44 research outputs found

    Turning a Band Insulator Into an Exotic Superconductor

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    Understanding exotic, non s--wave--like states of Cooper pairs is important and may lead to new superconductors with higher critical temperatures and novel properties. Their existence is known to be possible but has always been thought to be associated with non--traditional mechanisms of superconductivity where electronic correlations play an important role. Here we use a first principles linear response calculation to show that in doped Bi2_{2}Se3_{3} an unconventional p--wave--like state can be favored via a conventional phonon--mediated mechanism, as driven by an unusual, almost singular behavior of the electron--phonon interaction at long wavelengths. This may provide a new platform for our understanding superconductivity phenomena in doped band insulators.Comment: Published versio

    Computational Design of Axion Insulators Based on 5d Spinels Compounds

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    Based on density functional calculation with LDA+U method, we propose that hypothetical Osmium compounds such as CaOs2O4 and SrOs2O4 can be stabilized in the geometrically frustrated spinel crystal structure. They also show some exotic electronic and magnetic properties in a reasonable range of on-site Coulomb correlation U such as ferromagnetism and orbital magnetoelectric effect characteristic to Axion electrodynamics. Other electronic phases including 3D Dirac metal and Mott insulator exist and would make perspective 5d spinels ideal for applications.Comment: 5 pages, 3 figure

    Calculated Spin Fluctuational Pairing Interaction in HgBa2CuO4 using LDA+FLEX Method

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    A combination of density functional theory in its local density approximation (LDA) with k- and ω\omega dependent self-energy found from fluctuational-exchange-type random phase approximation (FLEX-RPA) is utilized here to study superconducting pairing interaction in a prototype cuprate superconductor HgBa2_{2}CuO4_{4}. Although, FLEX-RPA methodology have been widely applied in the past to unconventional superconductors, previous studies were mostly based on tight-binding derived minimal Hamiltonians, while the approach presented here deals directly with the first principle electronic structure calculation of the studied material where spin and charge susceptibilities are evaluated for a correlated subset of the electronic Hilbert space as it is done in popular LDA+U and LDA+DMFT methods. Based on our numerically extracted pairing interaction among the Fermi surface electrons we exactly diagonalize a linearized BCS gap equation, whose highest eigenstate is expectantly found corresponding to % d_{x^{2}-y^{2}} symmetry for a wide range of on-site Coulomb repulsions U and dopings that we treat using virtual crystal approximation. Calculated normal state self-energies show a weak k- and strong frequency dependence with particularly large electronic mass enhancement in the vicinity of spin density wave instability. Although the results presented here do not bring any surprisingly new physics to this very old problem, our approach is an attempt to establish the numerical procedure to evaluate material specific coupling constant λ\lambda for high Tc_{c} superconductors without reliance on tight-binding approximations of their electronic structures.Comment: 10 pages, 7 figure

    Origin of Low Thermal Conductivity in Nuclear Fuels

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    Using a novel many-body approach, we report lattice dynamical properties of UO2 and PuO2 and uncover various contributions to their thermal conductivities. Via calculated Grueneisen constants, we show that only longitudinal acoustic modes having large phonon group velocities are efficient heat carriers. Despite the fact that some optical modes also show their velocities which are extremely large, they do not participate in the heat transfer due to their unusual anharmonicity. Ways to improve thermal conductivity in these materials are discussed.Comment: 4 pages, 3 figures, 1 tabl

    Density-functional calculations of the electronic structure and lattice dynamics of superconducting LaO0.5_{0.5}F0.5_{0.5}BiS2_{2}: Evidence for an electron-phonon interaction near the charge-density-wave instability

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    We discuss the electronic structure, lattice dynamics and electron-phonon interaction of newly discovered superconductor LaO0.5_{0.5}F0.5_{0.5}BiS2_{2} using density functional based calculations. A strong Fermi surface nesting at k\mathbf{k}=(π\pi ,π\pi ,0) suggests a proximity to charge density wave instability and leads to imaginary harmonic phonons at this k\mathbf{k} point associated with in-plane displacements of S atoms. Total energy analysis resolves only a shallow double-well potential well preventing the appearance of static long-range order. Both harmonic and anharmonic contributions to electron-phonon coupling are evaluated and give a total coupling constant λ≃0.85\lambda \simeq 0.85 prompting this material to be a conventional superconductor contrary to structurally similar FeAs materials.Comment: Supplementary Materials is adde
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