We study a four-band model for the iron oxypnictides, in which the superconducting properties are assumed to be determined by the interband coupling between hole-like and electron-like Fermi sheets. We show that reasonable parameters can account for the angle-resolved photoemission spectra showing multiple gaps in Ba$_{1-x}$K$_x$Fe$_2$As$_2$, and for the temperature dependence of the superfluid density. At the same time, the zero-temperature value of the superfluid density shows a conventional scaling with the number of carriers.
The sign-reversing fully gapped superconducting state, which is expected to be realized in oxypnictide superconductors, can be prominently affected by nonmagnetic impurities due to the interband scattering of Cooper pairs. We study this problem based on the isotropic two-band BCS model: In oxypnictide superconductors, the interband impurity scattering $I'$ is not equal to the intraband one $I$. In the Born scattering regime, the reduction in Tc is sizable and the impurity-induced density of states (DOS) is prominent if $I\sim I'$, due to the interband scattering. Although impurity-induced DOS …
Employing the four-band tight-binding model we study theoretically the doping dependence of the spin response in the normal state of novel Fe-based pnictide superconductors. We show that the commensurate spin density wave (SDW) transition that arises due to interband scattering between the hole $α$-pockets and the electron $β$-pockets disappears already at the doping concentration $x \approx 0.04$ reflecting the evolution of the Fermi surfaces. Correspondingly, with further increase of the doping the antiferromagnetic fluctuations are suppressed for $x > 0.1$ and the Im$χ({\bf Q_{AFM}},ω)$ bec…
We report plane-polarized Raman scattering spectra of iron oxypnictide superconductor NdFeAsO$_{1-x}$F$_x$ single crystals with varying fluorine $x$ content. The spectra exhibit sharp and symmetrical phonon lines with a weak dependence on fluorine doping $x$. The temperature dependence does not show any phonon anomaly at the superconducting transition. The Fe related phonon intensity shows a strong resonant enhancement below 2 eV. We associate the resonant enhancement to the presence of an interband transition around 2 eV observed in optical conductivity. Our results point to a rather weak cou…
Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we studied SnS contacts in the layered oxypnictide superconductors Sm$_{1-x}$Th$_x$OFeAs with various thorium doping and critical temperatures $T_C = 21-54$ K. We observe a scaling between both superconducting gaps and $T_C$. The determined BCS-ratio for the large gap $2Δ_L/k_BT_C = 5.0-5.7$ and its eigen BCS-ratio (in a hypothetical case of zero interband coupling) $2Δ_L/k_BT_C^L = 4.1-4.6$ both exceeding the weak-coupling limit 3.52, and for the small gap $2Δ_S/k_BT_C = 1.2-1.6$ remain nearly constant within all the $T…
We show that in two-band $s$-wave superfluids it is possible to induce quantum phase transitions (QPTs) by tuning intraband and interband $s$-wave interactions, in sharp contrast to single-band $s$-wave superfluids, where only a crossover between Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein condensation (BEC) superfluidity occurs. For non-zero interband and attractive intraband interactions, we demonstrate that the ground state has always two interpenetrating superfluids possessing three spectroscopically distinct regions where pairing is qualitatively different: (I) BCS pairing in both b…
We analyze the evolution of two-band superfluidity from the weak coupling Bardeen-Cooper-Schrieffer (BCS) to the strong coupling Bose-Einstein condensation (BEC) limit. When the interband interaction is tuned from negative to positive values, a quantum phase transition occurs from a 0-phase to a $π$-phase state, depending on the relative phase of two order parameters. Furthermore, population imbalances between the two bands can be created by tuning the intraband or interband interactions. We also find two undamped low energy collective excitations corresponding to in-phase and out-of-phase mod…
The superconducting pairing state of LaPt$_{4}$Ge$_{12}$ is studied by measuring the magnetic penetration depth $λ(T,B)$ and the superfluid density $ρ_s(T)$ using a tunnel-diode-oscillator (TDO)-based method and by transverse field muon-spin rotation ($μ$SR) spectroscopy. $λ(T)$ follows an exponential-type temperature dependence at $T\ll T_{c}$, but its zero-temperature value $λ(0)$ increases linearly with magnetic field. Detailed analyses demonstrate that both $λ(T)$ and the corresponding $ρ_{s}(T)$, measured in the Meissner state by the TDO method are well described by a two-gap $γ$ model wi…
Superfluidity has recently been reported in double electron-hole bilayer graphene. The multiband nature of the bilayers is important because of the very small band gaps between conduction and valence bands. The long range nature of the superfluid pairing interaction means that screening must be fully taken into account. We have carried out a systematic mean-field investigation that includes (i) contributions to screening from both intraband and interband excitations, (ii) the low-energy band structure of bilayer graphene with its small band gap and flattened Mexican hat-like low-energy bands, …
Nonequilibrium conditions offer novel routes to superconductivity that are not available at equilibrium. For example, by engineering nonequilibrium electronic populations, pairing may develop between electrons in different energy bands. A concrete proposal has been made to photo-induce superconductivity in a semiconductor, where pairing occurs between electrons in the conduction and valence bands, even for repulsive interactions. Here, we calculate the superfluid density for such a nonequilibrium paired state, and find it to be positive for repulsive interactions and interband pairing. The pos…
We investigate Bardeen-Cooper-Schrieffer (BCS)- Bose-Einstein condensation (BEC) crossover in a two-band superfluid Fermi gas with an energy shift between the bands. When the intraband coupling in the cold (first) band is fixed as weak, we find that in the case of vanishing interband interaction and in the strong-coupling limit of the hot (second) band the system undergoes a transition to a single-component configuration with the full suppression of the first energy gap and with the full redistribution of particles between bands. For non-vanishing interband interaction we reveal the non-monoto…
We investigate the optical response properties of three-dimensional Luttinger semimetals with the Fermi energy close to a quadratic band touching point. In particular, in order to address recent experiments on the spectroscopy of Pyrochlore Iridates and half-Heusler superconductors, we derive expressions for the optical conductivity in both the normal and general superconducting states in the linear response regime within the random phase approximation. The response functions can be decomposed into contributions from intraband and interband transitions, the latter comprising a genuine signatur…
The free energy expansion of a two-band pair-transfer superconductor is developed. A critical in temperature and a noncritical coherence length appear. The effective in-plane $ξ_{ab}$, $H_{c2}\sp c\sim ξ_{ab}\sp{-2}$ and the thermodynamic critical magnetic field ($H_{c0}$) on the whole hole doping ($p$) scale are calculated for a "typical'' cuprate. A doping-prepared bare spectrum with normal state gaps quenched by doping has been used. The coherence length $ξ_{ab}$ falls with $p$ first rapidly. A moderate enhancement starts when the "cold'' defect band overlap with the valence band is reached…
To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of magnetic torque $τ(θ)$ in the mixed state of SmO$_{0.8}$F$_{0.2}$FeAs single crystals as functions of temperature $T$ and high magnetic field $H$ up to 30 T. We show that the effective mass anisotropy parameter $γ$ extracted from $τ(θ)$, can be greatly overestimated if the strong paramagnetism of Sm or Fe ions is not properly taken into account. The correctly extracted $γ$ depends on both $T$ and $H$, saturating at $γ\simeq 9$ at lower temperatures. Neither the London penetration depth…
We present a theory of the superfluid weight in multiband attractive Hubbard models within the Bardeen-Cooper-Schrieffer (BCS) mean field framework. We show how to separate the geometric contribution to the superfluid weight from the conventional one, and that the geometric contribution is associated with the interband matrix elements of the current operator. Our theory can be applied to systems with or without time reversal symmetry. In both cases the geometric superfluid weight can be related to the quantum metric of the corresponding noninteracting systems. This leads to a lower bound on th…
We report the first direct measurements of superfluid density, ns(T) \propto λ-2(T), in films of Fe-pnictide superconductors. The magnetic penetration depth, λ(0), in our epitaxial, single-crystal Ba(CoxFe1-x)2As2 films near optimal doping (x=0.08) is 350 nm to 430 nm, comparable to bulk single crystals. The T-dependence of λ-2 indicates a small s-wave gap, 2Δ(0)/kBTc = 2.2 \pm 0.1. In detail, λ has power-law behavior at low T: λ(T)/λ(0) - 1 \approx 0.60*(T/Tc)2.5\pm0.1. The small gap, together with power-law behavior at low T, suggests strong intraband scattering on the larger-gap Fermi surfa…
We report on the dramatic effect of random point defects, produced by proton irradiation, on the superfluid density $ρ_{s}$ in superconducting Ca$_{0.5}$Na$_{0.5}$Fe$_2$As$_2$ single crystals. The magnitude of the suppression is inferred from measurements of the temperature-dependent magnetic penetration depth $λ(T)$ using magnetic force microscopy. Our findings indicate that a radiation dose of 2$\times10^{16}$cm$^{-2}$ produced by 3 MeV protons results in a reduction of the superconducting critical temperature $T_{c}$ by approximately 10%. % with no appreciable change in the slope of the upp…
We propose to increase the superconducting transition temperature Tc of strongly correlated materials by designing heterostructures which exhibit a high pairing energy as a result of magnetic fluctuations. More precisely, applying an effective theory of the doped Mott insulator, we envisage a bilayer Hubbard system where both layers exhibit prominent intralayer (intraband) d-wave superconducting correlations. Introducing a finite asymmetry between the hole densities of the two layers such that one layer becomes slightly more underdoped and the other more overdoped, we evidence a visible enhanc…
High-quality single crystals of NdFeAsO$_{0.65}$F$_{0.35}$ (the transition temperature $T_{\rm c} \simeq 30.6$~K) were studied in zero-field (ZF) and transverse-field (TF) muon-spin rotation/relaxation ($μ$SR) experiments. An upturn in muon-spin depolarization rate at $T\lesssim 3$~K was observed in ZF-$μ$SR measurements and it was associated with the onset of ordering of Nd electronic moments. Measurements of the magnetic field penetration depth ($λ$) were performed in the TF geometry. By applying the external magnetic field $B_{\rm ex}$ parallel to the crystallographic $c$-axis ($B_{\rm ex}\…
We show, based on microscopic models, that fermionic band insulators with multiple bands and strong interband attraction are generically unstable towards nonzero momentum Cooper pairing leading to a pair density wave (PDW) superfluid state. Our first model considers a band insulating state of fermionic atoms in a three-dimensional cubic optical lattice. We show that this insulator is unstable towards an incommensurate PDW in the vicinity of a Feshbach resonance. Our second model is a two-band tight binding model relevant to electrons in solids; we show that the insulating state of this model h…
The temperature-dependence of the in-plane optical properties of (CaFe$_{1-x}$Pt$_{x}$As)$_{10}$Pt$_{3}$As$_{8}$ have been investigated for the undoped ($x=$0) parent compound, and the optimally-doped ($x=$0.1) superconducting material ($T_{c}\simeq$ 12 K) over a wide frequency range. The optical conductivity has been described using two free-carrier (Drude) components, in combination with oscillators to describe interband transitions. At room temperature, the parent compound may be described by a strong, broad Drude term, as well as a narrow, weaker Drude component. Below the structural and m…
We study the superfluid critical temperature in a two-band attractive Fermi system with strong pairing fluctuations associated with both interband and intraband couplings. We focus specifically on a configuration where the intraband coupling is varied from weak to strong in a shallow band coupled to a weakly-interacting deeper band. The whole crossover from the Bardeen-Cooper-Schrieffer (BCS) condensation of largely overlapping Cooper pairs to the Bose-Einstein condensation (BEC) of tightly bound molecules is covered by our analysis, which is based on the extension of the Nozières-Schmitt-Rink…
We studied iron-based superconductors of various families with critical temperatures covering almost all range $T_C = 9 - 53$ K. In natural arrays of contacts formed in these materials we observed intrinsic multiple Andreev reflections effect (IMARE). By using IMARE spectroscopy, we detected the two-gap superconductivity, determined the value of the large and the small superconducting gaps, and the corresponding BCS-ratios. The temperature dependencies of the large and the small gaps $Δ_{L,S}(T)$ are similar for various families of the Fe-based superconductors and could be well-fitted in the f…
