Pair-Hopping Mechanism for Layered Superconductors

We propose a possible charge fluctuation effect expected in layered superconducting materials. In the multireference density functional theory, relevant fluctuation channels for the Josephson coupling between superconducting layers include the interlayer pair hopping derived from the Coulomb repulsion. When interlayer single-electron tunneling processes are irrelevant in the Kohn-Sham electronic band structure calculation, the two-body effective interactions stabilize a superconducting phase. This state is also regarded as a valence-bond solid in a bulk electronic state. The hidden order parameters coexist with the superconducting order parameter when the charging effect of a layer is comparable to the pair hopping. Relevant materials structures favorable for the pair-hopping mechanism are discussed.Comment: 24 pages, 2 figures, to be published in J. Phys. Soc. Jpn. (2009

Self-consistent description of Andreev bound states in Josephson quantum dot devices

We develop a general perturbative framework based on a superconducting atomic limit for the description of Andreev bound states (ABS) in interacting quantum dots connected to superconducting leads. A local effective Hamiltonian for dressed ABS, including both the atomic (or molecular) levels and the induced proximity effect on the dot is argued to be a natural starting point. A self-consistent expansion in single-particle tunneling events is shown to provide accurate results even in regimes where the superconducting gap is smaller than the atomic energies, as demonstrated by a comparison to recent Numerical Renormalization Group calculations. This simple formulation may have bearings for interpreting Andreev spectroscopic experiments in superconducting devices, such as STM measurements on carbon nanotubes, or radiative emission in optical quantum dots.Comment: 12 pages, 11 figures. Last version: we added several extra references, modified two figures, and discussed recent proposals for Andreev spectroscop

The K2-ESPRINT Project VI: K2-105 b, a Hot-Neptune around a Metal-rich G-dwarf

We report on the confirmation that the candidate transits observed for the star EPIC 211525389 are due to a short-period Neptune-sized planet. The host star, located in K2 campaign field 5, is a metal-rich ([Fe/H] = 0.26$\pm$0.05) G-dwarf (T_eff = 5430$\pm$70 K and log g = 4.48$\pm$0.09), based on observations with the High Dispersion Spectrograph (HDS) on the Subaru 8.2m telescope. High-spatial resolution AO imaging with HiCIAO on the Subaru telescope excludes faint companions near the host star, and the false positive probability of this target is found to be <$10^{-6}$ using the open source vespa code. A joint analysis of transit light curves from K2 and additional ground-based multi-color transit photometry with MuSCAT on the Okayama 1.88m telescope gives the orbital period of P = 8.266902$\pm$0.000070 days and consistent transit depths of $R_p/R_\star \sim 0.035$ or $(R_p/R_\star)^2 \sim 0.0012$. The transit depth corresponds to a planetary radius of $R_p = 3.59_{-0.39}^{+0.44} R_{\oplus}$, indicating that EPIC 211525389 b is a short-period Neptune-sized planet. Radial velocities of the host star, obtained with the Subaru HDS, lead to a 3\sigma\ upper limit of 90 $M_{\oplus} (0.00027 M_{\odot})$ on the mass of EPIC 211525389 b, confirming its planetary nature. We expect this planet, newly named K2-105 b, to be the subject of future studies to characterize its mass, atmosphere, spin-orbit (mis)alignment, as well as investigate the possibility of additional planets in the system.Comment: 11 pages, 9 figures, 4 tables, PASJ accepte

Composite-Fermion Picture for the Spin-Wave Excitation in the fractional quantum Hall system

Spin-wave excitation mode from the spin-polarized ground state in the fractional quantum Hall liquid with odd fractions ($\nu=1/3,1/5$) numerically obtained by the exact diagonalization of finite systems is shown to be accurately described, for wavelengths exceeding the magnetic length, in terms of the composite-fermion mean-field approximation for the spin-wave (magnon) theory formulated in the spherical geometry. This indicates that the composite picture extends to excited states, and also provides the spin stiffness in terms of peculiar exchange interactions.Comment: 10 pages, typeset in LATEX, NA-94-05, 2 figures available upon request at [email protected]

The X^- Solution to the ^6Li and ^7Li Big Bang Nucleosynthesis Problems

The $^6$Li abundance observed in metal poor halo stars appears to exhibit a plateau as a function of metallicity similar to that for $^7$Li, suggesting a big bang origin. However, the inferred primordial abundance of $^6$Li is $\sim$1000 times larger than that predicted by standard big bang nucleosynthesis for the baryon-to-photon ratio inferred from the WMAP data. Also, the inferred $^7$Li primordial abundance is 3 times smaller than the big bang prediction. We here describe in detail a possible simultaneous solution to both the problems of underproduction of $^6$Li and overproduction of $^7$Li in big bang nucleosynthesis. This solution involves a hypothetical massive, negatively-charged leptonic particle that would bind to the light nuclei produced in big bang nucleosynthesis, but would decay long before it could be detected. We consider only the $X$-nuclear reactions and assume that the effect of decay products is negligible, as would be the case if lifetime were large or the mass difference between the charged particle and its daughter were small. An interesting feature of this paradigm is that, because the particle remains bound to the existing nuclei after the cessation of the usual big bang nuclear reactions, a second longer epoch of nucleosynthesis can occur among $X$-nuclei. We confirm that reactions in which the hypothetical particle is transferred can occur that greatly enhance the production of $^6$Li while depleting $^7$Li. We also identify a new reaction that destroys large amounts of $^7$Be, and hence reduces the ultimate $^7$Li abundance. Thus, big-bang nucleosynthesis in the presence of these hypothetical particles, together with or without an event of stellar processing, can simultaneously solve the two Li abundance problems.Comment: 18 pages, 7 figures, minor changes and references added, ApJ accepte

Quantum phase transition in a minimal model for the Kondo effect in a Josephson junction

We propose a minimal model for the Josephson current through a quantum dot in a Kondo regime. We start with the model that consists of an Anderson impurity connected to two superconducting (SC) leads with the gaps $\Delta_{\alpha}=|\Delta_{\alpha}| e^{i \theta_{\alpha}}$, where $\alpha = L, R$ for the lead at left and right. We show that, when one of the SC gaps is much larger than the others $|\Delta_L| \gg |\Delta_R|$, the starting model can be mapped exactly onto the single-channel model, which consists of the right lead of $\Delta_R$ and the Anderson impurity with an extra onsite SC gap of $\Delta_d \equiv \Gamma_L e^{i \theta_L}$. Here $\theta_L$ and $\Gamma_L$ are defined with respect to the starting model, and $\Gamma_L$ is the level width due to the coupling with the left lead. Based on this simplified model, we study the ground-state properties for the asymmetric gap, $|\Delta_L| \gg |\Delta_R|$, using the numerical renormalization group (NRG) method. The results show that the phase difference of the SC gaps $\phi \equiv \theta_R -\theta_L$, which induces the Josephson current, disturbs the screening of the local moment to destabilize the singlet ground state typical of the Kondo system. It can also drive the quantum phase transition to a magnetic doublet ground state, and at the critical point the Josephson current shows a discontinuous change. The asymmetry of the two SC gaps causes a re-entrant magnetic phase, in which the in-gap bound state lies close to the Fermi level.Comment: 23 pages, 13 figures, typos are correcte

Contrast of LiFeAs with isostructural, isoelectronic, and non-superconducting MgFeGe

Stoichiometric LiFeAs at ambient pressure is an 18 K superconductor while isoelectronic MgFeGe is not, despite their extremely similar electronic structures. To investigate possible sources of this distinctively different superconducting behavior, we quantify the differences using first principles density functional theory, establishing first that the Fe total 3d occupations are identical in the two compounds. Individual 3d orbital occupations also differ very little ($\sim 0.01$). The differences in Fermi surfaces (FSs) do not seem significant; however a redistribution of bands just above the Fermi level does represent a possibly significant distinction. Because the bands and FSs of LiFeAs are less in agreement with experiment than for other iron-pnictides, we study the effects of additional exchange-correlations effects beyond GGA (the generalized gradient approximation) by applying the modified Becke-Johnson potential (mBJ) exchange potential, which gives much improved bandgaps in insulators compared to GGA and might be useful for semimetals such as the Fe-based superconductors. Overall, we conclude that the mBJ corrections do not improve the description of LiFeAs as compared to experiment