Normal state bottleneck and nematic fluctuations from femtosecond quasi-particle relaxation dynamics in Sm(Fe,Co)AsO

We investigate temperature and fluence dependent dynamics of the photoexcited quasi-particle relaxation and low-energy electronic structure in electron-doped 1111-structure Sm(Fe_{0.93}Co_{0.07})AsO single crystal. We find that the behavior is qualitatively identical to the 122-structure Ba(Fe,Co)_{2}As_{2} including the presence of a normal state pseudogap and a marked 2-fold symmetry breaking in the tetragonal phase that we relate to the electronic nematicity. The 2-fold symmetry breaking appears to be a general feature of the electron doped iron pnictides

Quasiparticle relaxation dynamics in spin-density-wave and superconducting SmFeAsO_{1-x}F_{x} single crystals

We investigate the quasiparticle relaxation and low-energy electronic structure in undoped SmFeAsO and near-optimally doped SmFeAsO_{0.8}F_{0.2} single crystals - exhibiting spin-density wave (SDW) ordering and superconductivity respectively - using pump-probe femtosecond spectroscopy. In the undoped single crystals a single relaxation process is observed, showing a remarkable critical slowing down of the QP relaxation dynamics at the SDW transition temperature T_{SDW}\simeq125{K}. In the superconducting (SC) crystals multiple relaxation processes are present, with distinct SC state quasiparticle recombination dynamics exhibiting a BCS-like T-dependent superconducting gap, and a pseudogap (PG)-like feature with an onset above 180K indicating the existence of a pseudogap of magnitude 2\Delta_{\mathrm{PG}}\simeq120 meV above T_{\mathrm{c}}. From the pump-photon energy dependence we conclude that the SC state and PG relaxation channels are independent, implying the presence of two separate electronic subsystems. We discuss the data in terms of spatial inhomogeneity and multi-band scenarios, finding that the latter is more consistent with the present data.Comment: Replaced by the correct versio

Inter-site pair superconductivity: origins and recent validation experiments

The challenge of understanding high-temperature superconductivity has led to a plethora of ideas, but 30 years after its discovery in cuprates, very few have achieved convincing experimental validation. While Hubbard and t-J models were given a lot of attention, a number of recent experiments appear to give decisive support to the model of real-space inter-site pairing and percolative superconductivity in cuprates. Systematic measurements of the doping dependence of the superfluid density show a linear dependence on superfluid density - rather than doping - over the entire phase diagram, in accordance with the model's predictions. The doping-dependence of the anomalous lattice dynamics of in-plane Cu-O mode vibrations observed by inelastic neutron scattering, gives remarkable reciprocal space signature of the inter-site pairing interaction whose doping dependence closely follows the predicted pair density. Symmetry-specific time-domain spectroscopy shows carrier localization, polaron formation, pairing and superconductivity to be distinct processes occurring on distinct timescales throughout the entire superconducting phase diagram. The three diverse experimental results confirm non-trivial predictions made more than a decade ago by the inter-site pairing model in the cuprates, remarkably also confirming some of the fundamental notions mentioned in the seminal paper on the discovery of high-temperature superconductivity in cuprates.Comment: Dedicated to Prof. K. A. Mueller on the Occasion of his 90th Birthda

Polaron and bipolaron transport in a charge segregated state of doped strongly correlated 2D semiconductor

The 2D lattice gas model with competing short and long range interactions is appliedused for calculation of the incoherent charge transport in the classical strongly-correlated charge segregated polaronic state. We show, by means of Monte-Carlo simulations, that at high temperature the transport is dominated by hopping of the dissociated correlated polarons, where with thetheir mobility is inversely proportional to the temperature. At the temperatures below the clustering transition temperature the bipolaron transport becomes dominant. The energy barrier for the bipolaron hopping is determined by the Coulomb effects and is found to be lower than the barrier for the single-polaron hopping. This leads to drastically different temperature dependencies of mobilities for polarons and bipolarons at low temperatures

Diamagnetism of real-space pairs above Tc in hole doped cuprates

The nonlinear normal state diamagnetism reported by Lu Li et al. [Phys. Rev. B 81, 054510 (2010)] is shown to be incompatible with an acclaimed Cooper pairing and vortex liquid above the resistive critical temperature. Instead it is perfectly compatible with the normal state Landau diamagnetism of real-space composed bosons, which describes the nonlinear magnetization curves in less anisotropic cuprates La-Sr-Cu-O (LSCO) and Y-Ba-Cu-O (YBCO) as well as in strongly anisotropic bismuth-based cuprates in the whole range of available magnetic fields.Comment: 4 pages, 4 figure

Dependence of the superconducting effective mass on doping in cuprates

Using a doping-determined multiband model spectrum of a "typical'' cuprate the effective mass of the paired carriers is calculated on the whole doping scale. Large $m_{ab}$ values quench rapidly with leaving the very underdoped region. Further slower diminishing of $m_{ab}$ reproduces the trend towards restoring the Fermi-liquid behaviour in cuprates with progressive doping. The interband superconducting condensate density ($n_s$) shows similar behaviour to the transition temperature and superconducting gaps. The $n_s(0)/m_{ab}$ ratio has an expressed maximum close to optimal doping as also the thermodynamic critical field. All the overlapping band components are intersected by the chemical potential at this. The pairing strength and the phase coherence develop simultaneously. In spite of its simplicity, the model describes the behaviour of various cuprate characteristics on the doping scale.Comment: 9 pages, 5 figure