a simple time-domain electric susceptibility model for a BCS type superconductor, valid for the spectral range spanning the optical energy gap frequency $\hbar\omega$$\sim$2$\Delta$ and $\it{T}\ll\it{T}_C$. The expression can be used in Finite Difference Time Domain (FDTD) calculations for propagating electromagnetic waves through systems containing superconductor materials, including meta-materials. Since the energy gap appears explicitly, it can be varied as a function of time to describe non-linear and non-equilibrium effects as observed in microwave experiments. We use the expression in a FDTD calculation for the transmission through and reflection from a thin film of NbN on a substrate, and compare with both conventional frequency domain calculations as well as actual experimental results.   

Influence of the interplay between de Gennes boundary conditions and cubicity of Ginzburg-Landau equation on the properties of superconductors

Solutions of the Ginzburg-Landau (GL) equation for the film subjected to the de Gennes boundary conditions (BCs) with extrapolation length $\Lambda$ are analyzed with emphasis on the interaction between $\Lambda$ and the coefficient $\beta$ of the cubic GL term and its influence on the temperature $T$ of the strip. Very substantial role is played also by the carrier density $n_s$. Physical interpretation is based on the $n_s$-dependent effective potential $V_{eff}({\bf r})$ created by the nonlinear term and its influence on the lowest eigenvalue of the corresponding Schr\"{o}dinger equation. For the large cubicities, the temperature $T$ becomes $\Lambda$ independent linearly decreasing function of the growing $\beta$ since in this limit the BCs can not alter very strong $V_{eff}$. The temperature increase produced in the linear GL regime by the negative de Gennes distance is wiped out by the growing cubicity. In this case, the decreasing $T$ passes through its bulk value $T_c$ at the unique density $n_s^{(0)}$ only, and the corresponding $\Lambda_{T=T_c}$ is an analytical function of $\beta$. For the large cubicities, the concentration $n_s^{(0)}$ transforms into the density of the bulk sample. Other analytical asymptotics are analyzed too.   

Numerical study of the magnetic and pair binding properties in aromatic hydrocarbon superconductors

We performed a systematic numerical study of the magnetic and pair binding properties in recently discovered aromatic hydrocarbon superconductors, by using exact diagonalization and quantum Monte Carlo methods. The $\pi $-electrons on the carbon atoms of a single molecule are modelled by the one-orbital Hubbard model, which takes into account the energy difference between carbon atoms with and without hydrogen bonds. Our results show that the spin polarized ground state is realized for charged molecules in the physical parameter region. This provides a reasonable explanation of local spins observed in experiments. In alkali-metal-doped picene and phenanthrene, the pairing binding energy is always negative for different electron doping densities, suggesting that electron correlation has no contribution to the formation of Cooper pairs. However, a positive pair binding energy for the charged dibenzopentacene molecule with one or three added electrons indicates that electron correlation may produce an effective attraction between electrons.